Extreme thermodynamics with polymer gel tori: harnessing thermodynamic instabilities to induce large-scale deformations

TL;DR

This paper explores how rapid heating of polymer gel tori induces phase separation and stress-driven buckling, demonstrating a novel way to actuate large-scale deformations through thermodynamic instabilities.

Contribution

It reveals how thermodynamic instabilities can be harnessed to induce controlled mechanical deformations in polymer gels, especially in toroidal shapes.

Findings

Rapid heating causes phase separation and inhomogeneous stress.

Stress induces buckling of the torus out of plane.

Thermodynamic instabilities can be used for actuation.

Abstract

When a swollen, thermoresponsive polymer gel is heated in a solvent bath, it expels solvent and deswells. When this heating is slow, deswelling proceeds homogeneously, as observed in a toroid-shaped gel that changes volume whilst maintaining its toroidal shape. By contrast, if the gel is heated quickly, an impermeable layer of collapsed polymer forms and traps solvent within the gel, arresting the volume change. The ensuing evolution of the gel then happens at fixed volume, leading to phase-separation and the development of inhomogeneous stress that deforms the toroidal shape. We observe that this stress can cause the torus to buckle out of the plane, via a mechanism analogous to the bending of bimetallic strips upon heating. Our results demonstrate that thermodynamic instabilities, i.e., phase transitions, can be used to actuate mechanical deformation in an extreme thermodynamics of materials.