Adsorption and Thermoresponsive Behavior of Poly (N

May 24, 2007 - Ellane J. Park, Danielle D. Draper, and Nolan T. Flynn* ... Liang Hu , Jinghua Zhu , Dan Li , Yafei Luan , Wenwen Xu , and Michael J. S...
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Langmuir 2007, 23, 7083-7089

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Adsorption and Thermoresponsive Behavior of Poly(N-isopropylacrylamide-co-N,N′-cystaminebisacrylamide) Thin Films on Gold Ellane J. Park,† Danielle D. Draper, and Nolan T. Flynn* Department of Chemistry, Wellesley College, 106 Central Street, Wellesley, Massachusetts 02481-8203 ReceiVed March 4, 2007. In Final Form: April 4, 2007 We describe the synthesis of thermoresponsive polymers made from N-isopropylacrylamide and varying amounts of a thiol-containing co-monomer, N,N′-cystaminebisacrylamide (P(NIPAm-co-CBAm)). Infrared spectroscopy revealed a backbone similar to that seen with pure PNIPAm. UV-vis spectroscopy showed that P(NIPAm-co-CBAm) undergoes a thermoresponsive phase transition around 32 °C in aqueous solution. The presence of the thiol groups enabled the polymer to adsorb onto gold surfaces. Following adsorption onto a gold surface, X-ray photoelectron spectroscopy showed a carbon/gold atomic ratio of 0.93 for a sample without CBAm and a ratio of 1.61 for a P(NIPAm-co-CBAm) sample with 0.20% CBAm. Quartz crystal microbalance (QCM) analysis showed increases in the mass of polymer adsorbed when the CBAm content in the polymer increased. The thermoresponsive behavior of the thin films on gold was investigated with contact angle and dissipative QCM analysis. Contact angles were measured for polymer films at both 25 and 60 °C. The largest temperature-induced alteration in the contact angle was seen with the 1.00% CBAm sample. Similarly, QCM-D results showed a significantly greater change in frequency and dissipation following a temperature change when CBAm was present than in pure NIPAm polymers.

Introduction The creation of responsive, or smart, surfaces has drawn significant attention.1 In response to a specific stimulus, these surfaces can undergo a dramatic change in physicochemical propertiessoften the strength of their interaction with water (wettability). Surfaces that respond to stimuli such as temperature, pH/ionic strength, electric fields, light, and pressure have been created.1 One means of fabricating such surfaces is by modifying a substrate with a responsive polymer or other molecule. Poly(N-isopropylacrylamide) (PNIPAm), a polymer that undergoes a dramatic change in hydrophobicity/hydrophilicity with temperature, is a prime example of a responsive polymer. The thermoresponsive behavior of PNIPAm in solution has been well-known and widely studied for decades.2 PNIPAm undergoes a coil-to-globule-like transition at a lower critical solution temperature (LCST) near 32 °C. The inclusion of comonomers enables some tuning of the LCST. This thermoresponsive behavior of PNIPAm and the ability to make NIPAmbased gels has led to many potential applications, particularly in drug delivery and tissue engineering.3-5 Recently, interest has grown in creating responsive thin films based on PNIPAm. Two main methods have been developed for creating these films. First, surface-initiated polymerization can be used to create PNIPAm-coated silica or gold substrates.6-9 * Author to whom correspondence should be addressed. Tel.: (781) 2833097. Fax: (781) 283-3642. E-mail: [email protected]. † Current address: Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027. (1) Lahann, J.; Langer, R. MRS Bull. 2005, 30, 185-188. (2) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163-249. (3) Hoffman, A. S. AdV. Drug DeliVery ReV. 2002, 54, 3-12. (4) Jeong, B.; Kim, S. W.; Bae, Y. H. AdV. Drug DeliVery ReV. 2002, 54, 37-51. (5) Lee, K. Y.; Mooney, D. J. Chem. ReV. (Washington, DC, U.S.) 2001, 101, 1869-1879. (6) de las Heras Alarcon, C.; Farhan, T.; Osborne, V. L.; Huck, W. T. S.; Alexander, C. J. Mater Chem. 2005, 15, 2089-2094. (7) Ista, L. K.; Mendez, S.; Perez-Luna, V. H.; Lopez, G. P. Langmuir 2001, 17, 2552-2555.

