Micropatterned Surfaces through Moisture-Induced Phase-Separation

Jun 30, 2010 - Micropatterned Surfaces through Moisture-Induced Phase-Separation of Polystyrene−Clay Nanocomposite Particles. Bindu P. Nair and ...
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Micropatterned Surfaces through Moisture-Induced Phase-Separation of Polystyrene-Clay Nanocomposite Particles Bindu P. Nair and Chorappan Pavithran* Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Thiruvananthapuram 695019, India Received May 7, 2010. Revised Manuscript Received June 12, 2010 We report micropatterned polystyrene-clay nanocomposite (PCN) surfaces with concavities by moisture-induced phase separation of PCN particles. Micropatterned film with concavity size of 800 nm to 1.3 μm and a high number density of 2108 features/cm2 was obtained by drop-casting PCN solution (20 mg/mL PCN/THF) under ambient relative humidity of 70-80%. It is proposed that water droplets were channeled through the hydrophilic interfaces between the PCN particles, and the two-dimensional array of concavities was formed by spontaneous phase separation due to the presence of rigid clay platelets. The concavity size and number density can be tuned by varying the solvent for PCN. Micropatterned film with concavity size in the range of 650 nm to 1.1 μm with a number density of 5 107 features/cm2 was obtained using chloroform as solvent, whereas a concavity size of 150-740 nm and number density of 108 features/cm2 were obtained using carbon disulfide.

Introduction In recent years, there has been a growing interest in two- or three- dimensional micropatterned polymer surfaces and monoliths for their use as scaffolds for tissue engineering,1,2 membranes for separation and purification,3,4 solid supports for sensors and catalysts,5 low-dielectric constant materials for microelectronic devices6 and photonic band gap materials,7,8 etc. In most cases, the fabrication of microstructured polymer films involves templating using colloidal particles,9,10 polymer microspheres,11 filter membranes,12 and emulsions or surfactants that self-organize and generate mesoporous silica.13,14 Self-assembly of microphaseseparated block copolymers is a template-free method, which is widely investigated for obtaining well-defined micropatterned structures.15,16 However, design and synthesis of starting copolymers has to be carried out carefully for effective phase separation and self-assembly of various segments in the copolymers. A dynamic *To whom correspondence should be addressed. Phone: 0091-471-2515244. Fax: 0091-471-2491712. E-mail: [email protected].

(1) Hubbell, J. A.; Langer, R. Chem. Eng. News 1995, 73, 42. (2) Nishikawa, T.; Arai, K.; Hayashi, J.; Hara, M.; Shimomura, M. Int. J. Nanosci. 2002, 1, 415. (3) Akolekar, D. B.; Hind, A. R.; Bhargava, S. K. J. Colloid Interface Sci. 1998, 199, 92. (4) Lewandowski, K.; Murer, P.; Svee, F.; Frechet, J. M. J. Anal. Chem. 1998, 70, 1629. (5) Lin, V. S. Y.; Motsharei, K.; Dancil, K. P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840. (6) Hedrick, J. L.; Miller, R. D.; Hawker, C. J.; Volksen, K. R.; Yoon, W.; Trollsas, M. Adv. Mater. 1998, 10, 1049. (7) Deutsch, M.; Vlasov, Y. A.; Norris, D. J. Adv. Mater. 2000, 12, 1176. (8) Jiang, K. S.; Hwang, D. M.; Mittleman, J. F.; Bertone, V. I.; Colvm J. Am. Chem. Soc. 1999, 121, 11630. (9) Holland, B. T.; Stein, A. Science 1998, 281, 538. (10) Yan, H. W.; Blanford, C. F.; Holland, B. T.; Parent, M.; Smyrl, W. H.; Stem, A. Adv. Mater. 1999, 11, 1003. (11) Li, J.; Zhang, Y. Chem. Mater. 2007, 19, 2581. (12) Martin, C. R. Science 1994, 266, 1961. (13) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (14) Kresge, C. T.; Leonowicz, M. E.; Roth, W. C.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (15) Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Khougaz, K.; Eisenberg, A.; Lennox, R. B.; Krausch, G. J. Am. Chem. Soc. 1996, 118, 10892. (16) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 64, 422.

