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Permeation Control through Porous Membranes Immobilized with Thermosensitive Polymer Yong Soon Park,† Yoshihiro Ito,*,†,‡,§ and Yukio Imanishi†,‡ Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, 606-01, Japan, Graduate School of Material Science, NAIST, Ikoma, 630-01, Japan, and PRESTO, JST, Keihanna Plaza, Hikaridai 1-7, Seika-cho, Kyoto, 619-02, Japan Received August 4, 1997. In Final Form: November 14, 1997 Permeation through a porous polycarbonate membrane, on which a thermosensitive polymer, poly(Nisopropylacrylamide), was immobilized, was investigated. For photoimmobilization of poly(N-isopropylacrylamide), photoreactive azidophenyl group was connected to the polymer either at a chain terminal or in side chains. The two types of derivatized polymers had different lower critical solution temperature (LCST). Prescribed amounts of the derivatized polymer were cast on the polycarbonate membrane and photoirradiated. When a small amount of polymer was used, a thin layer of immobilized polymer was not enough to cover pores of the polycarbonate membrane, while a thick gel layer of immobilized polymer was formed on the polycarbonate membrane to cover pores when a large amount of polymer was used. The former is represented by “porous membrane”, and the latter by “nonporous membrane”. The rate of water permeation through the porous membranes changed at different temperatures, although permeation through nonimmobilized membrane was independent of temperature. Water permeation through the porous membrane increased above the LCST of graft polymers. Hydraulic permeation through the nonporous membrane was not observed at any temperature. On the other hand, tryptophan permeation through the polymer-immobilized porous membrane became slower above the LCST, whereas that through the nonporous membrane became faster above the LCST. The permeation rate through the porous membrane was much higher than that through the nonporous membrane. The different temperature dependences of permeation can be explained as follows. In the case of the porous membrane, the graft chains expand below the LCST to close pores but contract at above the LCST to open pores. On the other hand, the nonporous membrane swells below the LCST to enhance diffusion of tryptophan and deswells above the LCST to reduce the diffusion. The present study demonstrated that the signal responsiveness of intelligent membrane can be controlled by the mode of device fabrication as well as by the nature of the sensoring unit.
Introduction Various types of signal-responsive membranes for regulated permeation have been devised.1 Signalresponsive hydrogels are typical examples. They contract or expand in response to environmental conditions such as pH, temperature, photoirradiation, the nature of solvent, or added chemicals.2-9 The hydrogel swell in the ionized state to facilitate permeation of solutes and deswell in the deionized state to suppress permeation. On the other hand, signal-responsive “polymer brushes”10-20 have been used to construct signal-respon* To whom correspondence may be addressed at NAIST. † Kyoto University. ‡ NAIST. § PRESTO. (1) Ito, Y. In Synthesis of Biocomposite Materials; Imanishi, Y., Ed.; CRC Press: Boca Raton, FL, 1992; p 137. (2) Irie, M. Adv. Polym. Sci. 1990, 94, 27. (3) Dong, L. C.; Hoffman, A. S. J. Controlled Release 1991, 15, 141. (4) Annaka, M.; Tanaka, T. Nature 1992, 355, 430. (5) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242. (6) Yu, H.; Grainger, D. W. Macromolecules 1994, 27, 4554. (7) Kinoshita, T.; Kakiuchi, T.; Takizawa, A.; Tsujita, Y.; Oya, M.; Iizuka, Y.; Iwatsuki, M. Macromolecules 1994, 27, 1389. (8) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi A.; Sakurai, Y.; Okano, T. Nature 1995, 374, 240. (9) Suzuki, A.; Yamazaki, M.; Kobiki, Y.; Suzuki, H. Macromolecules 1997, 30, 2350. (10) Milner, S. T. Science 1991, 251, 905. (11) Klein, J.; Perahia, D.; Warburg, S. Nature 1991, 352, 143. (12) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (13) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L. J. Nature 1994, 370, 634. (14) Sevick, E. M.; Williams, D. R. M. Macromolecules 1994, 27, 5285.
