Breath Figure: A Nature-Inspired Preparation Method for Ordered

Review pubs.acs.org/CR

Breath Figure: A Nature-Inspired Preparation Method for Ordered Porous Films Aijuan Zhang, Hua Bai, and Lei Li* College of Materials, Xiamen University, Xiamen, 361005, People’s Republic of China 5.5. Interfacial Tension between Water Droplets and the Polymer Solution 5.6. Deformation of BFA Films 5.7. Strategies for Perforated BFAs 5.8. Methods for Hierarchical BFA Films 5.9. Large-Scale Preparation of BFA Films 5.10. Other Methods 6. Chemical Modification and Functionalization of BFA Films 6.1. Functionalization of BFA Films 6.1.1. Hybridization with Inorganic Nanomaterials 6.1.2. Surface Chemical Modification 6.2. Cross-Linking of BFA Films 6.2.1. Thermal Cross-Linking 6.2.2. Chemical Cross-Linking 6.2.3. Photochemical Cross-Linking 7. Applications of BFA Films 7.1. Templates 7.2. Surface Enhanced Raman Scattering Substrates 7.3. Separation 7.4. Optical and Optoelectronic Devices 7.5. Chemical Power Source 7.6. Micropatterning of Biological Molecules 7.7. Cell Culture and Inhibition 7.8. Sensors 7.9. Miscellaneous Applications 8. Conclusion Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Preparation Methods for BFA Films 2.1. Conventional BF Methods: Dynamic and Static Processes 2.2. Spin-Coating and Dip-Coating 2.3. Generalized BF Methods 3. Formation Mechanism for BFA Films 3.1. Cooling of Solution and Nucleation of Moisture 3.2. Growth and Self-Assembly of Water Droplets 3.2.1. Shape of Water Droplets 3.2.2. Stability of Water Droplets 3.2.3. Growth of Water Droplets 3.2.4. Assembly of Water Droplets 3.3. Evaporation of Solvent and Water Droplets 4. Materials for BFA Films 4.1. Linear Block Copolymers 4.2. Star and Branch Polymers 4.3. Linear Homopolymers 4.4. Biodegradable Polymers 4.5. Microgels 4.6. Nonpolymeric Materials 4.6.1. Nanoparticles 4.6.2. Carbon Nanotubes and Graphene 4.6.3. Small Organic Molecules 5. Morphological Control of BFA Films 5.1. Solvents 5.2. Substrates 5.2.1. Materials of the Substrates 5.2.2. Shape of the Substrates 5.3. Solution Concentrations 5.4. Vapor Conditions 5.4.1. Water Vapor 5.4.2. Nonaqueous Vapors © XXXX American Chemical Society

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1. INTRODUCTION Nature provides inexhaustible inspiration to mankind. It continues to encourage us to develop new methods and approaches to the construction of artificial advanced materials. For example, the self-cleaning of the lotus leaf originates from the fractal structure features on the leaves combined with their waxy surface.1,2 Key advances in the understanding and fabrication of surfaces with controlled wettability are poised to make the dream of a contamination-free (or “no-clean”) Received: February 3, 2015

A

DOI: 10.1021/acs.chemrev.5b00069 Chem. Rev. XXXX, XXX, XXX−XXX

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surface come true (Figure 1a−c).3−5 In another example, the mimicking of nacre enables the fabrication of bulk ceramics

Figure 2. (a) Dew on leaves. (b) Pouring collodion on the clean glass substrate before making a wet-collodion negative.16 Reprinted with permission ref 16. Copyright Freestyle Photographic Supplies.

contamination on glass substrates. In Figure 2b, a historical picture demonstrates how to pour collodion onto a clean sheet of glass to make negatives.16 For a long time, BF was considered an ordinary natural phenomenon without practical applications in materials science. Then, in 1994, François et al. reported the preparation of polymeric porous film with honeycomb structures by exposing a drop of polystyrene-b-polyparaphenylene (PS-b-PPP, 1) (Scheme 1) solution in carbon disulfide (CS2) to a flow of

Figure 1. Examples of natural materials and nature-inspired materials with high performance. (a, d, g) Photographs of natural materials: lotus leaf (a), shell (d), and male Morpho didius butterfly (g). (b, e, h) Multiscale structures of the corresponding natural materials.1,6,9 (b) Reprinted with permission from ref 1. Copyright 2009 The Royal Society. (e) Reprinted with permission from ref 6. Copyright 2008 Elsevier. (h) Reprinted with permission from ref 9. Copyright 2002 The Royal Society. (c, f, i) Nature-inspired materials with superhydrophobicity and self-cleaning capacity (c),4 high mechanical strength (f),7 and structure-based colors (i).10 (c) Reprinted with permission from ref 4. Copyright 2002 Wiley-VCH. (f) Reprinted with permission from ref 7. Copyright 2008 American Association for the Advancement of Science. (i) Reprinted with permission from ref 10. Copyright Donna Sgro.

Scheme 1. Chemical Structure of PS-b-PPP (1)

without a ductile phase but with a unique combination of strength (470 MPa), toughness (22 MPa m1/2), and stiffness (290 GPa).6,7 These ceramics retain their mechanical properties at high temperature (600 °C) (Figure 1d−f). MORPHOTEX is an unstained, structurally colored fiber.8 This technology is based on the mimicking of the microscopic structure of the morpho butterfly’s wings.9 No dye or pigment is used. Instead, color is created by varying the thickness and structure of the fibers (Figure 1g−i). The use of the breath figure (BF) method in materials science, which is the topic of this review, is another example where we learn from nature. BF refers to the fog that forms when water vapor contacts a cold surface. BF is a commonly observed phenomenon in daily life. One example is the fog that appears on a window when we breathe on it. This is also the origin of the name “breath figure”. Another example in nature is the formation of dew, as shown in Figure 2a.11 The formation of BF was first investigated by John Aitken12,13 in 1895 and later, in 1911, by Lord Rayleigh,14 who revealed the morphology of water droplets on cold surfaces. Depending on the wetting properties of the surface, the condensed fluid can form either a uniform film, which appears dark (black BF), or an assembly of droplets that scatter light and appear white (gray BF).15 On clean hydrophilic glass, a uniform black BF is expected. For the wet-plate photographic technique, a scrupulously clean piece of clear glass is required. Therefore, the BF method has been used since the 1850s by photographers as a simple and effective way to detect oil

Figure 3. SEM image of PS-b-PPP BFA film.17 Reprinted with permission from ref 17. Copyright 1994 Nature Publishing Group.

moist air.17 As shown in Figure 3, the micrometer-sized pores exhibited a highly regular hexagonal arrangement after removal of the upper layer of these films. The relationship between the works of Lord Rayleigh and François was immediately recognized: the hexagonally packed pores were the result of BFs on the surface of the solution. Therefore, the ordered pore arrays on the polymer film prepared with this method were named breath figure arrays (BFAs), and the films were called BFA films. In the past two decades, this simple process has been widely utilized as a versatile soft-template method for the fabrication of honeycomb-patterned films with controlled pore size and different potential functions and applications, starting from a variety of organic and inorganic materials. Compared with the existing lithography techniques, including e-beam lithography and scanning probe lithography, BF technique has the following advantages: (1) very low costno complicated instrument is needed, and the operation cost is also very low; (2) time savingthousands of pores can be fabricated rapidly B


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2.1. Conventional BF Methods: Dynamic and Static Processes

with one step in less than 5 min, in sharp contrast to e-beam lithography and photolithography in which multistep processes and much more time are involved; (3) easy implementation no complex setup or environment control, such as vacuuming, scanning electron microscopy (SEM), or clean room, is necessary. Accordingly, the BF process has become a versatile and handy strategy for the preparation of micropatterns and porous materials in laboratories. Additionally, the BF process has aroused the interest of industry: a patent by Fuji Film Co. demonstrated their interest in equipment that could produce BFA films continuously on a commercial scale.18 The progress of the BF technique in materials science up to early 2013 has been summarized in several remarkable reviews.19−28 However, in the past two years, a great number of reports (over 100 papers) have been published, and these provide many new insights into the mechanism of the BF process and show novel and exciting applications of BFA films. These publications have demonstrated the rapid development of the BF technique and have, to some extent, changed our understanding of this technique. Therefore, it is necessary to give an overall review of the BF technique based on the new findings from recent reports. In this review, we will present a comprehensive summarization on the state of the art in the preparation and applications of porous materials (mainly films) based on the BF process. A historical account of important early studies is included in this section, and the methods for preparing BFAs are summarized in section 2. Although the preparation of BFAs is quite simple, the mechanism of BFA formation is rather abstruse due to the complex mass and heat transfer. In section 3, a comprehensive description of the formation mechanism is presented. In the past 20 years, a number of polymeric and nonpolymeric materials have been developed for the construction of BFAs. This not only expands the applications of BFAs but also facilitates understanding of the formation mechanism. In section 4, the materials used in the literature to fabricate BFAs are summarized. The morphological control of BFAs, including the influence of solvents, substrates, humidity, gas flow, atmosphere, and solution concentration, and the formation of BFAs with perforation and hierarchy, along with other control methods, are discussed in section 5. The functionalization and chemical modification of BFAs that are described in section 6 are critical for the exploration of further practical applications. The applications of BFAs in templating, surface enhanced Raman scattering (SERS), separation, optical and optoelectronic devices, electrode materials, and biology are covered in section 7. Finally, a summary and an outlook on this exciting field are presented at the end of this review in section 8.

In a conventional BF process, a drop of polymer (or other substance) solution is cast onto a solid substrate and allowed to evaporate in a humid environment.17 After the solvent and the condensed water droplets evaporate completely, a porous film forms on the substrate. The process can be divided into dynamic and static BF processes according to how the humid environment is established. In a dynamic BF process, the moisture is supplied by a gas flow blown over the substrate (Figure 4a). This process was

Figure 4. Conventional BF methods. (a) Dynamic and (b) static BF processes.

first used by François et al. in their first publication on BFAs.17 They produced BFAs by evaporating a solution of polymer 1 in CS2 under a flow of moist gas. In a dynamic process, the velocity of the flow and the humidity can be tightly controlled, thus allowing the fine-tuning of the evaporation rate of the solvent and the morphology of the BFAs17,29−32 (see section 3.2.3 for details). In a static BF process, the evaporation of solvent is completed spontaneously without the aid of gas flow. Several researchers, including Shimomura33−35 and Stenzel,19,36 prepared BFAs in their early studies with different polymers in a static humid environment. Recently, our group optimized this method and used it to fabricate various BFA films.37−40 In a typical static BF process, the humid environment can be established by filling water in a sealed vessel or chamber. After a sufficiently long period, the air in the vessel is saturated by water vapor. The humidity depends on the vapor pressure of water, which can be tuned by dissolving a certain amount of inorganic salts in the water (Figure 4b). Because the uncertainties caused by flow disturbance are minimized, the preparation of BFA films is repeatable in each batch with a static BF process. Occasionally, dynamic and static BF processes yield different results. For example, in a dynamic BF process, linear polystyrene (PS) resulted in regular BFA films only under very specific conditions, and the formation of the BFA was very sensitive to the polar end group, molecular weight, and casting solvents.41−44 However, in a static BF process, we have shown that linear PS without a polar end group could also form welldefined BFA films under a wide range of temperatures, solution concentrations, and molecular weights.45 These differences may come from the discrepancy in the evaporation rate of the solvent or the disturbance of the gas flow. Usually, however, both dynamic and static BF processes are efficient ways to produce BFA films.

2. PREPARATION METHODS FOR BFA FILMS BFA refers to the pore array generated by a water droplet array condensed on a cold substrate. Therefore, the BF technique can be defined as the process of preparing an ordered porous film with condensed water droplets as the template. According to the above definition, the condensation of water vapor is the key step in the BF process. There are other similar methods for preparing porous films in which the water droplet template is generated by adding a certain amount of water. These are not BF methods in the strictest sense, so they are set aside for a brief discussion as complements at the end of the section. C


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2.2. Spin-Coating and Dip-Coating

However, for these generalized BF methods, a high atmospheric humidity is not required. Water-in-oil emulsion has been used to construct porous polymer films.69−71 The water droplets in the emulsion can self-assemble during the evaporation of organic solvents, yielding a hexagonally packed array. For example, Wu et al. prepared a water-in-oil reversed hybrid microemulsion by mixing a poly(methyl methacrylate) (PMMA) dichloromethane solution containing P123 with a polyoxometalate (POM) aqueous solution through shaking.69 When the microemulsion was cast onto the substrate and allowed to dry under 30−40% relative humidity, a porous film with an ordered pore array was obtained. The size of the pores could be adjusted by changing the volume fraction of water in the microemulsion, indicating that the water droplets in the microemulsion acted as the templates for the pores. Similarly, porous films can be obtained with a THF/water mixture,49,50,72−75 dichloromethane/tertiary amyl alcohol,76 dichloromethane/propylene glycol,77 or an acetone/toluene78 mixture as the solvent. The pore size will increase with the amount of water in the solution.79 For example, Pourabbas et al. soaked a PMMA slab in THF containing 5 or 10% water to swell the surface of the slab.80 They then transferred the slab into a dry atmosphere. After the solvent fully evaporated, pores with a diameter of several micrometers were found on the PMMA slab. The template of the pores was believed to be the water droplet array left after THF evaporation. Because water is the nonsolvent of PMMA, upon the evaporation of THF, the water fraction in the solution continued to increase, leading to phase separation between water and the PMMA solution. These water droplets self-assembled into an ordered array and produced hexagonally packed pores on the surface of the PMMA slab. Bormashenko et al. developed a novel method for preparing porous polymeric films with a bubble array as the template.66,81 They used a dip-coating technique to coat the substrate with a polymer solution. The dip-coating was performed with an unusually high pulling speed of V = 47 cm min−1, and the pulling direction was vertical. When the substrate was pulled out of the solution, the film was immediately dried with a hot air current or dried nitrogen (3 m s−1, 20−100 °C). Pores were found on the dry film. The authors claimed that a water droplet array was not responsible for the formation of the porous structure because the humidity of the gas had no effect on the morphology of the porous film. Instead, they proposed that because the drying process took place at a temperature close to or higher than the boiling point of the solvent, there should be extensive formation of bubbles filled with the solvent vapor. These bubbles acted as a template for the formation of pores on the surface of the polymer film.

Spin-coating provides another way to control the BF process, as shown in Figure 5a.46−58 When spin-coating is employed, the

Figure 5. Preparation of BFAs by (a) spin-coating and (b) dip-coating processes.

nucleation rate of water droplets and the influx of water droplets into the solution become important in determining the packing of water droplets because a large amount of the solution dropped on the substrate is removed at the beginning of spin-coating, and the polymer solution solidifies quickly during spin-coating. Kim et al. spin-coated cellulose acetate butyrate (CAB) in a mixed solvent of tetrahydrofuran (THF) and chloroform and studied the influence of concentration on the pore morphology.49,59 Using pure THF as the solvent, a uniform BFA film over a large area was obtained. RodriguezHernandez et al. prepared hierarchically micro- and nanostructured polymer surfaces with block copolymer/homopolymer blends using the spin-coating method.48 Small pores with diameters of 100−300 nm were found in the film and were attributed to the BF process occurring during the spin-coating process. The pores were significantly smaller than those formed in conventional BF processes because the evaporation time was much shorter for the spin-coating process (see section 3.2.3 for details). Dip-coating is a simple way to coat a substrate with polymer film, especially when the shape of the substrate is complex.60−67 To perform dip-coating, a substrate is first soaked in a polymer solution and then removed at a certain speed, as shown in Figure 5b. A thin layer of solution will remain on the surface of the substrate, and after it dries, a uniform polymer film forms and covers the substrate. Mansouri et al. prepared BFA films on a nylon mesh with the dip-coating method.68 They used a polysulfone (PSF) solution in dichloromethane and performed the dip-coating process under high humidity to allow the formation of a template of water droplets. A thin, robust, porous PSF film formed on the surface of the nylon mesh due to the film drainage and the BF process. Dip-coating conditions such as withdrawal speed and holding time can be used to control the morphology of the film.

3. FORMATION MECHANISM FOR BFA FILMS In this section, the formation mechanism for BFA films will be discussed. It should be noted here that the formation of BFAs is a very complicated process, and no general mechanisms have adequately explained all experimental results. In fact, results from different researchers often seem to conflict with one other,43,45,82−88 adding to the difficulties of revealing the mechanism of the BF process. One important reason for this confusing state is that the BF process is controlled by complex heat and mass transfer, and these transfer processes are further dependent upon various experimental parameters, including temperature, humidity, gas flow velocity, the physical properties

2.3. Generalized BF Methods

As mentioned above, in the BF process, water droplet templates are generated by the condensation of moisture. There are similar processes that produce ordered porous films from a template not involving water droplets or in which the water droplet templates are not generated from water vapor. These methods are called “generalized” BF methods. For conventional BF methods, to prepare regular patterns, the relative humidity of the atmosphere should be maintained at a high level. D


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Figure 6. Formation mechanism for BFA.

quite different, they all predicted that, after the solution was cast, the temperature of the solution first decreased sharply to reach a minimum value and then gradually increased, as shown in Figure 7.93−95 It should be noted here that the substrate

of the solvents and the solution, the physical and chemical properties of the solute (typically polymers), and the nature of the substrate. Slight variation in any of the above parameters can significantly change the final BFA films. Moreover, there is no standard BF procedure, and the reported experimental results have all been obtained using homemade equipment. Details regarding these apparatuses, which played important roles in BF processes, were usually not described exhaustively in the reports. Therefore, although great efforts have been devoted to uncovering the mechanism of the BF process, some details remain unknown. In this section, we will summarize the results of previous studies and give an overview of the mechanism of BFA formation. There have been various approaches to producing BFAs, but the essence of all these approaches is the construction of a cold solution surface under a humid atmosphere, under which the moisture can condense on the cold surface. The well-accepted mechanism of the entire BF process contains the following steps, as shown in Figure 6: (1) cooling of the solution and nucleation of the moisture, producing small but disordered water droplets on the solution surface; (2) growth and selfassembly of the water droplets, forming an ordered and closely packed water droplet array that covers the entire surface of the solution; and (3) evaporation of the solvent and water droplets, leaving a hexagonal pore array on the dry film. These steps will be discussed in detail in the following sections. 3.1. Cooling of Solution and Nucleation of Moisture

A cold surface is crucial for the BF process. Here, “cold” indicates a temperature sufficiently lower than the dew point of the atmosphere, such that water vapor can condense onto the surface spontaneously. Although directly cooling the substrate and the solution by using a cold stage have occasionally been reported,89,90 evaporation cooling is the most widely used and convenient method for producing a cold solution surface. Evaporation cooling involves casting a solution with a high solvent evaporation rate onto a substrate. The solvent immediately begins to evaporate, and the temperature of the solvent will decrease rapidly due to the enthalpy of vaporization. It was reported that the temperature of a chloroform solution fell to 0−6 °C during evaporation,91 and for the more volatile CS2, the temperature could reach −6 °C.92 These temperatures are significantly lower than the dew point of the atmosphere; thus, the water vapor is able to condense onto the surface of the solution. Although the evaporation cooling process is easy to perform experimentally, it is difficult to describe theoretically. Several researchers have conducted preliminary theoretical investigations of the solvent cooling process and compared their results with the experimental data.93,94 Although their models were

Figure 7. Temperature profiles of the polymer solutions during evaporation. (a) 0.05 mL of pure chloroform evaporating in a dry environment with the substrate placed directly onto a cold plate to maintain the constant boundary condition. 93 Reprinted with permission from ref 93. Copyright 2011 Royal Society of Chemistry. (b) 0.3 mL of 1% PS (molecular weight (Mw) = 50 000) in CS2 on a glass substrate with a thickness of 0.7 mm, evaporating under 60% relative humidity.95 No gas flow was used in either experiment. Reprinted with permission from ref 95. Copyright 2010 The Optical Society.

plays an important role in the evaporation cooling process. The mass of the substrate is usually larger than that of the solution; thus, the heat exchange between the solution and the substrate will apparently influence the temperature of the solution. Bormashenko and Battenbo reported that the thickness of the substrate could influence the amount of condensed water by changing the temperature of the solution, which demonstrated the importance of the substrate effect.93,96,97 Unfortunately, this issue was usually neglected by other researchers when they tried to discuss the effect of the substrate on the BF process.41,98−102 E


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Nucleation of the water vapor is the first step in the formation of a water droplet array. As suggested by Nepomnyashchy et al., two different types of nucleation may occur: homogeneous nucleation, in which the water nucleus appears in the gas phase near the cold liquid surface, and heterogeneous nucleation, in which the water nucleus is formed at the liquid−gas interface as a small liquid lens.103 The nucleation work (ΔE) for homogeneous nucleation is ΔE = −(P′ − P)V + σ13A13

Scheme 2. Chemical Structure of PEO-b-PFOMA (2)

(1)

and for heterogeneous nucleation is ΔE = −(P′ − P)V + σ13A13 + σ12A12 − σ23πR2

(2)

elucidate the mechanism of the BF process, one must interpret the growth of water droplets and the formation of the hexagonal droplet lattice. In this section, we will briefly present the results of investigations in recent years on the growth and self-assembly of the water droplets. 3.2.1. Shape of Water Droplets. It is necessary to investigate the shape of the water droplets suspended on the surface of an organic solution because the shape of the water droplets determines the shape of the pores in the BFAs and is related to the self-assembly process. When a small volume of liquid is dropped onto the surface of another immiscible liquid, it may form a droplet or spread as a thin film. The spread coefficient S can be used to predict the behavior of the droplet from the point of view of thermodynamics. It is defined as108

where P′ − P is the pressure difference between the nucleus and the bulk phase, V is the volume of the spherical nucleus, σij are the interfacial tensions, Aij are the interfacial areas between phases i and j (water, solution, and vapor phases are referred to as phases 1, 2, and 3, respectively), and R is the radius of the three-phase contact line, as shown in Figure 8. From eqs 1 and

S = γs − (γl + γls)

(3)

where γs and γl are the surface tensions of the substrate solution and the droplet liquid, respectively, and γls is the interfacial tension between the liquid and the solution. For nonspread behavior, S < 0, and for spread behavior, S ≥ 0. The surface tension of water (72.75 mN m−1) is greater than that of organic solvents (20−30 mN m−1); thus, water droplets do not spread on the surface of organic solvents. However, for organic droplets, the situation is complicated because γs and γl are comparable. A small amount of a surface-active agent may severely change the spread behavior. For example, our group observed that methanol droplets spread on the surface of PS solution in CS2 because the surface tension of methanol was not large enough to yield a negative S. However, after polystyrene-b-poly(dimethylsiloxane) (PS-b-PDMS, 3) (Scheme 3) was added to the PS solution, we found that

Figure 8. Simplified sketch of a liquid droplet at a liquid−gas interface used to calculate the nucleation work by Nepomnyashchy et al.103 Reprinted with permission from ref 103. Copyright 2006 American Physical Society.

2, the free energy barriers of the two nucleation processes can be calculated and compared. Nepomnyashchy et al. found that the free energy barrier for heterogeneous nucleation was always smaller than that for homogeneous nucleation.103 Therefore, the nucleation of water droplets occurs at the liquid/gas interface. Moreover, eq 2 demonstrates that the nucleation work for heterogeneous nucleation is governed by interfacial tensions. Thus, the polymers in the solutions can influence the nucleation because they determine the surface tension of the solution. If the surface tension of the solution is sufficiently low (or the solution is very hydrophobic), the nucleation will be suppressed.104 Johnston et al. used poly(ethylene oxide)-bpoly(fluorooctyl methacrylate) (PEO-b-PFOMA, 2) (Scheme 2) to produce BFAs. They found that when the molecular weight of the hydrophobic PFOMA block was increased from 70 to 140 kDa, the number of pores in the final BFA decreased markedly.105 A possible reason is that, with a longer PFOMA block, the solution becomes too hydrophobic, such that the nucleation of the water vapor is suppressed. Some other examples of the pore density of BFA decreasing with the hydrophobicity of the solution can be explained in a similar way.106,107

Scheme 3. Chemical Structure of PS-b-PDMS (3)

methanol formed nonspread droplets on the surface of the solution. Thus, BFA could be prepared with a methanol droplet array as the template.37 This phenomenon can be sufficiently explained by eq 3: PS-b-PDMS significantly decreased γs, leading to a negative S, and thus, methanol droplets did not spread on the polymer solution. The shape of the water droplets suspended on the surface of the organic liquid was further investigated with microscopy. Photographs of 10-μL water droplets on the surface of paraffin oil are shown in Figure 9.109 Only a small part of the droplet can be observed when viewed from above (Figure 9a). The

3.2. Growth and Self-Assembly of Water Droplets

The principal feature of BFAs is the hexagonal closely packed pores, which indicate the existence of an ordered water droplet array on the surface of the unsolidified polymer solution. To F


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Figure 10. Cylindrical pores prepared from PS-b-PDMS in methanol vapor.39 Reprinted with permission from ref 39. Copyright 2012 Royal Society of Chemistry.

direction. For water droplets, this deformation is difficult due to the large surface tension, but for alcohol droplets, it is much easier. As a result, the elliptical or cylindrical pores were obtained under alcohol vapors. 3.2.2. Stability of Water Droplets. Two typical morphologies are usually observed on BFA films, as shown in Figure 11.42 Figure 11a shows irregular packed pores with a

Figure 9. Shape of water droplets. (a, b) Photographs of a 10-μL drop of water deposited on the surface of paraffin oil observed from (a) above the surface and (b) below the surface. (c) Sketch of the shape of a water droplet.109 Reprinted with permission from ref 109. Copyright 1988 IOP Publishing.

radius of the three-phase line R is smaller than the radius of the droplets r12. Therefore, to accurately measure the diameter of the pores in BFAs, it is necessary to peel off the superficial layer to reveal the pores underneath.110 It should be noted that water is denser than paraffin oil, but the droplets are able to stay atop the surface of the oil. This is because, for small droplets, the strength of interfacial tensions is much greater than that of gravity.111 Princen et al. have shown that when the droplets are sufficiently small, gravity can be neglected, and the droplets are composed of two spherical segments whose radii are completely determined by the interfacial tensions:112 1+ cos θij =

⎛ γij ⎞2 ⎛ γik ⎞2 ⎜ ⎟ − ⎜ ⎟ ⎝ γjk ⎠ ⎝ γjk ⎠ 2γij γjk

rij R

hij R

=

=

1 sin θij

(4) Figure 11. SEM images of BFAs prepared from 2 mg mL−1 CS2 solutions of (a) PS−Br and (b) PS−NHCH2CH2OH.42 Reprinted from ref 42. Copyright 2014 American Chemical Society.

