Stimuli-Responsive Polyelectrolyte Block Copolymer Brushes

Kai Yu, Hanfu Wang, Longjian Xue, and Yanchun Han*. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,...
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Langmuir 2007, 23, 1443-1452

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Stimuli-Responsive Polyelectrolyte Block Copolymer Brushes Synthesized from the Si Wafer via Atom-Transfer Radical Polymerization Kai Yu, Hanfu Wang, Longjian Xue, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Graduate School of the Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed July 23, 2006. In Final Form: October 15, 2006 Surface-tethered oppositely charged weak polyelectrolyte block copolymer brushes composed of poly(2-vinyl pyridine) (P2VP) and poly(acrylic acid) (PAA) were grown from the Si wafer by atom-transfer radical polymerization. The P2VP-b-PAA brushes were prepared through hydrolysis of the second PtBA block to the corresponding acrylic acid. The P2VP-b-PAA brushes with different PAA block length were obtained. The P2VP-b-PAA brushes revealed a unique reversible wetting behavior with pH. The difference between the solubility parameters for P2VP and PAA, the changes of surface chemical composition and surface roughness, and the reversible wetting behavior illustrated that the surface rearrangement occurred during treatment of the P2VP-b-PAA brushes by aqueous solution with different pH value. The reversible properties of the P2VP-b-PAA brushes can be used to regulate the adsorption of the sulfonated PS nanoparticles.

Introduction Polymer brushes refer to an assembly of polymer chains which are tethered by one end to a surface or interface.1 Covalent attachment of polymer brushes can be achieved by either “grafting to” or “grafting from” techniques. The “grafting to” techniques often lead to low grafting density and low thickness due to the steric hindrance. To overcome this problem, the “grafting from” approach involving covalent attachment of the initiator layer and subsequent polymerization can be used to grow thick covalent tethered polymer brushes. To achieve maximum control over brush density, polydispersity, and composition, atom-transfer radical polymerization (ATRP) with a living/controlled character has been widely used to grow polymer brushes.2 Polyelectrolyte (PEL) brushes consist of surface-grafted polymer chains containing charged monomer. The structures and properties of PEL brushes are strongly dependent on the Coulombic interaction between polymer chains. The use of a variety of environmental triggers, including pH and ion strength, to control the conformation of PEL brushes demonstrated very pronounced responsive properties of the thin films.3-8 These responsive properties can be exploited in the development of “smart” surface and nanoactuators.9-11 The properties of tethered * Corresponding author footnote: State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China. Tel.: 86-431-5262175. Fax: 86-431-5262126. E-mail: [email protected]. † State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. (1) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (2) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724. (3) Ru¨he, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Grohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. AdV. Polym. Sci. 2004, 165, 79-150. (4) Biesalski, M.; Johannsmann, D.; Ru¨he, J. J. Chem. Phys. 2002, 117, 49884994. (5) Biesalski, M.; Johannsmann, D.; Ru¨he, J. J. Chem. Phys. 2004, 120, 88078814.

PEL brushes on the solid surface have attracted considerable theoretical12-14 and experimental interest.15,16 The theoretical investigation on PEL brushes was commenced with the seminal studies by Pincus12 and continued by Borisov, Birshtein, and Zhulina.17 They demonstrated that the local electroneutrality was the dominant effect in the PEL brushes. Specific complex formation and local change in the solubility of the polymer played a key role in describing the swelling behavior of such brushes. A particular case of the PEL brushes is represented by the weak PEL brushes in which the density of charge is not fixed but dependent strongly on the pH and salt concentration. The reversible change of conformation of the polymer chains and the responsive surface properties of homo weak PEL brushes have been intensively explored.3,4,18-22 Despite the extensive works on the homo PEL brushes, there were few studies23 on the block PEL brushes, which contain positively and negatively charged (6) Farhan, T.; Azzaroni, O.; Huck, W. T. S. Soft Matter 2005, 1, 66-68. (7) Biesalski, M.; Ru¨he. J. Macromolecules 1999, 32, 2309-2316. (8) Biesalski, M.; Ru¨he, J.; Johannsmann, D. J. Chem. Phys. 2000, 111, 70297037. (9) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950-15951. (10) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem., Int. Ed. 2005, 44, 4578-4581. (11) Jones, R. A. L. Soft Machines; Oxford University Press: Oxford, 2004. (12) Pincus, P. Macromolecules 1991, 24, 2912-2919. (13) Alexander, S. J. Phys. 1977, 38, 977. (14) de Gennes, P. G. Macromolecules 1980, 13, 1069-1075. (15) Klein, J.; Kumacheva, E. Science 1995, 269, 816-819. (16) Habicht, J.; Schmidt, M.; Ru¨he, J.; Johansmann, D. Langmuir 1999, 15, 2460-2465. (17) Borisov, O. V.; Birshtein, T. M.; Zhulina, E. B. J. Phys. (France) 1991, 1, 521-526. (18) Zhou, F.; Huck, W. T. S. Chem. Commun. 2005, 5999-6001. (19) Guo, X.; Ballauff, M. Phys. Rev. E 2001, 64, 051406/1-051406/9. (20) Lemieux, M.; Usov, D.; Minko, S.; Stamm, M.; Shulha, H.; Tsukruk, V. V. Macromolecules 2003, 36, 7244-7255. (21) Houbenov, N.; Minko, S.; Stamm, M. Macromolecule 2003, 36, 58975901. (22) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Luzinov, I.; Minko, S. Langmuir 2004, 20, 9916-9919. (23) Osborne, V. L.; Jones, D. M.; Huck, W. T. S. Chem. Commun. 2003, 1838-1839.

