Breath Figures and Coffee Stain - American Chemical Society

Article pubs.acs.org/Langmuir

Tuning the Pore Composition by Two Simultaneous Interfacial SelfAssembly Processes: Breath Figures and Coffee Stain Alberto S. de León,† Adolfo del Campo,‡ Marta Fernández-García,† Juan Rodríguez-Hernández,*,† and Alexandra Muñoz-Bonilla*,†,§ †

Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas (CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain ‡ Instituto de Cerámica y Vidrio (ICV), Consejo Superior de Investigaciones Científicas (CSIC), C/Kelsen 5, 28049 Madrid, Spain ABSTRACT: In the current paper, we prepared microstructured porous films by the breath figures approach using polymer blends consisting of polystyrene as the major component and an amphiphilic additive, either a synthetic block copolymer {two different polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] copolymers} or a series of commercial surfactants. Tetrahydrofuran was employed as the solvent. Confocal micro-Raman spectroscopy demonstrated the preferential location of the amphiphilic additives in the cavities of the film as a consequence of the breath figures mechanism. However, the distribution of the copolymer within the cavities varies depending upon the structure and, more precisely, the surface properties of the additives, leading to three different situations. First of all, the copolymer with a larger polystyrene segment, insoluble in the condensed water droplets, is homogeneously distributed along the whole surface of the cavities. On the contrary, when the copolymer is soluble in water (shorter polystyrene segment), it migrates inside the droplet and a coffee-stain phenomenon takes place during the water droplet evaporation, conducting to a ring-like deposition on the top edge of the cavities. Finally, when a water-soluble surfactant with high surface activity is used, the surfactant is solubilized inside the water droplets, which provokes a decrease on the surface tension and the coffee-ring effect is modified. In this situation, the copolymer covers the bottom of the pore.

■

INTRODUCTION Self-assembly processes have gained increasing attention to create nano- and microstructured materials and are expected to contribute importantly in the fabrication of the next generation of advanced materials and devices for many applications (e.g., microelectronics, biotechnology, photonics, sensors, and soft lithography).1−4 The most studied self-assembly process is the supramolecular or molecular self-assembly, in which entire molecules or segments of these molecules spontaneously aggregates into specific nanostructures guided by non-covalent interactions, forming, for instance, micelles or vesicles in solution. Self-assembly occurs not only at the nanoscale but also at the microscale and even at the macroscale. This is, for instance, the case of the self-assembly of spherical colloids to form highly ordered three-dimensional lattices that are good candidate materials for photonic crystals.5 The latter case is a representative example of the interfacial self-assembly process, i.e., the fabrication of self-assembled ordered structures induced by evaporation on solid surfaces.6 In drying-mediated selfassembly, non-molecular solutes, including polymers, nanocomposites, nanoparticles, clusters, or colloids dispersed/ solubilized in a solvent, are ordered through evaporation of a sessile droplet on a solid substrate. In addition to the colloidal crystal superlattices, the breath figures approach and coffee-ring © 2014 American Chemical Society

effect have received special attention as evaporative-induced self-assembly methods to fabricate structured patterns on a relative large scale. The breath figures method was first introduced by Francois et al.7 for the preparation of microstructured polymer films when a drop of polymer solution is cast under a moist airflow. The principle of this method is based on the rapid evaporation of the solvent that decreases the temperature of the solution/air interface and induces the condensation of water droplets on the top. The self-organization of the water droplets into an ordered hexagonal array is favored by the Benard−Marangoni convection because of thermal gradients and capillary attractive forces.8,9 After solvent evaporation, honeycomb-patterned films are formed, wherein the water droplets act as a sacrificial template that is finally removed by evaporation. Varying the experimental parameters, such as solvent, relative humidity, or concentration, the dimension, shape, and distribution of the pores can be tuned in a relatively controlled manner.10−16 Moreover, as a result of the formation mechanism, the hydrophilic segments of the polymers tend to self-aggregate Received: March 28, 2014 Revised: May 9, 2014 Published: May 9, 2014 6134