Alternatively, the polymer can be synthesized to contain some type of reactive group that adsorbs onto the desired substrate. PNIPAm end-functionalized with carboxyl, thiol, or long alkyl chains has been used for adsorption onto substrates such as gold, polymers, and supported lipid bilayers.10-14 Following the creation of these PNIPAm films, a variety of analytical tools, including X-ray photoelectron spectroscopy (XPS), surface force apparatus, atomic force microscopy, electrochemistry, contact angle analysis, and quartz crystal microbalance (QCM), has been used to characterize the thin films and their temperature-dependent behaviors. Several important conclusions have been drawn from these previous studies of PNIPAm-based films. Using PNIPAm grafted to polymer substrates, Takei et al. observed a change in the temperature at which the polymer film underwent a transition from hydrophilic to hydrophobic as measured by contact angle analysis.12 Specifically, a large reduction in the film’s LCST was observed (∼8 °C) for polymers with a single graft point, whereas those with multiple graft points exhibited LCST values unperturbed by the substrate. The dependence on the number of attachment points is one among many parameters, including molecular weight and graft density, known to affect the phase behavior of PNIPAm films.14 Other researchers have observed a continuous collapse of dense polymer brushes over 20-38 °C when grafted onto silica substrates.9 Additionally, the adsorptive behavior of PNIPAm end functionalized with thiols was observed to vary with the molecular weight of the polymer chain.11 (8) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. AdV. Mater. (Weinheim, Ger.) 2002, 14, 1130-1134. (9) Liu, G.; Zhang, G. J. Phys. Chem. B 2005, 109, 743-747. (10) Cho, E. C.; Kim, Y. D.; Cho, K. Polymer 2004, 45, 3195-3204. (11) Liu, G.; Cheng, H.; Yan, L.; Zhang, G. J. Phys. Chem. B 2005, 109, 22603-22607. (12) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163-6166. (13) Zhu, M.-Q.; Wang, L.-Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656-2657. (14) Zhu, X.; Yan, C.; Winnik, F. M.; Leckband, D. Langmuir 2007, 23, 162-169.

10.1021/la700624g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007

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Here, we report the synthesis of a NIPAm-based polymer that contains free thiols along the length of the polymer chain rather than only at one terminus. The thiols were introduced by copolymerizing NIPAm with N,N′-cystaminebisacrylamide (CBAm), a divinyl-containing disulfide-bridged species. The adsorption and thermoresponsive behavior of films created using P(NIPAm-co-CBAm) were investigated with a quartz crystal microbalance with dissipation, an X-ray photoelectron spectrometer, and a contact angle goniometer. Experimental Procedures Materials. All water was purified using a Nanopure Ultrapure Water System from Barnstead. N-Isopropylacrylamide (SigmaAldrich) was recrystallized from hexanes prior to use. Ammonium persulfate (APS, Sigma-Aldrich) and sodium metabisulfite (MBS, Sigma-Aldrich) were used as 10% (w/w) aqueous solutions. N,N′Cystaminebisacrylamide (Sigma-Aldrich), tris-(2-carboxyethyl)phosphine (TCEP, 0.5 M solution; Sigma-Aldrich), Oregon Green 488 maleimide (Molecular Probes), mercaptoethanol (SigmaAldrich), methanol (Fisher Scientific), ethanol (Pharmco), sulfuric acid (Fisher Scientific), and hydrogen peroxide (Fisher Scientific) were used as received. Phosphate buffer (pH ) 7.0, 0.05 M; Fisher Scientific) was used after diluting 1:4 with water. Gold-coated microscope slides (25.4 mm × 76.2 mm × 1.5 mm, with 100 nm Au film on a 10 nm Ti adhesion layer; Evaporated Metal Films, Inc.) were used for polymer deposition. Dialysis tubing (3500 molecular weight cutoff) was purchased from Fisher Scientific. Synthesis. The NIPAm monomer (0.50 g, 437 mmol) was dissolved in buffer to yield a final concentration of 4% (w/w). The appropriate mass of CBAm was dissolved in methanol and then added to the NIPAm solution to achieve the desired concentration with respect to the mass of NIPAm. The total volume of solution was 12.5 mL. The solution was put on ice before adding 125 µL of 10% APS stock solution as an initiator. The solution was purged with N2(g) for ∼20 min to remove dissolved O2(g). After 125 µL of the 10% MBS stock solution was added, the solution was stirred without light exposure at room temperature for a minimum of 24 h. Following the polymerization, the sample was dialyzed against 1 L of buffer for ∼24 h, changing the buffer at least once. Samples containing high concentrations of CBAm were often treated with TCEP, which reduces disulfides to thiols/thiolates.15 In this work, samples were labeled with the amount of CBAm (e.g., the 0.20% sample contains 0.20% (w/w) CBAm relative to NIPAm). To assay for the presence of thiols or disulfides in the polymer, a simple test using a thiol-reactive dye (Oregon Green 488 maleimide) was conducted. The polymer samples were first treated with excess TCEP. Next, the thiol-reactive dye was added in excess. After 24 h, mercaptoethanol was added to react with the remaining free dye. The polymer samples were then centrifuged at ∼16 000g and 40 °C, and the supernatant was removed and replaced with fresh phosphate buffer. This procedure was repeated 3 times to yield a colorless solution. Samples were diluted 5-fold, and the fluorescence intensity at 518 nm was recorded using a Varian Cary Eclipse fluorescence spectrometer with 491 nm excitation. Polymer Films on Gold Substrates. The gold-coated microscope slides were cleaned in piranha solution (3:1 concentrated sulfuric acid/30% hydrogen peroxide) for a minimum of 1 h at room temperature. (Caution: piranha solution reacts Violently with organic materials.) Slides were removed from the piranha bath, rinsed 3 times with both water and ethanol, and dried in an N2(g) stream. Polymer deposition was performed by immersing a clean, goldcoated microscope slide into a solution of P(NIPAm-co-CBAm) at room temperature. Samples were removed after a minimum of 24 h and cleaned by washing with water and ethanol 3 times and dried using a stream of N2(g). Polymer Characterization. Infrared Spectroscopy. Fourier transform-infrared (FT-IR) spectra were acquired with a PerkinElmer (15) Burns, J. A.; Butler, J. C.; Moran, J.; Whitesides, G. M. J. Org. Chem. 1991, 56, 2648-2650.

