J. Phys. Chem. B 2000, 104, 11667-11673
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Temperature-Sensitive Surfaces Prepared by UV Photografting Reaction of Photosensitizer and N-Isopropylacrylamide Liang Liang,* Peter C. Rieke, Glen E. Fryxell, Jun Liu, Mark H. Engehard, and Kentin L. Alford Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: NoVember 4, 1999; In Final Form: September 26, 2000
A thermal-sensitive surface was prepared on the surface of a silicone wafer (substrate) by an ultraviolet photografting reaction between a photosensitizer, (N,N′-diethylamino)dithiocarbamoylpropyl(trimethoxy)silane, and N-isopropylacrylamide. Ellipsometery measurement revealed a thin grafting layer (∼44 Å) consisting of a single terminal poly(N-isopropylacrylamide) (PNIPAAm) chain on the surface of the silicone wafers. X-ray photoelectron spectroscopy confirmed that the grafting layer was composed of the PNIPAAm structure. The properties of the grafting layer can be adjusted and manipulated by varying the photopolymerization time and the concentration of the monomer. Increasing the photopolymerization time and the concentration of the monomer increases both the thickness of the grafting layer and the wettability of the surface. The characteristics of the temperature-sensitive surface were investigated by dynamic contact angle as a function of temperature. The remarkable change on advancing contact angle can be observed around 32 °C. Compared with the substrate grafted by PNIPAAm gel, the substrate with a signal-terminal PNIPAAm chain exhibited a lower transition temperature and a narrower change range of transition temperature. This can be attributed to the mobile PNIPAAm chain with a single-terminal mode, which increases the characteristics of faster response to the temperature change.
Introduction Polymers can be used to modify silicon wafers to improve their surface properties, such as wettability, adhesion, friction, and biomedical compatibility.1 Many techniques have been explored to modify surfaces, including plasma treatment, corona discharge, ozone treatment, electron-beam bombardment, and ultraviolet (UV) and X-ray irradiation.2 Among these, a surface functionalized by UV photografting has several advantages. Mild reaction conditions and a low temperature may be employed. By choosing the photosensitizer, the solvent, and the excitation wavelength, special chemical compounds can be grafted on the surface.3 As poly(N-isopropylacrylamide) (PNIPAAm) exhibits thermally reversible soluble/insoluble changes in response to temperature change across a lower critical solution temperature (LCST) at 32 °C in an aqueous system,4 many researchers have attempted to develop intelligent materials by grafting PNIPAAm.5-8 The surfaces modified by PNIPAAm can be altered from hydrophilicity to hydrophobicity by external stimuli.9 This unique characteristic enables the materials to be modified by using PNIPAAm as the sensor,10 an intelligent biomaterial,11,12 a chemical memory unit,13 and a molecular separation system.14 Polystyrene films modified by PNIPAAm can be employed as the substrate for cell incubation.15 After being modified by PNIPAAm, the silica and glass beads can be constituted as the loaded stock for thermal-sensitive chromatography.16 Mutilfunctional separation membranes have been prepared by grafting PNIPAAm on the microfiltration mem* To whom correspondence
[email protected].
10.1021/jp9939017
would
be
addressed.
E-mail:
branes.17 A potentially intelligent switch and control valve can be prepared by grafting PNIPAAm within the capillary tube and sponge, respectively.18,19 In a previous paper,18 we reported on preparing glass plates by using UV photopolymerization to graft them with PNIPAAm in the presence of a cross-linking agent, N,N′-methylenebis(acrylamide). We described the surface properties of the glass plates as a function of temperature. Although the surface shows the hydrophilic/hydrophobic change with a change of external temperature, the thick grafting layer (∼1800 Å) shows a wider transition-temperature range that corresponds to a slower change of surface properties by thermal stimuli. The cross-linked network restricts the mobile rate of the polymer chain and therefore reduces the sensitivity of the polymer chain to temperature change. For useful applications, surfaces with rapid response to temperature changes are extremely desirable. The conformation of a PNIPAAm chain grafted on the substrate plays a critical role in controlling sensitivity to temperature change. The surface-attached PNIPAAm with a single terminal mode can improve the sensitivity of the surface to temperature change.20 The thickness of grafted polymer also affects surface properties. Modified surface layers that are too thick can change the mechanical and functional properties of the materials. Therefore, a thin grafting layer is desirable for useful applications. In the present study, we prepared thermal-sensitive surfaces by grafting PNIPAAm on the surface of silicone wafers without a cross-linking agent. It can be expected that a thin grafting layer with a single terminal mode can be developed, and the sensitivity of the PNIPAAm grafting layer to the variation of temperature can be increased.
