Load transfer between permanent and dynamic networks due to stress gradients in nonlinear viscoelastic hydrogels

Relaxation experiments are performed on single edge notch tension (SENT) and T shape specimens consisting of different variations of polyampholyte (PA) hydrogels. These specimens share a common feature of containing regions of high and low stress/strain. This feature enables us to probe the load transfer mechanisms between permanent and dynamic networks of different strengths. This load transfer mechanism is connected to viscoelastic behavior. PA gels are nonlinear viscoelastic, with time-dependent behavior controlled by the breaking and reforming of ionic bonds in the dynamic network. In contrast to the prediction of linear viscoelastic theory, the displacement and strain fields during stress relaxation are not fixed but change with time in a complex way depending on the strength of the dynamic bonds and the observation time window. These experimental results are explained by a nonlinear viscoelastic model. Additional physical insights are gained by appealing to the nonlinear rheology of dynamic bonds or stickers. Our result shows that load transfer between networks can be controlled by modifying the strength of dynamic bonds.