© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 26, JUNE 26, 1997
ARTICLES Effect of pH on the Volume Phase Transition of Copolymer Gels of N-Isopropylacrylamide and Sodium Acrylate Hideya Kawasaki, Shigeo Sasaki, and Hiroshi Maeda* Department of Chemistry, Faculty of Science, Kyushu UniVersity 33, Hakozaki, Higashi-ku, Fukuoka, 812, Japan ReceiVed: September 12, 1996; In Final Form: April 14, 1997X
A temperature-induced volume change of ionic gels was investigated for cross-linked copolymer gels of N-isopropylacrylamide and sodium acrylate (NIPA-AA) at various pH values. The volume change of the gel against temperature was discontinuous below pH 6.3, but it was continuous above pH 7.5. The continuous volume change observed for the completely ionized NIPA-AA gel suggests that an increase in the counterion osmotic pressure alters the discontinuous volume change of NIPA gel to the continuous one. Potentiometric titrations revealed that the dissociation constant of carboxyl groups of the linear copolymer of Nisopropylacrylamide and acrylic acid in solution decreased with increased temperature above 34 °C. The similar decrease in the carboxyl ionization with increasing temperature was also observed for NIPA-AA gel. The discontinuous volume change was correlated with the two effects arising from the decrease in the carboxyl ionization of NIPA-AA gel: the reduced Donnan osmotic pressure and the presence of a significant fraction of un-ionized carboxyl groups in the gel at the collapsed state.
1. Introduction The volume phase transition (i.e. discontinuous volume change) of a gel has been observed for various kinds of hydrogels while changing the temperature or solvent composition.1-9 N-isopropylacrylamide (NIPA) gel in water is known to show a volume phase transition in response to a temperature change at about 34 °C.10 The temperature-induced volume phase transition is closely related to the phase separation of the solutions of poly(N-isopropylacrylamide), which exhibits a lower critical solution temperature.11 Hirotsu et al. have studied the temperature-induced volume phase transition of copolymer gels of N-isopropylacrylamide and sodium acrylate (NIPA-AA) in water as a function of the copolymer composition.12 They observed an increase in the discontinuity of the volume transition with the ionization: as the ionization increased, both the X
Abstract published in AdVance ACS Abstracts, June 1, 1997.
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transition temperature and the volume change at the transition increased. Others have also reported the discontinuous volume change of the NIPA-AA gel.13,14 Prausnitz et al. have reported, however, that the volume change behavior of the NIPA-AA gel becomes continuous with increasing ionization.15 These different results have not been interpreted reasonably. It has been suggested that the driving force of the volume phase transition of the NIPA gel and the NIPA-AA gel is the hydrophobic interaction among the side chains.10,14,16 The volume phase transition of the NIPA-AA gel was explained qualitatively on the basis of the Flory-Huggins theory combined with the ideal Donnan osmotic pressure.12 In the theory, the χ parameter value (the Flory-Huggins polymer-solvent interaction parameter) is assumed to increase with temperature. This temperature dependence of the χ parameter is due to the fact that the hydrophobic interaction strengthens with temperature. According to the theory, large χ parameter values can induce © 1997 American Chemical Society
5090 J. Phys. Chem. B, Vol. 101, No. 26, 1997 the volume phase transition of the NIPA-AA gel even when the Donnan swelling pressure is operating. It is expected that the discontinuous volume change of the NIPA-AA gel occurs when the attractive hydrophobic interaction overwhelms the repulsive Donnan osmotic pressure. In the present study, we are concerned with effects of the ionization of acrylic acid (AA) groups on the volume phase transition of the NIPA-AA gel. Clarifying this effect is helpful for understanding the temperature dependence of the hydrophobic interaction of NIPA as well as the mechanism of the volume transition of the NIPA-AA gel. In the present study, we report the pH dependence of the temperature-induced volume change behavior of the NIPA-AA gel with various compositions of AA. Potentiometric titrations were carried out on solutions of the linear copolymer of N-isopropylacrylamide and acrylic acid (PNIPA-AA). The correlation between the volume change behavior of the NIPA-AA gel and the dissociation behavior of carboxyl groups in the gel will be examined.
Kawasaki et al.
