3744
Langmuir 2007, 23, 3744-3749
Stimuli-Responsive Surfaces Using Polyampholyte Polymer Brushes Prepared via Atom Transfer Radical Polymerization Neil Ayres, Crystal D. Cyrus, and William J. Brittain* The UniVersity of Akron, Department of Polymer Science, Goodyear Polymer Center, Akron, Ohio 44325-3909 ReceiVed August 15, 2006. In Final Form: December 20, 2006 The synthesis of AB diblock copolymer polyampholyte polymer brushes of the type Si/SiO2//poly(acrylic acidb-vinyl pyridine) prepared using atom transfer radical polymerization is reported. Both 2- and 4-vinyl pyridine have been used. The diblock polyampholyte polymer brushes demonstrate stimuli-responsive behavior with respect to pH, showing both polyelectrolyte and polyampholyte effects. Furthermore, we have quaternized the 4-vinyl pyridine segments to form a mixed weak/strong, or annealed/quenched, polyelectrolyte system. The quaternized polymer brush exhibits different pH-responsive behavior, with decreasing film thickness being observed with increasing pH.
Introduction Polymer brushes are assemblies of macromolecules that are tethered by one end to a surface or interface.1,2 Attachment of the polymeric chains in close proximity to one another forces the polymers to adopt a stretched conformation in order to minimize segment-segment interactions.3 As a result of these densely packed polymer topologies, the synthesis of polymer brushes has attracted increasing attention for applications in areas such as surface property tailoring,4 chemical gating,5-8 and nanolithographic patterning.9 Polymer brushes are typically prepared through either physisorption or covalent attachment; of these approaches, covalent attachment is often preferred as it overcomes the constraints of physisorption, namely thermal and solvolytic instability. Covalent attachment can be achieved using the “grafting to” and “grafting from” techniques. “Grafting to” requires the tethering of preformed polymer chains to a surface. However, this technique often suffers from low grafting density (surface coverage) and film thickness due to the requirement of polymer molecules to diffuse through an existing attached polymer layer to reach the reactive sites decorating the surface. Thus, steric hindrance for surface attachment increases as the thickness of the polymer film increases. As a result of these limitations, the “grafting from” approach has become the preferred option for the synthesis of polymer brushes. This approach utilizes a surface immobilized initiator layer and subsequent in situ polymerization to generate the polymer brush. This technique has been used to prepare thick polymer films that are covalently anchored to the substrate with a high grafting density. We,10-13 and others,14-18 have utilized atom transfer radical polymerization (ATRP) for the synthesis of polymer brushes, employing the “grafting from” methodology. The main advan* Corresponding author. E-mail:
[email protected]. (1) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Polym. Sci. 1992, 100, 31. (2) Milner, S. T. Science 1991, 251, 905. (3) Advincula, R. C.; Brittain, W. J.; Caster, K. C.; Ru¨he, J. Polymer Brushes: Synthesis, Characterization, Applications; Wiley: New York, 2004. (4) Mayes, A. M.; Kumar, S. K. MRS Bull. 1997, 22, 43. (5) Granville, A. M.; Brittain, W. J. Macromol. Rapid Commun. 2004, 25, 1298. (6) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739. (7) Ito, Y.; Nishi, S.; Park, Y. S.; Imanishi, Y. Macromolecules 1997, 30, 5856. (8) Park, Y. S.; Ito, Y.; Imanishi, Y. Chem. Mater. 1997, 9, 2755. (9) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837.
