Structure and Diffusive Properties of Water in Polymer Hydrogels

To investigate the diffusive properties of water in hydrogels, ab initio molecular orbital and molecular dynamics calculations of poly-N,N-dimethylacrylamide-water systems were performed. The results show that the mean diffusion coefficient of water drastically decreases in the middle dehydration stage. In this stage, the mobilities of water are restrained because part of the water forms hydrogen bonds to bind polymer chains due to a glass transition. In addition to the three well-known types of water (i.e., bound, intermediate, and free water) around hydrophilic polymers in hydrogels, our results suggest that the intermediate water can be further classified into two types: first and second intermediate water. The bound and first intermediate water acts as a cross-linker between polymer chains, even if the polymer does not form intra- or intermolecular bonds itself. The diffusive properties of water might have important implications for the interpretation of properties of polymer hydrogels.