Two-Dimensional Polymeric Nanomaterials through Cross-linking of

Two-Dimensional Polymeric Nanomaterials through Cross-linking of Polybutadiene-b-Poly(ethylene oxide) Monolayers at .... Langmuir 2008 24 (7), 3306-33...
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Langmuir 2007, 23, 649-658

649

Two-Dimensional Polymeric Nanomaterials through Cross-linking of Polybutadiene-b-Poly(ethylene oxide) Monolayers at the Air/Water Interface Rachid Matmour,†,‡ Thomas J. Joncheray,† Yves Gnanou,*,‡ and Randolph S. Duran*,† The Center for Macromolecular Science and Engineering, Department of Chemistry, UniVersity of Florida, P.O. Box 117200, GainesVille, Florida 32611-7200, and Laboratoire de Chimie des Polyme` res Organiques (LCPO), ENSCPB-CNRS-UniVersite´ Bordeaux 1, 16 AVenue Pey-Berland, 33607 Pessac Cedex, France ReceiVed July 31, 2006. In Final Form: September 18, 2006 Two-dimensional polymeric nanomaterials consisting of a continuously cross-linked polybutadiene (PB) twodimensional network with poly(ethylene oxide) (PEO) domains of controlled sizes trapped within the PB network were synthesized. To reach that goal, novel (PB(Si(OEt)3)-b-PEO)3 star block copolymers were designed by hydrosilylation of the pendant double bonds of (PB-b-PEO)3 star block copolymer precursors with triethoxysilane. The (PB(Si(OEt)3)-b-PEO)3 star block copolymers were characterized by 1H NMR and IR spectroscopy. Self-condensation of the triethoxysilane pendant groups under acidic conditions led to a successful cross-linking of the polybutadiene blocks directly at the air/water interface without any additives or reagents. This strategy was found more efficient than radical cross-linking of (PB-b-PEO)3 with AIBN to get a homogeneously cross-linked monolayer of controlled and fixed morphology as demonstrated by the easy mechanical removal of the cross-linked Langmuir film from the water surface. As shown by AFM imaging, this strategy allows the accurate control of the PEO “pore” size depending on the monolayer surface pressure applied during the cross-linking reaction. The subphase pH and surface pressure influence on the cross-linking kinetics and monolayer morphologies were investigated by Langmuir trough studies (isotherm and isobar experiments) and AFM imaging.

Introduction The idea of stabilizing amphiphilic self-assemblies by polymerization was introduced at least 30 years ago for monolayers and about 10 years later for bilayer vesicles.1,2 This approach to bridging the nanoscale world of labile, interfacially driven selfassemblies with the mesoscale has resulted in several examples of massively cross-linked 3D structures.3-7 For example, Bates and co-workers were the first to succeed in retaining the cylindrical morphology formed by gigantic wormlike rubber micelles of polybutadiene-b-poly(ethylene oxide) (PB-b-PEO) diblock copolymers in water by chemical cross-linking of the PB cores through their pendant 1,2-double bonds.3,7,8 However, relatively few groups have shown interest in stabilization by cross-linking of two-dimensional (2D) copolymer self-assemblies formed at the air/water interface; most studies have involved interfacial polymerization of small molecules in Langmuir monolayers.9-38 * Authors to whom correspondence should be addressed. R.S.D.: e-mail, [email protected]; fax, 352-392-9741. Y.G.: e-mail, [email protected]; fax, 33 (0)5 40 00 84 87. † University of Florida. ‡ ENSCPB-CNRS-Universite ´ Bordeaux 1. (1) Dorn, K.; Klingbiel, R. T.; Specht, D. P.; Tyminski, P. N.; Ringsdorf, H.; O’Brien, D. F. J. Am. Chem. Soc. 1984, 106, 1627. (2) Regen, S. L.; Czech, B.; Singh, A. J. Am. Chem. Soc. 1980, 102, 6638. (3) Won, Y. Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960. (4) Liu, G. J.; Qiao, L. J.; Guo, A. Macromolecules 1996, 29, 5508. (5) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (6) Doshi, D. A.; Huesing, N. K.; Lu, M.; Fan, H.; Lu, Y.; Simmons-Potter, K.; Potter, B. G., Jr.; Hurd, A. J.; Brinker, C. J. Science 2000, 290, 107. (7) Discher, B. M.; Bermudez, H.; Hammer, D. A.; Discher, D. E.; Won, Y. Y.; Bates, F. S. J. Phys. Chem. B 2002, 106, 2848. (8) Discher, D. E.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.; Discher, B. M.; Hammer, D. A. Science 1999, 284, 1143. (9) Bubeck, C. Thin Solid Films 1988, 160, 1. (10) Kloeppner, L. J.; Duran, R. S. Langmuir 1998, 14, 6734. (11) Kloeppner, L. J.; Duran, R. S. J. Am. Chem. Soc. 1999, 121, 8108.