10.1021/la062159g CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2006

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monomers within a single polymer chain which provides the responsive controllable interface with nanoscale features.24-26 Triblock copolymer brushes consisting of weak cationic and strong anionic polymers separated by a neutral moiety, poly(2-(methacryloyloxy)ethyl trimethylammonium chloride)-b-PMMA-b-poly(sodium methacrylate), were synthesized by the Huck group.23 The ability to alter PEL brushes in response to external stimuli such as salt concentration was investigated. Recently, a mixed PEL brush consisting of two oppositely charged weak polyelectrolytes, poly(2-vinylpyridine) (P2VP) and poly(acrylic acid) (PAA), was reported.21,22 This kind of brush provided new possibilities to control surface properties on a pH signal. In this paper, ATRP was used to synthesize weak PEL block copolymer brushes, P2VP-b-PAA, in which P2VP and PAA serve as cationic and anionic segments, respectively. These blocks are both weak PEL and have been widely used as model surface in protein adsorption.27-29 The PEL block copolymer brushes exhibited much more complex response than homo PEL brushes due to the larger varieties in combination of Coulombic interaction between the building blocks. We expect that the polymer brushes described in this study not only show responsive surface properties but also provide a model surface in studying the complexation in the PEL brushes and protein adsorption on the PEL brushes.

Yu et al. Scheme 1. Synthesis of P2VP-b-PAA Brushes from the Si Wafer

Experimental Section Materials. 2-Vinylpyridine (97%, contains 0.1 wt % p-tertbutylcatechol as inhibitor, Aldrich) was vacuum-distilled from calcium hydride. tert-Butyl acrylate (98%, contains 10-20 ppm monomethyl ether hydroquinone as inhibitor, Aldrich) was washed three times with 5 wt % sodium hydroxide solution and once with water. After being dried with magnesium sulfate, the monomer was obtained in pure form by distillation from calcium chloride. All of these monomers were stored in a refrigerator immediately after distillation. Toluene and tetrahydrofuran were purified by distillation from sodium/benzophenone ketyl. Acetonitrile was dried with anhydrous potassium carbonate and filtered, followed by distillation from phosphorus pentoxide. Acetone was dried with anhydrous potassium carbonate and filtered, followed by distillation. Tris(2aminoethyl)amine (TREN) (96%) and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) were purchased from Aldrich. Tris[2-(dimethylamino)ethyl]amine (Me6TREN) was prepared by the procedure reported previously.30 1H NMR (300 MHz, CDCl3): δ 2.25 (s, 18H); 2.38-2.43 (t, 6H); 2.6-2.63 (t, 6H) ppm. Trichlorosilane (99%) was obtained from Tangshan Zhongyou Silicon Corp., China. 10-Undecen-1-ol (g95.0%) was purchased from Fluka and used as received. 2-Bromoisobutyryl bromide (98%) and Karstedt’s catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution) were purchased from Aldrich and used as received. The highly polished silicon wafers were purchased from Shanghai Wafer Work Corp., China. They were first cleaned in a freshly prepared “piranha” solution (70/30 v/v, concentrated H2SO4/30% H2O2) at 100 °C for at least 2 h to remove contaminants and generate a hydroxyl functionalized surface. (Warning: Piranha solution is an extremely strong oxidant and should be handled Very carefully!) The silicon wafers were then removed from the solution, sequentially rinsed with copious amounts of distilled water, ultrasonically washed, and then dried with nitrogen gas. (24) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407-2408. (25) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. J. D. Macromolecules 2000, 33, 8821-8827. (26) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813-8820. (27) Hollmann, O.; Czeslik, C. Langmuir 2006, 22, 3300-3305. (28) Li, X.; Wei, X.; Husson, S. M. Biomacromolecules 2004, 5, 869-876. (29) Singh, N.; Husson, S. M. Biomacromolecules 2005, 6, 9-13. (30) Ciampolini, M.; Nardi, N. Inorg. Chem. 1966, 5, 41-44.

Polymerization. Scheme 1 outlined the procedure for preparation of the polyelectrolyte block copolymer brushes, including immobilization of the ATRP initiator silane 1 onto the Si wafer (I), synthesis of Si/SiO2/P2VP brushes (II), synthesis of Si/SiO2/P2VPb-PtBA diblock copolymer brushes (III), and hydrolysis of P2VPb-PtBA brushes to P2VP-b-PAA brushes (IV). Immobilization of the ATRP Initiator Silane onto the Si Wafer. The ATRP initiator used in this study is (11-(2-bromo-2-methyl)propionyloxy)-undecyl-trichlorosilane 1, Br(CH3)2CCOO(CH2)11SiCl3. The initiator was synthesized by using a similar procedure reported in the literature.2 1H NMR (300 MHz, CDCl3): δ 1.281.42 (br m, 16H); 1.55-1.70 (m, 4H); 1.93 (s, 6H); 4.15-4.19 (t, 2H) ppm. The initiator layer was formed on the Si wafer by placing it into an 8 mM initiator anhydrous toluene solution for 19 h without stirring at 60 °C. The initiator-modified Si wafer was thoroughly

