pubs.acs.org/Langmuir © 2009 American Chemical Society
Self-Organized Hierarchical Structures in Polymer Surfaces: Self-Assembled Nanostructures within Breath Figures Alexandra Mu~noz-Bonilla, Emmanuel Ibarboure, Eric Papon, and Juan Rodriguez-Hernandez*,† Laboratoire de Chimie des Polym eres Organiques (LCPO), CNRS, Universit e Bordeaux I, ENSCPB 16, Avenue Pey Berland, 33607, Pessac-Cedex, France. † Current Address: Instituto de Ciencia y Tecnologia de Polimeros (ICTP-CSIC) C/ Juan de la Cierva n 3 28006 Madrid. Received April 15, 2008. Revised Manuscript Received April 3, 2009 Herein we report the preparation of hierarchically micro- and nanostructured polymer surfaces in block copolymer/ homopolymer blends. The structural order at different length scales was obtained combining two methodologies, e.g., the breath figures method to produce porous microstructures (“top-down” approach) with block copolymer self-assembly to induce microphase separation at the nanometer length scale (“bottom-up” approach). The interplay of the breath figure formation during the spin-coating and self-assembly of the triblock copolymer allowed the preparation of polymer surfaces having micrometer-sized cavities decorated with nanostructured block copolymers. The system described herein possesses unique characteristics. First, the surface chemical composition can be varied by a surface rearrangement upon annealing either to dry or humid air. Moreover, surface rearrangement is accompanied with structural changes, i.e. both topography and nanostructuration can be reversibly modified upon annealing. In terms of topograghy, a transition between holes and hills was obtained upon soft annealing to water vapor and can be recovered upon annealing to dry air. Finally, the pore nanostructure can be modulated from a micellar array to a lamellar phase when the film is exposed either to air or to tetrahydrofuran vapor.
1. Introduction Nature is a source of knowledge in which scientists have found inspiration to construct artificial surfaces with unique properties. Detailed studies on natural surfaces have demonstrated that the special functionalities of organisms are not exclusively governed by the intrinsic property of materials but are more likely related to their unique micro- or nanostructures. That is the case of water repellence of many biological surfaces, particularly plant leaves, that have recently received great interest.1 Plant surfaces, usually rough on the micrometer scale and covered with cuticular wax, have the ability to make water bead off completely, thereby effectively washing off contamination. This property is known as the “Lotus effect”.2 Additional functions of the protective outer coverage of plants, also known as the “cuticle”, include transpiration barrier, protection against overheating by reflection of radiation or signaling, and recognition.3 Another example is the intriguing colors exhibited by butterflies (blue color of the Morpho sulkowskyi) originating from light diffraction and scattering, as a result of the ordered microstructure. This form of color is utilized by animals both for protection and warning.4 The examples described above are a few of the multiple instances found in nature. The superior adhesion of gecko feet via the enhanced adhesive interaction of nanohairs,5 reduced friction and *Corresponding author. Fax: (34) 91 564 48 53. Tel: (34) 91 258 75 05. E-mail:
[email protected]. (1) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (b) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943. (2) Nun, E.; Oles, M.; Schleich, B. Macromol. Symp. 2002, 187, 677. (3) Koch, K.; Bhushan, B.; Barthlott, W. Prog. Mater. Sci. 2009, 54(2), 137. (4) (a) John, S. Phys. Rev. Lett. 1987, 58, 2486. (b) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (c) Yablonovitch, E. Nature 1999, 401, 539. (5) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99(19), 12252. (6) Baumgartner, W.; Saxe, F.; Weth, A.; Hajas, D.; Sigumonrong, D.; :: Emmerlich, J; Singheiser, M.; Bohme, W.; Schneider, J. M. J. Bionic Eng. 2007, 4, 1.
