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J. Phys. Chem. 1996, 100, 16282-16284
Saccharide-Induced Volume Phase Transition of Poly(N-isopropylacrylamide) Gels Hideya Kawasaki, Shigeo Sasaki, and Hiroshi Maeda* Department of Chemistry, Faculty of Science, Kyushu UniVersity 33, Hakozaki, Higashi-ku, Fukuoka, 812, Japan
Satoshi Mihara, Masayuki Tokita, and Takashi Komai Department of Chemistry for Materials, Faculty of Engineering, Mie UniVersity, Tsu, 514, Japan ReceiVed: April 30, 1996X
Saccharide-induced volume phase transition of poly(N-isopropylacrylamide) (NIPA) gels was found for glucose, galactose, and sucrose, and the transition concentrations were about 1.5, 1.3, and 1.2 M, respectively. Temperature-induced volume phase transition of NIPA gels was affected by these saccharide, and the transition temperature decreased with increasing concentration of the saccharides. The reduced extent of the transition temperature increased linearly with the decrement of the chemical potential of water molecules due to the saccharides. This indicates that the change of the chemical potential of water induced by the addition of the saccharide plays an essential role in the saccharide-induced volume phase transition of NIPA gels.
Introduction Poly(N-isopropylacrylamide) (NIPA) gels have been known to exhibit a temperature-induced volume phase transition.1 This hydrogel changes its volume discontinuously between the swollen state and the collapsed state at about 34 °C. It has been suggested that the hydrophobic interaction is essential to the volume phase transition of NIPA gels.1,2 The temperatureinduced volume phase transition of the gel is closely related to the phase-separation phenomena for the solution of poly(Nisopropylacrylamide), which shows a lower critical solution temperature (LCST).3 Some additives have been known to affect the behavior of the temperature-induced phase transition of NIPA gels. The effect of various inorganic salts on the temperature-induced volume phase transition has been reported by Inomata et al.4 According to this study, addition of the salts reduces the transition temperature of the gels, and the decrement of the transition temperature increases linearly with the viscosity B coefficient of anions of the salts. The salt-induced volume phase transition of NIPA gels has been further investigated by Suzuki 5 and by Park et al.6 The effect of salts on the volume phase transition behavior of NIPA gels was explained by the reduction of the amount of the structured water around the hydrophobic moieties of NIPA gels with increasing salt concentrations.4-6 The mechanism of the effects of the salts on the structured water around the hydrophobic moieties (hydrophobic hydration), however, is not fully clarified. The purpose of the present investigation is to clarify the mechanism of the additives-induced volume phase transition of NIPA gels in thermodynamic terms. In this paper, the saccharide effect on both the swelling ratios and the phase transition temperature of NIPA gels was studied experimentally for glucose, galactose, and sucrose. Addition of the saccharide to NIPA gels is expected to affect the volume change behavior of the gels as observed in the salt-addition case, since the saccharide molecules are also considered to disturb the hydrophobic hydration of the gels. To examine how the interaction between water and the saccharides molecules affects the swelling behavior of the gel, the relation of the swelling X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01219-1 CCC: $12.00
behavior of the gels with the thermodynamic activities of water molecules in aqueous saccharides solutions was investigated. The activity of water molecules is a measure of the interaction between water and saccharide molecules. Saccharide binding to NIPA gel was also investigated to examine the possible direct interaction between the polymer networks and the saccharides, which may affect the volume change behavior of the gel. Experimental Section Samples. N-isopropylacrylamide (NIPA) gels were prepared by free radical copolymerization in the aqueous solutions of N-isopropylacrylamide (700 mM) and N,N′-methylenebis(acrylamide) (7.0 mM). Polymerization was initiated by ammonium persulfate (APS), accelerated by N′,N′,N′,N′-tetramethylethylendiamine (TEMED) and carried out at 5 °C for 24 h. The gel synthesized in a capillary (0.141 mm diameter) was cut into a rod form (20 mm length), was rinsed thoroughly with distilled water, and was dried gently. The dried gel fixed in a sample holder was immersed into the solvent. For the saccharide uptake experiment, the gel was synthesized in a plate form (1 mm thick) in the same manner as the preparation of the rods. Monosaccharides, D-(+)-glucose, D-(+)-galactose and disaccharide, sucrose, were of the reagent grade. Swelling Ratio. The gel volume V was estimated from the diameter d measured by using an optical microscope. Detail of the apparatus was described elsewhere.7 The swelling ratio was defined as V/V0 ) (d/d0)3 where d0 was a diameter of the capillary in which the gel was synthesized. Saccharide aqueous solution was supplied from a reservoir by using a peristaltic pump at the rate of about 0.5 mL/min. The concentration of the saccharide was altered by adding an appropriate amount of the saccharide or water to the reservoir. The concentration of the saccharide in the solution was evaluated from the refractive index of the solution. The temperature was controlled within (0.05 °C in the gel-swelling experiment. Activities of Water Molecules in the Solutions. Water activities in the saccharide aqueous solutions at various concentrations were evaluated from the osmolality of the solution at 37 °C, measured with a vapor pressure osmometer (VPO 5500, WESCOR). The relation between the osmolality and © 1996 American Chemical Society
Volume Phase Transition of NIPA Gels
J. Phys. Chem., Vol. 100, No. 40, 1996 16283
Figure 1. Swelling ratios of NIPA gels at 20 °C as a function of the concentrations of the various saccharides. The type of the saccharide is shown in the figure.