In the early 1970s Veyssie´ and co-workers20,30,31,33 were the first to demonstrate the formation of 2D cross-linked material by cross-linking polymerization of monolayers of dimethacrylates, and several other difunctional reactive amphiphiles under UV irradiation at a constant surface pressure at the air/water or oil/ (12) Kloeppner, L. J.; Batten, J. H.; Duran, R. S. Macromolecules 2000, 33, 8006. (13) Bruno, F. F.; Akkara, J. A.; Samuelson, L. A.; Kaplan, D. L.; Mandal, B. K.; Marx, K. A.; Kumar, J.; Tripathy, S. K. Langmuir 1995, 11, 889. (14) Kimkes, P.; Sohling, U.; Oostergetel, G. T.; Schouten, A. J. Langmuir 1996, 12, 3945. (15) Fichet, O.; Tran-Van, F.; Teyssie, D.; Chevrot, C. Thin Solid Films 2002, 411, 280. (16) Fichet, O.; Plesse, C.; Teyssie, D. Colloids Surf., A 2004, 244, 121. (17) Carino, S. R.; Totsmann, H.; Underhill, R.; Logan, J.; Weerasekera, G.; Culp, J.; Davidson, M.; Duran, R. S. J. Am. Chem. Soc. 2001, 123, 767. (18) Carino, S. R.; Duran, R. S.; Baney, R. H.; Gower, L. A.; He, L.; Sheth, P. K. J. Am. Chem. Soc. 2001, 123, 2103. (19) Carino, S. R.; Duran, R. S. Macromol. Chem. Phys. 2005, 206, 83. (20) Rehage, H.; Veyssie´, M. Angew. Chem., Int. Ed. Engl. 1990, 29, 439. (21) Sisson, T. M.; Lamparski, H. G.; Ko¨lchens, S.; Elayadi, A.; O’Brien, D. F. Macromolecules 1996, 29, 8321. (22) Fichet, O.; Teyssie´, D. Macromolecules 2002, 35, 5352. (23) Markowitz, M. A.; Bielski, R.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 7545. (24) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (25) Conner, M.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (26) Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (27) Dedek, P.; Webber, A. S.; Janout, V.; Hendel, R. A.; Regen, S. L. Langmuir 1994, 10, 3943. (28) Lee, W.; Hendel, R. A.; Dedek, P.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1995, 117, 6793. (29) Zhang, L.-H.; Hendel, R. A.; Cozzi, P.; Regen, S. L. J. Am. Chem. Soc. 1999, 121, 1. (30) Dubault, A.; Veyssie´, M.; Liebert, L.; Strzelecki, L. Phys. Sci. 1973, 245, 94. (31) Dubault, A.; Casagrande, C.; Veyssie´, M. J. Phys. Chem. 1975, 79, 2254. (32) Rehage, H.; Veyssie´, M. Angew. Chem., Int. Ed. Engl. 1990, 29, 497. (33) Rehage, H.; Schnabel, E.; Veyssie´, M. Makromol. Chem. 1988, 189, 2395. (34) Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J. Organometallics 1997, 16, 3401.

