Disentangling polymer network and hydration water dynamics in polyhydroxyethyl methacrylate physical and chemical hydrogels

The paper reports a chemical-physical study on the dynamic of polymer and water in physical and chemical polyhydroxyethyl methacrylate (pHEMA) hydrogels with various water content. To investigate these aspects, quasi-elastic neutron scattering with contrast variation was used to distinctly study the polymer and water dynamics. For the polymer network, a distribution of relaxation processes was observed, mainly related to the side chains. Water dynamics was found to occur as a H-bond governed process with a jump-diffusion mechanism. The interaction with the polymer matrix considerably slows the water dynamics with respect to bulk water and other confined systems and leads to a fraction of water molecules appearing as immobile. With a higher water content, the mobility of the water and the polymer network increases. When the hydration level is the same, physical gels present slower relaxation processes and a smaller explored space than their cg equivalents, as a result of side chains involvement in the formation of the three-dimensional network.