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 <i>ν</i>. Recent numerical investigations uncovered the occurrence of a hyper-auxetic transition (HAT) (<i>ν</i> = -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 <i>T</i> up to a maximum value of the crosslinker concentration <i>c</i>, thus being clearly distinct from the VPT, taking place at fixed <i>T</i> for all values of <i>c</i>. Our results provide novel fundamental insights on the interplay between network collapse and mechanical properties of these fascinating materials.