(766a) Understanding Rate Dependent Mechanical Properties of Supramolecular Hydrogels through Real Time SAXS Measurements during Stretching

Energy dissipation mechanisms are critical to the development of tough hydrogels with the double network being the most well established route. One challenge with the double network design is the toughness is derived from the breaking of covalent bonds in the sacrificial 2nd network. One alternative to the rupture of covalent bonds is the use of non-covalent interactions, such as ionic bonds, hydrogen bonds or hydrophobic aggregates, to act as the sacrificial network. In this case, the bonds can be reversible, so the energy dissipation should also be reversible. However, the exact mechanisms associated with the breaking and reforming of these non-covalent bonds are not clear. In this talk, I will discuss the use of a model hydrogel material that is based on a random amphiphilic copolymer. The hydrophobe is perfluorinated and this leads to aggregation of the fluorinated moieties into well-defined nanoscale aggregates. The higher electron density of the fluorinated domains provides x-ray contrast to enable the characterization of the nanostructure. As the crosslinks are dynamic and only based on hydrophobic association, the evolution of the nanostructure as the hydrogel is uniaxially stretched is strongly dependent on the stretching rate. Changing the stretching rate by 3 orders of magnitude leads to significant differences in the nanoscale anisotropy developed in the sample during deformation. This difference in the change in the nanostructure can be directly related to the mechanical response of the hydrogel (stress-strain curve). If time permits, a brief description of the structural recovery on cessation of deformation will be provided to provide insight into the reversibility of these tough hydrogels and the relevant time scales.