Abstract
Abstract H dependence of volume shrinking and swelling of a polyacrylamide (PAAm) gel in the mixed 1 : 1 solution of acetone and water is studied using positron annihilation spectroscopy. Relationships among macroscopic volume change, microscopic size, numerical concentration and size distribution of the free volumes formed in the PAAm gel are examined. A change in the free volume size is observed at pH 6.8, which coincides with the pH of the macroscopic volume transition induced by pH change. The numerical concentration of free volumes increases with increasing pH up to ∼5.0, beyond which it decreases. This indicates that the behavior of the number of free volumes in the PAAm gel is independent of that in the corresponding solvent and the number of carboxylate anions ([–COO-]). The change of the size and numerical concentration of free volumes is discussed in terms of electric attractive and repulsive interactions among [–COO-] anions in the gel network, the restrictive motion of the H+ counter cation, and the hydrogen bonding generated by amide groups and/or carboxyl groups incorporated in the PAAm gel. A change in the size distribution of the free volumes is observed. The present results show a peculiar increase of a dispersion of the free volume size distribution, and indicates an increase of inhomogeneity of a nanoscopic structure in the PAAm gel near the VPT point.
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