10.1021/la062256+ CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2006

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water interface. This idea was further followed by other groups. Regen and collaborators introduced the concept of a 2D network of molecular pores, i.e., “perforated monolayers” derived from calix[n]arene-based amphiphiles.23-28 By cross-linking with reagents such as malonic acid or via UV irradiation, they were able to synthesize porous and cohesive “perforated monolayers” with pore diameters in the range 2-6 Å with potential for gas permeation selectivity.26,27,29 Michl and collaborators synthesized grids through the coupling of arm-ends of star-shaped monomers forced to adhere to a mercury surface.34,35 After polymerization, well-defined covalent 2D square- or hexagonal-grid polymers could be synthesized34,35,39 and analogous supramolecular routes were also proposed.40-42 Palacin and co-workers reported on cross-linking porphyrins through molecular recognition between oppositely charged monomers at the air/water interface.36-38 Kloeppner and Duran11 were the first to demonstrate mechanical properties sufficient to allow removal of free-standing fibers from the water surface of 2D cross-linked 1,22-bis(2-aminophenyl)docosane polyanilines. Finally, Carino et al.17-19 followed the surface viscosity increase during 2D gelation of alkylalkoxysilane molecules under acidic conditions at the air/water interface. Concerning the cross-linking between true amphiphilic copolymeric chains at the air/water interface, only one example based on a polymerizable lipopolymer was previously proposed by O’Brien and colleagues involving network formation by photopolymerization.43 We are interested in cross-linking monolayers of block copolymers to achieve porosity at the sub-micrometer scale. Here, a 2D polymeric nanomaterial consisting of a continuously crosslinked polybutadiene material with poly(ethylene oxide) domains of controlled size trapped within the PB network is illustrated. This strategy opens up the possibility to retain a specific morphology at the mesoscopic scale defined by a given surface pressure (π). Such porous polymer thin films have potential applications in the preparation of membranes which will show large differences in permeability to water, methanol, and other polar compounds depending on the effective PEO “pore” size. With this in mind, two different methods were investigated for forming 2D cross-linked monolayers with a (PB-b-PEO)3 amphiphilic star block copolymer material based on a polybutadiene core and a poly(ethylene oxide) corona. In the first method, cross-linking of the PB hydrophobic block was achieved by using AIBN as a radical initiator under UV light directly at the air/water interface. The second method was based on the self-condensation of the triethoxysilane-functionalized polybutadiene blocks of the (PB(Si(OEt)3)-b-PEO)3 star block copolymer under acidic conditions. This latter route is extremely general and should be applicable to functionalization and coupling via other silanes and metal alkoxides. In both cases, the surface properties of the cross-linked materials were characterized by surface pressure measurements such as surface pressure-mean molecular area (MMA: interfacial area occupied by one polymer molecule) isotherms at different reaction times and isobar experiments (MMA evolution versus time for a given π) at (35) Michl, J.; Magnera, T. F. PNAS 2002, 99, 4788. (36) Palacin, S.; Barraud, A. Colloids Surf. 1991, 52, 123. (37) Porteu, F.; Palacin, S.; Ruaudel-Teixier, A.; Barraud, A. J. Phys. Chem. 1991, 95, 7438. (38) Palacin, S.; Porteu, F.; Ruaudel-Teixier, A. Thin Films 1995, 20, 69. (39) Magnera, T. F.; Pesherbe, L. M.; Ko¨rblova, E.; Michl, J. J. Organomet. Chem. 1997, 548, 83. (40) Brotin, T.; Pospisil, L.; Fiedler, J.; HKing, B. T.; Michl, J. J. Phys. Chem. B 1998, 102, 10062. (41) Magnera, T. F.; Michl, J. Atual. Fis. Quim. Org. 1998, 50. (42) Pospisil, L.; Heyrovsky, M.; Pecka, J.; Michl, J. Langmuir 1997, 13, 6294. (43) Brooks, C. F.; Thiele, J.; Frank, C. W.; O’Brien, D. F.; Knoll, W.; Fuller, G. G.; Robertson, C. R. Langmuir 2002, 18, 2166.

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different subphase pH values. The monolayer morphologies obtained at different surface pressures were studied by atomic force microscopy (AFM) imaging the Langmuir-Blodgett (LB) films obtained before and after cross-linking. Experimental Section Materials. Benzene for polymerizations and toluene for the hydrosilylation reaction were dried and distilled twice over CaH2 and polystyryllithium successively. THF was purified by distillation over CaH2 and then from a purple Na/benzophenone solution. 2-Methoxyethanol (Aldrich, 99%) was magnesium-dried and distilled. Solutions of sec-butyllithium (s-BuLi) (Aldrich) were used for halogen-lithium exchange reaction after double titration.44 The diphenylmethylpotassium (DPMK) solution was prepared and titrated following procedures described elsewhere.45 Butadiene (B) (Aldrich, 99%) was stirred over s-BuLi at -30 °C for 2 h and distilled prior to use. Ethylene oxide (EO) (Fluka, 99.8%) was stirred over sodium for 3 h at -40 °C and then distilled before use. For the synthesis of the tri- and tetrabromoinitiators, we followed the procedures described by Cheng et al.46 and Wolfe and Arnold,47 respectively. Triethoxysilane (Aldrich, 99%) and platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex (3 wt % solution in xylene) (Aldrich, 99%) were used as received. Instrumentation. High-purity argon (>99.5%) was rigorously dried and deoxygenated by passage through a column containing vermiculite. 1H NMR spectra were recorded on a Bru¨ker AC300 spectrometer using CDCl3, CD2Cl2, and CD3OD as deuterated solvents. Chemical shifts are reported in ppm (δ) downfield from tetramethylsilane (TMS) and referenced to residual solvent. Apparent molecular weights were determined with size exclusion chromatography (SEC) equipped with four TSK-gel columns (7.8 × 30 cm, 5 µm, G 2000, 3000, 4000, and 5000 HR with pore sizes of 250, 1500, 10000, and 100000 Å, respectively) and THF as the mobile phase (1 mL/min). This instrument was equipped with refractive index (RI) (Varian RI-4) and UV-vis (Varian 2550 variable λ) detectors. The SEC was calibrated using linear polystyrene samples. Absolute molecular weights of (PB-OH)3 stars were calculated using a multiangle laser light scattering (MALLS) detector (DLS) (Wyatt Technology) connected to an SEC line (abbreviated SEC/ DLS in the following). The dn/dc values for (PB-OH)3 stars were measured in THF at 25 °C with a laser source (λ ) 632.8 nm) (dn/dc ) 0.094 cm3/g). The tri- and tetrabromo initiators were characterized by mass spectrometry and elemental analysis. For photo-cross-linking, the Langmuir films were exposed to the radiation of a 80 W/cm medium-pressure mercury lamp, in the presence of air, at a passing speed of 50 m/min. Langmuir Films. Surface film characterization was accomplished using a Teflon Langmuir trough (width ) 150 mm and length ) 679 mm) system (KSV Ltd., Finland) equipped with two moving barriers and a Wilhelmy plate for measuring surface pressure. Between runs, the trough was cleaned with ethanol and rinsed several times with Millipore filtered water of ∼18 MΩ‚cm resistivity. The subphase temperature was maintained at 25 °C through water circulating under the trough. Samples were typically prepared by dissolving ∼1 mg of copolymer in 1 mL of chloroform and spread dropwise with a gastight Hamilton syringe on the Millipore water subphase. For the surface property studies of the (PB-b-PEO)3 star block copolymers, the chloroform was allowed to evaporate for 30 min to ensure no residual solvent remained and the isotherm experiments were run with barrier movement of 5 mm‚min-1. In the case of the (PB(Si(OEt)3-b-PEO)3 star block copolymers, the isotherms were recorded after different reaction times during cross-linking for various subphase pH values with a barrier compression speed of 100 mm‚min-1. The isobar experiments were carried out at a surface pressure of 5 mN/m for different subphase pH values. (44) Suffert, J. J. Org. Chem. 1989, 54, 509. (45) Francis, R.; Taton, D.; Logan, J. L.; Masse, P.; Gnanou, Y.; Duran, R. S. Macromolecules 2003, 36, 8253. (46) Cheng, K. J.; Ding, Z. B.; Wu, S. H. Synth. Commun. 1997, 27, 11. (47) Wolfe, J. F.; Arnold, F. E. Macromolecules 1981, 14, 909.

