Diameter decrement by the repeated exchange of solvent water has been reported in a polyelectrolyte hydrogel ionized by sodium acrylate in its swollen state. This behavior has been attributed to the exchange of counterions of carboxyl groups and the formation of hydrogen bonding. In this paper, the formation and destruction of hydrogen bonding in poly(N-isopropylacrylamide-co-sodium acrylate) (NIPA/SA) gel were studied using Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR) at 25 °C. Several swollen samples with different swelling ratios were prepared by changing the number of water exchanges at 25 °C. It was found that the ATR-FTIR spectra evidently depended on the initial swelling ratio. By sufficient water exchanges, new peaks appeared at ca. 1713 cm-1 (assigned to the carboxyl dimer) and at ca. 1650 cm-1 in the amide I region (assigned to a stretching vibration of the CO group that forms a hydrogen bonding with the N−H bond of a neighboring amide group), and the amide II band shifted to a smaller wavenumber by ca. 7 cm-1. From the detailed analysis of the IR spectra, the macroscopic polymer network shrinkage was attributed to the formation of three types of hydrogen bonding, that is, between two carboxyl groups of SA, between the carboxyl group of SA and the amide group of NIPA, and between two amide groups of NIPA. Moreover, the IR results indicated experimentally that the reswelling transition of the present system was caused by the destruction of hydrogen bonding on heating. It was concluded that not only the formation by the exchange of water but also the destruction of hydrogen bonding by heating is essentially important to determine the swelling ratio as well as the volume phase transition behavior in this system.

Poly(vinyl alcohol) cast gels (PVA cast gels) were prepared at different gelation temperatures, T gel , and the swelling ratio and mechanical properties were characterized by the equilibrium weight measurement, a uniaxial loading test, a tearing force measurement, and a cyclic compression test at room temperature. The results showed that the swelling ratio decreased and both the breaking stress and the tearing energy increased with an increase in T gel . The compression deformation energy of 50% reduction of its original size increased monotonically, while the hysteresis loss was not affected by T gel . To understand these macroscopic properties, the network structures of the samples were examined by X-ray diffraction and Fourier transform infrared measurements. As a result, the size of the microcrystallites increased with an increase in T gel and the average distance between the microcrystallites also increased. These results suggested that the structures of PVA networks, crosslinked by the microcrystallites, have essential roles in determining the mechanical properties of PVA cast gels. The macroscopic mechanical properties are discussed on the basis of the size, number, and distribution of microcrystallites as crosslinkers and the homogeneity of amorphous PVA networks, which were determined at gelation. Experimental procedure Sample preparationPVA powder was kindly supplied by Kuraray Co., Ltd. (cat. no. PVA117), and was used without further purification. The

Polyelectrolyte hydrogels, physically crosslinked by metal ions, were synthesized using poly(sodium acrylate) as the main constituent and Al ions as the crosslinker. The swelling ratio of the gel was measured whenever the solvent water was repeatedly exchanged in a constant interval. The as-synthesized gel exhibited two relaxation processes; the gel swelled at the first stage, then shrunk very slowly at the second stage, and recovered to the initial size just after the gelation (ultimately, the gel became smaller than that). The relaxation times of both processes were found longer (exceptionally longer for the shrinking process) than the conventional collective diffusion of polymer networks. The diffused amounts of Al ions and Na counter ions in the solvent were also measured at each water exchange. The diffusion of Al ions into the solvent was found to finish when the swelling ratio took the maximum (at the end of the first stage), while Na ions continued to diffuse until the diameter became the final one (at the end of the second stage). The microscopic structural changes by the repeated water exchange were obtained by the measurements of ATR FT-IR spectroscopy on the gels with different swelling ratios. The carboxyl groups were gradually protonated on both stages, and the formation of hydrogen bonding was accelerated on the second stage. Effects of the repeated water exchange on the swelling behavior are discussed in terms of the diffusion of Al ions into the solvent, the exchange of Na counter ions by protons, and the formation of hydrogen bonding.

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The effects of continuous water exchange on the swelling behavior of poly(N‐isopropylacrylamide‐co‐sodium acrylate) gel were studied. The diameter of gels and the Na+ concentration in the solvent were measured at several constant intervals after the solvent (the distilled deionized water) was exchanged. The diameter decreased at room temperature as the solvent was exchanged with water, and it continued to decrease even after more than about 97% of the initial Na+ diffused into the water. Thus, the final swelling ratio of the gel was only slightly larger than that of the neutral poly(N‐isopropylacrylamide) gel. To reveal the structural change in molecular level, solid‐state 13C cross‐polarization/magic‐angle spinning, solid‐state 1H combined rotation and multiple‐pulse spectroscopy, and swollen‐state 13C dipolar decoupled/magic‐angle spinning NMR experiments were carried out for several dried and swollen samples by varying the times of water exchange. As a result, the intensity and position of the carboxyl peak changed, and the relative intensity of the nonionized carboxyl groups of gels increased with an increasing number of water exchanges. These results indicated that hydrogen bonding was formed between the two, nonionized carboxyl groups (COOH) and/or between the COOH and CONH groups. The macroscopic polymer network shrinkage is discussed in terms of the replacement of counterions Na+ by H+ and of the formation of intermolecular hydrogen bonding. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1090–1098, 2004