Langmuir 2004, 20, 9315-9319
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Probing the Dielectric Environment Surrounding Poly(N-isopropylacrylamide) in Aqueous Solution with Covalently Attached Spirobenzopyran Mitsuyoshi Kameda, Kimio Sumaru,* Toshiyuki Kanamori, and Toshio Shinbo National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received February 10, 2004. In Final Form: July 1, 2004 The dielectric environment surrounding poly(N-isopropylacrylamide) in aqueous solution was investigated by probing with spirobenzopyran covalently attached as a side chain to the polymer main chain. Inherent characteristics of the spirobenzopyran chromophore were analyzed, and the chromophore was confirmed to be suitable to probe the local polar condition around the polymer. Measurements for an aqueous polymer solution at various temperatures elucidated that the dielectric environment surrounding the polymer changed continuously even in the temperature range far below the lower critical solution temperature. This result suggested that the local and weak orientation of water molecules around the polymer diminished continuously in a preliminary stage of shifting to thermally induced phase separation. The dielectric environment surrounding thermoresponsive polymer in aqueous solution was investigated by probing with spirobenzopyran covalently attached as a side chain to the polymer main chain.
Introduction Various solutions and gels of thermoresponsive polymers are being studied actively.1-12 In particular, aqueous solutions of poly(N-isopropylacrylamide) (pNIPAAm) are known to exhibit a distinctive phase transition at a critical temperature (the lower critical solution temperature, LCST). pNIPAAm is not soluble in water at temperatures higher than 32 °C, which is the LCST of the aqueous pNIPAAm solution, while it is soluble at lower temperatures. Interest in this property is increasing, and many experimental and theoretical works have been done to reveal the detailed mechanism of the phase transition.1,2 Many researchers have investigated the effects of incorporating various kinds of comonomer units into pNIPAAm,3-8 and linear copolymers and hydogels of N-isopropylacrylamide (NIPAAm) have already been used in numerous practical applications.9-12 As a pioneer study on probing polymer solutions with stimuli-responsive chromophores, Morishima et al. investigated the characteristics of polyelectrolytes by measuring the UV-visible absorption spectra of a pH-sensitive chromophore that was covalently attached as a side chain * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Ilman, F.; Tanaka, T.; Kokufuta, E. Nature 1991, 349, 400. (2) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T. Nature 1995, 374, 240. (3) Topp, M. D. C.; Dijkstra, P. J.; Feijen, J. Macromolecules 1997, 30, 8518. (4) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345. (5) Hu, T.; You, Y.; Pan, C.; Wu, C. J. Phys. Chem. B 2002, 106, 6659. (6) Maeda, Y.; Yamamoto, H.; Ikeda, I. Langmuir 2001, 17, 6855. (7) Charalambopoulou, A.; Bokias, G.; Staikos, G. Polymer 2002, 43, 2637. (8) Kang, M. S.; Gupta, V. K. J. Phys. Chem. B 2002, 106, 4127. (9) Ito, T.; Hioki, T.; Yamaguchi, T.; Shinbo, T.; Nakao, S.; Kimura, S. J. Am. Chem. Soc. 2002, 124, 7840. (10) Ohya, S.; Nakayama, Y.; Matsuda, T. J. Artif. Organs 2001, 4, 308. (11) Cammas, S.; Suzuki, K.; Sone, C.; Sakurai, Y.; Kataoka, K.; Okano, T. J. Controlled Release 1997, 48, 157. (12) Kanazawa, H.; Sunamoto, T.; Ayano, E.; Matsushima, Y.; Kikuchi, A.; Okano, T. Anal. Sci. 2002, 18, 45.
