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Langmuir 2007, 23, 182-189
Stimuli-Responsive Polyelectrolyte Polymer Brushes Prepared via Atom-Transfer Radical Polymerization† Neil Ayres,‡ Stephen G. Boyes,§ and William J. Brittain*,‡ Department of Polymer Science, Goodyear Polymer Center, 170 UniVersity AVenue, UniVersity of Akron, Akron, Ohio 44325, and Department of Chemistry and Geochemistry, 1500 Illinois Street, Colorado School of Mines, Golden, Colorado 80401 ReceiVed May 29, 2006. In Final Form: September 18, 2006 We present an account of our research into polyelectrolyte polymer brushes that are capable of acting as stimuliresponsive films. We first detail the synthesis of poly(acrylic acid) polymer brushes using ATRP in a “grafting from” strategy. Significantly, we employed a chemical-free deprotection step that should leave the anchoring ester groups intact. We have demonstrated how these polymer assemblies respond to stimuli such as pH and electrolyte concentration. We have used poly(acrylic acid) polymer brushes for the synthesis of metallic nanoparticles and review this work. We have used XPS, ATR-FTIR, and AFM spectroscopy to show the presence of silver and palladium nanoparticles within polymer brushes. Finally, we report the synthesis of AB diblock polyampholyte polymer brushes that represent an extension of polyelectrolyte polymer brushes.
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 with respect to 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 because 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 end-functionalized polymer chains to a surface under appropriate conditions. However, this technique often suffers from low grafting density (surface coverage) and film thickness because of the requirement that polymer molecules must 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 † Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom all correspondence should be addressed. E-mail:
[email protected]. Fax: (330) 972-5290. ‡ Goodyear Polymer Center. § Colorado School of Mines.
(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.; Ruhe, J. In Polymer Brushes: Synthesis, Characterization, Applications; Advincula, R., Brittain, W. J., Caster, K. C., Ruhe, J., Eds.; Wiley: Weinheim, Germany, 2004. (4) Mayes, A. M.; Kumar, S. K. MRS Bull. 1997, 22, 43. (5) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739. (6) Ito, Y.; Nishi, S.; Park, Y. S.; Imanishi, Y. Macromolecules 1997, 30, 5856. (7) Park, Y. S.; Ito, Y.; Imanishi, Y. Chem. Mater. 1997, 9, 2755. (8) Granville, A. M.; Brittain, W. J. Macromol. Rapid Commun. 2004, 25, 1298. (9) Kong, X.; Kawai, T.; Abe, J.; Iyoda, T. Macromolecules 2001, 34, 1837.
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 higher grafting density. We10-13 and others14-18 utilize atom-transfer radical polymerization (ATRP) for the synthesis of polymer brushes, employing the grafting from methodology. The main advantages 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, 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 Polymer brushes containing charged macromolecules (i.e., polyelectrolytes) have garnered increasing interest because they relate to stimuli-responsive or “smart” surface coatings,20 biosensors,21 and colloidal stabilization.22 One of the most common systems in polyelectrolyte polymer brushes utilizes acrylic acid, which as mentioned is problematic under standard ATRP conditions. (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.; Zhou, W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2000, 122, 2407. (14) Iwata, R.; Suk-In, P.; Hoven, V. P.; Takahara, A.; Akiyoshi, K.; Iwasaki, Y. Biomacromolecules 2004, 5, 2308. (15) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14. (16) Li, H.; Zhang, H.; Xu, Y.; Zhang, K.; Ai, P.; Jin, X.; Wang, J. Mater. Chem. Phys. 2005, 90, 90. (17) Ohno, K.; Morinaga, T.; Koh, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2005, 38, 2137. (18) Zhou, F.; Jiang, L.; Liu, W.; Xue, Q. Macromol. Rapid Commun. 2004, 25, 1979. (19) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921. (20) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem., Int. Ed. 2005, 44, 4578. (21) Tugulu, S.; Arnold, A.; Sielaff, I.; Johnsson, K.; Klok, H.-A. Biomacromolecules 2005, 6, 1602. (22) Jusufi, A.; Likos, C. N.; Ballauff, M. Colloid Polym. Sci. 2004, 282, 910.
