pubs.acs.org/Langmuir © 2009 American Chemical Society
Facile Method to Prepare Smooth and Homogeneous Polymer Brush Surfaces of Varied Brush Thickness and Grafting Density Shengqin Wang and Yingxi Zhu* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556 Received May 20, 2009. Revised Manuscript Received October 5, 2009 This Article describes a facile method to prepare smooth and homogeneous polymer brush surfaces of variable grafting density from a solid surface by combining Langmuir-Blodgett (LB) deposition with surface-initiated atom transfer radical polymerization (SI-ATRP). This method is successfully demonstrated by the preparation of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush surfaces on smooth silicon and quartz substrates. With the custom-synthesized inert diluent whose chemical structure, except end-functionality, is the same as that of the reactive initiator, smooth and chemically homogeneous mixed monolayers of initiators and inert diluents are immobilized on a solid surface by LB deposition, allowing the further variation of the grafting density of PNIPAM brushes grafted from the initiator monolayers of varied initiator coverage. With the optimized molar ratio of deactivator, Cu(II) in the Cu(I)ligand catalyst complex, the brush thickness of PNIPAM brushes at varied grafting density is controlled to grow nearly linearly with reaction time while smoothness and chemical homogeneity of PNIPAM brushes are achieved. For the demonstrated PNIPAM brush surfaces, the thermoresponsive characteristics of PNIPAM brushes are also verified. This combined LB-ATRP method can be applied to graft a variety of polymer brushes, including polyelectrolytes and block copolymers, from different solid substrates.
Introduction The polymer brush surface is made up of a thin layer of brushlike polymer chains with one end attached to a solid surface and the other extended away from the surface.1-4 The densely packed surface-tethered polymer chains in close proximity to each other result in a stretched, brushlike conformation to minimize segment-segment interactions. For decades, polymer brush surfaces have been widely used for coating, lubrication, biosensing, and many other chemical and biomedical applications.5-11 Among several different polymerization methods that have been developed to robustly produce polymer brush thin layers from various substrates,1,3 surface-initiated atomic transfer radical polymerization (SI-ATRP) has recently emerged as a well-controlled and versatile method to grow various polymer brushes from solid substrates because of its accurate control of brush thickness with a narrow distribution in molecular weight, as well as its “living” characteristic to synthesize block copolymer brushes by reactivating the dormant chain ends with a different monomer. Additionally, in comparison to other “living” (1) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25(5), 677–710. (2) Ruhe, 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. N. Polyelectrolyte brushes. In Polyelectrolytes with Defined Molecular Architecture I; Schmidt, M., Ed.; Springer-Verlag: Berlin, 2004; Vol. 165, pp 79-150. (3) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33(1), 14–22. (4) Toomey, R.; Tirrell, M. Annu. Rev. Phys. Chem. 2008, 59, 493–517. (5) Currie, E. P. K.; Norde, W.; Stuart, M. A. C. Adv. Colloid Interface Sci. 2003, 100, 205–265. (6) Nath, N.; Hyun, J.; Ma, H.; Chilkoti, A. Surf. Sci. 2004, 570(1-2), 98–110. (7) Tugulu, S.; Arnold, A.; Sielaff, I.; Johnsson, K.; Klok, H. A. Biomacromolecules 2005, 6(3), 1602–1607. (8) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U. I.; Nakamura, K.; Kawaguchi, H. Nat. Mater. 2004, 3(11), 829–836. (9) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119(11), 2739–2740. (10) Granville, A. M.; Brittain, W. J. Macromol. Rapid Commun. 2004, 25(14), 1298–1302. (11) Kim, D. J.; Lee, K. B.; Lee, T. G.; Shon, H. K.; Kim, W. J.; Paik, H. J.; Choi, I. S. Small 2005, 1(10), 992–996.
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polymerizations such as cationic and anionic polymerization, SI-ATRP is much less sensitive to the presence of the impurity in the reaction solution and compatible with a broad range of functional monomers.12-18 To prepare a smooth polymer brush surface by SI-ATRP, the immobilization of initiators to form a close-packed, smooth initiator monolayer on a solid substrate, such as glass and silicon substrate, is one of the most critical steps. It is often complemented by self-assembled monolayers (SAMs) of end-functionalized initiators on silicon, glass, and gold surfaces. The self-assembly method is simple yet could often produce rough and inhomogeneous initiator monolayers on smooth substrates such as mica, quartz, and silicon, especially when trifunctional silane-based initiators are used. First, the initiator aggregation that is induced by excess water in solution during the silane hydrolysis can introduce defects and aggregation islands on SAMs to subsequently lead to high roughness. Second, the reactivity of silane-based initiators could be considerably reduced when a fraction of functional groups are buried within the SAMs so as not to be accessible to the monomers in the commonly used SAM-ATRP method; for instance, previous X-ray photoelectron spectroscopy (XPS) measurements have shown that only 50% of end groups in amine end-functionalized SAMs are reactive,19,20 (12) 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(26), 8716–8724. (13) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33(2), 597–605. (14) Huang, X.; Wirth, M. J. Macromolecules 1999, 32(5), 1694–1696. (15) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33(23), 8813–8820. (16) Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35(4), 1175–1179. (17) Bontempo, D.; Tirelli, N.; Feldman, K.; Masci, G.; Crescenzi, V.; Hubbell, J. A. Adv. Mater. 2002, 14(17), 1239. (18) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38(5), 647–649. (19) Bierbaum, K.; Kinzler, M.; Woll, C.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11(2), 512–518. (20) Kristensen, E. M. E.; Nederberg, F.; Rensmo, H.; Bowden, T.; Hilborn, J.; Siegbahn, H. Langmuir 2006, 22(23), 9651–9657.