Stimuli-ResponsiVe PEL Block Copolymer Brushes rinsed sequentially with toluene, acetone, and anhydrous ethanol followed by drying with nitrogen gas. The initiator-modified Si wafer was either immediately used for surface polymerization or stored in a desiccator under vacuum. Synthesis of Si/SiO2/P2VP Brushes. CuBr2 (1 mg, 0.004 mmol), CuCl (4 mg, 0.04 mmol), and Me6TREN (22 µL, 0.094 mol) were added successively into a 30 mL glass tube followed by adding 15.7 mL of acetonitrile. After a homogeneous green Cu complex solution formed, 4.4 mL of 2-vinylpyridine (0.04 mol) was added and the homogeneous reaction solution was degassed with three freezepump-thaw cycles. In a second 30 mL Schlenk flask, the initiatormodified substrates were added and degassed via evacuation and nitrogen gas backfilling for three cycles. The homogeneous reaction solution was then transferred to the Schlenk flask by syringe. The surface-initiated polymerization with different reaction time was carried out separately under room temperature. After a determined reaction time, the substrates were removed from the Schlenk flask. To remove the untethered substance, the substrates were washed with ethanol and sonicated in ethanol for 10 min. The samples were then rinsed by ethanol thoroughly, followed by drying with nitrogen gas. Synthesis of Si/SiO2/P2VP-b-PtBA Diblock Copolymer Brushes. CuBr2 (6.4 mg, 0.029 mmol), CuBr (14.4 mg, 0.1 mmol), and PMDETA (54 µL, 0.26 mmol) were added into a 20 mL glass tube followed by addition of 5 mL of acetone. After the Cu complex solution formed, 7.1 mL of tert-butyl acrylate (0.049 mol) was added and the heterogeneous reaction solution was degassed with three freeze-pump-thaw cycles. In a second 30 mL Schlenk flask, the initiator-modified substrates were added and degassed via evacuation and nitrogen gas backfilling for three cycles. The reaction solution was stirred at 60 °C for at least 10 min until it became clear and homogeneous. It was then transferred to the Schlenk flask via syringe. The surface-initiated polymerization was allowed to proceed at 60 °C for a determined reaction time. The reaction was stopped by removing the substrates from the Schlenk flask. To remove the untethered substance, the modified substrates were washed with acetone and sonicated in acetone for 10 min. The samples were then rinsed by acetone thoroughly followed by drying with nitrogen gas. Hydrolysis of P2VP-b-PtBA Brushes to P2VP-b-PAA Brushes. To get P2VP-b-PAA brushes, the silicon substrates modified by the P2VP-b-PtBA copolymer brushes were placed into a flask, which contained a mixture of 20 mL of 1,4-dioxane and 3 mL of concentrated HCl (37%). The flask was connected to a condenser. The solution was heated to 80 °C. The samples were removed after 2 h and thoroughly washed with methanol and deionized water. Treatment of the P2VP-b-PAA Brushes by Different pH Aqueous Solution. The P2VP-b-PAA brushes were treated by aqueous solution with different pH values. (A desired amount of aqueous solution was removed and replaced with the corresponding amount of 0.1 M HCl or 0.001 M NaOH solution to adjust the pH value of the aqueous solution to a defined value.) The modified substrate was immersed in a water bath with defined pH value for 10 min and then withdrawn from the water bath and blown dry by nitrogen gas immediately. Adsorption of the Sulfonated PS Nanoparticles onto the P2VPb-PAA Brushes. The PS nanoparticles (mean diameter 238.6 ( 3.2 nm) synthesized by the emulsion method31 were sulfonated with concentrated sulfuric acid at 40 °C for 2 days. The hydrophilicity of the modified PS nanoparticles was due to the introduction of sulfonic acid group.32 A well-dispersed 0.05 wt % suspension of sulfonated PS nanoparticles in water with defined pH value was introduced into a vial, which contained the substrate modified by the P2VP-b-PAA brushes. After incubation for 10 min, the modified substrate was withdrawn from the vial and blown dry by nitrogen gas immediately. (31) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 171-183. (32) Yang, Z. Z.; Li, D.; Rong, J. H.; Yan, W. D.; Niu, Z. W. Macromol. Mater. Eng. 2002, 287, 627-633.

Langmuir, Vol. 23, No. 3, 2007 1445 Characterization. X-ray reflectivity (XRR) was applied to determine the thickness and roughness of the chemisorbed initiator layer and the grafted polymer layer. All measurements were performed with a diffractometer Brucker AXS D8 Discover (Brucker, Germany) operated in reflectivity mode. The X-ray source is an X-ray tube with copper anode working at 40 kV and 40 mA. The wavelength λ of Cu KR is 0.154 nm. The experimental data were analyzed by simulation of the reflectivity curves using the integrated software, “REFSIM”. The thickness and roughness of the initiator layer were evaluated by using a two-layer model Si/SiO2/initiator. (The thickness of the native SiO2 layer is usually 16 ( 2 Å. The native oxide is determined to have a density of 2.65 g/cm3.) The first grafted P2VP layer (typically 1-7 nm) was evaluated with a threelayer model Si/SiO2/initiator/P2VP. Finally, the thickness and roughness of the whole polymer layer after grafting of the second block were calculated using a three-layer model Si/SiO2/initiator/ polymer. The water contact angles were determined using a KRU ¨ SS DSA10MK2 contact angle measuring system (Kru¨ss, Germany) at ambient temperature. The static water contact angle was measured by using deionized water with drop sizes of 2 µL. The static contact angle was determined by using the Young/Laplace fitting method. The advancing water contact angle was read by injecting 2 µL of deionized water into 2 µL of sessile drops. The receding water contact angle was read by withdrawing 3 µL of deionized water from 4 µL of sessile drops. The static and dynamic water contact angles were averaged over three measurements. X-ray photoelectron spectra (XPS) were measured with Thermo ESCALAB 250 (Thermo Electron Corporation, UK) at room temperature by using an Al KR X-ray source (hV ) 1486.6 eV) at an emission angle of 9° originating from a region close to the surface. Pass energy of 50 and 20 eV were used to obtain the survey scan spectra and high-resolution spectra, respectively. Atomic force microscopy (AFM) studies were performed on a commercial scanning probe microscope (SPA300HV with a SPI3800N Probe Station, Seiko Instruments Inc., Japan). The tapping and phase modes were used to study the surface roughness at ambient conditions. Silicon tips with a spring constant of 2 N/m and a frequency of 63-70 kHz were used. Root-mean-square roughness (RMS) was evaluated using the integrated software.