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wear of the skin of sandfish via nanostructured scales,6 or turbulence reduction near shark scales by microgrooves are also examples of enhanced surface functionality by nanostructuring.7,8 In spite of the synthetic limits to obtain regularly structured surfaces compared to the perfection of nature, many attempts have been made to industrially manufacture hierarchically ordered surfaces by introducing, for instance, hydrophobicity and/ or roughness. The preparation of multiscale surface ordered materials9 requires the combination of different fabrication processes. “Bottom-up” techniques have been employed to obtain nanoscale order at surfaces, e.g., by self-assembly of block copolymers.10 However, even with the development of recent approaches such as controlled solvent annealing, or the use of crystallizable blocks, this approach provides limited in-plane ordering.11 On the contrary, microstructuration has been typically created by using “top-down” techniques such as lithography, writing, or printing techniques.12 In this case, diffraction depth focus and/or electrostatic interactions are important drawbacks resulting in expensive facilities and slow pattern writing.13 An alternative approach to overcome the limitations of both approaches is to combine their strengths, using a top-down fabrication mechanism to produce controlled surface features at the micrometer length scale, and a bottom-up approach based on self-assembly to create desired structures at the nanometer level. For instance, “bottom-up” and “top-down” strategies have been (7) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213. (8) Scherge, M. and Gorb, S. S. N. Biological Micro- and Nanotribology: Nature’s Solutions; Springer-Verlag: New York, 2001. (9) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (10) Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152–1204. (11) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505. :: :: (12) Nanotechnologie: Eine Einfuhrung in die Nanostrukturtechnik; M. Kohler, Ed.; Wiley-VCH: Weinheim, Germany, 2001. (13) Nie, Z.; Kumacheva, E. Nat. Mater. 2008, 7, 277.
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combined in the so-called templated self-assembly (TSA) approach.14 Hence, topographically patterned surfaces15 (lithographically defined grooved substrates, polydimethylsiloxane (PDMS) stamps or hard imprint molds) and/or chemically patterned substrates16 are typical templates used to order block copolymer films at larger length scales. Equally, hydrophobic surfaces in which the roughness has been artificially modified have been fabricated with hierarchical structures such as electrodeposition, nanowire arrays, colloidal systems, or photolithography.17 We propose a straightforward methodology to obtain hierarchical patterned polymer surfaces with controlled topography, chemical distribution, and nanostructured domains in one single step. For that purpose, breath figures formed by water condensation employed as a physical micropattern (“top-down” approach) to vary the topography were combined with the self-assembly of block copolymers (“bottom-up” approach) to modulate the chemical distribution and induce the formation of nanostructures within the cavities. Whereas several examples have been reported in which the pores formed have been decorated with different functional groups18 or nanoparticles,19 the preparation of nanostructured breath figures is unprecedented. Moreover, the polymer surfaces described herein exhibit unique adaptive properties: both chemical composition and the nanostructure formed can be, upon annealing, modified as a function of the environment.
2. Experimental Section 2.1. Materials. Styrene (St) (Aldrich, France, 99%) and tertbutyl acrylate (tBA) (Aldrich, France, 98%) were distilled under reduced pressure over calcium hydride prior to use. Copper (I) bromide (CuBr) (Aldrich, France, 98%), 2,20 -bipyridyl (bipy) (Aldrich, France, 99+%), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA) (Aldrich, France, 99%), phenylethyl bromide (PhEBr) (Aldrich, France, 97%) and other solvents were used as received.