sive membrane devices such pH responsive,21,22 glucose responsive,23 photo responsive24 porous membranes with the “polymer brushes”. The porous membrane with polymer brush is advantageous over the hydrogel membrane for mechanical strength and quick response to external signal. A polymer-brush membrane has been designed and synthesized, in which pores open or close in response to pH change.25 The atomic force microscopy revealed the pH-dependent gating of a porous membrane with pH-sensitive polymer brush.26,27 Apparently, the signal responsiveness of permeation through the porous (15) Misra, S.; Mattice, W. L.; Napper, D. H. Macromolecules 1994, 27, 7090. (16) Bergbreiter, D. E.; Bandella, A. J. Am. Chem. Soc. 1995, 117, 10589. (17) Webber, R. M.; van der Linden, C. C.; Anderson, J. L. Langmuir 1996, 12, 1040. (18) Kent, M. S.; Factor, B. J.; Satija, S.; Gallagher, P.; Smith, G. S. Macromolecules 1996, 29, 2843. (19) Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (20) Yamamoto, K.; Shimada, S.; Tsujita, Y.; Sakaguchi, M. Macromolecules 1997, 30, 1776. (21) Ito, Y.; Kotera, S.; Inaba, M.; Kono, K.; Imanishi, Y. Polymer 1990, 31, 2157. (22) Ito, Y.; Inaba, M.; Chung, D. J.; Imanishi, Y. Macromolecules 1992, 25, 7313. (23) Ito, Y.; M. Casolaro, M.; Kono, K.; Imanishi, Y. J. Controlled Release 1989, 10, 195. (24) Chung, D. J.; Ito, Y.; Imanishi, Y. J. Appl. Polym. Sci. 1994, 51, 2027. (25) Israels, R.; Gersappe, D.; Fasolka, M.; Roberts, V. A.; Balazs, A. Macromolecules 1994, 27, 6679. (26) Ito, Y.; Park, Y. S.; Imanishi, Y. Macromol. Rapid Commun. 1997, 18, 221. (27) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739.
S0743-7463(97)00866-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
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Figure 2. Schematic presentation of surface-immobilization of azidophenyl-derivatized polymer on porous membrane.
Figure 1. Synthetic scheme of photoreactive polymers, PNIPAAm-Az and PNIPAAm/AA-Az.
membrane was opposite to that through the hydrogel membrane. Although these membrane devices are useful for controlled release systems,28 the different signal responsiveness has been discussed only with membranes which were prepared by different methods. In this study, we attempted to compare the polymer-brush membrane and the hydrogel membrane having a thermosensitive polymer, poly(N-isopropylacrylamide), immobilized. Different amounts of the polymer were immobilized on a porous membrane, and the permeation through the grafted membranes in response to temperature was investigated. Materials and Methods Preparation of Azidophenyl-Modified Polymers. Two types of azidophenyl-derivatized poly(N-isopropylacrylamide) were prepared by the method as shown in Figure 1. Poly(N-isopropylacrylamide) carrying a phenylazido group at the chain terminal (PNIPAAm-Az) was synthesized by the method as shown in Figure 1a. N-Isopropylacrylamide (NIPAAm, 1.0 g) was polymerized in the presence of 4,4′-azobis(4-cyanovaleric acid) (10 mg) in methanol (5.0 mL). The polymer was precipitated in diethyl ether, dried in vacuo, and was referred to as PNIPAAm. PNIPAAm (200 mg), 4-azidoaniline (250 mg), and 1-ethyl-3-(3(dimethylamino)propyl)carbodiimide hydrochloride (water-soluble carbodiimide, WSC, 2 mg/mL) were dissolved in 2-morpholinoethanesulfonic acid (0.1 mM, MES)-buffered solution (20 mL, pH 7.0) and the solution was stirred in the dark for 48 h at 4 °C. After the reaction, the solution was subjected to ultrafiltration, and the polymeric product was washed twice with diluted hydrochloric acid (pH 2.0) and twice with distilled water. Copolymer of NIPAAm and acrylic acid was treated with 4-azidoaniline to obtain azidophenyl-derivatized copolymer (PNIPAAm/AA-Az) by the procedure as shown in Figure 1b. A mixture of NIPAAm (1.01 g) and acrylic acid (34 mg) was polymerized in the presence of 4,4′-azobis(isobutyronitrile) (20 mg) in methanol (5.0 mL). The reaction product (PNIPAAm/ AA) was precipitated in diethyl ether and dried in vacuo. (28) Chun, S.-K.; Kim, J.-D. J. Controlled Release 1996, 38, 39.
