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Completely Aqueous Procedure for the Growth of Polymer Brushes on Polymeric Substrates Parul Jain, Jinhua Dai, Sebastian Grajales, Sampa Saha, Gregory L. Baker,* and Merlin L. Bruening* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 ReceiVed June 12, 2007. In Final Form: September 10, 2007 The growth of polymer brushes on polymer substrates is often challenging because of substrate incompatibility with the organic solvents used for initiator attachment. This letter reports the use of layer-by-layer adsorption of macroinitiators and subsequent aqueous ATRP from these immobilized initiators to prepare polymer brushes on polymeric substrates. Polyethersulfone (PES) films and porous membranes were modified with polyelectrolyte multilayer films, and a previously developed polycationic initiator, poly(2-(trimethylammonium iodide)ethyl methacrylate-co-2-(2-bromoisobutyryloxy)ethyl acrylate), was then electrostatically adsorbed onto these polyelectrolyte films. The immobilized macroinitiator is very efficient in initiating the growth of polymer brushes on PES, as demonstrated by aqueous syntheses of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) films. PHEMA (250 nm thick) and PDMAEMA (40 nm thick) brushes were grown in 2 h from surfaces modified with polycationic initiators. Moreover, this procedure is effective for growing brushes in the pores of PES membranes.
Introduction Polymer brushes, which are assemblies of long-chain polymers attached by one end to a support and extended from the surface,1 are attractive for applications in areas such as colloid stabilization, adhesion, lubrication, tribology, and rheology.2,3 Highly swollen, functionalized brushes can also serve as a matrix for the immobilization and separation of biomacromolecules such as proteins and DNA.4-10 We are particularly interested in utilizing brush-modified polymer membranes as high-capacity protein absorbers.10 In these systems, it is vital to develop synthetic methods that afford both fine control over the rate of chain growth and compatibility with porous, polymeric substrates. Among the many methods used for the synthesis of polymer brushes (e.g., plasma polymerization, heat- or UV-assisted graft polymerization, nitroxide-mediated polymerization, and reversible addition-fragmentation chain-transfer polymerization), surface-initiated atom-transfer radical polymerization (ATRP) is appealing because it minimizes polymerization in solution and provides polymers with controlled growth rates and low polydispersities.3,11-14 The first step in the synthesis of polymer * Authors to whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. Phone: (517) 355-9715 ext. 237. Fax: (517) 353-1793. (1) Milner, S. T. Science 1991, 251, 905-914. (2) Husemann, M.; Morrison, M.; Benoit, D.; Frommer, K. J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844-1845. (3) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. AdV. Polym. Sci. 2006, 197, 1-45. (4) Singh, N.; Husson, S. M.; Zdyrko, B.; Luzinov, I. J. Membr. Sci. 2005, 262, 81-90. (5) Kurosawa, S.; Aizawa, H.; Talib, Z. A.; Atthoff, B.; Hilborn, J. Biosens. Bioelectron. 2004, 20, 1165-1176. (6) Kawai, T.; Saito, K.; Lee, W. J. Chromatogr., B 2003, 790, 131-142. (7) Ulbricht, M.; Yang, H. Chem. Mater. 2005, 17, 2622-2631. (8) Wittemann, A.; Haupt, B.; Ballauff, M. Phys. Chem. Chem. Phys. 2003, 5, 1671-1677. (9) Dai, J.; Bao, Z.; Sun, L.; Hong, S. U.; Baker, G. L.; Bruening, M. L. Langmuir 2006, 22, 4274-4281. (10) Sun, L.; Dai, J.; Baker, G. L.; Bruening, M. L. Chem. Mater. 2006, 18, 4033-4039. (11) 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. (12) Huang, W.; Kim, J.-B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 1175-1179.