water activity, aw, is given by7
aw ) exp(-V h ‚osmolality)
(1)
where V h ()0.018 [L mol-1]) is the partial molar volume of water. Partition of Saccharide Molecules Inside and Outside the Gel. NIPA gels in the plate form were immersed in the saccharide aqueous solutions of various concentrations. It took about 1 week to reach equilibrium. The concentration of saccharide in the solution was measured by use of the colorimetric method at a wavelength of 490 nm after the saccharide was reacted with phenol.9 The saccharide uptake experiment was carried out at 25.0 ( 0.5 °C. Results Figure 1 shows the saccharide concentration dependence of the swelling ratios of NIPA gels for three saccharides at 20 °C. The gel volume decreases discontinuously at a certain saccharide concentration, and the hysteresis of the discrete volume change was observed. The transition concentration increases in the following order: sucrose (1.21 M) < galactose (1.31 M) < glucose (1.47 M). Temperature dependence of the swelling ratios of NIPA gels in the saccharide aqueous solution of various concentrations is shown in Figure 2. The transition temperature of NIPA gels decreases as the saccharide concentration increases. The decrement of the transition temperature due to the addition of the saccharides, ∆T, is shown as a function of the added saccharide concentration in Figure 3. The decrement depends on the type of saccharide and is the greatest for sucrose, particularly at the high concentration examined. The effect of the saccharide on the transition temperature of NIPA gels is similar to the effect of glucose on LCST observed for poly(NIPA) solutions.10
Figure 4 shows the experimentally determined activities of water in the various saccharides aqueous solutions as functions of the saccharide concentration at 37 °C. The activities monotonically decrease with the saccharide concentration. The water activities at the transition concentration are approximately the same for sucrose and glucose but significantly higher for galactose. Galactose binding to the deswollen state is thus suggested. Binding of the saccharides to NIPA gels was examined by measuring the concentration of the saccharides in the solutions outside NIPA gels. The gel/solution partition coefficient of the saccharide Kp is defined as the ratio of the concentration of the saccharide inside the gel (Cin) to that outside the gel (Cout). The concentration inside the gel was evaluated by using the following relation.
CinVg + Cout(Vt - Vg) ) N
(2)
where Vg, Vt, and N, respectively, are the measured gel volume, the volume of the whole system, and the total amount of the added saccharide. Figure 5 shows the Kp values as a function of the saccharide concentration outside the gels. For glucose and sucrose, Kp values are slightly less than 1.0 in the measured range of the concentration. The value of Kp of galactose for the deswollen state was greater than unity. Binding of galactose to the deswollen state is consistent with the above suggestion. No uptake by NIPA gels and no exclusion from the gels, can be concluded for glucose and sucrose, but for galactose the conclusions can be made only for the swollen state of the gel. Discussion In this study, it was found that the phase transition temperature of NIPA gels is reduced by the addition of the saccharides. The effect of the saccharide is similar to that of other additives such as salts or alcohol;2,4-5,11 the additives reduce the transition temperature of NIPA gels. The following can be considered as possible mechanisms for the saccharide-induced volume phase transition of NIPA gels. (1) The hydration of the saccharide molecule induces a decrease of structured water around the gel network (dehydration with respect to hydrophobic hydration). (2) The swelling pressure is reduced by the exclusion of the saccharide molecules from NIPA gels. (3) Binding of the saccharide to the polymer network occurs. Mechanisms 2 and 3 can be ruled out, because the Kp values are nearly equal to 1 as shown in Figure 5 except for galactose in the deswollen state. Therefore, it can be concluded that the saccharide-induced volume phase transition of NIPA gels takes place mostly as a result of the dehydration of the networks caused by the saccharide. Chemical potential of water molecules decreases with increasing temperature due to the entropy contribution. At a given temperature, the chemical potential of water in the saccharide solution decreases with an increase in the saccharide concentra-
Figure 2. Swelling ratios of NIPA gels in the aqueous saccharide solution as a function of temperature. Their concentrations and the type of the saccharide are shown in the figure.