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resulting in considerably lower brush grafting density. Lastly, it is rather difficult to vary the grafting density of polymer brush surfaces by using mixed SAMs of reactive initiators and their inert diluents because of the voluntary segregation between silanes of distinct chemical structures, which is responsible for the island formation in SAMs.21 It should be noted that using monofunctional silanes for mixed initiator SAMs could be a possible solution22 to avoid the aggregation upon the hydrolysis of trifunctional silanes, yet it also suffers from the following shortcomings: The packing density of monofunctional silane monolayers23-25 is adversely affected by the presence of the side groups in all commercially available monofunctional silanes and often ∼50% lower than that of alkanethiol SAMs on gold26 and trifunctional silane SAMs,27,28 which ultimately prevents it from producing polymer brush surfaces of high grafting density; additionally, according to a recent systematic study29,30 of the reaction kinetics of both mono- and trifunctional silanes at solution-solid interfaces, it often takes much longer reaction time to produce monofunctional silane SAMs of high packing density than the time needed to produce trifunctional silane SAMs of similar density. Considering all the above possible problems associated with the SAM method, we instead employ the LB deposition method to produce molecularly smooth and homogeneous monolayers of mixed initiators and inert diluents on smooth solid substrates, where the aggregation of silanes is successfully prevented due to the absence of silanes in the aqueous subphase upon LB deposition.31 The application of the LB deposition method for immobilizing initiators for SI-ATRP was previously applied to graft hydrophobic polymer brush surfaces;32 however, no work has been done with the combined LB-ATRP method to produce smooth hydrophilic polymer brush surfaces in protic solvents, such as water or methanol, due to the instability of commonly used reactive initiators immobilized by LB deposition. Our method with tailor-made initiator and inert diluent is demonstrated as being applicable to graft both hydrophobic and hydrophilic polymer brushes from different substrates. For SI-ATRP, brush grafting density could be varied based on the initiator surface coverage in the monolayers by adjusting either reaction time or initiator concentration in the reaction solution. The strategy of controlling reaction time heavily relies on an adequate knowledge of surface reaction kinetics, which yet is difficult to determine accurately due to the insufficient quantity of reaction products at surface. Polyacrylamide33 and poly(glycidyl methacrylate) brush surfaces34 are among the few examples of controlling grafting density by varying reaction time. It was very recently reported that the grafting density of poly(tert-butyl acrylate) brushes on mica can be varied by properly (21) Fang, J. Y.; Knobler, C. M. Langmuir 1996, 12(5), 1368–1374. (22) Bao, Z. Y.; Bruening, M. L.; Baker, G. L. Macromolecules 2006, 39(16), 5251–5258. (23) Claudy, P.; Letoff, J. M.; Gaget, C.; Morel, D.; Serpinet, J. J. Chromatogr., A 1985, 329, 331–349. (24) Boksanyi, L.; Liardon, O.; Kovats, E. S. Adv. Colloid Interface Sci. 1976, 6 (2), 95–137. (25) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86(26), 5208–5219. (26) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111(1), 321–335. (27) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100(2), 465–496. (28) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111(15), 5852–5861. (29) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15(11), 3759–3766. (30) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16(18), 7268–7274. (31) Wood, J.; Sharma, R. Langmuir 1994, 10(7), 2307–2310. (32) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31(17), 5934–5936. (33) Wu, T.; Efimenko, K.; Genzer, J. J. Am. Chem. Soc. 2002, 124(32), 9394– 9395. (34) Liu, Y.; Klep, V.; Zdyrko, B.; Luzinov, I. Langmuir 2004, 20(16), 6710–6718.