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

or commercial polymer surfactant. Different blends were prepared by mixing high-molecular-weight PS matrix (80 wt %) with a variety of amphiphilic copolymers (20 wt %), maintaining constant the total concentration of polymer in the solution (30 mg/mL). Films were obtained from these solutions by casting onto glass wafers at room temperature under controlled humidity (70%) inside of a closed chamber. Measurements. The particle size was determined by dynamic light scattering (DLS) using a Malvern Zetasizer Nano ZS instrument at 25 °C. Malvern dispersion software was used for data acquisition and analysis, applying the general purpose algorithm for calculating the particle size distribution. Scanning electron microscopy (SEM) micrographs were taken using a Philips XL30 with an acceleration voltage of 25 kV. The samples were coated with gold/palladium (80:20) prior to scanning. The topography, chemical composition, and distribution of the different components of the blends on the polymeric films were determined using confocal Raman microscopy integrated with atomic force microscopy (AFM) on a CRM-Alpha 300 RA microscope (WITec, Ulm, Germany) equipped with Nd:YAG dye later (maximum power output of 50 mW power at 532 nm). The Raman spectra were taken point by point with a step of 100 nm. The topography was further analyzed with a Zeta 20 optical profiler (Zeta Instruments). The surface tension of the water solutions containing polymeric surfactants was measured by the pendant drop method using a KSV Theta goniometer.

preferentially toward the wall of the holes. Consequently, simultaneously with the microstructuration, the chemical functionality of the cavities can be precisely controlled. This method also allows for the preparation of hierarchical structures using, for instance, block copolymers.17−20 Another process that can occur during the drying of a sessile drop containing non-volatile solutes on a substrate is the well-known coffeestain effect that leads to a non-uniform deposition in a ring-like shape.21 This coffee-ring effect arises when the drop edges of the drying drop are pinned. The solvent evaporation rate is higher at the edge, and subsequently, radial capillary flows from the center to the edge are generated. The outward flow transports the solute to the perimeter where it is deposited, forming the solid ring. The coffee-ring effect has been investigated in a variety of systems, i.e., colloidal suspensions22 and polymer,23,24 DNA,25 and bacterial solutions.26 In recent years, the understanding of the mechanism has attained great attention from both scientific and technological points of view. As an example, in inkjet printing, this phenomenon is an important problem, and many efforts have been attempted to avoid it.27 To date, different strategies have been explored to ameliorate the coffee-stain effect, such as the use of ellipsoidal particles,28 electrowetting,29 addition of a surfactant,30,31 etc. These evaporative-induced self-assembly processes have been extensively studied, but only few examples are found, wherein both processes, breath figures and coffee stain, occur simultaneously. Up to date, literature examples have been limited to the deposition of nanoparticles32 and polymers19 in a ring-like shape within the cavities of the breath figures film. More precisely, we have previously reported the coffee-stain phenomenon occurring during the breath figures process in polymer blends containing polystyrene (PS) as the main component and a water-soluble amphiphilic copolymer as the additive.19 Confocal Raman microscopy allowed us to detect the unexpected localization of the amphiphilic copolymer on the top of the cavity edges, instead of covering the whole cavity. That is, a coffee-ring effect takes place in each hole formed during the breath figures process. This phenomenon observed in a range between 2 and 4 μm of pore size below the values observed in other coffee-stain systems (10 μm)33 was explained by the solubility and incorporation of the amphiphilic copolymer with a large hydrophilic block during the film formation. Herein, we intend to extend the previously mentioned preliminary results and study in detail this phenomenon, clarifying the parameters that are involved in this mechanism. Moreover, we will identify the role of the surface tension of the components in a binary polymer blend on the final pore distribution of the components.

