Synthesis, swelling behavior and surface microstructure of poly(sodium acrylate) gels cross-linked by aluminum ions

We report the synthesis and some unique properties of polymer gels cross-linked by metal ions. They are made of poly(sodium acrylate) and aluminum ions. Cylindrical and thin plate gels were synthesized in glass molds by mixing poly(sodium acrylate) with aluminum hydroxide of various concentrations. The swelling ratio of the gels was measured by changing the solvent pH and adding NaCl with different concentrations. Each gel was found to swell at the first stage and shrink thereafter by repeated solvent exchanges; the swelling ratio ultimately returned to the initial one just after gelation. These features could be explained using a qualitative model based on the ion exchange and the formation of hydrogen bonding. A characteristic surface microstructure was observed on the dehydrated gels using an atomic force microscope. The microstructures were analyzed in terms of the autocorrelation function and the root-mean-square roughness. The surface microstructure of this system was characterized by the microdomains, the shape of which was found to depend strongly on the amount of aluminum ions and the gel thickness at gelation. It could be controlled not only by the concentration of the cross-linker but also by the dehydration condition.