Figure 1. Temperature dependence of the swelling ratios for NIPAAA gels with various compositions at pH 5.6. The mole fraction of the acrylate, X, is shown in the figure.
2. Experimental Section NIPA-AA gels were prepared by radical copolymerization in aqueous solutions of N-isopropylacrylamide (700-600 mM), sodium acrylate (0-100 mM), and N,N′-methylenebisacrylamide (3.5 mM). Gels with a mole fraction of the acrylate at the gel preparation, X ) AA/(NIPA + AA), are denoted by NIPAAA(X). The polymerization was carried out at 5 °C for 24 h, initiated by ammonium persulfate (APS) and accelerated by N,N,N′,N′-tetramethylethylenediamine. The gel was synthesized in a capillary (0.296 mm diameter), cut into a rod form (20 mm length), rinsed thoroughly with distilled water, and then dried gently. The dried gel was fixed in a sample holder and was immersed into the solution whose pH was well stabilized. The sample holder was made of silicone rubber. Soluble copolymer of PNIPA-AA was prepared in the same way as that employed for the gel preparation without the addition of N,N′methylenebisacrylamide. The copolymers with the mole fraction of the acrylate X at the gel preparation are denoted by PNIPA-AA(X). The polymer recovered by precipitating at 60 °C was redissolved into 1 N HCl solution at a room temperature and dialyzed against distilled water. Soluble copolymer of acrylamide and acrylic acid (PAM-AA) was prepared by radical copolymerization in aqueous solutions of acrylamide (695 mM) and acrylic acid (5 mM) at 60 °C, using APS as initiator. The PAM-AA was precipitated into acetone, dried, and dialyzed against distilled water. All chemicals were of reagent grade. Molecular weights of PNIPA-AA and PAM-AA were determined by HPLC and were about 5 × 105. The column used was TSKGEL-5000 (TOSOH Co. Ltd.), and the mobile phase was NaCl in 100 mM aqueous solution. The molecular weight standards of poly(ethylene oxide) employed were 2.1 × 104, 8.5 × 104, and 3.4 × 105. The gel volume, V, was determined from the diameter, d, measured with an optical microscope. The swelling ratio was defined as V/V0 ) (d/d0)3 where d0 was the diameter of the capillary in which the gel was synthesized. The pH was controlled to a given pH by equilibrating the gel with partially neutralized 5 mM poly(acrylic acid) solution; [MW ∼ 1 × 106 according to the supplier (Wako Chemicals Co. Ltd.)]. To avoid the salt effect on the volume change of the gel,17 the polyelectrolyte solution was used as a pH buffer instead of the buffer of low molecular weight electrolytes. The pH of the polyelectrolyte buffer solutions did not change significantly with temperature, as found in the polyelectrolyte.18 Potentiometric titration of 1 wt % polymer solutions was carried out under nitrogen atmosphere. A blank correction was
Figure 2. Temperature dependence of the swelling ratios for NIPAAA gels with various compositions at pH 9.5. The mole fraction of the acrylate, X, is shown in the figure.
employed in the titration. The temperature in the gel-swelling experiment and the potentiometric titration was controlled by a circulating bath system within (0.1 °C. 3. Results and Discussion 3.1. Swelling Ratio. Figure 1 shows the temperature dependence of the swelling ratios of NIPA-AA (0 e X e 0.10) gels at pH 5.6 on heating. The gel volume changes discontinuously. The transition temperature and the volume change at the transition temperature both increase with X. The swelling ratio at a completely collapsed state (V/V0 ) 0.08) is constant irrespective of X. Hysteresis was also observed in most cases. These behaviors shown in Figure 1 are similar to the experimental result reported before.12-14 Figure 2 shows the temperature dependence of swelling ratios of NIPA-AA (0 e X e 0.10) gels at pH 9.5 on heating. The NIPA-AA gels show continuous volume changes and no hysteresis was observed. The gels with X greater than 0.020 do not reach the completely collapsed state even at 70 °C. As to nonionic pure NIPA gel, the discontinuous volume change was observed at 33.8 °C at both pH 5.6 and 9.5 as shown in Figures 1 and 2. The temperature dependence of swelling ratios of NIPA-AA (0.007) gels at various pH values on heating is shown in Figure 3. The volume change behavior of the NIPA-AA gel becomes continuous with increasing pH. At pH < 6.3, the volume
Volume Phase Transition of Copolymer Gels
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5091
Figure 3. Temperature dependence of the swelling ratios for NIPA-AA gels with a mole fraction of the acrylate of X ) 0.007 at various pH values.
