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Thermal Response of Poly(N-isopropylacrylamide) Brushes Probed by Surface Plasmon Resonance S. Balamurugan, Sergio Mendez, Sreelatha S. Balamurugan, Michael J. O’Brien II, and Gabriel P. Lo´pez* Center for Micro-Engineered Materials, Department of Chemical and Nuclear Engineering and Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131 Received November 1, 2002. In Final Form: January 17, 2003 The thermally induced hydration transition of surface-grafted poly(N-isopropyl acrylamide) (PNIPAAm) brushes was probed by surface plasmon resonance spectroscopy (SPR) and contact angle measurements. Data are presented for a PNIPAAm brush film with a dry thickness of ∼50 nm that was synthesized by atom radical transfer polymerization on the surface of a self-assembled monolayer on gold. SPR measurements were taken as a function of temperature in two modes: the quasi-static mode, in which the sample was equilibrated at each temperature for ∼15 min prior to measurement, and the real-time mode, in which SPR reflectivity data were collected as the sample was heated and cooled at ∼4.5 °C/min. Both types of measurement indicate that the hydration transition for the PNIPAAm brush occurs over a broad range of temperatures (∼10-40 °C). This result is in accordance with theoretical predictions that have suggested that polymer brush structures on planar surfaces do not exhibit true critical solubility transitions. Contact angle measurements revealed a discontinuity in the surface wettability at a temperature (∼32 °C) that corresponds to the dilute aqueous critical solution temperature. Taken together, these results suggest that the polymer segments in the outermost region of the brush remain highly solvated until the dilute solution lower critical solution temperature (∼32°), while densely packed, less solvated segments within the brush layer undergo dehydration and collapse over a broad range of temperatures.
Introduction Poly(N-isopropylacrylamide) (PNIPAAm), one of the best studied environmentally responsive polymers, undergoes a phase transition in dilute aqueous solution at its lower critical solution temperature (LCST) of ∼32 °C.1-4 Below the LCST, water is a good solvent and the PNIPAAm is hydrated and adopts a random coiled conformation. Above the LCST, water is a poor solvent and the PNIPAAm is dehydrated and collapsed into a globular conformation.1 This transition, when it occurs in bulk water, is referred to as a coil-to-globule transition (as well as an LCST transition) and takes place over a narrow range of temperature (1-2 °C).4 When this polymer is grafted to a solid surface, the resulting surface shows temperaturedependent surface properties, such as wettability5,6 and film thickness.7 This property has been utilized in a number of applications, including chromatography,8 temperature sensitive membranes,9 and protein,10 bacterial biofilm,11 and mammalian cell release surfaces.12 When the grafting density of polymer chains on a solid surface is sufficiently high, the polymer chains are forced * Corresponding author. Fax: 505-277-5433. E-mail: gplopez@ unm.edu. (1) Heskins, M.; Guillet, J. E.; James, E. J. Macromol. Sci., Chem. 1968, A2, 1441. (2) Snowden, M.; Murray, M.; Chowdry, B. Chem. Ind. 1996, 15, 531. (3) Hoffman, A. S. MRS Bull. 1991, 16, 42. (4) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (5) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163. (6) Zhang, J.; Pelton, R.; Deng, Y. Langmuir 1995, 11, 2301. (7) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. Adv. Mater. 2002, 14, 1130. (8) Kanazawa, H.; Matsushima, Y.; Okano, T. Adv. Chromatogr. 2001, 41, 311. (9) Rao, G. V. R.; Krug, M. E.; Balamurugan, S.; Xu, H.; Xu, Q.; Lopez, G. P. Chem. Mater. 2002, 14, 5075. (10) von Recum, H.; Okano, T.; Kim, S. W. J. Controlled Release 1998, 55, 121. (11) Ista, L. K.; Perez-Luna, V. H.; Lopez, G. P. Appl. Environ. Microbiol. 1999, 65, 1603. (12) Yamada, N.; Okano, T.; Sakai, H.; Karikusa, F.; Sawasaki, Y.; Sakurai, Y. Makromol. Rapid Commun. 1990, 11, 571.