2D Polymeric Nanomaterials from PB-b-PEO Atomic Force Microscopy (AFM). LB films of the star copolymers were transferred onto freshly cleaved mica at various surface pressures. Once the film had equilibrated at a constant π, the mica was then pulled at a rate of 1 mm‚min-1. The transferred film was dried in a desiccator for 24 h and subsequently scanned in tapping mode at a scan rate of 1 Hz with a Nanoscope III AFM (Digital Instruments, Inc., Santa Barbara, CA) using silicon probes (Nanosensor dimensions: T ) 3.8-4.5 µm, W ) 27.6-29.2 µm, L ) 131 µm). The images were processed with a second-order flattening routine (Digital Instruments software). Synthesis of (PB-b-PEO)3 Amphiphilic Star Block Copolymers. Polymerization and End Capping of Butadiene. In a flamed and vacuum-dried three-neck flask, 0.135 g (2.5 × 10-4 mol) of tribromoinitiator was freeze-dried. Benzene (5.0 mL) was added to make a precursor solution concentration of 5.3 × 10-2 mol‚L-1, followed by 0.295 mL (3.75 × 10-3 mol) of 2-methoxyethanol. sec-Butyllithium (4.2 mL, 5.25 × 10-3 mol) at a concentration of 1.3 M was added to the solution. After 20 min of reaction, butadiene was added. The polymerization was allowed to proceed for 24 h and then end-capping was accomplished by addition of a large excess of ethylene oxide. The reaction was deactivated by degassed acidic methanol (3 mL of concentrated HCl in 50 mL of methanol). The reaction mixture was then concentrated on a rotary evaporator, the LiCl inorganic salts were removed by extraction of a dichloromethane solution of the star polymer with distilled water, and the polymer solution was dried over sodium sulfate and concentrated. The star polymer was finally precipitated in methanol to give 7.0 g of a crude product (98%). Mw (SEC/DLS in THF) ) 32500 g/mol; Mw/Mn ) 1.02. 1H NMR (δppm; CD2Cl2): 8.0-6.9 (m, 15H, aromatic resonances from the trifunctional initiator), 5.4 (m, 3H, CH2-CHdCH-CH2and CH2dCH-CH-), 4.9 (s, 2H, CH2dCH-CH-), 3.6 (s, 6H, -CH2-OH), 2.0 (b, 5H, CH2-CHdCH-CH2- and CH2dCHCH-), 1.2 (b, 2H, CH2dCH-C(R)H-CH2-). Polymerization of Ethylene Oxide Initiated by a HydroxylTerminated Polybutadiene Star. With use of a flamed and vacuumdried three-neck flask, the (PB-OH)3 polybutadiene star was freezedried in benzene and then dissolved in anhydrous THF. This solution was titrated by a solution of diphenylmethylpotassium in dried THF of known concentration to give the corresponding multioxanionic species. This was then used as macroinitiator for the polymerization of ethylene oxide at 45 °C for 48 h. Deactivation of the polymerization was accomplished by addition of degassed acidic methanol (3 mL of concentrated HCl in 50 mL of methanol). For relatively low molecular weight PEO blocks, a white precipitate (KCl) was observed. The crude reaction mixture was concentrated and the resulting solid was redissolved in dichloromethane. This dichloromethane solution was washed repeatedly with distilled water to remove inorganic salts and concentrated on a rotary evaporator. The star copolymers were precipitated in methanol, diethyl ether, or a mixture of the two solvents depending on the block lengths. 1H NMR (δppm; CDCl3): 3.6 (s, 4H, -CH2-CH2-O-) and the polybutadiene region is similar to the corresponding hydroxyl-terminated polybutadiene star resonances. Hydrosilylation of the (PB200-b-PEO326)3 Star Block Copolymer. In a flamed and vacuum-dried three-neck flask equipped with a magnetic stirrer, a reflux condenser, a dry argon inlet, and a heating mantle, 164 mg (2.17 × 10-6 mol) of (PB200-b-PEO326)3 star block copolymer were freeze-dried and dissolved in 15 mL of dry toluene. Then 0.285 mL (1.562 × 10-3 mol) of triethoxysilane and 0.1 mL of the catalyst platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (3 wt % solution in xylene) were added, and the reaction was carried out under argon for 24 h at 80 °C. At the end of the reaction, the solvent and the unreacted triethoxysilane were removed by evaporation under vacuum and small samples were taken for NMR, FTIR, and Langmuir trough studies. The product was stored under argon (crude product: Mn (1H NMR) ) 135500 g/mol; m ) 160 mg). 1H NMR (δppm; CDCl3): 5.4 (m, 3H, CH2-CHdCHCH2- and CH2dCH-CH-), 4.9 (s, 2H, CH2dCH-CH-), 3.8 (s, -CH2-Si(OCH2CH3)3), 3.6 (s, -CH2-CH2-O-), 2.0 (b, 5H, CH2-CHdCH-CH2- and CH2dCH-CH-), 1.5-0.9 (b, 11H,