Park et al. Instruments Spectrum One FT-IR spectrophotometer fitted with a PerkinElmer universal attenuated total reflectance (ATR) sampling accessory. All spectra were acquired with 32 scans at a resolution of 4.00 cm-1 over the 650-4000 cm-1 range. Buffer was used as the background. The polymer samples were centrifuged for 10 min at 16 000g and a temperature of 40 °C to concentrate the sample. Excess buffer was removed, and the sample was transferred to the ATR crystal for analysis. UV-vis Spectroscopy. Spectra were recorded on a Cary Scan 500 UV-vis-NIR spectrophotometer over the 400-800 nm range with a 1.0 nm resolution in the double-beam mode with baseline correction. The cuvette temperature was controlled using the Cary Varian PCB 150 Water Peltier System, which had an uncertainty of (0.3 °C. Spectra were recorded a minimum of 10 min after the water bath had reached the desired temperature. Polymer samples were shaken 2 min prior to spectral acquisition to homogenize the sample. Baseline spectra were recorded with a cuvette containing buffer at 25.0 °C. Polymer samples were diluted 50-fold in buffer prior to investigation. To show the spectral changes caused by heating, the average absorbance over the 400-800 nm range was calculated as a measure of turbidity.16,17 This value was then converted into the fractional absorbance using the following equation: FA ) (Atemp - Amin)/(Amax - Amin), where Atemp, Amin, and Amax represent the average absorbance values at any temperature, the minimum absorbance, and the maximum absorbance, respectively. Gel Permeation Chromatography (GPC). GPC analysis was performed by the Bodycote Polymer-Broutman Laboratory (Melrose Park, IL) using a Waters 150-C system with an Ultrasyragel column and THF as the solvent. GPC was performed at 40 °C with narrow distribution polystyrene as the standard. P(NIPAm-co-CBAm) samples were first dried at 90 °C to remove water and subsequently redissolved in THF for chromatographic analysis. All samples were analyzed 3 times, and reported values are the average of the three injections. X-ray Photoelectron Spectroscopy. Spectra were acquired on a Physical Electronics PHI 5400 spectrometer, employing a Mg KR (1253 eV) cathode X-ray source operated at 300 W. Photoelectrons were collected at an angle of 45° relative to the surface normal. Spectra were recorded, baseline corrected, and fit using the RBD AugerScan program (RBD Enterprises, Inc.). For survey spectra, three acquisitions over the 0-1100 eV binding energy range were recorded with 1.0 eV resolution at a pass energy of 178.95 eV. High-resolution spectra were acquired for the Au 4f, C 1s, N 1s, and O 1s peaks at a pass energy of 35.75 eV and a resolution of 0.1 eV. Polymer samples were prepared on piranha-cleaned, gold-coated microscope slides. Samples were mounted onto the sample holder using a metal clip to ensure electrical contact with the gold surface. QCM Analysis. QCM experiments were conducted on a QCMZ500 system with a flow cell (QCM500M) and temperature controller (QCM501) all from KSV Instruments. The instrument chamber was cleaned with multiple washings with water and ethanol and dried with a stream of compressed air before and after each experiment. Gold-coated AT-cut quartz crystals with a fundamental resonance frequency of ∼5.0 MHz and a diameter of 14 mm were purchased from KSV Instruments. Crystals were cleaned using the piranha procedure outlined previously for gold-coated microscope slides. For each experiment, the changes in frequency and dissipation were monitored for the fundamental frequency and the third and seventh harmonics. All data reported and shown here are for the seventh harmonic to avoid noise problems associated with the fundamental frequency.9 Contact Angle Analysis. Contact angles were measured using a VCA Optima XE surface analysis system and accompanying software (AST Products, Inc.). A 0.25 µL water droplet was dispensed onto the substrate using the instrument’s software control. An image of the water droplet was then recorded. The AutoFast Calculate software method was used to determine the contact angles. To (16) Winnik, F. M. Macromolecules 1987, 20, 2745-2750. (17) Winnik, F. M. Macromolecules 1990, 23, 233-242.

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measure temperature-dependent contact angles, slides were immersed in a beaker of water placed in a thermostatic bath at either 25 or 60 °C. After a minimum of 10 min, samples were withdrawn, and contact angles were measured within 30 s.

