Characterizing state of chain entanglement in entangled polymer solutions during and after large shear deformation

Using four entangled 1,4-polybutadiene solutions as model systems, we carry out rate-switching startup shear and elastic recovery tests to probe the state of chain entanglement during startup continuous shear and after shear cessation, respectively. Specifically, we apply a second startup shear of higher rate either during the first startup shear or after cessation of the first shear. The nonmonotonic response (overshoot) to the startup shear is evaluated to elucidate the effect of any preceding shear on the entanglement structure. In a second type of experiments, at different strains during and after shear or relaxing for a certain amount of time after shear cessation we set the shear stress free to determine the amount of elastic recoil, which can also reveal the status of chain entanglement interactions. Consistent with the emerging theoretical interpretations, these multistep rheometric procedures allow us to learn more about (a) elastic yielding during relaxation after cessation of shear that is produced at a low rate, (b) existence of a critical strain amplitude for the elastic yielding, (c) nonmonotonic evolution of the chain entanglement upon startup shear, and (d) yielding of the entanglement network at the point of shear stress overshoot.