pubs.acs.org/Langmuir © 2010 American Chemical Society
Stereoregulation of Thermoresponsive Polymer Brushes by Surface-Initiated Living Radical Polymerization and the Effect of Tacticity on Surface Wettability. Naokazu Idota,† Kenichi Nagase,‡ Keiji Tanaka,§ Teruo Okano,‡ and Masahiko Annaka*,† †
Department of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, ‡Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawadacho, Shinjuku-ku, Tokyo 162-8666, Japan, and § Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Received June 14, 2010. Revised Manuscript Received October 13, 2010 In this study, stereocontrolled poly(N-isopropylacrylamide) (PIPAAm) brushes were grafted from surfaces by atom transfer radical polymerization (ATRP) in the presence of a Lewis acid, and the effect of PIPAAm brush tacticity on the thermoresponsive wettabiliy was investigated. PIPAAm grafted by ATRP in the presence of Y(OTf)3 showed high isotacticity, while the control brush polymerized in the absence of Y(OTf)3 was clearly atactic. The isotacticity and molecular weight of PIPAAm brushes were controlled by polymerization conditions. The wettability of isotactic PIPAAm-grafted surfaces decreased slightly below 10 °C, although the phase transition temperature of atactic surface was 30 °C, and the bulk isotactic polymer was water-insoluble between 5 and 45 °C.
Introduction In the past decade, controlled/living radical polymerization (CRP) methods1-5 have been developed to regulate polymer chain growth by reversible activation/deactivation of radical species. Functional macromolecules with controlled nanostructures can be obtained simply by optimizing the CRP conditions through the selection of initiator and monomer and the control of active/dormant species.3-5 Although conventional CRP has not been successful in producing stereoselective chain growth, useful methods for providing significant tacticity control have been achieved with the help of a polar solvent6-8 and Lewis acid.9-13 These reagents interact catalytically with the polar moieties of monomers and/or propagating polymers around radical species for continuous stereospecific chain growth.14,15 These new CRP methods can be used to synthesize polymers with well-defined primary structures and physicochemical properties. *To whom correspondence should be addressed. E-mail: annaka@chem. kyushu-univ.jp. Telephone: þ81-92-642-2594. Fax: þ81-92-642-2607.
(1) Favier, A.; Charreyre, M.-T. Macromol. Rapid Commun. 2006, 27, 563. (2) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661. (3) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (4) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. Adv. Polym. Sci. 2006, 197, 1. (5) Matyjaszewski, K.; Tsarevsky, N. V. Nat. Chem. 2009, 1, 276. (6) Isobe, Y.; Yamada, K.; Nakano, T.; Okamoto, Y. Macromolecles 1999, 32, 5979. (7) Hirano, T.; Okumura, Y.; Kitajima, H.; Seno, M.; Sato, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4450. (8) Hirano, T.; Kamikubo, T.; Okumura, Y.; Bando, Y.; Ymaoka, R.; Mori, T.; Ute, K. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2539. (9) Isobe, Y.; Fujioka, D.; Habaue, S.; Okamoto, Y. J. Am. Chem. Soc. 2001, 123, 7180. (10) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2003, 36, 543. (11) Lutz, J.-F.; Neugebauer, D.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6986. (12) Ray, B.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M.; Seno, K.; Kanaoka, S.; Aoshima, S. Polym. J. 2005, 37, 234. (13) Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Saha, S.; Ray, B. Langmuir 2010, 26, 6775. (14) Habaue, S.; Okamoto, Y. Chem. Rec. 2001, 1, 46. (15) Satoh, K.; Kamigaito, M. Chem. Rev. 2009, 109, 5120.