This article not subject to U.S. Copyright. Published 2000 by the American Chemical Society Published on Web 11/10/2000
11668 J. Phys. Chem. B, Vol. 104, No. 49, 2000 Experimental Section Materials. N-Isopropylacrylamide (NIPAAm, 97%, Aldrich) was purified by recrystallization from hexane. Chloropropyltrimethoxysilane (CPTMS, 97%, Aldrich) was purified by reduced pressure distillation. Acetone (99.9%, Aldrich) was dried by CaCl2 (Aldrich) and a 4 Å molecular sieve (Aldrich) for 4 days, respectively, and then refluxed in the presence of AlH4Li (Aldrich) for 48 h before distillation. Anhydrous toluene (99%, Aldrich) was purified by distillation. Sodium N,N′diethyldithiocarbamate (SDDC) was recrystallized from methanol. Ultrapure water with a conductivity of 18 S cm-1 was used in all experiments. Silicone wafers that were polished and p-doped were obtained from Silicone-Source Technol. Inc. Preparation of (N,N′-Diethylamino)dithiocarbamoylpropyl(trimethoxy)silane (DATMS).21 The following typical synthesis was used to synthesize DATMS. We dissolved 12.0 g of CPTMS (60 mmol) and 4.0 g of SDDC (23 mmol) in 100 mL of acetone by stirring at room temperature. The solution was transferred into one 250 mL round-bottom flask and allowed to be refluxed for 48 h. During this process, DATMS as the photosensitizer was synthesized by the reaction between CPTMS and SDDC with white precipitation of NaCl as the byproduct. The solution was cooled to room temperature, and NaCl was separated by centrifugation. The unreacted CPTMS and the residence solvent were evaporated by reduced pressure distillation. The final product was a yellow-colored viscous liquid with a yield of 69%. Modification of Silicone Wafers. The silicone wafers as substrates were cut to 25 mm × 10 mm. The substrates were immersed in 2-propanol and cleaned by a supersonic wave for 15 min. After the substrates were thoroughly rinsed with water, they were dipped into 0.1 N KOH for 2 min and 0.1 N HNO3 for 10 min, respectively. Then they were washed with an excess of water and dried by blowing dry nitrogen gas for 2 h before reaction with the photosensitizer. To immobilize the photosensitizer on the surface of the substrates, the substrates were immersed in 100 mL of toluene containing 5 g of DATMS. The reaction was performed in a plate-bottom flask connected to a reflex condenser for 24 h at 110 °C. Finally, the substrates were rinsed with dichloromethane for 5 min and dried under a stream of dry nitrogen gas overnight at room temperature. Preparation for Grafting PNIPAAm on Substrates. The following procedure was used to prepare the surfaces for grafting the PNIPAAm chain. We dissolved 5 g of NIPAAm in 15 mL of water by stirring for 1 h. The solution was poured into one 100 mL quartz flask with one rubber septum and deoxygenated by nitrogen gas bubbling for 30 min. Finally, the upper space of the flask was filled with nitrogen gas. UV light was used for photopolymerization energy. Polymerization was started by irradiating the solution using two 100 W high-pressure mercury lamps and a UV wavelength of 254 nm. The quartz flask was put inside UV lamps, and the distance of the substrate to the UV lamps was 10 cm. After finishing photopolymerization, the substrates were washed with water and sonicated in water for 30 min to clean the surfaces. The substrates were completely dried in a vacuum oven at room temperature. Fourier Transfer Infrared (FTIR) Spectrum. The FTIR spectrum of the photosensitizer was measured by FTIR instrument (Magan-IR 860, Nicolet). The scan number and spectra resolution were 100 and 4 cm-1, respectively. Spectrum of Nuclear Magnetic Resonance (NMR). NMR data were collected on a Chemagnetics CMX300 NMR spec-