10.1021/la061526l CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2006
Polymer Brushes Prepared Via Radical Polymerization
This article is separated into three sections. First, we will describe an alternative, chemical-free method for the conversion of tert-butyl acrylate to acrylic acid and the response of the corresponding products to stimuli such as pH and salt concentration;23 next, the synthesis of a diblock polymer containing poly(acrylic acid) segments that can be used for the preparation of metal nanoparticles;11 and finally, the synthesis of an AB diblock polyampholyte polymer brush and its response to added electrolyte. Experimental Section Materials. Styrene (Sty, Aldrich 99%), methyl acrylate (MA, Aldrich 99%), tert-butyl methacrylate (Aldrich, 98%), and 2-(N,Ndimethylamino)ethyl methacrylate (Aldrich 98%) were passed through a column of activated basic alumina and degassed with high-purity nitrogen for 1 h prior to use. CuBr (Aldrich, 98%) was purified as described in the literature. N,N,N′,N′,N′′-Pentamethyldiethylenetriamine (PMDETA, Aldrich 99%), 1,1,4,7,10,10hexamethytriethylenetetramine (Aldrich, 97%), ethyl 2-bromoisobutyrate (E2Br-iB, Aldrich 98%), silver acetate (Ag(Ac), Aldrich 99.999%), sodium tetrachloropalladate(II) (Na2PdCl4, Aldrich 98%), anhydrous anisole (Aldrich), and anhydrous acetone (Aldrich, 99.8%) were used as received. Silicon wafers were purchased from Umicore Semiconductor Processing and cleaned prior to use in accordance with the published procedure. The synthesis of the surface-bound initiator, (11-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane, has been reported previously.24 All other reagents were purchased from either Aldrich or Fisher Scientific and used as received. Instrumentation. 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 on a Gaertner model L116C ellipsometer with a He-Ne laser (λ ) 632.8 nm) and a fixed angle of incidence of 70°. AFM studies were made at ambient conditions on a multimode scanning probe microscope (Digital Instruments) in tapping mode with a silicon tip (Multi-75) that was purchased from NanoDevices, Inc. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer instrument using Al KR radiation at the MATNET Surface Analysis Center at Case Western Reserve University. The takeoff angle was 45° with respect to the surface normal, making the X-ray-to-electron angle 90°. Substrate Preparation. ATR crystals and silicon wafers were cleaned by treatment with freshly prepared piranha solution (70/30 v/v concentrated H2SO4/30% H2O2(aq)) at 100 °C for 2 h and were then rinsed with distilled water and dried with a stream of clean air. It should be noted that piranha solution is extremely reactiVe and should be handled with appropriate care. General Procedure for Deposition of Surface Bound Initiator. Into a dry round-bottomed flask was placed a freshly cleaned silicon wafer and an ATR crystal. The flask was sealed using a septum and flushed with high-purity nitrogen for 30 min. Dry toluene (10 mL) and a 25 vol % solution of the trichlorosilane initiator in toluene (0.2 mL) were added to the flask via syringe, and the flask was held at 60 °C for 4 h under an atmosphere of nitrogen. The silicon wafer and ATR crystal were then removed, sequentially washed with toluene and EtOH, and then dried in a stream of air. The syntheses of Si/SiO2//poly(tert-butyl acrylate) polymer brushes and their subsequent pyrolysis have been previously reported.23 The syntheses of Si/SiO2//polystyrene and Si/SiO2//poly(methyl acrylate) polymer brushes, subsequent chain extension with tert-butyl acrylate, and acid hydrolysis have been previously reported.11 The reaction of the afforded polyelectrolyte polymer brushes with aqueous metal (23) Treat, N. D.; Ayres, N.; Boyes, S. G.; Brittain, W. J. Macromolecules 2006, 39, 26. (24) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.
Langmuir, Vol. 23, No. 1, 2007 183 salts and reduction to form metallic nanoparticles is detailed in the same reference.11 Typical ATRP Procedure for 2-(N,N-Dimethylamino)ethyl Methacrylate (DMAEMA) Polymerization from Initiator-Coated Silicon Substrates. A Schlenk flask was charged with anhydrous 1,2-dichlorobenzene (15 mL), DMAEMA (15 mL, 89 mmol), and the ligand HMTETA (0.20 mL, 0.74 mmol). The contents of the flask were then degassed using the freeze/pump/thaw method. Initiator-coated silicon wafers, CuBr (0.052 g, 0.36 mmol), and CuBr2 (0.016 g, 0.072 mmol) were placed into a second Schlenk flask, and the contents were deoxygenated by three vacuum purge/ nitrogen fill cycles. The monomer solution was heated to 60 °C for 15 min and added to the wafers via a degassed cannula. The free initiator ethyl bromoisobutyrate (22 µL, 0.15 mmol) was added, and the polymerization was conducted for 5 h at 60 °C. After this time, the polymerization was quenched by exposure to air, and the wafer was removed and rinsed with methylene chloride, ethanol, and THF before being placed in a Soxhlet apparatus charged with THF overnight. The wafers were then recovered and rinsed with toluene and THF before being dried in a stream of air. Typical ATRP Procedure for tert-Butyl Methacrylate Polymerization from Poly(DMAEMA)-Coated Silicon Substrates. A Schlenk flask was charged with tert-butyl methacrylate (18.9 mL, 0.116 mol), acetone (13 mL), and CuBr (0.043 g, 0.0003 mol), and the contents were degassed by the freeze/pump/thaw method. Poly(DMAEMA)-coated silicon wafers and CuBr were placed into a separate Schlenk flask, and the flask was deoxygenated with three vacuum purge/nitrogen fill cycles. The monomer solution was then heated to 60 °C, and PMDETA was added (0.094 mL, 0.00045 mol). After stirring for 15 min, the monomer solution was then added to the wafers via a degassed cannula. The free initiator ethyl bromoisobutyrate was added, and the polymerization was allowed to proceed for 4 h at 60 °C. After this time, the polymerization was quenched by exposure to air, and the wafer was removed and rinsed with methylene chloride, ethanol, and THF before being placed in a Soxhlet apparatus charged with THF overnight. The wafers were then recovered and rinsed with toluene and THF before being dried in a stream of air. Typical Procedure for Deprotection of tert-Butyl Methacrylate to Methacrylic Acid. A poly(DMAEMA-b-tert-BMA)-modified silicon wafer was placed on a watch glass and then in an oven preheated to 200 °C for 30 min. The wafer was then rinsed thoroughly with DI water, immersed in DI water overnight, and blown dry with a stream of air.