2.2. Synthesis of the block copolymers. Synthesis of Poly(2,3,4,5,6-pentafluorostyrene) (PS5F). S5F was polymerized in bulk using (1-bromoethyl) benzene as the initiator and CuBr/PMDETA as the catalyst with the following stoichiometry: [monomer]/[initiator]/[ligand]/[CuBr] = 50/1/1/1. The Schlenk was charged with 3 mL (22.1 mmol) of S5F, 0.09 mL (0.44 mmol) of PMDETA, 0.0630 g (0.44 mmol) of CuBr, and 0.06 mL (0.44 mmol) of initiator. The reaction mixture was degassed during three freeze-thaw cycles and then immersed into a thermostatically controlled oil bath at 90 °C under stirring. (14) (a) Gorzolnik,, B.; Mela, P.; Moeller, M. Nanotechnology 2006, 17, 5027. (b) Seagalman, R. A.; Yokoyama, H.; Kramer, E. J. Adv. Mater. 2001, 13, 1152. (c) Cheng, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Vancso, G. J. Adv. Mater. 2003, 15, 1599. (d) Sundrani, D.; Darling, S. B.; Sibener, S. J. Nano Lett. 2004, 4(2), 274. (e) Sundrani, D.; Darling, S. B.; Sibener, S. J. Langmuir 2004, 20, 5091. (15) (a) McCord M. A. ; Rooks M. J. Handbook of Microlithography, Micromachining, and Microfabrication, Vol. 1; Choudhury, P. R., Ed.; SPIE: Bellingham, WA, 1997.(b) Xia, Y. N.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (c) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. J. Vac. Sci. Technol., B 1996, 14, 4129. (d) Resnick, D.; Sreenivasan, S. V.; Wilson, C. G. Mater. Today 2005, 8(2), 34. (16) Fasolka, M. J.; Mayes, A. M. Annu. Rev. Mater. Res. 2001, 31, 323. (17) (a) Chong, M. A. S.; Zheng, B.; Gao, H.; Tan, L. K. Appl. Phys. Lett. 2006, 89, 233104. (b) del Campo, A.; Greiner, A. J. Micromech. Microeng. 2007, 17, R81. (c) Ming, W.; Wu, R.; van Benthem, R.; de With, G. Nano. Lett. 2005, 5, 2298. (d) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929. (18) (a) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. J. Polym. Sci. Part A. Polym. Chem. 2006, 44, 2363. (b) Barner-Kowollik, C.; Dalton, H.; Davis, T. P.; Stenzel, M. H. Angew. Chem., Int. Ed. 2003, 42, 3664. (c) Nygard, A.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Aust. J. Chem. 2005, 58, 595. (d) Stenzel, M. H.; Davis, T. P. Aust. J. Chem. 2003, 56, 1035. :: (19) Boker, A.; Lin, K .; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick, T.; Rusell, T. P. Nat. Mater. 2004, 3, 302.
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After 60 min, the reaction mixture was diluted in tetrahydrofuran (THF) and passed through a neutral alumina column to remove the catalyst. The solution was concentrated under reduced pressure, and the mixture was poured into ethanol, precipitated, and dried under vacuum (MnNMR = 4100 g/mol, Mw/MnSEC = 1.07).
Synthesis of Poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene (PS5F-b-PS-Br) Diblock Copolymer. The diblock copolymer was prepared via atom-transfer radical polymerization (ATRP) of styrene from PS5F-Br used as macroinitiator. A 1.1 g portion of PS5F-Br (MnNMR = 4100 g/mol, 0.26 mmol) was placed in a Shlenk tube and dissolved in 1 mL of acetone and 3 mL (26.18 mmol) of styrene. Then 0.05 mL (0.26 mmol) of PMDETA and 0.038 g (0.26 mmol) of CuBr were added. The mixture was degassed three times by freezing and thawing, and was subsequently heated at 90 °C. After 240 min, the polymerization was stopped and purified over an alumina column. Finally, the block copolymer was obtained by precipitation into ethanol and dried under vacuum (MnNMR = 7300 g/mol, Mw/MnSEC = 1.09).
Synthesis of Poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene-b-poly[poly(ethylene glycol) methyl ether methacrylate] (PS5F-b-PS-b-PPEGMA) triblock copolymer. The polymerization of PEGMA was carried out using PS5F-b-PS-Br as a macroinitiator in toluene solution (75% v/v) at 90 °C with [monomer]/[initiator]/[ligand]/[CuBr] = 40/1/1/1. We employed 0.3195 mg (MnNMR = 7300 g/mol, 0.04 mmol) of diblock copolymer, which was added to a Shlenk tube and dissolved in 1.5 mL of toluene. Afterwards, 0.5 mL (1.7 mmol) of PEGMA, 0.01 mL (0.04 mmol) of PMDETA, and 0.0063 g (0.04 mmol) of CuBr were added to the solution. The mixture was deoxygenated by three freeze-pump-thaw cycles and heated at 90 °C for 150 min. The solution was subsequently diluted with THF prior to purification through an alumina column. The pure triblock copolymer was obtained by evaporation and precipitation in heptane (MnNMR = 18700 g/mol, Mw/MnSEC = 1.25). Characterization. The homo- and