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controlling both the immobilization reaction time and the concentration of ATRP initiator.35 The simpler strategy of varying the brush grafting density is to adjust the molar ratios of the reactive initiators to their inert diluents.36-38 However, when the chemical structure of an inert diluent is highly dissimilar to that of its initiator chain, segregation in mixed monolayers could lead to the island formation and high surface roughness and inhomogeneity;39,40 to avoid the island formation, an inert diluent with high similarity in chemical structure to the reactive initiator should be used to control the brush grafting density.22 In this work, we thus custom synthesize the particular inert diluent, whose chemical structure, except end-functionality, is the same as that of the reactive initiator, to minimize the segregation in mixed monolayers. By using the LB deposition of hydrolyzed initiator and inert silanes to a solid surface at the water-air interface, the initiator surface coverage in the mixed monolayer can be simply controlled by the molar ratios of the reactive silane initiators to their inert diluents in the mixture solution without knowing their respective surface reaction kinetics as required by the SAM method. Therefore, we can easily control the grafting density of polymer brush surfaces by LB deposition of mixed reactive initiator and its analogous inert diluent at varied molar ratios on solid substrates, from which homogeneous polymer brushes can be subsequently grafted without island formation. In this Article, we demonstrate our facile LB-ATRP method by preparing smooth and homogeneous thermoresponsive PNIPAM brush surfaces of varied grafting density, from 0.04 to 0.62 chains/nm2, on silicon and quartz substrates under optimized ATRP catalytic conditions. The thermoresponsive characteristics and lower critical solution temperature (LCST) of PNIPAM brushes are verified. Our method of combining LB and SI-ATRP is also applicable to graft various hydrophobic (e.g., polystyrene) and hydrophilic (e.g., polymethacrylic acid) polymer brushes from different substrates such as silica and indium tin oxide surfaces.
Experimental Section Materials. Amino-undecyltriethoxylsilane (AUTES) was purchased from Gelest Inc. and used as received. All other reagents used in this work were purchased from Aldrich and used as received unless specified otherwise. Tetrahydrofuran (THF) and chloroform were distilled by calcium hydride to eliminate water. Monomer N-isopropylacrylamide (NIPAM) was recrystallized twice from the mixture of benzene and n-hexane (35/65, v/v) before use. Deionized water (18 MΩ cm) used in this work was purified through a Barnstead Nanopure II system. Silicon wafer used as the substrate in this work was cut into 1 cm 1 cm pieces and cleaned via sonication in acetone, methanol, and water each for 5 min subsequently. The quartz coverslip (ESCO) was also cleaned in the same procedure. After being dried with a flow of nitrogen gas (Purity >99.9%), the substrate was exposed in a UV/ozone chamber (Jelight 144AX) for 10 min to ensure a hydrophilic surface with water contact angle e 0 ( 2°. All glassware used in this work was cleaned by being soaked in piranha solution (30% H2O2 and 70% H2SO4) overnight, thoroughly rinsed with copious deionized water, and dried with nitrogen gas before use. (35) Lego, B.; Francois, M.; Skene, W. G.; Giasson, S. Langmuir 2009, 25(9), 5313–5321. (36) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18(4), 1265–1269. (37) Nagase, K.; Kobayashi, J.; Kikuchi, A. I.; Akiyama, Y.; Kanazawa, H.; Okano, T. Langmuir 2008, 24(2), 511–517. (38) Jordi, M. A.; Seery, T. A. P. J. Am. Chem. Soc. 2005, 127(12), 4416–4422. (39) Ejaz, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2000, 33(8), 2870–2874. (40) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13(6), 1558– 1566.
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Scheme 1. Synthesis of (a) the ATPR Initiator, 2-Bromo-2-methyl-N-(11-(triethoxysilyl)undecyl)propanamide (BMTUP), (b) Inert Diluents, N-(11-(Triethoxysilyl)undecyl)pivalamide (MTUP) in Two Steps with the Synthesis of (i) 2-Methylpropionyl bromide (2-MPB) and then (ii) MTUP, and (c) Grafted PNIPAM Brushes from (i) BMTUP Monolayer and (ii) Mixed BMTUP/MTUP Monolayer by SI-ATRP
Synthesis of ATRP Initiator and Its Inert Analogue. The reactive initiator, 2-bromo-2-methyl-N-(11-(triethoxysilyl)undecyl)propanamide (BMTUP), was synthesized as illustrated in Scheme 1a. AUTES (1.0 g, 3.0 mmol) and anhydrous pyridine (242 μL, 3.0 mmol) were dissolved in 20 mL of THF under continuous stirring. When R-bromoisobutyryl bromide (370 μL, 3.0 mmol) was added dropwise, the solution became cloudy with the appearance of white precipitation due to the reaction product of salt. After the completion of the reaction at room temperature in 1 h, the suspension was filtered through a 0.2 μm PTFE filter to obtain the oily BMTUP in light yellow color after removing the solvent. 1H NMR δ: 3.83 (m, 6H, CH2), 3.22 (m, 2H, CH2), 1.97 (s, 6H, CH3), 1.55 (m, 2H, CH2), 1.21-1.27 (m, 23H, CH2, CH3), 0.61 (t, 2H, CH2). 13C NMR δ: 170.8 (CdO), 58.5 (CH2), 40.7 (CH2), 33.5 (CH), 32.9 (CH2), 29.8 (CH2), 29.5 (CH2), 27.1 (CH2), 18.6 (CH3), 17.5 (CH2), 10.6 (CH2). The inert analogue to BMTUP, 2-methyl-N-(11-(triethoxysilyl)undecyl)pivalamide (MTUP), was synthesized as illustrated in Scheme 1b. Phosphorus tribromide (15.2 mL, 0.164 mol) was added dropwise to isobutyric acid (6.4 mL, 0.068 mol) under continuous stirring first at 0 °C for 3 h and then at room temperature for 15 h to synthesize 2-methylpropionyl bromide (2-MPB), as illustrated in Scheme 1b-(i). Fractional distillation of the reaction mixture under continuously purging nitrogen gas finally produced 2-MPB as a clear liquid with 70% yield in mass. As illustrated in Scheme 1b-(ii), the procedure and reaction 13450 DOI: 10.1021/la901785t
conditions to synthesize the inert analogue, 2-methylN-(11-(triethoxysilyl) undecylpropanamide (MTUP), were the same as those for the BMTUP synthesis except for the replacement of R-bromoisobutyryl bromide with 2-MPB. MTUP was also obtained as an oily liquid in light yellow color after filtering. 1 HNMR δ: 3.83 (m, 6H, CH2), 3.22 (m, 2H, CH2), 1.56 (m, 2H, CH2), 1.15-1.26 (m, 29H, CH2, CH3), 0.61 (t, 2H, CH2). 13C NMR δ: 170.2 (CdO), 58.4 (CH2), 40.0 (CH2), 36.0 (CH), 33.6 (CH2), 29.7 (CH2), 27.2 (CH2), 20.0 (CH3), 18.5 (CH3), 15.4 (CH2), 10.4 (CH2).