Results and Discussion 1. Synthesis of Si/SiO2/P2VP-b-PAA Block Copolymer Brushes. Initiator Immobilized onto the Si Wafer. The initiator was immobilized onto the Si wafer by the condensation reaction between the hydroxy groups on the treated Si wafer and the trichlorosilane ends of the initiator. Thin film X-ray reflectivity measurements, AFM, water contact angle measurements, and XPS results confirmed that a self-assembled monolayer of the initiator was formed on the Si wafer after 19 h. The thin film X-ray reflectivity measurements for the initiator layer (Br(CH3)2CCOO(CH2)11SiCl3) yielded a thickness of 1.9 ( 0.2 nm with a roughness of 0.49 nm. The result was in good agreement with the theoretical calculation (2.0 nm) based on the normal bonds angles and lengths and assuming a 10° tilt angle for the initiator chain.33 AFM topographical image (Figure 1a) showed that the roughness (RMS ) 0.82 nm) of the initiator layer was larger than that of the underlying Si wafer (RMS ) 0.2 nm). The high surface roughness was contributed mostly by the clusters of polymerized and precipitated initiator (due to the well-known polycondensation reaction of chlorosilanes). The advancing and receding water contact angles (θa and θr) were determined to be 79° ( 2.1° and 70.3° ( 2°, respectively, indicating that the trichlorosilane was deposited in a horizontal polymerization or covalent attachment mechanism. XPS data collected from the initiator layer were presented in Figure 1b. The Br(3d) peak was (33) Granville, A. M. Ph.D. Dissertation, Akron University, Akron, OH, 2004.

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Yu et al.

Figure 2. Dependence of (a) P2VP and (b) P2VP-b-PtBA brushes thickness on the polymerization time.

Figure 1. (a) AFM topographic image and the corresponding cross section of the chemisorbed initiator on the Si wafer; (b) XPS spectrum of the chemisorbed initiator on the Si wafer: Br (3d).

presented at 69.7 eV, which confirmed the formation of the initiator layer on the Si wafer. Synthesis of Si/SiO2/P2VP Brushes. The ATRP of 2-vinyl pyridine (2VP) from the initiator-modified Si wafer was conducted first. The surface-initiated polymerization of 2VP was performed using a modified procedure of Husson’s work.28 The silicon wafer was used as the substrate instead of the Au substrate in Husson’s work since the silane/silicon interface (the Si-O bond dissociation energy is 90-133 kcal‚mol-1 34-36) is stronger than that of the thiol/gold interface. (The S-Au bond energy is 3040 kcal‚mol-1 and will be broken when the temperature rises to 60 °C.) Polymerization of 2VP poses a very challenging problem for ATRP because both 2VP and P2VP are strong coordinating ligands that can compete for binding of the metal catalyst in these systems. The possibility of formation of pyridinecoordinated metal complex during polymerization turns out to be much stronger since the monomer is in a large excess over the employed ligand. To overcome the significant coordination, tris[2-(dimethylamino)ethyl]amine (Me6TREN) was used as the ligand as proposed by Matyjaszewski’s work on ATRP of (34) Tu, H.; Heitzman, C. E.; Braun, P. V. Langmuir 2004, 20, 8313-8320. (35) Dyer, D. J. AdV. Funct. Mater. 2003, 13, 667-670. (36) Walsh, R.; Becerra, R. In Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2 (Pt. 1), 153-180.

