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Photoresponsive Properties of Poly(N-isopropylacrylamide) Hydrogel Partly Modified with Spirobenzopyran Kimio Sumaru,* Katsuhide Ohi, Toshiyuki Takagi, Toshiyuki Kanamori, and Toshio Shinbo Research Center of AdVanced Bionics, National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed October 28, 2005. In Final Form: February 22, 2006 A photoresponsive hydrogel was prepared by radical copolymerization of N-isopropylacrylamide, a vinyl monomer having a spirobenzopyran residue and cross-linker. By the observation of photoresponsive shrinking and the conductance change, it was confirmed that the hydrogel in an acidic condition exhibited drastic and rapid volume shrinkage and proton dissociation when it was irradiated with blue light. Further, to examine its application to the mass transfer control, we prepared a photo- and thermoresponsive gate membrane by introducing this photoresponsive hydrogel to the surface of a porous membrane. As the first demonstration of the photocontrol of membrane permeation for liquid, it was observed that its permeability for 1 mM HCl aqueous solution increased by 2 times in response to the blue light irradiation, and this photoresponse of the permeability was confirmed to be repeatable.
Introduction The control of the mass transfer by outer stimuli, such as temperature,1-4 light,5-9 pH,10,11 or specific chemical species3,4 has been one of the most important challenges in the research field of artificial membranes. Among these studies, various kinds of photoresponsive membranes, which enable us to control the mass transfer by light, have been developed. As a pioneering study on such membranes, Shinkai et al. developed the crown ether functionalized by azobenzene, of which the inclusion property can be modulated by light.5,6 Using this compound as a K+ carrier, they composed a liquid membrane and proved that the ion transport through the membrane was effectively switched by light. Around the same time, Shimidzu et al. successfully realized the uphill transport of ions driven only by light irradiation utilizing a liquid membrane with an ion carrier functionalized with a spirobenzopyran.7 In addition to these studies, there have been several reports on similar ion transport based on photoresponsive carriers. In contrast, the photocontrol of permeation of substances other than ions has scarcely been reported until now.8,9 On the other hand, photoresponsive polymer materials have been studied actively by many research groups, and many polymers and polymer gels functionalized with azobenzene,12,13 * Corresponding author. E-mail:
[email protected]. (1) Reber, N.; Ku¨chel, A.; Spohr, R.; Wolf, A.; Yoshida, M. J. Membr. Sci. 2001, 193, 49. (2) Kim, S. Y.; Kanamori, T.; Shinbo, T. J. Appl. Polym. Sci. 2002, 84, 1168. (3) Yamaguchi, T.; Ito, T.; Sato, T.; Shinbo, T.; Nakao, S. J. Am. Chem. Soc. 1999, 121, 4078-4079. (4) Ito, T.; Hioki, T.; Yamaguchi, T.; Shinbo, T.; Nakao, S.; Kimura, S. J. Am. Chem. Soc. 2002, 124, 7840-7846. (5) Shinkai, S.; Nakaji. T.; Ogawa, T.; Shigematsu, K.; Manabe, O. J. Am. Chem. Soc. 1981, 103, 111-115. (6) Kumano, A.; Niwa, O.; Kajiyama, T.; Takayanagi, M.; Kano, K.; Shinkai, S. Chem. Lett. 1983, 1327-1330. (7) Shimidzu, T.; Yoshikawa, M. J. Membr. Sci. 1983, 13, 1-13. (8) Aoyama, M.; Watanabe, J.; Inoue, S. J. Am. Chem. Soc. 1990, 112, 55425545. (9) Kameda, M.; Sumaru, K.; Kanamori, T.; Shinbo, T. J. Appl. Polym. Sci. 2003, 88, 2068-2072. (10) Kono, K.; Kimura, S.; Imanishi, Y. J. Membr. Sci. 1991, 58, 1-9. (11) Turner, J. S.; Cheng, Y.-L. J. Membr. Sci. 1998, 148, 207-222. (12) Irie, M.; Kungwatchakun, D. Macromol. Chem. Rapid Commun. 1988, 9, 243-246. (13) Kro¨ger, R.; Menzel, H.; Hallensleben, M. L. Macromol. Chem. Phys. 1994, 195, 2291-2298.