16284 J. Phys. Chem., Vol. 100, No. 40, 1996
Figure 3. Decrement of the transition temperature of NIPA gels due to the addition of saccharide ∆T as a function of the concentration of the saccharide. ∆T ) (transition temperature in pure water) - (transition temperature in the saccharide solution). The type of the saccharide is shown in the figure.
Figure 4. Concentration dependence of the activities of water molecules in the saccharide aqueous solution at 37 °C. The type of the saccharide is shown in the figure.
Kawasaki et al.
Figure 6. The relationship between ∆µ, the decrement of the chemical potential of water molecules with addition of the saccharide, and the transition temperature decrement ∆T of NIPA gels in the saccharide solution. The type of the saccharide is shown in the figure.
of water in the saccharide aqueous solution are assumed to be independent of temperature in the range of 10-35 °C. This assumption was shown to be valid since the activity at 37 °C obtained in the present study agreed with the values in the literature at 25 °C12 within the experimental error (3%) in the case of glucose and sucrose. As shown in Figure 6, good linear relations are found between ∆µ and ∆T for the three saccharides. The observed slope for galactose is, however, significantly greater than those for glucose and sucrose as shown in Figure 6. This difference will be caused by preferential binding of galactose to the deswollen state of the gel. The similar linear relation between ∆µ and ∆T for glucose and sucrose indicates that the decrease in the chemical potential of water molecules by adding the saccharides causes the saccharide-induced volume phase transition of NIPA gels. Decrement of the chemical potential of water by any means (saccharide, salt, temperature, and so on) may destabilize the hydration on NIPA gel chains, resulting in the shrinkage of the gels. The effect of the addition of the saccharide (glucose, sucrose and galactose) on the volume of a hydrophilic gel and saccharide binding to the gel were also examined on acrylamide (AAm) gel in the present experiment (not shown). Neither volume change of AAm gels nor saccharide binding to AAm gels was observed with an increase in the saccharide concentration up to 2 M. This clearly indicates that the hydrophobic hydration around the isopropyl groups of NIPA plays an essential role in determining the gel volume. References and Notes
Figure 5. Gel/solution partition coefficient Kp of the saccharides for NIPA gels as a function of the saccharide concentration outside the gels at 25 °C. The Kp value is defined as the ratio of the concentration of the saccharide inside the gel to that outside the gel. The type of the saccharide is shown in the figure. Closed symbols refer to the completely deswollen state, while open symbols refer to the swollen state.
tion due to the mixing free energy. In this way, adding saccharides and raising temperature have the same effect of decreasing the chemical potential of water. The decrement of the chemical potential of water caused by adding saccharides ∆µ () µw° - µw ) (J mol-1) is given by -RT ln aw in terms of the activity aw. µw° and µw represent the chemical potential of water in pure water and that in the saccharide aqueous solution, respectively. Figure 6 shows the relation between ∆µ and ∆T, the decrement of the transition temperature of NIPA gels due to the saccharide. In the calculation of ∆µ, T is the transition temperature at a given concentration. However, the activities
(1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (2) Ohtake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (3) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (4) Inomata, H.; Goto, S.; Ohtake, K.; Saito, S. Langumir 1992, 8, 687. (5) Suzuki. A. AdV. Polym. Sci. 1993, 110, 226. (6) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 5045. (7) Kawasaki, H.; Nakamura, T.; Miyamoto, K.; Tokita, M.; Komai, T. J. Chem. Phys. 1995, 103 (14), 6241. (8) Dick, D. A. Int. ReV. Cytol. 1959, 8, 387. (9) Dubois, M.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28 (3), 350. (10) Kim, Y. H.; Kwon, I. C.; Bae, Y. H.; Kim, S. W. Macromolecules 1995, 28, 939. (11) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 34, 4352. (12) Weast, C. W. CRC Handbook of Chemistry and Physics, 58th ed.; CRC Press, Inc.: Boca Raton, FL, 1978; p D219.
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