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templating method commonly called breath figures, exploiting selforganization of water droplets is a facile technique and generally yields honeycomb-patterned porous polymer films.17-20 Microphase separation of polymer solution spread on a nonsolvent surface is another simple route to micropatterned polymer surfaces, and tuning of the surface patterns can be achieved by changing the conditions of phase separation.21,22 The two-dimensional micropatterned film with concavities obtained through the above method has potential applications as micro-optical components, as microcontainers, and in controlled release. Polymer-clay nanocomposites is an area of great research interest during the past two decades,23-25 and recent investigations have proven that control of organization of the clay layers on a larger scale can produce nanocomposites with radically different morphologies, properties, and applications.26-29 More recently, we have reported microvesicles useful for guest encapsulation through the self-assembly of polystyrene-clay nanocomposite particles.30 The present article highlights the use of polymer-clay nanocomposite particles as successful substrates for producing two-dimensional micropatterned surfaces with concavities through moisture induced phase separation. The mechanism of formation of a two-dimensional micropatterned film from polystyrene-clay (17) Rayleigh, L. Nature 1911, 86, 416. (18) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (19) Bunz, U. H. F. Adv. Mater. 2006, 18, 973. (20) Stenzel, M. H.; Kowollik, C. B.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 2363. (21) Wang, Y.; Liu, Z.; Han, B.; Gao, H.; Zhang, J.; Kuang, X. Chem. Commun. 2004, 800. (22) Wang, Y.; Liu, Z.; Han, B.; Sun, Z.; Zhang, J.; Sun, D Adv. Funct. Mater. 2005, 15, 655. (23) Okada, A.; Usuki, A. Macromol. Mater. Eng. 2006, 291, 1449. (24) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539. (25) Triantafillidis, C. S.; LeBaron, P. C.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 4088. (26) Lim, Y. T.; Park, J. H.; Park, O. O. J. Colloid Interface Sci. 2002, 245, 198. (27) Haraguchi, K.; Ebato, M. Adv. Mater. 2006, 18, 2250. (28) Herrera, N. N.; Putaux, J. L.; David, L.; Haas, F. D.; Lami, E. B. Macromol. Rapid Commun. 2007, 28, 1567. (29) Toombes, G. E. S.; Mahajan, S.; Thomas, M.; Du, P.; Tate, M. W.; Gruner, S. M.; Wiesner, U. Chem. Mater. 2008, 20, 3278. (30) Nair, B. P.; Pavithran, C.; Sudha, J. D.; Prasad, V. S. Langmuir 2010, 26, 1431.

Published on Web 06/30/2010

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Figure 1. (a) SEM image of the top surface of PCN micropatterned film from 20 mg/mL PCN/THF and (b) broken edges of the film showing the smooth nonpatterned bottom surface.

Figure 2. SEM images of mixed morphologies from PCN for solution concentration of (a and b) 5 mg/mL (c and d) 10 mgmL-1 PCN/THF. Inset in panel a shows fluorescent micrograph of giant hollow spheres encapsulated with dye 8-anilinonapthlaenesulfonic acid.

nanocomposite particles is also discussed. The synthetic strategy is more facile compared to that of star and comb polymers for obtaining micropatterned films31 and can be applicable to clays of different aspect ratios and olefinic monomers other than styrene.