Deposition of Initiator and Initiator/Inert Monolayers to Silicon Surfaces. Monolayers of BMTUP, MTUP, and their mixtures of varied BMTUP/MTUP molar ratios from 10/90, 25/ 75, and 50/50 to 75/25 were deposited to cleaned silicon substrates in the nitric acid subphase of pH = 2 by using a Langmuir trough (Nima 301A). Solutions of BMTUP, MTUP, and their mixtures in chloroform of typically ∼2 mg/mL in concentration were spread onto the freshly aspirated subphase and allowed to sit for 30 min to complete the hydrolysis of silanes before compressing them at a constant rate of 15 cm/min. Surface pressure (π)-area (A) isotherms were measured upon compression at 25 °C. All isotherms were repeated at least twice in the direction of compression with freshly spreading films before the monolayers were transferred to the silicon surfaces at a constant rate of 1 mm/min.
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N-isopropylacrylamide (NIPAM) was performed using a modified literature procedure,41 as illustrated in Scheme 1c. In a clean Schelenk tube, NIPAM (2.5 g, 22 mmol) and N,N,N0 ,N0 -pentamethyldiethylenetriamine (PMDETA) (108 μL, 0.55 mmol) were dissolved in 10 mL of MeOH/H2O mixture (1/1, v/v) and degassed by two freeze-thaw cycles. Under the continuous purge of nitrogen gas, 0.11 mmol of CuBr (or CuCl) as the catalyst in the CuBr2 (or CuCl2)-PMDETA complex was quickly added to control the Cu(II)/Cu(I) molar ratios varied from 5/100 to 30/100 in the reaction solution. The solution was subsequently transferred via a clean cannula to another degassed Schelenk tube including an initiator-coated silicon wafer inside. ATRP under the stream of nitrogen gas was conducted at room temperature over 5-240 min to vary PNIPAM brush thickness. The reaction was concluded by disconnecting the nitrogen stream from the Schlenk tube. Surface-tethered PNIPAM brush surfaces were first thoroughly rinsed with deionized water, followed with sonication in ethanol and water, and finally dried under the flow of nitrogen gas before being stored in a drybox. Characterization of Polymer Brush Surfaces. The surface morphology of all initiator monolayers of varied initiator surface coverage and PNIPAM brush surfaces of varied grafting density and thickness were characterized by tapping-mode atomic force microscopy (AFM; Veeco, Multimode) using a silicon tip (Veeco). The root-mean-square (rms) roughness was measured over a scanning area of 10 μm 10 μm. The thickness of dry PNIPAM brush surfaces (index of refraction, n = 1.49) was determined by ellipsometry (Gaertner, model L116C) at the fixed laser wavelength of λ = 632.8 nm and incidence angle of 70°. The reported thickness in this paper was averaged over measurements at five different spots on PNIPAM brush surfaces. The dry thickness, hdry was also confirmed by AFM using the section analysis of the scratches cut through the PNIPAM brush surfaces by a clean and sharp blade (see Figure 5a). The wet thickness, hwet, of swollen PNIPAM brushes when immersed in deionized water in a custom-built fluid cell was measured by phase-modulated ellipsometry (Beaglehole). The measurements of hwet commenced at least 30 min after the immersion of the PNIPAM brush surface in the fluid cell to ensure the full swelling of PNIPAM brush chains. The ratio of measured dry and wet thicknesses of polymer brush chains was used to estimate the grafting density of PNIPAM brush surfaces by using the mean-field theoretical model42 as discussed below. The thermoresponsive characteristics of PNIPAM brush surfaces were examined with the temperature-dependence of surface hydrophobicity and wet brush thickness. The static contact angle of sessile drops of water placed on PNIPAM brush surfaces in the sample cell of a heating stage (INSTEC, model HCS60) was measured by using a goniometer (Rame-Hart, model 250) as temperature, T, was varied from 25 to 40 °C. As mentioned above, the wet thickness of PNIPAM brushes in a custom-built fluid cell with a temperature controller (Lake Shore) was measured by phase-modulated ellipsometry (Beaglehole) at T = 25-40 °C. The thickness measurement was repeated 10 min after a new equilibrium temperature was reached.