tages to using a controlled/“living” radical technique for polymer brush synthesis are control over the brush thickness via control of molecular weight, polymers possessing narrow molecular weight distributions (MWDs), and the ability to prepare block copolymers by reinitiation of dormant chain ends and subsequent extension of the polymer chains. However, ATRP cannot be used for some functional monomers, for example, (meth)acrylic acid. The acid functionality inherent in the monomer will poison the ATRP catalyst.19 Polyampholytes are described as charged macromolecules that contain both positively and negatively charged groups on the polymer chain.20 When synthetic copolymers are used as polyampholytes, commonly weak acids and bases are employed. Thus, varying the pH can change the net charge of the polyampholyte in an aqueous solution. The isoelectric point is the pH value where there is an equal number of positive and negative charges on the copolymer, resulting in a zero net charge.21 Around this isoelectric point the copolymer will show the characteristics of a polyampholyte. When a charge asymmetry exists, at either high or low pH values, the polymers will instead exhibit polyelectrolyte behavior. Diblock polyampholytes correspond to the case where one block of the polymer may be rendered anionic and the other block cationic. The properties of block polyampholytes are quite different from those of random polyampholytes, as the localization of like charges within one segment increases the electrostatic interactions between oppositely charged blocks; therefore, only research on diblock polyam(10) Boyes, S. G.; Brittain, W. J.; Weng, X.; Cheng, S. Z. D. Macromolecules 2002, 35, 4960. (11) Boyes, S. G.; Akgun, B.; Brittain, W. J.; Foster, M. D. Macromolecules 2003, 36, 9539. (12) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790. (13) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813. (14) Zhou, F.; Jiang, L.; Liu, W.; Xue, Q. Macromol. Rapid Commun. 2004, 25, 1979. (15) Ohno, K.; Morinaga, T.; Koh, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2005, 38, 2137. (16) Li, D.; Sheng, X.; Zhao, B. J. Am. Chem. Soc. 2005, 127, 6248. (17) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14. (18) Iwata, R.; Suk-In, P.; Hoven, V. P.; Takahara, A.; Akiyoshi, K.; Iwasaki, Y. Biomacromolecules 2004, 5, 2308. (19) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921. (20) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3513. (21) Shusharina, N. P.; Zhulina, E. B.; Dobrynin, A. V.; Rubinstein, M. Macromolecules 2005, 38, 8870.
10.1021/la062417+ CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
Synthesis of Polyampholyte Polymer Brushes
pholytes is considered in this manuscript. While the solution behavior of polyampholytes has been studied for a considerable time, there has been relatively little work conducted on surfaceattached polyampholytes. Stamm and co-workers22-27 conducted one of the most active efforts. Here, AB diblock copolymers of poly(2-(dimethylamino)ethyl methacrylate-b-methacrylic acid) (poly(DMAEMA-b-MAA) were prepared through anionic polymerization, and the absorption of these polymers onto surfaces was studied. Bhat et al.28 recently reported the synthesis of diblock copolymer brushes of poly(DMAEMA-b-acrylic acid) in a “grafting from” fashion and observed the response of film thickness to pH. They saw a decrease in the thickness of the polymer brush around the isoelectric point. Shusharina and Linse29 performed mean-field lattice theory calculations of a diblock polyampholyte grafted onto an uncharged planar surface and examined the effect of varying the polyelectrolyte charge, the polyelectrolyte length, and the amount of added low molecular weight electrolyte (salt) on the brush structure. They proposed that these systems should display a complicated behavior, where the interplay between electrostatic repulsions of like charges and attractions of opposite charges leads to a variety of possible chain conformations including a coexistence of stretched and coiled chains. Monte Carlo simulations30 have also been used with lattice mean-field theory to examine diblock polyampholytes grafted onto spherical particles. These results showed a strong dependence on the net charge of the diblock. When only one of the blocks were charged, the chains