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Langmuir 2000, 16, 8016-8023
Surfaces with Reversible Hydrophilic/Hydrophobic Characteristics on Cross-linked Poly(N-isopropylacrylamide) Hydrogels Liang Liang,* Peter C. Rieke, Jun Liu, Glen E. Fryxell, James S. Young, Mark H. Engelhard, and Kentin L. Alford Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Received July 31, 2000 Temperature-sensitive surfaces were prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) hydrogel on the surface of silicone wafers. The silicone wafer substrates were modified by organosilane (vinyltriethoxylsilane). They were further reacted with N-isopropylacrylamide (NIPAAm), using N,N′methylenebisacrylamide as the cross-linking agent, to generate the cross-linked PNIPAAm layer on the surface of the substrate. The surfaces modified by the cross-linked PNIPAAm layer were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and attenuated total reflectanceFourier transform infrared (FTIR) spectroscopy. The reversible hydrophilic/hydrophobic properties of the surface were evaluated by the dynamic contact angle. The morphology of the surface modified by a crosslinked PNIPAAm layer can be turned from “sea island” to “mountain valley” by changing the molar ratio of NIPAAm to BisAAm and by varying the polymerization time. A completely hydrophilic surface (advancing contact angle ) 0°) was observed below 25 °C, and the surface became extremely hydrophobic (advancing contact angle ) 92°) above 40 °C. The sensitivity of the surfaces to temperature change can be improved by increasing the cross-linking density of the polymer layer and varying the polymerization time. The water meniscus height in a capillary tube, whose wall was coated by a cross-linked PNIPAAm layer, went up or down as the temperature changed to below or above the lower critical solution temperature of PNIPAAm. The differences in the water meniscus heights are 10 and 5 mm for a capillary tube with a diameter of 2 and 3 mm, respectively, corresponding to a change in temperature from 23 to 50 °C. The temperature-sensitive characteristics, which produce remarkable and rapid changes of surface properties, make this technology applicable for use as actuators, modulators, sensors, and switches.
Introduction Environmentally sensitive polymers, which can alter their properties with a change in environmental stimuli, have been extensively investigated. These polymers can be fine-tuned for a wide variety of applications, such as drug delivery,1,2 biocatalysis,3-5 size-selective separation,6-8 and energy transducers.9-11 Temperature-sensitive polymers have received much attention because it is easy to control and regulate the temperature during processing. Among thermally sensitive polymers, poly(N-isopropylacylamide) (PNIPAAm) has been of great interest because aqueous solutions of PNIPAAm undergo fast, reversible changes around their lower critical solution temperature of 32 °C.12 PNIPAAm chains show an expanded conformation in water below the lower critical solution temperature (LCST) because of strong hydration and change to the compact forms above the LCST by sudden dehy* Correspondence to L. Liang. E-mail:
[email protected]. (1) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291. (2) Hoffman, A. S. J. Controlled Release 1987, 6, 297. (3) Bergbreiter, D. E.; Case, B. L.; Liu, Y. S.; Caraway, J. W. Macromolecules 1998, 31, 6053. (4) Bhattacharya, S.; Moss, R. A.; Ringsdorf, H.; Simon, J. Langmuir 1997, 13, 1869. (5) Takeuchi, S.; Omodaka, I. Makromol. Chem. 1993, 194, 1991. (6) Ito, Y.; Kotera, S.; Inaba, M.; Kono, K.; Imanish, Y. Polymer 1990, 31, 2157. (7) Mika, A. M.; Childs, R. F.; Dickson, J. M.; McCarry, B. E.; Gagnon, D. R. J. Membr. Sci. 1995, 108, 37. (8) Park, Y. S.; Ito, Y.; Imanishi, Y. Langmuir 1998, 14, 910. (9) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345. (10) Tanaka, T.; Nishio, I.; Sun, S. T.; Nishio, S. U. Science 1982, 218, 467. (11) Irie, M. Macromolecules 1996, 19, 2890. (12) Heskins, M.; Guillet, J. E.; James, E. J. Macomol. Sci., Chem. 1968, A2, 1441.