leukochromophore,14,15 and spirobenzopyran16,17 have been examined. Among these studies, we newly developed a photoresponsive copolymer by introducing spirobenzopyran chromophores to side chains of poly(N-isopropylacrylamide) (pNIPAAm) and studied its properties in detail.18-20 In an acidic aqueous solution at a certain temperature, the copolymer was observed to dehydrate and precipitate only in a half minute responding to the visible light irradiation. Further, we revealed that most spirobenzopyrans at side chains of the copolymer, which had been fully protonated and positively charged in darkness, dissociated protons and isomerized into nonionic and hydrophobic formd in response to the light irradiation in the process of the photoinduced dehydration. In this study, we prepared a photoresponsive hydrogel by radical copolymerization of N-isopropylacrylamide, a vinyl monomer having a spirobenzopyran residue and cross-linker. Since the swelling volume of the hydrogel and proton dissociation from the chromophores were suggested to be photoresponsive, we monitored the response of its swollen state and specific conductance to light irradiation. Further, to examine its application to the mass transfer control, we prepared a photo- and thermoresponsive gate membrane (PTGM) by introducing this photoresponsive hydrogel to the surface of a porous membrane and investigated the influence of light irradiation and temperature change on its permeability for dilute HCl aqueous solution. Experimental Section Materials. N-Isopropylacrylamide (NIPAAm, Wako Chemical Industries, Ltd., Osaka, Japan) was used after purifying by recrystallization in a mixed solution of hexane and toluene (40/60 vol %) and dried under a vacuum. 1′,3′,3′-Trimethyl-6-hydroxyspiro (2H-1-benzopyran-2,2′-indoline) (Acros Organics, Geel, Belgium), acryloyl chloride (Wako), N,N-methylene-bis(acrylamide) (MBAAm, (14) Irie, M.; Kungwatchakun, D. Macromolecules 1986, 19, 2476-2480. (15) Irie, M.; Hosoda, M. Macromol. Chem. Rapid Commun. 1985, 6, 533536. (16) Irie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176-1180. (17) Menju, A.; Hayashi, K.; Irie, M. Macromolecules 1981, 14, 755-758. (18) Sumaru, K.; Kameda, M.; Kanamori, T.; Shinbo, T. Macromolecules 2004, 37, 4949. (19) Sumaru, K.; Kameda, M.; Kanamori, T.; Shinbo, T. Macromolecules 2004, 37, 7854. (20) Kameda, M.; Sumaru, K.; Kanamori, T.; Shinbo, T. Langmuir 2004, 20, 9315.
10.1021/la052899+ CCC: $33.50 © 2006 American Chemical Society Published on Web 03/24/2006
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Figure 1. Chemical structure of cross-linked pSPNIPAAm gel. Wako), 2,2′-azobisisobutyronitrile (AIBN, Wako), and dimethyl sulfoxide (DMSO, Wako) were used without further purification. Porous hydrophilized poly(tetrafluoroethylene) (HPTFE) membrane filter, with diameter of 25 mm, thickness of 150 µm, and pore size of 1.0 µm, was purchased from Millipore Co. Ltd (Bedford, MA) and used without further treatment. Synthesis of Photoresponsive Hydrogel. We synthesized the acrylated spirobenzopyran monomer by treating 1′,3′,3′-trimethyl6-hydroxyspiro(2H-1-benzopyran-2,2′-indoline) with acryloyl chloride. The cross-linked photoresponsive gel was synthesized in a DMSO solution by free-radical polymerization with AIBN as an initiator. The NIPAAm monomer (566 mg, 5.0 mmol), the acrylated spirobenzopyran monomer (17 mg, 0.05 mmol), MBAAm (8 mg, 0.05 mmol), and AIBN (49 mg, 0.30 mmol) were dissolved in DMSO (5.0 mL). The reaction mixture was degassed by subjecting it to freeze-thaw cycle three times and then was polymerized at 65 °C for 20 h. After the polymerization, resultant cross-linked gel of pNIPAAm partly modified with spirobenzopyran (pSPNIPAAm, Figure 1) was washed with a large amount of 1 mM HCl aqueous for many times to remove non-cross-linking copolymers, nonreacted monomers, and DMSO completely. Observation of Photoresponse of pSPNIPAAm Hydrogel. A volume change of the cross-linked pNIPAAm hydrogel in response to light irradiation was observed with a microscope system (IX-71, Olympus Corp., Tokyo, Japan), which was equipped with a CCD video camera (DXC-C33, Sony Corp., Tokyo, Japan). The