Liang et al. trometer operating at 298.3 MHz for the proton frequency. We used 13C-labeled chloroform (Aldrich) without further purification. X-ray Photoelectron Spectroscopy (XPS). XPS data were collected on a Phi Quantum 2000 Scanning ESCA Microprobe using a monochromatic Al KR X-ray source. The analysis area was 1.5 mm × 0.2 mm. A typical mutilplex pass energy is 23.5 eV, and a typical survey pass energy is 117.4 eV. The takeoff angle is 45°. The charge correction was calculated from the difference of the observed carbon 1s binding energy from the calculated 284.8 eV value for the carbon 1s binding energy. The composition of the PNIPAAm hydrogel layer on the surface of silicone wafer was analyzed by the relative peak area. Before measurement of XPS, the silicone wafers with and without grafting PNIPAAm were dried in a vacuum at room temperature. The clean method of silicone wafer without grafting PNIPAAm is same as the silicone wafer with grafting PNIPAAm. Static Contact Angle. The static contact angle of water on the surface of the substrates was measured by a contact-angle goniometer (100-00, rame-hart, inc.). One water drop (10 µL) was deposited on the dry surface of a silicone wafer. The angle was measured as soon as possible after a sessile drop of water formed on the surface of the substrates. The static contact angle could be read directly from the goniometer. All measurements were done at room temperature and about 40% humidity. The five points on the surface were chosen at random for the measurement. The mean data were taken as final data as discussed in the Results and Discussion, and they have standard deviations less than 5%. Dynamic Contact Angle. The dynamic contact of silicone wafers with and without the grafted PNIPAAm gel was measured using a dynamic Wilhelmy plate technique (DCA312, Cahn Instrument Inc.). The temperature of the test chamber (100 mL) was controlled by a thermostated circulator to (0.2 °C. The advancing contact angles were measured at an immersion speed of 10 mm/min with a 10 mm immersion depth. A computer automatically recorded the hysteresis curve for the surface tension immersion depth in water. Each measurement took five consecutive strokes. Because the cross-linked PNIPAAm layer on the surface of silicone wafer will be wetted gradually in water at room temperature, the real dynamic contact angle will be evaluated after second stroke, which shows stable values. We use the standard calculation procedure (DCA4A, Cahn Instrument Inc.) to calculate the contact angle. The mean data with a standard deviation less than 5% are shown in the Results and Discussion. Ellipsometery. Spectroscopic ellipsometery (Gaertner Ellipsometer) was used to determine the thickness of the oxide layer, the immobilized photosensitizer layer, and the grafting PNIPAAm layer on the surface of the silicone wafers. The lateral size of ellipsometery spot is around the diameter of 0.5 mm. All samples were dried in a vacuum oven at 30 °C overnight before measurement. The measurements were done immediately after the samples were taken out of the oven. At least five points on the sample surface were measured, and the thickness was calculated by a standard procedure (Auto, Gaertner Ellipsometer). The refractive index of the monolayer and underlying oxide was taken as 1.46.22 The thickness of the organosilane layer was calculated by subtracting the thickness of the oxide layer from the total thickness. Results and Discussion Figure 1 shows the FTIR spectrum of the photosensitizer. The C-H stretching bands of the propyl group and the
Temperature-Sensitive Surfaces
Figure 1. FTIR spectrum of photosensitizer (DATMS).
Figure 2.
13C
NMR spectrum of photosensitizer (DATMS).
asymmetric stretch of the methoxy group fall together in one broad peak at 2950 cm-1.23 The CH2 scissoring vibration can be found at 1410 cm-1. The strong bands at 1090 and 820 cm-1 were attributed to the Si-O and Si-O-C stretching mode. The peaks at 1280 and 1350 cm-1 were assigned to the dithiocarbamate group.24 Figure 2 shows the photosensitizer’s NMR spectrum. The peaks at 40 and 195 ppm were attributed to carbon in C-S and the CdS group (C5, C6).25 The peaks at 48 and 51 ppm were assigned to the carbon in the methoxyl group (C9, C10, C11). The peaks at 47 and 50 ppm were attributed to carbon in the methylene group (C2, C4). Figure 3 shows the effect of reaction time between the photosensitizer and substrates on the static contact angle and the thickness of the photosensitizer coated on the substrate. Both the static contact angle and the thickness of the photosensitizer coated on the substrate increase with increasing reaction time.