change is discontinuous, while it is continuous at pH >7.5. At pH 7.0, it appears to be nearly discontinuous. The similar pH dependence of the volume change behavior of the gels was also observed for NIPA-AA (0.029) gels (not shown). 3.2. Potentiometric Titration of the Soluble Copolymer NIPA-AA and the Temperature Dependence of the Degree of Carboxyl Ionization in the NIPA-AA Gel. An increase in X results in an increase in the number of acrylate in the gel, which enhances the swelling pressure due to the increased Donnan osmotic pressure. Yet, the volume change behavior of the NIPA-AA gel shown in Figure 1 is different from that shown in Figure 2: at pH 5.6 an increase in X enhances the discontinuity of the volume transition, while at pH 9.5 it makes the transition continuous. This indicates that the pH determines the type of the volume transition of the NIPA-AA gel (continuous or discontinuous) as shown in Figure 3. It is not obvious why a big difference resulted between pH 9.5 and 5.6, since a large fraction of carboxyl groups are expected to dissociate at these two pHs in aqueous environment. To obtain the degree of ionization of the carboxyl group at various pH values, potentiometric titration was performed for PNIPA-AA. Figure 4 shows a plot of pK ) pH - log[R/(1 R)] versus R for salt-free solutions of PNIPA-AA (0.007) at 25, 30, and 35 °C. Here, R represents the degree of ionization of the acid. The pK values are independent of R at these temperatures in the range of R below 0.7. This indicates that the electrostatic interaction among ionized groups scarcely affects the ionization, which is a natural consequence of low X values. The intrinsic dissociation constant of the carboxyl group pK0 can be determined as the pH value at R ) 0.5. At 25 and 30 °C, pK0 values of PNIPA-AA are similar and about 4.9, while at 35 °C the pK0 is about 5.9. Similar titration behavior was also observed for PNIPA-AA (0.029). In the case of gels, the pK0 values were evaluated as the pH values at R ) 0.5. Small pieces of gels were employed for the pH measurements in order to enhance the equilibration speed. The suspensions of small pieces of gels with R ) 0.5 were incubated at a given temperature for 1-2 h before the pH measurements. Temperature dependence of pK0 thus deter-
Figure 4. Potentiometric titration curves for PNIPA-AA solutions with a mole fraction of the acrylate of X ) 0.007. The polymer concentration was 1 wt %.
mined for NIPA-AA gel is shown in Figure 5 together with the results on the soluble copolymers (PNIPA-AA). Approximately identical temperature dependence of pK0 was observed for both the gel and the soluble copolymers. The pK0 values are independent of temperature below about 33 °C, while they increase with temperature above about 34 °C. It should be noted that this onset temperature (34 °C) is close to the transition temperature of NIPA gel (33.8 °C) but it is lower than that of the NIPA-AA (0.007) gel (37.0 °C) and NIPA-AA (0.029) gel (44.0 °C) at pH 5.6. The increment of the pK0 becomes small for gels with high acrylate contents such as the NIPA-AA (0.029) gel or PNIPA-AA (0.029). For the hydrophilic PAMAA (0.007), the pK0 values were independent of temperature as shown in Figure 5. The R value at a given pH can be obtained from the relation of pK0 ) pH - log[R/(1 - R)] from a known pK0. Figure 6 shows the temperature dependence of R for NIPA-AA (0.007)
5092 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Kawasaki et al. TABLE 1: Donnan Osmotic Pressures at the Completely Deswollen State X
pH
XCOOHa
Rb
ΠD/RT (mM)
0.007
5.6 5.9 6.3 5.6 5.9 6.3
0.0046 0.0029 0.0022 0.021 0.016 0.016
0.34 0.58 0.69 0.26 0.44 0.44
18.8 32.2 38.3 59.8 93.9 91.9
0.029
b
Figure 5. Temperature dependence of the pK0 value for PNIPA-AA, PAM-AA solution, and NIPA-AA gel. The mole fraction of the acrylate, X, is shown in the figure.