Langmuir 2010, 26(23), 17781–17784
Surface modification of a solid material with polymer is a valuable technique for improving physicochemical properties including wettability and molecular interactions.16,17 Thermoresponsive poly(N-isopropylacrylamide) (PIPAAm) is well-known to exhibit a thermoresponsive soluble-insoluble phase transition across its lower critical solution temperature (LCST) at 32 °C in aqueous solution,18 and, when employed as a surface modification polymer, it is a good candidate for dynamic control of surface properties by external stimuli. Several reports have described PIPAAm modified solid substrates that show temperaturedependent reversible changes in wettabilitiy and the adsorption/ release of molecules.19-23 Using surface-initiated atom transfer radical polymerization (ATRP), chain length, graft density, and monomer sequence have been investigated as PIPAAm surface graft parameters that affect temperature-dependent reversible changes.24-27 However, there are few reports showing the control of stereoregularity of grafted polymers using this method. Tacticity in bulky PIPAAm is known to largely influence its thermally induced phase transition in water.7-9,12,13,28 In addition, (16) Lampin, M.; Warocquier-Clerout, R.; Legris, C.; Degrange, M.; Sigot-Luizard, M. F. J. Biomed. Mater. Res. 1997, 36, 99. (17) Vogler, E. A. Adv. Colloid Interface Sci. 1998, 74, 69. (18) Heskins, M.; Guillet, J. E.; James, E. J. Macromol. Sci., Chem. 1968, A2, 1441. (19) Kikuchi, A.; Okano, T. Prog. Polym. Sci. 2002, 27, 1165. (20) Nath, N.; Chilkoti, A. Adv. Mater. 2002, 14, 1243. (21) Huber, D. L.; Manginell, R. P.; Samara, M. A.; Kim, B.-I.; Bunker, B. C. Science 2003, 301, 352. (22) Ebara, M.; Hoffman, J. M.; Hoffman, A. S.; Stayton, P. S. Lab Chip 2006, 6, 843. (23) Yang, J.; Yamato, M.; Shimizu, T.; Sekine, H.; Ohashi, K.; Kanzaki, M.; Ohki, T.; Nishida, K.; Okano, T. Biomaterials 2007, 28, 5033. (24) Idota, N.; Kikuchi, A.; Kobayashi, J.; Akiyama, Y.; Sakai, K.; Okano, T. Langmuir 2006, 22, 425. (25) Yim, H.; Kent., M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420. (26) Li, D. X.; Cui, Y.; Wang, K. W.; He, Q.; Yan, X. H.; Li, J. B. Adv. Funct. Mater. 2007, 17, 3134. (27) Nagase, K.; Kobayashi, J.; Kikuchi, A.; Akiyama, Y.; Kanazawa, H.; Okano, T. Langmuir 2008, 24, 511. (28) Katsumoto, Y.; Kubosaki, N. Macromolecules 2008, 41, 5955.
Published on Web 11/01/2010
DOI: 10.1021/la1024229
17781
Letter
Idota et al.
stereoregulated polymers can show unique characteristics, such as molecular recognition29 and strong mechanical properties.30 Therefore, tacticity control of grafted PIPAAm can promise to modulate thermoresponsive surface properties. In this study, a stereocontrolled PIPAAm brush surface was prepared, and the effects of polymer brush tacticity on thermally induced aqueous wettability changes were investigated. PIPAAm brushes were grafted from both silica nanoparticles and silicon surfaces by surface-initiated ATRP with isospecific catalysis. By cleaving the polymer brushes from the silica nanoparticles, optimized conditions for controlling tacticity and molecular weight were found. Grafting polymer brushes from planar silicon surfaces enabled the investigation of the role of tacticity in temperature-dependent wettability changes.
Experimental Section Surface Modification with PIPAAm by Surface-Initiated ATRP in the Presence of a Lewis Acid. Silicon wafers (10
30 mm2; average thickness, 525 μm) (SUMCO) were cleaned by vacuum ultraviolet (VUV) exposure (wavelength, 172 nm; intensity, 20 W) for 10 min by using an Excimer VUV irradiation system (H0011, USHIO), and were immediately put in the bottom of a separable flask with an ATRP initiator activated silane, 3-(chloromethylphenyl)ethyltrimethoxysilane (2 mL) (Gelest). Silanization on silicon wafers proceeded for 1 h at 70 °C under reduced pressure (100 mmHg). The ATRP initiator-immobilized surfaces were baked at 110 °C for 1 h. Fumed silica nanoparticles (average diameter, 12 nm; specific surface area, 200 ( 25 m2/g) (Evonik Degussa) were also treated with ATRP initiator according to previous reports.27 Isospecific bulk polymerization methods for N-isopropylacrylamide (IPAAm)9-13 were performed on ATRP initiator-immobilized surfaces. Briefly, IPAAm (4.52 