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11669
Figure 3. Effect of reaction time on static contact angle and thickness of photosensitizer coated on the substrate. Concentration of DATMS: 5 wt % in toluene.
At initial reaction, a rapid increase in the static contact angle and the thickness of the photosensitizer coated on the substrate can be observed, which indicates that the condensation reaction to immobilize the photosensitizer on the substrate is the faster process. The schematic diagram of photografting NIPAAm onto a silicone wafer is shown in Figure 4, which includes the mechanism of immobilizing the photosensitizer on the substrate. Four steps are used to immobilize the photosensitizer onto the surface of the substrate.26 At the beginning of the reaction, the Si(OCH3)3 group in the photosensitizer is hydrolyzed, and the condensation to oligomer with hydroxyl groups follows. The oligomer forms a hydrogen bond with the hydroxyl groups on the substrates. Finally, a covalent linkage among hydroxyl groups is formed. The photosensitizer’s shorter chain length and good solubility in the solvent may be the reason the reaction rate of both hydrolysis and the condensation of the photosensitizer increases, which results in the faster increase of both the static contact angle and the thickness of the photosensitizer coated on the substrate. The static contact angles of the substrates before and after reacting with a photosensitizer are 37° and 70°, respectively. The increase of the contact angle can be attributed to the fact that the substrates with a more hydrophilic OH group (before reacting with photosensitizer) were covered by a more hydrophobic group in the photosensitizer. The thickness of the photosensitizer coated on the substrate is listed in Table 1, which was calculated by subtracting the thickness of the oxide layer from the total thickness of the coating layer. From Table 1, it can be seen that the thickness of photosensitizer coated on the substrate and the thickness difference between the experimental and calculated data27 increase with increasing reaction time. The thickness difference is small at the shorter reaction time, and it increases with increasing reaction time. This is because a multimolecular layer with a loose network structure could be formed on the surface of the substrate. This possibility increases especially with a longer reaction time.28 The free radicals on the substrate can be generated by fission of the S-C bond in the photosensitizer by irradiation with UV light.29 The free radicals generated on the substrate can further attack the double bond of NIPAAm in solution to graft PNIPAAm on the substrate. Figure 5 shows the effect of photopolymerization time on static contact angle and grafting layer thickness. As the photopolymerization time increases, the thickness of the grafting layer increases, but the static contact angle decreases. Before photopolymerization, the thickness of the photosensitizer coated on the substrate is 54 Å. It increases to 90 Å after 24 h of photopolymerization. Increasing the
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Figure 5. Effect of photopolymerization time on static contact angle and thickness ofgrafting layer on the substrate. Concentration of NIPAAm: 1.2 mol/L; solvent: water.
Figure 6. Effect of monomer concentration on static contact angle and thickness ofgrafting layer on the substrate. Photopolymerization time: 24 h. Solvent: water
Figure 4. Schematic representation of grafting PNIPAAm on the surface of a silicone wafer.
TABLE 1: Thickness of Substrate Grafted by Photosensitizer reaction time (h)
thickness of photosensitizer (Å)
thickness differencea (Å)
0.5 1.0 2.5 6.0 24.0
20.4 22 24 27 28
3.2 4.8 6.8 9.8 10.8
a Calculated by subtracting the theoretical thickness of the photosensitizer from the experimental thickness of the photosensitizer.