observation was carried out at 28 °C for a small fragment of the hydrogel in a 1 mM HCl aqueous solution. Conductometric measurement was carried out by using a conductance cell with Pt electrodes (cell constant: 0.95 cm) placed in the hydrogel including 0.5 mM HCl aqueous solution at 25 °C. In both experiments, the sample was irradiated with blue light in a wavelength range from 400 to 440 nm after the systems were equilibrated. For the light irradiation, the light from an Hg-Xe light source (LC6, Hamamatsu Photonics K. K. Hamamatsu, Japan) was guided to the samples through a set of filters and a liquid-fiber light guide. Preparation of Photo- and Thermoresponsive Gate Membrane (PTGM). The NIPAAm monomer (566 mg, 5.0 mmol), SP (17 mg, 0.05 mmol), MBAAm (8 mg, 0.05 mmol), and AIBN (49 mg, 0.30 mmol) were dissolved in DMSO (5.0 mL). After this mixture was shaken for 5 min at ambient temperature, HPTFE membrane was sunk into the solution, and the reaction mixture was degassed by subjecting it to freeze-thaw cycle three times. After being heated for 20 h at 65 °C, the resultant mixture was washed with water to remove non-cross-linking copolymers, nonreacted monomers, and DMSO; excess pNIPAAm hydrogel adhering to the surface of the porous HPTFE membrane was scraped off carefully to obtain the PTGM (Figure 2). Permeability Measurement of PTGM. The experimental set up for permeability measurement of PTGM is illustrated schematically in Figure 2. The PTGM was set in a permeation cell equipped with a quartz window allowing occasional light irradiation onto the membrane during the measurement. The permeation cell was connected with a vertical capillary through the intermediary of a
Figure 2. Schematic illustrations indicating the structure of PTGM and the experimental set up for the permeability measurement. reservoir containing 1 mM HCl aqueous solution. The permeation cell and the reservoir were set in a thermostatic bath to regulate the temperature of the system. By measuring the time (t) and the movement of a meniscus of the HCl solution from a certain height (h1) to another certain height (h2), we estimated the permeability (P) of the PTGM corresponding to the following equation: P)
r ln(h1/h2) Fg(t - t0)
where r, F, g and t0, are the ratio of the capillary cross section to the effective area of the PTGM in the permeation cell, the density of the HCl solution, the acceleration of gravity, and the blank value of t obtained in the condition without the PTGM, respectively.
Results and Discussions Photoresponsibility of pSPNIPAAm Hydrogel. The chemical structure of pSPNIPAAm hydrogel and the photoisomerization of the spirobenzopyran residue are illustrated in Figure 1. In an acidic aqueous system in the dark, most of the spirobenzopyrans functionalizing the hydrogel is in a protonated open-ring form. When the hydrogel is irradiated by blue light , however, they are isomerized immediately to a free closed-ring form, dissociating protons and losing positive charges.20 After the light is turned off, the chromophore returns spontaneously to the protonated open-ring form, which is more stable than the free closed-ring form in the dark. In our previous studies,19 it was confirmed that this photoisomerization affects greatly the hydration of pSPNIPAAm in a certain temperature range at around 30 °C. Figure 3 shows the deformation process of the cross-linked pSPNIPAAm hydrogel at 28 °C in 1 mM HCl under the blue light irradiation. Before the irradiation, the hydrogel was yellowish due to the chromophores in a protonated open-ring form. When the hydrogel was irradiated with the blue light, it started to shrink immediately, and the significant deformation took place in the first 9 s. After the irradiation for 24 s in total, the hydogel was decolorized indicating that most chromophores were isomerized to the closed-ring form and dissociated protons. Due to the drastic change in the property of the chromophores, which was brought about by the light irradiation, the dehydration of the hydrogel was induced as reported in terms of pSPNIPAAm aqueous solution,8 and the volume of the gel decreased to about 1/5 of that of the initial state.