Figure 6. The degree of ionization, R, of the carboxyl group in the NIPA-AA (0.007) gels at various pH values is plotted as a function of temperature.
gels at various pH values. At pH < 6.3, the R decreases with increasing temperature above 34 °C, and a significant fraction of the carboxyl group is un-ionized at high temperature (i.e. at the collapsed state of the gel). As pH decreases, the decrement of R becomes large and the discontinuity of the volume transition of NIPA-AA gels increases as shown in Figure 3. Thus, the decrease in R is correlated with the discontinuous volume change of the NIPA-AA gel. This suggests that the sharp decrease in the carboxyl ionization with increasing temperature plays an essential role in the discontinuous volume change of the NIPA-AA gel. At pH > 7.5, R is greater than 0.8 in the temperature range examined. The continuous volume change of the NIPA-AA gel is only observed at high pH as shown in Figures 2 and 3. It can be concluded that the introduction of ionized acrylate residues into NIPA gel chains (i.e. an increase in the Donnan osmotic pressure) alters the discontinuous volume change of the gel to the continuous one.19 This suggests that the hydrophobic interaction of NIPA chains in the NIPA-AA gel at high temperatures is not strong enough to overcome the increased Donnan osmotic pressure. The volume change of the NIPA-AA gel at a high pH becomes continuous due to the increased Donnan pressure. On the other hand, the NIPA-AA
a XCOOH: Mole fraction of un-ionized carboxyl groups in the gel. R: degree of the ionization of carboxyl groups in the gel.
gels at pH < 6.3 undergo the discontinuous volume change, despite the weak hydrophobic interaction of NIPA chains. The decrease in R leads to the corresponding decrease in the Donnan osmotic pressure of counterions, which favors the shrinkage of the gel. Furthermore, the resultant un-ionized carboxyl group can form a hydrogen bond to the amide group, which also favors the shrinkage of the gel. The coupling of these two effects with the hydrophobic interaction of NIPA groups is considered to induce the discontinuous collapse of NIPA-AA gel. 3.3. A New Contracting Force Contribution at pH 5.6, 5.9, and 6.3. To examine a relation between the volume change behavior of NIPA-AA gel and the above two effects, we have calculated the Donnan osmotic pressure at the completely deswollen state, as done in the previous study.19 In the previous study, the two regimes (discontinuous or continuous) of the volume transition for completely ionized NIPA-AA gels are unambiguously discriminated by a critical value of the Donnan osmotic pressure ΠD of about ΠD/RT ) 18 mM.19 The calculations were done for the case of the discontinuous volume changes at the transition temperatures, and the results are shown in Table 1. As seen from Table 1, the discontinuous transition was observed for the gels characterized with ΠD/RT values greater than 20 mM if the pH is 5.6 ∼ 6.3. The result is inconsistent with the previous result that the critical value of ΠD/RT is about 18 mM. If the effect of protonation can be approximated with the corresponding decrease of ΠD, this inconsistency should not appear. The present data at pH 5.6 ∼ 6.3 indicate that a new contractile force contribution should be present. The critical value of ΠD/RT is expected to be much greater than 18 mM, due to this new contribution at these pH values. In the previous copolymer gels of NIPA with completely ionized AA, the main contracting force contributions were hydrophobic and rubber elasticity. As the most probable candidate for the new contracting force, a hydrogen bond between COOH and CONH group is suggested. 3.4. Temperature Dependence of the pK0 for the Carboxyl Group of the NIPA-AA Gel. The free energy difference of carboxyl group between the ionized and the un-ionized state determines the pK0. There are two possible physicochemical mechanisms for the increase of pK0 with temperature found in the present study. One is due to the increase in the free energy of the ionized state because of a decrease in the dielectric constant.20 The other is due to the decrease in the free energy of the un-ionized state stabilized by hydrogen bonding between -COOH and -CONH groups. Water molecules associated with the “hydrophobic hydration” around NIPA groups are released when the gels shrinks to the deswollen state at the transition temperature of NIPA gels. The dehydration is suggested by the endothermic heat of NIPA gel 16and NIPA-AA gels14 with differential scanning calorimetry. Since the dehydrated NIPA groups are less polar than water, the dielectric constant surrounding the charges of the carboxylate is expected to decrease with increases in temperature. This unfavorable situation will be relaxed by protonation. The