to the polymer main chain.13 They estimated the electrostatic potential at the polymer surface from the pK shifts of the pH-sensitive chromophore and analyzed the dielectric environment surrounding the polymers based on the wavelength absorption maxima of the chromophores. Simultaneously, they proved in that study that the detailed condition near the polymer main chain can be investigated by using covalently attached chromophores, which have more than two different colored states. Also most organic photochromic molecules have two different stable states with different colors, and some of them are expected to be useful in probing the local space of the nanometer scale as mentioned previously. Above all, spirobenzopyrans have attracted considerable attention owing to their potential use in various optical applications.14-18 They have at least two isomerization states, whose physical properties are remarkably different: the zwitterionic merocyanine (MC) state and nonionic spiropyran (SP) state. MC has a strong absorption band in the visible light region due to its extensive π-electron conjugation, while SP is basically colorless. With respect to several derivatives of spirobenzopyran whose activation energies of isomerization are relatively low, isomerization between MC and SP is induced not only by the light irradiation but also by the solvent effect; the zwitterionic MC state is stabilized in protic solvents, while the nonionic SP state is predominant in less polar solvents. The thermal and photoinduced isomerization behaviors of the spirobenzopyrans in various solvents have been studied using NMR techniques and UV-visible spectroscopy.19-23 (13) Morishima, Y.; Kobayashi, T.; Nozakura, S. Macromolecules 1988, 21, 101. (14) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 1741. (15) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421. (16) Anzai, J.; Sakamura, K.; Hasebe, Y.; Osa, T. Anal. Chim. Acta 1993, 281, 543. (17) Rosario, R.; Gust, D.; Hayes, M.; Jahnke, F.; Springer, J.; Garcia, A. A. Langmuir 2002, 18, 8062. (18) Fissi, A.; Pieroni, O.; Angelini, N.; Lenci, F. Macromolecules 1999, 32, 7116. (19) Anzai, J.; Osa, T. Tetrahedron 1994, 50, 4039.
10.1021/la049649y CCC: $27.50 © 2004 American Chemical Society Published on Web 09/16/2004
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Scheme 1. Isomerization among SP, MC, and Protonated MC States of Spirobenzopyran
In the present work, we introduced spirobenzopyran residues to pNIPAAm as a side chain to probe the characteristics of thermoresponsive polymer in dilute aqueous solutions. Because the SP and MC states have remarkably different chemical structures and are quite different from each other in electronic characteristics and solvent affinities, this chromophore is expected to be sensitive to the change in the dielectric environment around thermoresponsive polymers. First, we investigated the inherent characteristics of spirobenzopyran in the various dielectric environments and at various temperatures utilizing its monomeric ester. Then we measured the UV-visible absorption spectra of an aqueous solution of pNIPAAm partially modified with the spirobenzopyran chromophore at various temperatures and investigated the dielectric environment surrounding the polymer main chain in a preliminary stage of shifting to thermally induced phase separation. Experimental Section Materials. NIPAAm (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was purified by recrystallization from toluenehexane followed by drying under a vacuum and was stored at -20 °C. Tetrahydrofuran (THF, Wako) was dried on sodium under reflux in the presence of a small amount of benzophenone until a blue color persisted, and it was used immediately after distillation. 1′,3′,3′-Trimethyl-6-hydroxyspiro(2H-1-benzopyran2,2′-indoline) (Acros Organics, Geel, Belgium), acryloyl chloride (Wako), acetyl anhydride (Wako), azobisisobutyronitrile (AIBN, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and triethylamine (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used without further purification. For measurements of UV-visible absorption spectra, Milli-Q reagent grade water (Millipore, Bedford, MA) was used. Synthesis of the Monomeric Spirobenzopyran Ester. For estimating the inherent characteristics of the spirobenzopyran chromophore, the acetylated spirobenzopyran monomer was synthesized as a monomeric ester. A solution of 252 mg (0.86 mmol) of 1′,3′,3′-trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,2′indoline) and 0.14 mL (1 mmol) of triethylamine in dry THF (5 mL) was stirred at 0 °C. A total of 0.1 mL (1 mmol) of acetyl anhydride was added to the mixture, and then the mixture was stirred at 25 °C for 15 h. After the solvent was removed in vacuo, ethyl acetate and saturated aqueous sodium hydrogen carbonate were added and the aqueous phase was extracted with ethyl acetate. The combined organic phase was washed with brine, (20) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2003, 68, 4158. (21) Hobley, J.; Malatesta, V. Phys. Chem. Chem. Phys. 2000, 2, 57. (22) Go¨rner, H. Phys. Chem. Chem. Phys. 2001, 3, 416. (23) Drummond, C. J.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1990, 86, 3613.