■

■

RESULTS AND DISCUSSION In an earlier study, we prepared breath figures films from polymer blends containing an amphiphilic copolymer as the minor component and PS matrix as the main component. In this study, we observed that the amphiphilic copolymer was not homogeneously distributed, covering the whole surface of the cavities, but was located on the top edge of cavities. This special distribution was previously explained by the coffee-stain process taking place simultaneously with the breath figures. That is, once the water condenses on the top of the solution surface, the amphiphilic copolymer changed from the THF solution phase to the water droplets. Then, the block copolymer could aggregate into micelles within the aqueous droplets, and as the water evaporated, the micelles are deposited on the edge of the pores guided by the coffee-stain mechanism. To obtain further insight in this process and understand the role of the molecular structure on this phenomenon, herein, we study first the self-assembly behavior of the water-soluble amphiphilic block copolymer PS40-b-P(PEGMA300)48 and the employed commercial surfactants in aqueous solution. Because it is expected that the amphiphilic additives migrate toward the condensed water droplets, this study will provide additional information about their behavior on this phase. It has to be mentioned that the block copolymer was not directly soluble in water; thus, the copolymer was first dissolved in THF, which is a co-solvent for both segments. Then, deionized water was added dropwise with vigorous stirring to reach a concentration of 1 mg/mL. After that, the mixture was exposed to air overnight to allow for the evaporation of THF. This route also simulates, at least to some extent, the breath figure process, in which the copolymer is dissolved in THF before the water condensation occurs at the surface of the solution. The DLS experiment shows that the copolymer indeed forms micelles in aqueous solution, with a diameter of ∼24 nm (see Figure 1). It is also observed that the commercial surfactants generate smaller sizes than that obtained by this block copolymer. For comparative purposes, an amphiphilic copolymer, PS45-bP(PEGMA300)34, with a shorter hydrophilic segment was synthesized by ATRP. In this case, the copolymer was not

EXPERIMENTAL SECTION

Materials. The amphiphilic block copolymers, polystyrene-bpoly[poly(ethylene glycol) methyl ether methacrylate]s [PS40-bP(PEGMA300)48 and PS45-b-P(PEGMA30034)] (copolymers are labeled with the degree of polymerization of each block), were synthesized via atom transfer radical polymerization (ATRP) as previously reported.34 The polymeric surfactants, Triton-X 114, polyoxyethylene (10) lauryl ether, Pluronic L64, and Pluronic F127, were all purchased from Aldrich. High-molecular-weight polystyrene (PS2400) (Aldrich; Mw = 2.50 × 105 g/mol) was used as the polymeric matrix, while tetrahydrofuran (THF, Scharlau) was employed as the solvent. Round glass coverslips of 12 mm in diameter were supplied from Ted Pella, Inc. Film Preparation. Polymer blend solutions were obtained by dissolving in THF the PS matrix and either the amphiphilic copolymer 6135

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

within the pores, the XZ images give appropriate information on the surface covering. Figure 3 shows XZ micro-Raman mapping of the two polymer blends mentioned above, performed using the signals corresponding to the ring breathing mode of PS (Raman shift at 1012 cm−1) and the strong band at 1735 cm −1 attributed to the carbonyl groups of the P(PEGMA300) segment. Figure 3a reveals, as expected, that the water-soluble amphiphilic copolymer is mainly accumulated on the top edge of the pores rather than at the bottom. This preferential enrichment of the pore edges in copolymer chains in a ring-like fashion was, however, not observed when a waterinsoluble amphiphilic copolymer was employed (Figure 3b). Instead, a rather regular distribution inside the pore wall has been evidenced by Raman spectroscopy. This result supports the hypothesis proposed previously for the coffee-ring phenomenon; that is, to obtain a coffee-ring distribution, the block copolymer needs to be solubilized into the condensed water droplets. To further corroborate this premise, other water-soluble amphiphilic polymers were employed as components of the polymer blends in the breath figures process. In this sense, four commercially available surfactants with different molecular weights and chemical structures were used: Triton-X 114 (537 g/mol), polyoxyethylene (10) lauryl ether (627 g/mol), Pluronic L64 (2900 g/mol), and Pluronic F127 (12600 g/ mol). Similarly, blends consisting of 80 wt % PS and 20 wt % surfactant were prepared with a total concentration of 30 mg/ mL, and the films were formed at 70% RH. The SEM images (Figure 4) indicated that only the Pluronic surfactant with a higher molecular weight is able to form regular porous films in those particular experimental conditions. Shimomura et al. have previously investigated the influence of the addition of surfactant on the formation of honeycombpatterned porous polymer films.35,36 Because surface-active molecules stabilize the water droplet during the solvent evaporation, the variation of the interfacial tension between a water droplet and the polymer solution, as a result of the surfactant, strongly affects the regularity of the structure. They demonstrated that this decrease in the interfacial tension is dependent upon the chemical structure of the surfactants. Thus, we have centered the investigation from this point exclusively to the Pluronic F127 as the water-soluble surfactant because it allows the preparation of regular arrays under these experimental conditions. More interestingly, a higher magnification of the films prepared using Pluronic F127 shows a