(5)

1 − cos θij sin θij

(6)

wide size distribution, and Figure 11b shows a perfect hexagonal lattice of monodispersed pores. It is widely accepted that the morphology shown in Figure 11a results from the coalescence of water droplets during the BF process.42,114,115 This coalescence is easy to understand: when two droplets touch each other, they tend to merge.116,117 However, experimental results show that, in most BFAs, pore arrays are mostly regular over a larger area, as shown in Figure 11b.42 It is difficult to imagine such uniform pores being templated from water droplets that underwent coalescence. Thus, the water droplets on the solution surface are quite stable; they rarely coalesce even when they are closely packed. In other words, the water droplets on the surface of a solution behave like a “solid ball”. Thus, the first problem when trying to elucidate the mechanism of the BF process is how to explain the stability of the droplets. Several hypotheses have been proposed to explain the stability of the water droplets. One notable phenomenon is that, on the surface of pure oil without any solute, the water droplets can also keep themselves from coalescing for a long period.118 An important effect that prevents water droplets from coalescing is the Marangoni convection caused by a

where θij, rij, hij, and R are shown in Figure 9c. By changing the surface tension of the polymer solution, one can tune the shape of water droplets from sphere to lens.113 Note that θ23 cannot be zero; otherwise, the forces in the vertical direction will be unbalanced. Consequently, the liquid surface near the water droplets is curved. This curvature is the origin of the attractive force between droplets, which will be discussed later. Small water droplets can maintain a spherical shape because the surface tension of water is relatively large. However, for other liquids, the droplets may deform from spheres when they are pressed. Our group compared the BFAs prepared under the vapors of water, methanol, and ethanol and found that the pores in the BFAs prepared under water vapor were nearly spherical, whereas those in alcohol vapors were elliptical or cylindrical with large depths and radii, as shown in Figure 10.37,39 The reason for these phenomena is that the surface tensions of methanol and ethanol are much smaller than that of water. When the droplets are closely packed, the growth of a droplet is limited by the neighboring droplets. Further increasing the volume of the droplets will lead to deformation of the droplet, and the droplets will grow in the vertical G


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temperature gradient.119,120 This gradient causes convective motion in any two droplets that suppresses their coalescence. A thin layer of air between two water droplets may act as a lubricant, which will also suppress the coalescence. Another hypothesis is that the condensing water droplets are kept apart and made to interact elastically by the vapor of the evaporating solvent as it escapes from the surface.121 The above hypotheses are helpful in understanding the behavior of water droplets on another liquid surface, but they are not able to fully explain the stability of droplets on the surface of a polymer solution. The behavior of droplets on the surface of a solution is highly dependent on the properties of the solute. While water droplets on a pure chloroform surface coalesce after a certain period, water droplets on a polymer solution surface remain stable during the entire BF process. Thus, we can conclude that the polymer (or other solute) in the solution must contribute to the stability of the water droplets. Pitois and François demonstrated that when the droplets came into contact with a polymer solution, the polymer precipitated around the droplets and formed a thin layer.122 This layer of polymer is like an envelope that can prevent water droplets from coalescing. Direct evidence of the polymer envelope layer was discovered on a water drop hanging on a syringe needle in a CS2 solution of PS-b-PPP (Figure 12). The

Scheme 4. Chemical Structure of Polymer Bearing AIE Moieties (4)

Figure 13. Fluorescence microscopy images taken at (a, b) 0 s and (c, d) 30 s after casting the solution on the substrate. Scale bar: 3 mm.123 Reprinted with permission from ref 123. Copyright 2014 Wiley-VCH.

the polymer should be insoluble in water. The precipitation of the polymer around water droplets relies on the fact that water is the nonsolvent for the polymer. If the polymer is too hydrophilic, it will dissolve into the water droplets and, thus, will not effectively protect the droplets. In this circumstance, disordered pores such as those shown in Figure 11a are usually found.124−126 Second, the rate of polymer precipitation is also important. Star polymers give better BFAs compared with their linear counterparts.127,128 A possible reason is that star polymers have higher segment density and are easy to precipitate. Similarly, some block copolymers (BCPs) can form starlike aggregates in the solvent and thus also precipitate quickly around the water droplets, giving high quality BFAs.17,129 Stenzel et al. used a light-scattering technique to investigate the relationship between molecule aggregation and the formation of BFAs. They found that those polymers that could form large aggregates were able to produce better BFAs with higher regularity.72 Moreover, if the concentration of the polymer solution is low, the precipitation of the polymer is slow, and the polymer protective layers around water droplets are thin and weak. Thus, the produced BFA will be nonuniform due to the coalescence of water droplets.42,114,115,130 In fact, solid polymer films are not the only type of protective layer; an interfacial adsorption layer of the solute at the water/ solution interface also effectively protects the water droplets. In many cases, there is no evidence for the solid polymer protective film, but the interfacial adsorption is easy to confirm both in theory and experimentally. Many researchers have reported that polymers with amphiphilic molecular structure formed ordered BFAs much more easily.131 For example, carboxyl terminated PS could form regular BFAs, but PS without a hydrophilic end group failed to produce regular BFA films under the same conditions.111 Obviously, amphiphilic

Figure 12. Photograph of a water droplet in PS-b-PPP solution in CS2. A solid layer at the interface between the two liquids is visible when the solution contained in the drop is partially removed.122 Reprinted with permission from ref 122. Copyright 1999 EDP Science, SpringerVerlag.

concept of a protective layer is quite often invoked in the literature to explain the formation of BFAs, and its existence is widely accepted and believed to be the principal reason for the stability of water droplets during the BF process. Recently, Qin and Tang et al. directly observed the polymer film around water droplets during the BF process using a fluorescence microscope.123 They performed the BF experiments with a polymer bearing aggregation-induced-emission (AIE) moieties (4) (Scheme 4), which were nonluminescent when dissolved in the solution and became highly emissive when aggregated. Under the fluorescence microscope, weakly emissive water droplets were clearly observed after the polymer solution was cast onto the substrate (Figure 13), suggesting that water droplets were covered by a polymer film. They also found that at the early stage of the BF process, before the emissive water droplets appeared, dark water droplets were first observed under a bright-field microscope. This phenomenon indicates that the formation of a polymeric protective film is subsequent to the formation of water droplets. According to the published literature, the formation of the polymer envelope layer is controlled by several factors. First, H


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of the particles, such as amphiphilicity, shape, and size, show effects on the stability of the water droplets. The stabilization energy ΔE, which is defined as the energy required to remove a particle from the interface, is given by

molecules tend to assemble at the water/solution interface and form a protective layer. The amphiphilic-molecule-protected water droplets on the surface of a polymer solution comprise a two-dimensional emulsion that is quite stable. If the polymer is not amphiphilic, the water droplets can be stabilized by adding a small amount of amphiphilic molecules into the solution.51,114,132,133 For example, Shimomura et al. demonstrated that, with the aid of the surfactant CAP (copolymer of dodecylacrylamide and carboxyhexylacrylamide, 5) (Scheme 5),

ΔE = πr 2γ(1 − |cos θ|)2

(7)

where r is the particle radius, γ is the interfacial tension, and θ is the contact angle (CA), as shown in Figure 14. The larger the

Scheme 5. Chemical Structure of Polymer CAP (5)

Figure 14. A small gold particle at the water/oil interface.151 Adapted from ref 151. Copyright 2004 American Chemical Society.

stabilization energy, the higher the stability of the water droplets will be and the more easily the ordered BFA can be obtained.149,151 Judging from eq 7, the largest stabilization energy is obtained when the CA is 90°, which requires that the particles have a balanced hydrophobicity and hydrophilicity. For example, using amino-functionalized zeolite to stabilize water droplets only resulted in disordered pores with varied size, but with carboxyl-functionalized zeolite as the stabilizer, monodispersed pores were achieved.152 The reason for this result was that the former zeolite was too hydrophilic to stabilize the water droplets. Other researchers have also reported that by carefully adjusting the amphiphilicity of NPs with a surface modification technique, better BFAs could be obtained.153,154 3.2.3. Growth of Water Droplets. Pore size is an important parameter of a BFA film because many applications depend on the average pore size. Thus, it is important to investigate the size evaluation process for the water droplets. The condensation of water on solid substrates has already been investigated comprehensively.116,155−158 If we assume that the temperature of the substrate remains unchanged throughout the growth process and further ignore the gradient near the hemispherical droplets, we will find that condensation on the substrate should induce a steady-state condensation gradient of water molecules parallel to the substrate, whose length scale can be predicted by hydrodynamic theory. The rate of droplet volume (V) growth is proportional to the concentration gradient in the boundary layer, the mass diffusivity, and the saturation pressure difference:159

ordered BFA films could be prepared from most liposoluble polymer solutions.134,135 Similarly, when tetraethyl orthosilicate (TEOS), whose hydrolysis product is also amphiphilic, was added into PS solution, perfect BFA films were obtained.136 These experimental results clearly demonstrate the important function of amphiphilic molecules in the formation of BFAs. If the interfacial adsorption occurs, the hydrophilic groups in the amphiphilic molecules should aggregate primarily inside the pores of the BFA. This has been confirmed by substantial experimental results. For example, Raman spectrum mapping of BFA films revealed that the hydrophilic molecule or segments were enriched in the pores.137,138 Similarly, time-of-flight secondary ion mass spectrometry (TOF-SIMS) mapping results also demonstrated that polar groups were primarily located inside the pores.139−141 X-ray photoelectron spectroscopy (XPS) was also used to characterize the distribution of elements on BFA film.142,143 Stenzel et al. compared the element composition of the top surface layer and the pores of a BFA film made from a PS-based amphiphilic BCP and found that the content of oxygen in the pores was higher than that on the top surface layer.142 Thus, the hydrophilic blocks were enriched on the walls of the pores. By labeling the hydrophilic segments with fluorescent moieties, the enrichment of hydrophilic groups inside the pores could also be directly observed with a fluorescence microscope.144,145 Another widely used method to stabilize water droplets during the BF process is the addition of nanoparticles (NPs) into the polymer solution. Many inorganic particles can adsorb onto the interface of water/oil and form a mechanically strong protective layer around the water droplets. The emulsion stabilized by particles is called a Pickering emulsion. Some researchers have investigated the influence of solid particles on the formation of polymer BFAs.146−148 For example, gold NPs were reported to be able to promote the formation of ordered PS BFAs.147,148 The particles can be observed directly inside the pores by SEM or transmission electron microscopy (TEM),146,149,150 revealing that they were located at the interface of water and the polymer solution. The properties

dV ∼ DΔpC0δ −1 dt

(8)

where D is the diffusion coefficient of water molecules in carrier gas, Δp is the saturation pressure difference of water between the environment and the surface of substrate, C0 is the concentration of water in the carrier gas, and δ is the length scale of the condensation gradient, which is related to viscosity, the velocity of the gas flow, and D. Therefore, the growth of the radius R of water droplets should follow a power law: R ∼ (DΔpC0δ −1t )1/3 I

(9) DOI: 10.1021/acs.chemrev.5b00069 Chem. Rev. XXXX, XXX, XXX−XXX

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Near room temperature, the saturation pressure difference Δp is related to the temperature difference ΔT:

Δp ∝ ΔT 0.8

(10)

Thus, the growth of water droplets is described as R ∼ (DC0ΔT 0.8δ −1t )1/3

(11)

From eq 11, we can conclude that when the concentration of the water vapor C0, the temperature difference, or the growth time is increased, the average radius of the water droplets will increase. Here, we summarize some experimental results that can be explained well by the above equations. A great number of researchers have reported that the size of pores in BFAs increased with the relative humidity (RH).43,82,110,129,160−163 The RH is defined as p RH = · 100% ps (12)

Figure 15. (a) Average apparent diameter ⟨D⟩ and (b) apparent surface coverage ε2 of water droplets on paraffin oil as a function of time. The line is simply a guide for the eye. Experimental conditions: gas flow velocity F = 9.6 cm3 s−1, ΔT = 8.0 K, and T0 = 12.2 °C.109 Reprinted with permission from ref 109. Copyright 1988 IOP Publishing.

where p is the partial pressure of water vapor and ps is the saturated vapor pressure of water. Thus, relative humidity is proportional to C0, and higher relative humidity will lead to a faster growth rate. It should be noted here that relative humidity is also dependent on the temperature, because ps increases with temperature. Some researchers found that BFAs obtained at higher temperature had larger pores, although the relative humidity values were identical.87,164,165 These results can be explained by the larger C0 at higher temperatures. There are also some experimental results that confirm the relationship between temperature differences and the pore size. The cold stage technique is valuable for these investigations because it allows the temperature of the solution to be controlled, whereas in the conventional BF process, the temperature is always changing. Chen et al. reported that the pore size in BFA was significantly increased when the temperature of the substrate decreased.89 Stenzel et al. also reported similar results.90 In conventional BF processes, the temperature of the solution is determined by evaporation cooling and is difficult to control. A possible method for adjusting the surface temperature is to control the evaporation rate. For example, by reducing the environment pressure, the evaporation process was accelerated markedly, leading to a lower surface temperature and larger pores in the final BFA.166 The gas velocity,90,121,129,161,167,168 the boiling point of the solvent, 87,110,162,169−174 and the solution concentration106,115,154,169,172 can also influence the evaporation rate. Generally speaking, a higher gas velocity, lower boiling point, and lower solution concentration will lead to a higher evaporation rate and lower surface temperature. However, increasing the evaporation rate will decrease evaporation time, which has an opposite effect on the size of water droplets. Which factor dominates the growth process may depend on the nature of the polymer or solvent. This may explain the contradictory experimental results in the literature. Substantial efforts have been devoted to investigating the relationship between water droplet size and growth time.175−180 Knobler et al. were the first to investigate the growth rate of water droplets on oil.109 They found that, at early times, the average radius of the droplets R followed a power law with an exponent of ∼0.3, but at a later time, the exponent became ∼1, as shown in Figure 15. The existence of two growth regimes is distinct, as shown in Figure 15a. Figure 15b also represents the surface coverage as a

function of time. It should be noted that the surface coverage is the apparent surface coverage because the observed diameter is smaller than the diameter of the portion of the droplets beneath the surface. Therefore, the transition time t0 is the time point when water droplets achieve hexagonal close packing. Before t0, the surface coverage is low, and the growth of the droplets is controlled by the behavior of individual droplets. After t0, droplets contact each other, and the coalescence of droplets becomes a dominating process, thus increasing the growth rate. In some experiments, the growth of water droplets was found to follow a power law with an exponent between 0.3 and 1; Knobler et al. noted that this was because the growth occurred at the transition stage between growth of individual droplets and coalescence-dominating growth.109 Therefore, the growth of water droplets on liquid evolves through three stages: the initial stage, the crossover stage, and the coalescence-dominated stage. Knobler et al., in their above-mentioned experiments, created the cold surface by using nonvolatile oil and Peltier elements. To examine the dynamics of BF formation on volatile surfaces, Limaye et al. discarded the Peltier element and replaced the oil with volatile benzene and chloroform.181 Thus, the condensation of moisture was driven by evaporation cooling. On a benzene surface, the growth of water droplets followed the same power law described by Knobler et al. At the initial times, the exponent was ∼0.3, and at the later time, it was ∼0.95. On a chloroform surface, the exponent was ∼0.5 during the entire growth period. To explain the differences between experimental results on chloroform and benzene, the authors argued that the surface perturbations induced by convection were important. Chloroform evaporates faster than benzene. Thus, the convection in chloroform is stronger and can promote the coalescence of water droplets and increase their growth rate.181 The results demonstrate that, on a dynamic and deformable surface such as a polymer solution, the behavior of water droplets is more complex than it is on solid substrates. The above growth laws were popularly invoked in the literature to explain the growth of pore diameters in BFAs. However, it is not safe to directly apply these conclusions to the J


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same reason, the pores in the center of the BFA films are larger than those near the edge of the film because the thickness of the solution at the center of the drop is larger.33,186 3.2.4. Assembly of Water Droplets. The nucleation of water droplets on the surface of a polymer solution is random. Thus, during the growth process, the water droplets floating on the liquid surface self-assemble and finally form a closely packed array. Marangoni convection, air flow, and the capillary effect are believed to drive the self-assembly of water droplets. These effects are summarized in this section. One of the main differences between the formation of BFs on a solid substrate and their formation on a liquid surface is that the water droplets on a liquid surface are moving.187,188 This motion allows the separated water droplets to collide with each other, which enables the self-assembly of the water droplets. Two forces are responsible for the motion of water droplets: air flow and Marangoni convection. In a dynamic BF process, the carrier gas flows above the polymer solution, thus exerting forces on the water droplets along its flow direction. Direct evidence for the interaction between gas flow and water droplets is that the pores in the BFA deform from a spherical to elliptical shape, as depicted in Figure 17. The long axes of the

formation of BFA films because they do not include the effects of the polymer in the solution. Without the protection of the polymer layer, the water droplets forming on the oil surface tend to coalesce at high surface coverage. For the droplets on the surface of the polymer solution, as discussed above, the coalescence is prevented by a polymer protective layer; thus, it is expected that there is no coalescence-dominated stage. Many researchers have recorded the growth of water droplets on the surface of polymer solutions.31,42,95,182 Using a light-scattering technique, Pitois et al. found that the growth of water droplets on the surface of PPP-b-PS solution in CS2 followed a power law with an exponent of 0.35, a typical behavior for the growth of individual droplets.31 As expected, they did not observe the coalescence-dominated stage. In fact, the power law cannot describe the entire growth process of water droplets on a volatile polymer solution. The growth rate of water droplets is expected to decrease and finally approach zero because both the temperature and the viscosity of the solution will increase at the late stage of evaporation. The real diameter growth curve measured by Servoli et al. is shown in Figure 16.55 Fitting the

Figure 17. SEM image of the deformed BFA caused by gas flow at the blowing velocity of 20 m min−1. The arrow indicates the direction of gas flow.189 Reprinted with permission from ref 189. Copyright 2007 Elsevier.

Figure 16. Data fitting of experimental data points reporting the average radius of water droplets as a function of drying time on a poly(lactic acid) (PLA) solution in chloroform.55 Reprinted with permission from ref 55. Copyright 2010 Elsevier.

elliptical pores are parallel to the direction of gas flow.189 This suggests that the force exerted by the gas flow changes the shape of the water droplets. Obviously, this force can also make the droplets move on the surface of the solution. Hao et al. found that if the humid gas flow was stopped before the polymer solution was completely dried, the ordered packed pores were found near the edge of the film and no pores were observed in the center of the film.190 It is believed that the gas flow carries the water droplets to the contact line and promotes the rearrangement. However, in a static BF process, the effects of gas flow can be neglected because no extra gas flow is induced. Marangoni convection (or thermocapillary convection) is another driving force of assembly. The Marangoni effect is the mass transfer along an interface between two fluids due to a surface tension gradient. For a drop of polymer solution on the substrate, the evaporation rates near the contact line and near the center of the drop are different. Thus, there will be a temperature gradient on the surface of the solution caused by the evaporation cooling. Because surface tension is dependent on the temperature, the temperature gradient will lead to a surface tension gradient and Marangoni convection.191 The experimental and predicted flow field in a drying octane drop is shown in Figure 18.192 The velocity of the flow in the region above the center of the vortex was measured at 7.2 mm s−1. This velocity was rather high because the contact-line radius of

data with eq 11 gave poor results (dashed line) due to the existence of a plateau in the curve after ∼60 s. Servoli et al. proposed a Boltzmann type equation that can better fit the experimental data:55 R=

1+e

Rf −[(t − t 0)/ k]

(13)

where Rf is the final average diameter of the water droplets, t0 is the time at which R = Rf/2, and k is a parameter related to the slope of the linear part of the diameter growth curve. However, considering that the growth of water droplets at different stages is controlled by different factors, eq 13 should be a phenomenological equation; despite this, the author attempted to give the physical explanation of the parameters.55 All the above growth laws predict that a longer growth time will lead to larger water droplets and larger pores in the final BFAs; thus, changing the growth time is the most convenient way to control the pore size of BFAs. In the BF process, water droplets continue to grow until the temperature of the solution rises above the dew point of the environment or the viscosity of the solution becomes too high. Thus, the growth time is decided by the evaporation time. Shimomura and Yan et al. reported that the diameter of the pores in BFA was easy to regulate by the thickness of the cast polymer solution because the evaporation time varied with the thickness.183−185 For the K


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Figure 18. Flow field in a drying octane droplet, (a) imaged experimentally and (b) predicted.192 Reprinted from ref 192. Copyright 2006 American Chemical Society.

the octane droplets was only 2 mm. The strong Marangoni convection is able to drive the motion of water droplets floating on the surface of the solution. Moreover, it is believed that Marangoni convection can draw the water droplets into the solution, which will yield a multilayered BFA film.193,194 The gas flow and Marangoni convection allow the water droplets on the surface of a polymer solution to approach each other. When they are close enough, capillary interactions begin to play a dominant role. As discussed above, the water droplets are floating on the surface of a liquid through a subtle balance of buoyancy, gravity, and capillary forces. The liquid surface is curved near the water droplets. The bending of the liquid surface can cause long-range interactions between the droplets. With some approximations, the attractive force F between two droplets separated by a distance l is as follows:118,195 F=

Figure 19. (a) Optical micrograph of the initial stage of water droplets’ condensation after the evaporation of toluene, showing the droplet islands.43 Reprinted with permission from ref 43. Copyright 2003 Elsevier Science. (b) SEM image of a BFA film made of PSF. The irregular border between domains is noted with two white ellipses.197 Reprinted with permission from ref 197. Copyright 2005 Elsevier.

2 4πR6ω 2 ⎡ 1 2 1.5 2 0.5 ⎤ + 0.25(1 − p ) − 0.75(1 − p ) ⎢ ⎦⎥ 3lσ ⎣ d

(14)

where R is the radius of the water droplet, ω = ρg is the specific weight of the liquid, g is the earth’s gravitational acceleration, ρ is the density of the liquid, σ is the oil−air surface tension, d is the oil-to-air relative density, and p is a key parameter defined as the ratio of the apparent droplet radius (r) above the surface to the droplet radius (R) below the surface. Equation 14 is valid when bond number B0 = (R/lc)2 is relatively small. Here, lc is the capillary length defined as lc = (σ/ω)1/2. For a droplet with a radius of 10 μm, B0 is of the order 10−4. Thus, eq 14 can be used to describe the interaction between water droplets on the liquid surface. From this equation, the diameter of the water droplets strongly influences the attractive force (F ∼ R6), and the surface tension of the liquid is also very important (noting that p is also determined by surface tension). After elucidation of the motion and interaction of water droplets, the self-assembly of water droplets can be understood. Two other points related to the defects of BFAs are worth noting here. First, water droplet islands form at the beginning of assembly. As shown in Figure 19a,43 at the initial stage of self-assembly, the water droplets form many isolated islands, and the droplets are closely packed in the islands.43,118,196 These islands expand gradually after they admit new droplets, and the droplets in the islands grow larger simultaneously. Finally, the islands touch each other and merge, producing a large assembly of water droplets. This process coincides with the evolution of a light-scattering pattern in the polymer solution.31 As shown in Figure 20, with the evaporation of solvent, the scattering pattern changed from a ring to hexagonal dots, indicating the development from islands to monocrystalline water droplets. A direct consequence of such a process is the polydomain of the final BFAs, as depicted in Figure

Figure 20. Typical scattering patterns of the water droplet array during the process of BFA formation. (a) Island droplets with low surface coverage. (b) Water droplets with short-range order. (c) Polycrystal. (d) Monocrystal.31 Reprinted with permission from ref 31. Copyright 1999 Springer-Verlag.

