Kinetics of water swelling and development of porous structure in ionic poly(acrylonitrile-acrylamide-acrylic acid) hydrogels

Abstract

The kinetics of water uptake into the dry form of, and benzoic acid diffusion through, swollen inhomogeneous poly(acrylonitrile—acrylamide—acrylic acid) gets were investigated with gravimetry and ultra-violet visible spectroscopy, respectively. The goal is to understand the effects of ionizable moieties and water polymer affinity on the rate of water diffusion during the formation of hydrogels from glassy polymers and on the free-volume mechanism for solute transport through the resulting swollen networks. The xerogel with intermediate acid content or minimal affinity with water has the maximum water diffusion coefficient in the initial swelling stage and the maximum deviation from Fickian transport. The molecular diffusion of benzoic acid through hydrogels was found to be controlled by the swelling, and the size of voids for solute diffusion does not remain constant over all the range of compositions. This discrepancy from the simple free-volume theory is due to a wide range of molecular interactions for gels with amide/acid moieties. In conclusion, dynamic swelling is influenced by water—polymer affinity and subsequently causes various types of steady transport in hydrogels.