Relaxation modes in chemically cross-linked poly(2-methacryloyloxyethyl phosphorylcholine) hydrogels

We investigated the relaxation modes of autocorrelation functions obtained by dynamic light scattering (DLS) of chemically cross-linked zwitterionic hydrogels comprising poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) as the main chain of the network. A time- and space-averaged (i.e., an ensemble-averaged) autocorrelation function was obtained using a home-built DLS apparatus enabling automated positioning of a sample. The equilibrated hydrogels exhibited fast and slow relaxations; the former was derived from thermodynamic fluctuations of the polymer network whose cross-link distances were modeled by the Stokes-Einstein equation, whereas the latter depended on cross-linking densities, cross-linker type, and external electrolytes in the hydrogels, suggesting a mesoscopic weak segment-segment interaction driven by electrostatic forces or by the salting-out effect. These observations may aid in the regulation of network integrity and hydration state of the polymer in PMPC hydrogels.