Experimental Section Materials. Styrene monomer was purchased from Aldrich Chemicals and benzoyl peroxide from S.d Fine Chem Ltd., India. The clay used was Cloisite-Naþ (Cation exchange capacity (CEC) 92.6 mequiv/100 g) from Southern Clay Products. Toluene and carbon disulfide were of extra pure grade and chloroform and tetrahydrofuran (THF) of HPLC grade from Merck Specialties Pvt. Ltd., India. Methods. Polystyrene-clay nanocomposite (PCN) particles were synthesized by in situ intercalative polymerization of styrene with the polyhedral oligomeric silsesquioxane (POSS)-modified organoclay as reported elsewhere.30,32 POSS-modified organoclay was synthesized using POSS solution from hydrolytic condensation (31) Guerrero, H.; Barner-Kowollik, C.; Davis, T. P.; Stenzel, M. H. Eur. Polym. J. 2005, 41, 2264. (32) Nair, B. P; Pavithran, C. Langmuir 2010, 26, 730.

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Figure 3. Optical transmission micrograph of (a) nonpatterned PCN film (20 mg/mL PCN/THF) cast in nonhumid condition in glovebox and (b) corresponding micropatterned film obtained by casting in humid atmosphere. of 3-aminopropyltriethoxysilane and vinyltriethoxysilane in the mole ratio 1:3.32 In POSS-modified organoclay, POSS with their amino groups interacting with the clay surfaces formed a bilayer within the clay gallery with a corresponding d001 spacing of 26.35 A˚. In brief, POSS-modified clay (10 wt % of monomer) was dispersed in styrene by sonication for 20 min (Branson 3510 Sonicator, 100 W, DOI: 10.1021/la1018295

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Figure 4. Scheme of formation of PCN micropatterned film from PCN particles through breath figure method. 40 kHz), benzoyl peroxide was added (2 wt % of the monomer), and under nitrogen atmosphere and magnetic stirring (∼ 300 rpm), the solution was heated at 70 °C for 30-60 min during which gelation of the mix occurred and then heated at 90 °C for 10-12 h to obtain the nanocomposite. Free polymer present was removed by Soxhlet extraction of the nanocomposite using toluene. The residue was dispersed in toluene and centrifuged to remove insoluble/ suspended particles. The PCN particles were precipitated by adding methanol, filtered, and dried at 100 °C in vacuum. The PCN particles showed an inorganic content of ∼3 wt % by thermogravimetric analysis, intercalated morphology with d-spacing of 29.35 A˚, and particulate nature.30 Micropatterned PCN surfaces were obtained by drop-casting one drop of PCN particles either in THF, chloroform, or carbondisulfide (20-35 mg/mL) on a 0.3  0.3 cm rectangular glass slide under ambient conditions (28-30 °C, relative humidity of 70-80%), followed by evaporation of the solvent and then drying at 60 °C. The PCN micropatterned film was characterized using optical transmission microscopy (OTM) and scanning electron microscopy (SEM). Samples for OTM and SEM were prepared by drop-casting solutions of PCN on glass slides followed by evaporation of the solvent at ambient temperature. SEM images were taken in JEOL JSM-5600 LV scanning electron microscope using samples provided with a thin gold coating using JEOL JFC-1200 fine coater. OTM micrographs were taken using Leica DMRX microscope. Dye-encapsulated giant spheres were prepared by drop-casting solutions containing fluorescent dye 8-anilinonaphthalene sulfonic acid (ANS) and unencapsulated guest-molecules were removed by repeated washing using methanol. Fluorescent micrographs were taken using Leica DM LB2 fluorescence microscope.

Results and Discussion Formation of PCN particles through in situ polymerization of styrene with POSS-modified clay having reactive vinyl groups on POSS was reported earlier.30 The PCN particles possessed a sandwich structure consisting of stacked clay layers of thickness 12.6 nm at the core and polystyrene layers of thickness 36.7 nm on either side, exposing the hydroxylated edges of the silicate layers. While the primary PCN particles from dilute solution of 0.001 mg/mL/ THF showed a lateral dimension of 190-800 nm and thickness of 120 nm, microvesicles of diameter 2.5-3.5 μm and average membrane thickness of 85 nm were produced from solution concentration of 2.5 mg/mL, by consuming all of the PCN particles. In solvents like THF, the PCN particles assumed the characteristic features of bilayers from amphiphilic block copolymers and formed microvesicles by lateral association through H-bonding interactions between the hydroxylated edges of the silicate layers.30 The PCN particles yielded uniform micropatterned film from higher solution concentrations >15 mg/mL PCN/THF. Figure 1 12950 DOI: 10.1021/la1018295

