Onset of criticality in hyper-auxetic polymer networks

A Ninarello and J Ruiz-Franco and E Zaccarelli, NATURE COMMUNICATIONS, 13, 527 (2022).

DOI: 10.1038/s41467-022-28026-z

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 ultralow-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.

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