4-vinylpyridine.37 For ATRP from a surface, the small amount of initiator tethered to the substrate provides a too low concentration of Cu(Π) to control the polymerization. To ensure a sufficient concentration of deactivating Cu(Π) species, CuBr2 was added to the reaction solution. With use of the CuCl/Me6TREN complex as the catalyst and CuBr2 as the deactivator, the growth of P2VP brushes (Figure 2a) showed a linear increase in brush thickness with time. Deviation from linearity increase of thickness occurred at longer reaction time (>5 h). It was possibly due to the progressive deactivation of the copper catalyst through complexation with the pyridine group38 or biomolecular termination.28 To elucidate this problem, we added the fresh catalyst39 at the 6 h timepoint where the deviation from linear increase of thickness occurred. At the 8 h timepoint, there was no obvious increase of thickness compared with the one without fresh catalyst added (Figure S1, Supporting Information). Furthermore, the growth of thickness did not resume when using the P2VP brushes (polymerized for 8 h) as macroinitiator to initiate the second P2VP block. We concluded that the nonlinear behavior at longer polymerization time was a result of bimolecular termination not catalyst deactivation. Compared to the other polymer brushes (PMMA brushes) synthesized by the “grafting from” approach (usually have thickness of tens of nanometers40), the limited thickness of P2VP brushes was due to the low ratio of propagation constants (kp) to termination constant (kt) for 2VP compared with those for other monomers.41 The static water contact angle of the P2VP brushes was 62° ( 1.3° after 5 h ATRP of 2-vinyl pyridine. AFM was performed to examine the surface roughness of the P2VP brushes. Figure 3b showed a typical 3D topographic scan of the P2VP brushes. The surface was relatively smooth and uniform with a rootmean-square roughness (RMS), 0.9 nm. X-ray reflectivity was also used to study the nature of P2VP brushes on the Si wafer with different polymerization time (Figure 3c). The data gave a roughness of 0.78 nm (X-ray emission slitter, 0.2 cm) after 6 h ATRP of 2-vinyl pyridine, confirming that the surface was smooth. (37) Xia, J.; Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 35313533. (38) Xia, Y.; Yin, X.; Burke, N. A. D.; Stover, H. D. H. Macromolecules 2005, 38, 5937-5943. (39) Wei, X.; Li, X.; Husson, S. M. Biomacromolecules 2005, 6, 1113-1121. (40) Jones, D. M.; Huck, W. T. S. AdV. Mater. 2001, 13, 1256-1259. (41) Bao, Z.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2006, 128, 9056-9060.

Stimuli-ResponsiVe PEL Block Copolymer Brushes

Figure 3. AFM (a) topography and (b) 3D image of the P2VP brushes with 52 Å thickness. (c) X-ray reflectivity curves of the P2VP brushes growth from (A) chemisorbed initiator on the Si wafer as a function of reaction time: (B) 1 h, (C) 4 h, and (D) 6 h.

Synthesis of Si/SiO2/P2VP-b-PtBA Brushes. The PtBA block can be successively grown from the P2VP macroinitiator by the “living” characterization of ATRP. One of those substrates modified by P2VP serving as macroinitiator was chain extended with 4 M solution of tert-butyl acrylate in acetone solution that contained 8 mM CuBr/PMDETA and 2.4 mM CuBr2/ PMDETA catalyst with a Cu-to-ligand molar ratio of 1:2. Figure 2b showed the thickness as a function of polymerization time for the PtBA brushes grown from the substrates modified by P2VP. Up to 8.5 h, 4.4 nm thick PtBA block was grown on top of 5.2 nm thick P2VP brushes. The static water contact angle increased progressively to the characteristic value of homo PtBA brushes (86° ( 2°) with increasing the PtBA block length from 1 to 4.4 nm. Figure 4a showed the AFM image for the surface morphology of the P2VP-b-PtBA brushes. The surface roughness (RMS) was 0.8 nm over a 1 µm × 1 µm scan area. The XRR spectra of the P2VP-b-PtBA brushes on the Si wafer with different reaction times were shown in Figure 4c. For the PtBA brushes growth on P2VP layer up to 8.5 h, the roughness was 0.72 nm, confirming that the surface was smooth. Hydrolysis of P2VP-b-PtBA Brushes to P2VP-b-PAA Brushes. The PtBA block was converted to PAA by hydrolysis of the tert-ester group. The thickness and water contact angle change were provided to confirm that the hydrolysis was complete after reaction for 2 h. After reaction for 2 h, the initial thickness of the whole layer (P2VP-b-PtBA brushes with 5.2 nm P2VP and 4.4 nm PtBA) decreased rapidly by 23%. With time elapsed, the thickness of the whole layer showed a slower decrease. For example, the decrease of thickness in the next 2 h was about 14%. Wu has estimated the change of thickness of the PtBA film after complete hydrolysis is about 52%.42 The 23% decrease of thickness for the whole layer (P2VP-b-PtBA brushes) corresponded to 50% decrease of thickness for the PtBA layer, which agreed well with Wu’s estimation. Since the reaction rate for tert-ester hydrolysis is much faster than that for initiator primary ester hydrolysis, we attributed the decrease of the film (42) Wu, T. Ph.D. Dissertation, North Carolina State University, 2003.

Langmuir, Vol. 23, No. 3, 2007 1447

Figure 4. AFM (a) topography and (b) 3D image of the P2VPb-PtBA brushes with 96 Å thickness. (c) X-ray reflectivity curves of the P2VP-b-PtBA brushes growth from (A) a silicon substrate composed of a 5.2 nm thick P2VP layer as a function of reaction time: (B) 4 h and (C) 8.5 h.