dried over anhydrous magnesium sulfate, and filtered. After evaporation of the solvent, the residue was purified by silica gel column chromatography (10% ethyl acetate/n-hexane as the eluent) to give the acetylated spirobenzopyran monomer (121 mg, 0.36 mmol, 42%). Synthesis of the p(SP-NIPAAm). We synthesized the acrylated spirobenzopyran monomer by treating 1′,3′,3′-trimethyl-6-hydroxyspiro(2H-1-benzopyran-2,2′-indoline) with acryloyl chloride. The practical procedure of the synthesis is similar to the method to synthesize monomeric spirobenzopyran ester as described in the previous subsection. The yield of the acryl ester was 71%. The p(SP-NIPAAm) copolymer was synthesized in a dry THF solution by free-radical polymerization with AIBN as an initiator. The NIPAAm monomer (2.24 g, 19.8 mmol), acrylated spirobenzopyran monomer (69.5 mg, 0.2 mmol), and AIBN (32.8 mg, 0.2 mmol) were dissolved in dry THF (10 mL). After the mixture was deaerated by a freezing and thawing method, the solution was polymerized at 60 °C for 12 h. The resultant solution was poured into diethyl ether, and the precipitate was collected and dried under a vacuum to give p(SP-NIPAAm) (2.28 g; monomer inversion rate, 97%) as a white powder. The molecular weight was determined by gel permeation chromatography (column, Shodex KF-805) at 25 °C with THF as the eluent. The number-average molecular weight, Mn, estimated from polystyrene standards was 7800. NMR and UV-visible absorbance measurement revealed that the molar fraction of the spirobenzopyran units in the synthesized p(SP-NIPAAm) was 1.1 mol %, which agreed with the molar ratio of the initial monomers in the solution. Measurements of UV-Visible Absorption Spectra for the Characterization of the Spirobenzopyran Chromophore in Aqueous Solution. We measured UV-visible absorption spectra of the acetylated spirobenzopyran monomer (0.2 mM) in acetonitrile (MeCN)/water solutions to evaluate the inherent characteristics of the spirobenzopyran chromophore. The isomerization and protonation of spirobenzopyran are summarized in Scheme 1. Because the spirobenzopyran was partly protonated in the solution due to an influence of CO2 from the air, a small amount of NaOH (0.03 mM in concentration) was added to the sample solution just before the measurement. In investigating the influence of dielectric environment surrounding the chromophore, the measurement was carried out at 25 °C for various MeCN fractions (20-80 wt %). Also the temperature dependence of the absorption spectra was measured in the temperature range from 0 to 40 °C at a 30 wt % MeCN fraction. Measurements of UV-Visible Absorption Spectra for the Characterization of the Aqueous p(SP-NIPAAm) Solution. We measured UV-visible absorption spectra of an aqueous 0.2 wt % solution of p(SP-NIPAAm) at several proton concentrations and various temperatures. We found in the preliminary experiments that all of the chromophore of p(SP-NIPAAm) was converted to the protonated MC under the proton concentration above 1 mM, and absorption spectra were hardly influenced by the change in temperature in the range from 0 to 25 °C. Therefore,
Probing pNIPAAm with Spirobenzopyran
Figure 1. Solvent effect on the UV-visible absorption spectra of the acetylated spirobenzopyran monomer solution at constant temperature (25 °C). MeCN fractions in MeCN/water solvent are 20, 30, 40, 60, and 80 wt %.
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Figure 2. Maximum absorbance values (Amax, squares) and the wavelength absorption maxima (λmax, circles) of the acetylated spirobenzopyran monomer solution plotted against the relative dielectric constant () of the mixed solvents.
by measuring the UV-visible spectra of the p(SP-NIPAAm) solution just after the addition of an excess amount of NaOH to the acidic solution at 0 °C (MC isomerized to SP gradually after addition of NaOH), we obtained the absorption spectrum of the p(SP-NIPAAm) solution in which all the chromophores in solution were in the MC state. For the measurement to investigate the temperature dependence, a small amount of NaOH (0.3 mM in concentration) was added to the sample solution just before the measurement to eliminate an influence of CO2 from the air as described in the former subsection. To monitor the thermoresponsive characteristics of the polymer solution at equilibrated conditions, the measurement was carried out raising the temperature of the sample solution stepwise at the rate of 1 °C/15 min typically.