block copolymers were characterized by 1H NMR with a Bruker Advanced 400 MHz spectrometer at room temperature using deuterated chloroform as solvent. Whereas average molecular weights were calculated by 1 H NMR, dispersities were determined by size exclusion chromatography (SEC) using a Jasco system equipped with two PL gel 5 μm (300 7.5 mm) mixed-C columns and a PL gel 5 μm (50 7.5 mm) guard column, a Jasco 1530 differential refractive index detector and a Jasco 875 UV detector. N,N-Dimethylformamide (HPLC grade) was used as the eluent containing 0.1 M LiBr with a flow rate of 0.8 mL/min at 60 °C. Calibration was obtained from narrowly distributed polystyrene standards. Contact Angle Measurements. Water contact angles were :: determined at room temperature using a Kruss DSA100 (Germany) contact angle goniometer. A water droplet of 2 μL was placed on the specimens at two different pH values: 2 and 6. A charge coupled device camera was used to capture the images of the water droplets for the determination of the contact angles. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with a 220i-XL ESCALAB from VG. The films supported on the silicon tip were put under ultrahigh vacuum (UHV) to reach the 10-8 Pa range. The Nonmonochromatized Mg X-ray source was used at 100 W, as well as a flood gun to compensate for nonconductive samples. The core-level spectra were obtained at a photoelectron takeoff angle (R, measured with respect to the sample surface) of 90°. The spectra were calibrated in relation to the C1s binging energy, which was applied as the internal standard. Fitting of the high resolution was provided by the ADVANTAGE program from VG. Atomic Force Microscopy (AFM). AFM images were recorded in air at room temperature with a Nanoscope IIIa microscope operating in tapping-mode. The probes were commercially available silicon wafer with a spring constant of 42 N/m, a resonance frequency of 285 kHz and a typical radius curvature Langmuir 2009, 25(11), 6493–6499
~ Munoz-Bonilla et al. in the 10-12 nm range. Both the topography and the phase signal images were recorded with a resolution of 512 512 data points. Dynamic Light Scattering (DLS). DLS experiments were performed using an ALV Laser goniometer, which consisted of a 22 mW HeNe linear polarized laser with a wavelength of 632.8 nm and an ALV-5000/EPP Multiple Tau Digital Correlator with 125 ns initial sampling time. The samples were kept at a constant temperature of 25.0 ( 0.1 °C during all of the experiments. The measurements were carried out at 90°. The solutions were introduced into 10 mm diameter glass cells. The minimum sample volume required for the experiment was 1 mL. The data acquisition was done with ALV Correlator Control Software, and the counting time varied for each sample from 300 s up to 600 s. Distilled THF was thoroughly filtered through 0.1 μm filters and directly employed for the preparation of the solutions. Film Preparation. Solution mixtures having 1 to 20 wt % triblock copolymer and 99 to 80% high molecular weight homopolystyrene (Mn 250 000 g/mol) were prepared in THF. The overall polymer solution concentration was varied from 1 to 50 mg/mL. The polymer solutions were filtered with a 0.1 μm Millipore membrane and spin-coated (4000 rpm for 60 s) onto silicon wafers purchased from Siegert Consulting, e.K. (SC, Germany). The silicon wafers were cleaned prior to use in piranha solution (3:1 v/v of H2SO4 in 30%H2O2) and rinsed several times with ethanol. For the preparation of samples under low relative humidity, the spin coater was flooded with argon before and during the spin-coating process. Beakers containing water and saturated aqueous solutions of sodium bromide were placed inside the spin coating chamber in order to obtain values of relative humidity of ∼40 and ∼57%, respectively. The resulting films had thicknesses of the order of 300-400 nm. To further study the variations of the surface chemical composition as a function of the environment, different experiments were performed. Upon analyzing (by AFM imaging) the films obtained after spin-coating, the samples were exposed either to air at 90 °C for 3 days or placed in a chamber saturated in THF solvent vapors. After each treatment, the samples were dried under vacuum at room temperature.
