Characterizing hydrogel behavior under compression with gel-freezing osmometry

TL;DR

This paper introduces Gel-Freezing Osmometry (GelFrO), a novel technique for precisely measuring hydrogel mechanical and osmotic properties across a wide water content range, revealing power-law behaviors linked to fractal network structures.

Contribution

The study develops GelFrO, enabling small-sample, wide-range characterization of hydrogels, and connects fractal microstructure to their mechanical and osmotic responses.

Findings

Hydrogels exhibit power-law behavior in compression and osmotic pressure.

Classical Flory-Huggins theory does not fully capture observed behaviors.

Fractal structure influences hydrogel mechanical and osmotic properties.

Abstract

Hydrogels are particularly versatile materials that are widely found in both Nature and industry. One key reason for this versatility is their high water content, which lets them dramatically change their volume and many of their mechanical properties -- often by orders of magnitude -- as they swell and dry out. Currently, we lack techniques that can precisely characterize how these properties change with water content. To overcome this challenge, here we develop Gel-Freezing Osmometry (GelFrO): an extension of freezing-point osmometry. We show how GelFrO can measure a hydrogel's mechanical response to compression and osmotic pressure, while only using small, $O(100\mu$L$)$ samples. Because the technique allows measurement of properties over an unusually wide range of water contents, it allows us to accurately test theoretical predictions. We find simple, power-law behavior for both mechanical response to compression, and osmotic pressure, while these are not well-captured by classical Flory-Huggins theory. We interpret this power-law behavior as a hallmark of a microscopic fractal structure of the gel's polymer network, and propose a simple way to connect the gel's fractal dimension to its mechanical and osmotic properties. This connection is supported by observations of hydrogel microstructures using small-angle x-ray scattering. Finally, our results motivate us to propose an updated constitutive model describing hydrogel swelling, and mechanical response.