19b.197,198 Clear domain borders are observed in the BFA. These one-dimensional defects are caused by lattice mismatch when two islands merge. The second type of defect is the absence of pores. Han et al. claimed that this was because the water droplets were carried away by the vapor flow at the late stage of the evaporation.199 However, as mentioned before, only a small part of the suspended droplet is above the surface of the solution, thus making it difficult for the gas to remove the droplets. Wang et al. provided another explanation.187 They believed the water droplets could completely sink into the polymer solution. Their evidence was that, under microscopy, they found that one droplet surrounded by six other droplets suddenly disappeared but then reappeared after 0.4 s. A potential explanation is that this droplet submerged into the L


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4. MATERIALS FOR BFA FILMS Polymers are considered ideal materials for producing BFA films because (1) most soluble polymers can form robust thin films when their molecular weight is high enough,204 and (2) many polymers are water immiscible and can efficiently stabilize water droplets. In the past two decades, a wide spectrum of polymeric materials with different topographical morphologies has been used to prepare BFA films. With the development of the BF technique, any solutes that are able to stabilize the water droplets and form a continuous film can be used to produce BFA films called nonpolymeric BFA films. In this section, the materials employed in the literature to prepare BFA films and the influences of the structures and properties of the materials on the formation and function of the BFAs are summarized.

polymer solution temporarily and then reemerged after a period of time. If the submerged droplet fails to emerge before drying of the polymer solution, a vacancy will be left in the BFA film. 3.3. Evaporation of Solvent and Water Droplets

With the evaporation of solvent, the viscosity of the solution increases, and the movement of the water droplets is limited. When the temperature of the solution is above the dew point, the water droplets stop growing, and the evaporation of the water droplets dominates. The evaporation process will take several minutes. The results obtained on a quartz crystal microbalance (QCM) revealed that, for a 1% PS solution in chloroform, it took 2.5 min for the solvent to completely evaporate, and another 4.5 min was needed to allow all the water to leave the polymer film.200 After the complete evaporation of the water droplets, pores are left on the polymer film. During the drying process, although the positions of the droplets are not likely to change, the shape of the pores evolves. The polymer film plasticized by residual solvent offers sufficient fluidity for the evolution of the pores. Wang et al. found that, during the evaporation process, the apparent diameter of the water droplets was maintained at 1.5 μm for approximately 76 s and then increased rapidly to 2.8 μm in approximately 12 s. It was believed that the polymer film covering the water droplets receded suddenly when they began to cure.187 Another phenomenon worthy of mention is that, in many BFA films, the pores are interconnected. Obviously, these interconnected holes between neighboring pores must appear after the solution has sufficiently high viscosity. Otherwise, the droplets should coalesce and leave a large pore. Consequently, the interconnected holes are generated during the solidification of the solution due to volume shrinkage and the surface tension of the drying solution. The fluidity of the solution during evaporation plays a decisive role in the formation of BFAs on nonplanar substrates. Qiao et al. investigated the preparation of BFA films on a curved substrate.56,128,201,202 They used several star polymers with different glass transition temperatures (Tg’s) to prepare BFA arrays on a TEM grid128 and found that polymers with Tg’s higher than room temperature were unable to form ordered BFAs. Instead, several cracks were found in the film. They speculated that polymers with high Tg’s were rigid and brittle and thus could not cover the curved substrate. However, our group obtained different results.203 We found that a BFA film of PS-b-PAA (6) (Scheme 6), which has Tg’s of 100 and

4.1. Linear Block Copolymers

As discussed in section 3, linear BCPs easily form ordered BFA films compared with the linear homopolymers due to their unique self-assembling behavior in solutions and at interfaces. Many efforts have been made to investigate the self-assembly behaviors and theories of BCPs,205−213 which has promoted the development of BF technique. At the same time, with the rapid development of controlled radical polymerization techniques, the synthesis of BCPs with precisely adjustable volume fractions and chain architectures is no longer a difficult task. Thus, several BCPs with unique structures and functions have been prepared and used to fabricate BFA films. Some of these BCPs can produce regular BFAs, and some of the BCPs with designed topology are valuable for investigating the influence of molecular structures on the formation of BFAs. BCPs for BFA films can be divided into two categories: amphiphilic and nonamphiphilic BCPs. As discussed in section 3, nonamphiphilic BCPs will precipitate around the water droplets, form a solid film, and stabilize the water droplets. However, amphiphilic BCPs can stabilize water droplets through specific adsorption in the interface due to their surface activity. PS-based BCPs are predominant in the available literature because styrene is easy to polymerize via different methods, and PS has good solubility in various organic solvents. Thus, in the following paragraphs, we will first summarize PSbased BCPs for BFA preparation. The first BFA film was prepared with a nonamphiphilic diblock copolymer of PS-b-PPP (1) by François et al.17 In their work, CS2 was used as the solvent, in which the PS block was soluble but the PPP block was insoluble. Therefore, in the CS2 solution, PS-b-PPP chains formed quasi-spherical micelles with an insoluble PPP core surrounded by PS arms. The authors claimed that these star-shaped aggregates of PS-b-PPP were crucial for the formation of BFAs because they possessed high segment density and, consequently, could effectively protect the condensed water droplets. Another important example of a nonamphiphilic diblock copolymer is PS-b-PDMS (3). Our group prepared a PS-b-PDMS BFA film using a static method.214 We found that a highly ordered BFA was obtained when the concentration of PS-b-PDMS was approximately 40 mg mL−1, and either higher (70 mg mL−1) or lower (10 mg mL−1) concentrations resulted in irregular pore arrays. Furthermore, the PS-b-PDMS BFA films were converted into a SiO2 honeycomb porous structure by pyrolysis (see section 7 for details). Recently, it was found that BCPs with PDMS blocks could generate ordered BFAs not only in water vapor but also in methanol and ethanol vapors. The size and shape of pores formed in alcohol vapors were different from those in

Scheme 6. Chemical Structure of PS-b-PAA (6)

102 °C for PS and PAA, respectively, was perfectly contoured on the TEM grid. Thus, when the surface of the polymer solution touched the TEM grid, the solution was fluid because of the plasticization of the solvent. One piece of evidence was that the pores on the grid were smaller than those in the mesh, indicating that the water droplets were growing and the solution was still deformable at that time. M


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Scheme 7. Chemical Structures of Nonamphiphilic BCPs (7−15)

Scheme 8. Chemical Structures of Amphiphilic BCPs (16−27)

water vapor.39 A detailed description is presented in section 5. Other PS-based nonamphiphilic BCPs that have been used to prepare BFA films include (Scheme 7) polystyrene-b-poly(ethoxyethyl acrylate) (PS-b-PEEA, 7),215 polymethylene-bpolystyrene (PM-b-PS, 8),216,217 polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA, 9),218 polystyrene-b-poly(tertbutyl acrylate) (PS-b-PtBA, 10),101,171 polystyrene-b-poly(n-

butyl methacrylate) (PS-b-PnBMA, 11),218 polystyrene-bpolyfluorene (PS-b-PFO, 12),219 polystyrene-b-poly(2,5-dioctyloxy-phenylenevinylene) (PS-b-PPV, 13),92 PPV-b-P(S-statC60MS) (14),220 and poly(phenylquinoline)-b-polystyrene (PPQ-b-PS, 15).221 Amphiphilic PS-based BCPs are a large class of materials for fabricating BFA films. PS-b-PAA (6)175and its cousins (16, N


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17)142,144 (Scheme 8) are typical amphiphilic BCPs. They can efficiently stabilize water droplets during the BF process and, thus, can produce highly regular BFAs. Hu et al. prepared PS-bPAA BFA films using a dynamic BF method.82 They systematically investigated the influence of critical factors, such as the concentration of the BCP solution, the relative humidity of the atmosphere, the properties of the solvent, the spreading volume, and the substrates, on the morphology of the films. They also found that the water CA of the prepared BFA film was determined by the porosity and the ionization degree of PAA. Our group first used a static BF method to prepare PSb-PAA BFA films.222 We demonstrated that PS-b-PAA BFA films prepared with this static method had high regularity even on nonplanar substrates.203 We also used PS-b-PAA BFA films to guide the growth of other inorganic materials, such as carbon nanotubes (CNTs), ZnO, and SiO2, by mixing lipo-soluble precursors or catalysts into the casting solution.223−225 These studies are described in section 7. Ma et al. systemically investigated the preparation of PS-b-PAA BFA films using a static BF method. 110 They changed the experimental conditions, such as the concentration of the polymer solution, the solvent, the temperature, and, particularly, the molar ratio of the PAA and PS segments, to control the morphology of PSb-PAA BFA films. Their results demonstrated that PS-b-PAA BFA films could be obtained easily from CS2 or chloroform solutions under broad experimental conditions. Moreover, with PS-b-PAA as the dopant, a composite of PS-b-PAA and polypyrrole (PPy) (18) was synthesized and used to prepare BFA films.226 Because of the introduction of conductive polymer, the obtained BFA films were suitable for the growth of cultured cells, showing potential application as scaffolds for tissue engineering. Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP, 19) (Scheme 8) is another widely used PS-based amphiphilic BCP for the fabrication of BFA films.166,227,228 For example, Zhang et al. used PS-b-P4VP as a model polymer to investigate the effect of a vacuum on the formation of BFAs.166 They modified the conventional static BF process as follows: after the polymer solution was cast on the substrate, a vacuum was immediately applied to the chamber to achieve an appropriate pressure. They found that, by decreasing the pressure of the chamber, the pore size significantly increased. This size increase occurred because, under reduced pressure, the temperature difference between the solution surface and the atmosphere was larger, leading to faster condensation of water (see section 3 for details). Save and Billon et al. prepared PS-b-P4VP BFA films from a chloroform solution using a dynamic BF method.227 The pore size of the obtained BFAs was approximately 700 nm. Because PS and PVP segments are immiscible, the authors observed a PVP domain generated by microphase separation on the surface of the film. Moreover, PVP is pH-responsive because of the reversible protonation and deprotonation of its pyridine rings. As a result, the PS-b-P4VP BFA films showed a pH-responsive water CA. Other PS-based amphiphilic BCPs include polystyrene-bpoly(2-vinylpyridine) (PS-b-P2VP, 20),229 polystyrene-b-poly(ethylene oxide) (PS-b-PEO, 21),198,218 polystyrene-b-poly(N,N-dimethylacrylamide) (PS-b-PDMA, 22),230 polystyreneb-poly(N-isopropylacrylamide) (PS-b-PNIPAm, 23),231,232 polystyrene-b-poly(N,N-dimethylaminoethyl methacrylate) (PS-bPDMAEMA, 24),32,233 polystyrene-b-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA, 25),143 PS-b-POTI (26),234 and dextran-b-PS (27)235 (Scheme 8). These BCPs are all

reported to produce regular BFAs due to their amphiphilicity. Recently, Russell et al. reported a unique “BFA film” prepared from PS-b-PEO.218 They cast a 70 mg mL−1 cyclohexanone solution of PS-b-PEO (21) under 85% humidity. The typical pores in the obtained BFAs were approximately 56 nm, which was much smaller than those in any other BFAs. The authors noted that because the evaporation rate of cyclohexanone was relatively slow, the condensation of water droplets was suppressed, and only water absorption by the amphiphilic PSb-PEO occurred. Thus, these small pores were generated from the swollen PEO microdomains. Other BCPs have also been used to fabricate BFA films. In the following paragraphs, some typical classes of BCPs are discussed. Poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) are widely used hydrophilic chains in the construction of amphiphilic BCPs. For example, Stenzel et al. synthesized polystyrene-b-poly(ethylene glycol) (PS-b-PEG) via atom transfer radical polymerization (ATRP) with a brominated PEG chain as the macroinitiator.236 They then converted the PS block into poly(vinyldiphenylquinoline) (PVQ),237 which is a blue-light-emitting polymer. The authors found that the obtained PVQ-b-PEG (28) (Scheme 9) could Scheme 9. Chemical Structures of BCPs Containing PEO Block (28−30)

form ordered fluorescent BFAs due to its amphiphilic nature. Another PEO-based BCP for the preparation of BFA is a rod− coil polymer, PDMA-b-PEO (29), reported by Xi et al.238 It was also synthesized via ATRP with a brominated PEO chain as the macroinitiator. The authors compared the PDMA-b-PEO BFA films prepared on mica and a water surface. They found that, for BFAs prepared on the water surface, the pores were smaller and the regularity was lower. They attributed these differences to the smaller thickness of the polymer solution on the water surface. A liquid-crystalline (LC) diblock copolymer, poly(ethylene oxide)-b-poly(azobenzene-modified methacrylic acid) (PEO-b-PAz, 30), was also used to prepare BFA films by spin-coating method.239 Fluoride BCPs are a group of unique polymers with high thermal stability and chemical inertness and low dielectric constants and surface energy.240 Johnston and Korgel et al. prepared PEO-b-PFOMA (2) and used it to prepare BFA films.105 They found that high-quality BFAs with long-range order could be obtained from a PEO-b-PFOMA/Freon solution. However, because the PFOMA block was rather hydrophobic, when the molecular weight of this block was too large (140 kDa, with a PEO block molecular weight of 2 kDa), O


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Scheme 10. Chemical Structures of Fluoride BCPs (31−37)

the nucleation of water droplets was suppressed. As a result, under these circumstances, the number of pores in the BFA film was obviously small. In other examples, fluoride BCPs of polystyrene-b-poly(perfluorooctyl ethyl methacrylate) (PS-bPFMA, 31), 241 polystyrene-b-poly(2,3,5,6-tetrafluoro-4(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecaoxy)styrene) (PS-b-PFSF, 33),241 polystyrene-b-polypentafluorostyrene (PS-b-PFS, 33),241 PM-b-P(St-co-PFSt) (34),242 poly(tert-butyl acrylate)-b-poly(2-[(perfluorononenyl) oxy]ethyl methacrylate) (PtBA-b-PFNEMAs, 35), 243 PTFEMA-bPMMA-b-PEG-b-PMMA-b-PTFEMA (36),83,244 and poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF−HFP, 37)245 (Scheme 10) were synthesized and used to prepare BFA films with dichloromethane, chloroform, or CS2 as the solvent. The influences of the solvent, the molecular weight of copolymers, the relative humidity, and the temperature on the morphology of the BFA films were investigated. Shimomura et al. found that the BFA film of BCP 35 was hydrophobic with a water CA of 145° due to the low surface energy of the fluoride block. After peeling off the top layer of BFA film, the obtained pincushion structure increased the water CA to 170°.246 BCPs based on conjugated blocks are of great interest due to their unique supramolecular behaviors and photoelectric properties. In general, these BCPs are rod−coil polymers because the conjugated blocks are rather rigid. PS-based BCPs of PS-b-PPP (1),17 PS-b-PFO (12),219 PS-b-PPV (13),92 PPVb-P(S-stat-C60MS) (14),220 and PS-b-PPQ (15)221 are typical examples of such BCPs. There have been many non-PS-based rod−coil polymers, such as PPQ-b-PMMA (38)247 (Scheme 11). Chen et al. synthesized a BCP containing a polyfluorene block, PF-b-PSA (39), via ATRP polymerization.91 It was found that a longer PSA block length, higher humidity, and a higher copolymer concentration created more regular BFAs. The BFA films could emit blue light with a maximum light intensity between 400 and 425 nm. Furthermore, when the top layer of the PF-b-PSA BFA film was peeled off, the film became superhydrophobic with a water CA of 163 ± 0.3°. Another representative conjugated block is poly(phenylenevinylene) (PPV). PPV-b-PMMA (40) has been successfully used to construct BFA films.248 Zhai et al. also synthesized a lightemitting BCP of poly(1,4-distyryl-3,5-dimenylbenzene-co-triethylene glycol) (PDTG, 41) and successfully used it to prepare BFA films.249 The introduction of BFA into the film greatly enhanced the photoelectric conversion efficiency (see section 7 for details).

Scheme 11. Chemical Structures of Rod−Coil BCPs (38− 41)

Some unique BCPs with functional groups were used to prepare BFA films. Stenzel and Davis et al. synthesized BCPs of PMMA-LC-b-STY-co-MAn (42) (Scheme 12), containing a side-chain LC segment and an amorphous styrene/maleic anhydride segment.250 They compared the BFAs prepared from BCP 42 and the LC homopolymers with a cold stage technique and found that the BCP could yield more regular BFAs. It was demonstrated that the BCPs formed aggregates in dichloromethane because poly(STY-co-MAn) was insoluble in dichloromethane. Therefore, the BCP precipitated faster around the water droplets and efficiently stabilized them. Photochromic BCPs of spiropyran-containing polymer (43),251 photosensitizer-containing BCPs of P(S-stat-VBC)-g-RB (44),252 and photoreconfiguration BCPs of poly(4-vinylpyridine)-b-poly[6[4-(4-butylphenylazo)phenoxy]hexyl methacrylate] (P4VP-bPAzoMA, 45)38 were synthesized and used to fabricate BFA films. The BFA films were endowed with unique optical properties because of the photoactive moieties. Multiblock copolymers were also employed to prepare BFA films.253,254 Our group investigated the formation of polystyrene-b-polybutylene-b-polystyrene (SBS, 46) and polystyrene-b-polyisoprene-b-polystyrene (SIS, 47)255,256 (Scheme 13) P


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BFA films under static BF conditions. After cross-linking, these films showed improved mechanical properties and chemical stabilities. Corresponding works will be discussed in section 6. Another PS-based multiblock copolymer, polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene(PSEBS) (48),166 was used by Zhang et al. to prepare BFA films. Moreover, a different series of poly(ether ester) thermoplastic elastomers, such as PTMN-b-PCLDN (49), PTMN-b-PTMEGN (50), and PTMN-b-(PCL-b-PTHF-b-PCL)DN (51), were synthesized using dimethyl-2,6-naphthalene dicarboxylate as the hard segment, 1,4-butanediol as the chain extender, and three different soft segments of different molecular weights, including polycaprolactonediol (PCLD), poly(tetramethylene ether glycol) (PTMEG), and polycaprolactone-b-polytetrahydrofuran-bpolycaprolactone (PCL-b-PTHF-b-PCL).257 These polymers were easily fabricated into regular BFA films due to their balanced hydrophobicity and hydrophilicity. In another example, an anion-conducting aromatic multiblock copolymer (QPE-b-AxBy, 52) was synthesized by a block copolycondensation reaction between fluorene- and phenolphthalein-

Scheme 12. Chemical Structures of BCPs with Functional Groups (42−45)

Scheme 13. Chemical Structures of Multiblock Copolymers (46−52)

Q


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Scheme 14. Chemical Structures of Star Polymers (53−61)

containing oligomers.258 BFA films with different pore sizes were obtained by varying the humidity.

demonstrated that the addition of 20 wt % of linear PS into the star PS led to less brittle films with fewer defects. Moreover, many other star polymers and BCPs based on the core of C60 (56),260 phenyl (57),170 polyhedral oligomeric silsesquioxane (POSS) (58),261 pentaerythritol (59),262 and porphyrin (60, 61)186 have been exploited as good materials for BFA films. Yan et al. used an amphiphilic hyperbranched polymer, poly(amidoamine) modified with palmitoyl chloride (62) (Scheme 15), to construct BFA films.263 They found that the diameter of the obtained pores was 5−6 μm, but the depth of the pores increased dramatically with the polymer concentration. Polymer 62 is highly photoluminescent, and thus, the obtained BFA films could be observed under a fluorescence microscope. Importantly, the dye-loaded polymer 62 could also form BFA films. Thus, BFA films with various colors were obtained. They also synthesized hyperbranched HBPO-star-PS (63) (Scheme 16) polymers by RAFT polymerization and used them to construct BFA films.185 The size of the pores could be controlled by changing the casting volume of the solution and the molecular weight and concentration of the polymer solution. The authors also found that inducing pores in the film significantly increased the water CA from 88° on smooth film to 130° on a BFA film with a pore diameter of 3.2 μm. Yoshie et al. constructed BFAs with fluorescent hyperbranched poly(phenylenevinylene) (hypPPV, 64).193 Mono- and multilayers of the pores on BFA films were obtained depending on the thickness of the films. Due to the conjugated structure of

4.2. Star and Branch Polymers

Star and branch polymers are believed to be good candidates for producing BFAs because they have high segment density and, consequently, are easy to precipitate around water droplets to stabilize them.19,259 For example, Stenzel and Davis et al. prepared star homoPS polymers with 5, 8, and 18 arms using initiators based on glucose, sucrose, and α-cyclodextrin (53) (Scheme 14) by metal-mediated radical polymerization. They also synthesized star PS polymers with six arms (54, 55) by reversible addition−fragmentation chain transfer (RAFT) polymerization using multifunctional derivatives of phenyl and β-cyclodextrin as the chain transfer agent.88,127 All the obtained star PS polymers were successfully used to construct BFA films. They found that the formation of BFA films was very sensitive to the residual copper/ligand, and extensive purification was required to ensure reproducible film formation. The star PS has the advantage of facile end-group functionalization, which allows the easy production of functional porous films. The pore size, which could be altered within the range 0.4−3 μm, was controlled by the molecular weight of the polymer, the number of arms, and the nature of the end groups. This result clearly indicates that the structure at the micrometer scale can be controlled by very small changes at the molecular level. The blending of star polymers with linear chains is also beneficial for the BFA formation. It was R


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Scheme 15. Chemical Structure of Hyperbranched Polymer (62)

Scheme 16. Chemical Structures of Hyperbranched Polymers (63, 64)

PPV, the BFA films emitted blue light under UV excitation. Furthermore, the as-prepared BFA films could be carbonized by simple thermal treatment without damaging the honeycomb structures. Tang et al. prepared a hyperbranched poly(tetraphenylethene) (PTPE) and used it to prepare BFA films.264 Depending on the obvious AIE activity of PTPE, the BFA formation process was observed in situ under a fluorescent microscope. 4.3. Linear Homopolymers

It was initially thought that only polymers with star-shaped architecture could generate BFA films, whereas polymers with linear architecture were unable to form regular patterns. However, linear homopolymers have their own advantages of easy synthesis and low cost. Thus, great effort has been devoted to fabricating BFAs using linear homopolymers. Many reports have clearly demonstrated the applicability of the BF process to linear homopolymers. The most widely used linear homopolymer for preparing BFAs is PS. PS is a commercially available plastic that is also easy to synthesize and modify in the lab. Thus, PS has long been employed as a model polymer to investigate the preparation method and formation mechanism of BFAs.265−269 For example, Limaye et al. employed PS to examine and compare the formation of BFAs on the surfaces of two volatile liquids: benzene and chloroform.181 Analysis of the observed dramatic differences in the pattern features revealed that surface perturbations induced by convective noise were important for the BFA formation. Srinivasarao et al. prepared two-dimensional (2D) and three-dimensional (3D) ordered BFAs with an atactic PS terminated with a carboxyl group (PS− COOH) by simply changing the casting solvents.121 A 3D network formed when benzene or toluene was the solvent, whereas only 2D porous films were obtained when CS2 was the solvent. The size of these structures could be tailored and dynamically controlled within the range 0.2−20 μm simply by changing the airflow velocity across the surface.

The hydrophilic end group of PS plays an important role in the formation and morphology of PS BFA films. Wan et al. synthesized a series of hydroxyl-end-functionalized PS (PS− OH) and used them to fabricate BFA films.42 According to their results, the hydrophilic end groups could dramatically improve the quality of PS BFA films. In another report, PS with an ionic chain end was synthesized by Billon et al.84 in a onestep reaction by nitroxide-mediated polymerization. They prepared BFA films with good pore regularity on a flexible poly(vinyl chloride) (PVC) sheet or rigid PMMA plate. An elegant technique based on reflected and transmitted light has been used to correlate the pore sizes inside and on the top of the film. Amino-terminated PS (PS−mNH2) can also produce ordered BFAs, which may confer additional biological applications to the BFAs.270 Linear PS without any polar groups can also generate regular BFAs by elaborately controlling the formation conditions. Han et al. investigated the influences of molecular weight, solvent properties, and the relative humidity of the atmosphere on the formation and pore size of PS BFAs.43 They claimed that BFAs formed when the solvent was toluene or chloroform, but S


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Scheme 17. Chemical Structures of Homopolymers (65−71)

disordered porous structures were obtained from PS solution in CS2. PS with an appropriate Mw was beneficial for the BF process because of the proper viscosity. In addition, in some cases, pore arrays with rhombohedral instead of hexagonal symmetry formed. However, our group prepared ordered BFAs from PS solution in CS2 using a static BF process.45 Moreover, it was found that ordered BFAs formed for a wide Mw range from 15 000 to 214 800 and over a broad temperature range. Recently, Shen et al. applied an electric field to assist the formation of BFA films with PS271 and PS/SiO2 NPs.272 For the PS BFA films, the pore size decreased from 2.3 to 0.35 μm when the voltage increased from 0 to 1000 V, which was attributed to the decrease of water surface tension under a stronger electric field. With the decrease of pore size, the water CA on these PS BFA films increased from 95 to 147°. In addition, it was reported that BFA films could also be directly prepared by casting a droplet of organic solvent on a PS Petri dish.273 Further, the ordered pores were modified with functional NPs by replacing the solvent with the NP suspension in organic solvent.274 Other traditional homopolymers, including polystyrenesulfonate (65),275 PMMA (66),79,199 PVC (67),276 poly(1,2butadiene) (PB) (68),277,278 PDMS (69),279 and ferrocenylbased oligomer (70)280 (Scheme 17) can also be used to prepare BFA films. Peng et al. prepared regular BFAs with linear PMMA and PMMA-crown-PMMA (71) casting from toluene.199 Pourabbas et al. directly fabricated BFAs by casting a mixture of solvent (THF) and a controlled amount of H2O on a PMMA plate in a dry atmosphere aided by sonication.80 The BFAs were then decorated with hydrophobic silica NPs grown by an in situ sol−gel reaction. As a result, the micronano hierarchical surface morphology endowed the BFA film with superhydrophobicity. A variety of engineering plastics, including polycarbonate (PC, 72),60,164,281,282 poly(ether ether ketone) with Cardo (PEK-C, 73),169,283 polyimide (PI),115,189,284 poly(phenylene oxide) (PPO),87,174,285 PSF,197 and poly(ether sulfone) (PESF),286 have also been employed to prepare BFA films (Scheme 18). PC has good mechanical properties, high thermal and oxidative stability, and the distinguishing optical property of a high refractive index (n ≈ 1.6). The high refractive index makes PC a suitable material for photonic band gap structures. Bormashenko et al. investigated the fabrication of regular PC BFA films using a fast dip-coating method.282 The PC BFA film

Scheme 18. Chemical Structures of PC (72) and PEK-C (73)

showed optical properties typical of a 2D photonic crystal. Li et al. systemically investigated the influence of humidity, concentration, temperature, and the solvent properties on the formation of PC BFA films.164 The results showed that highly ordered PC BFA films could be obtained from a chloroform or dichloromethane solution under certain conditions. However, BFA film formation was not observed with THF as the solvent. PIs are well-known engineering plastics with excellent thermal and chemical stability. They have been used in many areas such as microelectronics, solar cells, advanced composite materials, and high performance fibers. However, most common PIs are insoluble in organic solvents, which limits their use in constructing BFAs. Yabu et al. first constructed BFA films with polyion complexes of polyamic acids (74, 75) and dialkylammonium salts (76).287 After a subsequent chemical treatment, polyamic acids were converted to PIs, and dialkylammonium salts were removed from the BFA film. Simultaneously, the BFA structures were effectively preserved. Wang et al. directly synthesized fluorinated PIs of 6FDA-BDFA (77) and 6FDA-MDA (78), which are soluble in dichloromethane and chloroform.189,284 The excellent rigidity and thermal stability of PI were maintained well in these soluble polymers. Regular BFA films were successfully constructed with these PIs. The size and shape of the pores could be controlled by adjusting the solvent, concentration, humidity, and the velocity of gas flow. Gas flow with high velocity changed the shape of pores from circular to elliptical. Jiang et al. also prepared BFA films with soluble fluorinated poly(siloxaneimide) (PSI, 79)115 and BPDA/4FMA (80)288 (Scheme 19). T


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Scheme 19. Chemical Structures of PIs (74−80)

solution concentration, and the molecular weight of PSF. Longterm storage of the solution was necessary for the formation of regular BFAs. This phenomenon was explained by the aggregation of polymer chains during storage, which was beneficial for the stability of water droplets during the BF process. PESF (84) BFA films were also constructed by the conventional BF process.286 Due to the low solubility of PESF in volatile solvent, a small amount of amphiphilic BCP (85) (Scheme 21) was added to the casting solution to assist the stabilization of water droplets. A dip-coating BF process was also used to prepare BFA films with PSF and PESF, although the obtained patterns were irregular.68,291

PPO (81) (Scheme 20) is another type of high-performance thermoplastic polymer with desirable characteristics, such as Scheme 20. Chemical Structure of PPO (81)

good resistance to heat and water. Thus, PPO plastics are widely studied in the field of membrane science.289 Shi et al. found that PPO could form BFA films in a large range of solution concentrations in different organic solvents.87,174 A special structure with large pores surrounded by small pores was obtained when the solvent was trichloroethylene/dichloromethane (10/9, v/v). Reducing the environment temperature facilitates the formation of ordered BFAs. Wang et al. observed the formation process of BFA in a PPO/chloroform system in situ.187 Their observation suggested that water droplets were buried under the surface of the polymer solution during the BF process.122 Thus, they explained that missing pores in an ordered BFA appeared when the water droplets failed to erupt. PSF and PESF are two types of high-performance engineering plastics with excellent resistance to heat, acid, and base. PSF (82)197 and poly(phthalazionone ether sulfone ketone) (83)290 BFA films were prepared by coating their solutions in chloroform under a high humidity environment. The pore size of BFAs could be tightly controlled by the humidity, the

Scheme 21. Chemical Structures of Engineering Plastics (82−85)

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morphologies of the BFA films were reversibly stimuliresponsive to the external environment.311 When the BFA film was immersed in water, PVP extruded gradually from the pores, and the as-prepared porous structure changed into a sea−island structure. After drying by heating, the swollen PVP islands collapsed into pores, and the BFA structures were recovered. Lu et al. further found that the raised height of the PVP protrusion in the pores could be controlled by heating due to the heating-accelerated dewetting of PVP from the PS.312 If they pressed a patterned PDMS stamp on the composite BFA film when healing, various hierarchical morphologies were obtained dependent on the temperature, the time of healing, and the patterns of PDMS.