Volume Phase Transition of Copolymer Gels concomitant hydrogen bonds are expected to be formed. Thus, the pK0 of the NIPA-AA gel increases with increasing temperature greater than 34 °C due to the nonpolar nature of dehydrated NIPA group. The temperature dependence of pK0 should diminish when the hydrophobicity of the chain is weak. The increment of pK0 becomes smaller with the increase in the acrylate content of NIPA-AA gel and PNIPA-AA as shown in Figure 5. The hydrophilic nature of the acrylate may interfere with the hydrophobic interaction of the neighboring NIPA group. Indeed, the pK0 of both the NIPA-AA (0.14) gel and PNIPAAA (0.14) with a high acrylate content were independent of temperature (not shown). Finally, in the copolymer with the hydrophilic comonomer acrylamide, PAM-AA, pK0 has a normal value of 4.7 and is independent of temperature (Figure 5). 3.5. Different Volume Change Behaviors of the NIPAAA Gel with Ionization. From the results obtained in the present study, we can say that the different results of the volume change behavior of NIPA-AA gel reported previously 12-14 originates from different pH ranges used in these experiments. Hirotzu et al. performed the ionization of the NIPA gel by changing the copolymerization ratio, but all the sodium ions were expected to be replaced with protons in the process of the gel preparation.12 They did the swelling measurement of the NIPA-AA gel in pure water, in which the pH might be lower than 6. In the study of Prausnitz et al., on the other hand, the volume change behavior of the NIPA-AA gel is considered to be continuous due to the high pH (pH 8).15 Our experimental results suggest that the χ parameter of NIPA residues does not increase significantly with temperature. A new simple theory was proposed recently in which the coupling between the volume change and the dehydration of the gel chains induces the temperature-induced volume phase transition
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5093 of a nonionic gel.21 In this theory, the χ parameter values need not be greater than 0.5. To account for the continuous volume change of the NIPA-AA gel, we are trying to extend the theory to take into account the Donnan osmotic pressure.22 Acknowledgment. This work is partly supported by Grantin-Aids for Scientic Research (Numbers 08454184 and 08640742) from Monbusho, Japan. References and Notes (1) Tanaka, T. Phys. ReV. Lett. 1978, 40, 820. (2) Tanaka, T.; Fillmore, D. J.; Sun, S.-T.; Nishio, I.; Swislow, G.; Shah, A. Phys. ReV. Lett. 1980, 45, 1636. (3) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (4) Katayama, S.; Hirokawa, Y.; Tanaka, T. Macromolecules 1984, 17, 2641. (5) Rıˇcka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. (6) Ohmine, I.; Tanaka, T. J. Chem. Phys. 1982, 77, 5725. (7) Tanaka, T.; Nishio, I.; Sun, S.-T.; Nishio, U. Science 1982, 218, 467. (8) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 5045. (9) Irie, M.; Kunwatchakun, D. Macromolecules 1986, 19, 2476. (10) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (11) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 81, 3311. (12) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (13) Matsuo, E. S.; Tanaka, T. J. Chem. Phys. 1988, 89, 1695. (14) Shibayama, M.; Mizutani, S.; Nomura, S. Macromolecules 1996, 29, 2019. (15) Beltran, S.; Baker, J. P.; Hooper, H. H.; Blanch, H. W.; Prausnitz, J. M. Macromolecules 1991, 24, 2476. (16) Ohtake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (17) Suzuki, A. AdV. Polym. Sci. 1993, 110, 226. (18) Taylor, L. D.; Cerankowski, L. D. J. Polym. Sci. 1975, 13, 2551. (19) Kawasaki, H.; Sasaki, S.; Maeda, H. J. Phys. Chem., in press. (20) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496. (21) Sasaki, S.; Maeda, H. Phys. ReV. E 1996, 54, 2761. (22) Sasaki, S.; Maeda, H. J. Chem. Phys., in press.