Langmuir, Vol. 23, No. 2, 2007 651 Table 1. Data for (PB-b-PEO)3 and (PB(Si(OEt)3)-b-PEO)3 Star Block Copolymers Mna Mnb (1H Mnc Mw/ 1,2-PBb run (SEC) NMR) (theo) Mna (%) 1 45900 42500 40500 2 56000 75500 77500 3 58000 160500 164500 4 74000 320500 323000 5 135500

1.2 1.15 1.2 1.2

76 76 76 76 30

code (PB200-b-PEO76)3 (PB200-b-PEO326)3 (PB200-b-PEO970)3 (PB200-b-PEO2182)3 (PB72-co-PB(Si(OEt)3)128-bPEO326)3

a Apparent molecular weights determined by SEC in THF using a polystyrene calibration. b Estimated by 1H NMR analysis. c Mn,th ) MButadiene × ([butadiene]/[-PhLi]) × 3 + MEO × ([EO]/[(PB-OH)3].

-CH2-Si(OCH2CH3)3), and CH2dCH-C(R)H-CH2-), and 0.30.8 (s, 2H, -CH2-Si(OEt)3). Preparation of Cross-linked (PB-b-PEO)3 Star Block Copolymer Monolayer with AIBN. One hundred microliters (C ) 1 mg/ mL) of a chloroform solution of (PB200-b-PEO76)3 star block copolymer and 100 µL (C ) 0.2 mg/mL) of a chloroform solution of AIBN were spread dropwise with a gastight Hamilton syringe on the Millipore water subphase. The chloroform was then allowed to evaporate for 15 min to ensure no residual solvent remained. The subphase temperature was maintained at 25 °C with water circulating under the trough. The monolayer was then compressed up to a surface pressure of 20 mN/m with a barrier compression speed of 5 mm‚min-1, and the cross-linking reaction was carried out under UV light for 24 h at 25 °C. At the end of the reaction, the isotherm of the cross-linked monolayer was recorded after barrier expansion. The cross-linked monolayer was removed from the water surface for FTIR study and transferred on a mica substrate for AFM characterization. Preparation of Cross-linked (PB(Si(OEt)3)-b-PEO)3 Star Block Copolymer Monolayers under Acidic Conditions. One hundred microliters (C ) 1 mg/mL) of a chloroform solution of (PB(Si(OEt)3)-b-PEO)3 star block copolymer were spread dropwise with a gastight Hamilton syringe on the Millipore water subphase at pH ) 3.0. The monolayer was immediately compressed and held at the desired surface pressure with a barrier compression speed of 100 mm‚min-1. The cross-linking reaction was carried out for 10 h at 25 °C to ensure completion of the reaction and the cross-linked monolayers were subsequently transferred onto mica substrates for further AFM characterization. After cross-linking at 15 mN/m, the resulting material could be removed with a spatula from the interface after compressing the monolayer to a final area of ca. 2 × 15 cm2 and was dried in a desiccator for further solubility studies.