Results and Discussion Inherent Characteristics of the Spirobenzopyran Chromophore in Aqueous Solution. Figure 1 shows the solvent effect on the UV-visible absorption spectra of the acetylated spirobenzopyran monomer solution at a constant temperature (25 °C). The spectra of acetylated spirobenzopyran monomer were greatly dependent on the surrounding dielectric environment. Specifically, the absorbance value around 550 nm attributed to the MC state decreased drastically as the MeCN fraction in solvent increased. The maximum absorbance values (Amax) and the wavelength absorption maxima (λmax) of the solutions are plotted against the relative dielectric constant () of the mixed solvents in Figure 2. The value of Amax was fairly reduced in the solvent with the small less than 60 indicating that the nonionic SP state was predominant in the less polar condition,22 while the color of the solution was relatively deep at ) 71. On the other hand, the value of λmax decreased slightly with increasing . A similar tendency of λmax was observed experimentally also for 6-nitrospirobenzopyran in the previous studies.22,23 With respect to this shift in λmax, Drummond and Furlong suggested that the decrease in polarity of the solvent reduces the energy difference between the ground state and the exited state; the ground state based on the resonance between the zwitterionic structure and the quinoid structure shifts toward the nonpolar quinoid structure (Scheme 1).23 These experimental results obtained here for acetylated spirobenzopyran monomer showed that this chromophore is very sensitive to the polarity of the solvating medium and is expected to be suitable for a probe to detect the change in the dielectric environment surrounding thermosensitive polymers. UV-visible absorption spectra of the acetylated spirobenzopyran monomer in a MeCN/water solution at various
Figure 3. UV-visible absorption spectra of the acetylated spirobenzopyran monomer in MeCN/water solution (MeCN fraction in solvent: 30 wt %) at various temperatures.
temperatures are shown in Figure 3. The MeCN fraction in solvent was set to be 30 wt % because the monomer was not soluble at 0 °C in the solvent with the smaller MeCN fraction. As the temperature increased, the absorbance value around 550 nm increased, while that around 430 nm decreased. In this figure, an isosbestic point, which was not found in the change in the dielectric environment (Figure 1), was observed at 490 nm. In the preliminary experiments, we found that the protonated MC had an absorption band around 430 nm, and the isosbestic point existed at around 490 nm in the protonation equilibrium of MC. Considering that the proton activity and pKa of MC might be varied in the temperature change, the spectra change in Figure 3 was suggested to reflect the shift in the equilibrium of protonation/deprotonation of MC. The values of Amax and λmax are plotted against the temperature of the solution in Figure 4. λmax increased slightly with increasing temperature. Because decreases as temperature increases, we consider that λmax reflected this change in the dielectric condition. To the contrary, Amax decreased slightly with increasing temperature as mentioned above. This observation indicated that the increase of Amax brought about by the change in the polarity of the solvent and that brought about by the temperature change are different in nature. Thermoresponsive Characteristics of the Aqueous p(SP-NIPAAm) Solution. In Figure 5, we show the absorption spectra of the aqueous p(SP-NIPAAm) solution at 0 °C. Just after the addition of excess NaOH to an
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Figure 4. Maximum absorbance values (Amax, squares) and the wavelength absorption maxima (λmax, circles) of the acetylated spirobenzopyran monomer solution plotted against the temperature.
Figure 5. UV-visible absorption spectra of the aqueous 0.2 wt % p(SP-NIPAAm) solution just after the addition of excess NaOH to an acidic solution (solid line) and after 20 h (dashed line).
