Microstructural Rearrangement in Polyelectrolyte Complex Hydrogels*

In this talk, we discuss the swelling and dehydration response of polyelectrolyte complex (PEC) hydrogels by elucidating their microstructural evolution to establish them as equilibrium structures. These hydrogels are aqueous three-dimensional polymeric networks formed when triblock polyelectrolytes (PEs) of opposing charge micro-phase separate. These hydrogels can be tuned by varying intrinsic (charged-block length, PE concentration) and extrinsic (salt, pH) factors and have diverse microstructures that affect their physicochemical and mechanical properties. To examine if these microstructures are equilibrated or kinetically trapped structures, we probed microstructural rearrangements in PEC hydrogels with small-angle X-ray scattering. We discuss how controlled swelling of PEC hydrogels results in microstructural evolution opposite to that noted with increasing PE concentrations. The swelling experiments were extended to rehydration studies where PEC hydrogels with known PE concentrations and microstructures were de- and rehydrated to varying PE concentrations. The microstructures of the rehydrated gels were consistent with those of the as mixed PEC hydrogels with equal PE concentrations. Moreover, we discuss how mixtures of PEC hydrogels with distinct microstructures evolve into hydrogels with intermediate microstructures. Our data establish PEC hydrogels as equilibrium structures and reveal methods like dilution, dehydration, and mixing as methods for in situ tuning.