Results and Discussion Polymer Characterization. NIPAm-based polymers were synthesized using standard methods from the literature with APS as the initiator and MBS as the accelerant.2 As outlined in the Experimental Procedures, slight modifications to the literature procedure were made to incorporate CBAm, which has been used previously as a cross-linking agent for hydrogels.18,19 In our syntheses, the concentrations of NIPAm, 4% (w/w) in water, and CBAm, less than 1% (w/w) with respect to NIPAm, were low enough to avoid gelation of the sample. One concern with using CBAm as a co-momomer is that the sulfur in this species may act as a chain transfer reagent during polymerization.20 This behavior would result in the polymers possessing sulfur in the form of thioethers or thioesters instead of thiols or disulfides. Only the latter two species are known to interact strongly with metal surfaces through chemisorption.21 An assay with a thiol-reactive (maleimide-containing) dye was used to determine if free thiol/disulfide was present in the polymer samples. Results from this assay (data not shown) indicate that the dye was incorporated into samples that contain CBAm, whereas the 0.00% sample did not have dye levels above background. This assay demonstrates the presence of reactive thiols or disulfides in P(NIPAm-co-CBAm) but does not exclude the formation of some thioesters or thioethers. The P(NIPAm-co-CBAm) samples were analyzed with FTIR. For comparison, the spectra of the monomers, NIPAm and CBAm, are included in Figure A in the Supporting Information. The monomers both have peaks for secondary amines near 3300 cm-1, methyl and methylene C-H stretches are seen in the 29702870 cm-1 range, amide I and II bands are seen at ∼1650 and 1550 cm-1, respectively, and vinyl stretches are between 995 and 905 cm-1.22,23 Infrared spectra for 0.00% (A) and 0.20% (B) P(NIPAm-coCBAm) are shown in Figure 1. Spectra correspond to those observed previously for NIPAm-based polymers and hydrogels.22-24 The spectra for both samples are dominated by strong amide I and amide II bands at 1625 and 1558 cm-1.25 The richness of the fingerprint region below 1500 cm-1 has been reduced with the vinyl stretches between 995 and 905 cm-1 absent. These spectral features are good indicators of polymerization of NIPAm. In the 3700-3000 cm-1 range, both samples have a transmittance greater than 100%. This range is where water is typically observed. We believe this increased transmittance results from a reduction in the amount of water on the ATR crystal relative to that present when the background spectrum of the phosphate buffer was acquired. Specifically, the presence of the polymer in the samples displaces water that was in contact with the crystal during background acquisition, resulting in a higher percent transmit(18) Plunkett, K. N.; Kraft, M. L.; Yu, Q.; Moore, J. S. Macromolecules 2003, 36, 3960-3966. (19) Pong, F. Y.; Lee, M.; Bell, J. R.; Flynn, N. T. Langmuir 2006, 22, 38513857. (20) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 672. (21) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771-783. (22) Kim, S. J.; Lee, C. K.; Lee, Y. M.; Kim, S. I. J. Appl. Polym. Sci. 2003, 90, 3032-3036. (23) Petrovic, S. C.; Zhang, W.; Ciszkowska, M. Anal. Chem. 2000, 72, 34493454. (24) Percot, A.; Zhu, X. X.; Lafleur, M. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 907-915. (25) Liang, L.; Rieke, P. C.; Liu, J.; Fryxell, G. E.; Young, J. S.; Engelhard, M. H.; Alford, K. L. Langmuir 2000, 16, 8016-8023.

Figure 1. FT-IR spectra for 0.00% CBAm (A) and 0.20% CBAm (B) P(NIPAm-co-CBAm) samples. Spectra have been offset along the y-axis for clarity. The region from 3800 to 4000 cm-1 corresponds to ∼100% transmittance.

Figure 2. Thermoresponsive behavior of P(NIPAm-co-CBAm) samples with 0.00% (9), 0.04% (O), 0.20% (2), 0.50% (×), and 1.00% (]) CBAm. Polymer concentration is ∼0.8 mg/L.

tance. No FT-IR signatures for sulfur-containing moieties are seen with any of the CBAm-containing samples. This absence likely results from the low concentration of CBAm relative to NIPAm, 1.00% or less, in the samples. GPC analysis of the 0.00% P(NIPAm-co-CBAm) indicated a number average molecular weight, Mn, of 100 000. GPC analysis on the 0.04, 0.20, 0.50, and 1.00% P(NIPAm-co-CBAm) samples found all Mn values in the 80 000-90 000 range with no trend based on the amount of CBAm incorporated into the polymer. This reduction in Mn may result from a small amount of chain transfer caused by the thiol-containing CBAm. All samples were polydisperse with increases of ∼50% in the polydispersity index toward a higher CBAm content. We speculate that both the polymerization in water and the drying process contribute significantly to the polydispersity of the samples.2 Temperature-dependent UV-vis spectra were recorded for all samples. Spectra are shown in Figure B in the Supporting Information for 0.20% P(NIPAm-co-CBAm). The spectra show a nearly featureless increase in absorbance across the entire spectral region as the temperature is increased from 30.0 to 34.5 °C. Above 34.5 °C, a slight reduction in absorbance is seen, likely because of a slight aggregation of collapsed polymer. These spectra are characteristic for all samples studied. To determine the LCST using the cloud-point method,16,17 the fractional absorbance change (FA) was calculated and plotted versus temperature as shown in Figure 2. This plot defines the spectrum having the lowest average absorbance as zero and the spectrum having the highest average absorbance as one, aiding comparisons among samples. As the data show, an increase in FA is observed between 31.5 and 33.5 °C for all samples. This