photopolymerization time increases the probability that the monomer will be grafted on the substrate; therefore, the thickness of the grafting layer increases. Because of the flexibility of the polymer chain, the conformation of the polymer chain would be a random coil on the substrate, which reduces greatly the real length of the polymer chain on the substrate.30 On the other hand, the static contact angle of the substrate decreased from 78° to 44° when the reaction time changed from
0 to 24 h. This is because more hydrophilic PNIPAAm chains attached on the substrate increase the wettability of the substrate, which decreases the contact angle. Coating the substrate with PNIPAAm gel with a cross-linking agent generated an inhomogeneous surface and a thick grafting layer (∼1800 Å).18 However, thin and homogeneous surfaces were obtained by coating the substrate with PNIPAAm gel without a cross-linking agent. As the photopolymerization rate is proportional to the concentration of monomer, it can be expected that more polymer will be grafted on the substrate as the concentration of the monomer is increased.31 Figure 6 shows the effect of monomer concentration on the static contact angle and the thickness of the grafting layer when the photopolymerization time was kept at 24 h. When the monomer concentration was increased from 0.06 to 1.2 mol/L, the contact angle of the substrate decreased from 66° to 44°, but the thickness of the grafting layer increased from 54 to 90 Å. It is clear that the surface properties of substrates can be adjusted and manipulated by polymerization time and the monomer concentration. The substrates grafted by PNIPAAm were evaluated by XPS to prove the existence of the grafted PNIPAAm layer on the substrate. Figure 7 shows the XPS spectra of the original substrate and the substrate grafted by PNIPAAm. The peaks of oxygen (O1s), nitrogen (N1s), and carbon (C1s) at 532, 409, and 284 eV are attributed to the structure of PNIPAAm.32 Although we do not know the exact composition of surface of original silicone wafer, it is clear that the surface of original silicone
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Figure 7. XPS spectrum of different substrates. (1) Substrate grafted by PNIPAAm: concentration of NIPAAm: 1.2 mol/L; solvent, water; photopolymerization time: 24 h. (2) Original silicone wafer.
Figure 8. Relationship between repeat unit of PNIPAAm and element fraction in PNIPAAm chain.
TABLE 2: Composition of Surface Grafted by PNIPAAma element (%) tested points
C
N
O
Si
N/C
1 2 silicon wafer
62.5 70.6 12.6
9.2 11.3 0.7
17.1 14.3 40.7
11.2 3.78 45.9
0.147 0.159 0.06
a
N/C ) 0.166 (calculation by formulation of NIPAAm).
wafer was modified by some organic compound because of higher content of carbon and oxygen. The peak value of Si for substrate grafted by PNIPAAm was decreased dramatically. Two points on the same sample were chosen to measure the composition of surface grafted by PNIPAAm and the results were listed in Table 2, including with the N/C ratios calculated from XPS data. The composition of silicone wafer is also given for the purpose of comparison. A higher content of Si has been found on the substrate grafted by PNIPAAm. The polymer grafting layer with a loose coil structure that covered the surface increased the probability of Si being measured by XPS. However, a significant difference between the theoretical and experimental data of N/C ratio can be also found in Table 2. Figure 8 shows the relationship between the repeat unit of PNIPAAm attached on the surface and the element fraction in polymer chain if the polymer chains attached on the surface
Figure 9. Advancing contact angle of different substrates. Substrate grafted by PNIPAAm: concentration of NIPAAm: 1.2 mol/L; solvent: water; photopolymerization time: 24 h.
follow the single-terminal mode as shown in Figure 4. The N/C ratio increases with increasing repeat unit of PNIPAAm in polymer chain and will arrive at a critical value of 0.166. The test results shown in Table 2 are labeled by arrowheads in Figure 8, which indicate the repeat units of polymer chain are close to 4 and 10, respectively. The polymer chain with different length means that an inhomogeneous surface was generated. The reason can be attributed to the fact that the transfer reaction of macromolecular free radicals on the surface and the flexible polymer chains attached on the surface hinder further attacking of monomer from solution to the free radicals on the surface. The substrate grafted by a PNIPAAm chain exhibits temperature-sensitive characteristics; that is, a reversible change of the hydrophilic/hydrophobic surfaces can be observed by changing the temperature. Such characteristics were investigated by measuring the dynamic contact angle using ultrapure water as the probe liquid. The effect of temperature on the advancing and receding contact angles with standard deviation can be seen from Figures 9 and 10. Figure 9 shows the change that results as a consequence of advancing contact angle of different substrates as a function of temperature. The substrate with an OH group (before reacting with the photosensitizer) has a lower contact angle, while the substrate coated by photosensitizer
11672 J. Phys. Chem. B, Vol. 104, No. 49, 2000
Figure 10. Receding contact angle of different substrates. Substrate grafted by PNIPAAm: concentration of NIPAAm: 1.2 mol/L; solvent: water; photopolymerization time: 24 h.