Figure 1. Number-average size distribution of the PS40-b-P(PEGMA300)48 and the commercial surfactants, Pluronic F127, Pluronic L64, polyoxyethylene (10) lauryl ether, and Triton-X 114, in aqueous solution at a polymer concentration of 1 mg/mL.

analyzed by DLS because it remains insoluble in aqueous solution, following the two preparation approaches: i.e., either directly solubilizing the block copolymer in water or attempting to first solubilize it in THF and gradually change the solvent to water. This water-insoluble amphiphilic copolymer was mixed with the PS matrix and dissolved in THF solution to prepare the porous surfaces. As depicted in our previous contribution,19 the films were obtained from binary blends prepared by mixing 20 wt % block copolymers, PS45-b-P(PEGMA300)34 or PS40-bP(PEGMA300)48, with 80 wt % PS matrix and dissolved in THF with a total concentration of 30 mg/mL. Porous films from these polymer solutions were prepared by casting in a moist atmosphere with 70% relative humidity (RH). The films were then analyzed first by SEM. Figure 2 shows the SEM micrographs of the porous surfaces obtained from both blends containing either the water-soluble or -insoluble copolymers. Because the samples were prepared in water-miscible THF, moderately ordered patterns are obtained in both cases. Next, the chemical composition of these micropatterned surfaces was studied by confocal micro-Raman microscopy. The high resolution of this technique permits the XY and XZ imaging and allows us to observe the size and spatial distribution of the different phases or type of polymer used in the blend. To investigate the coffee-stain phenomenon

Figure 2. SEM images of breath figures films obtained at 70% RH from blends containing as amphiphilic bock copolymer: (a) PS40-bP(PEGMA300)48 or (b) PS45-b-P(PEGMA300)34. 6136

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

Figure 3. XZ Raman mappings of pores found in films containing PS (red color in panels a and b) and (a) water-soluble PS40-b-P(PEGMA300)48 (blue color) or (b) water-insoluble PS45-b-P(PEGMA300)34 (blue color). (c) Average Raman spectra and chemical structure of pure P(PEGMA300) (blue) and PS homopolymer (red).

Figure 4. SEM images of porous films obtained at 70% RH from blends containing as the minor component, the commercial polymeric surfactant: (a) Triton X-114, (b) polyoxyethylene (10) lauryl ether, (c) Pluronic L64, and (d) Pluronic F127.

Figure 5. High magnification of SEM images of porous films obtained at 70% RH from blends containing 20 wt % Pluronic F127 as the polymeric surfactant and 80 wt % PS matrix.

Figure 6. Optical micrograph and XZ Raman map of films obtained from the blend containing 20 wt % Pluronic F127 and 80% PS prepared at 70% RH.