Figure 5. Three-dimensional microporous film of neat polystyrene (20 mg/mL/THF).

shows SEM images of micropatterned PCN film prepared by dropcasting 20 mg/mL PCN/THF, on a glass slide under ambient conditions. The top surface of the film was patterned with concavities of diameters 800 nm to 1.3 μm with a high number density of up to 2  108 features/cm2 (Figure 1a). Furthermore, from the broken edges of the film, it can be seen that the film consists of closed pores and the bottom surface is nonpatterned (Figure 1b). The film is uniformly patterned with concavities (Supporting Information). Formation of micropatterned structures through vesicle fusion mechanism have been reported in literature.33,34 In order to find whether the micropatterned PCN film from higher solution concentration of PCN particles was produced through a gradual morphological transition involving vesicle fusion, we have analyzed the morphologies obtained from PCN solution of concentration between 2.5 and 20 mg/mL PCN/THF, respectively, the concentration at which uniform formation of vesicles and micropatterned film was observed. Mixed morphological features were observed between concentrations >2.5 and 10 μm were observed for 5 mg/mL PCN/THF (Figure 2a,b). Giant spheres were proven hollow by encapsulation with fluorescent dye 8-Anilinonaphthalenesulfonic acid, fluorescence was due to ANS adhering to the inner walls of the dried spheres (inset of Figure 2a). Isolated micropatterned film and (33) Deepak, V. D.; Asha, S. K. J. Phys. Chem. B 2006, 110, 21450. (34) Wang, H.; Zhou, X.; Yu, M.; Wang, Y.; Han, L.; Zhang, J.; Yuan, P.; Auchterlonie, G.; Zou, J.; Yu, C. J. Am. Chem. Soc. 2006, 128, 15992.

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Figure 6. SEM images of PCN micropatterned film prepared from 20 mg/mL PCN in (a) chloroform and (b) carbondisulfide.

vesicles were observed for 10 mg/mL PCN/THF (Figure 2c,d). As the concentration increases further, homogeneity of micropatterned film increased and uniform micropatterned film was observed for a concentration of 20 mg/mL PCN/THF. It is evident that as the concentration increases beyond 2.5 mg/mL PCN/THF, closure of the extended bilayer from PCN particles and formation of vesicles becomes difficult due to volume constraints. As a result, partially closed vesicles may collapse during drying to form circular discs, as observed in Figure 2a,b. The formation of giant hollow spheres having large membrane thickness could be due to closure of multilamellar assembly of extended bilayers during the drying process. Generally, size of the vesicle also increases as the bending modulus of the bilayer increases.35 Multilamellar assembly should show higher bending modulus than of unilamellar assembly. When the solution concentration is increased to 10 mg/mL, volume constraints becomes too high that the two-dimensional assembly of the extended bilayers in the solution assumes close-packed lamellar arrangement. Bending of the individual lamella being restricted, coalescence of the layers during drying leads to the formation of micropatterned or neat film along with vesicles depending on the localized concentration on the surface during drying. The above observations ruled out the formation of micropatterned PCN film through concentration dependent fusion of vesicles due to volume constraints. Besides, the PCN micropatterned film was two-dimensional and the vesicle fusion mechanism generally produces three-dimensional porous structures.33,34 The occurrence of two or three-dimensional micropatterned film can be explained by moisture induced phase separation or breath-figure (BF) mechanism.17-20 Due to evaporative cooling, water droplets nucleate on the surface of a polymer solution and grow subsequently, and arrays of pores are formed when drying of the remaining solvent leaves the imprint of the water droplets in the film. Pore size greatly depends on the concentration of the solution, evaporation rate of the solvent and the relative humidity. BF does not form in an atmosphere that has less than 4550% relative humidity. In the present case, micropatterned films were obtained at a high ambient humidity of 70-80%. BF mechanism was further confirmed by the fact that the micropatterned PCN film was not formed when casting and drying of the solution was carried out under nonhumid conditions in a glovebox. Figure 3a shows the optical transmission micrograph of nonpatterned PCN film from 20 mg/mL PCN/THF cast in a glovebox, and Figure 3b, the corresponding micropatterned film obtained by casting in humid atmosphere. The size of the concavities varied from 800 nm to 1.3 μm and the thickness of strut from 210 to 750 nm. The strut (35) Antonietti, M.; Forster, S. Adv. Mater. 2003, 15, 1323.