to stretch away from the surface to avoid overlap. This conformation of soluble polymer chains is referred to as a polymer brush.13-15 The polymer segments near to the substratum form a dense core, and the outer segments form a flexible solvated layer. The properties of polymer brushes are different from those of flexible polymer chains in solution where chains adopt random coil configurations. It has been theoretically predicted that the collapse of the surface-grafted polymer brush accompanying a solubility transition proceeds continuously as the solvent quality decreases.14,16 This effect becomes more pronounced with decrease in effective dimensionality.16 Experimental results obtained for PNIPAAm grafted on spherical nanoparticles17 and from other polymer brush systems18,19 have borne out this prediction. Most investigations of solubility transitions of surfacegrafted PNIPAAm on planar surfaces have employed contact angle measurements, which typically show a sharp solubility transition for surface-grafted PNIPAAm chains at about 32 °C.5,6 Force versus distance curves measured by atomic force microscopy (AFM) have shown reduced steric repulsion upon increase in temperature from below the LCST to above the LCST, suggesting a collapse of the grafted PNIPAAm layer.7,20 Typically these measurements have been carried out only at two temperatures, one below and one above the LCST. To our knowledge, the collapse of PNIPAAm brushes has not been studied systematically as a function of temperature or in real time. (13) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (14) Szleifer, I.; Cargnano, M. A. Adv. Chem. Phys. 1996, 94, 165. (15) Milner, S. T. Science 1991, 251, 905. (16) Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A.; Birshtein, T. M. Macromolecules 1991, 24, 140. (17) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1994, 164, 489. (18) Habitcht, J.; Schmidt, M.; Ruhe, J.; Johannsmann, D. Langmuir 1999, 15, 2460. (19) Auroy, P.; Auvary, L. Macromolecules 1992, 25, 4134. (20) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 2402.
10.1021/la026787j CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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Figure 1. Schematic of the temperature control cell used for the temperature-dependent SPR measurements.
In this paper, we report the use of surface plasmon resonance (SPR) spectroscopy to study the thermally induced hydration transition of PNIPAAm brushes. Grafting of NIPAAm was done by atom transfer radical polymerization. The SPR results show that the solubility transition behavior of PNIPAAm brushes is broad, as predicted by theoretical models. This behavior is different from that of its surface layer, as probed by contact angle measurements, which shows a sharp transition at 32 °C. To our knowledge, this is the first report of the real-time observation of a solubility transition of PNIPAAm brushes on the planar surface. Experimental Section N-Isopropyl acrylamide (NIPAAm) was grafted onto mixed self-assembled monolayers (SAMs) on gold by atom transfer radical polymerization (ATRP)21 using a CuBr and Me4Cyclam (1,4,8,11-tetramethyl 1,4,8,11-azacyclotetradecane) catalyst system at room temperature. All the chemicals were purchased from Aldrich Chemical Co., Inc. The monomer was recrystallized from hexane. Mixed SAMs of 11-mercaptoundecanol and 11-mercaptoundecane on gold were prepared by a reported procedure.22 As a substrate for graft polymerization, we used a surface mole fraction of hydroxyl-terminated alkylthiolates (χOHsurf) of 0.32, as estimated by X-ray photoelectron spectroscopy (XPS).22 Ellipsometric thickness of the monolayer was 12 ( 2 Å and the advancing water contact angle was 67 ( 2°. To perform ATRP, an initiator was immobilized on the SAM surface by reacting the hydroxyl groups with 2-bromopropionyl bromide in the presence of triethylamine in THF (anhydrous).23 Initiator immobilization was confirmed by XPS, ellipsometric measurements (which indicated a thickness increase of 4 ( 2 Å), and contact angle measurements (86 ( 1°). Polymerization was carried out by immersing the substrate in a solution containing NIPAAm (2 M), 0.2 mM CuBr, and 0.2 mM Me4Cyclam in DMF (anhydrous) at room temperature for 1 h. Polymerization was done inside a glovebox. After polymerization, the film was washed with DMF, ethanol, and deionized water. The ellipsometric thickness of the resultant film in the dry state was 517 ( 4 Å, and the advancing contact angle was 66 ( 1°. Ellipsometric measurements taken at many points along the surface indicated uniform polymer thickness. The presence of PNIPAAm on the surface was further confirmed by XPS analysis. Contact angles of water were measured using a Rame-Hart model 100 contact angle goniometer. The temperature-dependent contact angles were measured within a custom-built environ(21) Matyjaszewski, K. Controlled Radical Polymerization; ACS Symposium Series No. 685; American Chemical Society: Washington, DC, 1998. (22) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (23) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616.