brushes by ATRP entails the attachment of initiators to a surface, which is often realized by the immersion of a hydroxylfunctionalized substrate in an anhydrous organic solvent containing an initiator precursor such as 2-bromoisobutyryl bromide.3,11-14 However, hydroxylated versions of common membrane materials such as polysulfone and polyethersulfone (PES)15 are incompatible with organic solvents. For these polymers, organic solvents should be completely avoided in brush synthesis to preserve the pore structure of the membrane. This letter reports a general method for immobilizing ATRP initiators on polymer films and membranes and the subsequent surface-initiated growth of polymer brushes (Scheme 1). Initiator immobilization relies on the adsorption of a polycationic macroinitiator (PMI) as recently described by Armes and coworkers.16-18 To facilitate PMI adsorption, we first prime the surface through the adsorption of multilayer polyelectrolyte films composed of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC). Polycationic or polyanionic initiators readily adsorb onto oppositely charged surfaces through electrostatic interactions, as shown previously in adsorption on silicon,18 silica beads,17 and modified colloidal particles.19 This work builds on these previous studies to demonstrate macroinitiator adsorption and subsequent polymerization on PES films and membranes, which have a much different surface chemistry and morphology than previous substrates employed for macroinitiator adsorption and subsequent ATRP. Most importantly, the macroinitiator immobilization and polymerization occur from water, so damage to PES substrates caused by the use of organic solvents is avoided. The adsorbed macroinitiators readily initiate polymerization on PES surfaces to allow for the growth of 100-nm-thick poly(2-hydroxyethyl (13) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14-22. (14) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921-2990. (15) Baker, R. W. Membrane Technology and Applications, 2nd ed.; John Wiley & Sons: New York, 2004. (16) Chen, X. Y.; Armes, S. P. AdV. Mater. 2003, 15, 1558-1562. (17) Chen, X. Y.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 587-595. (18) Edmondson, S.; Vo, C.-D.; Armes, S. P.; Unali, G.-F. Macromolecules 2007, 40, 5271-5278. (19) Fulghum, T. M.; Patton, D. L.; Advincula, R. C. Langmuir 2006, 22, 8397-8402.
10.1021/la701735q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
Letters
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Scheme 1. Growth of Polymer Brushes by ATRP from Macroinitiators Adsorbed in a Membrane Pore
methacrylate) (PHEMA) brushes in 30 min, and the procedure can be applied to porous PES membranes. Experimental Section Materials. 3-Mercaptopropionic acid (MPA), 11-mercapto-1undecanol (MUD), 2-bromoisobutyryl bromide, CuCl (99.999%), iodomethane (99%), poly(sodium 4-styrene sulfonate) (PSS, Mw ≈ 70 000), poly(diallyldimethylammonium chloride) (PDADMAC, Mw ≈ 150 000), CuBr2 (99.999%), 2,2′-bipyridyl (g99%, bpy), and 2,2′-azobis(2-methylpropionitrile) (AIBN) were obtained from Sigma-Aldrich and used as received. 2-Hydroxyethyl methacrylate (97%, HEMA, Aldrich) and 2-(dimethylamino)ethyl methacrylate (98%, DMAEMA, Aldrich) were purified before use by passing the monomer through a column of activated basic alumina (Spectrum). 2-(2-Bromoisobutyryloxy)ethyl acrylate (BIEA) was synthesized according to a modified literature procedure described in Supporting Information.20,21 PES membranes with a nominal 0.45 µm cutoff were obtained from GE Osmonics (cat. no. S04WP02500). The internal surface area of these membranes was determined using N2 adsorption measurements (Micromeritics ASAP 2010 sorptometer) and an analysis of the data using a BET model (Supporting Information, Figure SI-2). PES coatings on Au-coated Si wafers (200 nm of gold sputtered on 20 nm of Cr on Si(100)) were prepared by dissolving a PES membrane (17 mg) in 10 mL of CH2Cl2 and spin coating the solution (1.0 mL) onto a substrate (1.1 × 2.4 cm2) at a spin rate of 500 rpm. Characterization Methods. NMR spectra were collected on a Varian Gemini-300 spectrometer. Polymer molecular weights were determined by GPC (gel permeation chromatography) at 35 °C using two PL gel 10 mm mixed-B columns in series and an Optilab rEX differential refractometer (Wyatt Technology Co.) as a detector. THF was the eluting solvent at a flow rate of 1 mL/min. All samples were filtered through a 0.2 µm Whatman PTFE syringe filter prior to GPC analysis. Films were examined by reflectance FTIR spectroscopy and ellipsometry after each step leading to brush growth. Reflectance FTIR spectra were acquired with a Nicolet Magna-IR 560 spectrophotometer containing a PIKE grazing angle (80°) attachment, and a UV/O3-cleaned gold slide served as a background. Film thicknesses were determined using a rotating analyzer ellipsometer (model M-44, J. A. Woollam) at an incident angle of 75° and assuming a film refractive index of 1.5. The films were imaged in tapping mode with a Dimension 3100 scanning probe microscope equipped with a Nanoscope IIIA control station. Film growth inside PES membranes was verified using transmission FTIR spectroscopy (Mattson Galaxy Series 3000) with an air background as well as (20) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M. Macromolecules 1997, 30, 5192-5194. (21) Zhai, G.; Kang, E. T.; Neoh, K. G. Macromolecules 2004, 37, 72407249.
