Polymer networks which locally rotate to accommodate stresses, torques, and deformation

Abstract

Polymer network models construct the constitutive relationships of a broader polymer network from the behavior of a single polymer chain (e.g. viscoelastic response to applied forces, applied electromagnetic fields, etc.). Network models have been used for multiscale phenomena in a variety of contexts such as rubber elasticity, soft multifunctional materials, biological materials, and even the curing of polymers. For decades, a myriad of polymer network models have been developed with differing numbers of chains, arranged in different ways, and with differing symmetries. To complicate matters further, there are also competing assumptions for how macroscopic variables (e.g. deformation) are related to individual chains within the network model. In this work, we propose a simple, intuitive assumption for how the network locally rotates relative to applied loading (e.g. stresses, external fields) and show that this assumption unifies many of the disparate polymer network models–while also recovering one of the most successful models for rubber elasticity, the Arruda–Boyce 8-chain model. The new assumption is then shown to make more intuitive predictions (than prior models) for stimuli-responsive networks with orientational energies (e.g. electroactive polymers), which is significant for shape morphing and designing high degree of freedom actuators (e.g. for soft robotics). Lastly, we unveil some surprising consequences of the new model for the phases of multistable biopolymer and semi-crystalline networks.