mental chamber as described earlier.9 Analyses by ellipsometry (M-44 multiwavelength ellipsometer, J. A. Woollam Co., Inc.) and XPS (AXIS-HSi, Kratos Analytical Inc.) were carried out as described previously.24 Surface plasmon resonance experiments were performed on a custom-built spectrometer that has been described previously.25 To control the temperature of water, we used a custom-built SPR cell in which a mixture of ethylene glycol and water from the controlled temperature bath was flowed through an outer compartment and used to control the temperature of water in a thin sample compartment (see Figure 1). The temperature of the water in the inner compartment was measured with a thermocouple. The thermocouple output was directly recorded by a personal computer along with the SPR data. To measure the effect of temperature on the refractive index of water, we used a SAM formed by adsorption of 11-mercaptoundecane on gold.
Results and Discussion Recently, the technique of ATRP has been used in the synthesis of homo-23,26,27 and copolymer28,29 brushes in what is referred to as the “grafting from” method of surface tethering. In the ATRP technique, the polymerization is surface confined and proceeds in a controlled manner such that it is possible to tailor the length and molecular weight of the grafted polymer chain. We used a catalyst system of CuBr and Me4Cyclam to prepare PNIPAAm brushes. After polymerization for 1 h at the conditions described above, a grafted polymer layer with a dry thickness of ∼500 Å was obtained. The XPS data confirm the presence of the PNIPAAm layer on the gold substrate. The C/N/O ratio for the grafted layer obtained was 70:14:16, which is close to that expected for PNIPAAm (75:12.5:12.5). Highresolution C1s spectra obtained for the grafted layers were also as expected for PNIPAAm. No signal from the underlying gold substrate was detected, indicating complete coverage of the gold surface. Surface plasmon resonance spectroscopy can be used to measure the changes in the refractive index near a noble metal-liquid interface,30 including those associated with (24) Ista, L. K.; Mendez, S.; Perez-Luna, V. H.; Lopez, G. P. Langmuir 2001, 17, 2552. (25) O’Brien, M. J.; Brueck, S. R. J.; Perez-Luna, V. H.; Tender, L. M.; Lopez, G. P. Biosens. Bioelectron. 1999, 14, 145. (26) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (27) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265. (28) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813. (29) Bo¨rner, H. G.; Duran, D.; Matyjaszewski, K.; da Silva, M.; Sheiko, S. S. Macromolecules 2002, 35, 3387. (30) Frutos, A. G.; Corn, R. M. Anal. Chem. 1998, 70 (13), 449A.
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structural changes in tethered polymer layers31 and tethered cross-linked PNIPAAm gel films.32 The collapse of the PNIPAAm toward the gold surface upon its transition to a water-insoluble state results in an increase in the concentration of PNIPAAm segments near the gold surface, which in turn results in an increase in the refractive index. This increase in interfacial refractive index is measured as a shift in the resonance curve, such that the minimum in reflected intensity is observed at higher angles from the surface normal. The temperature response of PNIPAAm brushes was probed by SPR spectroscopy in two modes, in quasi-static mode and in real-time mode. In quasi-static mode, the resonance curves were collected at discrete temperature intervals. The sample was equilibrated at each temperature for 15 min before the resonance curve was recorded. In real-time mode, the temperature of the water was changed continuously by heating the circulating liquid at a steady rate. In this mode, the reflected intensity of the laser source at a fixed angle of reflection (74.5°) corresponding to a near linear portion of the resonance curve was measured as a function of time. In this type of measurement, changes in intensity at this fixed angle reflect shifts in the resonance angle as the refractive index near the surface changes.33 Figure 2A shows the change in the resonance angle (quasi-static mode) as a function of temperature for the PNIPAAm layer and for a SAM formed from 11-mercaptoundecane. The data for the polymer film show a distinctive feature compared to that of untethered PNIPAAm in bulk aqueous solution: the transition is quite broad. In bulk aqueous solutions, the LCST transition of PNIPAAm typically occurs within 1-2 °C at ∼ 32 °C.1 In contrast, the SPR results show that the PNIPAAm brush collapses gradually over a range of temperature from ∼10 to 40 °C. This behavior is in accordance with the theoretical prediction that the collapse of PNIPAAm brushes grafted on planar surfaces does not occur as a true second-order thermodynamic phase transition, as observed for PNIPAAm chains in solution.16 Instead the transition occurs such that the polymer layer