pNIPAAm Hydrogel Functionalized with Spirobenzopyran
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Figure 4. Change in the specific conductance of a cross-linked pSPNIPAAm hydrogel including 0.5 mM HCl aqueous solution at 25 °C in response to blue light irradiation.
Figure 5. Temperature-dependent permeability of the PTGM for 1 mM HCl aqueous solution in the dark (closed circles) and under blue light irradiation (open circles).
Figure 3. Photoresponsive deformation of a cross-linked pSPNIPAAm hydrogel in 1 mM HCl at 28 °C: (a) before irradiation, (b) after irradiation for 9 s, and (c) after irradiation for 24 s.
Figure 4 shows the change in the specific conductance of a cross-linked pSPNIPAAm hydrogel including 0.5 mM HCl aqueous solution in response to blue light irradiation at 25 °C. The specific conductance, which had been about 0.65 mS/cm before the light irradiation, increased after the irradiation by 20% suggesting, the increase of free protons brought about by the efficient proton dissociation from the hydrogel. After the light was turned off, on the other hand, the specific conductance approached gradually to the former value before the irradiation due to the spontaneous isomerization of the chromophore from
the free closed-ring form to the protonated open-ring form as described above. Photoresponsive Permeability of PTGM. Figure 5 shows the temperature dependence of the permeability of PTGM, which was composed of cross-linked pSPNIPAAm, for 1 mM HCl aqueous solution. Whether it was irradiated or not, the permeability increased drastically as the temperature increased since the pNIPAAm main chain was basically thermoresponsive having a transition temperature at around 30 °C. However, the permeability under irradiated condition was always larger than that measured in the dark, and the difference was maximized at around 30 °C. Figure 6 shows the change in the permeability of the PTGM monitored at 30 °C as irradiated occasionally with the blue light. As expected from the experimental result shown in Figure 5, it was observed that its permeability for 1 mM HCl aqueous solution increased by twice in response to the blue light irradiation. This result is the direct evidence indicating that the photoresponsive shrinking of the hydrogel was reflected in the membrane permeability for HCl aqueous solution. After the light was turned off, the permeability decreased gradually back to the former state through the previously mentioned mechanism, and this photoresponse of the permeability was confirmed to be repeatable. Although this tendency was similar to that observed in the conductometric measurement shown in Figure 4, the time scale of the recovery from the photostimulated state was much longer.
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the permeability measurement at 30 °C, is considered to surround and stabilize the free closed-ring form of the chromophore.
Conclusion
Figure 6. Change in permeability of the PTGM for 1 mM HCl aqueous solution at 30 °C in response to occasional blue light irradiation.
In terms of this point, the dehydration of the polymer main chain was suggested to slow drastically the isomerization of the chromophore;19,20 in contrast to the condition in which the conductometric measurement was carried out at 25 °C and the polymer chain remained thoroughly to be hydrated stably, the polymer chain, which was dehydrated after light irradiation in
We synthesized a photoresponsive hydrogel composed of pNIPAAm partly modified with spirobenzopyran and observed that the hydrogel in acidic conditions exhibited drastic and rapid volume shrinkage and proton dissociation when it was irradiated with blue light. Further, we prepared a photo- and thermoresponsive gate membrane by introducing this photoresponsive hydrogel to the surface of a porous membrane and demonstrated the reversible photocontrol of permeation for liquid for the first time. Since the light can be irradiated in a precision of micrometerscale, this hydrogel is considered to be a feasible material out of which to construct photoresponsive devices to control the mass transfer or ionic conditions in microfluidic systems. Acknowledgment. A part of this work was financially supported by Industrial Technology Research Grant Program in 2005 from the New Energy Development Organization (NEDO) of Japan. LA052899+