3. Results and Discussion The preparation, at larger scales, of polymer surfaces with controlled hierarchical nano- microstructure and topography is still a challenge. With more and more researchers becoming interested in this field, a number of methods have been reported to produce multistructured surfaces mainly by combination of “bottom-up” and “top-down” approaches. However, the preparation of such surfaces is usually bound up with time-consuming steps and expensive lithographic techniques. Herein, we explore an alternative approach using simpler and cheaper means to produce hierarchically structured surfaces in a polymer blend combining the self-assembly of block copolymers (to induce nanostructuration at the surface) with the breath figures method (physical micropatterning) to vary the topography. Hence, we obtained, in one single step, topographically modified surfaces created by water condensation with nanostructured domains. Moreover, as we will describe below, surface chemical composition and nanostructuration can be modified depending on the environment of exposure. As additive for the blend, we employed a triblock copolymer, more precisely, PS5F21-b-PS31-b-PPEGMA38 (TB), which was prepared by sequential ATRP steps20 The TB copolymer contains a fluorinated (low surface energy) segment able to migrate toward the polymer-air interface and a hydrophilic block to induce the orientation toward the condensed water droplets. Moreover, the (20) S. Mu~noz-Bonilla, E. Ibarboure, E. Papon, J. Rodriguez-Hernandez J. Polym. Sci. Part A. Polym. Chem., in press.
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A and C blocks (hydrophobic and hydrophilic, respectively) are incompatible and may induce microphase separation in nanostructured domains. The preparation of regular porous films by water condensation during casting is a simple methodology and was first described by Franc-ois et al.21 This technique involves casting of a polymer solution under humid conditions. Cooling the solution surface, caused by solvent evaporation, produces the condensation of water droplets on the surface. Simultaneously, amphiphilic copolymers (typically having either a single hydrophilic functional group or a polar segment) surround the water droplets and precipitate, and after complete evaporation an array of holes is obtained. The TB was blended with linear high molecular weight polystyrene and dissolved in THF. Homogeneous films with controlled thicknesses were obtained by spin coating from solutions having either different concentration of polymer (1-50 mg/mL) or varying ratio of TB copolymer/homopolymer (1-20%). In the first series of experiments, films of the blends containing 1-20% of TB and 99-80% of high molecular weight linear polystyrene were prepared by spin coating in a sealed chamber in which the relative humidity can be controlled. As depicted in Figure 1, blends having 10% of triblock copolymer and 90% of polystyrene formed surfaces with hole sizes between 300 and 400 nm were obtained in a 40% relative humidity environment. Decreasing the relative humidity below 14% by introducing Argon in the chamber during spin-casting impedes the formation of holes and leads to a rather smooth surface. As a consequence, a relatively high humidity is required in the environment during spin coating in order to improve the water condensation and formation of holes at the surface.26 Since the average pore size in surfaces prepared by the breath figures methodology determines the final application of the material, it is of high interest to investigate the parameters involved in the pore formation.22 Whereas porous polymer surfaces with very small pore sizes (20-50 nm) are appropriated for energy transfer materials in photovoltaic uses, surfaces with pore sizes ranging between 100 and 200 nm are desirable for the elaboration of porous conjugated materials for sensor uses. Equally, micrometer-sized porous films have found interest in biomedical applications, for instance, as templates for cell growth. The average pore diameter is influenced by factors such as type of polymer, molecular weight, but also the preparation conditions: humidity, solvent, or polymer concentration. These parameters have been studied and, as an example, the effect of polymer concentration on the pore size is illustrated in Figure 2. The experiments were performed with a 10% TB/90% homopolymer blend, varying the total polymer blend/solvent concentration, 1, 5, 25, and 50 mg 3 ml-1. Figure 2 shows height AFM images and cross sections of the four blends obtained by spin-coating under 40% of humidity. In these images, both the size of the pores and, as a consequence, the surface roughness increased linearly with the polymer concentration within this range of concentrations. As a result, varying the polymer blend concentration is an straightforward way to modify the average pore size, depth, and distribution and consequently the average roughness of the surface. The surface chemical composition has been determined by XPS analysis. In Figure 3 are illustrated the XPS spectra of the spincoated films (10% TB and 90% PS) obtained without any further treatment (i), annealed to air (ii), and annealed to water vapor (iii). The atomic chemical composition of the polymer surfaces as a (21) Widawski, G.; Rawieso, M.; Franc- ois, B. Nature 1994, 369, 397. (22) Bunz, U. H. F. Adv. Mater. 2006, 18, 973.
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Figure 1. AFM topography images (Left images size: 10 μm 10 μm. Right images size: 2 μm 2 μm) of polymer blend films containing 10 wt % of triblock copolymer PS5F21-b-PS31-b-PPEGMA38 in polystyrene matrix. Whereas the film in panel a was prepared in a moist atmosphere (40% relative humidity), the film in b was prepared under Argon (