Results and Discussion Chemical-Free Deprotection Strategy to Yield StimuliResponsive Poly(acrylic acid) Polymer Brushes: Pyrolysis. Acrylic acid is difficult to polymerize under ATRP control because of the chelating effect of the monomer. A strategy to circumvent this problem is to polymerize tert-butyl acrylate, which proceeds to high conversion and yields polymers of narrow polydispersity that will readily reinitiate upon further addition of monomer.25-30 The tert-butyl group acts as a protecting group and can be can be removed by acid hydrolysis, giving the desired acrylic acid functionality. It should be noted that the direct aqueous polymerization of the sodium salt of acrylic acid from a surface (25) 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. (26) Zhang, Q.; Remsen, E. E.; Wooley, K. L. J. Am. Chem. Soc. 2000, 122, 3642. (27) Ma, Q.; Wooley, K. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4805. (28) Cheng, G.; Boeker, A.; Zhang, M.; Krausch, G.; Mueller, A. H. E. Macromolecules 2001, 34, 6883. (29) Francis, R.; Lepoittevin, B.; Taton, D.; Gnanou, Y. Macromolecules 2002, 35, 9001. (30) Burguiere, C.; Chassenieux, C.; Charleux, B. Polymer 2002, 44, 509.
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Ayres et al.
Scheme 1. Synthetic Route for the Formation of Si/SiO2//Poly(acrylic acid) Brushes from a Silica Substrate
has been reported;31 however, the chemical deprotection of the tert-butyl ester is a more common strategy for forming acrylic acid polymer brushes. The acid hydrolysis approach for the conversion of tert-butyl acrylate to acrylic acid is problematic for polymer brushes because the anchoring moiety often contains an ester functional group. Thus, this group can also be cleaved, leading to the loss of the polymer brush from the surface. It is desirable to combine the benefits and control of the tert-butyl acrylate polymerization with a deprotection strategy that does not cleave the brush from the surface. This problem motivated us to examine a pyrolysis approach where the tert-butyl ester groups are converted to acrylic acid upon heating.32 The experimental procedure for the synthesis of a poly(tert-butyl acrylate) brush and subsequent transformation to a poly(acrylic acid) brush is outlined in Scheme 1.23 The synthesis of the poly(tert-butyl acrylate) brush was confirmed by ATR-FTIR, contact angle analysis, and ellipsometry. The ATR-FTIR spectrum (Figure 1) contained the expected
Figure 1. ATR-FTIR spectra of Si/SiO2//poly(tert-butyl acrylate) (solid line) and Si/SiO2//poly(acrylic acid).
peaks at 1730 cm-1 (CdO stretch) and 2977 cm-1 (asymmetric CH3 stretching vibration) and a doublet at 1368/1392 cm-1 (symmetric methyl deformation mode). The contact angles recorded (93° advancing and 71° receding) correlated well with literature values whereas the thickness of the film was 36 nm (Table 1). (31) Osborne, V. L.; Jones, D. M.; Huck, W. T. S. Chem. Commun. 2002, 1838. (32) Klemm, L. H.; Antoniates, E. P.; Lind, C. D. J. Org. Chem. 1962, 27, 519.
Table 1. Water Contact Angle and Ellipsometry Data for the Si/SiO2//Poly(tert-butyl Acrylate) Polymer Brush and the Si/SiO2//Poly(acrylic acid) Polymer Brush Derived from Its Pyrolysis water contact anglea (deg) sample
Θa
Θr
thicknessb (nm)
Si/SiO2//poly(tert-butyl acrylate) Si/SiO2//poly(acrylic acid)
93 48
71 34
36 16
a Standard deviation of the contact angles is