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Park et al. Table 1. Atomic Percentages for 0.00 and 0.20% P(NIPAm-co-CBAm) Determined from Fitting of XP Spectra Shown in Figure 3 P(NIPAm-co-CBAm)

Figure 3. XP spectra of (a) 0.00% and (b) 0.20% P(NIPAm-coCBAm) deposited on clean gold substrates at 25 °C. Spectra have been scaled so that the Au 4f7/2 peaks are the same height.

range has been observed previously for the LCST of pure PNIPAm.2 A trend toward higher LCST with increasing CBAm is seen in Figure 2 with 0.20% P(NIPAm-co-CBAm) not following this trend. An increase in LCST for copolymers of NIPAm is typically attributed to increases in the hydrophilicity of the copolymer.2 This increased hydrophilicity often results from the co-monomers possessing charge, changing the strength of the polymer hydration. Here, CBAm may impart charge to the polymer, depending on the extent of protonation/deprotonation of the thiol group. These characterization results indicate that a thermoresponsive polymer was synthesized. CBAm was incorporated into the polymer, yielding thiol or disulfide groups, but it did not significantly affect the LCST of the samples. GPC results show some variation upon the addition of CBAm, yielding slightly lower Mn and greater polydispersity. However, variation in Mn among CBAm-containing samples is small. Polymer Adsorption onto Gold. XP spectra of gold-coated microscope slides after exposure to 0.00 and 0.20% P(NIPAmco-CBAm) are shown in Figure 3 a,b, respectively. Peaks are assigned to Au 4f7/2 (84.0 eV), C 1s (285.0 eV), N 1s (400.0 and 409.0 eV), and O 1s (532.0 eV).26 These peaks correspond to Au0, C bound predominately to other carbons and hydrogens, N bound to carbons and hydrogens (400.0 eV), strongly oxidized N (409.0 eV), and O2-.26 Several differences are seen between the two spectra in Figure 3. First, the 0.00% P(NIPAm-co-CBAm) contains only strongly oxidized nitrogen with a binding energy of 409.0 eV. (The absence of N bound to hydrocarbons (∼400.0 eV) was also seen in highresolution spectra acquired for the 0.00% P(NIPAm-co-CBAm) sample in the N 1s regionsdata not shown.) Second, changes in the relative peak heights are seen in Figure 3. These changes (26) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R. NIST X-ray Photoelectron Spectroscopy Database; Measurement Services Division of the National Institute of Standards and Technology (NIST) Technology Services: Washington, DC, 2003.