stretched away from the surface, forming a polyelectrolyte brush, whereas with zero net charges, the chains collapsed, forming a polyelectrolyte complex close to the substrate. Furthermore, at intermediate charge compensation, both collapsed, and extended chains were proposed. Herein, we report the synthesis of diblock copolymer brushes comprising acrylic acid (AA) and 2- or 4-vinyl pyridine (2VP or 4VP, respectively). The AA segments were obtained by polymerizing tert-butyl acrylate (tert-BA) as a protected form of the monomer, followed by subsequent deprotection by the pyrolysis method. We have demonstrated that these polyampholytes show stimuli-responsive behavior with pH. Furthermore, the behavior will markedly change based upon the nature of the polyampholyte, i.e., an annealed system or a mixed annealed/ quenched system. Experimental Section Materials. tert-BA (Aldrich 98%) was passed through a column of basic alumina immediately before use. 2VP (Aldrich 97%) and 4VP (Aldrich 95%) were distilled prior to use. CuBr (Aldrich, 98%) was purified as described in the literature.31 Silicon wafers were purchased from Umicore Semiconductor Processing and cleaned using “piranha” solution (30/70 30% aqueous hydrogen peroxide solution/sulfuric acid) at 90 °C for 2 h. It should be noted that (22) Mahltig, B.; Walter, H.; Harrats, C.; Muller-Buschbaum, P.; Jerome, R.; Stamm, M. Phys. Chem. Chem. Phys. 1999, 1, 3853. (23) Mahltig, B.; Gohy, J. F.; Jerome, R.; Bellmann, C.; Stamm, M. Colloid Polym. Sci. 2000, 278, 502. (24) Mahltig, B.; Jerome, R.; Stamm, M. Phys. Chem. Chem. Phys. 2001, 3, 4371. (25) Mahltig, B.; Mueller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Wiegand, S.; Gohy, J. F.; Jerome, R.; Stamm, M. J. Colloid Interface Sci. 2001, 242, 36. (26) Mahltig, B.; Gohy, J. F.; Jerome, R.; Buchhammer, H. M.; Stamm, M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 338. (27) Mahltig, B.; Gohy, J. F.; Jerome, R.; Pfuetze, G.; Stamm, M. J. Polym. Res. 2003, 10, 69. (28) Bhat, R. R.; Tomlinson, M. R.; Wu, T.; Genzer, J. AdV. Polym. Sci. 2006, 198, 51. (29) Shusharina, N. P.; Linse, P. Eur. Phys. J. E 2001, 6, 147. (30) Akinchina, A.; Shusharina, N. P.; Linse, P. Langmuir 2004, 20, 10351. (31) Keller, R. N.; Wycoff, H. D. Inorg. Synth. 1947, 2, 1.
Langmuir, Vol. 23, No. 7, 2007 3745 piranha solution is extremely reactiVe and as such should be handled with great care. The synthesis and deposition of the surface-bound initiator, (11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane, has been reported previously.32 Briefly, ω-undecylenyl alcohol was esterified with an equimolar amount of 2-bromoisobutyryl bromide and then hydrosilated using trichlorosilane and Karstedt’s catalyst. The product was dissolved as a 10% solution in toluene and stored until use. This was deposited onto freshly cleaned wafers by adding 0.2 mL of the toluene solution in an additional 10 mL of toluene and heating to 60 °C for 4 h. All other reagents were purchased from either Aldrich or Fisher Scientific and used as received. Instrumentation. Fourier transform infrared grazing angle attenuated total reflection (FTIR-GATR) spectra were recorded using a Bruker Tensor 27 spectrometer using a GATR accessory with a Ge ATR crystal (Harrick Scientific). Spectra were recorded at 2 cm-1 resolution, and 256 scans were collected. Contact angles were determined using a Rame Hart NRL-100 contact angle goniometer equipped with a tilting stage. Drop volumes were 10 µL. Ellipsometric measurements were performed in the dry state on a Gaertner model L116C ellipsometer with a He-Ne laser (λ ) 632.8 nm) and a fixed angle of incidence of 70°. Refractive index values (n) of 1.46 (poly(tert-BA)), 1.527 (poly(acrylic acid), poly(AA)), 1.595 (poly(vinyl pyridine), poly(VP)) were used. Composite values were used for the block copolymers. Molecular weight analysis (poly(VP)) was performed with a Waters 501 pump, two PLgel (Polymer Laboratories) mixed D columns (5 µm), and a Waters 410 differential refractometer. The eluent was dimethylformamide, and the flow rate was 0.7 mL/min. Molecular weights were calibrated by comparison to narrow MWD poly(methyl methacrylate) standards (200 to 1.0 × 106 g/mol) (Polymer Laboratories). Data analysis was performed with the E-Z chrom software package. Molecular weight analysis (tert-BA) was performed