acidic solution (solid line), most chromophores in the solution were in the MC state, and a large absorption band, which is attributed to the MC state, was observed around 530 nm. As the time proceeded after the addition of NaOH, the absorbance decreased gradually and was reduced to be 12% of the initial value after 20 h (dashed line), indicating the spontaneous isomerization from MC to SP. We confirmed that SP can be isomerized gradually but efficiently to protonated MC again by adding an excess amount of HCl to the solution, and this cycle is repeatable. Figure 6 shows the temperature dependence of UVvisible absorption/turbidity spectra of aqueous p(SPNIPAAm) solution. In the measurement, it was confirmed that the system reached equilibrium at each temperature. As the temperature increased from 0 to 20 °C, the absorbance value around 530 nm decreased continuously while the solution was consistently clear. With increasing the temperature further, the solution started to become turbid at 28 °C, while the LCST of the aqueous solution of NIPAAm homopolymer has been reported to be 32 °C. With respect to this point, Irie and Kungwatchakun reported that small amount of hydrophobic chromophores introduced to pNIPAAm as side chains decreases the LCST of the aqueous polymer solution drastically.24 However, the molar fraction of the spirobenzopyran units in the synthesized p(SP-NIPAAm) was minimized (about one (24) Irie, M.; Kungwatchakun, D. Macromolecules 1986, 19, 2476.
Kameda et al.
Figure 6. Temperature dependence of UV-visible absorption/ turbidity spectra of the aqueous 0.2 wt % p(SP-NIPAAm) solution.
Figure 7. Maximum absorbance values (Amax, squares) and the wavelength absorption maxima (λmax, circles) of the aqueous 0.2 wt % p(SP-NIPAAm) solution plotted against the temperature.
chromophore per polymer chain on an average) in this study, and the influence of the polymer modification on the LCST was limited to several degrees Celsius. At 35 °C, the solution was completely turbid, and the absorption peak at around 530 nm disappeared. The values of Amax and λmax are plotted against the temperature of the solution in Figure 7. While Amax of the acetylated spirobenzopyran monomer in MeCN/water increased slightly as the temperature increased, indicating that the protonation equilibrium of MC shifted in the direction of protonation (Figure 4), efficient ring-closing isomerization with increasing temperature was observed for spirobenzopyran covalently attached as a side chain to the thermoresponsive polymer; an isosbestic point as observed at 490 nm in Figure 4 was not found in Figure 6. Therefore, we suggest that the drastic decrease of Amax with increasing temperature observed for the aqueous p(SP-NIPAAm) solution was not due to the inherent characteristics of the chromophore but due to the change in the dielectric environment around the chromophore. This continuous change in Amax was observed even in the temperature range far below the LCST of the system (28 °C). Several experimental results of the measurements of IR spectra elucidated that the hydrogen bonding between the subunit of pNIPAAm and water changes drastically only around the LCST, and no significant change has been detected in the other temperature range.25 However, the experimental results shown in Figures 6 and 7 indicated that the (25) Maeda, Y.; Nakayama, T.; Ikeda, I. Macromolecules 2001, 34, 8246.
Probing pNIPAAm with Spirobenzopyran
dielectric environment surrounding the polymer changed continuously even in the temperature range far below the LCST. This observation suggested that the local orientation of water molecules around the polymer based on the very weak dipole-dipole interaction (much weaker than hydrogen bonding) diminished gradually in a preliminary stage of shifting to thermally induced phase separation. Conclusions We investigated the hydration change of thermoresponsive polymer in aqueous solution by probing with spirobenzopyran covalently attached as a side chain to the polymer main chain. As a result of the experiments to specify the inherent characteristics of the chromophore, it was confirmed that the isomerization of the chromophore is considerably sensitive to the surrounding dielectric environment and is feasibly detectable by measuring UVvisible absorption spectra. Measurements for an aqueous polymer solution at various temperatures elucidated that
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the dielectric environment surrounding the thermoresponsive polymer changed continuously even in the temperature range far below the LCST. This result suggested that the local and weak orientation of water molecules around the polymer diminished gradually in a preliminary stage of shifting to thermally induced phase separation. These observations obtained in this study were expected to provide an important clue to reveal the detailed mechanism of the phase transition of thermoresponsive polymers in solution. Acknowledgment. This work was supported by Industrial Technology Research Grant Program in 2002 from the New Energy Development Organization (NEDO) of Japan, to which our sincere gratitude is due. We thank Dr. Toshiyuki Takagi and Mr. Tomonori Endo (AIST) for their helpful discussion about the synthesis of the materials. LA049649Y