thickness decreases smoothly as solvent is excluded from the interfacial segments.14,16 Previous experimental efforts to examine the change in thickness of tethered PNIPAAm layers as temperature is increased have utilized the polymer grafted on latex particles as measured by light scattering.17 This method is hampered by the fact that as the surface becomes hydrophobic, the particles begin to flocculate at ∼30 °C.17 This flocculation can be avoided by addition of small amounts of surfactant, and the PNIPAAm transition observed in this manner is broader than that observed for the coil-to-globule transition of PNIPAAm free in solution. Nevertheless, there is a clearly observed sharp change (“jump”)16 in the PNIPAAm layer thickness at ∼32 °C.34 By comparison, our data for PNIPAAm brushes on flat surfaces show no sharp changes at any point throughout the transition. These observations are consistent with theoretical predictions that, as compared to the spherical surface, strong interchain interactions are present in the brush grafted to the planar surface.16 These enhanced interactions are thought to result in a broadening of the transition of polymer tethered on the planar surface.16 (31) Wischerhoff, E.; Zacher, T.; Laschewsky, A.; Rekaı¨, E. D. Angew. Chem., Int. Ed. 2000, 39, 4602. (32) Harmon, M. E.; Jakob, T. A. M.; Knoll, W.; Frank, C. W. Macromolecules 2002, 35, 5999. (33) Mayo, C. S.; Hallock, R. B. Rev. Sci. Instrum. 1989, 60, 739. (34) Zhu, P. W.; Napper, D. H. J. Chem. Phys. 1997, 106, 6492.
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Figure 2. Temperature-dependent SPR. (A) Change in the minimum of the SPR curve (∆θ) for the PNIPAAm layer (9) and for the control surface (a SAM formed from 11-mercaptoundecane) (b) as a function of temperature. The sample was equilibrated at every temperature for 15 min before the resonance curves were obtained. The plotted values are averages of three values. The average standard deviation in the resonance angle measurements was 0.0047 for the polymer and 0.0043 for the control. (B) Real-time measurement of change in the reflected intensity at a constant angle (74.5°) for the PNIPAAm layer as a function of temperature. The heating rate was 4.8 °C/min, and the cooling rate was 4.3 °C/min.
The cooling curve in Figure 2A shows that the transition is reversible and that the hydration of collapsed polymer layer occurs over a broad temperature range as well. Another interesting point is that during heating, the collapse of the polymer chain proceeds up to T ∼ 41 °C. However, during cooling the polymer does not start expanding until T ∼ 37 °C. This indicates that during the reverse process, the hydration of the polymer is hindered, perhaps by the intra- and interchain interactions formed
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in the collapsed state.35 Furthermore, the difference in heating and cooling curves is observed throughout the temperature range of the transition, resulting in a overall hysteresis in the temperature-induced expansion and collapse. This behavior is also observed for PNIPAAm in dilute solution.35 Other investigators have noted that the hysteresis is time dependent with larger differences observed when the polymer is kept in the collapsed state for longer times.36 The changes in the resonance angle measured as a function of temperature for the control SAM surface in our experimentation (see Figure 2A) are consistent with the known temperature dependence of the refractive index of water.37 As temperature is increased, the index of refraction of water steadily decreases in a nonlinear manner that is precisely mirrored by the resonance angle shift for the SAM. It is important to note that this temperature dependence complicates the measurement of the temperature response of the PNIPAAm layer. This effect can be easily seen, for example, in the resonance angle versus temperature curve for the polymer layer at temperatures above those where the polymer collapse is complete (∼>40 °C). At these higher temperatures, the downward slope of the curve is likely due to the fact that the evanescent field generated by SPR is probing the water beyond the collapsed polymer layer. Wischerhoff et al. used SPR to study the temperature response of a polymer that also exhibits an LCST in aqueous solution (partially acetylated poly-N-[tris(hydroxymethyl)methyl]acrylamide).31 Though they were able to use SPR to detect the LCST transition, the grafting procedure used to tether the polymer to the gold surface (a “grafting to” technique) resulted in a very thin, less dense polymer layer for which the SPR response was too small to extract detail on the nature of the solubility transition. The SPR result obtained in the real-time or continuous mode is shown in Figure 2B. The result is similar to that obtained in quasi-static mode, except that the hysteresis in the SPR response is