The construction of BFA films with conjugated polymers such as 86−99 (Scheme 22)has been thoroughly investigated Scheme 22. Chemical Structures of Conjugated Polymers (86−99)

4.4. Biodegradable Polymers

Biodegradable polymers (Scheme 23), including PLA (100), poly(ε-caprolactone) (PCL, 101), poly(lactic-co-glycolic acid) Scheme 23. Chemical Structures of Biodegradable Polymers (100−105)

because of their potential applications in optical and electrochemical devices.20,35,292−298 For example, Valiyaveettil et al. synthesized amphiphilic poly(p-phenylene) (PPPOH, 95) by Suzuki polycondensation and constructed BFA films with this conjugated polymer.293 They found that the length of the alkoxy groups on the polymer backbone had a significant influence on the formation of patterned films. Highly ordered, defect-free BFA films were obtained by incorporating a 12carbon chain on the PPP backbone (C12PPPOH). Both the decrease and the increase in the length of the alkoxy chain led to the formation of irregular porous structures with defects. Importantly, the BFA films could emit blue light with a very high quantum yield. In addition, various polymer blends have been used to construct BFAs with multifunctional and hierarchical morphologies.299−305 If the blended polymers are miscible, the composite BFA films will have the multifunction inherited from the component polymers. Poly(N-vinylcarbazole) (PVK)/ PS composites showed excellent processability because of the introduction of PS and were used to prepared BFAs.306 A PVK component endowed the composite film with excellent photoelectrochemical behaviors. The photocurrent of the patterned thin films increased with increasing PVK concentration. Immiscible polymer blends can exhibit various morphologies and properties that cannot be provided by a single polymer.50,54,96,307,308 Han et al. employed a PS/PVP system to prepare BFA films.309 When the humidity was higher than 30%, regular BFAs appeared, and the walls of pores were mainly composed of the hydrophilic component of PVP induced by the water droplets. This result was ascribed to the strong hygroscopic characteristics of PVP and the strong immiscibility of PS and PVP (χ = 0.1).310 Moreover, the

(PLGA, 102), poly(3-hydroxybutyrate) (PHB, 103),313 and their copolymers314 and derivatives, are popular materials for the preparation of BFA films due to their potential applications in bioengineering.58 However, the preparation of BFA films based on these polymers is difficult because the hydrophobic ester group cannot effectively stabilize water droplets. Shimomura et al. fabricated BFA films using PLA and PCL.113,315−317 They added amphiphilic copolymers or surfactants into the polymer solution to facilitate the formation of BFA films. Introducing polar and hydrophilic groups to the biodegradable polymers is another effective way to promote the formation of BFAs. For example, Chen and Ellis synthesized a series of PLGA polymers with different portions of GA to control the hydrophilicity of the copolymers.89,91,94,96,174,176,179,180,318 When the molar ratio of LA to GA was as high as 1:1, highly ordered BFA films were obtained. Cao et al. introduced a hydrophilic unit of L-lysine into PLA and successfully obtained BFA films with this modified PLA.165 Incorporation of PEG also increased the hydrophilicity of the resulted copolymer of PEG-b-PLA (104). The diblock copolymers with proper block ratio between PEG and PLA produced highly ordered BFAs.319 Star and branched polymers have been shown to contribute to the stability of the water droplets. Accordingly, star-shaped PLA (105) was synthesized and successfully used to construct BFA films.130,320 V


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Other researchers have attempted to directly fabricate PLA BFA films without adding surfactant or modifying the molecular structure. Li et al. prepared ordered BFA films using pure PLA in THF by tightly controlling the experimental conditions, including the molecular weight, the solution concentration, and the humidity.163 Similarly, Servoli et al. obtained PLA BFA films by selecting appropriate solvents and processing conditions. The physicochemical and kinetic factors of the BF process have been evaluated.55 Recently, to avoid the use of surfactants, Choi et al. presented a novel two-step approach that involved precoating a polymer solution onto a substrate and subsequently dipping the substrate in a binary mixture of a solvent and a nonsolvent, generating a highly ordered BFA film of PLA with a large area, as shown in Figure 21.321 Chloroform and methanol were used as the solvent and

Scheme 24. Chemical Structures of Cellulose-Based Polymers (106−111)

of the PEG chain illustrated the potential biological applications of the BFA films. Moreover, Wang et al. synthesized biodegradable amino acid ester-substituted polyphosphazenes (PGAP, 112) (Scheme 25)

Figure 21. SEM images of PLA BFA films prepared by a two-step BF process. (a) Low magnification with large area. (b) High magnification with an inserted FFT pattern. (c) Tilt angle of 60°.321 Reprinted with permission from ref 321. Copyright 2014 Royal Society of Chemistry.

Scheme 25. Chemical Structures of PGAP (112) and UFDP (113)

nonsolvent, respectively. The size, shape, and number density of pores could be tightly controlled by the ratio of chloroform to methanol. Cellulose is one of the most abundant natural biodegradable polymers on earth. It provides a platform for the preparation of bioactive membranes by attaching bioactive molecules for applications such as the detection and deactivation of bacteria. The preparation of BFAs based on cellulose and its derivatives is attracting increasing attention.322 The greatest difficulty is that cellulose is insoluble in most organic solvents. Kondo and Yu et al. selected cellulose triacetate (106), which is soluble in organic solvents, to construct BFA films.323,324 The honeycomb structure of the BFA films was preserved well after deacetylation, and thus, cellulose BFA films were successfully obtained. It was reported that, when assisted by CoCl2, cellulose acetate solutions in acetone could also form regular BFAs.325 BFA films have also been prepared from nitrocellulose (107) by casting its solution in amyl acetate on a cold water surface.326 Cellulosed-based graft copolymers have also been used to prepare BFA films.72,88 Liu et al. synthesized three types of cellulose graft copolymers: EC-g-PS (108), EC-g-PtBA, and EC-g-P(PEGMA).106 However, only EC-g-PS was successfully used to construct BFA films due to the low aggregation level of PtBA and P(PEGMA) chains around the water droplets. To obtain highly ordered BFAs, a suitable polymer concentration and side chain length and a comparably high relative humidity were needed. By controlling the above experimental parameters, the pore size could be regulated within the range 100 nm− 2 μm. Kadla et al. also prepared three cellulose copolymers grafted with PEG chains (109−111) (Scheme 24) terminated with functional groups.131,327 Through a BF process, highly ordered BFAs were obtained. The bioactive groups at the end

to prepare BFA films, which accordingly enhanced protein adsorption and apatite deposition in simulated body fluid and showed great advantages in promoting osteogenous differentiation.328 Urushiol-formaldehyde diethylenetriamine polymer (UFDP, 113), a renewable and eco-friendly biopolymer, was also synthesized and used to fabricate BFA films.329,330 4.5. Microgels

A microgel is an intramolecularly cross-linked macromolecule dispersed in solution. Depending on the degree of cross-linking and the nature of the solvent, the microgel is more or less swollen.331 Various microgels have been used to prepare BFA films.332 Davis et al. reported the first preparation of BFA films based on a microgel, which was synthesized using a two-step method.333 They first synthesized the linear prearm, (polystyrene)-b-(polydivinylbenzene) (PS-b-PDVB), via RAFT polymerization. Then, the arms were coupled together via free radical polymerization of the residual double bonds in the polydivinylbenzene block, and the core-cross-linked star (CCS) microgels were prepared. The cross-linking time could be used to control the number of arms, which could reach up to 16 (on average) per cluster. When these microgels were used to prepare BFAs, the number and the component ratio of the linear arms played important roles in determining the pore size distribution and the regularity of the array. In another recent work, Yao et al. prepared BFA films using CCS nanogels with the middle block P(St-alt-MAn) as the core and two side block PS polymers as the corona.334 Poly et al. also obtained wellW


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defined BFA films by spin-coating THF solutions of nanogels based on poly(vinyl acetate) (PVAc).335 Qiao’s group conducted a series of investigations on the preparation of BFA films with CCS polymers (Figure 22).56,126,128,129,201,202,336,337 For example, they reacted living,

Figure 22. Schematic illustration of the arm-first method to synthesize CCS polymers.

linear, low molecular weight PMMA with ethylene glycol dimethacrylate (EGDMA, as cross-linker) and varying amounts of methyl methacrylate (MMA, as spacer) under ATRP conditions. A series of CCS polymers with Mn ranging from 0.3 × 106 to 1.0 × 106 and with 11−74 arms were produced. The casting conditions could be used to control the pore size of the BFAs. In this experiment, the pore diameters increased with humidity and decreased with flow rate. It was also found that the pore diameters decreased with the number of PMMA arms and the molecular weight of the star microgel.93 Qiao et al. also synthesized CCS polymers with PDMS chains as arms following the “arm first” method and found that these polymers could form perfect BFAs on a nonplanar substrate.202 They believed the starlike molecular shape and low Tg were the key factors for the formation of such BFA films. In another paper, they found that only those CCS polymers with Tg’s less than 48 °C could form ordered BFAs on a nonplanar substrate (see section 5.2 for details).128 They also found that the end group of the CCS polymer had an important effect on the formation of BFAs (see section 5.6 for details).126

Figure 23. (a, b) SEM (a) and high-resolution (HR) SEM (b) images of a BFA film of gold nanocrystals.151 Reprinted from ref 151. Copyright 2004 American Chemical Society. (c, d) TEM (c) and HR TEM (d) images of a perforated monolayer BFA film of gold NPs deposited on a water surface. The inset in (d) is the electron diffraction pattern of the BFA film.347 Reprinted with permission from ref 347. Copyright 2010 Wiley-VCH.

ordered structure at the nanometer scale. The superhydrophobic capping ligand provided adequate polarity and enabled nanocrystals to adsorb at the water−solvent interface and prevent water droplets from coalescing, which was critical for the formation of the gold nanocrystal BFAs. This concept has also been applied to other systems.343−346 Hao et al. employed dodecanethiol-capped gold NPs to prepare BFAs on a water surface and obtained the perforated pore structure (Figure 23c,d).347 The prepared gold NP-based macroporous thin films were easily transferred onto different substrates for various applications. In another study, Han et al. prepared elliptical pores with dodecanethiol-capped gold NPs by carefully controlling the direction and velocity of the airflow.348 Other NPs have also been used to prepare BFAs, such as various metal oxide NPs,349 Mn12 single-molecule magnet,350,351 PS modified clay,352 and horseradish peroxidase (HRP) NPs.353 For example, Yonezawa et al. prepared a BFA film with 1H,1H,2H,2H-perfluorodecanethiol (F8)-stabilized Ag NPs.354 Sanchez et al. investigated the formation of BFA films with different NPs as building blocks, including TiO2, Co, CdS, and SiO2.340 They found that optimum surfactants were CnH2n+1NH4Br (n = 12−14) for SiO2, CnH2n+1COOH (n = 7− 11) for TiO2 and Co, and CnH2n+1SH (n = 10−12) for CdS. The C11H23COOH/TiO2-based films maintained their honeycomb morphologies after calcination at 500 °C, and they exhibited higher photocatalytic activity than the calcinated nonporous films. Moreover, a heat treatment at 110 °C provided C9H19COOH/Co particles with magnetic properties, and an external magnetic field was able to influence the formation of the C9H19COOH/Co-based BFA film.355 In other reports, Wu153,356 and Hao357,358 demonstrated that a POM/ surfactant complex produced in chloroform could form regular BFA films by a dynamic BF process or at the air/water interface. Saito et al. prepared BFA films using Al2O3 NPs stabilized with a polymer containing catechol groups.359 The film retained its 3D structure after sintering at 600 °C. The

4.6. Nonpolymeric Materials

Nanomaterials including NPs, CNTs, and graphene possess a variety of unique catalytic, photic, and electric properties. The introduction of ordered patterns into these nanomaterials usually enhances their inherent properties and leads to new functions. In addition, inorganic materials have many other excellent properties, such as high resistance to heat and solvent. Therefore, the BFAs prepared with the mentioned nonpolymeric materials have attracted more and more interest and been widely investigated. 4.6.1. Nanoparticles. Ordered assemblies of NPs are attractive because of their unique photonic, electronic, and catalytic properties.338,339 A variety of NPs have been used to prepare BFAs, including NPs of metal and metal oxide and quantum dots (QDs).340 Usually, the NPs are decorated by organic ligands341 or polymers 151,342 to achieve good dispersibility in organic solvents and adequate polarity for NP adsorption at the water/solvent interface to prevent water droplet coalescence during evaporation (see section 3.2 for details). The most-studied NP for BFAs is the gold NP. For example, Korgel et al. prepared gold nanocrystals decorated with fluorooctyl methacrylate thiol (FOMA-SH) and used them to fabricate BFA films.151 Notably, nanocrystal BFA films with a long-range order at both the micrometer and nanometer length scales were obtained, as observed in Figure 23a,b. Here, the BFA is one level of ordered micrometer-scale structure, while a superlattice of monodispersed gold nanocrystals forms another X


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surfaces of the sintered BFA films contained nanosized pores among the NPs. Thus, they were hierarchical ordered films and were thermally and chemically stable. Moreover, based on the BF technique and the interfacial assembly of porphyrin vesicles at the interface of water/organic solution and air/water, Wang et al. prepared flower-shaped porphyrin Janus particles with an oriented arrangement.360 Due to the hierarchical structures, the obtained flower-shaped particles had a multiwavelength optical signal. Further, Hao et al. studied the direct assembly of NPs on the interface between BF water droplets and organic solution, and prepared ordered fluorescent rings with CdSe QDs.361 In this process, ordered arrays of water droplets were condensed on a chemically patterned substrate prepared by microcontact printing (μCP). Then, water droplet arrays were dip-coated with an organic solution of CdSe QDs. The NPs self-assembled onto the interface between water droplets and organic solution, and gave ring patterns after evaporation of organic solvent and water droplets. By this process, ordered ring arrays of NaYF4:Yb,Er NPs362 and organic molecules of oligo(pphenylenevinylene)s (OPVs)363 were also successfully prepared. Especially, arrays of square-in-ring binary structures363 and bull’s-eye-like microstructures364 were obtained by two steps of dip-coating process. Owing to the enhanced fluorescent properties, the ordered rings can be used as sensors.363 Based on a similar self-assembly process, Wang et al. also obtained porphyrin rings.365 Besides, other morphologies, such as wheels and hole−shells, were also prepared by controlling the hysteresis behaviors of substrates and solvent ratios. 4.6.2. Carbon Nanotubes and Graphene. The above NPs are all zero-dimensional nanomaterials. In fact, onedimensional (1D) or 2D nanomaterials can also be made into BFA films. CNTs and graphene are typical 1D and 2D carbon nanomaterials, respectively. The construction of CNTs and graphene films with regular porous structures has attracted increasing interest because of their potential applications in the areas of nanoelectronics, energy storage/conversion, and catalysis. To prepare CNT or graphene BFA films, dissolving CNTs and graphene sheets in a suitable organic solvent is the critical step. Nakashima et al. used lipids to help dissolve shortened single-walled CNTs (s-SWNTs) in chloroform and successfully constructed BFA films with this solution on glass and flexible poly(ethylene terephthalate) (PET) film.162,366 After an ionexchange process, the lipids were removed and pure CNT BFA films were obtained, as shown in Figure 24. Meanwhile, the CNT BFA films on PET with thinner skeletons exhibited a dramatic decrease in the surface resistivity, changing from insulating to conducting after ion exchange. CNTs are conductive and biocompatible, and thus, the BFA films based on them may have potential applications in electrochemistry and tissue engineering. Kim et al. grafted polymer chains on graphene oxide (GO) platelets to improve their solubility in organic solvents.367 Then, the modified GO was used to prepare flexible BFA films, as shown in Figure 25. The pore size and the number of porous layers were effectively controlled by the concentration of the dispersion and the length of polymer chains grafted onto the GO surface. Then, the GO was reduced by pyrolysis at high temperature, yielding reduced GO (rGO) BFA films. The conductivity of the BFA films was enhanced by pyrolysis and N-doping, as shown in Figure 25e.

Figure 24. (a) Schematic illustration for preparing complexes of sSWNTs−COON3C12 in chloroform and removing the lipid from the CNT films. (b, c) SEM images of BFA films of SWNTs.162 Adapted with permission from ref 162. Copyright 2007 Wiley-VCH.

Figure 25. (a) Schematic illustration of the formation of rGO BFA film. (b) SEM and (c) TEM images of rGO BFA film. (d) SEM image of the N-doped rGO BFA film decorated with gold NPs by ionic interaction. (e) Electrical behavior of GO, rGO, and N-doped rGO BFA films.367Adapted with permission from ref 367. Copyright 2010 Wiley-VCH.

GO can also be transferred into the organic phase from its aqueous solution with the assistance of a cationic surfactant because of the electrostatic interactions.368 Accordingly, Chen et al. prepared a complex solution of GO and dimethyl dioctadecyl ammonium (DODA) using the phase-transfer methods.369 BFA films based on this complex were then prepared via an “on-water spreading” BF method. The asprepared freestanding films exhibited superior broad-spectrum antibacterial activity, as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli (E. coli) as model pathogens. Upon decoration by TiO2 NPs, the composite BFA film electrode showed a fast, stable, and completely reversible photocurrent response that was approximately 2 times higher than that of the pure TiO2 film electrode. Wei et al. found that the complex of GO and DODA showed interfacial activities and could adsorb at water/solution interfaces.370 Therefore, the complexes were used as additives to promote the formation of BFA films of gold NPs and PS. Y


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(PACA) BFA films were obtained. In another work, Maniglio et al. irradiated the solution of acrylate (133, 134) and diacrylate (135) by UV under a moist atmosphere382,383 (Scheme 28). Thus, the polymerization and fabrication of BFAs occurred simultaneously, yielding a copolymer BFA film.

4.6.3. Small Organic Molecules. Small organic molecules are believed to be unsuitable for the BF process because their crystallization during the drying process will prevent the formation of BFAs. However, in some special cases, where the crystallization of molecules has been suppressed, ordered BFA films based on small organic molecules can also be successfully prepared. Karthaus et al. first demonstrated that small organic molecules could form ordered BFA films.371 They used a solution containing a cationic or anionic surfactant (114, 115) and metal complex (116−120) (Scheme 26) to cast the film and successfully obtained ordered BFAs. They speculated that the interaction between surfactants and the metal complex could inhibit the crystallization.

5. MORPHOLOGICAL CONTROL OF BFA FILMS The practical applications of BFAs are related to their morphologies. As demonstrated above, the BF process is very sensitive to the experimental conditions, and thus, great efforts have been devoted to controlling the morphology of BFAs. Many experimental variables, such as solvents, polymer− solvent interactions, substrates, the concentration of the polymer solution, humidity, gas flow velocity, atmospheric conditions, and solution−droplet interactions, have been identified as particularly crucial factors that determine the morphologies and properties of the BFA films. By controlling these variables, the regularity, pore size and shape, spacing between pores, and thickness of the membranes can be tuned and optimized. Moreover, mechanical and optical postprocessing has been used to control the morphology of BFAs. Many unique and useful BFA films have been prepared based on these strategies. In this section, the control of the morphology of BFA films is discussed.

Scheme 26. Chemical Structures of Small Organic Molecules (114−120)

5.1. Solvents

Solvents used to dissolve polymers have been identified as one of the most important factors influencing the formation of BFAs.81,384,385 Based on the mechanism of BFA formation, solvents with low boiling point, high vapor pressure, and water immiscibility are suitable for the BF process. In addition to these inherent properties of a solvent, the interactions, including interfacial tension between the solvent and the water droplet55 and the thermodynamic affinity between the polymer and the solvent,87,174 have also been shown to have obvious effects on the regularity and shape of the pores. Therefore, the most often used solvents are CS2 and chloroform. Other solvents, including dichloromethane, benzene, toluene, THF, dichloroethane, xylene, and Freon, have also been used in the BF process.20 PS serves as an example. Ferrari et al. selected a series of solvents to assess the influence of the boiling point, density, miscibility, interfacial tension with water, and thermodynamic affinity with the polymer on the formation of BFA films.41 Their results showed that no or poorly ordered BFAs or other morphologies were obtained using acetone, ethyl acetate, THF, or toluene. Chloroform, CS2, and dichloromethane allowed the formation of regular patterns (Figure 25). These results are in agreement with those obtained by Billon and Tian.101,174 Hansen solubility parameters were used to quantitatively evaluate the thermodynamic affinity between PS and the solvents by Ferrari.41 They found that only solvents with a relative energy difference (RED) number lower than 1 were able to generate regular BFAs. Therefore, the boiling point, miscibility with water, interfacial tension with water, and thermodynamic affinity with the polymer can be used as criteria to predict which solvent can generate BFAs. However, at present, it is difficult to state which criterion is the most important. The use of humid airflow instead of static conditions should make solvent evaporation easier and allow BFA films to form even when using solvents with relatively high boiling points.121

Another type of small molecule amenable to the BF process are those that can form supramolecules (121−131)372−380 (Scheme 27). Kim et al. used 121 to produce BFA films. 121 is a gelator that can form organogel in chloroform.372 Thus, during solution drying, it did not crystallize, and ordered BFAs were successfully prepared. Jiang et al. used a ribbonlike assembly of a small molecule, alkylated guanosine derivatives (122, 123), to fabricate ordered BFA films.373 These BFA films showed excellent capacities to encapsulate organic fluorescent dyes, which made the films fluorescent and thus allowed their direct visualization by fluorescence microscopy. Li et al. prepared BFA films with diphenylalanine (FF, 124) as the raw material.374 Interestingly, it was found that 124 underwent a hydrogen bond configuration transition from an antiparallel β sheet to a parallel β sheet during the BF process, and the aggregates were also transformed from laminar stacking to the hexagonal structure. The authors claimed that these phenomena could be attributed to the condensed water during the BF process. The water molecules were believed to intervene in the formation of hydrogen bonds during the self-assembly of 124 molecules. Huang et al. prepared highly ordered BFA films with 125, benzo-21-crown-7, and dialkylammonium salt via the static BF method.381 The reversible host−guest interaction between the two compounds was critical for achieving suitable viscosity and stabilizing the water droplets. In addition, monomers and small organic molecules, which can be polymerized or transformed into insoluble materials during the BF process, have also been used directly to prepare BFAs. For example, Xu et al. used an alkylcyanoacrylate (ACA, 132) solution in chloroform to prepare BFAs.102 Condensed water droplets on the solution surface acted not only as templates to generate pores but also as initiators triggering the polymerization of ACA. After the evaporation of solvent and the polymerization of monomers, polyalkylcyanoacrylate Z


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Scheme 27. Chemical Structures of Small Organic Molecules (121−131)

5.2. Substrates

on mica, glass, and silicon. Ferrari et al. believed that the surface energy of the substrate could be an important parameter determining the morphology of the BFAs.41 Thus, they tried to prepare BFA films on various inorganic and organic substrates with a broad range of surface energies, including glass (G), glass washed with a piranha solution (GW), silicon wafer (WSi), glass silanized with 3-glycidoxypropyltrimethoxysilane (GWG), glass silanized with octyltriethoxysilane (GWO), fluorinated glass (GWF), polyethylene (PE), PVC, and PET. As shown in Figure 26, substrates with higher surface energy promoted the

5.2.1. Materials of the Substrates. During the BF process, the substrate or surface where the polymer solution is deposited plays a very important role in the regularity of the pore array in the membranes.99,101,386,387 However, it is still unclear how and why the substrate influences BFA formation. Xi et al.100 and Hu et al.82 proposed that a good wettability of solid substrates by a polymer solution benefited the periodicity and regularity of pores based on a comparison of BFAs of the amphiphilic copolymers PDMA-b-PEO (29) and PS-b-PAA (6) AA


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products are of interest in many areas because they can be used as catalysts, sensors, adsorbents, scaffolds, photonic band gap materials, and optical stop-bands. Theoretically and experimentally, the formation of BFAs is not limited by the shape of substrates, and thus, BF is a promising technique for patterning on nonplanar substrates. Qiao et al. first reported the formation of BFAs on nonflat surfaces by using a solution containing a CCS polymer with PDMS arms (Figure 27).56 This polymer has a very low Tg

Scheme 28. Chemical Structures of Monomers (132−135)

formation of long-range-ordered BFAs for the PS/chloroform solution, but a completely opposite behavior was observed for the PS/CS2 solution. In this case, the best results were obtained on PVC (with a lower surface tension). In fact, the thickness, mass, and specific heat of the solid substrate also influence the formation of BFA because of the various heat exchange processes discussed in section 3.1.93 Therefore, there is no quantitative correlation between the substrate surface energy or wettability and the pore size and order. The combined effect of solvent, substrate, and polymer can possibly affect the nucleation and arrangement of water droplets. In addition to the above solid substrates, BFAs can be obtained on the surface of a liquid. Perforated BFAs have been prepared on various organic solvents with high surface tension, as discussed in section 5.8. Recently, Ma et al. found that liquid substrates with higher surface tension and lower viscosity produced smaller pores.83,244 5.2.2. Shape of the Substrates. Patterning a nonplanar substrate is challenging for lithography techniques, but the

Figure 27. SEM images of star PDMS BFA films on nonplanar substrates. (a) Spherical kaolin particle. (b) “Doughnut”-shaped kaolin particle. (c) Silica chromatography particle.56 Reprinted with permission from ref 56. Copyright 2007 Royal Society of Chemistry.

