In situ SAXS/SANS Characterization of Supramolecular Hydrogels under Deformation

Double network hydrogels were the first synthetic tough hydrogels with their toughness attributed to energy dissipation through a sacrificial covalent network to prevent catastrophic failure, but this leads to significant hysteresis. Supramolecular hydrogels can overcome this limitation as energy is dissipated by rearrangement of physical associations (hydrophobic, ionic, H-bond). These associations are reversible, but the mechanisms associated with the network rearrangement are not well understood. Here, we examine a family of hydrogels based on amphiphilic statistic copolymer that are crosslinked by hydrophobic association during uniaxial elongation where the nanostructure evolution during deformation is elucidated by time-resolved SAXS. Nanostructures of hydrogels are dependent on both the strain rate and the crosslink density (as defined by copolymer composition) with the relative change in the nanostructure evolving from rubber-like (ν=0.5) to glass-like (ν=0.33) as rate increases. These differences are related to time scales for stress relaxation processes relative to the strain rate. Combined SANS and SAXS measurements demonstrate plastic deformation of hydrophobic aggregates and segmental pull-out of hydrophobes contribute to the energy dissipation of these hydrogels.