keeps a higher contact angle in the whole range of temperature variation. The different contact angles can be attributed to the hydrophilic and hydrophobic properties of the OH and photosensitizer groups, respectively. Because neither the OH nor the photosensitizer groups respond to temperature stimuli, the contact angle of the substrate with OH and photosensitizer groups is independent of temperature change. The substrate grafted by PNIPAAm shows a typical characteristic of thermal response. At a lower temperature, the substrate can be wetted as the PNIPAAm chain is swollen by water; therefore, a lower advancing contact angle was observed. When the temperature is increased to above the LCST of the PNIPAAm, an abrupt shrinkage of the PNIPAAm chain on the surface causes the surface to become hydrophobic. Subsequently, the advancing contact angle of the surface increases. The contact angle of the substrate with the PNIPAAm chain is higher than that with OH groups, but lower than that with photosensitizer groups below LCST. This is because the PNIPAAm chain with a loose coil structure on the substrate may result in a substrate with incomplete wettability. The contact angle of the substrate with PNIPAAm is higher than that with a photosensitizer above LCST. The PNIPAAm chain was dehydrated and exhibited more hydrophobicity than the photosensitizer group at a higher temperature. Therefore, the advancing contact angle of the substrate grafted by PNIPAAm is higher than that with a photosensitizer group above LCST. From Figure 9, we can also see the transition temperature, that is, the temperature at which a marked change of advancing contact angle can be observed. This temperature is around 32 °C. This value is lower than that of the substrate grafted by PNIPAAm gel (35 °C).18 Meanwhile, the range of temperature to alter the contact angle for the substrate with a single terminal PNIPAAm chain is narrower compared with the substrate modified by PNIPAAm gel. The conformation of the PNIPAAm chain plays a critical role in controlling the LCST of the polymer and the range of temperature transition. It has been found that the surface modified by terminal grafting of a PNIPAAm chain exhibits a lower transition temperature and narrower range of temperature change than that surface modified by multipoint grafting on the surface with increasing temperature.18 The multipoint grafting conformation constrains the dehydration of polymers and prevents the dehydrated polymers. Therefore, the polymer chain can only shrink at a higher temperature, which
Liang et al. results in the transition temperature being shifted to a higher temperature and a wider transition temperature. The fact that the advancing contact angles are lower than the receding contact angles at the temperature range from 25 to 30 °C can be observed by comparing the data shown in Figures 9 and 10. Morra et al.33 investigated the effect of temperature and velocity on the shape of hysteresis loops and contact angles measured by the Wilhelmy plate experiments. Based on their studies, two reasons may result in the fact that the advancing contact angles are lower than the receding contact angles. One is that water film remains on fully wetted materials and its interaction with liquid bulk in advancing corresponds to a lower contact angle than previous receding. Another is that water evaporation has the effect to produce a lower temperature, which also reduces the advancing contact angles. Figure 10 shows the effect of temperature on the receding contact angle. It is clear that the receding contact angle is independent of the change of temperature for all substrates. This result is same as that observed by the substrates grafted with cross-linked PNIPAAm gels.18 Conclusions The UV polymerization of NIPAAm on the surface of a silicone wafer coated by photosensitizer generates a thin grafting layer with a single terminal PNIPAAm. Single terminal PNIPAAm chains attached on the surface turn the surface to a thermal-sensitive surface. Dramatic changes of the hydrophilic/ hydrophobic surface were observed by investigating the variation of the advancing contact angles. The surface properties of a thermal-sensitive surface can be adjusted by concentration of monomer and by varying the photopolymerization time. The advancing contact angles which are less than 60° below LCST of PNIPAAm and higher than 90° above LCST of PNIPAAm can be achieved by changing the temperature. Thin surfaces with the characteristics of temperature sensitivity are highly desirable and are especially important in application where reducing volume is critical, for example the temperaturesensitive microchannels and mesoporous materials. Acknowledgment. This research work was supported by a grant from the Microtechnology Initiative at Pacific Northwest National Laboratory. A portion of the research described in this paper was performed in the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DEAC06-76RLO 1830. References and Notes (1) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surface: From Physics to Technology; John Wiley and Sons: New York, 1998; Chapter 9. (2) Chan, C. M. Polymer Surface Modification and Characterization; Hanser Publisher: New York, 1994; Chapter 1. (3) Roffey, C. G. Photopolymerization of Surface Coating; John Wiley and Sons: New York, 1982; Chapter 3. (4) Heskins, M.; Guillet J. E.; James, E. J. Macromol. Sci., Chem. 1968, A2, 1441. (5) Feil, H.; Bae, Y. H.; Jan, F.; Kim, S. W. J. Membr. Sci. 1991, 64, 283. (6) Dong, L. C.; Hoffman, A. C. J. Controlled Release 1986, 4, 223. (7) Trank, S. J.; Johnson, D. W.; Cusser, E. L. Food Technol. 1989, 43, 78. (8) Susuki A.; Tanaka, T. Nature 1990, 346, 345.