(−125 °C), and thus, it is soft, mobile, and able to replicate the complex contours of substrates.388 The versatility of the system was demonstrated by fabricating the BFA films on a range of inorganic particles, including chromatography-grade silica, glass microbeads, kaolin particles, and salt or sugar crystals, which have various shapes, sizes, and surface properties. These structures may be useful in a range of areas, such as chromatographic media and superhydrophobic/self-cleaning coating. During the preparation of BFA films on nonplanar substrates, cracking or fracture of the films caused by the stress

Figure 26. BFAs prepared from PS solution with various organic solvents on different substrates (relative humidity = 75%, 23 °C, scale bar 100 μm).41 Reprinted from ref 41. Copyright 2011 American Chemical Society. AB


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of the curved surface is a major issue.389 Qiao’s group systemically investigated the influence of the Tg and Young’s modulus (E) of the polymers on the formation of cracks on the BFA films prepared on nonplanar substrates. They synthesized a library of CCS polymers with different arm compositions, including PDMS, poly(ethyl acrylate) (PEA), poly(methyl acrylate) (PMA), PtBA, and PMMA, with the Tg’s ranging widely from −123 to 100 °C.128 It was found that only star polymer compositions with a Tg below 48 °C could form homogeneous BFA films on the surface of nonplanar substrates. In another series of β-cyclodextrin (β-CD)-based star polymers with varying Tg and E, it was found that the E of a polymer was a more important factor than Tg in determining the occurrence of cracking during the formation of BFA on nonplanar surfaces.390 With appropriate design, a star polymer with a high Tg and suitable E could potentially be used to form noncracking BFA films on nonplanar substrates, and this approach may ultimately lead to more and new industrial applications of this technology.201 Recently, our group further confirmed that neither the molecular topography nor the Tg of polymers was the key factor in the formation of uniform BFAs on nonplanar substrates. A linear homopolymer of PS, a BCP of PS-b-PAA, or a commercially available triblock copolymer of SIS can be used to produce BFA films that perfectly replicate nonplanar substrates (Figure 28).203,391 To explain why a brittle polymer

Figure 29. SEM images of representative BFA films prepared from PSb-PDMS on sugar particles. (b−d) Magnifications of the corresponding areas in (a).39 Reprinted with permission from ref 39. Copyright 2012 Royal Society of Chemistry.

micropipets with a curvature gradient.392 It was found that the BFA films formed on the micropipets changed remarkably with the gradually increased surface curvature. At low surface curvatures, the pores were mostly hexagonally packed, like those on flat substrates, but at high surface curvatures, the pores became irregularly shaped and more randomly dispersed. The variation of the pores on the micropipet at different sections was similar to that of BFAs at different heights on a vertical substrate, which was valuable for investigating the mechanism of the BF process. 5.3. Solution Concentrations

The pore size and regularity of BFAs are sensitive to the concentration of the solution. No ordered pores can form when the solution concentration is too low because the small amount of solute is not able to stabilize the droplets and prevent their coalescence.100,379,393 Additionally, sometimes the film is so thin that it becomes discontinuous because there is too little solute.99,284 With the increase of the solution concentration, water droplets can be effectively stabilized so that narrowly dispersed pores appear. However, the increased viscosity will weaken the convection in the solution, which is not beneficial to the self-assembly of droplets, and the pores will become disordered when the concentration is very high.163 It has been confirmed that increasing the solution concentration will reduce the evaporation rate of the solvent. A lower evaporation rate has two consequences: the temperature difference between the atmosphere and the solution surface will decrease, and the total evaporation time will increase. As discussed in section 3, the former will result in smaller pores, while the latter will result in larger pores. As a result, contradictory experimental results have been reported. In many cases, a higher concentration of polymer solution led to smaller pores.82−86 However, in some other, the pore size did not change significantly with the concentration of the solution.87 Moreover, the pore size was found to increase with the polymer concentration for some polymers.88 Therefore, the influence of concentration on the pore size depends on the structure and the property of the polymers as well as the casting condition.

Figure 28. SEM images of BFAs on nonplanar substrates prepared from (a, b) PS-b-PAA, (c) PS, and (d) SIS. (b) Magnification of (a).203,391 Reprinted with permission from ref 203. Copyright 2012 Elsevier. Reprinted with permission from ref 391. Copyright 2010 Royal Society of Chemistry.

film can contour the nonplanar structures, a hypothesis involving polymer plasticization by solvent during the BF process has been proposed. As discussed in section 3, at the late stage of the evaporation of the solution, the polymer film is still deformable due to the plasticization effect of the residual solvent. Thus, the film is able to contour to the nonplanar substrate without cracks. Importantly, the morphology of BFAs on nonplanar substrates can be used to expose the mechanism of BFA formation. Our group investigated the formation of BFA films on vertical substrates.39,391 BFAs at different stages can be fixed onto the wall at different heights with the help of a capillary force between the descending solution surface and the vertical wall. From top to bottom, both the pore size and the degree of order of the pore array increased (Figure 29), indicating the evolution of the water droplet array in the whole BF process. Gu et al. also investigated the BF process occurring on

5.4. Vapor Conditions

5.4.1. Water Vapor. In water vapor, high humidity is beneficial for the formation of ordered BFA films.160,330,394,151,296,351 No ordered pores can be generated when the relative humidity of the environment is too low, but AC


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it also reduces the total evaporation time of the solution. As described in section 3, these two processes have opposite effects on the pore size. As a result, while some researchers have reported that a high gas velocity led to smaller pores,121,161,167 others reported opposite results.168 In addition, the gas flow was also able to distort the water droplets.189 When the casting was performed at high gas velocities (>60 m min−1) with a horizontally tilted nozzle, the BFA pores were not spherically shaped and hexagonally ordered but were elliptically distorted and showed more of a brick-wall-type rectangular arrangement. 5.4.2. Nonaqueous Vapors. Usually, the BF process is performed in a water vapor atmosphere, and thus, the condensed water droplets on the surface of the solution act as templates to produce pores. For most of the polymers, ordered BFAs can be obtained in water vapor in suitable conditions (Figure 30a). Besides water, other vapors can also form BFs under the right conditions.397,398 However, the influence of the type of vapor atmosphere on the formation and morphology of pores has long been neglected by most researchers. Only a few publications discuss the preparation of BFAs in nonaqueous vapor.399−401 The morphologies of BFA films prepared under nonaqueous vapor are summarized in Figure 30. Disordered shallow holes with very large size (Figure 30b) or nonuniform spheres (Figure 30c) were found after casting a polymer solution in an ethanol or methanol atmosphere, an effect that could be attributed to the low surface tension of alcohols. Condensed alcohol spreads but does not form stable droplets on the surface of the polymer solution. If alcohol cannot dissolve in the polymer solution, such as methanol in a PS solution of CS2, a flat film with very large holes will be obtained. Otherwise, if alcohol is miscible with the solvent of the polymer solution, nonuniform spheres will be produced in a process similar to the nonsolvent induced phase separation because the alcohol acts as the nonsolvent.402 When mixed solvents of water and alcohol were used as the atmosphere, with the increase of the fraction of alcohol, the morphologies of the obtained films showed a conversion from porous films to spherical segments and microspheres. Recently, our group prepared BFAs with PS-b-PDMS (3) in methanol and ethanol vapor and systematically investigated the influence of the atmosphere on the morphology of the BFAs,

the threshold value depends on the properties of the polymer and the additives. The more stable the water droplets, the lower the threshold. For polymer solutions in THF, the threshold humidity is as high as 75% for PVC (67),276 but for amphiphilic copolymers of PS-b-PAA (6) and poly(styrene-co-acrylonitrile) (SAN, 136) (Scheme 29), 60 and 48% humidities, respectively, Scheme 29. Chemical Structure of SAN (136)

are sufficient to enable the formation of regular BFAs.82,394 The addition of Ag NPs into the solution of polyurethane and PLGA (102) decreased the threshold from 90 to 30%.395 A certain degree of control of the pore size can be achieved by regulating humidity.82 A trend is observed where the size of the pores in the films increases in a nearly linear fashion with humidity.43,163,262 However, for those materials that cannot efficiently stabilize water droplets, high humidity may result in the coalescence of water droplets, yielding a polydisperse pore size distribution.160 In a dynamic BF process, gas flow is used to adjust the humidity and accelerate the solvent evaporation to create a temperature gradient between the solution surface and the bulk solution. Systematic investigation revealed that the presence of the gas flow could speed the evaporation of the solvent and that a larger temperature gradient could be achieved between the surface of the solution and the substrate.190,396 As a result, Marangoni convection was promoted. The convection was able to pull the water droplets into the solution, and multilayered pores in the BFAs formed. When the gas flow was removed, the convection effect was suppressed, and only monolayered pores were observed.194 Therefore, the presence of gas flow should facilitate the formation of multiple-layered honeycomb structures. Although a high gas flow velocity can generate a large temperature difference between the solution surface and the atmosphere and can accelerate the condensation of water vapor,

Figure 30. Schematic illustration and representative SEM images of various polymer films prepared by the BF method in water and alcohol atmospheres.37,39,40,401 Adapted from ref 37. Copyright 2014 American Chemical Society. Adapted with permission from ref 39. Copyright 2012 Royal Society of Chemistry. Adapted with permission from ref 40. Copyright 2009 Elsevier. Adapted with permission from ref 401. Copyright 2014 Elsevier. AD


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including the shape, size, and regularity of the pores.39 With the decrease of the surface tension and evaporation enthalpy of the atmospheric solvent, the volume of the pores increased, and the shape of the pores changed from spheroid to ellipsoid and cylindrical (Figure 30e,g). This is the first report of a successful preparation of ordered polymeric BFA films under a nonaqueous atmosphere. The formation of ordered BFAs in alcohol vapors can be ascribed to the preferential aggregation of low-surface-energy siloxane segments of PS-b-PDMS at the air/ solution interface, thus preventing the spread of CS2-saturated methanol droplets. Based on the results, a universal modified BF process was devised for preparing BFAs in methanol vapor with conventional polymers by adding a small amount of a surface-active agent, such as siloxane- and fluorine-containing BCPs, to the casting solution.37 The pores in the PS films prepared with this method were cylindrical with large depth− diameter aspect ratios (Figure 30d), and the diameter and depth of the pores could be easily controlled by the experimental conditions. 5.5. Interfacial Tension between Water Droplets and the Polymer Solution

Because of the inherent mechanism of utilizing water droplets as the templates of pores in BFAs, the interfacial interaction between water droplets and the solution plays an important role in the morphology of BFAs.55 The interfacial tension between water droplets and the polymer solution is decided by both the solvent and the solute of the solution. Therefore, changing the hydrophilicity of the polymers and adding surfactants to the solution can modulate the interfacial tension. Changing end groups can modify the hydrophilicity of polymers, and a suitable hydrophilicity is needed for the formation of highly ordered BFAs. Qiao et al. systematically investigated the manipulation of film features by controlling the polymer end groups.126 They first synthesized the star polymers end-functionalized with acetonide-protected dendrons (generations 1−4) via the “arm first” method. Deprotecting the acetonide groups yielded hydroxyl-functionalized CCS polymers, and modifying these hydroxyl groups with pentadecafluorooctanoyl chloride gave a third series of functionalized CCS polymers. When these CCS polymers were used to prepare BFAs, the authors found that the morphologies of the BFAs were dramatically different (Figure 31). The CCS polymers with amphiphilic character (with hydrophilic end groups) afforded interconnected porous morphologies with multiple layers of pores, whereas the CCS polymers with pentadecafluorooctanoyl end groups showed highly ordered monolayers of cylindrical pores with extremely thin walls. This work provided a possible method for tuning the morphology of BFAs. These results agree well with those reported by Wan et al.,186 who used porphyrin-core star polymers with PS arms (60) and PS-b-PHEMA (61) arms to construct BFA films. Monolayers of highly ordered pores and multilayers of pores were generated from star PS and star PS-b-PHEMA, respectively. The small amount of hydrophilic HEMA units had a significant influence on the morphology of the BFA. Linear PS with different hydrophilic end groups, including bromo, hydroxyl, sodium carboxylate, ethanolamine, diethanol amine, 2-amino-1,3-propanediol, and 2-(2-aminoethoxy)ethanol, showed similar results.42,403 For PS-b-PDMAEMA, only block copolymers with appropriate block ratios obtained ordered BFAs because of the suitable interface tension between copolymer solution and water droplets.404

Figure 31. SEM images of BFA films prepared from a CCS polymer with various end groups. The insets are the chemical structures of the end groups: (a) acetonide; (b) hydroxyl; (c) perfluoroalkyl.126 Reprinted with permission from ref 126. Copyright 2008 Wiley-VCH.

Using small molecule surfactants or hydrophilic polymers as additives is also an effective way to control the morphologies of BFA films owing to the decrease of the interfacial tension between the polymer solution and the water droplets.405 Shimomura et al. added various surfactants with different hydrophile−lipophile balance (HLB) values into the PLA solution for the BF process.113 They found that, with the increase of the HLB value, the morphology of the casting film changed from highly ordered arrays with spherical pores to disordered arrays with ellipsoid pores. Further increasing the HLB value caused the disappearance of pores (Figure 32a−c). Wan et al. also controlled the pore shape by adding hydrophilic

Figure 32. SEM images of BFA films with various surfactants and hydrophilic polymers. (a−c) PLA films with assistance of (a) dioleoylphosphatidyl ethanolamine (DOPE), (b) dioleoylphosphatidyl choline (DOPC), and (c) dilauroylphosphatidyl choline (DLPC). HLB value: DOPE < DOPC < DLPC.113 Adapted with permission from ref 113. Copyright 2009 Royal Society of Chemistry. (d, e) PS films prepared from a chloroform solution without (d) and with (e) PDMAEMA (7 mg mL−1). (f) PS films prepared from a THF solution with 4 mg mL−1 PDMAEMA. The insets are cross-section views.406 Adapted from ref 406. Copyright 2011 American Chemical Society. AE


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Figure 33. (a) Schematic illustration of shrinking of BFA films using a thermally shrinkable substrate. (b−i) SEM images of PB BFA films before (b, c) and after (d−i) shrinking. The shrinking directions are shown in (c).277 Adapted with permission from ref 277. Copyright 2008 Wiley-VCH.

polymer into a PS solution.406 When chloroform was used as the solvent, with the increase of the amount of poly(N,Ndimethylaminoethyl methacrylate) (PDMAEMA), the pores were changed from near-spherical to ellipsoid in shape (Figure 32d,e). However, when THF was used as the solvent, ellipsoid pores were always obtained regardless of the amount of hydrophilic polymers due to the low interfacial tension between water droplets and the polymer solution in THF (Figure 32f). Recently, they also reported an easy and effective method for manipulating the pore size of BFAs by adding various amounts of amphiphilic BCPs into the solution of PS with long alkyl or fluorinated end groups.114

Recently, our group developed a noncontact photomanipulation technique to modulate the shape of the pores in BFA films made of an azobenzene-containing polymer of P4VP-bPAzoMA (45).38 This technique is based on the mass migration caused by the photoreconfiguration of azobenzene units in the azobenzene-containing polymers. It allows nanoscopic elements of materials to be precisely manipulated with good reproducibility and high throughput, and it causes massive motion at the micro scale (Figure 34a).407 Under the

5.6. Deformation of BFA Films

Usually, the pores obtained by the BF process are circular. However, pores with other shapes are expected in many special applications. Therefore, a variety of physical secondary processes have been used to create novel BFAs with nonconventional pores. Both mechanical stretching and shrinking can transform hexagonally arranged round pores. For example, Shimomura et al. demonstrated that when a BFA film of PCL, a viscoelastic polymer, was mechanically stretched, the isotropic array of hexagonal pores was transformed into an anisotropic alignment of stretched pores.315 Geometric patterns observed on the stretched film were elongated hexagons, rectangles, squares, and triangles, depending on the stretching direction. The arrays of stretched pores could guide the spreading of cardiac myocytes along the long axis of the pores. Shrinking is also able to control the shapes and sizes of the pores. The same group also illustrated the topological changes in BFA films on a thermally shrinkable substrate, as shown in Figure 33a.277 After thermal shrinking, porous films with various geometrical patterns (ellipsoids, rectangles, and triangles) were fabricated (Figure 33b−i). The sizes of the pits were reduced from several micrometers to hundreds of nanometers by repeated cycles of thermal shrinking.

Figure 34. Schematic and SEM images of the photoreconfiguration of P4VP-b-PAzoMA copolymer BFA film. (a) Schematic illustration of the transition movement of azobenzene. (b) Two types of irradiation with different types of polarization light (S and V). (c, d) SEM images of BFA film after irradiation along the S and V directions for 30 min, respectively.38 Reprinted with permission from ref 38. Copyright 2014 Wiley-VCH. AF


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Figure 35. (a) Calculated curves of differential pressure and critical differential pressure vs the radius. Insets are pore structures of ordered membranes prepared from solutions using benzene, chloroform, and CS2 (from left to right) as the solvent and an illustration of the formation of perforated pores.409 (b) Illustration of the formation of BFA films with perforated pores on a glass surface.255 (c−f) SEM images of the typical BFA film with perforated pores prepared on an ice surface (c, d)409 and glass substrate (e, f).255 Adapted from ref 409. Copyright 2011 American Chemical Society. Adapted from ref 255. Copyright 2012 American Chemical Society.

a very thin water film on the melted ice surface, the condensed water droplets on top could run through the solution and contact the water film underneath, forming perforations during the BF process. Based on these findings, perforated BFA films of brominated PEO, miktoarm star copolymers and cellulose triacetate were obtained on ice substrates.171,324,411 Goedel et al. fabricated microsieves with a hierarchical porous structure by creating BFA films on a structured substrate.412 Usually, the structured substrates were made of a layer of spherical glass beads protruding out of a planar surface or plates structured via embossing, engraving, or etching. During the BF process, water droplets partially penetrated into the layer and created pores in the final solid polymer layer. The trenches of the structured substrate were filled with polymer at a level much deeper than the penetration depth of the water droplets. After the separation of the vitrified layer from the substrate, thin polymer membranes were obtained with a hierarchical structure consisting of an ultrathin active separation layer with submicrometer pores and a supporting layer with larger pores. Recently, our group also developed a versatile method to prepare perforated BFA films on solid substrates.255 The key step of this method is to rapidly aspirate the excess solution underneath the floating droplets array during the evaporation process (Figure 30b). With the descent of the solution surface, the floating droplets touched the substrate, only separated by a thin layer of polymer. The thin interfacial polymer layer ruptured from the pressure between the water droplet and the substrate. As a result, the water droplets adhered to the substrate, and perforated pore structures appeared after the complete evaporation of the solvent and water, as shown in Figure 30e,f.

irradiation of linearly polarized light, circular pores in the BFAs were converted into rectangles, rhombuses, and parallelograms in 30 min, depending on the polarization direction of incident light (Figure 34b−d). After a secondary irradiation by rotating the sample 90°, the transformed circular pores were recovered, demonstrating good reversibility for this photodeformation process. 5.7. Strategies for Perforated BFAs

Polymer films with uniform perforation can act as separation membranes and masks. In most studies, there is always a bottom layer under the pores of BFA films, and thus, the pores cannot perforate the film. Shimomura and Parisi et al. prepared freestanding perforated BFA films by depositing a dilute solution of an amphiphilic polymer (5) and nitrocellulose (107) on the surface of water.326,408 This method is called “onwater spreading”. Wan et al. demonstrated that the surface tension of soft substrate was essential for the formation of perforated BFAs.409 They proposed that, with the evaporation of the casting solvent, the thickness of the polymer solution became thinner than the diameter of the water droplets. Consequently, a meniscus formed under the water droplet, where the water droplets and the substrate liquid were separated by a thin polymer film (Figure 35a). The pressure difference induced by the surface tension of the substrate liquid across the meniscus during the evaporation of water droplets should exceed the critical pressure at which the thin polymer film ruptures, generating perforated pores (Figure 35c,d). Therefore, perforated pores could form on the surfaces of glycerol and formic acid with high surface tension but not on those with low surface tension, such as acetic acid, TEOS, ethyl acetate, ethanol, isopropanol, and methanol. Furthermore, Hao et al. prepared highly ordered freestanding BFA films with asymmetric perforated pore structures from PS/dodecanethiolstabilized gold NPs at an air/ice interface.410 Because there was AG


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5.8. Methods for Hierarchical BFA Films

Scheme 30. Chemical Structure of PS5F-b-PS-b-PPEGMA (137)

Hierarchical structures of materials are necessary for many device applications. For BFA films, the hexagonal BFAs make up one level of the hierarchical structures, and another level can be either a smaller scale structure induced by microphase separation413 or a larger scale patterns generated by external masks or substrates.414 Besides, multilevel BFA films can be produced by two steps of spraying atomized water droplets.415 Hayakawa and Horiuchi et al. synthesized a semi-rod−coil BCP of PS-b-POTI (26) containing mesogenic oligothiophene on side chains.234 After casting the polymer solution under a moist atmosphere, a film with hierarchical structures of three tiers from angstroms to micrometers was obtained: a liquid crystal phase and phase-separated nanodomain structures formed by BCPs and ordered BFAs (Figure 36a−c).

prepared BFA films with PS-b-PnBMA (11) to construct regular micrometer-scale pores and degraded the component of PnBMA by UV irradiation to produce nanoscale pores. Thus, a multilength scale porous polymer film was obtained.218 Additionally, Schubert et al. demonstrated a hierarchically porous film via a surfactant-assisted BF process.417 They added a prehydrolyzed silica sol containing the nonionic surfactant to the casting solution. Thus, during the BF process, water droplets produced regular macropores, and the surfactant selfassembly induced the microphase separation at the nanometer scale. After removing organics at high temperature, micro-, meso-, and macroporous SiO2 was obtained. The pores were not strictly ordered; however, the poor regularity had no influence on the adsorption applications. Sel et al. used the surfactant-assisted BF process to prepare BFA films of PVDF− HFP (37) and silica sol.245 Subsequently, the surfactant was removed by Soxhlet extraction, and organic−inorganic films with hierarchical structures of macro- and mesopores were obtained. After introducing −SO3H into the silica particles, the prepared films showed high proton conductivity owing to the hierarchical porous structures. In another example, Saito et al. prepared BFA films with NPs of SiO2, TiO2, Al2O3, and ZnO stabilized by mussel-inspired copolymers.349 After removing the polymers by calcination at a high temperature, mesopores were produced among the NPs. Therefore, hierarchical porous organic films were produced. The hierarchically ordered BFA films were also prepared with the assistance of masks or patterned substrates. Kim et al. prepared diverse hierarchical structures by placing a gratingtype mask on the polymer solution during the BF process, physically confining the nucleation and the growth of water droplets.198 After evaporation was complete, the grating structure itself and the hexagonally packed pores constituted independent multiscaled orderings in the polymer film (Figure 37a,b). Kim et al. reported the combination of top-down and bottom-up approaches to produce dual-patterned honeycomb lines.372 They first prepared a BFA film with a photoresponsive small molecule (121) and then patterned the film by exposing it to UV irradiation through a photomask, inducing selective chemical cross-linking in the exposed region. After removing the non-cross-linked polymer with organic solvent, honeycomb lines were obtained at predefined locations (Figure 37c,d). Qiao et al. also used the combined method of BF and photolithography to prepare hierarchical BFAs using a photocross-linkable polymer of star PMA terminated with 9anthracene.201 Based on the reversible cross-linking reaction of anthracene under UV irradiation, different patterns could be generated on BFA films by photolithography. Moreover, hierarchical BFA films can be produced by using patterned substrates, such as a TEM grid, as mentioned in section 5.2. Therefore, the combination of the BF process, patterned

Figure 36. Hierarchically self-organized BFAs. (a−c) BFA film made from semi-rod−coil BCP PS-b-POTI. (a) SEM image of hierarchical BFA film. (b) TEM image of the corresponding areas in (a). (c) Corresponding sulfur-distribution image.234 Adapted with permission from ref 234. Copyright 2003 Wiley-VCH. (d−g) BFA films made from a blend of PS and triblock copolymers of PS5F-b-PS-b-PPEGMA. (d, e) Atomic force microscopy (AFM) topograph images of BFA films (d) before and (e) after annealing. (f, g) Corresponding AFM phase images of the holes.48 Reprinted from ref 48. Copyright 2009 American Chemical Society.