with three Waters HR Styragel columns and a Wyatt DAWN EOS multiangle laser light scattering (MALLS) detector with a Waters 410 differential refractometer. The eluent was tetrahydrofuran (THF) at 35 °C, and a flow rate of 1 mL/min was used. Data analysis was performed with the Wyatt ASTA v4.73.04 software package. Typical Synthesis of a Si/SiO2//Poly(2VP) Polymer Brush. A Schlenk flask was charged with 2VP (15 mL, 0.139 mol) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (0.14 mL, 5.1 × 10-4 mol) and degassed using the freeze-pump-thaw method (three cycles). Cu(I)Br (0.067 g, 4.63 × 10-4 mol) and Cu(II)Br2 (0.01 g, 4.63 × 10-5 mol) were added to a second Schlenk flask, and three vacuum purge/nitrogen fill cycles were performed. The solution from the first flask was then added to the inorganic salts via a degassed cannular and the mixture was heated to a reaction temperature of 80 °C until a homogeneous solution had formed. A silicon wafer modified with an ATRP initiator ((11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) was placed into a third Schlenk flask, and three vacuum purge/nitrogen fill cycles were performed. Free initiator, ethyl 2-bromoisobutyrate, (0.68 mL, 4.63 × 10-4 mol) was added to the flask containing the wafer, followed by the reaction solution from the second flask via a degassed cannular. The reaction was allowed to proceed for the desired time before being quenched by exposure to oxygen (air) and cooling. The wafer was isolated and rinsed with dichloromethane (DCM), ethanol, and THF before being placed in a Soxhlet extractor with DCM for 24 h, then sonicated in DCM for 30 min to remove any physisorbed polymer, and dried under a stream of air. When 4VP was used, the same procedure was followed with the addition of anhydrous 2-propanol (15 mL), and the reaction was conducted at 60 °C, except in the case of the diblock copolymer brush that was quaternized. Here, the polymerization was run in bulk at 80 °C. Typical Synthesis of a Si/SiO2//Poly(tert-BA) Polymer Brush. A Schlenk flask was charged with tert-BA (14 mL, 9.65 × 10-2 mol), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (0.2 mL, 9.57 × 10-4 mol), and anhydrous acetone (7 mL) and degassed using the freeze-pump-thaw method (three cycles). Cu(I)Br (0.069 g, 4.81 (32) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.
3746 Langmuir, Vol. 23, No. 7, 2007 × 10-4 mol) was added to a second Schlenk flask, and three vacuum purge/nitrogen fill cycles were performed. The solution from the first flask was then added to the Cu(I)Br via a degassed cannular, and the mixture was heated to a reaction temperature of 60 °C until a homogeneous solution had formed. A silicon wafer modified with an ATRP initiator ((11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) was placed into a third Schlenk flask, and three vacuum purge/nitrogen fill cycles were performed. Free initiator, ethyl 2-bromoisobutyrate, (0.05 mL, 3.37 × 10-4 mol) was added to the flask containing the wafer, followed by the reaction solution from the second flask via degassed cannular. The reaction was allowed to proceed for the desired time (typically 18 h) before being quenched by exposure to oxygen (air) and cooling. The wafer was isolated and rinsed with DCM, ethanol, and THF before being placed in a Soxhlet extractor with THF for 24 h and then sonicated in THF for 30 min to remove any physisorbed polymer. The wafer was then dried under a stream of air. Typical Synthesis of a Si/SiO2//Poly(AA-b-vinyl pyridine) Polymer Brush. A Si/SiO2//poly(tert-BA) polymer brush was prepared as described above, and the chain was extended with 2or 4VP using the same procedure used for the homopolymer brush. This neutral diblock copolymer brush was then placed into a clean vacuum oven for 30-90 min before being placed into a beaker of deionized (DI) water overnight. ATR-FTIR spectroscopy of the wafer after removal from the DI water and drying under a stream of air revealed no peaks that could be attributed to a possible acid anhydride formation. Quaternization of a Si/SiO2//Poly(AA-b-4VP) Polymer Brush. A silicon wafer bearing a Si/SiO2//poly(AA-b-VP) polymer brush was immersed in a 2 M solution of methyl iodide in nitromethane and heated to 40 °C for 5 h. The wafer was then retrieved, rinsed with methanol, and sonicated in methanol for 45 min.