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thickness was of the order of the lateral dimension of the PCN particles. This suggests that the water droplets were channeled through the hydrophilic interfaces between the PCN particles as schematically shown in Figure 4, which leads to the polydispersity in the size of the concavities and strut thickness. An array of concavities was formed during drying or precipitation of the particles around the water droplets. Since linear polystyrene can produce micropatterned polymer film under humid condition,36 we cast 20 mg/mL linear polystyrene/THF synthesized under identical conditions of PCN. Interestingly, the micropatterned film obtained was three-dimensional due to sinking of multi arrays of water droplets, with a broad pore size distribution of 460 nm to 2.8 μm (Figure 5). It was evident that spontaneous phase separation due to the presence of rigid clay platelets in PCN particles prevented the sinking of multiarrays of water droplets, forming a two-dimensional micropatterned film. The above mechanism also limits the pore size distribution in the PCN film when compared to that of neat polystyrene. As the concentration increases, the number density and size of the concavities decreased. At very high concentration of g50 mg/mL PCN/THF, the sinking of water droplets and formation of concavities could be restricted due to high viscosity of the solution and a nonpatterned PCN film was observed (Supporting Information). The size and number density of the concavities can be tuned by varying the solvent for PCN. The concavity size and number density was less when chloroform and carbon disulfide were used as the solvent. The film produced from 20 mg/mL/chloroform showed concavities with diameters ranging between 650 nm to 1.1 μm, with a number density of 5107 features/cm2 (Figure 6a). Whereas, the PCN film cast from 20 mg/mL/carbondisulfide has a number density of 108 features/cm2 with concavity size ranging between 150 and 740 nm (Figure 6b). The difference in the size and density of the concavities using the above solvents is attributed to their difference in miscibility with water. Dipole moment of water, THF, chloroform, and carbon disulfide were respectively 1.85, 1.63, 1.15, and 0 D. The high miscibility of THF with water promotes the formation of a uniformly patterned film, whereas low miscibility with water and fast evaporation leads to concavities of smaller diameter with less number density in the case of chloroform and carbon disulfide.

Conclusions In conclusion, micropatterned polystyrene-clay nanocomposite films with relatively uniform concavities were fabricated using moisture-induced phase separation in solvents like THF, chloroform, and carbon disulfide. The variation in the size and number (36) Peng, J.; Han, Y.; Li, B. Polymer 2004, 45, 447.

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density of concavities with solvent can be explained based on the difference in the affinity of the solvents toward water. The nanocomposite micropatterned film can find application as templates for patterned polymer films, in the crystallization of inorganic salts, or as scaffolds. Acknowledgment. The authors thank Dr. B. Krishnakumar and Mr. M. R. Chandran, NIIST respectively for fluorescence

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microscopy and SEM micrographs. P.N.B. is also thankful to CSIR, New Delhi for research fellowship. Supporting Information Available: Low and high magnification SEM images of PCN micropatterned film from 20 mg/ mL PCN/THF and OTM image of nonpatterned film obtained from g50 mg/mL PCN/THF. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(15), 12948–12952