g), which was kindly provided by Kohjin, and Y(OTf)3 (1.07 g) (Aldrich) were dissolved in dried methanol(20 mL), and this solution was then degassed by freezepump-thaw cycles. After being degassed, the monomer solution was poured into another flask containing CuCl (40 mg), CuCl2 (5.4 mg) (Wako Pure Chemicals), and Me6TREN (126 μL), which was synthesized from tris(2-aminoethyle)amine (Acros),24 under Ar gas flow. Either the ATRP-initiator immobilized silicon wafers or silica nanoparticles were added to the solution after stirring for 30 min, and the polymerization proceeded at a specific temperature (-20 to 40 °C) for 24 h. The polymerization was stopped by exposing the solution to air, and the substrates were then rinsed with methanol, hot dimethyl sulfoxide (DMSO), and distilled water and dried at 25 °C under vacuum. Characterization of Surface-Grafted PIPAAm. For estimating the amount of immobilized ATRP initiator on the silica nanoparticles, elemental analysis was performed by using a CHN elemental analyzer (VarioEL, Elementar). The amount of immobilized ATRP initiator on the surfaces was calculated by the following equation:27 immobilized ATRP initiator ¼
%Cl %Cl S %ClðcalcdÞ 1 %ClðcalcdÞ
ð1Þ where %Cl is the amount of chlorine (%) as determined by elemental analysis, %Cl(calcd) is the calculated weight percent of chlorine in the initiator subtracted by the weight of the methoxy groups, and S (m2/g) is the specific surface area of the silica support. For retrieving surface-grafted PIPAAm from silica nanoparticles, PIPAAm-grafted nanoparticles were treated overnight with 10 mol/L sodium hydroxide aqueous solution. After (29) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013. (30) Rosa, C. D.; Auriemma, F. Prog. Polym. Sci. 2006, 31, 145.
17782 DOI: 10.1021/la1024229
Figure 1. 1H NMR spectra of grafted poly(N-isopropylacrylamide) (PIPAAm) brush prepared by surface-initiated ATRP in (A) the absence and (B) the presence of Y(OTf)3. NMR conditions: 600 MHz, DMSO-d6, 150 °C. Chemical structure in the figure is PIPAAm. The numbers [(1)-(5)] near peaks of the spectra correspond to the numbers near the moieties of PIPAAm structure. Markers “m” and “r” of (1) peaks indicate meso and racemo dyads in the polymer, respectively. Asterisks identify the aromatic protons of ATRP initiator in PIPAAm chain ends.
being neutralized by adding hydrochloric acid, the solution was dialyzed against distilled water using a dialysis membrane (Spectra/Por standard regenerative cellulose dialysis membrane; MWCO, 1000) for 2 days with several changes of distilled water. The grafted polymer was then obtained from the dialyzed solution by lyophilization. The molecular weight and tacticity of the grafted PIPAAm were determined by 1H NMR measurement in DMSO-d6 using a 600 MHz spectrometer (DRX-600, Baker). An attenuated total reflection/Fourier transform infrared (ATR/ FT-IR) spectrometer (FT/IR-4200, JASCO) was used to characterize PIPAAm-grafted silicon wafers. The wettability of PIPAAm-grafted surfaces was measured by using a contact angle meter (type CA-DT, Kyowa Interface Science). Sample temperature was regulated via a circulating water bath within a deviation of (0.1 °C.
Results and Discussion The amount of immobilized ATRP initiator on the silica nanoparticles was determined to be 3.34 μmol/m2 by elemental analysis. In this study, the molar ratio of IPAAm to ATRP initiator was fixed ([IPAAm]0/[initiator]0 = 100), where the fed ATRP initiator was calculated from the amount of immobilized initiator on the silica nanoparticles. In ATRP, the molar ratio of monomer to initiator largely affects the chain length of polymer brush.3 Because the amount of ATRP initiator on the silicon wafer was less than that on the silica nanopartices, ethylbenzyl chloride was added to normalize the fed initiator concentration to that of the silica nanoparticles. The surface-initiated ATRP system for the isospecific polymerization of IPAAm was demonstrated in the presence of a Lewis acid as a stereocontrol catalyst. Y(OTf)3 and methanol were chosen as a Lewis acid and polymerization solvent, respectively. They are reported to control polymer isotacticity in radical polymerizations.9-13 The stereochemical features of polymers Langmuir 2010, 26(23), 17781–17784
Idota et al.