marginally less and the reflected intensity returns to its original value upon cooling to low temperature. This may be due to the fact that the polymer layer was kept in the collapsed state for only a short time. However, since the temperature was changed rapidly there may be a delay in the thermocouple response that may result in a difference in the measured temperature from the actual value. Furthermore, in the quasi-static experiments we measured a slight decrease in the minimum reflected intensity in the resonance curves as the temperature was increased, which is likely due to the known temperature dependence of the absorption of water at 632.8 nm.38 Thus, in this case, the reflected intensity at a fixed angle is not a linear function of the resonance angle. Therefore it is difficult to compare the SPR data obtained in two different modes especially if the differences are marginal. Nevertheless, the similar nature of the SPR data obtained in real-time mode and in quasi-static mode shows that the PNIPAAm brush responds in real time to the temperature changes imposed. We further analyzed the PNIPAAm grafted surface by contact angle measurements taken as a function of temperature. For each data point, the sample was (35) (a) Wu, C.; Wang, X. Phys. Rev. Lett. 1998, 80, 4092. (b) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 2972. (36) Nath, N.; Chilkoti, A. J. Am. Chem. Soc. 2001, 123, 8197. (37) Robinson, G. W.; Cho, C. H.; Gellene, G. I. J. Phys. Chem. B 2000, 104, 7179. (38) Langford, V. S.; McKinley, A. J.; Quickenden, T. I. J. Phys. Chem. A 2001, 105, 8916.
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Figure 3. Advancing water contact angle of the PNIPAAm surface as a function of temperature. Data points are an average of six measurements taken at different points along the sample surface.
equilibrated at the measurement temperature for 15 min before measurement of the advancing contact angle. Figure 3 shows a sharp change in the wettability at 32 °C. For highly ordered SAMs, contact angle measurements have been shown to be sensitive to the outermost 5-10 Å of the surface.39 The data in Figure 3, when compared to the SPR data discussed above, suggest that the collapse behavior of the outermost surface of the PNIPAAm brush is very different from that for the whole film. It may be that the polymer segments in the outermost region of the brush remain highly solvated until the dilute solution LCST (∼32 °C), while densely packed, less solvated segments within the brush layer undergo dehydration and collapse over a broad range of temperatures.16 Conclusions In conclusion, PNIPAAm was grafted on the monolayer surface by the ATRP technique to create relatively thick polymer brush architecture. We have shown that SPR is a useful technique to follow hydration transitions of such thermoresponsive polymer brushes grafted to gold surfaces. SPR measurements showed that the PNIPAAm brush collapses over a temperature range of ∼10-40 °C. This result is consistent with previous theoretical models. In contrast, contact angle measurements indicate a sharp wettability transition at a temperature (∼32 °C) consistent with the LCST of PNIPAAm in aqueous solution. These results suggest that PNIPAAm brushes exist in a partially collapsed state at room temperature. Though the results presented here were for polymer grown from mixed SAMs with χOHsurf ) 0.32, nearly identical results were obtained for PNIPAAm brushes formed on 100% OH terminated SAMs. This is consistent with an earlier report that described the formation of poly(methyl methacrylate) on 100% hydroxyl SAMs by ATRP which indicated that “only 10% of the surface bound initiator actually initiate PMMA chains.”23 The results reported here are significant to the design and use of PNIPAAm surface-grafted layers in technological applications. Such thermoresponsive materials (39) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897.
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have been suggested for many applications including control of cell attachment, protein adsorption, biofouling, colloidal aggregation, fluidic transport, and molecular adsorption (chromatography). For some applications, a switchable surface property (e.g., wettability, interfacial energy) may be of interest, while in others switching of a property of the entire polymer film (e.g., thickness, volume, and absorbance) may be desired. Optimization of the molecular architecture of the grafted polymer layer
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for a particular application will require a detailed understanding of the nature of the thermal transition. Acknowledgment. Support for this work was provided by the Office of Naval Research, the Air Force Office of Scientific Research, Sandia National Laboratories, and the Department of Energy through the U.S./Mexico Materials Corridor Initiative. LA026787J
