Onset of criticality in hyper-auxetic polymer networks

TL;DR

This paper demonstrates that ultra-low-crosslinked polymer networks exhibit hyper-auxetic behavior and undergo a critical-like transition between states of different densities, revealing a novel phase behavior in such materials.

Contribution

The study introduces the concept of hyper-auxeticity in polymer networks and identifies a critical transition triggered by mechanical instability at a specific crosslinker concentration.

Findings

Hyper-auxetic behavior occurs at finite crosslinker concentration.

A critical-like transition between density states is observed.

The model aligns with real hydrogel behavior and can be experimentally tested.

Abstract

Against common sense, auxetic materials expand or contract perpendicularly when stretched or compressed, respectively, by uniaxial strain, being characterized by a negative Poisson's ratio $\nu$. The amount of deformation in response to the applied force can be at most equal to the imposed one, so that $\nu=-1$ is the lowest bound for the mechanical stability of solids, a condition here defined as "hyper-auxeticity". In this work, we numerically show that ultra-low-crosslinked polymer networks under tension display hyper-auxetic behavior at a finite crosslinker concentration. At this point, the nearby mechanical instability triggers the onset of a critical-like transition between two states of different densities. This phenomenon displays similar features as well as important differences with respect to gas-liquid phase separation. Since our model is able to faithfully describe real-world hydrogels, the present results can be readily tested in laboratory experiments, paving the way to explore this unconventional phase behavior.