Unconstrained dynamic gel swelling generates transient surface deformations

TL;DR

This study investigates how internal network constraints and external diffusive pressure influence transient surface deformations during the unsteady-state swelling of polymer gels, revealing mechanisms to control dynamic surface properties.

Contribution

It introduces a framework for manipulating swelling kinetics and surface deformations in poly(ethylene glycol) gels through network design and solvent choice, focusing on the unsteady-state regime.

Findings

Fewer internal constraints lead to more solvent imbibition over time.

Swelling in ethanol increases time to equilibrium while reducing solvent uptake.

Surface features relax rapidly within five minutes, with their persistence affected by solvent quality.

Abstract

Polymer gels are comprised of a three-dimensional, cross-linked network that can typically withstand the mechanical deformation associated with both swelling and de-swelling. Thus, gels can be designed with smart behaviors that require both stress generation and dissipation, making them well-suited to many applications including membrane technology, water capture devices, and drug delivery systems. In contrast to the fully swelled equilibrium state, limited research characterizes the unsteady-state swelling regime prior to equilibrium. It is in this regime where unique surface deformations can occur. Here we show how internal network constraints and external diffusive pressure can be leveraged to manipulate swelling kinetics and surface deformations in poly(ethylene glycol) gels during unconstrained, three-dimensional swelling. We find that increasing cross-linker molecular weight and swelling in ethanol, as opposed to water, are both effective routes to increase the time it takes to reach equilibrium but do so through different mechanisms. Networks with fewer internal constraints, manipulated via cross-linker chain-length, imbibe more solvent over a longer time. In contrast, swelling in ethanol reduces the amount of solvent imbibed by the network while increasing the time to reach equilibrium. Measurements of surface patterns during swelling establishes that an immediate, fast relaxation at the surface occurs during the first five minutes of swelling. However, the density and persistence of these features varies with solvent quality. These results serve establish a framework for how soft materials undergo dynamic deformation. Engineering transient surface properties while mitigating unwanted instabilities opens the door for emerging technologies such as smart anti-fouling and sensors.