Varying States and Dynamics of Water within Hydrogels across Rubbery-to-Glassy Transition

Water is essential for survival and functionality of biological organisms. However, the effects of temperature and water content on the dynamics of water molecules within cells and organs remain elusive due to the complexity of biological systems. In this study, we use hydrogels as a model system to investigate the effects of temperature and water content on the state and dynamics of water within polymer networks, especially when the gels undergo the rubbery-to-glassy transition. Pulsed nuclear magnetic resonance (pulsed-NMR) results indicate impeded diffusion of water within the gel matrix; the diffusion of water is too slow to be detectable in glassy hydrogels with relatively low water content or at low temperatures. Quasi-elastic neutron scattering measurements reveal the transition of water from jump diffusion to local motion when the gel changes from a rubbery to a glassy state. The varying dynamics and fractions of water in different states are further examined by pulsed-NMR with multiple echo sequences. The elastic gels have confined, intermediate, and free water, whereas the glassy gels have only confined and intermediate water. The spin–spin relaxation time of water in different states and their fractions depends on the temperature and water content of the gel. Temperature primarily affects the dynamics of water, while water content influences the fractions of water in different states. This work deepens the understanding of the varying states of water and their fractions as the hydrogel undergoes the glass transition. It also provides new insights into the interactions between the polymer network and water molecules within gels under varying conditions, which is crucial for understanding the essential features of glassy gels and for studies of water-mediated functionalities in biological systems.