Results and Discussion We recently investigated the surface properties of monolayers of a new set of (PB-b-PEO)n (n ) 3 or 4) amphiphilic three- and four-arm star block copolymers at the air/water interface.48 A divergent anionic polymerization method yielded copolymers with well-defined architecture, molecular weights, and block volume fractions. Different samples of well-defined (PB-b-PEO)n (n ) 3) amphiphilic star block copolymers exhibiting narrow molecular weight distributions were prepared with poly(ethylene oxide) coronas over a broad range of volume fractions (Table 1). Isotherm experiments at the air/water interface showed three characteristic regions: a “pancake” region (I) at high mean molecular areas where π slowly increases as the monolayer is compressed, a pseudoplateau at a pressure of ca. 10 mN/m (II) that corresponds to the dissolution of the PEO chains, and finally a compact brush region (III) at low surface areas affected only by the PB segments (Figure 1). The dotted lines in Figure 2 also show the extrapolations used to estimate the three corresponding parameters Apancake, Ao, and ∆A. A fit of the pseudoplateau data (48) Matmour, R.; Francis, R.; Duran, R. S.; Gnanou, Y. Macromolecules 2005, 38, 7754.

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Figure 1. Surface pressure-area per polymer molecule isotherms for (PB200-b-PEOn)3 star block copolymers (n ) 76, 326, 970, and 2182).

Figure 2. Isotherm of (PB200-b-PEO970)3 depicting how measurements of molecular areas for the three principal regions are obtained.

Figure 3. Linear dependence of ∆Apseudoplateau on the total number of ethylene oxide units.

revealed a linear dependence of ∆A with the number of EO units (y ) 12.351x - 0.4889; R2 ) 0.99), indicating that the “pseudoplateau” region area was largely dependent on the PEO block length (Figure 3). The extrapolated curve falls almost perfectly through the origin with an x-axis intercept corresponding to less than 1 EO unit, indicating a strong phase separation between blocks compared with slight intermixing observed with PSPEO systems. Our data indicate that, along the pseudoplateau,

Matmour et al.

Figure 4. Surface pressure-area isotherms for (PB200-b-PEO76)3 star block copolymer before (blue line) and after (red line) crosslinking in the presence of AIBN under UV light (π ) 20 mN/m).

a monomer of EO occupies 12.4 Å2, which is in good agreement with the value found for PS-PEO systems.49,50 The monolayers were also transferred as Langmuir-Blodgett films on mica at various surface pressures and analyzed by atomic force microscopy (AFM), showing different morphologies from analogous (PS-b-PEO) star copolymers.49,50 Reaction of the Polybutadiene Block at the Air/Water Interface in the Presence of AIBN. Initially, photocleavage of AIBN under UV light was attempted to cross-link the hydrophobic PB blocks of the (PB-b-PEO)3 copolymer monolayers directly at the air/water interface. For this purpose, 100 µL of a solution of (PB200-b-PEO76)3 star block copolymer in chloroform at a concentration of C ) 1 mg/mL and 100 µL of a solution of AIBN in chloroform at a concentration of C ) 0.2 mg/mL were successively deposited on the water surface. The monolayer was then compressed up to the desired surface pressure (20 mN/m) and the radical polymerization reaction of the 1,2-PB double bonds initiated by the photoinduced dimethylcyano radicals was carried out for 24 h. At the end of the reaction, the surface properties of the reacted material were investigated through isotherm and AFM studies. As shown in Figure 4, a shift toward the low mean molecular area region was observed after reacting the polybutadiene blocks. Furthermore, the progression of the reaction was followed by IR analysis of an aliquot of the material removed directly from the water surface (Figure 5). The disappearance of the peaks at 3100 and 1600 cm-1 corresponding to the 1,2-PB double bonds (dCH2 antisymmetric stretch and alkenyl -HCdCH2 stretch, respectively) confirmed significant consumption of the pendant double bonds. AFM characterization of the morphologies obtained before and after the reaction strengthened the previous observations. Figure 6 shows images of monolayers transferred to mica substrates at π ) 20 mN/m at different reaction times. The images reveal sub-micrometer-sized white circular domains that enlarge with reaction time, but remain separated. Due to the hydrophilic nature of mica, we assume PEO transfers as the bottom layer, represented in the images as the continuous dark phase, whereas PB occupies the top portion of the film corresponding to the white higher elevation domains shown in Figure 6A. We suppose that the bright domains correspond to cross-linked PB regions. The fraction of the surface occupied by the bright domains (49) Logan, J.; Masse, P.; Dorvel, B.; Skolnik, A. M.; Sheiko, S. S.; Francis, R.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005, 21, 3424. (50) Logan, J.; Masse, P.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005, 21, 7380.

