Hyper-Auxeticity and the Volume Phase Transition of Polymer Gels

TL;DR

This study explores the relationship between the volume phase transition and hyper-auxetic behavior in thermoresponsive hydrogels, revealing that hyper-auxeticity occurs independently of the volume phase transition at varying temperatures and crosslinker concentrations.

Contribution

The paper demonstrates through simulations that hyper-auxeticity in hydrogels occurs at different conditions than the volume phase transition, providing new insights into their mechanical behavior.

Findings

Hyper-auxetic transition occurs at different temperatures than VPT.

HAT is influenced by crosslinker concentration.

HAT exhibits critical-like behavior.

Abstract

Thermoresponsive hydrogels exhibit reversible deswelling at the Volume Phase Transition (VPT), associated to a minimum of the Poisson's ratio $\nu$. Recent numerical investigations uncovered the occurrence of a Hyper-Auxetic Transition (HAT) ($\nu=-1$) 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 $T$ and for all $c$. Our results provide novel fundamental insights on the interplay between network collapse and mechanical properties of these fascinating materials.