From thermally activated to viscosity controlled fracture of biopolymer hydrogels

TL;DR

This study investigates the rate-dependent fracture mechanisms in biopolymer hydrogels, revealing a transition from thermally activated unzipping at slow crack speeds to viscosity-controlled fracture at higher speeds, supported by an extended theoretical model.

Contribution

It introduces an extended model that captures both thermally activated unzipping and viscous drag effects in hydrogel fracture, with experimental validation across a wide range of crack velocities.

Findings

Unzipping dominates at slow crack velocities due to thermal activation.

Viscous solvent drag becomes significant at higher crack speeds.

The extended model accurately predicts the transition between mechanisms.

Abstract

We report on rate-dependent fracture energy measurements over three decades of steady crack velocities in alginate and gelatin hydrogels. We evidence that, irrespective of gel thermo-reversibility, thermally activated "unzipping" of the non-covalent cross-link zones results in slow crack propagation, prevaling against the toughening effect of viscous solvent drag during chain pull-out, which becomes efficient above a few mm.s$^{-1}$. We extend a previous model [Baumberger {\it et al.} Nature Materials, {\bf 5}, 552 (2006)] to account for both mechanisms, and estimate the microscopic unzipping rates.