Hyper-auxeticity and the volume phase transition of polymer gels.

Thermoresponsive hydrogels exhibit reversible deswelling at the volume phase transition (VPT), associated to a minimum of the Poisson's ratio ν. Recent numerical investigations uncovered the occurrence of a hyper-auxetic transition (HAT) (ν = -1) for low-crosslinked hydrogels at low temperature, accompanied by a critical-like behavior. Here, we perform extensive numerical simulations to unveil the relation between these two transitions. We find that the HAT occurs at different temperatures T up to a maximum value of the crosslinker concentration c, thus being clearly distinct from the VPT, taking place at fixed T for all values of c. Our results provide novel fundamental insights on the interplay between network collapse and mechanical properties of these fascinating materials.