peak

0.00%

0.20%

Au 4f C 1s N 1s O 1s C/Au

46.6 43.1 0.0 10.3 0.93

31.0 49.9 7.7 11.4 1.61

are quantified by fitting the spectra and determining the percent atomic concentrations using appropriate instrumental sensitivity factors.27 Results from this analysis for these two samples are shown in Table 1. The 0.20% P(NIPAm-co-CBAm) has a N/O/C ratio of 1.0:1.5:6.5, which is close to the theoretical value of 1:1:6 and to values obtained in previous XPS studies of PNIPAmbased films.10,28 Incorporation of CBAm into the polymer clearly increases the C, N, and O content relative to 0.00% P(NIPAmco-CBAm). This change is most noticeable in comparing the C/Au ratios for the two samples in the bottom line of Table 1. A 73% increase in the C/Au ratio is observed with the 0.20% sample relative to the CBAm-free sample. Fitting of high-resolution spectra of the C 1s binding energy region is helpful in identifying chemical environments (oxidation states) for the elements observed in XPS. Spectral fitting is shown in panels a and b in Supporting Information Figure C for 0.00 and 0.20% P(NIPAm-co-CBAm), respectively. The C 1s region for the 0.00% CBAm sample is most well fit by two carbon components at 284.8 ( 0.3 and 287.1 ( 0.3 eV. However, the 0.20% CBAm sample is best fit by three carbon components at 284.7 ( 0.3, 285.6 ( 0.3, and 287.6 ( 0.3 eV. The difference in carbon content may result from less polymer being deposited with the 0.00% P(NIPAm-co-CBAm) and much of this signal resulting from carbon contamination of the surface. The 0.20% P(NIPAm-co-CBAm) fitting corresponds to that observed previously for P(NIPAm)-based thin films on gold.10 The incorporation of CBAm into the polymer was expected to alter the affinity of the polymer for gold substrates. XPS of gold surfaces following exposure to CBAm-containing or CBAmabsent polymers supports this theory. Specifically, CBAmcontaining polymers produce a number of changes in XPS, such as a relative increase in the C, N, and O signals, a higher C/Au ratio, and high-resolution spectra that match those previously obtained for P(NIPAm)-based thin films on gold. These XPS characteristics indicate that more polymer is deposited on the gold substrate when CBAm is copolymerized with NIPAm. QCM analysis can also provide information concerning the role CBAm plays in polymer deposition. With this technique, the adsorption process can be monitored in real-time by observing the time-dependent frequency changes for the gold-coated quartz crystal.29 Figure 4 shows the change in frequency for the seventh harmonic of the gold-coated QCM crystal following exposure to P(NIPAm-co-CBAm) containing 0.00-1.00% CBAm. The 0.00% CBAm sample (Figure 4A) shows an abrupt change in the frequency of approximately -170 Hz within 30 s of the addition of polymer. This rapid change is followed by minimal alteration in frequency as seen by the final frequency change of approximately -175 Hz at 550 s. (27) Moulder, J. F.; Stickle, W. F.; Sobol, P. E. Handbook of X-Ray Photoelectron Spectroscopy; PerkinElmer, Physical Electronics Division: Eden Prairie, MN, 1993; p 275. (28) Tu, H.; Heitzman, C. E.; Braun, P. V. Langmuir 2004, 20, 8313-8320. (29) Rodahl, M.; Dahlqvist, P.; Hook, F.; Kasemo, B. Biomol. Sens. 2002, 304-316.

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Figure 4. Frequency change of the seventh harmonic for P(NIPAmco-CBAm) samples with 0.00% (A), 0.04% (B), 0.20% (C), 0.50% (D), and 1.00% (E) CBAm. The time at which the polymer was introduced was set to t ) 0 s. Each data set (A-E) represents the average of at least two separate QCM adsorption experiments.

We believe two phenomena lead to this initial change in frequency for 0.00% P(NIPAm-co-CBAm). First, segments of the polymer chain may interact with the gold substrate through weak interactions such as van der Waals forces, leading to physisorption of the polymer. Other researchers have seen this behavior for NIPAm-based polymers that lack a free thiol.11 A second contribution may result from the difference in viscosity between water and polymer solutions. This contribution would not be indicative of significant polymer adsorption. The effects of the viscosity of polymer solutions on QCM have been examined previously.30 The polymer samples that contain CBAm (Figure 4B-E) exhibit different adsorption behavior in QCM experiments. Again, a large initial change in the frequency of the seventh harmonic is observed following polymer introduction. However, unlike 0.00% P(NIPAm-co-CBAm), samples with 0.04-1.00% CBAm exhibit a continued decrease in frequency. The 0.04% P(NIPAmco-CBAm) has a ∆f of -160 Hz at 30 s after polymer addition but drops to -185 Hz at 500 s. As the CBAm content increases, the magnitude of this continued decrease in crystal frequency grows. For example, the 1.00% P(NIPAm-co-CBAm) sample has a ∆f of -180 Hz at 30 s but decreases to a value below -270 Hz at 500 s. Thus, the incorporation of 1.00% CBAm into the polymer leads to a ∆f at longer times that is 50% greater than the ∆f observed immediately after polymer introduction. The greater frequency changes for higher CBAm content polymers indicate that a larger amount or more adherent film is deposited as compared to the CBAm-free polymer. This result confirms the observations using XPS to analyze the films. QCM results with our samples are similar to those previously observed with other sulfur-containing NIPAm polymers. Liu et al. investigated the adsorption process of short (∼18 000 g/mol) and long (∼34 000 g/mol) PNIPAm chains that were endfunctionalized with a thiol.11 QCM analysis of the adsorption of these polymers onto gold showed adsorption with similar magnitudes of frequency change as observed in our work. Additionally, after the initial drop in frequency, a greater continued change occurred with the short chain PNIPAm than with the long chain species. The authors attribute this to more complete coverage being obtained with the shorter PNIPAm chains.11 Thin Film Thermoresponsive Behavior. The thermoresponsive behavior of the deposited films was investigated using QCM and temperature-dependent contact angle measurements. Following deposition within the QCM chamber, the effects of a (30) Munro, J. C.; Frank, C. W. Macromolecules 2004, 37, 925-938.