2D Polymeric Nanomaterials from PB-b-PEO

Langmuir, Vol. 23, No. 2, 2007 653 Scheme 1. Hydrosilylation of the Pendant Double Bonds of the (PB-b-PEO)3 Star Block Copolymer

Figure 5. IR spectra of (PB200-b-PEO76)3 before (blue line) and after (red line) cross-linking in the presence of AIBN under UV light.

Figure 6. AFM topographic images of the (PB200-b-PEO76)3 star block copolymer transferred to mica substrates (π ) 20 mN/m) before (A) and after cross-linking (B, C, D, and E) at different reaction times. The films are scanned with a scale of 10 µm × 10 µm (A, B, C, and D) and 40 µm × 40 µm (E).

increases with the reaction time until large portions of the surface is covered with cross-linked and hydrophobic PB after 24 h. However, the picture scanned at 40 µm (Figure 6E) reveals that the bright domains remain isolated from each other and do not interconnect across the film. The formation of these domains is consistent with a chain polymerization occurring precisely at the spots of a radical initiation, which are also the nucleation spots.

The continuous PEO phase indicates that while some areas may be reacted, this strategy does not lead to homogeneously crosslinked PB covering the entire surface. Cross-linking with Hydrosilylated Polybutadiene Blocks at the Air/Water Interface. Hydrosilylation of (PB-b-PEO)3 Star Block Copolymers. Unsaturated polymers, especially diene polymers, are ideal for selective chemical modification because of the technological importance associated with the parent materials and the different reactivities of the double bonds. A particularly interesting reaction involves the hydrosilylation of diene polymers to obtain silane-modified rubber materials. Many papers and patents have appeared on the hydrosilylation of polymers.51-62 In most cases, the hydrosilylated polydienes were used for the synthesis of macromolecular complex architectures such as arborescent graft polybutadienes,63 multigraft copolymers of polybutadiene and polystyrene,64 or side-loop polybutadiene.65 We apply here the hydrosilylation reaction on the polybutadiene segments of the (PB200-b-PEO326)3 star block copolymer. Triethoxysilane was used as the hydrosilylating agent in stoichiometric amount with the total molar amount of double bonds in the polybutadiene block (1,2 and 1,4 units), and platinum(0)divinyltetramethyldisiloxane complex (Karstedt catalyst) was used as the catalyst (Scheme 1). The reaction was heated under argon for 24 h at 80 °C in dry toluene (water-free environment). After workup, the hydrosilylated copolymer was analyzed by 1H NMR and FTIR spectroscopy (Figures 7 and 8). Figure 7 shows the 1H NMR spectra of the (PB-b-PEO)3 star block copolymer before and after triethoxysilane hydrosilylation. (51) Guo, X.; Farwaha, R.; Rempel, G. L. Macromolecules 1990, 23, 5047. (52) Guo, X.; Rempel, G. L. Macromolecules 1992, 25, 883. (53) Gabor, A. H.; Lehner, E. A.; Mao, G.; Schneggenburger, L. A.; Ober, C. K. Chem. Mater. 1994, 6, 927. (54) Witte, J.; Guenter, L.; Pampus, G. (to Bayer A. G.) Ger. Offen. 2,344,734, 1975. (55) Kimitake, K. (to Shin-Etsu Chemical Industry Co.) Japan Kokai 7,777,194, 1977. (56) Voigt, H.; Winter, R. (to Kahel und Metallwerke Gutehoffnungshutte) Germ. Offen. 2,646,080, 1978. (57) Uemiya, T.; Osawa, Y.; Shibita, Y.; Nishimura, A.; Niwa, S. (to Sumitomo Electric Industries, Ltd.) Japan Kokai 61,254,625, 1986. (58) Lien, O. S.; Humphreys, R. W. (to Loctite Corp.) U.S. Patent 4,587,276, 1986. (59) Fontanille, M.; Krantz, N.; Gautier, C.; Raynal, S. (Societe Nationale des Poudres et Explosifs) Fr. Demande FR 2,579,982, 1986. (60) Voigt, H.; Muller, G. (to Kahel und Metallwerke Gutehoffnungshutte A. G.) Germ. Offen. 2,554,944, 1977. (61) Tsai, T. (to Copolymer Rubber and Chemical Corp.) U.S. Patent 4,158,765, 1979. (62) Streck, R.; Zerpner, D.; Haag, H.; Nordsick, K. H. (to Chemistry Werke Huels A. G.) Germ. Offen. 2,635,601, 1979. (63) Hempenius, M. A.; Michelberger, W.; Mo¨ller, M. Macromolecules 1997, 30, 5602. (64) Xenidou, M.; Hadjichristidis, N. Macromolecules 1998, 31, 5690. (65) Baum, K.; Baum, J. C.; Ho, T. J. Am. Chem. Soc. 1998, 120, 2993.