PNIPAAm/AA (200 mg), 4-azidoaniline (250 mg), and WSC (2 mg/mL) were dissolved in 0.1 mM MES-buffered solution (20 mL, pH 7.0), and the solution was shaken in the dark for 48 h at 4 °C. After the reaction, the solution was subjected to ultrafiltration, and the polymeric product was washed twice with diluted hydrochloric acid (pH 2.0) and twice with distilled water. Gel-permeation chromatography was carried out using Cosmosil 5GPC-100 and 5GPC-300 gel (Nacalai Tesque, Kyoto, Japan). The average molecular weight was calculated using polystyrene as standard sample. The amount of carboxyl groups in the polymer was determined by using rhodamine 6G as reported previously.21,22 Surface-Immobilization of Azidophenyl-Derivatized Polymers. The preparative scheme is shown in Figure 2. An aqueous solution containing different amounts of azidophenylderivatized polymer was cast on a porous polycarbonate membrane (DuPont Nuclepore membrane; average pore diameter, 200 nm). The membrane having 2 mg of PNIPAAm immobilized was referred to as PNIPAAm-grafted porous membrane. The membranes having 2 or 200 mg of PNIPAAm/AA-Az immobilized were referred to as PNIPAAm/AA-2 or PNIPAAm-200, respectively. After air-drying at room temperature, the membrane was irradiated with UV light for 1 min and washed with distilled water until the 280-nm absorption due to the polymer released in the washing liquid became undetectable. Measurement of Water and Tryptophan Permeation. Water permeation through the polycarbonate membrane was investigated using an apparatus previously reported.21,22 The sample membrane was mounted on an ultrafiltration cell (Toyo Roshi UHP-25) which was placed 200 cm below a water reservoir. The reservoir was filled with a distilled water. After equilibration, the aqueous solution was allowed to flow under a constant hydraulic pressure. The rate of water permeation was calculated from the weight of water permeated in a minute. For tryptophan permeation through the polymer membrane, the sample membrane was interposed between two chambers. Three hundred milliliters of distilled water containing tryptophan (2.0 × 10-3 mol/mL) was added in one chamber, and distilled water (300 mL) was added to the other. Permeated solution containing tryptophan was taken out after given periods, and the absorbance at 278 nm was measured by a spectrophotometer. Measurement of Polymer Properties. The polymer was dissolved in distilled water (pH ) 5.0) to a concentration of 1.0 mg/mL. The turbidity change of polymer solution at various temperatures was monitored by the optical transmittance at 750 nm. The 750-nm transmittance was plotted against the solution temperature to determine the lower critical solution temperature (LCST). Determination of carboxyl group in polymer was carried out by the rhodamin method. Fourier transform attenuated total reflectance infrared (FT-ATR-IR) spectroscopy was measured using a KRS-5 prism with an incident angle of 45° (Perkin-Elmer infrared spectrometer).
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Figure 4. Scanning electron micrograph of PNIPAAm-immobilized porous membrane (PNIPAAm/AA-2). Figure 3. Transmittance at 750 nm of aqueous solution containing (b) PNIPAAm homopolymer, (O) PNIPAAm-Az, (2) PNIPAAm/AA copolymer and (4) PNIPAAm/AA-Az at several temperatures. The concentration of the solution was 5.0 mg/ mL.