Figure 5. Frequency change of the seventh harmonic before (A) and after (B) exposure to a P(NIPAm-co-CBAm) sample with (a) 0.00% and (b) 0.20% CBAm, following a temperature change from 34 to 31 °C at time 0 s. Spikes in the data are the result of flushing the chamber with thermally equilibrated polymer solution.

small temperature step were monitored. The polymer was deposited at 31 °C, just below the LCST determined for the polymer in solution. The temperature was then increased from 31 to 34 °C. After both the frequency and the dissipation stabilized, the temperature was decreased from 34 to 31 °C, and the changes in frequency and dissipation were recorded. Figure 5 shows the frequency changes following the temperature step from 34 to 31 °C for crystals exposed to 0.00 and 0.20% P(NIPAm-co-CBAm) in panels a and b, respectively. Comparison of the data to that obtained for the same crystal prior to exposure to polymer is made in both cases. This comparison is necessary because of the coupling of temperature and crystal frequency observed in QCM experiments.9,31 To separate the effects of the temperature change on solution viscosity from those on the conformation of the deposited polymer, ∆f for both the clean crystal prior to polymer exposure, spectrum A, and the same crystal after polymer deposition, spectrum B, are included. As Figure 5a shows, ∆f for 0.00% P(NIPAm-coCBAm) (B) sample upon temperature change is nearly identical to that seen with the clean crystal (A). The final frequency change for both samples is approximately -80 Hz. The similarity in behavior between the clean gold-coated crystal and that same crystal following exposure to 0.00% P(NIPAm-co-CBAm) indicates that no significant variation in bound polymer interaction with water occurs. This behavior likely results from little of the 0.00% P(NIPAm-co-CBAm) sample adhering to the gold surface. With 0.20% P(NIPAm-co-CBAm), a different behavior is observed as Figure 5b illustrates. Here, ∆f values for the crystal (31) Plunkett, M. A.; Wang, Z.; Rutland, M. W.; Johannsmann, D. Langmuir 2003, 19, 6837-6844.

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Figure 6. Dissipation change of the seventh harmonic before (A) and after (B) exposure to a P(NIPAm-co-CBAm) sample with (a) 0.00% and (b) 0.20% CBAm, following a temperature change from 34 to 31 °C at time 0 s.

before and after exposure to polymer vary significantly. The clean crystal has a ∆f value of about -75 Hz 3000 s after the temperature change, whereas 0.20% P(NIPAm-co-CBAm) has a ∆f value of -125 Hzsan increase in ∆f of approximately 66%. Cooling a PNIPAm-based thin film is expected to increase the mass and, therefore, decrease the crystal resonator frequency as the polymer rehydrates in solution. This behavior was observed in thermal cycling experiments conducted by Liu et al. on PNIPAm brushes grafted to silica-coated QCM substrates.9 Correcting for differences in the harmonic used for the studies, Liu et al. measured frequency changes on the same order of magnitude as those seen with our samples in the current study. The rehydration of the 0.20% P(NIPAm-co-CBAm) film between 34.0 and 31.0 °C is near the LCST observed for the polymer in solution. This proximity of temperature may result from the number of attachment points imparted by CBAm in the polymer. As described in the Introduction, previous studies demonstrated little variation in the temperature for this transition when multiple graft points are present in a PNIPAm chain.12 Differences in behavior between 0.00 and 0.20% P(NIPAmco-CBAm) samples are seen in the dissipative data following a temperature step from 34 to 31 °C. Figure 6a shows the change in dissipation for a gold-coated QCM crystal before (spectrum A) and after (spectrum B) exposure to 0.00% P(NIPAm-coCBAm). As with the frequency change, no difference in ∆D is observed for the crystal before and after exposure to the 0.00% CBAm polymer; both possess a ∆D value of ∼2.5 × 10-6. The similarity in dissipation changes for the crystal before and after exposure to 0.00% P(NIPAm-co-CBAm) indicates that the crystal is responding only to changes in solution viscosity and not changes in the viscoelastic properties in the film adhering to the crystal surface. Such changes in the viscoelastic properties of a thin film should lead to large changes in the measured dissipation.29

Park et al.

Figure 7. (a) Relative contact angles for water on clean gold and on 0.00, 0.04, 0.20, 0.50, and 1.00% P(NIPAm-co-CBAm)-coated gold substrates, from bottom to top, following soaking in 25 and 60 °C water baths. Values have been offset along the y-axis for clarity. Error bars represent the 95% confidence interval based on a minimum sample size of n ) 10. (b) Change in water contact angle for clean gold (A) and 0.00, 0.04, 0.20, 0.50, and 1.00% (F) P(NIPAm-coCBAm) samples upon steps from 25 to 60 and 60 to 25 °C.