Temperature-Sensitive Surfaces (9) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai Y.; Okano, T. Macromolecules 1994, 27, 6163. (10) Ecerhart, D. S. ChemTech 1999, 4, 30. (11) Takeuchi, S.; Omodake, I. Macromol. Chem. 1993, 194, 1991. (12) Shiroya, T.; Yasui, M.; Fujimoto, K.; Kawaguchi, H. Colloids Surf. B: Biointerfaces 1995, 4, 275. (13) Snowden, M.; Murray, M.; Chowdry, B. Chem. Ind. 1996, 15, 531. (14) Iwata, H.; Oodate, M.; Uyama, Y.; Ameminya H.; Ikada, Y. J. Membr. Sci. 1991, 55, 119. (15) Okano, T.; Yamada, N.; Okuhara, M.; Sakai H.; Sakurai, Y. Biomaterials 1995, 16, 297. (16) Kanazawa, H.; Kashiwase, Y.; Yamamoto, K.; Matsushima, Y.; Kikuchi, A.; Sakurai, Y.; Okano, T. Anal. Chem. 1997, 69, 823. (17) Liang, L.; Feng, X. D.; Perrung, L. M.; Viswanathan, V. J. Membr Sci. 1999, 162, 235. (18) Liang, L.; Feng, X. D.; Liu, J.; Peter P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845. (19) Liang, L.; Feng, X. D.; Martin, P. F. C.; Peurrung, L. M. J. Appl. Polym. Sci. 2000, 75, 1735. (20) Yakushiji, T.; Sakai, K.; Kikuchi, A.; Aoyaai, T.; Sakurai Y.; Okano, T. Langmuir 1998, 14, 4657. (21) Kobayaki, T.; Takahashi S.; Fujii, N. J. Appl. Polym. Sci. 1993, 49, 417.
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11673 (22) Fryxell, E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Wiliford, R. E.; Graff, G. L.; Campbell, A. A.; Wiacek, R. J.; Lee, L.; Halverson, A. Langmuir 1996, 12, 5064. (23) Roeges, N. P. G. A Guid to the Complete Interpretation of Infrared Spectra of Organic Structures; John Wiley and Sons: New York, 1994; Chapter 2. (24) Lee K. W.; McCarthy, T. L. Macromolecules 1988, 21, 3553. (25) Kem, W. Organic Spectroscopy; Macmillan: London, 1991; Chapter 3. (26) Arkels, B. ChemTechnology 1977, 7, 766. (27) Lide, D. R., Freserikse H. P. R., Eds. CRC Handbook of Chemistry and Physics; CRC Press: New York, 1997. (28) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982; Chapter 4. (29) Kuriyama, A.; Otsu, T. Polym. J. 1984, 16, 511. (30) Sperling, L. H. Introduction to Physical Polymer Science; John Wiley and Sons: New York, 1992; Chapter 2. (31) Odian, G. Principles of Polymerization; John Wiley and Sons: New York, 1991; Chapter 3. (32) Clark, D. R.; Saresh, S.; Ward, I. W. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge Press: New York, 1998; Chapter 6. (33) Della Volpe C.; Cassinelli, C.; Morra, M. Langmuir 1998, 14, 4650.
