Swelling behavior of poly(sodium acrylate) gels crosslinked by aluminum ions

The cylindrical poly(sodium acrylate) gel (SA gel) was synthesized in the glass capillary using aluminum ions as the crosslinker. The swelling ratio of the gel was measured after the repeated exchange of solvent (distilled deionized water, about pH 5.8). The gel exhibited two relaxation processes; at first the gel swells rapidly as exchange of water (the swelling process), then shrinks very slowly (the shrinking process). In order to reveal the microscopic structural change (especially, the formation of hydrogen bonding) by water exchange, attenuated total refraction (ATR) Fourier transform infrared (FT-IR) spectroscopy was applied to the gels with different swelling ratio. The IR absorption peaks of the gel were assigned based on those of poly(sodium acrylate) aqueous solutions at different pH. On the swelling process, the carboxyl groups were gradually protonated, and the intermolecular hydrogen bonding started to form in the gel with maximum swelling ratio. On the shrinking process, the formation of hydrogen bonding gradually increased with long-time repeated water exchange which resulted in the shrinkage of the gel. Effects of the repeated water exchange on the swelling behavior were discussed in terms of the exchange of counter ions and the formation of hydrogen bonding.