Differential Dynamic Microscopy of Polymer Hydrogels

ABSTRACT Polymer hydrogels are covalently or physically crosslinked polymer networks. The mechanical and rheological properties of these materials are linked to thermodynamics and local chain fluctuations. Measuring the local network dynamics can help understand material performance locally and in a non‐destructive way, and this is typically done with dynamic light scattering (DLS). Here, we report that gel fluctuations can also be measured directly with differential dynamic microscopy (DDM). We compare the local chain motion of both physically and chemically crosslinked hyaluronic acid (HA) gels in phosphate‐buffered saline. Dynamics are extracted from both DLS and DDM for 1.0 wt% HA gels chemically crosslinked with 0.5 and 1 mol% cysteamine. Similarly, 0.5, 1, and 2 wt% HA solutions are analyzed with DDM and DLS. In physical gels, motion corresponds to translation of polymer chains in a transient polymer matrix. In chemically crosslinked gels, motion corresponds to the solid vibrations in the material. The time and distance scales are vastly different in DDM compared to DLS, showing that DLS and DDM are complementary techniques that are able to measure the mutual diffusion of the freely mobile chains and aggregates and the cooperative motion of the strands in the network structures.