Letter
Figure 2. Effects of the ratio of Y(OTf)3 to monomer (A), polymerization temperature (B), and monomer concentration (C) on the meso dyad value and the molecular weight of grafted PIPAAm brush.
can be quantitatively determined only by NMR spectroscopy. However, it is difficult to apply this technique to the measurement of thin layers of grafted polymers on solid surfaces. Thus, the grafted PIPAAm was harvested from the surfaces by immersing the substrates in a concentrated alkaline solution. Silica nanoparticles, with large specific surface area, were used as a modification substrate for obtaining the amount of grafted polymer required for characterization by NMR spectroscopy. The grafted PIPAAm was harvested from the surface as a mixture with residual silica by lyophilization after dialysis against water, because the PIPAAm prepared in the presence of Y(OTf)3 showed water-insolubility (e.g., 23 mg of mixture was obtained from 50 mg of PIPAAm brush nanoparticles). Figure 1 shows the 1H NMR spectra of grafted PIPAAm prepared by surface-initiated ATRP in the absence and presence of Y(OTf)3. Both grafted polymers were able to provide characteristic chemical shifts and the proton ratios in each functional group originated from PIPAAm chains as well as their bulky polymers. The degree of isotacticity in each grafted PIPAAm sample was estimated from the peaks of backbone methylene protons at 1.8-1.1 ppm (2H), which are assigned to be meso and racemo dyads in the polymers.9,10 It is well-known that meso (m) and racemo (r) dyads indicate the percentage of parallel and alternate directions of side chains of two connected monomers in three-dimensional configuration, respectively. In the absence of Y(OTf)3, the grafted PIPAAm showed 52% meso and 48% racemo dyads, indicating that the polymer was antactic, whereas ATRP in the presence of Y(OTf)3 provided high isotacticity (m = 79%). Previously, Y(OTf)3 has been reported to provided efficient stereoregulation in bulk free radical polymerizations.9-13 To the best of our knowledge, this is the first report of Y(OTf)3 providing tacticity control in surface-initiated polymerization of polymer brushes. Surface-initiated polymerization enables an increase in the graft density compared with “grafting to” methods,4 and the graft architectures of polymer brushes can be precisely modulated by living radical polymerization techniques.5 To confirm the chain length of polymer brushes made by surface-initiated ATRP, the number-average molecular weight (Mn) of the grafted PIPAAm was determined by comparing the peaks of the aromatic protons of the ATRP initiator (the asterisks in Figure 1) with that of the methine protons of the polymer’s isopropyl groups ((4) in Figure 1). The effect of the ratio of Y(OTf)3 and IPAAm on the tacticity and molecular weight is shown in Figure 2A. The meso dyad content of the surfacegrafted PIPAAm increased with increasing Y(OTf)3/IPAAm Langmuir 2010, 26(23), 17781–17784
ratio up to approximately 0.1 and then began to plateau. Because Y(OTf)3 catalytically coordinated with monomers and allowed monomers to react preferentially with the growing polymer terminals,9 the polymer stereoregulation was able to be influenced by only a small amount of Y(OTf)3 relative to monomer. As Y(OTf)3 was added (Y(OTf)3/IPAAm ratio of 0.025), the Mn of the grafted PIPAAm decreased and remained low up to a Y(OTf)3/IPAAm ratio of 0.18. In our method, PIPAAm-grafted surfaces were prepared with two different polymerization catalysts: Y(OTf)3 and the Cu/Me6TREN complex. In ATRP, it is proposed that Y(OTf)3 acts to increase the radical deactivation rate, while Cu/Me6TREN controls the rate of radical formation. In contrast, increasing Y(OTf)3 concentration induced only a slight change in the PIPAAm brush molecular weight. Thus, other possible polymerization conditions for controlling polymer brush molecular weight should be investigated. Figure 2B and C shows the molecular weight and tacticity changes of PIPAAm brushes at various polymerization temperatures and monomer concentrations, respectively. The Mn of grafted PIPAAm increased linearly with increasing polymerization temperature and monomer concentration, both of which acted to enhance the polymerization rate. The use of methanol as a solvent in ATRP of IPAAm was previously reported to give a narrow molecular weight distribution in living manner.31,32 However, methanol gives a low the degree of polymerization with poor conversion at early time points. As a result, PIPAAm molecular weight can be modulated by temperature and monomer concentration instead of polymerization time. On the other hand, the meso dyad of grafted PIPAAm showed little change and maintained a high isotacticity as polymerization temperature and monomer concentration increased. Therefore, the isotacticity and molecular weight of the surface-grafted PIPAAm can be simultaneously controlled by optimizing the polymerization conditions in surface-initiated ATRP. The immobilization of ATRP initiator and surface-initiated polymerization of IPAAm on silicon wafers were confirmed by ATR/FT-IR spectra (Figure 3). The ATRP initiator-immobilized surface showed peaks at 1540 and at 2950 cm-1, which can be assigned to the aromatic CdC and alkyl C-H bond, respectively. Spectra of PIPAAm-grafted surfaces were notably different from those of the ATRP initiator-immobilized surface in a region (31) Xia, Y.; Yin, X.; Burke, N. A. D.; Stover, H. D. H. Macromolecules 2005, 38, 5937. (32) Ye, J.; Narain, R. J. Phys. Chem. B 2009, 113, 676.
