Dependence of structure of polymer side chain on water structure in hydrogels

To investigate the effects of structure of polymer side chains on structure of water in hydrogels, Raman spectra of polyacrylamide (PAA) and poly-N,N,-dimethylacrylamide (PDMAA) hydrogels were measured. PAA and PDMAA have similar chemical structures, except for the side chain structure. The result shows that the frequency of the O–H stretching mode for the PAA hydrogel decreases as the water content decreases, while that for the PDMAA hydrogel increases. Furthermore, the analysis of the relative intensities of the O–H stretching modes shows that the number density of tetragonal water structures differs between the hydrogels. We conclude that most of the bound water molecules in the PAA hydrogel form four strong hydrogen bonds with the hydrophilic groups in the side chain of PAA, whereas those in the PDMAA hydrogel form weak hydrogen bonds with surrounding water molecules. The water structure is an important factor governing the physical and chemical properties of gel materials.