[Paper Review] Independent characterization of the elastic and the mixing parts of hydrogel osmotic pressure

Zefan Shao, Qihan Liu|arXiv (Cornell University)|Aug 8, 2023

Hydrogels: synthesis, properties, applications4 citations

TL;DR

This study introduces a novel fully constrained swelling test to independently measure the elastic and mixing contributions to hydrogel osmotic pressure. By varying crosslink density at constant polymer content, the method decouples elastic and mixing effects, revealing that the Flory-Rehner model accurately predicts the mixing part but fails by orders of magnitude in predicting the elastic component for polyacrylamide hydrogels across diverse swelling ratios.

ABSTRACT

Osmotic pressure is the driving force for the swelling of hydrogels. The hydrogel osmotic pressure can be decomposed into two parts: the mixing part due to polymer-solvent interaction and the elastic part due to polymer chain stretching. While the two components are distinguished in existing constitutive models, they have not been independently characterized in experiments. This paper reports a novel method to independently measure these two components using a fully constrained swelling test. The test allows the crosslink density to be varied at a fixed polymer content, thus varying the elastic part independently of the mixing part. Our measurement shows that the widely used Flory-Rehner model predicts the mixing part accurately for polyacrylamide hydrogel of a wide range of swelling ratios but predicts the elastic part with orders-of-magnitude error.

Motivation & Objective

• To independently measure the elastic and mixing contributions to hydrogel osmotic pressure, which are typically conflated in experiments.
• To develop an experimental method that decouples crosslink density variation from polymer concentration, enabling independent control of elastic and mixing effects.
• To test the accuracy of the widely used Flory-Rehner model in predicting both the elastic and mixing components of osmotic pressure.
• To quantify the deviation of theoretical predictions from experimental measurements for polyacrylamide hydrogels across a range of swelling ratios.

Proposed method

• A fully constrained swelling test is employed, where hydrogel samples are confined to prevent volume change during swelling, isolating osmotic pressure contributions.
• Crosslink density is systematically varied while maintaining constant polymer concentration, allowing independent tuning of the elastic contribution without altering the mixing contribution.
• The osmotic pressure is measured as a function of swelling ratio under these constrained conditions, enabling direct separation of elastic and mixing components.
• The Flory-Rehner model is applied to the experimental data to compare theoretical predictions with measured values for both components.
• The method relies on the assumption that the mixing part depends only on polymer-solvent interactions and swelling ratio, while the elastic part is governed by network elasticity and crosslink density.

Experimental results

Research questions

• RQ1 Can the elastic and mixing contributions to hydrogel osmotic pressure be experimentally separated using a controlled swelling protocol?
• RQ2 To what extent does the Flory-Rehner model accurately predict the mixing component of osmotic pressure in polyacrylamide hydrogels?
• RQ3 How does the Flory-Rehner model perform in predicting the elastic component of osmotic pressure across varying swelling ratios?
• RQ4 What is the role of crosslink density in decoupling elastic and mixing effects in hydrogel swelling?

Key findings

• The Flory-Rehner model accurately predicts the mixing part of osmotic pressure across a wide range of swelling ratios in polyacrylamide hydrogels.
• The Flory-Rehner model underestimates the elastic part of osmotic pressure by orders of magnitude, indicating a fundamental discrepancy in the theoretical treatment of network elasticity.
• The experimental method successfully decouples elastic and mixing contributions, enabling independent quantification of each component.
• The study demonstrates that crosslink density can be varied independently of polymer concentration, validating the method's ability to isolate the elastic contribution.