Results and Discussion Surface Characterization of Initiator Monolayers. The packing density and homogeneity of initiator monolayers are critical to produce smooth and homogeneous polymer brush surfaces from a solid substrate. The LB deposition condition is optimized from the measured surface pressure (π)-area (A) isotherms for the monolayer deposition of BMTUP initiators, MTUP inert diluents, and their mixtures of varied molar ratios. (41) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. Adv. Mater. 2002, 14(16), 1130–1134. (42) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121(5), 1016–1022.
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Figure 1. Surface pressure-area (π-A) isotherms of BMTUP (black), BMTUP/MTUP mixed in molar ratios of 75/25 (red), 50/50 (green), and 25/75 (blue), and MTUP (purple), spread to the nitric acid aqueous solution of 10-2 M at 25 °C. Inset: Close-up view of π-A isotherms.
As shown in Figure 1, all the π-A isotherms appear similar to each other and are consistent with the results reported for other similar silanes.32 The critical pressure for the transition from the liquid condensed region to the solid condensed region upon compression is 9.7 mN/m for 100 mol % BMTUP and shifts to 10.3, 9.5, 8.8, and 8.0 mN/m with increasing molar ratio of MTUP in the spreading mixture from 25, 50, 75 to 100 mol %, respectively; the increase in the measured critical pressure with increasing BMTUP molar ratio results from the stronger repulsion between two BMTUP head groups than that between BMTUP and MTUP ones. The actual pressures to deposit initiator monolayers to a silicon or quartz substrate are optimized to be 1 mN/m lower than their corresponding critical pressures at varied BMTUP/MTUP molar ratios to prevent the collapse of monolayers before thin film transfer. The morphology of LB-deposited initiator monolayers was characterized by tapping-mode AFM. AFM height micrographs are exhibited for 100 mol % BMTUP and 10 mol % BMTUP monolayers in Figure 2a and b, respectively; the deposited thin films appear very smooth as indicated by the measured rms roughness of 0.2 nm for both cases, and no island formation is observed with both monolayers, in sharp contrast to the aggregation domains that are often observed with silane SAMs43 or LBdeposited monolayers of long alkane silanes such as octadecyltriethoxylsilane.31 Height and phase AFM micrographs are shown for the mixed monolayer of 50 mol % BMTUP in Figure 2c and d, respectively; the topographical smoothness of the mixed monolayer is achieved with the measured rms roughness of 0.2 nm, and no evident segregation is exhibited in the AFM phase micrograph, suggesting a chemically homogeneous monolayer in which BMTUP and MTUP are well mixed and closepacked. Similar morphology is observed with all the mixed monolayers where the molar ratio of BMTUP in the BMTUP/ MTUP mixtures is varied from 90 mol % down to 10 mol %. In contrast to the segregation resulting from chemically distinct initiator and diluent, for instance, 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane and octadecyltriethoxylsilane used in the mixed monolayer,44 we expect that the high similarity in alkane chain length and silane chemistry, except end-functionality, plays (43) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102(37), 7190–7197. (44) Ejaz, M.; Yamamoto, S.; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35 (4), 1412–1418.
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Figure 2. AFM height micrographs of (a) 100 mol % BMTUP initiator monolayer, (b) 10 mol % BMTUP, and (c) 50 mol % BMTUP in mixed BMTUP/MTUP monolayers, all exhibited in the same height scale from 0 to 5 nm. (d) AFM phase micrograph of the mixed monolayer of 50 mol % BMTUP. All images were obtained over a scanning area of 10 μm 10 μm.