Rodriguez-Hernandez et al. reported the preparation of hierarchically micro- and nanostructured polymer films with blends of poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene-bpoly[poly(ethylene glycol) methyl ether methacrylate] (PS5Fb-PS-b-PPEGMA, 137)/PS48,416 (Scheme 30). The interplay of the formation of BFAs and the self-assembly of the triblock copolymer allowed the preparation of polymer films with micrometer-sized pores decorated with nanostructured BCPs. The chemical composition of the surface could be varied by a surface rearrangement upon annealing in either dry or humid air (Figure 36d−g), and the porous nanostructure could be modulated from a micellar array to a lamellar phase when the film was exposed either to air or to THF vapor. Russell et al. AH


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highly volatile solvent and a nonsolvent followed by exposure to ambient air, as shown in Figure 38a. The evaporation of the mixture under ambient air eventually induced phase separation and spontaneously created a highly ordered honeycomb micropattern on the thin polymer film, as shown in Figure 38b−g. The pore size of the patterned film could be further fine-tuned through modulating the experimental conditions such as nonsolvent content, ambient humidity, and evaporation temperature. Patterned film was observed throughout the coated surface, as large as 150 cm2. The advantages of the newly developed strategy over the conventional BF approaches rely on that the nonsolvent (usually methanol) can induce the transport of the polymer to stabilize the droplets, and enhance the lateral capillary force between the adjacent droplets, promoting the formation of the ordered structures. By this strategy, large areas of highly ordered porous films of PMMA and PLA that are difficult to prepare using conventional BF methods have been successfully obtained. The other strategy was devised by Yamazaki et al. in Fuji Film Co.18,419,420 They examined the formation mechanisms of the line defects in BFA, and clarified two types of formation mechanisms of the divergent mode line defects and the convergent mode line defects caused by the “tectonics” of the water droplet arrays. The calculation indicated that an optimum water droplet growth time was necessary for the formation of defect-free polymer film. An experimental apparatus shown in Figure 39a was designed based on the theory, and A4-sized defect-free BFA polymer films (Figure 39b) were fabricated on this apparatus. A European patent was also applied by Fuji Film Co. to protect this new bottom-up microfabrication technique.18 More industrial application information can be found on the Web site of Fuji Film Co.421

Figure 37. SEM images of well-ordered hierarchical BFA structures. (a, b) Using a parallel grating (a) before and (b) after the nucleation of water droplets.198 Reprinted with permission from ref 198. Copyright 2007 Wiley-VCH. (c, d) BFA lines prepared by UV lithography. (d) Thinnest lines achieved by UV lithography.372 Reprinted with permission from ref 372. Copyright 2009 Wiley-VCH.

substrates, and photolithography with masks can create hierarchical porous polymer films with three or more levels, and these have great application potential.201 5.9. Large-Scale Preparation of BFA Films

The high-throughput and large-scale preparation of ordered BFAs is still a tough task since the first publication of BFA films by François two decades ago.17 This is why their commercial application is hardly reported. Recently, two breakthroughs have been achieved, and are expected to promote the industrialization of the BFA films. The first strategy involving a two-step process without the addition of the surfactants was developed by Bui et al.321,418 First, the polymer solution was coated on a solid substrate to fabricate a large-scale thin polymer film with a uniform thickness. Then the film was dip-coated with a mixture of a

Figure 38. (a) Preparation of BFA films with large areas. (b−d) SEM images of the PMMA BFA films prepared with chloroform/methanol (85/15, v/v) under a relative humidity of 55% and a temperature of 23 °C. The inset in (b) is the FFT pattern. The inset in (c) is the patterned pincushion film. (e, f) Two-dimensional and 3D AFM images, respectively, and (g) cross-sectional profile of (e). The red line in (e) indicates the location of the cross-sectional profile.418 Reprinted from ref 418. Copyright 2015 American Chemical Society. AI


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shown in Figure 40a,b. Chin et al. developed a novel method to control the separation between pores and to prepare non closely packed arrays of micrometer and submicrometer pores.422 When water droplet arrays formed on the surface of the polymer solution, the authors began to heat the substrate and the solution to a temperature above the dew point (Figure 40c). This caused rapid evaporation of both the solvent and the water droplets. The water droplets quickly became immobilized due to the increasing viscosity of the solution, but the solution remained deformable. As a result, the pores on the surface continued to shrink along with the water droplets, as shown in Figure 40d−g. With this method, the authors prepared BFAs with feature separation-to-size ratios of up to 12.44. Porous fibers423−426 and microspheres427 have been prepared by combining the BF technique with other methods. For example, Li et al. developed a simple method called “nonsolvent assisted electrospraying” to prepare hierarchically porous polymer microspheres, as shown in Figure 41a.428 A PMMA solution in mixed solvents of dichloromethane (solvent) and hexanol (nonsolvent) was electrosprayed, and the formed microdroplets were deposited onto a substrate. After drying the solvents, the porous microspheres were obtained, as shown in Figure 41b,c. Nonsolvent-induced phase separation and the BF process were responsible for the formation of the hierarchical porous structures. The nonsolvent not only induced phase separation but also stabilized the interface between the droplet and the air, thus preventing the droplet from undergoing strong deformation. Therefore, the nonsolvent was beneficial to the formation of regular and uniform microspheres. The hierarchically porous microsphere significantly increased the surface roughness and, consequently, the hydrophobicity; thus, the CA could become as high as 152.2°. Cho et al. prepared porous microspheres with a nonsolvent-assisted microfluid method.429 Briefly, an oil phase droplet, which was composed of PLGA (102), 2-methylpentane (PCM, nonsolvent), and dichloromethane (solvent), was

Figure 39. (a) Schematic illustration of the experimental apparatus. (b) Photograph of a defect-free BFA film prepared on an A4-sized PET film. The inset image shows a laser diffraction pattern of the BFA film.419 Adapted with permission from ref 419. Copyright 2013 Royal Society of Chemistry.

5.10. Other Methods

As discussed in section 3, the attraction between water droplets comes from interfacial tension, and their repulsion is caused by the slight deformation of water droplets when they touch. Therefore, the pores in BFAs are usually packed closely, as

Figure 40. BFA film with non closely packed (NCP) pores. (a, b) SEM (a) and AFM (b) images of the closely packed BFAs prepared without a heating stage. (c) Illustration of the formation of transition-type morphologies. (d−g) SEM images of corresponding BFA areas in (c).422 Adapted with permission from ref 422. Copyright 2013 Royal Society of Chemistry. AJ


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Figure 41. (a) Schematic illustration of porous microspheres prepared by combined BF and electrospraying processes. (b, c) SEM images of porous microspheres prepared with different flow rates: (b) 2 and (c) 3 mL h−1. Insets are magnified SEM and TEM images.428 Adapted with permission from ref 428. Copyright 2013 Royal Society of Chemistry.

discontinuously introduced into a 1 wt % poly(vinyl alcohol) (PVA) aqueous solution that flowed continuously throughout the tube-type fluidic apparatus. After solidification, porous microspheres with a surface dimple pattern were obtained. The surface dimples were demonstrated to be replicas of PCM droplets that underwent a similar process as in BF, whereas the internal pores contributed to the nonsolvent-induced phase separation. The particle size can be well controlled by the flow rate of the PVA aqueous solution, and the size of the surface dimples can be adjusted by the ratio between PLGA and PCM. Other microporous polymer microspheres, such as PS, PMMA, PLA, and PCL, have also been prepared by this method. Zhang Newby et al. developed a modified BF approach to prepare a porous polymer film with different surface features.430,431 As shown in Figure 42a−d, they first produced an ordered water droplet array on a SiOx or Si substrate by drying pure ethanol in a humid air flow. During the evaporation of ethanol, water condensed on the ethanol surface and accumulated near the receding contact line. This induced the formation of water fingers at the receding contact line and consequently ordered arrays of water droplets after detachment. They then coated a layer of PS solution onto the water droplet array by dip-coating. After the PS solution dried, porous films were obtained. Hexagonal, square, and other pore arrays could be fabricated by varying the substrate wettability, the receding velocity of the contact line, and the thickness of the cast ethanol liquid film (Figure 42e−h). The interpore distance (L) and pore size (D) could be controlled by tuning the humidity of the air flow during the formation of the water droplet arrays. Moreover, the pore size could be readily tuned by further water condensation or evaporation, yielding porous polymer films with pore arrays with different D/L ratios.

Figure 42. (a−d) Schematic illustration of BFA formation using the Marangoni flow-induced arrays of water droplets as templates. (e−h) Typical images of porous PS films with hexagonal (e, f) and square arrays of pores (g, h) formed by the modified BF process on the SiOx and Si substrates, respectively. The corresponding relative humidity is shown at the upper right corner of each image.430 Reprinted from ref 430. Copyright 2009 American Chemical Society.

and physical modification, which may endow the film with new functionalization. Accordingly, a variety of chemical and physical methods have been developed to modify and functionalize BFAs. Moreover, polymeric matrixes consisting of linear structures can be cross-linked by postchemical modification to increase their dimensional stability. This provides another way to improve the performance of BFAs. These postmodification methods are summarized in this section. 6.1. Functionalization of BFA Films

Physical blending and surface chemical modification strategies have been employed to build functionalized BFA films. By casting a mixture of polymer and NPs in solution, the Pickering effect combined with the BF process can afford additivefunctionalized porous BFA films. Moreover, the surface chemical modifications are widely performed after the BF process, and these modifications endow the BFA films with new functions. 6.1.1. Hybridization with Inorganic Nanomaterials. To prepare hybrid BFAs, the inorganic nanocomponent is mixed with a polymer solution, and the blend solution is cast following a standard BF process. As discussed above, the nanocomponent may self-assemble at the polymer solution/ water droplet interface and help stabilize the water droplets (Pickering effect) (Figure 43a).150,432 The Pickering effect in

6. CHEMICAL MODIFICATION AND FUNCTIONALIZATION OF BFA FILMS Controlling surface properties and chemical functions is one of the essential goals in the construction of porous materials. During the BF process, polar groups or segments and NPs will be selectively enriched on the wall of pores due to the hydrophilic−lipophilic balance and the Pickering effect. This spontaneous surface heterogeneity benefits further chemical AK


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functionalization of the zeolite surface resulted in different organizations inside the pores of the polymer BFA film. The NPs can also be incorporated into BFA films by in situ reactions. Xu et al. prepared hemispherical or mushroomlike TiO2 microparticles from a homogeneous solution of TiCl4/ PS/CHCl3 using the BF method (Figure 44).439 Water droplet

Figure 43. (a) Illustration of the formation of hybrid BFA film. The particles assemble on the water−solution interface due to the Pickering effect. (b, c) TEM images of cross section of BFA film. The CdSe NPs can be observed as a thin black film at the polymer−air interface.150 Adapted with permission from ref 150. Copyright 2004 Nature Publishing Group. (d, e) Fluorescence optical microscopic images of BFA films of polymer embedded with Ag NPs and CdSe QDs (ratio 2:1) (d) and with only CdSe QDs (e).437 Reprinted with permission from ref 437. Copyright 2013 Royal Society of Chemistry.

Figure 44. SEM images of ordered composite films filled with (a) hemispherical and mushroomlike (b)TiO2 NPs.439 Reprinted from ref 439. Copyright 2011 American Chemical Society.

arrays condensed on the solution surface acted as the “microreactors” for the hydrolysis of TiCl4. At the same time, PS self-organized around the arrays of “microreactors” to form an ordered porous matrix. This method is versatileother polymers can be employed, and other hemispherical or mushroomlike particles can be obtained by using corresponding precursors. 6.1.2. Surface Chemical Modification. As mentioned above, polar segments always localize on the inner surface of the pores, whereas the hydrophobic segments exist inside and on the top layer of the film.319,440 Therefore, it is possible for us to selectively modify the inner surface of the pores or the top surface of the BFA films.441 Juang et al. prepared polyurethane BFA films from dendritic side-chain polymers with reactive pendant units.160 Subsequent chemical modification through reaction with a hydrophobic poly(oxyalkylene) amine increased the CA from 113 to 134°. Further physical modification through a peeling-off process gave the film surface a 3D rod-co-valley structure. This film exhibited superhydrophobicity with a CA of 151°. Stenzel et al. sequentially grafted the thermoresponsive monomers Nisopropylacrylamide (NIPAAm) and N-acryloyl glucosamine (AGA) onto BFA films by RAFT polymerization.442 As a result, the films displayed switchable hydrophilic/hydrophobic characteristics and were turned into a thermodependent switch for the selective recognition of biomolecules. The patterning of biomolecules is an important and challenging technique. Cortajarena et al. prepared BFA films from a mixture of a PS-b-PAA (6) diblock copolymer and a PS homopolymer.443 They then performed a straightforward functionalization using ethyl(dimethylaminopropyl) carbodiimide as a catalyst to esterify carboxyl groups inside the pores with different polypeptide sequences. Wan et al. prepared BFA films with an amphiphilic BCP, PS-b-PHMA (25).143 Three-

the BF process leads to the formation of hierarchically structured NP arrays. Thus far, a great number of nanomaterials, such as NPs of Au,433 CdSe,150 Fe2O3,434,435 and SiO2,44,149 CNTs,436 hydrophilic nanoclays,288 surfactant-encapsulated POMs,146 and amino modified zeolite L crystals,152 have been reported to be able to form hybrid BFAs. Russell et al. confirmed the preferential segregation of the CdSe NPs to the polymer solution−water interface, where they formed a 5−7-nm-thick layer and functionalized the walls of the holes (Figure 43b,c).150 This process opened a new route to the fabrication of highly functionalized ordered microarrays of NPs that are potentially useful in sensors, separation membranes, or catalytic applications. Cavallini et al. reported that a 6-fold enhancement of the QD emission was observed when the QDs were coupled with the near field from Ag NPs, which were spatially organized in an ordered BFA (Figure 43d,e).437 Sun et al. proved that silica NPs, PS colloid crystals, and poly(NIPAm-co-acrylic acid) microgels could be easily transported to the oil/water interfaces, where they would stabilize the water droplets and assist the formation of BFAs.149,438 It was found that an increase in the amount of particles resulted in an increase in the pore size and a decrease in the size of the pore intervals. Jiang et al. prepared a bioinspired BFA film with PI (80) as a basic structure and nanoclay as the enhanced layer in the walls. 288 The introduction of the clay into the film not only increased the thermal stability of the materials but also improved the hardness of the BFA films 5-fold. In another work, Vohra et al. prepared electroluminescent polymeric BFA films with zeolite nanocrystals of 40 nm diameter that self-organized in the surface.152 To increase the solubility of zeolite, they chemically modified their external surface. The different AL


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dimensional fluorescence results demonstrated that the hydroxyl groups were primarily inside the pores (Figure 45).

demonstrated that the surface modification was successful. This group also found unprecedented morphologies in BFAs prepared with an incompatible ternary blend consisting of high-molecular-weight PS, an amphiphilic BCP of polystyreneb-poly[poly(ethylene glycol) methyl ether methacrylate] (PS-bP(PEGMA), 138) (Scheme 31), and a fluorinated homopolymer, P5FS.138,447 The fluorine homopolymer was preferentially situated at the pore edge but formed spherical domains with narrow polydisperse sizes (Figure 46b−d). This morphology was attributed to the two simultaneously occurring processes, i.e., the BF process and the phase separation process. Thiolated glucose molecules could be specifically attached to the P5FS21 domains via a thiol−para fluorine “click” reaction. 6.2. Cross-Linking of BFA Films

Cross-linking is a common method for polymer modification because of its profound influence on the mechanical properties and other characteristics, such as segment mobility, solubility, and permeability, of polymers. Therefore, cross-linking can effectively improve the polymer performance in a harsh environment. Usually, cross-linking reactions can be performed by thermally, chemically, or photochemically treating the polymers with cross-linkable reactive groups. As described below, by carefully choosing suitable reactions, BFA films can be efficiently cross-linked without damaging their honeycomb morphology. 6.2.1. Thermal Cross-Linking. A one-step thermal treatment will inevitably cause the linear polymeric microstructure to completely or partially collapse before the temperaturesensitive groups totally cross-link. Bunz et al. synthesized azidesubstituted, soluble poly(p-phenyleneethynylene) (PPE, 88) and prepared its BFA film.448 Upon heating to 300 °C, the polymer cross-linked, and the thin film was turned into an insoluble matrix with isolated pores. However, the pore-filled structures collapsed on the substrate. To avoid the collapse, a two-step thermal treatment was adopted by Galeotti and coworkers.47 First, the pores on the BFA films were filled with a liquid PDMS precursor. Then, the PDMS precursor was solidified at a temperature lower than the Tg of the polymer matrix. When the temperature was increased further, the thermal cross-linking of the polymer matrix was initiated. The solidified PDMS elastomer covering the surface and filling the pores of the film worked as a memory mask for maintaining the microstructure of the BFAs. After cooling and peeling off the PDMS mask, the polymer film became insoluble, while the surface structure was perfectly preserved. 6.2.2. Chemical Cross-Linking. Many simple chemical reactions can efficiently cross-link BFA films, thus improving their mechanical strength and chemical stability. Wong et al. reported the synthesis of silicone-based random branched copolymers (139, 140) (Scheme 32) containing 3(trimethoxysilyl)propyl methacrylate (MPS) and 3-[tris(trimethylsiloxy)silyl]propyl methacrylate (TRIS).449 After the BF process, the presence of alkoxysilane precursors inside BFA films enabled the cross-linking of the film via the sol−gel process. Cross-linked BFA films had enhanced properties, including higher flexibility and better solvent resistance and thermal stability, which may enable their applications as thermal sensors or low-cost conformal digital displays when coupled with thermally sensitive or light-emitting materials inside the pores. Karthaus et al. prepared mesoporous BFA films from poly(styrene-co-maleic anhydride) (P(St-co-MAH), 141)450

Figure 45. (a) Schematic illustration of site-selective grafting polymerization from the pore surface of PS-b-PHMA BFA films and the subsequent lectin recognition. (b) Three-dimensional confocal fluorescence image of PS-b-PHEMA BFA film after staining with 5aminofluorescein.143 Reprinted from ref 143. Copyright 2010 American Chemical Society.

2-(2,3,4,6-Tetra-O-acetyl-β-D-glucosyloxy)ethyl methacrylate was selectively grafted in the pores by a surface-initiated ATRP. Further specific recognition of the carbohydrate microarrays by lectin (Con A) can lead to an organized microarray of protein. Another widely used type of reaction for surface modification is the “click” reaction.444,445 Kadla et al. fabricated robust, region selectively modified amphiphilic cellulose azide BFA films.29,446 Due to the existence of azide groups, the BFA film can be postfunctionalized through the “click” reaction as follows: using the Cu(I)-catalyzed alkyne− azide [2 + 3] cycloaddition reaction, biotin is “clicked” onto the azide functionalized area in BFA films without any effect on the structure of the film. These results indicated that this system could serve as a platform for the design and development of biosensors. The applications of patterned biomolecules will be further discussed in section 7. More interestingly, Muñ oz-Bonilla et al. reported the selective functionalization of the top surface of BFA films while maintaining the functionality of the pores.107 For this purpose, they prepared BFA films with a blend of PS and PS-bP5FS (33). The P5FS blocks assembled on the surface of the film due to their low surface energy, leading to a hydrophobicity of the BFA films. Thus, the measured water contact angle of the BFA films was similar to that predicted by the Cassie−Baxter model, indicating that water could not go into the pores on the BFA films. Therefore, the top surfaces were selectively modified with glucose through the “click” chemistry reactions between the fluorine atoms in the para position of the pentafluorostyrene units along the surface and thiol-protected glucose (Figure 46a). After deprotecting the hydroxyl groups, an obvious decrease of the water CA on the surfaces and the change of elemental composition in the XPS measurements AM


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Figure 46. (a) Schematic illustration of the selective functionalization of the external surface of BFA films by a mixture of PS and PS-b-P5FS with glucose moieties and subsequent recognition of Con A.107 Reprinted with permission from ref 107. Copyright 2013 Royal Society of Chemistry. (b) Illustration of the composition distribution of a BFA film prepared from a blend of PS, PS-b-P(PEGMA), and P5FS. (c, d) XY (c) and XZ (d) Raman mappings of the BFA films prepared from the mixture. Red, PS; blue, P(PEGMA); green, P5FS.138 Adapted from ref 138. Copyright 2013 American Chemical Society.

formation of effective cross-links. After 15 min of vulcanization, the resulting SIS BFA films showed high mechanical strength and good resistance to a variety of organic solvents and strong acidic and basic solutions (Figure 47b−e). The cross-linked film also remained stable at up to 350 °C for 2 h. The versatility of the methodology was also demonstrated by applying it to other vinyl group containing polymers.256 6.2.3. Photochemical Cross-Linking. Photochemical cross-linking is an attractive technique because it is easier to perform but leaves little or no contamination. Generally, photochemical cross-linking requires the existence of a photosensitizer that can generate radicals upon exposure to visible light, UV light, or high energy radiation. Macroradical recombination occurs when two radicals meet and form a covalent bond. Double-bond-containing polymers are ideal candidates for photochemical cross-linkage. Karthaus et al. demonstrated that highly ordered BFA films can be prepared from a photo-crosslinkable poly(vinyl cinnamate) (142)/polyion complex (143) mixture (Scheme 34), and UV cross-linking could be conducted while retaining the 3D honeycomb structure.451

Scheme 31. Chemical Structure of PS-b-P(PEGMA) (138)

(Scheme 33). Cross-linking was achieved by immersing the film in an ethanol solution of alkyldiamine. The cross-linked honeycomb structure was stable at up to 350 °C, which is more than 150 °C higher than the stability threshold for the non-cross-linked films. Inspired by the vulcanization of rubbers, our group developed a simple technique of cross-linking the double bonds in SIS by exposing the BFA film to S2Cl2 vapor.256 The attack of S2Cl2 on the internal vinyl groups of the polymer leads to the formation of pendant sulfur chloride groups on the polyimide chains that can act as potential crosslinking sites (Figure 47a). The reaction of these groups with the internal vinyl groups on other chains is responsible for the AN


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Scheme 32. Chemical Structures of Polymers Containing Cross-Linkable Moieties (139, 140)

Scheme 34. Chemical Structures of Photochemical CrossLinkable Polymers (142−144)

Scheme 33. Chemical Structure of P(St-co-MAH) (141)

Figure 48. SEM images of BFA films after UV cross-linking and treatment with heat or organic solvent. (a, b) BFA films of a poly(vinyl cinnamate)/polyion complex immersed in chloroform after UV irradiation for 0 (a) and 30 min (b).451 Reprinted with permission from ref 451. Copyright 2007 Wiley-VCH. (c−f) BFA films of PS thermally treated at 250 °C (c−e) and immersed in chloroform (f) after UV irradiation. UV irradiation times: (c) 2 and (d−f) 4 h.45 Reprinted with permission from ref 45. Copyright 2009 Royal Society of Chemistry.

films were noncytotoxic and were suitable as cell scaffolds. Recently, Wang et al. prepared and photocured poly(εcaprolactone) triacrylate (144) BFA films.167 Cell behavior and gene expression were investigated on these two series of BFA films and their flat counterparts. Shimomura et al. prepared bas-relief patterns by selectively UV irradiating BFA films containing PB through photomasking.453 After heat treatment, the non-cross-linked film was melted, and the remaining pattern showed a very distinct edge with a 4% resolution error. PS and PS-containing BCPs can also be photochemically cross-linked, although the mechanism is still in dispute.45,222 Under UV exposure, hydrogen atoms at the α position of the PS backbone are extracted, yielding macroradicals along the polymer chain. A covalent bond then forms when two macroradicals annihilate by recombination. The cross-linked PS BFA films are chemically and thermally stable compared with their linear cousins (Figure 48c−f), which is important for

Figure 47. (a) Cross-linking mechanism of polyisoprene chains. (b−e) SEM images of SIS BFA films thermally treated at 350 °C for 2 h (b− d) and immersed in chloroform for 1 min after cross-linking (e). Vulcanization times: (b) 4 and (c−e) 24 h.256 Reprinted with permission from ref 256. Copyright 2010 Elsevier.