thickness in the first 2 h to the removal of the bulky tert-butyl groups. The slower decrease at the later time may be associated with possible cleavage of the polymer from the substrate. We also characterized the reaction of hydrolysis by water contact angle. For simplicity, the wetting behavior of homo PtBA brushes associated with conversion time was investigated. The static water contact angle of homo PtBA brushes after hydrolysis for 2 h dropped from 86° ( 2° to 46° ( 3.1° (characteristic value of PAA layer). With time elapsed, the static water contact angle almost stayed constant. The result of water contact angle also confirmed that the reaction of hydrolysis almost completed in the first 2 h. 2. Wettability Reversible Behavior of P2VP-b-PAA Brushes. The dependence of static water contact angle (θw) on pH was studied for P2VP-b-PAA brushes, homo P2VP brush, and homo PAA brush, respectively. For homo P2VP brush, the static water contact angle increased from 31° ( 0.7° at pH ) 1.5 to 62° ( 1.3° at pH ) 8, and then it stayed at a constant value with further increasing the pH value (Figure 5a). The homo PAA brush demonstrated an inverse behavior (Figure 5b). The static water contact angle decreased from 46° ( 3.1° at pH ) 1.5 to 15° ( 1.8° at pH ) 8, and then it almost stayed at a constant value with further increasing the pH value. The P2VP-b-PAA brushes revealed a unique wetting behavior with pH (Figure 6). Three samples with the same thickness of P2VP, 5.2 nm, while the thickness of PAA changed from 1 to 1.4 and 2.2 nm, respectively, demonstrated similar wetting behavior with pH. For example, for the P2VP-b-PAA brushes with 5.2 nm P2VP and 2.2 nm PAA, the static water contact angle increased from 33° ( 1.5° at pH ) 1.5 to 54.7° ( 0.9° at pH ) 5.6-8, and then it decreased to 38° ( 1.5° at pH ) 11. The pH-responsive wettability of the P2VP-b-PAA brushes was reversible for repeated cycle of alternating treatment by aqueous solution with pH ) 1.5, 5.6, and 11 (Figure 7). The reversible change of static water contact angle for the P2VP-b-PAA brushes was correlated with the chemical nature of the PEL brushes. Each block of the PEL brushes is weak PEL.

1448 Langmuir, Vol. 23, No. 3, 2007

Figure 5. Static water contact angle as a function of pH for P2VP brushes (a) and PAA brushes (b), respectively.

Figure 6. Dependence of the static water contact angle on pH for the P2VP-b-PAA brushes with different PAA block length (PAA/ P2VP): (a) 2.2 nm/5.2 nm, (b) 1.4 nm/5.2 nm, and (c) 1 nm/5.2 nm.

The charge density of the weak PEL depends on pH. The isoelectric point (IEP) of P2VP and PAA is 6.7 and 3.2, respectively.21 P2VP is protontated at pH < 6.7. The charge density of P2VP chains and the degree of protonation increase with a decrease of pH. PAA is dissociated at pH > 3.2. The charge density of PAA chains and the degree of dissociation increase with an increase of pH. At pH ) 1.5 and pH ) 11, the static water contact angle was low because the polymer-air interface was preferentially occupied by the protonated P2VP block and dissociated PAA block, respectively, which was convinced by the comparison of water contact angle between the block PEL brushes and homo PEL brushes. In the range 5.6 < pH < 8, the protonated P2VP and dissociated PAA formed the polyelectrolyte complex43 which led to a maximum value of static water contact angle.44 The stepwise increase of static water contact angle from pH)3.2 to pH)5.6 and stepwise decrease (43) Gohy, J.-F.; Khousakoun, E.; Willet, N.; Varshney, S. K.; Jerome, R. Macromol. Rapid Commun. 2004, 25, 1536-1539. (44) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Angew. Chem., Int. Ed. 2006, 45, 1770-1774. (45) Akgun, B.; Baum, M.; Bickle, C.; Boyes, S. G.; Granville, A. M.; Mirous, B.; Zhao, B.; Brittain, W. J.; Foster, M. D. AdV. Polym. Sci. 2006, 198, 125-147. (46) Hansen, C. M.; Beerbower, A. Solubility Parameters. In Kirk-Othmer Encyclopedia of Chemical Technology, 2nd ed.; Standen, A., Ed.; Interscience: New York, 1971: Supp. Vol., pp 889-910.

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Figure 7. Reversible change of the static water contact angle for the P2VP-b-PAA brushes (with 5.2 nm P2VP and 2.2 nm PAA) under the alternating treatment by aqueous solution with pH 1.5, 5.6, and 11, respectively.

of static water contact angle from pH)8 to pH)11 were associated with the dependence of charge density of the P2VP and PAA block on pH. Zhao and Brittain reported that the tethered PS-b-PMMA brushes on the silicon substrate underwent reversible change on contact angle as the diblock copolymer brushes were treated with different solvents.26 Initially, the film exhibited a contact angle characteristic of PMMA; following treatment with cyclohexane (a better solvent for PS than for PMMA), the water contact angle increased to a characteristic value for PS. Subsequently, treatment of the same sample with CH2Cl2 (a good solvent for PMMA and PS) reversed this change. They attributed the reversible wetting behavior to the surface rearrangement during solvent treatment. Treating the polymer brushes with a good solvent for two blocks enriched the outer PMMA block at the topmost layer, while treating the polymer brushes with a solvent that is a good solvent for the inner block and a poor solvent for the outer PMMA block would induce the inner PS block to migrate to the solvent interface to form a shield around the PMMA aggregates. We also studied the relation between the reversible wetting behavior and the surface rearrangement during treatment of the P2VP-b-PAA brushes by aqueous solution with different pH values. The difference between the solubility parameters for P2VP and PAA, the surface chemical composition by XPS analysis, and surface roughness by AFM and XRR were used to illustrate the surface rearrangement during treatment of the P2VP-b-PAA brushes by aqueous solution with different pH value. We calculated the respective solubility parameters for P2VP and PAA to elucidate the possibilities of surface rearrangement. Brittain and co-workers showed that, for block copolymer brushes, the larger the difference in solubility parameters, the greater the interaction parameter for the system and the lower the chance for complete rearrangement.45 The solubility parameters of P2VP and PAA calculated by the group contribution method as outlined by Hansen and van Krevelen are 21.1 and 23.6 (J/cm3)1/2, respectively.46,47 The difference between the solubility parameters for P2VP and PAA is relatively small. The two blocks can become (47) van Krevelen, D. W. Properties of Polymers: Their Estimation and Correlation with Chemical Structure; Elsevier Science: Amsterdam, 1976; pp 136-143.

