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

The cross-linker nature and water content may impact the microscopic dynamics of hydrogels components and thus their properties in applications such as drug delivery and water confinement. To investigate these aspects, we used quasi-elastic neutron scattering with contrast variation to distinctly study the polymer and water dynamics in polyhydroxyethyl methacrylate (pHEMA) chemical (cg) and physical (pg) hydrogels with various water contents. 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 hydration level, the mobility of both the water and the polymer network increases. For the same water content, pg presents 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 typical of hydrogels. In the less hydrated gels, water mobility is sensibly reduced in the cg compared with pg, whereas at higher hydration the mobilities are similar but with shorter residence times in cg.