Results Azidophenyl-Derivatized Polymers. The average molecular weight of PNIPAAm was found to be about 4000 by gel permeation chromatography. Determination by rhodamin 6G showed that 40% of the end groups were reacted with azidoaniline. The average molecular weight of PNIPAAm/AA was about 7000. An elemental analysis showed that the content of acrylic acid in the copolymer was 7 mol %, which was a little higher than the 5 mol % in the feed. Sixty percent of carboxylic acid was found to have been capped with azidophenyl group. When PNIPAAm-Az was cast on glass plate and photoirradiated, the product was soluble in water. The average molecular weight of polymer increased to about 11000, which could be due to formation of branched polymer produced by intermolecular cross-linking reaction on photoirradiation of cast polymer. On the other hand, when PNIPAAm/AA-Az was cast on glass plate and photoirradiated, no water-soluble polymers were found. The photoirradiation of PNIPAAm/AA-Az should lead to a network structure, because the copolymer contains four to five azidophenyl groups in one polymer chain. Thermal Sensitivity of Polymers in Aqueous Solution. Figure 3 shows the transmittance at 750 nm of aqueous solutions (pH 5.0) containing different types of polymers at various temperatures. The transmittance of the solutions sharply decreased on warming. The LCST of PNIPAAm-Az was 33.5 °C, which is almost the same as that of PNIPAAm. On the other hand, the LCST of PNIPAAm/AA was 35.5 °C, which is higher by a few degrees centigrade than that of PNIPAAm, and the LCST of azidophenyl-derivatized PNIPAAm/AA (PNIPAAm/AAAz) was 30.5 °C, which is lower by a few degrees centigrade than that of PNIPAAm. Chen and Hoffman29 reported that statistical copolymerization of NIPAAm with hydrophilic monomers such as acrylic acid resulted in rise of LCST. In addition, Yoshida et al.30 reported that (29) Chen, G.; Hoffman, A. S. Nature 1996, 373, 49. (30) Yoshida, R.; Sakai, K.; Okano, T.; Sakurai, Y. J. Biomater. Sci., Polym. Ed. 1994, 5, 585.
statistical copolymerization of NIPAAm with hydrophobic monomers such as methyl methacrylate resulted in lowered LCST. Copolymerization of NIPAAm and acrylic acid should be statistical, which could explain the deviations of LCSTs of PNIPAAm/AA and PNIPAAm/AA-Az from that of PNIPAAm. The LCST of the present PNIPAAm/AA was lower than that (38.5 °C) previously prepared by us.31 The determination of LCST in the present study at pH 5 instead of pH 7 in the previous study should have led to a lower value of LCST. Chen and Hoffman29 reported that the LCST of the copolymer significantly depended on the pH of the solutions. In addition, the LCST of PNIPAAm-Az in the present study was higher than that (21.5 °C) in the previous study. This difference should have arisen from the lower hydrophobicity of the present copolymer owing to the lower content (60 mol %) of azidophenyl groups than that (100 mol %) in the previous copolymer. Photoimmobilization of Azidophenyl-Derivatized Polymer on Porous Membrane. PNIPAAm-Az or PNIPAAm/AA-Az was cast on the porous membrane and photoirradiated. Release of PNIPAAm/AA-Az was not found in the washing liquid of the photoirradiated membrane. However, some PNIPAAm-Az was released in washing liquid of the photoimmobilized membrane. As mentioned before, photoirradiation of PNIPAAm/AA-Az coated on the membrane should produce network polymer, and photoirradiation of PNIPAAm-Az coated on the membrane should produce branched polymers. The different structures of polymers should explain the different solubilities of nonimmobilized polymers in the washing liquid. Figure 4 shows a scanning electron micrograph of a polymer-immobilized membrane. Pores are observed in the PNIPAAm-grafted membrane immobilized with 2 mg of PNIPAAm/AA-Az (PNIPAAm/AA-2), whereas pores were not observed in the membrane immobilized with 200 mg of PNIPAAm/AA-Az (PNIPAAm/AA-200) (photograph not shown). The FT-ATR-IR spectrum of PNIPAAm/AA-2 shows absorptions at 1650 and 1550 cm-1, which are ascribed to amide bond of PNIPAAm, as well as an absorption at 1750 cm-1 due to the ester group of the polycarbonate membrane (Figure 5). However, the spectrum of PNIPAAm/AA-200 showed only the absorptions ascribable to PNIPAAm, (31) Ito, Y.; Chen, G.; Guan, Y.; Imanishi, Y. Langmuir 1997, 13, 2756.
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Figure 5. FT-ATR-IR spectra of (A) polycarbonate membrane, (B) PNIPAAm/AA-2, and (C) PNIPAAm/AA-200, and (D) PNIPAAm-Az.