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Figure 8. XPS survey scan spectra of the P2VP-b-PAA brushes after treatment by aqueous solution with pH 1.5 (a) and pH 11.3 (b). Table 1. Relative Molar Ratios of Carbon to Oxygen (C/O) and Carbon to Nitrogen (C/N) in Different pH Condition molar ratios of C/N and C/O for P2VP-b-PAA brushes molar ratio molar ratio ofC/N for ofC/O for PAA P2VP pH condition pH 1.5 pH 11.3

7

1.5

C/N

C/O

9.6 14.7

9 5.3

miscible with each other, and therefore the P2VP-b-PAA brushes have a chance to rearrange. Because the reversible wetting behavior of the P2VP-b-PAA brushes was correlated with the chemical nature of the topmost layer, XPS was used to determine the chemical composition at the topmost layer after the PEL brushes were treated by aqueous solution with different pH values. In the XPS experiments, the emission angle relative to the surface is 9°. The sampling depth is about 0.8-1.6 nm based on the equation t ) d sin θ, where t is the sampling depth at an emission angle of θ and d is the sampling depth at an emission angle relative to the surface of 90°. (The sampling depth at 90° is approximately 5-10 nm depending on the core level binding energy.) Figure 8 depicted the XPS survey spectra for the P2VP-b-PAA brushes with 5.2 nm P2VP and 2.2 nm PAA after treatment with pH 1.5 (Figure 8a) and pH 11.3 (Figure 8b), respectively. The relative molar ratios of carbon to oxygen (C/O) and carbon to nitrogen (C/N) in different pH conditions (Table 1) were calculated based on the mole percentage of each element (Table S1, Supporting Information) as quantified from the survey scan spectra. The molar ratio of C/N increased from 9.6 for pH ) 1.5 to 14.7 for pH ) 11.3, while the molar ratio of C/O decreased from 9 for pH ) 1.5 to 5.3 for pH ) 11.3. At low pH, the molar ratio of C/N at the topmost layer was close to the theoretical value for P2VP, while the molar ratio of C/O was far above the theoretical value for PAA. It indicated that, at low pH, the topmost layer was preferentially occupied by P2VP chains, consistent with the result of water contact angle. While at high pH, the molar ratio of C/O at the topmost layer was close to the theoretical value for PAA, but the molar ratio of C/N was far above the theoretical value for P2VP. It indicated that, at high pH, PAA chains tended to preferentially occupy the topmost layer, which was consistent with water contact angle observations.

Figure 9. XPS spectra of (a) N(1s) and (b) C(1s) of the P2VPb-PAA brushes after treatment by aqueous solution with different pH values.

Figure 9 showed the high-resolution spectra of N(1s) and C(1s) of the same sample (P2VP-b-PAA brushes with 5.2 nm P2VP and 2.2 nm PAA) treated by aqueous solution with pH 1.5 and pH 11.3, respectively. The N(1s) spectrum (Figure 9a) was decomposed into two component peaks, which were attributed to pyridine group (399.1 eV) and protonated pyridine group (401.7 eV), respectively. The corresponding relative percentage of the two peaks was 92.5% and 7.5%. (The protonation percentage of the whole layer was 21% based on analysis of the XPS spectra obtained at an emission angle of 60° (Figure S2, Supporting Information). At this emission angle, the sampling depth is 4.38.7 nm, which is close to the thickness of the P2VP-b-PAA brushes.). The intensity of the N(1s) peak increased by 320% after treatment with pH 1.5 as compared with that after treatement with pH 11.3. It indicated that the percentage of P2VP present at the topmost layer at pH 1.5 increased as compared with that at pH 11.3. In addition, the presence of peak situated at 401.7 eV demonstrated that the topmost layer was partially occupied by the protonated pyridine groups and therefore the surface became positively charged. The C(1s) spectrum (Figure 9b) can be decomposed into two component peaks. They were attributed to C-C group (284.6 eV) and COOR group (chemical shift varies from 287.9 to 288.9 eV, depending strongly on the chemical environment of element), respectively. The intensity of the C(1s) peak (Figure 9b) decreased by 213% after treatment with pH 11.3 as compared with that after treatment with pH 1.5, while

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Figure 10. The (a, b, c) height and (d, e, f) phase images of the P2VP-b-PAA brushes after treatment by aqueous solution with different pH values: (a, d) 1.5, (b, e) 5.6, and (c, f) 11.

the intensity ratio of the COOR component to C-C component increased by 170% (intensity of the COOR component was normalized to that of the C-C component) when the pH value was increased from 1.5 to 11.3. It indicated that the percentage of PAA present at the topmost layer at pH 11.3 increased as compared with that at pH 1.5. In addition to an increase of intensity ratio at pH 11.3, there was a shift of peak position from 288.9 to 287.9 eV, which corresponded to acrylic acid group and sodium acrylate group, respectively. It demonstrated that, at pH 11.3, the topmost layer was partially occupied by the dissociated carboxylic acid groups and therefore the surface became negatively charged. Surface rearrangement not only brought the reversible change of water contact angle but also the change of surface roughness.25 To further investigate the surface rearrangement, AFM and XRR were used to study the change of surface roughness after the P2VP-b-PAA brushes were treated by aqueous solution with different pH values. Figure 10 compared the surface morphologies of the P2VP-b-PAA brushes with 5.2 nm P2VP and 2.2 nm PAA after treatment by aqueous solution with different pH values. At low and high pH, the surface fluctuating amplitude and the phase contrast were 5 nm and 4°, respectively. At pH 5.6 (between the

Figure 11. X-ray reflectivity curves of the P2VP-b-PAA brushes after treatment by aqueous solution with different pH values: (a) 1.5, (b) 5.6, and (c) 11.