Results and Discussion We previously demonstrated that Si/SiO2//poly(tert-BA) polymer brushes can be converted to the corresponding poly(AA) polymer brushes through pyrolysis.33 This facile strategy has the added advantage of not requiring a chemical solution using acid. Therefore, no loss of product should occur through cleavage of the ester moiety present in the anchoring (initiator) layer. These brushes are responsive to both solution pH and added electrolyte concentration. We extended this work by showing that block copolymer polymer brushes of poly(DMAEMA-b-MAA) could be prepared, and demonstrated that systems were responsive to added electrolyte, exhibiting the “antipolyelectrolyte” effect.34 Here, we report the synthesis of polyampholyte polymer brushes that utilize vinyl pyridine segments. We used both 2VP and 4VP. We initially prepared homopolymer brushes of vinyl pyridine, and the results of the polymerizations are shown in Table 1. As can be seen, after polymerization, the silica wafers possessed lower contact angles due to the presence of the hydrophilic poly(VP) chains. Poly(VP) polymer brushes are known to be pH responsive. We have shown that the polymer brushes prepared in this study also exhibit a pH response, and these results are given in Figures 1 and 2. As can be seen, with decreasing pH there is an increase in film thickness. As the pyridine segments become protonated, the polymer chains increase their fractional charge and experience an electrostatic repulsion. This effect forces the chain to adopt an extended coil conformation, which corresponds to the increase in film thickness. It is known that the vinyl pyridines are challenging to polymerize in a controlled fashion under ATRP conditions (33) Treat, N. D.; Ayres, N.; Boyes, S. G.; Brittain, W. J. Macromolecules 2006, 39, 26. (34) Ayres, N.; Boyes, S. G.; Brittain, W. J. Langmuir 2007, 23, 182.
Ayres et al. Table 1. Ellipsometry and Aqueous Contact Angle Results for the Si/SiO2//Poly(VP) Polymer Brushes Used in This Study contact angleb sample Si/SiO2 Si/SiO2//SAMd Si/SiO2//poly(2VP) Si/SiO2 Si/SiO2//SAMd Si/SiO2//poly(4VP)
film thickness (nm)a 2 7 41 3 5 16
θa
θr c
88 80
70 66 c
85 68
72 54
a Thickness measurement of the whole film ( 1 nm. b Contact angle measured with a 10 µL water droplet. c Contact angle not measured for freshly cleaned wafer. d SAM refers to deposited initiator layer.
Figure 1. Polymer brush thickness vs pH for a Si/SiO2//poly(2VP) polymer brush.
Figure 2. Polymer brush thickness vs pH for a Si/SiO2//poly(4VP) polymer brush.
depending on the ligand employed, and few examples exist in the literature.19 This is because the pyridine can ligate the inorganic salt displacing the amine ligands. We observed similar results in our work. Broad polydispersities (>2) were seen when free polymer obtained from the sacrificial initiator was examined using size exclusion chromatography. It was because of these issues that we chose to extend a Si/SiO2//poly(tert-BA) polymer brush with vinyl pyridine rather than attempt the reverse blocking order (Scheme 1). We have prepared a range of polyampholyte polymer brushes of the type Si/SiO2//poly(AA-b-VP). From this point, they will be referred to as Si/SiO2//poly(AA-b-2(4)VP) depending on whether 2- or 4VP was used. In each case, we have employed tert-BA as a protected form of AA and used the pyrolysis approach to convert the ester to the carboxylic acid. After each tert-BA
Synthesis of Polyampholyte Polymer Brushes
Langmuir, Vol. 23, No. 7, 2007 3747
Scheme 1. Reaction Scheme for the Preparation of Si/SiO2// poly(AA-b-4VP)
Table 2. Ellipsometry and Aqueous Contact Angle Results for the Si/SiO2//Poly(AA-b-VP) Polymer Brushes Used in This Study contact angleb sample
film thickness (nm)a
θa
θr
Si/SiO2//poly(tert-BA) Si/SiO2//poly(tert-BA-b-2VP) Si/SiO2//poly(AA-b-2VP) Si/SiO2//poly(tert-BA) Si/SiO2//poly(tert-BA-b-4VP) Si/SiO2//poly(AA-b-4VP)
13 20 9c 13 18 14c
89 82 76 89 82 68
78 61 56 77 62 57
Figure 3. Polymer brush thickness vs pH for a Si/SiO2//poly(AAb-2VP) polymer brush.
a Thickness measurement of the whole film ( 1 nm. b Contact angle measured with a 10 µL water droplet. c Thickness measured in bulk after exposure to an aqueous solution at pH 7.
polymerization, the free polymer was analyzed by gel permeation chromatography (GPC), and narrow polydispersity (