6137

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

Figure 7. ZX profiles obtained by optical profilometry of breath figures films prepared at 70% RH from blends containing 80 wt % PS matrix and 20 wt % (a) PS40-b-P(PEGMA300)48 and (b) Pluronic F127.

the surfactant is deposited at the bottom of the cavities, as seen in Figure 6. Next, profilometer analysis were carried out to gain further insight into the cavity of films using the commercial surfactant and compared to that obtained from PS40-bP(PEGMA300)48. Effectively, there is a significant difference between each film, as seen in the profiles displayed in Figure 7. While the interior of the cavities is rather smooth in the films containing PS40-b-P(PEGMA300)48, the cross-section of holes having the Pluronic commercial surfactant shows a ring-like profile at the bottom of the holes. It is well-known that the presence of a surfactant in aqueous solution has a significant effect on the surface tension gradients within a droplet and, therefore, has a dramatic impact on the coffee-stain process.23,31,37 Surfactants reduce the surface tension of the water droplets, and when the surfactant molecules increase at the pinned line because of the coffeestain effect, the surface tension decreases locally, generating a gradient in surface tension. This drives the Marangoni flow inward that, in general, leads to a more uniform deposition. In previous works, the surfactant was added as an additional component to the polymer solution or colloidal suspension to suppress the coffee-ring effect.23,26,31,37 In this study, the polymeric surfactant is its own solute. To investigate the role of the amphiphilic structures used on the pore distribution, in this work, the surface tension of water was measured as a function of the concentration of the water-soluble copolymer. In Figure 8, the surface tensions are represented as a function of the concentration of the additive [Pluronic F127 or PS40-bP(PEGMA300)48] employed. As observed for both cases, the surface tension diminishes with the addition of the amphiphilic additive. However, the surfactant Pluronic F127 conducts a decrease in the surface tension much larger than that of PS40-b-

particular pattern inside the cavities, as seen in Figure 5. Instead of a homogeneous distribution of the additive, the SEM images indicated the formation of a ring-like structure at the bottom of the pore. Because the formation mechanism of the breath figures implies the arrangement of the hydrophilic components of the polymeric solution toward the water droplet, a preferential localization of the Pluronic surfactant inside the cavities is expected. Besides, because the surfactant is soluble in water, it might migrate toward the condensing droplets during the breath figures process. The chemical distribution of the two blend components (PS and Pluronic F127) within the film was then analyzed by micro-Raman spectroscopy. The XZ crosssection Raman mapping of the film containing 20 wt % Pluronic F127 surfactant (Figure 6) was contracted taking point by point with a step of 100 nm and using the ring breathing mode associated with PS (Raman shift at 1012 cm−1) and the band attributed to the −CH2− scissor of the surfactant (at 1491 cm−1). Likewise, to the water-soluble PS40-bP(PEGMA300)48, it can be seen that the surfactant is not homogeneously distributed in the pore. This corroborates our previous suggestion that water-soluble polymers can be directed into the droplets during the breath figures process rather than being reoriented along the THF/water interface. However, in this case, the surfactant is located only at the bottom of the cavity, contrary to what was observed in the amphiphilic block copolymer PS40-b-P(PEGMA300)48, located at edge of the pores. Although the non-homogeneous distribution around the pore wall seems to be an indication of its solubilization into the droplet, the coffee-stain phenomenon is not occurring, evidence of the presence of another mechanism. In this particular case, 6138

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

on the top edge of the cavities or at the bottom of the cavities. This coffee-stain phenomenon occurs inside the condensed water droplets simultaneously during the breath figures process. As is well-known, the coffee-ring effect occurs in an evaporating droplet with suspended or solubilized matter when its contact line is pinned and the solvent evaporation in the vicinity of the droplet edge is enhanced. The solvent lost at the edge by evaporation is replaced by solvent drawn from the center, generating an outward capillary flow that carries solute to the edges of the droplet and results in deposition near the contact line. Finally, as already reported, several factors can alter the above-mentioned flow that usually leads to coffee-stain formation. All of these factors are related to the occurrence of gradients in surface tension that create a flow directed toward the center of the droplet and counteract the outward flow, the so-called Marangoni effect.38 This phenomenon reduces to some extent or completely prevents the formation of coffee-stain structures. At first, the local concentration of the surfactant molecules at the pinned contact increased because of the coffee-stain effect, resulting in a local decrease of the surface tension. This gradient in surface tension produces a Marangoni flow toward the center of the drop. In this situation, the deposition on the edge is suppressed and the shape of the droplet remains spherical as drying proceeds. Therefore, at the end of the evaporation, the surfactant is deposited in a thin ring at the bottom of the holes. On the other hand, with PS40-bP(PEGMA300)48, the shape of the droplets might start to deviate from spherical during the drying process as a consequence of the outward flow that carries the copolymer to the edge, where it is accumulated. The internal region remains fluid, whereas the edge starts to solidify. Thus, the central region of the drop decreases and the shape turns to concave. In this particular case, when an amphiphilic copolymer with poor surface activity is used in the breath figures process,