Figure 6b shows the temperature-induced change in dissipation for the gold-coated QCM crystal before (spectrum A) and after (spectrum B) exposure to 0.20% P(NIPAm-co-CBAm). With CBAm present, the differences in dissipation are evident between the crystal before and after polymer deposition. Dissipation changes of ∼1.0 × 10-6 and ∼3.6 × 10-6 are observed for the clean and polymer-coated QCM crystal, respectively. We postulate that this difference in dissipation results from rehydration of the 0.20% P(NIPAm-co-CBAm) film on the crystal as it cools. Cooling the sample increases the polymer’s interaction with water. This rehydrated polymer is significantly more viscoelastic than the clean crystal, leading to greater dampening and a larger dissipation of energy by the film. Again, similar trends in dissipation were observed in the studies of Liu et al.9 Cooling the polymer through the solution-phase transition temperature resulted in an increased dissipation. The changes we observe with our 0.20% P(NIPAm-co-CBAm) film are similar in magnitude to the changes observed previously for grafted PNIPAm on silica.9 Contact angle analysis is a second convenient method to analyze the effects of temperature on PNIPAm-based thin films.6,10,12 Figure 7a shows the relative contact angles during temperature cycling from 25 to 60 °C for all five samples (0.00-1.00% P(NIPAm-co-CBAm)) plus bare gold. Initial contact angles (data not shown) for all samples fell between 67 and 76° without any clear correspondence to the amount of CBAm in the polymer. These initial, low-temperature angles are in agreement with those previously seen for PNIPAm thin films on gold or polymer surfaces, which fell in the range from 62 to 80°.6,10,12

BehaVior of P(NIPAm-co-CBAm) Thin Films on Au

The contact angle variation for the gold substrate, line A in Figure 7a, is independent of temperature over three cycles from 25 to 60 °C. The 0.00% CBAm polymer, line B, also exhibits behavior that is largely independent of the temperature. However, introduction of even small amounts of CBAm introduces temperature-dependent changes in contact angles. The 0.04% P(NIPAm-co-CBAm), line C, has an initial contact angle of 73.5° at 25 °C and a contact angle of 79.0° after 10 min at 60 °C, a change of 5.5°, after 10 min at 60 °C. As the amount of CBAm in the polymer increases, the magnitude of the contact angle change following the temperature step increases, culminating in a change of 9.0° for 1.00% P(NIPAm-co-CBAm). The change in contact angle for 1.00% P(NIPAm-co-CBAm) is near the range (10-25°) previously observed for PNIPAm-based thin films prepared using other methods.6,10 The slightly lower contact angles in our work may result from the higher number of attachment points imparted by thiols along the polymer chain length. These attachment points may serve to anchor the polymer more closely to the substrate, preventing extensive rehydration in solution. The changes in contact angle following temperature steps are illustrated in Figure 7b as a bar graph. Here, the relationship between the amount of CBAm and the change following the first step from 25 to 60 °C is evident. This relationship is generally followed for all subsequent temperatures steps, whether positive or negative. Two trends are evident from Figure 7b. First, the change in contact angle upon cooling is generally smaller than upon heating. This behavior may result from differences in the rates of the hydration versus dehydration processes, a phenomenon known with PNIPAm-based hydrogels32 that was not investigated in our studies. Second, the magnitude of change in contact angle is generally largest for the first temperature step with a sample and decreases slightly thereafter. The trend is evident looking at the three steps from 25 to 60 °C. We postulate that this may result from changes in the number of bonds between the polymer and the substrate following heating. Specifically, the collapse of the P(NIPAm-co-CBAm) at high temperature would bring the polymer and substrate in more intimate contact. Under these (32) Matsuo, E. S.; Tanaka, T. J. Chem. Phys. 1988, 89, 1695-1703.

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conditions, any free (unbound) thiols along the polymers’ length might be able to bond with the substrate, increasing the number of P(NIPAm-co-CBAm)-gold chemisorption interactions. This increased bonding would serve to limit the mobility of the polymer during subsequent temperature steps. Notably, previous work with temperature-dependent contact angles of PNIPAm-based films on substrates did not investigate thermal cycling but instead reported only a single temperature step.6,10,12

Conclusion Thermoresponsive adsorbing polymers were synthesized using CBAm as a co-monomer with NIPAm. The presence of small amounts of CBAm did not alter the structure or the phase transition temperature of the polymer in solution. XPS and QCM results indicate that polymer containing CBAm adsorbs more effectively to gold substrates than pure PNIPAm analogues. Thermoresponsive behavior is also observed with the P(NIPAm-co-CBAm) thin films on gold. QCM indicates rehydration of the polymer as the temperature is decreased from 34 to 31 °C. Contact angle analysis shows variations in the hydrophobicity/hydrophilicity of the polymer film on gold as the temperature is stepped between 25 and 60 °C. The magnitude of this variation increases with the amount of CBAm in the polymer sample. Acknowledgment. The authors thank Dr. Richard T. Haasch for his assistance with the XPS data acquisition and interpretation and Prof. Craig M. Teague for review of the manuscript. This work was partially supported by the Donors of the American Chemical Society Petroleum Research Fund through Grant PRF 42899-GB10, by the National Science Foundation (NSF grant no. CHE-0353813), and by the Brachman Hoffman Program, Wellesley College. Some research for this publication was carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which are partially supported by the U.S. Department of Energy under Grant DEFG02-91ER45439. Supporting Information Available: Figures A-C referred to in text. This material is available free of charge via the Internet at http://pubs.acs.org. LA700624G