a crucial role for the formation of smooth and homogeneous mixed monolayers, where two similar silanes are compatible to each other and thus can be well dispersed at the water-air interface rather than segregate from each other. Similar morphology is also obtained with mixed initiator monolayers on a quartz surface. Thus, the success in preparing smooth and homogeneous initiator monolayers of varied initiator molar ratios by LB deposition allows us to graft PNIPAM polymer brushes of varied grafting density from BMTUP initiator monolayer by SI-ATRP. Surface Characterization of PNIPAM Surfaces of Varied Brush Thickness. In this work, PNIPAM brush surfaces from smooth BMTUP monolayers were prepared under an optimized catalytic condition of 5 mol % CuBr2 in the CuCl/ CuBr2/PMDETA complex, in which a nearly linear polymerization rate for synthesizing PNIPAM brushes was achieved (see the Supporting Information). The thickness of PNIPAM brushes is thereby varied by controlling polymerization time (see Figure S1 in the Supporting Information). The morphology of grafted PNIPAM brushes from 100 mol % BMTUP initiator monolayer under the optimal reaction condition is characterized by AFM as shown in Figure 3. The rms roughness measured for the PNIPAM brush surface of dry thickness, d = 20 nm, after ATRP over 15 min is 0.7 nm over a scan area of 10 μm 10 μm as shown in Figure 3a, indicating a smooth PNIPAM brush surface. As the reaction time is extended to 240 min to obtain a PNIPAM brush surface of d = 112 nm, the surface roughness slightly increases to 0.9 nm as shown in Figure 3b. It is also observed that the rms roughness decreases considerably with decreasing polymerization rate with the increased concentration of deactivating catalyst (under a nonoptimized ATRP condition as shown in Figure S1 in the Supporting Information); for instance, for the PNIPAM brush surface of d = 23.5 nm, which is produced after ATRP for 240 min in the reaction solution with 30 mol % deactivating CuBr2, the measured rms roughness is ∼0.4 nm over the micrograph shown in 13452 DOI: 10.1021/la901785t
Figure 3c. It is suggested that the linear growth of polymer brush thickness via SI-ATRP could be possibly compromised if achieving a smooth morphology of polymer brush surfaces is highly preferred. It should also be noted that, in this work, we focus on preparing smooth and chemically homogeneous PNIPAM brush thin films, not the aspect of PNIPAM molecular weight distribution, which is also practically difficult to determine for tethered PNIPAM brushes on a silicon or quartz substrate. However, we expect that narrow molecular weight distribution in polymer brushes grafted from a molecularly smooth initiator monolayer could be manifested by the characterized topographical smoothness. Control the Grafting Density of PNIPAM Brush Surfaces. The grafting density of PNIPAM brushes is controlled to decrease by increasing the molar ratio of the inert diluent, MTUP, in the BMTUP/MTUP mixed monolayer. As summarized in Table 1, the BMTUP initiator surface coverage is obtained from the corresponding π-A isotherms in Figure 1 to be 9.0 molecules per nm2 for 100 mol % BMTUP initiator monolayer and decreases to 6.5, 4.0, 1.8, and 0.7 molecules per nm2 for 75, 50, 25, and 10 mol % BMTUP in the mixed BMTUP/MTUP monolayers, respectively. As we focus on the morphology of polymer brush surface, not the polymerization kinetics of the polymer brush, the effect of brush grafting density on polymerization kinetics is just briefly examined as shown in Figure 4. The polymerization rate of PNIPAM brushes decreases with decreasing molar ratio of BMTUP initiator in the mixed monolayer; PNIPAM brush thickness saturates over the reaction time of 60-120 min when the molar ratio of BMTUP decreases to e50 mol %. We contribute the reduction in PNIPAM polymerization rate with the decreased grafting density to the collapse of surface-tethered “living” PNIPAM chains during ATRP, which could lead to the immersion of terminal C-Cl bonds into the collapsed polymer network. The inset of Figure 4 indicates a linear relationship of Langmuir 2009, 25(23), 13448–13455
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Figure 3. AFM height micrographs of dry PNIPAM brushes of (a) 20 nm thick and (b) 112 nm thick, both grafted from 100 mol % BMTUP monolayers in the reaction solution with [CuCl] = 11 mM and CuBr2/CuBr = 5/100 in molar ratio, and (c) dry PNIPAM brushes of 23.5 nm thick, polymerized in the reaction solution with [CuCl] = 11 mM and CuBr2/CuBr = 30/100. All the AFM micrographs are exhibited in the same height scale from 0 to 10 nm. Table 1. Initiator Surface Coverage and PNIPAM Brush Grafting Density for Varied Initiator Concentrations in the Mixed Solution of Initiator and Inert Silanes initiator molar ratio in the mixture (mol %) 10 25 50 75 100 0.7 1.8 4.0 6.5 9.0 initiator surface coverage (molecule/nm2) PNIPAM brush grafting density (chain/nm2) 0.04 0.13 0.33 0.52 0.62
Figure 4. Growth of the dry brush thickness of PNIPAM brush surfaces versus the polymerization time, from the 100 mol % BMTUP initiator monolayer (squares) and mixed BMTUP/ MTUP monolayers of (a) 75 mol % (circles), (b) 50 mol % (triangles), (c) 25 mol % (down triangles), and (d) 10 mol % BMTUP (tilted squares). SI-ATRP is conducted in the reaction solution of [NIPAM] = 2.2 M, [CuCl] = 11 mM, [CuBr2] = 0.22 mM, and [PMDETA] = 55 mM in 10 mL of 1/1 (v/v) MeOH/ H2O at room temperature. Thickness data were averaged over ellipsometry measurements at five different spots on one PNIPAM brush surface sample. Inset: Dry thickness of PNIPAM brushes versus initiator grafting density at a given polymerization time of 30 min (squares) and 60 min (circles).