Figure 8. Plot of the surface tension of water against the concentration of the amphiphilic copolymer added to water (blue), Pluronic F127 (red), or PS40-b-P(PEGMA300)48 (black).

P(PEGMA300)48. According to other recent examples, the decrease in the surface tension has been associated with a reduction in the formation of coffee-stain structures.23 In summary, as depicted in this paper, depending upon the molecular structure of the additive employed to produce the porous films, three different situations can be observed. Scheme 1 illustrates the situations observed for the structures employed and their distribution inside of the pore. First of all, if a water-insoluble amphiphilic copolymer is blended with a polymer matrix, the copolymer segregates to the water/polymer solution interface and forms a uniform layer around the water droplets. Second, when a water-soluble copolymer is used, the copolymer not only is oriented to the water droplets but also can be solubilized in the condensed water. This fact can cause an inhomogeneous distribution of the copolymer within the holes in a ring-like fashion, located either

Scheme 1. Schematic Illustration of the Breath Figures and Coffee-Stain Processes Occurring Simultaneously from Polymer Blends Based on a PS Matrix and Three Different Amphiphilic Polymers: Water-Insoluble PS45-b-P(PEGMA300)34 and Two Water-Soluble Polymers, PS40-b-P(PEGMA300)48 (Low Surface Activity) and Pluronic F127 (High Surface Activity)

6139

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

at the end of the film formation, the copolymer is almost localized near the top edge with a ring-like structure.

