3D observations discover a new paradigm in rubber elasticity

TL;DR

This study reveals that rubbers and similar polymers exhibit significant local volume changes during deformation, challenging classical assumptions, and introduces a revised thermodynamic framework based on 3D X-ray observations.

Contribution

The paper introduces a novel 3D X-ray measurement approach that uncovers volume changes in rubbers, leading to a revised understanding of their elasticity.

Findings

Rubbers undergo significant local volume changes during deformation.

Overall specimen volume remains constant despite local volume variations.

The presence of a mobile phase explains negative local bulk moduli.

Abstract

The mechanical response of rubbers has been ubiquitously assumed to be only a function of the imposed strain. Using innovative X-ray measurements capturing the three-dimensional spatial volumetric strain fields, we demonstrate that rubbers and indeed many common engineering polymers, undergo significant local volume changes. But remarkably the overall specimen volume remains constant regardless of the imposed loading. This strange behaviour which also leads to apparent negative local bulk moduli is due to the presence of a mobile phase within these materials. Combining X-ray tomographic observations with high-speed radiography to track the motion of the mobile phase we have revised classical thermodynamic frameworks of rubber elasticity. The work opens new avenues to understand not only the mechanical behaviour of rubbers but a large class of widely used engineering polymers.