PNIPAM polymerization rate versus the brush grafting density, with a simple assumption that the molecular weight of PNIPAM brushes is constant at varied brush grafting density. The smoothness and chemical homogeneity of PNIPAM brushes of varied grafting density were systematically examined as AFM height and phase micrographs are shown in Figure 5. The AFM phase micrographs shown in Figures 2 and 5 confirm that both mixed monolayers and grafted PNIPAM brushes from the mixed monolayers are smooth and chemically homogeneous. The AFM micrographs in Figure 5 also confirm that no aggregation islands are observed over a large area of 50 μm 50 μm for PNIPAM brushes grafted from 50% BMTUP monolayers. The rms roughness for PNIPAM brush surfaces of varied grafting density, which are all prepared under the high polymerization rate with 5 mol % deactivator CuBr2 in the catalytic complex, ranges from 0.8 to 1.4 nm and exhibits a slight increase with reduced grating density. Noting that the roughness of all mixed Langmuir 2009, 25(23), 13448–13455
monolayers is within 0.2 ( 0.2 nm, we thus consider that the increased roughness could possibly result from partially collapsed polymer chains at lower grafting density. Nevertheless, the achieved chemical homogeneity and smoothness of PNIPAM brushes grafted from molecularly smooth mixed monolayers indicate that using an inert analogue with the highest chemical similarity to its reactive initiator is crucial to minimize the segregation in mixed initiator monolayers and polymer brushes of varied grafting density. The grafting density, σ, of PNIPAM brushes is estimated45 from the measured thickness of PNIPAM brush thin films in the dry and swollen states by using the theoretical model. According to the mean-field theory for polymer brushes in a good solvent,42 the configuration of a polymer brush chain in a good solvent could be considered as an alignment of blobs, whose diameter approximates to the distance between two neighboring grafting sites; thus, the wet thickness, hwet, of swollen PNIPAM brush chains in water can be simply estimated as hwet ¼ ξðN=N1 Þ
ð1Þ
-1/2
is the distance between the two neighboring where ξ = 2(πσ) grafting sites, N1 is the number of monomers inside one blob, and N is the total number of monomers in a polymer brush chain. The chain segments inside each blob can be considered to be selfavoiding walking and hence follow the scaling law of ξ ∼ N13/5. According to the reported radius of gyration, Rg, of PNIPAM chains of molecular weight Mn = 1.3 107 g/mol in water at 20 °C measured to be 185 nm by light scattering,46 we could obtain the approximate relation of Rg with N for a PNIPAM chain as Rg = 0.17N3/5 Thus, the wet brush thickness can be described as hwet ¼ 2ðπσÞ -1=2
N -1=2
ðπσÞ 0:17
1=3 5=3 ¼ 0:153Nσ
ð2Þ
(45) The direct measurement of brush grafting density is rather difficult due to the very small amount of polymer brush chains at surfaces. Furthermore, it is nearly impossible for our PNIPAM brush preparation method to cleave the grafted PNIPAM brush chains from the substrate and conduct further direct analysis (ex situ). Instead, the grafting density reported in this paper is indirectly obtained from brush thickness data and model-based.
DOI: 10.1021/la901785t
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Wang and Zhu
Figure 5. AFM micrographs of PNIPAM brushes grafted from the mixed monolayer of 50 mol % BMTUP in the reaction solution with [CuCl] = 11 mM and CuBr2/CuBr = 5/100 in molar ratio after a polymerization time of 15 min: (a) height image (left panel) over a scanning area of 50 μm 50 μm with scratches to measure the PNIPAM brush thickness of 12.1 nm by the section analysis (right panel), which is consistent with the measured of 10.8 nm by ellipsometry as reported in Figure 4; (b) height and (c) phase images over a scanning area of 10 μm 10 μm of the same sample.
while the dry brush thickness is given by hdry ¼ σNMn =NA F ¼ 0:149Nσ
ð3Þ
where Mn (=113.2 g/mol) is the molecular weight of NIPAM monomer, NA is Avogadro’s number, and F (=1.269 g/cm3) is the density of PNIPAM.47 By comparing eqs 2 and 3, the grafting density, σ, can be estimated from the measured hwet and hdry as hwet =hdry ¼ 1:03=σ 2=3
ð4Þ
The thickness of PNIPAM brushes grafted from the 100% initiator monolayer was measured by phase-modulated ellipsometry as hdry = 20.2 nm in the dry state and increased to hwet = 28.5 nm when the PNIPAM brushes swelled in deionzed water at T = 21.5 °C; the grafting density is thus estimated as 0.62 chains/ nm2 or an area of 1.6 nm2/chain. Compared to the grafting density of typically 0.28-0.31 chains/nm2 for polymer brushes grafted from the initiator monolayer by the SAM method,42 our LBATRP method clearly produces a polymer brush surface of much higher grafting density. As the polymerization degree, N, can be assumed as the same for polymer brushes of varied grafting density under the same reaction conditions and the same reaction time, a linear dependence of the dry brush thickness with the grafting density is expected and indeed confirmed with the PNIPAM brush synthesis by our method as shown in the inset of Figure 4. Therefore, with the comparison of the measured dry (46) Wu, C.; Wang, X. H. Phys. Rev. Lett. 1998, 80(18), 4092–4094.
