Glass-like dynamics of the strain-induced coil/helix transition on a permanent polymer network

TL;DR

This study investigates the slow, glass-like stress relaxation in gelatin gels caused by strain-induced coil to helix transitions, revealing temperature-dependent dynamics and long-range elastic couplings.

Contribution

It provides new insights into the glass-like slow dynamics of polymer networks driven by strain-induced conformational changes, with detailed temperature dependence analysis.

Findings

Stress relaxation exhibits stretched exponential followed by exponential decay.

Characteristic times follow Arrhenius-like temperature dependence.

Relaxation dynamics are influenced by long-range elastic couplings.

Abstract

We study the stress response to a step strain of covalently bonded gelatin gels in the temperature range where triple helix reversible crosslink formation is prohibited. We observe slow stress relaxation towards a $T$-dependent finite asymptotic level. We show that this is assignable to the strain-induced coil $\rightarrow$ helix transition, previously evidenced by S. Courty, J.L. Gornall and E.M. Terentjev (PNAS, {\bf 102}, 13453 (2005)), of a fraction of the polymer strands. Relaxation proceeds, in a first stage, according to a stretched exponential dynamics, then crosses over to a terminal simple exponential decay. The respective characteristic times $\tau_{K}$ and $\tau_{f}$ exhibit an Arrhenius-like $T$-dependence with an associated energy $\cal E$ incompatibly larger than the activation barrier height for the isomerisation process which sets the clock for an elementary coil $\to$ helix transformation event. We tentatively assign this glass-like slowing down of the dynamics to the long-range couplings due to the mechanical noise generated by the local elementary events in this random elastic medium.