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Figure 9. Surface pressure-area isotherms for (PB200-b-PEO326)3 star block copolymer and the corresponding hydrosilylated (PB(Si(OEt)3)-b-PEO)3 star block copolymer before (pH ) 3.0, t ) 0 h) and after (pH ) 3.0, t ) 10 h) cross-linking. Scheme 2. Cross-linking Mechanism Involving Hydrolysis and Condensation of the Triethoxysilane Groups of the Polybutadiene Backbone

Figure 7. 1H NMR spectra (CDCl3; 300 MHz) of (PB200-b-PEO326)3 star copolymer (Run 2, Table 1) and the corresponding hydrosilylated (PB(Si(OEt)3)-b-PEO)3 star block copolymer (Run 5, Table 1).

Figure 8. IR spectra of the (PB200-b-PEO326)3 star block copolymer (Run 2, Table 1) and the corresponding hydrosilylated (PB(Si(OEt)3)b-PEO)3 star block copolymer (Run 5, Table 1).

The 1H NMR spectrum of the (PB-b-PEO)3 starting material was used to determine the distribution of 1,2- and 1,4-units in the polybutadiene block. The two hydrogens of the pendant vinyl carbon in the 1,2-units (dCH2) and the other hydrogens in the double bonds (-CHdCH- and -CHdCH2) having chemical shifts of 4.9 and 5.4 ppm, respectively, the PB block turned out to be composed of 76 mol % of 1,2-PB units. The 1H NMR spectrum of the (PB(Si(OEt)3)-b-PEO)3 star block copolymer revealed a strong decrease in the intensity of the signal corresponding to the -CHdCH2 (δ ) 4.9 ppm) protons of the pendant 1,2-double bonds. Furthermore, the fact that the signal of the -Si-OCH2CH3 methyl protons increased in intensity (δ ) 1.2 ppm) indicated that the reaction occurred with a high efficiency. However, as shown by Figure 7, some pendant double bonds remain after hydrosilylation. Based on the integration values of the signals at δ ) 4.9 ppm (-CHdCH2) and δ ) 0.5 ppm (-CH2-Si-), a conversion of 85% of the 1,2-PB pendant double

bonds was found, knowing that triethoxysilane reacts predominantly with the 1,2-PB units as previously demonstrated.66 This result was confirmed by IR spectroscopy (Figure 8). The absorbance peak at 3100 cm-1 originating from -CHdCH2 double bonds (dCH2 antisymmetric stretch) disappears after hydrosilylation, but there are still remaining unreacted pendant double bonds as demonstrated by the signal at 1640 cm-1 (alkenyl -HCdCH2 stretch), indicating 10 mN/m) to freeze the “thicker” conformation (PEO sublayer underneath a PB network) of the cross-linked material.

The main objective of this study was to propose a new and general method to synthesize a novel two-dimensional polymeric nanomaterial consisting of a continuous cross-linked polybutadiene network containing poly(ethylene oxide) pores of controlled sizes. To reach that goal, novel (PB(Si(OEt)3)-b-PEO)3 star block copolymers were synthesized by hydrosilylating the PB pendant double bonds of (PB-b-PEO)3 star block copolymers with triethoxysilane. The hydrolysis and condensation of the triethoxysilane pendant groups of the (PB(Si(OEt)3)-b-PEO)3 star block copolymer under acidic conditions allowed us to easily cross-link the polybutadiene blocks directly at the air/water interface without any additives or reagents. This demonstrated the improved efficiency of this method compared to the radical polymerization in the presence of AIBN to get a homogeneously cross-linked material with controlled and fixed morphologies. This strategy permits the control of the PEO pore size by simply adjusting the surface pressure during the cross-linking reaction as shown by AFM imaging of the LB films. The characterization of these 2D amphiphilic cross-linked materials are currently under investigation (permeability, small angle scattering, and 2D viscometry) to understand the benefits provided by 2D self-assembly at the air/water interface over conventional solution self-adsorption and other processes. At stake is the possibility to use 2D self-organization as a means to construct materials with anisotropic structures, to reproducibly engineer such structures, and to target defined functions with these materials. In addition, such copolymer silane monolayers could be easily transferred and grafted through covalent bonds to inorganic surfaces (glass support such as silicon wafer) for polymer/inorganic composite synthesis. Work is also in progress to introduce triethoxysilane groups and other metal alkoxides on other polydiene block copolymers of more complex architectures such as triblock copolymers with the aim toward the stabilization of other original 2D and 3D morphologies. Acknowledgment. The authors are grateful to CNRS, the French Ministry of Research, DOE (BES), the University of Florida, and NSF for support of this research. LA062256+