field-emission scanning electron microscopy (FESEM, Hitachi S-4700II, acceleration voltage of 15 kV). Synthesis of Poly(2-(dimethylamino)ethyl methacrylate-co2-(2-bromoisobutyryloxy)ethyl acrylate) (Poly(DMAEMA-coBIEA), the Precursor of the Polycationic Initiator. DMAEMA (2.97 g, 18.9 mmol), BIEA (1.45 g, 5.47 mmol), and AIBN (82 mg, 0.5 mmol) were added to 5 mL of dry THF. The mixture was degassed via three freeze-pump-thaw cycles, and then polymerization was carried out at 60 °C with stirring for 2 h. The highly viscous polymer mixture was diluted with 15 mL of THF, and the polymer was precipitated into pH 11 water. After filtration, the polymer was dried under vacuum, redissolved in 15 mL of THF, and precipitated into hexane. Filtration and drying under vacuum at room temperature gave 2.1 g of the copolymer. The 1H NMR spectrum of the polymer is shown in Figure SI-3 of the Supporting Information and indicates approximately 20% BIEA in the copolymer. Synthesis of Poly(2-(trimethylammonium iodide)ethyl methacrylate-co-BIEA) (Poly(TMAEMA-co-BIEA)), the Polycationic Initiator. Poly(DMAEMA-co-BIEA) (1.71 g) was dissolved in 25 mL of THF, and 1.0 mL of CH3I was added to the stirred solution at room temperature. Within 2 min, the reaction mixture became turbid and had a butterlike color. After 1 h of stirring, the solution was added dropwise to vigorously stirred hexane to precipitate the polymer as a fine powder. Washing with hexane and drying under vacuum at room temperature for 12 h provided 2.52 g of polymer. The 1H NMR spectrum of the polymer is shown in Figure SI-3 of the Supporting Information. Polymer Brush Synthesis. Poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) were grown from poly(TMAEMA-co-BIEA)-modified Au and polyethersulfone surfaces. In the case of Au surfaces, an Au-coated silicon wafer was UV/ozone cleaned and immersed overnight in a 5 mM MPA solution in ethanol to form an MPA self-assembled monolayer. This substrate was then alternatively immersed in aqueous solutions of 0.02 M PDADMAC (containing 0.5 M NaCl) and 0.02 M PSS (containing 0.5 M NaCl) for 5 min, with a 1 min water rinse between each immersion. (Polymer concentrations are given with respect to the repeating unit, and the pH of these solutions was ∼7.0). After the deposition of a (PDADMAC/PSS)2 film, the substrate was immersed in a solution of poly(TMAEMA-co-BIEA) (2.0 mg/mL in water) for 10 min, rinsed with water, and dried in a stream of N2. PES substrates (both membranes and spin-coated films) were modified similarly but with slightly thicker polyelectrolyte films, (PSS/PDADMAC)4PSS. (The deposition on polymers begins with PSS instead of PDADMAC because PSS adsorption on polyethersulfone should be more favorable as a result of hydrophobic interactions.) For poly(TMAEMA-coBIEA)/PSS multilayer films on gold, we used the same deposition solutions and times as above. Brush growth from adsorbed initiators followed previously reported procedures.12 Briefly, 10 mL of monomer (HEMA or DMAEMA) was mixed with 15 mL of water in a Schlenk flask, and this solution was degassed via three freeze-pump-thaw cycles. Catalyst and ligand were added, and the solution was subjected to one additional freeze-pump-thaw cycle. The molar ratio of the reagents was 50:1:0.3:2.6 monomer/CuCl/CuBr2/bpy. In a glovebag, initiator-modified substrates were immersed in the degassed solution, and polymerization was allowed to proceed at room temperature for the desired times. Polymer-coated substrates were then removed from the glovebag and rinsed sequentially with ethanol and water.
Results and Discussion Synthesis of the Polycationic Macroinitiator. The macroinitiator described in this article, poly(TMAEMA-co-BIEA), is closely related to that reported by Armes et al.16,17 The primary difference is the method used to incorporate the ATRP initiator into the macroinitiator. Armes used a post-polymerization strategy, acylating a HEMA-DMAEMA copolymer, whereas we copolymerized DMAEMA with 2-(2-bromoisobutyryloxy)ethyl acrylate (BIEA), a monomer capable of initiating ATRP.
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Quaternization of the resulting copolymer is the final step in both syntheses. The copolymer intermediate, poly(DMAEMA-co-BIEA), is insoluble in water but easily dissolves in acetone and THF. The mole fraction of BIEA in the copolymer determined from the 1H NMR integration ratios (Supporting Information Figure SI-3, top) was ∼20%, in reasonable agreement with the initial ratio of monomers in the polymerization solution (3.46:1 DMAEMA/ BIEA). The broad resonances in the 1H NMR spectra imply a high molecular weight for the copolymer, but GPC data acquired in THF and calibrated with polystyrene standards reveal a bimodal molecular weight distribution with molecular weights of