The honeycomb structure was stable against organic solvents after a threshold cross-linking time of 30 min (Figure 48a,b). Our group prepared BFA films with the commercial triblock copolymer SBS.452 After 1 h of UV irradiation, the SBS films became resistant to a wide range of organic solvents and became thermally stable at up to 350 °C. Another beneficial effect of the photochemical process was the formation of polar groups on the film surface, which changed the surface wettability from hydrophobic to hydrophilic. The resulting AO


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Figure 49. Preparation of inorganic honeycomb patterns with BFA templates. (a) SEM image of CNT bundles synthesized from as-prepared PS-bPAA/ferrocene BFA film by chemical vapor deposition (CVD). (inset) Cross-sectional SEM images. (b−d) SEM images of silica (b), Fe2O3(c), and ZnO (d) patterns by pyrolysis of cross-linked PS-b-PAA/precursor composite BFA films. (b, inset) AFM image of silica pattern. (e) SEM of aligned CNTs grown with Fe2O3 pattern shown in (c) as a catalyst by a CVD. (inset) Cross-sectional SEM images. (f) ZnO nanorods grown from the ZnO pattern shown in (d) by a hydrothermal method.223,224 Adapted from ref 223. Copyright 2009 American Chemical Society. Adapted from ref 224. Copyright 2010 American Chemical Society.

direct the synthesis of other materials with ordered structures.457−460 In general, the BFA films have been used as templates, stamps for soft lithography, and masks for etching. In this section, we focus on the fabrication of inorganic and polymeric ordered materials with BFA films as templates. The patterning of biological molecules will be discussed in section 7.6. One of the appealing applications of polymer BFA films is in directing the growth of inorganic materials, such as ZnO,461 CdS QDs,462 and TiO2.463 To do this, the particles of the inorganic materials or their precursors should be incorporated into the BFA films. After removing the polymer components by pyrolysis, inorganic BFA films are obtained. Because the BFA films are usually prepared from organic solutions, the particles or precursors must have good dispersibility or solubility in these solvents. Many inorganic/organic complexes are desirable candidates for the precursors. For example, by adding precursors of 3-amino-propyltrimethoxysilane (APETS), zinc acetylacetonate (Zn(acct)2), and ferrocene into the solution of PS-b-PAA, BFAs of SiO2, ZnO, and Fe2O3 were obtained by subsequently cross-linking and mineralizing the as-prepared BFA films (Figure 49).223−225 Importantly, the inorganic BFAs could be further used as catalysts or nuclear sites to direct the growth of other materials. BFAs of vertically aligned CNTs (Figure 49a,e) and ZnO nanorods (Figure 49f) were obtained on the patterned Fe2O3 and ZnO substrates, respectively. Based on this templating technique, Wan et al. developed a nonlithographic method to prepare micropatterned ZnO nanowires (ZnO NWs) with through-pore BFAs as a mask.32 The hexagonally ordered BFA films with perforated pore structures were transferred onto Si wafers coated with a ZnO seed layer. In the subsequent hydrothermal growth, vertically aligned ZnO NW arrays were formed in the mesh areas. Roomtemperature photoluminescence spectra indicated that the micropatterned ZnO NWs showed greatly enhanced nearband-edge emission.

some special applications. For example, the UV cross-linked PSb-PAA BFA film had a char yield of 40% at 450 °C, and thus, it could work as the structure-directing agent to grow CNT and ZnO nanorod arrays.223 Details of these applications can be found in section 7. UV irradiation of PS can also induce the formation of hydrophilic groups and change the surface properties of the BFA film. Yabu et al. prepared a pincushion-patterned film by removing the top layer of PS BFA film.454 Then, they treated the film with UV and obtained a hydrophilic surface. Further investigation showed that the obtained strongly hydrophilic pincushion surface exhibited an excellent bubble repellency under water. Additionally, Shimomura et al. prepared BFAs with a mixture of a photo-cross-linkable epoxy oligomer, a cationic iodide-type curing agent, and an amphiphilic copolymer.455 By controlling the UV irradiation time, they successfully regulated the pore size of the ordered BFAs. After the immersion of the crosslinked BFA film in chloroform and ultrasonication, honeycomb mesh and microdot structures were obtained due to the crosslinking of the surface.456

7. APPLICATIONS OF BFA FILMS Great attention has been given to the applications of BFAs. The characteristic of BFA films is the hexagonally packed pores on the films. Therefore, the applications of BFAs have focused on how to make use of this unique morphology. Combined with the different functions of various materials, BFA films have demonstrated their applications in templating, biology, and optical and photoelectronic devices, among others. These applications are categorized and summarized in this section. 7.1. Templates

The BF method is an efficient and cheap method of fabricating ordered pore arrays. However, the materials that are suitable for the BF process are limited to soluble polymers and a small group of other materials. Many other valuable functional materials cannot be used in the BF process. Therefore, some researchers have used polymeric BFA film as the template to AP


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tionally high thermal and chemical stability.193 Furthermore, simple thermal treatment converted the BFA film of hypPPV into a carbonaceous film without destroying the ordered structures. PS-b-PDMS BFA films have also been used to prepare a SiO2 honeycomb structure through UV cross-linking and pyrolysis.214 Gong et al. used another silicon-containing cocopolymer, PDMS-graf t-polyacrylates (PDMS-g-PAs), to prepare BFA film.466 After plasma treatment and pyrolysis, freestanding silicon oxycarbide sheets with ordered throughpores were obtained. BFA films have also been used as templates for in situ polymerization. For example, Li et al. prepared conducting polymer polyaniline (PAni) patterns using PC281 or sulfonated polysulfones (SPSF)467 BFA films as the template. They first soaked the BFA films in aniline solution for 24 h and then placed the film in oxidant solution to perform the polymerization. After polymerization, the PAni/PC or PAni/SPSF composite films became electrically conductive while maintaining the honeycomb structure. The BFA structure even survived after PC or SPSF was removed by soaking in chloroform. Using a similar method, they also prepared a conductive microlens of PPy.468 PS-b-PAA BFA film was also used as a template for the electrochemical deposition of PAni.168 The highly ordered structure of BFAs has also been transferred to other materials through soft lithography.268,469−471 As shown in Figure 50a, concave and convex arrays can be obtained by one or more steps of lithography, respectively. One representative example of this templating application was reported by Galeotti et al., who prepared silk fibroin films with structure color, as shown in Figure 50b.472 Concave or convex microlens arrays, as well as packed micrometric bump arrays, were molded from PS BFA films (Figure 50c,d). This strategy is attractive for the development of new biocompatible photonic devices. They also found that when a PDMS replica was peeled away from the PS BFA film, a thin layer of PS residues covered the surface of the PDMS replica.473 The authors released the PS residue onto a Si substrate by placing the PDMS stamp on the Si substrate and

In some special cases, polymers are themselves precursors of inorganic materials, and thus, they can produce inorganic honeycombs without additional reactants.464 For example, Bunz et al. prepared cyclobutadiene (cyclopentadienyl) cobaltcontaining PPE (145, 146) (Scheme 35) polymers and used Scheme 35. Chemical Structures of Cobalt-Containing PPE Polymers (145, 146)

them to prepare BFA films.465 As a result of the high content of cross-linkable ethynylene and the rigid molecular chains, the structures of the BFA films persisted well during cross-linking and mineralization at temperatures as high as 600 °C. Under nitrogen, a Si−C−Co ceramic with low amounts of oxygen was obtained, whereas in air, all of the carbonaceous matter was burned off and a Si−Co−O ceramic of the approximate composition Co2Si4O11 was formed instead. Yoshie et al. demonstrated that hypPPV (64) BFA films exhibited excep-

Figure 50. (a) Schematic diagram of soft lithography to prepare concave and convex arrays of various materials. (b) Digital picture of patterned silk fibroin film showing structural color. (c, d) AFM 3D rendering of silk fibroin films patterned with cavity (c) and bump (d) templated from BFA films.472 Reprinted with permission from ref 472. Copyright 2012 Royal Society of Chemistry. AQ


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Figure 51. (a) Schematic diagram of the preparation of the SERS substrate with pincushion structure. (b) Raman scattering spectra of the R6G measured on the silver flat and pincushion films, respectively. Inset shows the chemical structure of R6G. (c) SEM image of the silver pincushion films. Inset shows close-up image of the top of a spike. Scale bar: 500 nm.480 Adapted with permission from ref 480. Copyright 2010 Royal Society of Chemistry.

deposited onto the pincushion film by electroless plating or an evaporation technique (Figure 51).184,479 Accordingly, Shimomura et al. deposited silver on pincushion films and used these films as SERS substrates. Raman spectra demonstrated that the obtained SERS substrate could detect rhodamine 6G (R6G) at concentrations as low as 0.5 nM (Figure 51b).480 In addition, the silver pincushion films can be prepared on a wide surface area, and they are flexible and freestanding after removal from the glass substrate. Wan et al. developed another approach to prepare highly sensitive SERS substrates with BFA films decorated by Ag NPs.233 The silver NPs, with diameters from 18 to 30 nm, were generated in situ by reducing Ag+ adsorbed on the PDMAEMA BFA film. Substrates prepared under optimal conditions had enhancement factors as high as 106−108, depending on the ratio of the number of analytes to Ag NPs. Hao et al. directly prepared substrates for SERS using blends of polymers and Au NPs modified by mercaptan.154 Because of the Pickering effect, the Au NPs were enriched on the pore walls. They investigated the relationship between the morphology of the Au BFA films and their performance as the substrates for SERS and found that for R6G, the more regular the BFA films, the stronger the SERS signals. They claimed that the more regular BFA films had large surface areas that allowed them to adsorb more analytes from the surrounding solution.

heating it over the Tg of PS. As a result, a hydrophobic PS pattern was created on the hydrophilic Si surface. Finally, the patterned Si surfaces were treated with the water solution of CdTe QDs; the nanocrystals aggregated only on the hydrophilic areas such that an array of hexagonally ordered fluorescent spots was produced. Another application of BFA films is in inductively coupled plasma reactive ion etching (ICP-RIE), wet etching, and electrodeposition, where BFA films are employed as masks to transfer the honeycomb pattern to the substrates.474,475 For example, Hirai peeled off the top layer of a BFA film and used it as a mask for lithography.57,476 The nanospike arrays were transferred to the silicon with various crystal facets, and the obtained surfaces showed antireflective properties and superhydrophobicity. A metal mask with a honeycomb pattern could also be prepared by sputter-coating a BFA film. When the metals were deposited at vertical incidence, an ordered array of metal spots could be obtained by peeling off the top layer of the BFA films. After the ICP-RIE process, an array of silicon cylinders was obtained, which could be used as a photonic crystal filter.477 Our group deposited metal at a title incidence on the BFA film.40 After removal of the polymer with organic solvent, a metal layer with a honeycomb pattern formed on the substrate, and the pattern could be transferred onto the substrate surface by etching. Using this method, various substrates were patterned, including silicon, glass, and Tedoped silicon.

7.3. Separation

Size-based separation is an important application of porous polymeric membranes. Microsieves are advanced filtration membranes characterized by uniform pore size, high pore density, and a thickness smaller than the pore diameter. Microsieves provide high selectivity, high flux, and the efficient removal of filter cake by back flushing. To be used as microsieves, BFA films should have perforated pores of uniform size with a hydrophilic surface and high mechanical and thermal stability. Such perforated BFA films were prepared with modified BF methods (using ice substrate, liquid substrate,

7.2. Surface Enhanced Raman Scattering Substrates

Periodically arranged nano- and microstructures of noble metals have proved to be good candidates as substrates for SERS, which is based on enhanced electromagnetic fields due to localized surface plasmon resonance.478 The nanostructures, such as sharp edges and NPs, are the factors that enhance SERS. Polymer pincushion films with hexagonally arranged spike structures can be prepared by simply peeling off the top layer of the BFA films with adhesive tape. Metals can be AR


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Figure 52. Mechanical properties and separation performance of cross-linked SIS BFA films with perforated pores. (a) Typical stress−strain curves of the SIS membranes with different vulcanization times, commercial filter papers, and PTFE filter membranes. (b) Graphical comparison showing the resistance of the SIS membrane to the organic solvents, acid, and base. (c−j) Results of separation experiments. (c−f) Size distribution of the feeding particles composed of 500 nm and 3 μm PS microspheres (c) and the particles in filtrate obtained by filtering the feeding dispersion solution in water at 25 (d) and 90 °C (e) and in THF (f) through the SIS microsieve. (g−j) Corresponding SEM images of the particles.255 Adapted from ref 255. Copyright 2012 American Chemical Society.

dip-coating, or rapidly removing excess solution), as discussed in section 5.8. The performance of these BFA films is summarized in this section. Cong et al. prepared perforated brominated PPO (82) BFA films with controllable pore sizes ranging from 4.5 to 1.0 μm by adjusting the solution concentration.411 A maximum water flux of 25 m3 h−1 m−2 was achieved under a feed pressure of 0.13 MPa. With a similar method, Gao et al. produced microsieve films from a miktoarm star copolymer containing PS and PtBA components.171 Partial hydrolysis of the PtBA arm species in the BFA film converted the surface wettability from hydrophobic to hydrophilic, facilitating the size sorting of colloidal particles in aqueous dispersion. The microsieve film, with a pore size of 1.4 μm, could effectively separate PS microspheres of 650 nm from those of 2 μm. Wan et al. prepared a PS-bPDMAEMA (24) BFA film with perforated pores on a soft substrate with a pore size of 3 μm.409 The film showed a highresolution separation performance of PS particles with sizes of 2.0 and 5.0 μm. They also introduced an elastic copolymer (SIS, 47) into the membrane to enhance their tenacity and the interfacial strength between the membrane and the support.481 Unfortunately, the non-cross-linked polymer membrane is not freestanding, and thus, the filter membrane must be supported by a stainless steel woven wire mesh. Recently, our group demonstrated that vulcanized SIS BFA films with perforated pores showed good mechanical properties and excellent chemical and thermal stability.255 Due to the cross-linked chemical structure, this type of microsieve was freestanding and resistant to a variety of polar organic solvents and acidic, basic, and hot water (Figure 52a,b). The pores on SIS BFA films were highly ordered and tunable, with sizes ranging from 1 to 7 μm. The vulcanized SIS microsieve successfully separated microparticles not only in water at room temperature but also in hot water and THF (Figure 52c−h). These characteristics make the vulcanized SIS membrane an

ideal microsieve with potential applications in industry. Mansouri et al. also fabricated robust microsieves with the BF method by using engineering PSF plastics (82).68 In their work, filtration membranes with isoporous structures on the surface of a nylon mesh were prepared by a combination of dip-coating and the BF approach. Although the flux was lower than that of commercial membranes, their separation effect was comparable. 7.4. Optical and Optoelectronic Devices

Periodic microstructures are frequently used in optical and optoelectronic devices, including microlens arrays (MLAs), micropatterned light emitting diodes (LEDs), antireflective (AR) coatings, solar cells, photonic crystals, and optical sensors, due to their unique interaction with light. The size of the periodic microstructures in BFAs falls within the range of hundreds of nanometers to tens of micrometers, matching the requirements of optical and optoelectronic devices.482,483 Therefore, the BF process provides a simple and cost-effective route to prepare ordered microstructures with large areas for optical and optoelectronic devices. MLA, an important component for devices used in optical telecommunication, displays, and solid-state lighting, has been prepared by molding the pores in BFAs. Spherical and hemispherical MLAs (Figure 53a,b) were obtained by molding the as-prepared BFA films and BFA films with the top layer peeled off, respectively.484 Anamorphic MLAs have also been prepared by molding the anamorphic BFA films, which were fabricated by mechanically stretching BFA films at a temperature above their Tg (Figure 53c).484,485 Considering the demolding process, PDMS is the most suitable material for preparing MLAs because it has low surface energy and is easily peeled away from the mold after solidification. To prepare MLAs with other materials, Chari et al. created a low-surfaceenergy “nonstick” surface on BFA film via a surface reconstruction process after heating the BFA based on a AS


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and 60°.488 Similarly, in another work, BFA films directly enhanced the efficiency of a blue LED by 17.7%.489 Microstructured LED arrays, including dot arrays and network arrays, have been prepared based on BFA films.490 For example, a dot-array OLED was prepared by a microcontact printing method using an MLA-like PDMS stamp molded from BFA film and an electroluminescent polymer solution as ink (Figure 54). Red, green, and blue OLEDs were all obtained simply by changing the polymer inks. The dot array had a diameter as small as 1 μm, indicating that a high display resolution of 640 000 pixels mm−2 could be achieved. To avoid the degradation of emissive performance caused by direct contact between the active polymer and the stamp, Bradley et al. developed a new method. They added an insulating layer of polyimide between the active layer and the metal layer.491 By patterning the PI layer with a PDMS stamp, a dot-array OLED was also obtained, which had current and power efficiencies twice as high as those for nonpatterned OLEDs. Another important application of surface microstructure control is in AR coating, which is used to maximize the transmission of light through an optical surface. The principle of AR is based on the destructive interference between the reflected light from the air−coating and coating−substrate interfaces, which can be produced by the porous structure in the coating due to the decreased refractive index. Therefore, BFA films are potential candidates for AR coating. Kim et al. reported that a CAB BFA film with two-layer pores demonstrated broad-band antireflection at near-infrared wavelengths.492 The reflectivity of glass was reduced from over 4% to less than 1% by the AR coating (Figure 55a,b). Generally speaking, it is difficult for conventional BFA film to be used as an AR coating for visible and UV light because the pore size is much larger than the wavelength of visible and UV light. However, Shimomura et al. prepared a BFA film with 300 nm pores by reducing the thickness of the cast polymer solution to less than 100 μm.493 The as-prepared BFA film was transparent; the 300 nm pores did not scatter visible light. Over a wide range of UV−vis spectroscopy, the transmittance of the film with 300 nm pores was higher than 80%, whereas that of the film with 2 μm pores was only 50%, demonstrating the high antireflective effect of small pores on visible light. Recently, Trespidi et al. constructed a nanostructured PDMS AR coating molded from a BFA with pore sizes less than 300 nm (Figure 55c,d).494 The AR coating increased the glass transmission to more than 2% at normal incidence in the

Figure 53. (a−c) SEM images of spherical (a), hemispherical (b), and anamorphic MLA (c).484,485 Adapted from ref 484. Copyright 2005 American Chemical Society. Adapted with permisssion from ref 485. Copyright 2008 AIP Publishing. (d) Setup for measuring optical diffusion. (e, f) Optical diffusion through plane glass substrate, MLAs with diameters of 6 and 3 μm, respectively.486 Reprinted with permission from ref 486. Copyright 2008 the Optical Society.

PDMS-containing copolymer in dry air.485 During this process, the low-surface-energy PDMS segments migrated to the surface of the BFA film, yielding a “nonstick” surface. This “nonstick” template enables the fabrication of MLA using a variety of polymer precursor materials. For example, polyurethane MLA, which showed a relatively high refractive index (1.54) and E (138 MPa), was prepared with the above method. The main applications of MLAs are in light modulation, which is affected by the shape and size of the microlens in the MLAs. For example, the projection images from hemispherical MLAs are sharper than those from spherical MLAs.484 Anamorphic MLAs effectively redistribute the luminance angularly when illuminated by a collimated beam.485 Another important application of MLAs is as light diffuser films to spread and homogenize nonuniform illumination. As shown in Figure 53d−g, the obtained PDMS MLAs molded from BFA film distributed the point light source effectively. The haze distribution propagated through MLAs with a lens diameter of 3 μm was more uniform than that with a diameter of 6 μm.486 Further studies showed that 3D BFA films could be directly used as a diffuser of microcavity organic light emitting diodes (OLEDs), effectively reducing the viewing angle dependency of OLEDs.487 When PDMS MLAs were patched outside the OLED, it was found that the external quantum efficiency (EQE) of a green OLED was enhanced by 32%, and the enhancement was found widely, at viewing angles between 30

Figure 54. (a) Schematic illustration of the microcontact printing method for the fabrication of patterned OLEDs. (b−d) Electroluminescence and (e−g) photoluminescence of the red, green, and blue OLEDs fabricated by microcontact printing. (h) SEM image of the aluminum electrode surface.490 Reprinted with permission from ref 490. Copyright 2005 Elsevier. AT


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wavelength of the reflection minimum shifted linearly with the surrounding refractive index, demonstrating the potential of BFA films as optical sensors. 7.5. Chemical Power Source

The lithium ion battery is one of the most important energy storage devices. The porous structure of the BFA film anode can promote the mass transfer in the electrolyte and provide sufficient space to release the volume expansion in the charge/ discharge process.496,497 For example, Tu et al. prepared a Si/ honeycomb reduced graphene oxide (Si/H-rGO) hybrid film with the BF method.498 Si NPs were uniformly dispersed in the pores of the BFA films. rGO not only improved the electric conductivity of the hybrid BFA film but also avoided the aggregation of the Si NPs during cycling (Figure 56a,b).

Figure 55. (a) SEM images of a CAB film with two layers of BFA. Inset shows the cross-sectional image. (b) Reflectance of glass coated with CAB film and bare glass. The symbols and solid line represent the reflectance measured and calculated from the characteristic matrix theory, respectively.492 Reprinted from ref 492. Copyright 2005 American Chemical Society. (c) SEM tilted view of the PDMS AR coating. (d) Single surface reflection curves taken at normal incidence for bare glass, typical MgF2 AR coating, and patterned PDMS-coated glass.494 Reprinted from ref 494. Copyright 2014 American Chemical Society.

visible/near-IR spectrum, which corresponds to 50% reduced reflection. Furthermore, effective improvement was observed at a wide range of incident angles from 90 to 50°. The performance of this PDMS AR coating is better than the commercial high-performance broad-band AR coating of MgF2. Because BFA films can reduce reflectivity, they are expected to enhance the light harvest and improve the performance of solar cells and photocatalysts. For example, the BF method was used to prepare a highly ordered porous film of the lightemitting rod−coil BCP PDTG (41).249 When irradiated by white light, the BFA films showed a photocurrent 3 times higher than that of the smooth film. Chen et al. prepared a porous photovoltaic electrode of rGO/DODA/TiO2 NP (rGO/DODA/P25) by the BF method.369 The photoactive TiO2 NPs were enriched on the walls of the pores because of the Pickering effect. The introduced porous structure and NPs reduced the reflection of the film to as low as approximately 4% in the UV range and 10% in the visible range. Because of the low reflection and hybrid structure of the TiO2 NPs strongly anchored on rGO, the BFA film showed a photocurrent 2 times that of smooth TiO2 film under the same conditions. In addition, BFA films have also shown potential applications in other optical devices, such as photonic crystals and novel optical sensors. Bormashenko et al. prepared 2D tunable photonic crystals by coating a PC BFA film on a PVDF film.282 Investigation of the transmittance spectra demonstrated that the photonic band gap of the PC/PVDF film was located at a wavelength of 0.95 μm. Combining the excellent thermal and mechanical properties of PC and PVDF, the obtained structures may have various optical engineering applications. Furthermore, Cusano et al. constructed an optical sensor with high sensitivity to the surrounding environment by coating metallodielectric BFA films on the tip of optical fiber.495 The

Figure 56. (a) SEM and (b) TEM images of Si/H-rGO composite BFA film. (c) Rate capability of Si/H-rGO composite film and pure silicon at various current densities ranging from 50 to 1000 mA g−1.498 Reprinted with permission from ref 498. Copyright 2014 Royal Society of Chemistry.

Meanwhile, the BFA structure offered abundant pathways for efficient Li+ diffusion and balanced the volume variation of Si during the Li alloying and dealloying processes. Therefore, the Si/H-rGO film had both a high reversible capacity (1118 mA h g−1 at 50 mA g−1 for up to 50 cycles) and good high-rate capability (Figure 56c). Lee et al. prepared an Fe-doped MnxOy membrane with hierarchically connected macropores and mesopores using a BFA film of amine-functionalized bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) as the sacrificial template.499 When used as the anode of a lithium ion battery, Fe-doped MnxOy showed large reversibility (620 mA h g−1 at 200 mA g−1 for up to 100 cycles), a good rate (up to 1600 mA g−1), and an outstanding cycle life even at very high current density (900 cycles at 1500 mA g−1). In addition to above electrode materials, BFA films have also shown potential application as polymer electrolyte membranes for lithium ion batteries. For example, Wang et al. prepared PVDF−HFP BFA films with interconnected multisized pores as shown in Figure 57a.500 The BFA films had a porosity of 78%, which led to a high electrolyte uptake of 86.2%. The pore size and porosity decreased from the front side to the back side of the film, which prevented lithium dendrite growth and short circuit when applied in lithium ion batteries. Moreover, the AU


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cell culture, biomedical devices, and tissue engineering, among others. BFA films are promising substrates for patterning biomolecules because of the ordered packed pores on the surface and the differences in surface chemical composition inside and outside the pores. Water-soluble blocks and polar groups of amphiphilic polymers are enriched on the pore wall and provide the chemical sites for the modification of biomolecules.503 For example, Zhang et al. used a blend of PS and amino-terminated PS to prepare BFA films with pores decorated with amino groups. Proteins were successfully conjugated to the amino-terminated pore surface through the cross-linking of glutaraldehyde (Figure 58).270 Stenzel and

Figure 57. (a) SEM images of PVDF−HFP BFA films. The inset is the back side. The front side is ordered porous structure, while the back side has much less density of pores with smaller size. (b) Combustion test of the BFA film after set on fire, showing its fireproof property. (c, d) Electrochemical performances of lithium ion cells with LiFePO4 as cathode materials. (c) Cycling performances. The cutoff voltages are 2 and 4.2 V. The current density is 0.2 C (1 C = 170 mA g−1). (d) Rate behavior. Reprinted with permission from ref 500. Copyright 2014 Nature Publishing Group.

BFA films were also thermally stable up to 350 °C and noncombustible in fire (fireproof), as shown in Figure 57b. In order to make better polymer electrolyte membranes, the authors combined two BFA films to one membrane by putting the front sides together with the back sides exposed to the outside. The combined membranes exhibited a high ionic conductivity of 1.03 mS cm−1 at room temperature, which was much higher than that of commercial polymer membranes of Celgard 2400. When the combined films were applied in lithium ion batteries with LiFePO4 as cathode materials, the voltage difference between charge and discharge curves was smaller than 0.1 V, and the reversible capacity was much higher than that of a battery with Celgard 2400 as electrolyte, as shown in Figure 57c. In addition, the higher capacity at lower current density could be repeated after several cycles at higher current density as shown in Figure 57d, illustrating that the PVDF−HFP polymer electrolyte membranes could support outstanding rate performance. Porous structures in electrodes are also very important for the performance of solid oxide fuel cells (SOFCs) because they can provide more electrochemical sites and supply a transportation path for oxygen and the generated electrons. Accordingly, Sun et al. dispersed La0.8Sr0.2MnO3/8YSZ (8 mol % Y2O3-doped ZrO2) (LSM/YSZ) and La0.6Sr0.4Co0.2Fe0.8O3−δ−Gd0.2Ce0.8O2−δ (LSCF−GDC) in an organic solution of SIS and prepared porous BFA films on the YSZ substrate with those solutions by a conventional BF process.501,502 Then, the porous composite BFA films were used as cathodes of SOFCs. Electrochemical impedance spectroscopy (EIS) measurement showed that the porous BFA films effectively reduced the polarization resistance because of the improved diffusion of oxygen and the easy transportation of electrons.

Figure 58. Fluorescence images of PS BFA film after fluorescein isothiocyanate conjugated bovine serum albumin (FITC-BSA) is attached to the amino-terminated surfaces. (a) Two-dimensional top view. (b) Three-dimensional confocal image. (c) Confocal slice images at different z positions (s1, s2).270 Reprinted with permission from ref 270. Copyright 2007 Wiley-VCH.