13454 DOI: 10.1021/la901785t
thickness of varied PNIPAM brushes after 15 min of polymerization as shown in Figure 4 to that of 100 mol % PNIPAM brushes, the grafting density for PNIPAM brushes grafted from 75, 50, 25 and 10 mol % mixed monolayer is estimated to be 0.52, 0.33, 0.13, and 0.04 chains/nm2, respectively, as summarized in Table 1. Swelling and Thermoresponsive Characteristics of PNIPAM Brushes in Water. Temperature-dependent brush thickness and surface hydrophobicity of PNIPAM brush surfaces on a silicon substrate were examined to determine the lower critical solution temperature (LCST) of PNIPAM brushes.48 Figure 6a shows the static water contact angle on PNIPAM brushes of d = 60 nm grafted from the 100 mol % BMTUP monolayer. As T increases from 28 to 45 °C, the water contact angle gradually increases from 55° to 73°, indicating a broad LCST transition. The thermoresponsive behavior of PNIPAM brushes from a hydrophilic surface to a partially hydrophobic one is consistent with the LCST behavior of PNIPAM in solution, where intermolecular H-bonding between PNIPAM chains and water becomes intramolecular H-bonding within PNIPAM chains as T increases across the LCST. It is noted that polymer brushes often exhibit a broader LCST range than that of polymers in solution,49 which is also observed with our measured LCST of ∼30-40 °C for PNIPAM brushes in comparison to that of ∼32-34 °C for bulk PNIPAM in solution. (47) Ishida, N.; Biggs, S. Langmuir 2007, 23(22), 11083–11088. (48) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. Langmuir 2006, 22(9), 4259–4266. (49) Balamurugan, S.; Mendez, S.; Balamurugan, S. S.; O’Brien, M. J.; Lopez, G. P. Langmuir 2003, 19(7), 2545–2549.
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brushes measured in air increases to 28.5 nm after the PNIPAM brush surface is immersed in water for 30 min at constant T = 21.5 °C, indicating a swelling ratio of 141%. As shown in Figure 6b, the measured thickness of swollen PNIPAM brushes decreases with increasing T from 21.5 to 48 °C in the water fluid cell, due to the collapse of PNIPAM brushes with increasing T across the LCST. The LCST range determined from the temperature-dependent swollen PNIPAM brush thickness is ∼30-44 °C and is consistent with that obtained from the measurements of temperature-dependent PNIPAM surface hydrophobicity.
Figure 6. Thermoresponsive characteristics of PNIPAM brush surfaces. (a) Water contact angle on PNIPAM brush surfaces against temperature, T. PNIPAM brushes of 60 nm thick in air were grafted from the 100 mol % BMTUP initiator monolayer in the same reaction solution described in Figure 4. Inset: Digital photographs of water sessile drops on PNIPAM brush surfaces taken at T = 25 °C (lower left) and 45 °C (upper right). (b) Thickness of swollen PNIPAM brushes immersed in water against T. PNIPAM brushes of 20.2 nm in air were grafted from the 100 mol % BMTUP initiator monolayer in the same reaction solution described in Figure 4. Inset: Measured ellipticity of swollen PNIPAM brushes immersed in water against temperature. Dashed lines are a guide to the eye.
The broad LCST range for PNIPAM brushes is also confirmed by the temperature-dependent wet thickness of PNIPAM brush surfaces immersed in water using a phase-modulated ellipsometry. The swelling of water-immersed PNIPAM brushes is observed in Figure 6b: The thickness of 20.2 nm for dry PNIPAM
Langmuir 2009, 25(23), 13448–13455
Conclusions In summary, we report a facile method by combining LB deposition of initiator monolayers with SI-ATRP to prepare smooth and homogeneous polymer brush surfaces of varied brush thickness and grafting density from a smooth solid substrate. A tailor-made inert silane diluent, whose chemical structure and chain length, except end-functionality, are the same as those of the reactive silane initiator, makes it possible to produce molecularly smooth and homogeneous mixed initiator monolayers of varied initiator coverage by LB deposition. As a result, smooth and homogeneous polymer brush surfaces of varied grafting density are produced from the mixed initiator monolayers by SI-ATRP. We have successfully demonstrated our method by grafting thermoresponsive PNIPAM brushes from smooth silicon and quartz substrates. With the versatility of the LB deposition method to immobilize various initiators on a solid substrate and the well-controlled polymer brush growth by SI-ATRP under an optimized catalytic condition, our method can be applied to graft a variety of hydrophilic (e.g., polymethylacrylic acid) and hydrophobic (e.g., polystyrene) polymer brushes, also including polyelectrolyetes and block copolymers, from different solid substrates (e.g., silica, gold, and indium tin oxide). Acknowledgment. We thank Ashis Mukhopadhyay and Christopher Grabowski for their assistance in the ellipsometry characterization of PNIPAM brushes. This work is supported by DoE (DE-FG02-07ER46390). S.W. acknowledges the Bayer postdoctoral fellowship through the Center of Environment Science and Technology at the University of Notre Dame, and Y.Z. acknowledges a 3M nontenured faculty award. Supporting Information Available: Experimental details and data of thickness control of PNIPAM brushes. This material is available free of charge via the Internet at http:// pubs.acs.org.
DOI: 10.1021/la901785t
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