DOI: 10.1021/la1024229
17783
Letter
Figure 3. ATR/FT-IR spectra of bare, initiator-immobilized, and PIPAAm-grafted silicon wafers.
Idota et al.
to that used in polymerizations from nanoparticles, and the tacticiy of grafted PIPAAm on silicon wafers was estimated to be m=84% (Figure 2). The water-solubility of PIPAAm prepared in bulk polymerizations has been reported to decrease with increasing meso dyad value, and PIPAAm with a high isotacticity (m>72%) is unable to be dissolved in water regardless of temperature.9,12 In contrast to these findings, as shown in Figure 4, the isotactic PIPAAm-grafted surface exhibited hydrophilic characteristics below 10 °C. Thus, in contrast to bulk polymers free in solution, surface wettability changes may be influenced by surface modification, such as anchoring of polymer ends to the substrate, and interaction between neighboring chains due to close packing on the substrate surface. Detailed studies for understanding the effect of tacticity on thermal-induced wettability changes by analyzing the microscopic structure and hydration state of isotactic PIPAAm brushes are now in progress.
Conclusions
Figure 4. Temperature-dependent contact angle changes (cos θ) on initiator-immobilized and PIPAAm brush surfaces: ([) atactic PIPAAm brush without Y(OTf)3, (0) isotactic PIPAAm brush with Y(OTf)3, and (9) initiator.
between 1500 and 1650 cm-1, which are assigned to CdO stretching and N-H deformation in the amide group of PIPAAm, respectively. Thus, ATR/FT-IR studies indicated that both molecules were successfully immobilized on silicon wafer. Although PIPAAm is well-known to exhibit a reversible soluble/insoluble change in water at 32 °C, the high isotactic PIPAAm (m = 84%) was shown to be water-insoluble across the range of 5-45 °C (shown in Figure S1 in the Supporting Information). In this study, the effect of grafted PIPAAm brush tacticity on surface aqueous wettability at various temperatures was examined by contact angle measurements (Figure 4). Although conventional PIPAAm-grafted surfaces polymerized in the absence of Y(OTf)3 showed a hydrophilic/hydrophobic phase change around 30 °C, the cos θ value of PIPAAm-grafted surface prepared in the presence of Y(OTf)3 increased slightly below 10 °C. In this study, ethylbenzyl chloride was added to silicon wafer polymerizations to normalize initiator concentration
17784 DOI: 10.1021/la1024229
Stereoregularity of thermoresponsive polymer brushes was regulated through surface-initiated ATRP in the presence of Y(OTf)3, and their thermoresponsive wettabilities were investigated. Addition of Y(OTf)3 in the surface-initiated ATRP of PIPAAm increased the meso dyad content of polymer brushes, and tacticity was controlled by varying Y(OTf)3 concentrations (m = 50-85%). The polymerization temperature and monomer concentration in ATRP in the presence of Y(OTf)3 were available factors for controlling the molecular weight of a PIPAAm brush with high isotacticity. The isotactic PIPAAm brush surface exhibited a small wettability change in response to temperature below 10 °C, and this temperature response is different from not only the atactic polymer brush surfaces that transition at 30 °C but also for isotactic bulk polymers, which are water-insoluble. The isotactic PIPAAm brush surface wettability change was caused by the effect of surface modification. Acknowledgment. This work was financially supported in part by a Grant-in-Aid for the Global COE Program, Science for Future Molecular Systems from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by the Development of New Environmental Technology Using Nanotechnology Project of the National Institute of Environmental Science (NIES), commissioned from the Ministry of Environment, Japan. The first author is supported by Research Fellowships of the Japan Society for Promotion of Science (JSPS) for Young Scientists. The authors are grateful to Dr. Norio Ueno (Tokyo Women’s Medical University) and Dr. John M. Hoffman (National Institute for Materials Science) for their valuable comments and text editing. Supporting Information Available: Plot showing the effect of tacticity on PIPAAm phase transition. This material is available free of charge via the Internet at http://pubs. acs.org.
Langmuir 2010, 26(23), 17781–17784