Stimuli-ResponsiVe PEL Block Copolymer Brushes

Figure 12. The sulfonated PS nanoparticles were adsorbed onto the P2VP-b-PAA brushes (5.2 nm P2VP and 2 nm PAA) after incubation in the aqueous solution with different pH values: (a) 1.5 and (b) 11.

IEP of PAA and P2VP), the surface fluctuating amplitude and the phase contrast were both remarkably enhanced to 10 nm and 7°, respectively. At low and high pH, the topmost layer was partially occupied by protonated P2VP block and dissociated PAA block as probed by XPS measurement, respectively. The protonation and dissociation caused Coulombic repulsion occurring between the same charges (cation for P2VP or anion for PAA) on the neighbor polymer chains. While at pH 5.6, there (48) Mei, Y.; Wittemann, A.; Sharma, G.; Ballauff, M.; Koch, Th.; Gliemann, H.; Horbach, J.; Schimmel, Th. Macromolecules 2003, 36, 3452-3456.

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was a favorable interaction between the protonated P2VP and negatively charged PAA block due to the Coulombic effect, leading to formation of the polyelectrolyte complex, which reduced the Coulombic repulsion between the neighboring chains. The ion pairing between the protonated pyridine groups and dissociated carboxylic acid groups in the same chains or neighbor chains led to a collapsed state. Therefore, the PEL brushes showed enhanced surface fluctuating amplitude and phase contrast as compared with those at low and high pH. The change of surface roughness was also seen nicely from X-ray reflectivity curves (Figure 11) obtained after the sample was exposed to aqueous solution with different pH value. The film roughness of the P2VP-b-PAA brushes with 5.2 nm P2VP and 2.2 nm PAA after treatment with pH 1.5 (a), 5.6 (b), and 11 (c) were 0.87, 1.15, and 0.86 nm, respectively. 3. Reversible Adsorption of the Sulfonated PS Nanoparticles onto the P2VP-b-PAA Brushes. The reversible properties of the P2VP-b-PAA brushes can be used to regulate the adsorption of charged nanoparticles. After sulfonation, the PS nanoparticles were modified inwardly, forming a layer of poly(styrene sulfonic acid). The degree of sulfonation of PS nanoparticles was 69% based on XPS analysis (Figure S3, Supporting Information). The average size of the sulfonated PS nanoparticles was 199.3 ( 4.7 nm (Figure S4, Supporting Information). Poly(styrene sulfonic acid) is a strong polyelectrolyte. It always carries negative charges despite the pH value of the aqueous solution. After the P2VP-b-PAA brushes modified substrate was immersed in an aqueous solution (pH 1.5) which contained the sulfonated PS nanoparticles, the protonated P2VP block segregated to the topmost layer and the surface got positively charged. The Coulombic interaction between the positively charged surface and negatively charged nanoparticles led to the preferential adsorption of the modified PS nanoparticles from the solution to the P2VP-b-PAA brushes. The preferential adsorption owing to Coulombic interaction was also confirmed by the evidence that the sulfonated PS nanoparticles were not adsorbed onto the bare Si substrate since there is no Coulombic interaction between the charged nanoparticle and the uncharged surface. Figure 12a showed that rather small aggregated PS nanoparticles were deposited on the modified substrate. Once a particle came into contact with the P2VP-b-PAA brushes, it was attached to the substrate by the Coulombic attraction. The attachment partially counterbalanced the strong capillary forces that led to a dense packing of the particles during the drying process. As a consequence of the sticking of the particles to the surface, rather small aggregates of the modified PS nanoparticles were formed upon drying.48 While at high pH value, the topmost layer was partially occupied by the dissociated PAA segments. The Coulombic repulsion between the negatively charged surface and negatively charged nanoparticles led to the nonadsorption of the modified nanoparticles from the solution to the substrate (Figure 12b). The regular adsorption also verified the reversible charge change of the P2VP-b-PAA brushes after treatment by aqueous solution with different pH value.

Conclusion In summary, ATRP was used to synthesize P2VP-b-PAA brushes from the Si wafer with different PAA block length. The wetting property of the P2VP-b-PAA brushes was regulated by pH reversibly. The difference between the solubility parameters for P2VP and PAA, the changes of surface chemical composition and surface roughness, and the reversible wetting behavior illustrated the surface rearrangement occurred during the treatment of the P2VP-b-PAA brushes by aqueous solution with different

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pH value. The responsive change of surface charge of the P2VPb-PAA brushes was explored to regulate the adsorption of sulfonated PS nanoparticles. Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (20334010, 50573077). Supporting Information Available: X-ray reflectivity curves

Yu et al. of P2VP brushes with and without addition of fresh catalyst during polymerization; XPS spectra of N(1s) of P2VP-b-PAA brushes after treatment by aqueous solution with pH 1.5 at an emission angle of 60°; XPS survey scan spectra of modified PS nanoparticles; TEM image of the modified PS nanoparticles; The mole percentages of carbon, nitrogen, and oxygen as quantified from the survey scan spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA062159G