applications and future perspectives. Biomacromolecules 2005, 6, 2427−2448. (4) Park, W. J.; Choi, K. J.; Kim, M. H.; Koo, B. H.; Lee, J.-L.; Baik, J. M. Self-assembled and highly selective sensors based on air-bridgestructured nanowire junction arrays. ACS Appl. Mater. Interfaces 2013, 5, 6802−6807. (5) Lee, H. S.; Shim, T. S.; Hwang, H.; Yang, S.-M.; Kim, S.-H. Colloidal photonic crystals toward structural color palettes for security materials. Chem. Mater. 2013, 25, 2684−2690. (6) Ma, H.; Hao, J. Ordered patterns and structures via interfacial self-assembly: Superlattices, honeycomb structures and coffee rings. Chem. Soc. Rev. 2011, 40, 5457−5471. (7) Widawski, G.; Rawiso, M.; François, B. Self-organized honeycomb morphology of star-polymer polystyrene films. Nature 1994, 369, 387−389. (8) Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Evidence for convective effects in breath figure formation on volatile fluid surfaces. Phys. Rev. Lett. 1996, 76, 3762−3765. (9) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Threedimensionally ordered array of air bubbles in a polymer film. Science 2001, 292, 79−83. (10) Bunz, U. H. F. Breath figures as a dynamic templating method for polymers and nanomaterials. Adv. Mater. 2006, 18, 973−989. (11) Hernández-Guerrero, M.; Stenzel, M. H. Honeycomb structured polymer films via breath figures. Polym. Chem. 2012, 3, 563−577. (12) Escalé, P.; Rubatat, L.; Billon, L.; Save, M. Recent advances in honeycomb-structured porous polymer films prepared via breath figures. Eur. Polym. J. 2012, 48, 1001−1025. (13) Muñ o z-Bonilla, A.; Fernán dez-García, M.; RodríguezHernández, J. Towards hierarchically ordered functional porous polymeric surfaces prepared by the breath figures approach. Prog. Polym. Sci. 2014, 39, 510−554. (14) Ma, H.; Hao, J. Evaporation-induced ordered honeycomb structures of gold nanoparticles at the air/water interface. Chem.Eur. J. 2010, 16, 655−660. (15) Fan, D.; Jia, X.; Tang, P.; Hao, J.; Liu. Self-patterning of hydrophobic materials into highly ordered honeycomb nanostructures at the air/water interface. Angew. Chem., Int. Ed. 2007, 46, 3342−3345. (16) Ma, H.; Cui, J.; Song, A.; Hao, J. Fabrication of freestanding honeycomb films with through-pore structures via air/water interfacial self-assembly. Chem. Commun. 2011, 47, 1154−1156. (17) Hayakawa, T.; Horiuchi, S. From angstroms to micrometers: Self-organized hierarchical structure within a polymer film. Angew. Chem., Int. Ed. 2003, 115, 2387−2391. (18) Muñoz-Bonilla, A.; Ibarboure, E.; Papon, E.; RodriguezHernandez, J. Self-organized hierarchical structures in polymer surfaces: Self-assembled nanostructures within breath figures. Langmuir 2009, 25, 6493−6499. (19) de León, A. S.; del Campo, A.; Fernández-García, M.; Rodríguez-Hernández, J.; Muñoz-Bonilla, A. Hierarchically structured multifunctional porous interfaces through water templated selfassembly of ternary systems. Langmuir 2012, 28, 9778−9787. (20) Escalé, P.; Save, M.; Lapp, A.; Rubatat, L.; Billon, L. Hierarchical structures based on self-assembled diblock copolymers within honeycomb micro-structured porous films. Soft Matter 2010, 6, 3202−3210. (21) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827−829. (22) Hu, H.; Larson, R. G. Analysis of the microfluid flow in an evaporating sessile droplet. Langmuir 2005, 21, 3963−3971. (23) Kajiya, T.; Kobayashi, W.; Okuzono, T.; Doi, M. Controlling the drying and film formation processes of polymer solution droplets with addition of small amount of surfactants. J. Phys. Chem. B 2009, 113, 15460−15466. (24) Li, F. I.; Thaler, S. M.; Leo, P. H.; Barnard, J. A. Dendrimer pattern formation in evaporating drops. J. Phys. Chem. B 2006, 110, 25838−25843.

■

CONCLUSION Although the coffee-ring effect is extensively investigated nowadays in polymer solution or colloidal suspension, it is poorly studied in more complex systems involving polymer interfaces, as studied in this current paper. Herein, we have investigated the coffee-stain phenomenon occurring inside the pores during the breath figures process. We have demonstrated that only water-soluble copolymers added to the initial blend are capable of migrating to the condensed water droplet and forming an inhomogeneous distribution within the cavities of the breath figures films. Besides, it has been demonstrated that the surface activity of the amphiphilic macromolecules strongly influences the final position of the copolymer in the holes. Whereas the surfactant is deposited at the bottom of the pore, the soluble block copolymer produced a ring-like structure as a consequence of the coffee stain. The use of confocal Raman microscopy permitted us the analysis of the spatial distribution of different components within a film requiring a rather low concentration. Thus, this technique is postulated to be a great alternative to contribute in the understanding of the breath figures process, coffee-stain phenomenon, and other interfacial self-assembly processes. This work may be of fundamental as well as practical interest. In addition of putting some insight into the reduction of the coffee-stain phenomenon using surfactant, the combination of evaporative-induced selfassembly processes presents great potential to create more complex patterns of, i.e., biomolecules or nanoparticles, with potential applications as sensors, photonic devices, or genomics.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] and/or alexandra.munnoz@uam. es. Present Address

́ ́ Departamento de Quimica Fisica Aplicada, Facultad de Ciencias, Universidad Autónoma de Madrid, C/Francisco Tomás y Valiente 7, Cantoblanco, 28049 Madrid, Spain. §

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the Ministerio de Economiá y Competitividad (MINECO) (Projects MAT201017016, MAT2010-21088-C03-01, and COST Action MP0904 SIMUFER). Alexandra Muñoz-Bonilla gratefully acknowledges the MINECO for her Juan de la Cierva postdoctoral contract, and Alberto S. de León thanks the Ministerio de Educación for his Formación Personal Universitario (FPU) predoctoral fellowship.