dration. Pentagonal ring structures form around water molecules, and the hydrophilic groups in the PNIPAAm chains13 result in reversible formation and cleavage of hydrogen bonds between hydrophilic groups and surrounding water molecules with a change in temperature. Recent developments on surfaces modified by grafting PNIPAAm have opened up an exciting possibility to develop new smart materials. Silica grafted by PNIPAAm has been synthesized as column packing materials for temperature-sensitive chromatography,14 by which a series of chemicals with similar properties can be separated and analyzed. Polystyrene films modified by PNIPAAm have been employed as a substrate to culture cells.15,16 Above the LCST of PNIPAAm, the cells were attached and proliferated on the substrate, and the cultivated cells were readily detached from the substrate without the usual damage associated with trypsinization below the LCST of PNIPAAm. Microfiltration membranes grafted by PNIPAAm have been used as the valve to control liquid transfer.17 Below the LCST of PNIPAAm, the pores of the membrane were blocked by the swelling of PNIPAAm chains and opened as the PNIPAAm chains shrank above the LCST of PNIPAAm. The membrane can allow liquid to pass through or block it, depending on the temperature. Multifunctional separation membranes have (13) Urry, D. W. Sci. Am. 1995, 1, 64. (14) Kanazawa, H.; Kashiwase, Y.; Yamamoto, K.; Matsushima, Y.; Kikuchi, A.; Sakurai, Y.; Okano, T. Anal. Chem. 1997, 69, 823. (15) Okano, T.; Kikuchi, A.; Sakurai, Y.; Takei, Y.; Ogata, N. J. Controlled Release 1995, 36, 125. (16) Ito, Y.; Chen, G.; Guan, Y.; Imanishi, Y. Langmuir 1997, 13, 2756. (17) Iwata, H.; Oodate, M.; Uyama, Y.; Amemiya, H.; Ikada, Y. J. Membr. Sci. 1991, 55, 119.
10.1021/la0010929 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/21/2000
Cross-linked Poly(N-isopropylacrylamide) Hydrogels
been fabricated by introducing PNIPAAm at different locations on the separation membrane.18 The membranes exhibit characteristics of a microfiltration membrane above the LCST and those of an ultrafiltration membrane below the LCST. Several strategies have been developed to graft PNIPAAm on the surfaces. The PNIPAAm layer can be immobilized on the surface of substrates by chemical19,20 and physical grafting.21,22 Linear and cross-linked PNIPAAm layers can be obtained, depending on the approach. However, the characteristics of temperaturedependent PNIPAAm in solution are maintained regardless of the grafting method.23,24 PNIPAAm graft architecture strongly influences the wettability of a modified surface because of the dynamic motion of the grafted polymer chain with different architectures.19 By introducing PNIPAAm chains with freely mobile ends, we can effectively alter the surface properties within a narrow temperature range, and the surface can respond to the temperature change rapidly. In a previous paper,22 we reported on surfaces modified by a cross-linked PNIPAAm layer using ultraviolet (UV) photopolymerization. The surfaces show temperature-sensitive characteristics, but the temperature change range is wider, which implies that the sensitivity of the surface to the temperature is not strong. A cross-linked PNIPAAm layer that was dense and smooth was observed by scanning electron microscopy (SEM), which restricts the motion of polymer chains and results in a slow response of polymer chains to a change of temperature. In this paper, we report a different method to generate a cross-linked PNIPAAm layer on the surface of silicone wafers. This new approach offers two advantages: the morphology of the grafting layer can be modified, and the surfaces respond with a remarkably rapid change in hydrophilic/hydrophobic properties. Experimental Section Materials. N-Isopropylacrylamide (NIPAAm, 97%, Aldrich) was purified by recrystallization from hexane. Vinyltriethoxysilane (VTES, Gelest) was purified by reduced pressure distillation. We purified 2-isopropanol (99%, Aldrich), diethyl ether (99%, Aldrich), anhydrous toluene (99%, Aldrich), tetrahydrofuran (99%, Aldrich), and methanol (99%, Aldrich) by distillation. We used N,N′-methylenebisacrylamide (BisAAm, 95%, Aldrich), 3-mercaptopropionic acid (MCPA, 99%, Aldrich), dichloromethane (99%, Aldrich), and 2,2-azobisisobutyronitrile (AIBN, Aldrich) without purification. Ultrapure