́ Rodriguez-Herná ndez also developed several BFA film substrates for protein patterning.443,504 They used PS-b-PAA to prepare BFA films. The high hydrophilicity inside the pores of the BFA films ensured the penetration of all reactants into the pores. Consequently, biotin molecules were selectively bonded on the pore wall. An organized array of proteins was obtained based on the molecular recognition between biotin and streptavidin. They also prepared copolymers containing βgalactose moieties to construct BFA films with bioactivity. Most of the peanut agglutinin protein was conjugated to the glycopolymer inside the pore, indicating the excellent selectivity of the template.124 Furthermore, glycopolymer BFA films could be used to recognize Con A,125,143,241 and their bioactivity can be tuned between “on” and “off” states based on temperature due to the thermoresponsive property of PNIPAm.505 Another strategy to pattern proteins is to fill the pores of BFA films with microspheres decorated with biomolecules or bioactive molecules.506−508 PS microbeads with comparable size have been successfully trapped within the pores of BFA films (Figure 59).136 Because the microbeads can be modified with various biomolecules, bead-based protein bioassays can be constructed following this strategy. Wan et al. also assembled microspheres decorated with glucose in the pores of BFA films. The functional arrays produced were used to recognize Con A.509 The other way to construct biomolecule patterns is to directly use these molecules to prepare BFA films. To do so, water-soluble biomolecules must be decorated to increase their

7.6. Micropatterning of Biological Molecules

Micropatterns of proteins have attracted increasing interest because of their potential application in biosensors, biocatalysis, AV


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was subsequently used to selectively adsorb proteins through their electrostatic interactions with POMs, thus producing protein patterns (Figure 60b−d).69 7.7. Cell Culture and Inhibition

Cell behaviors, including cellular adhesion, spreading, proliferation, differentiation, and gene expression, are regulated by the interactions between cells and their microenvironments. Therefore, the chemical, mechanical, and topological properties of scaffold surfaces play important roles in the control of cell behaviors. BFA films have surface chemical heterogeneity and adjustable surface topology, and thus, both the as-prepared and the chemically modified BFA films have been demonstrated to be promising scaffolds for cell culture and the investigation of cell behaviors.513−515 The behaviors of various types of cells have been investigated systematically on BFA films of amphiphilic and biodegradable polymers, including PLA (100), PCL (101), PLGA (102), and others.319,516,517 Compared with flat films, BFA films with an appropriate pore size can promote the adhesion and proliferation of most normal cells, including endothelial cells, 518,519 fibroblasts, 520 chondrocytes, 521,522 hepatocytes,183,523 neural stem cells (NSCs),524 preosteoblastic cells,525,526 and human mesenchymal stem cells.527 BFA films have also demonstrated capacities to control the morphology and differentiation of cultured cells, which was crucial for tissue engineering. These cells include hepatocytes,528 NSCs,529 breast epithelial cells,530 osteoblasts,167,328 stem cells,527 endothelial cells, 531 mesenteric-stromal vascular cells (mSVCs),532 and mast cells.533 For example, on a flat film, hepatocytes had a typical monolayer spreading morphology after culturing for 72 h (Figure 61a).528 However, on BFA films

Figure 59. (a, b) Optical (a) and fluorescence (b) images showing the patterning of fluorescent PS microbeads on BFA film. (c) Line-profile graph with peaks corresponding to the microbeads indicated by the white line in (b). (d) Fluorescence image of patterned PS microbeads immobilized with FITC-BSA on BFA film.136 Reprinted with permission from ref 136. Copyright 2006 Wiley-VCH.

solubility in organic solvents. For example, Wu et al. developed a strategy for preparing DNA micropatterns using DNA/ cationic surfactant (DTDA) complexes as a raw material (Figure 60a).510 The electrostatic interaction between the DNA

Figure 60. (a) SEM image of DNA−DTDA complex BFA film (tilted 60°).510 Reprinted from ref 510. Copyright 2009 American Chemical Society. (b−d) Confocal images of POM-patterned PMMA film before (b) and after encountering the immersion in hemoglobin-Cy3 (c) and FITC-BSA (d) aqueous solutions.69 Reprinted with permission from ref 69. Copyright 2013 Elsevier.

Figure 61. (a, b) CLSM images of actin localization in adhered hepatocytes after 72 h of culture on (a) flat film and (b) BFA film.528 Reprinted with permission from ref 528. Copyright 2005 Elsevier. (c, d) SEM images of neuron cell morphologies after culturing for 5 days on flat film (c) and BFA film with pore sizes of 3 μm (d).524 Reprinted with permission from ref 524. Copyright 2004 The Society of Polymer Science.

and the cationic surfactant allowed them to modify the DNA with noncovalent interactions and to render it soluble in chloroform. Ordered BFA films were successfully prepared with this chloroform solution. Galeotti et al. also directly patterned biological molecules by using microprinting methods with patterned PDMS as a stamp, which was a replica of BFA films.511 Microemulsion BFAs are another way to pattern water-soluble biomolecules.512 Wu et al. prepared an inverse microemulsion with an aqueous solution of bovine serum albumin (BAS) and polymer organic solution. The microemulsion was then cast onto a substrate and allowed to dry, yielding BFA films. BAS was found anchored on the pores of the BFA films. The obtained BAS patterns could be locally used to detect other proteins through specific recognition.71 Using a similar method, they also constructed the POM pattern, which

with a pore size of 5 μm, the cell spreading was inhibited, and the hepatocytes showed a spherical morphology (Figure 61b). The size of the spheroids was approximately 100 μm after 24 h. Importantly, the urea level of spherical hepatocytes was higher than that of the monolayer hepatocytes. Thus, the hepatocytes on the BFA films formed as tissuelike structures and displayed appropriate liver-specific functions. Further investigation showed that the differentiation of NSCs could be controlled by the pore size of BFA films.529 The NSC differentiation was suppressed by the BFA film with pore sizes of 3 μm, specifically AW


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evaluated through NIH 3T3 fibroblast cell-culture experiments. Compared with the spherical morphology on titanium foil, a flat morphology of cells was found on the TiO2 BFA films with pore sizes of 4.6 μm, showing enhanced cell adhesion and spreading. The improved adhesion, spreading, and proliferation of the cells demonstrated excellent biocompatibility for the TiO2 BFA films. The inhibition of cancer cells and bacteria by BFA films has been investigated by some research groups.514,537,538 Tang et al. prepared BFA films with polymers (147) containing TPE units (Scheme 36), which had an AIE property.539 They found that

due to the adhesion arrangement of the NSCs (Figure 61c,d). Recently, it was reported that BFA film could promote the epithelial differentiation and growth of breast epithelial cells (MCF-7) because of the increased expression of mammary differentiation genes (GATA3, EMA, and INTEGB4) and the repressed expression of a mesenchymal gene (CALLA) (Figure 62).530 In addition, it was found that the pattern geometry greatly influenced the morphology of cells, because the pore walls guided the spreading of the cells.315,534

Scheme 36. Chemical Structure of Polymer Containing TPE Unit (147)

the BFA film was not beneficial for the growth of cancer cells because of a relatively large hydrophobic property and the presence of air in the pores (Figure 63). Shiratori et al.

Figure 62. Morphology of MCF-7 cells cultured on different scaffolds. (a, c) On a PLGA plate. (b, d) On dip-coated PLGA BFA film. (a, b) Optical images. (c, d) SEM images. (e) Representative plot of semiquantitative RT-PCR expression data for GATA3, EMA, INTEGB4, B2MG, Notch1, and CALLA. 530 Reprinted with permission from ref 530. Copyright 2013 Wiley-VCH.

Figure 63. (a−c) Fluorescence and (d−f) SEM images of HeLa cells grown on a glass surface (a, d), smooth film (b, e), and BFA film (c, f).539 Reprinted from ref 539. Copyright 2013 American Chemical Society.

The surface chemical and mechanical properties of BFA films, such as hydrophobicity, surface charge, and hardness, can also have a tremendous influence on cell behaviors.33,452,535 Generally, a charged, hydrophilic, hard surface is desirable for cell culture. Stenzel et al. prepared a soluble PPy composite using an amphiphilic BCP of PS-b-PAA as a dopant and stabilizer (18).226 The BFA films based on this PPy composite were noncytotoxic and were suitable as scaffolds for tissue engineering. Initial fibroblast cell culture studies on these scaffolds demonstrated a dependency of cell attachment on the pore size of the scaffolds. BFA films based on small molecules and inorganic materials were also employed as substrates for the investigation of cell behavior. Li et al. prepared FF (124) BFA films and cultured human embryonic skin fibroblasts (ESFs) on them.374 ESFs adhered, spread, and grew better on FF BFA scaffolds than on FF organogels due to the honeycomb structure and surface roughness. They also prepared TiO2 BFA films consisting of anatase nanocrystals, starting from a titanium n-butoxide (TBT) solution.536 The cell adhesive and proliferative capacities on TiO2 BFA films with different pore sizes were

demonstrated that PS BFA films with pore sizes of 5−11 μm showed effective inhibition to bacterial adhesion, growth, and biofilm formation. According to bacterial proliferation tests, the area covered by bacteria on the BFA film was only 0.59% that of the area covered on flat film.540 In another work, dodecyloxyazo-zinc phthalocyanine (daZnPc, 130) was chosen to fabricate a nanoscale BFA film.380 This novel BFA film could generate excited-state singlet oxygen (1O2) under irradiation, which caused irreversible oxidative lethal damage to bacterial cells. Therefore, the daZnPc BFA films exhibited a robust photodependent antibacterial effect against E. coli. According to the same mechanism, BFA films containing Mn(III) meso-tetra(4sulfonatophenyl) porphine chloride (MnTPPS) also showed an improved antibacterial effect compared to the flat film.379 BFA films made of cellulose with a quaternary ammonium (QA) structure also exhibited effective antibacterial activity against E. coli.327 Recently, Rodriguez-Hernandez also found that BFA films of polyimide-g-PEO/polyimide could effectively prevent the adhesive Staphylococcus aureus, which may attribute to hydrophilicity of pore walls.440 Further investigation showed AX


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that the BFA films demonstrated antifouling properties in addition to antibacterial activity. Antifouling BFA films, which had a multigradient porous surface, were also reported by Rodriguez-Hernandez et al. in their recent work.541 In addition, BFA films could potentially be used as a physical shield to prevent intraperitoneal adhesion after abdominal surgery.542

rapidly and selectively quenched by a nitroaromatic. Therefore, the fluorescent BFA films can be applied as explosive sensors. In another work, porphyrinated polyimides (148) (Scheme 37) Scheme 37. Chemical Structure of Porphyrinated Polyimide (148)

7.8. Sensors

BFA films have been used as biosensors with high sensitivity. For example, Wang et al. fabricated a biosensor of nanoelectrode ensembles (NEEs) by constructing a PS-b-PAA BFA film on the surface of a glass carbon electrode.543 Cyclic voltammograms demonstrated that the NEEs drastically suppressed the response of ascorbic acid (AA) and resolved the overlapping voltammetric response of uric acid (UA) and AA into two well-defined peaks with a large anodic peak difference (ΔEpa) of approximately 310 mV (Figure 64). Thus,

were used to prepare to BFA films, which had a similar fluorescence emission property with porphyrin.546 Especially, the porphyrinated BFA films showed remarkable fluorescence quenching behavior toward HCl gas because HCl protonated the porphyrin moieties and changed their conformation from planar to saddle structure, demonstrating high sensitivity to HCl. Also, the quenched fluorescence emission property could be recovered by ammonia gas, showing excellent reusability of the BFA films. In addition, the BFA films had a high thermal stability and kept high sensitivity to HCl gas even at a temperature as high as 300 °C. 7.9. Miscellaneous Applications

In addition to the aforementioned applications, BFA films have also been used in catalysis, stimuli-response devices, water collection, and other applications. Some representative applications are reviewed below. Catalysts are usually loaded on a porous matrix to enhance their activity.280 Accordingly, Save et al. introduced a Rose Bengal (RB) photosensitizer into BFA film by using a special copolymer of P(S-stat-VBC)-g-RB (44).252 Due to the hydrophilicity of RB molecules, they were enriched on the pore walls of the BFA film, which enhanced their contact with reactive molecules. Thus, the photoactivity of RB immobilized in BFA film was 5 times greater than that of nanoporous RB film. A BFA film of palladium-containing polymer also showed excellent catalytic activity.547 Karthaus et al. first adsorbed photocatalytic TiO2 nanocrystals onto surface-functionalized and cross-linked BFA films.548 After calcination, BFA films of TiO2 nanocrystals were obtained. Owing to the porous structure, the TiO2 BFA films showed a photocatalytic activity 6.8 times higher than those of flat TiO2 films when decomposing a cyanine dye in aqueous solution. In another ́ work, Rodriguez-Herná ndez et al. incorporated bioactive compound of alkaline phosphatase (ALP) on the BFA films by layer-by-layer technique.549 The BFA films always showed higher catalytic activity than that of plate surfaces. More importantly, ALP was selectively deposited inside the pores in the case of only the pore wall containing carboxylic acid groups, allowing the potential applications as microreactors. Huh et al. prepared patterned stimuli-responsive hydrogel films of poly(acrylic acid) (PAA) and PNIPAm using BFA film as a template.550,551 The hydrogel films were then used to reversibly

Figure 64. (a, b) Cyclic voltammograms of 1 mM AA (black line) and 0.5 mM UA (red line) on a glassy carbon electrode (a) and on NEEs (b) in 0.10 M PBS (pH 6.86). Scan rate: 50 mV s−1. (c) Plot of the anodic peak current of cyclic voltammograms against the concentration of UA. (d, e) Top view (d) and cross-section view (e) of PS-bPAA BFA film on the electrode.543 Reprinted with permission from ref 543. Copyright 2006 Elsevier.

the precise detection of UA could be achieved in the presence of AA. The peak current obtained from differential pulse voltammetry (DPV) increased linearly with the UA concentration within the range 0.25−50 μM, and the detection limit was as low as 0.04 μM (S/N = 3). Furthermore, the NEEs showed excellent sensitivity and selectivity in the detection of UA in serum and urine samples. In another example, Wan et al. constructed a glucose sensor with a BFA film based on polymers containing phenylboronic acid (PBA).144 It was demonstrated that the PBA groups aggregated on the pore walls of BFA because of their hydrophilicity, which could benefit the contact between the PBA and the glucose. These PBA-functionalized BFA films had high sensitivity toward glucose, as shown by the QCM results. Furtherly, Wu et al. loaded insulin into PBA-modified BF pores, and based on the sensing property of PBA to glucose, glucose-responsive release of insulin was realized.544 Besides the above biosensors, BFA films have also shown potential applications in other kinds of sensors. For example, Sun et al. prepared BFA films with a mixture of PS and fluorophore pyrene.545 The fluorescence of BFA films could be AY


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The pH response of PAA was also utilized to actuate the conductivity of a porous semiconductor interface that was obtained by assembling CNTs on the surface of BFA film through sequential layer-by-layer (LBL) deposition.552 Investigation of the electronic properties revealed that the electrical conductivity of the interface depended on the connectivity rather than the amount of CNTs on the surface of the BFA films. The conductivity of the film was dependent on the pH. When the pH was close to the pKa of PAA, the PAA chains were swollen, leading to an expansion of the polymer gaps between CNT filaments. As a result, the conductivity of the film decreased. The highest conductivity was obtained at pH 1. Kim et al. confirmed the high electrical conductivity and excellent field-emission properties of BFA films made of CNTs.436 This BFA film was prepared with a BF process using entangled CNTs and a PS blend solution, followed by pyrolyzing PS (Figure 66a). The surface resistance was decreased from 137 to 4.3 kΩ/□ after calcination, indicating efficient connectivity of the CNTs. The obtained BFA substrate of the CNTs demonstrated excellent field-emission properties with a turn-on field of 1.79 V μm−1 at an emission current density of 10 μA cm−2 and a current density of 450 μA cm−2 at an electric field of 2.8 V μm−1 (Figure 66b,c). These performances were comparable with those of CNT field emitters grown by CVD. Hou et al. combined the Rayleigh instability technique and the BF method and prepared porous bead-on-string (BOS) fibers with epoxy E-44.553 A variety of BOS fibers with different microstructures that were smooth, less porous, homogeneously porous, gradiently porous, and dented could be easily prepared by controlling the experimental conditions, such as the reaction time, temperature, and humidity (Figure 67a−c). The authors found that the BOS fiber could be used for fog harvesting. Due to the Laplace force and the wettability force induced by the ellipsoid shape, the gradient-porous BOS fibers exhibited better water-collecting capacity compared with the smooth and homogeneous porous BOS counterparts (Figure 67d−f). Bormashenko et al. demonstrated that BFA films had the potential for use in low-voltage electrowetting.60 After a BFA film of PC was impregnated with silicone, the onset of reversible electrowetting occurred at voltages as low as 35 V. Heng et al. demonstrated a photoelectric cooperative induced patterned wetting on a hydrophobic BFA film.554 In this process, BFA films of 149 (Scheme 38) were first modified with a photosensitive dye of cis-bis(4,4′-dicarboxy-2,2′-bipyridine) dithiocyanato ruthenium(II) (N3) by dip-coating method and

adsorb Ag NPs. Depending on the pH responsiveness of PAA, at lower pH, the PAA hydrogel was hydrophobic and adsorbed Ag NPs, whereas at higher pH, the hydrogel became hydrophilic and released Ag NPs. Importantly, the PAA hydrogel showed an oscillatory performance of adsorption− desorption with a periodically changed pH, demonstrating the rapid and reversible response to external stimuli (Figure 65). Due to the thermosensitivity of PNIPAm, the patterned films showed a rapid and reversible adsorption−desorption of Ag NPs with changes in temperature.

Figure 65. (a−c) pH oscillation patterns at various ratios of [HCO3−]0/[H2SO4]0 in the pH oscillatory system of CaSO3− H2O2−NaHCO3−H2SO4. [HCO3−]0/[H2SO4]0 corresponds to (a) 0.5, (b) 1.0, and (c) 2.0. (d, e, f) Oscillations of the adsorption− desorption of Ag NPs occurring at the hydrogel surface derived by the pH oscillations of (a), (b), and (c), respectively.551 Reprinted from ref 551. Copyright 2013 American Chemical Society.

Figure 66. (a) SEM image of CNT field emitter after calcination. (b) Plot of the emission current density of CNT BFA films versus applied electrical field. (c) Fowler−Nordheim plot representing the conventional field-emission behavior of CNT emitters.436 Reprinted with permission from ref 436. Copyright 2008 Royal Society of Chemistry. AZ


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However, several challenges remain before we can push the BF technique into real practical applications. The first challenge is how to make very small and very large pores reliably from arbitrary polymeric and nonpolymeric materials. To address this issue, an unambiguous elucidation of the mechanism of BFA formation is needed, as is the exploitation of novel BF methods. Although great efforts have been made to expose the BF mechanism, there are many conflicts and puzzles, including the nucleation and growth process of the droplets on the polymer solution, the interaction between the polymer solution and the water-droplet template, the stabilization of droplets, and the driving force behind the droplet arrangement. The clarification of these confounding details will benefit the development of new BF methods and improve the control of pore size. The second challenge is how to precisely control the surface morphology of the BFA to rival the expensive lithography technique for a variety of pore shapes and arrangements. The physical deformation techniques, such as mechanical stretching or shrinking and photomanipulation, offer a simple strategy to construct BFA films with novel pore shapes.38,277,315 However, these techniques are only available for certain types of polymers. Therefore, developing new manipulation methods is essential in future research. The last challenge is how to further exploit the new functions and applications of BFA films. To achieve this goal, apart from making progress in solving the above two problems, we must exploit new functional materials in the BF process and devise novel postmodification methods to expand the functions of BFA films. In summary, the recent rapid development of the BF method has demonstrated its merits of low cost, easy operation, and wide application scope. Furthermore, the complexity of BF that emerges from the relationship between experimental parameters and its morphology compels us to discover more novel physical details of the process. We believe breakthroughs in both fundamental and practical aspects will be achieved upon better understanding the formation mechanism and with the development of new materials.

Figure 67. (a−c) SEM images of the polymer BOS fibers with smooth (a), homogeneously porous (b), and gradient-porous (c) microstructures. (d−f) Aggregation of water droplets on a polymer of BOS fibers with different microstructures: (d) smooth, (e) homogeneously porous, and (f) gradient porous.553 Reprinted with permission from ref 553. Copyright 2013 Royal Society of Chemistry.

Scheme 38. Chemical Structure of Polymer (149)

subsequently treated with hydrophobic heptadecafluorodecyltrimethoxysilane (FAS) vapor, producing photosensitive and hydrophobic BFA films. The wetting threshold voltage of the BFA film was about 24.9 V without light, and it decreased to about 10.8 V with light because of the photoconductity property. Relying on this difference of threshold voltages with and without light, photoelectric cooperative induced patterned wetting was achieved with assistance of a photo mask. Another study revealed that BFA film could be used as a strongly adhesive surface for water droplets, and the adhesive force could be controlled by the pore diameter and density.555

8. CONCLUSION The preparation of highly ordered porous films using the BF method is another example of humans devising a technique inspired by nature. The seemingly simple process endows the resulting films with both an ordered surface structure and patterned distributions of functional groups on the surface, which provides more opportunities to further modify and functionalize the films, thus broadening their applications. After two decades of development, this technique has now made a large impact in the area of materials and has created a general interest that has extended to a large scientific community at the frontiers of chemistry, physics, biology, and medicine. The BF technique has particularly aroused the interest of industry. A recent patent by the Fuji Film Co. demonstrated their interest in equipment that could produce BFA films continuously on a commercial scale.18

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +86-592-2186296. Fax: +86592-2183937. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval. Notes

The authors declare no competing financial interest. BA


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Biographies

moved to the College of Materials, Xiamen University, as a full professor. He was a visiting scholar at the University of Akron in 2013. His research interests are living radical polymerization, functional polymers, and porous materials.

ACKNOWLEDGMENTS L.L. gratefully acknowledges the National Natural Science Foundation of China (Nos. 51373143 and 21174116), the Natural Science Foundation of Fujian Province (No. 2014J0105), and the Fundamental Research Funds for the Central Universities (Nos. 2013SH003 and 201312G004). REFERENCES (1) Koch, K.; Barthlott, W. Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials. Philos. Trans. R. Soc., A 2009, 367, 1487−1509. (2) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (3) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38, 644−652. (4) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Super-hydrophobic surfaces: from natural to artificial. Adv. Mater. 2002, 14, 1857−1860. (5) Feng, X. J.; Jiang, L. Design and creation of superwetting/ antiwetting surfaces. Adv. Mater. 2006, 18, 3063−3078. (6) Rousseau, M.; Meibom, A.; Geze, M.; Bourrat, X.; Angellier, M.; Lopez, E. Dynamics of sheet nacre formation in bivalves. J. Struct. Biol. 2009, 165, 190−195. (7) Munch, E.; Launey, M. E.; Alsem, D. H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Tough, bio-inspired hybrid materials. Science 2008, 322, 1516−1520. (8) Tabata, H.; Yoshimura, M.; Shimizu, S. Development of the light interference colored fibers “MORPHOTEX®”. Sen’i Gakkaishi 2001, 57, 248−251. (9) Kinoshita, S.; Yoshioka, S.; Kawagoe, K. Mechanisms of structural colour in the morpho butterfly: cooperation of regularity and irregularity in an iridescent scale. Proc. R. Soc. London, Ser. B 2002, 269, 1417−1421. (10) http://www.asknature.org/. (11) Beysens, D. The formation of dew. Atmos. Res. 1995, 39, 215− 237. (12) Aitken, J. Breath Figures. Nature 1911, 86, 516−517. (13) Aitken, J. Breath figure. Proc. R. Soc. Edinburgh 1895, 20, 94−97. (14) Rayleigh. Breath figures. Nature 1911, 86, 416−417. (15) Spurr, R. T.; Butlin, J. G. Breath figure. Nature 1957, 179, 1187−1187. (16) http://www.freestylephoto.biz/the-lure-of-collodion. (17) Widawski, G.; Rawiso, M.; François, B. Self-organized honeycomb morphology of star-polymer polystyrene films. Nature 1994, 369, 387−389. (18) Washizu, S.; Yamazaki, H.; Yamanouchi, J.; Naruse, H. EP Patent 20,060,010,988, 2012. (19) Stenzel, M. H. Formation of regular honeycomb-patterned porous film by self-organization. Aust. J. Chem. 2002, 55, 239−243. (20) Bunz, U. H. F. Breath figures as a dynamic templating method for polymers and nanomaterials. Adv. Mater. 2006, 18, 973−989. (21) Hoa, M. L. K.; Lu, M.; Zhang, Y. Preparation of porous materials with ordered hole structure. Adv. Colloid Interface Sci. 2006, 121, 9−23. (22) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. Formation of honeycomb-structured, porous films via breath figures with different polymer architectures. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2363−2375. (23) Ma, H. M.; Hao, J. C. Ordered patterns and structures via interfacial self-assembly: superlattices, honeycomb structures and coffee rings. Chem. Soc. Rev. 2011, 40, 5457−5471.

Aijuan Zhang received her B.E. (2008) and M.S. degrees (2011) from Xiamen University and Nankai University, respectively. She is currently a Ph.D. candidate at the College of Materials, Xiamen University, under the supervision of Prof. Lei Li. Her research interests are macromolecular self-assembly and porous materials.

Hua Bai received his B.S. (2004) and Ph.D. (2009) degrees in Department of Chemistry at Tsinghua University, under the supervision of Prof. Gaoquan Shi. From 2009 to 2011 he was a postdoctoral fellow in the Department of Chemical Engineering at Tsinghua University. He is currently an associate professor in the College of Materials at Xiamen University. His research interests mainly focus on organic conjugated materials, solar cells, and graphene.

Lei Li received his Ph.D. in 2001 from China Textile University. After two-year postdoctoral experience at Peking University, he joined the National Institute of Advanced Industrial Science and Technology (AIST) in Japan as a NEDO and JSPS research fellow. In 2007 he BB


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