Phase Transition Dynamics of Three Types of Water within Poly(N,N-dimethylacrylamide) Hydrogels

Water in hydrogels has been classified into three types: bound, intermediate, and free water. To investigate the individual phase transition dynamics for each type of water, differential scanning calorimetic (DSC) curves and Raman spectra of poly(N,N-dimethylacrylamide) hydrogels were measured with heating from 130 to 310 K. Bound and intermediate water showed glassy initial states at 130 K, whereas free water became hexagonal ice (Ih) structure. Intermediate water in glassy state undergoes four phase transition steps: glass-to-liquid transition (at 160–190 K), crystallization from liquid state to cubic ice (Ic) (at 200–230 K), Ic–Ih transition (at 240–250 K), and melting (at 250–273 K). It is concluded that pre-melting of ice, which has been observed in various polymer hydrogels, results from the exothermic Ic–Ih transition of intermediate water.