water with conductivity 18 S cm-1 was used in all experiments. Silicone wafers and glass capillary tubes were obtained from Silica-Source Technology Inc. and Quartz Sci. Inc. Modification of Silicone Wafers. Polished and p-doped silicone wafers were cut along the crystal axes into 25 mm × 10 mm samples. The wafers were sonicated for 15 min in 2-isopropanol to remove any trace organic residue and to ensure a uniform oxide coat. The precleaned silicone wafers were immersed into 0.1 N KOH for 2 min and 0.1 N HNO3 for 10 min, respectively, and the wafers were washed with an excess of water. The silicon wafers were further dried under flowing nitrogen gas for a minimum of 2 h before immersion in the organosilane solution. (18) Liang, L.; Feng, X. D.; Peurrung, L. M.; Viswanathan, V. J. Membr. Sci. 1999, 162, 235. (19) Yakushiji, T.; Sakai, K.; Kikuchi, A.; Aoyagi, T.; Sakurai, Y.; Okano, T. Langmuir 1998, 14, 4657. (20) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163. (21) Volpe, D.; Cassinelli, C.; Morra, M. Langmuir 1998, 14, 4650. (22) Liang, L.; Feng, X. D.; Liu, J.; Peter, P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845. (23) Kawasaki, H.; Sasaki, S.; Maeda, H. J. Phys. Chem. 1997, 101, 4184. (24) Walter, R.; Ricka, J.; Quellet, Ch.; Nyftenegger, R.; Binkert, Th. Macromolecules 1996, 29, 4019.
Langmuir, Vol. 16, No. 21, 2000 8017
Figure 1. Schematic representation of grafting procedure to create a cross-linked PNIPAAm layer on the surface of a silicone wafer. Table 1. Recipe of Polymerization Solution Used to Generate the Cross-linked PNIPAAm Gela NIPAAm to BisAAm (molar ratio)
NIPAAm (g)
BisAAm (g)
water (mL)
50 100 200 400
7 7 7 7
0.191 0.095 0.048 0.024
100 100 100 100
a
Polymerization time: 24 h.
To generate functional groups on the surface of the silicone wafers, the dried silicone wafers were immersed into anhydrous toluene containing 5 vol % vinyltriethoxysilane. The reaction was performed in a plate-bottom flask connected to a reflux condenser for 24 h. The silicone wafers were washed with dichloromethane and sonicated in 2-isopropanol to quench any residual organosilane remaining after the reaction. The silicone wafers were dried under a stream of dry nitrogen gas overnight at room temperature to allow interstitially trapped solvent to evaporate. Using the same procedure, the glass capillary tubes were coated with VTES. Preparation of Cross-linked PNIPAAm Layer on Surface of Silicone Wafer. The cross-linked PNIPAAm layer was grafted on the surface of a silicone wafer coated with VTES with thermal free-radical polymerization. The schematic representation of the grafting procedure is shown in Figure 1. Table 1 lists the recipe to graft the cross-linked PNIPAAm layer on the surface of the silicone wafer. We used the following preparation procedure. We dissolved 7.0 g of NIPAAm (62 mmol) and 0.0477 g of BisAAm (0.62 mmol) into 100 mL of water by stirring. The solution was transferred into a 250 mL plate-bottom flask with three necks and equipped with a reflux condenser. The silicone wafers coated with VTES were immersed in the solution. The solution was then deoxygenated by nitrogen gas bubbling for 30 min. Finally, the upper space of the flask was filled with nitrogen gas. The polymerization was carried out with reflux at 100 °C for 24 h. During this process, free radicals were generated by the thermal decomposition of double bonds immobilized on the surface of the silicone wafer. The free radicals on the surface of the silicone wafer further attack the monomer and the cross-linking agent to generate the cross-linked PNIPAAm layer on the surface of the silicone wafer. The silicone wafers, grafted by a cross-linked PNIPAAm layer, were rinsed with methanol and water to remove any unreacted monomer, cross-linking agent, and unimmobilized polymers. Before measurements, the silicone wafers grafted by a cross-linked PNIPAAm layer were immersed in water for 24 h and dried in a vacuum at room temperature to completion. Scanning Electronic Microscopy. The surfaces of silicone wafers grafted by a cross-linked PNIPAAm layer were observed by SEM (SEM, LEO 982). The thickness of the grafted PNIPAAm was measured at seven randomly chosen points. The mean data were taken as the final data, as seen in the Results and Discussion section. They have a standard deviation of