■

REFERENCES

(1) Black, C. T.; Ruiz, R.; Breyta, G.; Cheng, J. Y.; Colburn, M. E.; Guarini, K. W.; Kim, H.-C.; Zhang, Y. Polymer self assembly in semiconductor microelectronics. IBM J. Res. Dev. 2007, 51, 605−633. (2) Jenekhe, S. A.; Chen, X. L. Self-assembly of ordered microporous materials from rod−coil block copolymers. Science 1999, 283, 372− 375. (3) Senaratne, W.; Andruzzi, L.; Ober, C. K. Self-assembled monolayers and polymer brushes in biotechnology: Current 6140

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141

Langmuir

Article

(25) Zhang, L.; Maheshwari, S.; Chang, H.-C.; Zhu, Y. Evaporative self-assembly from complex DNA−colloid suspensions. Langmuir 2008, 24, 3911−3917. (26) Sempels, W.; De Dier, R.; Mizuno, H.; Hofkens, J.; Vermant, J. Auto-production of biosurfactants reverses the coffee ring effect in a bacterial system. Nat. Commun. 2013, 4, 1757. (27) Soltman, D.; Subramanian, V. Inkjet-printed line morphologies and temperature control of the coffee ring effect. Langmuir 2008, 24, 2224−2231. (28) Yunker, P. J.; Still, T.; Lohr, M. A.; Yodh, A. G. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 2011, 476, 308−311. (29) Eral, H. B.; Augustine, D. M.; Duits, M. H. G.; Mugele, F. Suppressing the coffee stain effect: How to control colloidal selfassembly in evaporating drops using electrowetting. Soft Matter 2011, 7, 4954−4958. (30) Deegan, R. D. Pattern formation in drying drops. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 6, 475−485. (31) Still, T.; Yunker, P. J.; Yodh, A. G. Surfactant-induced Marangoni eddies alter the coffee rings of evaporating colloidal drops. Langmuir 2012, 28, 4984−4988. (32) Sun, W.; Ji, J.; Shen, J. Rings of nanoparticle-decorated honeycomb-structured polymeric film: The combination of pickering emulsions and capillary flow in the breath figures method. Langmuir 2008, 24, 11338−11341. (33) Shen, X.; Ho, C.-H.; Wong, T.-S. Minimal size of coffee ring structure. J. Phys. Chem. B 2010, 114, 5269−5274. (34) Muñoz-Bonilla, A.; van Herk, A. M.; Heuts, J. P. A. Preparation of hairy particles and antifouling films using brush-type amphiphilic block copolymer surfactants in emulsion polymerization. Macromolecules 2010, 43, 2721−2731. (35) Fukuhira, Y.; Yabu, H.; Ijiro, K.; Shimomura, M. Interfacial tension governs the formation of self-organized honeycomb-patterned polymer films. Soft Matter 2009, 5, 2037−2041. (36) Kojima, M.; Hirai, Y.; Yabu, H.; Shimomura, M. The effects of interfacial tensions of amphiphilic copolymers on honeycombpatterned films. Polym. J. 2009, 41, 667−671. (37) Truskett, V. N.; Stebe, K. J. Influence of surfactants on an evaporating drop: Fluorescence images and particle deposition patterns. Langmuir 2003, 19, 8271−8279. (38) Hu, H.; Larson, R. G. Marangoni effect reverses coffee-ring depositions. J. Phys. Chem. B 2006, 110, 7090−7094.

6141

dx.doi.org/10.1021/la5011902 | Langmuir 2014, 30, 6134−6141