Figure 7. Rate of tryptophan permeation through (a) (O) PNIPAAm-grafted membrane, (2) PNIPAAm/AA-2, and (0) ungrafted porous membrane and through (b) PNIPAAm/AA200 at different temperatures. The concentration of tryptophan was 2 × 10-3 M (pH 5.0).
Figure 6. Rate of water permeation through (O) PNIPAAmgrafted membrane, (2) PNIPAAm/AA-2, (4) PNIPAAm/AA200, and (0) ungrafted porous membranes at several temperatures. The pH of permeating water was adjusted to 5.0.
indicating that the cross-linked polymer layer is too thick for the infrared beam to transmit into the bulk. Water Permeation through the Polymer-Immobilized Membrane. Figure 6 shows the water permeation through the polymer-immobilized membranes at various temperatures. Ungrafted porous membrane showed a high rate of water permeation independently of temperature. PNIPAAm/AA-200 did not permeate water at any temperatures. On the other hand, PNIPAAm-
grafted membrane and PNIPAAm/AA-2 showed a temperature-dependent water permeation, which became faster above LCST. Since PNIPAAm-Az and PNIPAAm/ AA-Az have different LCSTs, the rate of water permeation varied at different temperatures. Tryptophan Permeation through the PolymerImmobilized Membrane. Permeation of tryptophan through the membranes is shown in Figure 7. PNIPAAmgrafted membrane and PNIPAAm/AA-2 permeated tryptophan in a high rate, which slowed below LCST. On the other hand, PNIPAAm/AA-200 showed a very slow permeation of tryptophan, which increased below LCST. Apparently opposite temperature dependence of tryptophan permeation was found with the present membranes. Discussion A number of signal-responsive polymer membranes have been prepared, but the mechanism of signal re-
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sponsiveness has not been made perfectly clear.28,32 Although Iwata et al.33 and the present authors1,34 proposed a permeation mechanism, the explanations do not stand on the experimental results using the same material. The present study demonstrated that the different regulation mechanisms operate in permeation through membranes prepared using the same kind of polymer but by different fabrication methods. Figure 8 summarizes the experimental results and explains different mechanisms of permeation through polymerimmobilized membranes. In the porous PNIPAAmgrafted membrane and PNIPAAm/AA-2 membrane, a valve mechanism was operating. These membranes permeate water and tryptophan at high temperatures as a result of aggregation of surface-graft polymer brush or deswelling of surface-graft polymer network. However, the permeation becomes slow at low temperatures by extension of polymer brush or swelling of polymer network. On the other hand, a diffusion-in-gel mechanism operated with the nonporous (gel-packed) PNIPAAm/AA-200 membrane. Hydraulic permeation through the membrane was not observed at any temperatures. The diffusional permeation of tryptophan increased, through in a low rate, with lowering temperature as a result of swelling of gel, while it became very slow at high temperatures by deswelling of gel. Recently, Yoshida et al.35 reported the preparation of a thin film with cylindrical nanopores that open and close depending on temperature. They prepared the film by γ-irradiation grafting of thermoresponsive polymer onto track-etched membrane. This membrane should be equivalent to the present PNIPAAm/AA-2. The apparently opposite signal responsiveness in per(32) Okahata, Y.; Noguchi, H.; Seki, T. Macromolecules 1986, 19, 493. (33) Iwata, H.; Oodate, M.; Uyama, Y.; Amemiya, H.; Ikada, Y. J. Membr. Sci. 1991, 55, 119. (34) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 1619. (35) Yoshida, M.; Asano, M.; Safran, A.; Omichi, H.; Spohr, R.; Vetter, J.; Katakai, R. Macromolecules 1996, 29, 8987.
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Figure 8. Thermoresponsive permeation of water or tryptophan through polymer-immobilized membranes: (a) PNIPAAm-grafted membrane, (b) PNIPAAm/AA-2, and (c) PNIPAAm/AA-200.
meation experiments through intelligent membranes that has been reported could be explained in terms of different mechanisms of permeation due to differently fabricated devices. Fabrication of micro- or nanocomponents is necessary for construction of a micro- or nanomachine. The photoimmobilization method, including photolithography, is an important technique for fabrication of signalresponsive permeation membrane.31 The development of a precise method of fabrication will lead to the development of new intelligent devices soon. LA970866R
