Measurement of Dynamic Light Scattering Intensity in Gels

TL;DR

This paper demonstrates that dynamic light scattering can reliably measure the thermodynamic fluctuations in gels, providing insights into osmotic pressure and polymer-gel interactions, with considerations for scattering effects and instrumental corrections.

Contribution

It shows that DLS can be used to measure scattered light intensity in gels, revealing thermodynamic information and polymer interactions, which was previously underexplored.

Findings

DLS yields reliable measurements of scattered light intensity in gels.

Separation of heterodyne components allows accurate osmotic pressure estimation.

Gel environment alters virial coefficients, indicating polymer-gel interactions.

Abstract

In the scientific literature little attention has been given to the use of dynamic light scattering (DLS) as a tool for extracting the thermodynamic information contained in the absolute intensity of light scattered by gels. In this article we show that DLS yields reliable measurements of the intensity of light scattered by the thermodynamic fluctuations, not only in aqueous polymer solutions, but also in hydrogels. In hydrogels, light scattered by osmotic fluctuations is heterodyned by that from static or slowly varying inhomogeneities. The two components are separable owing to their different time scales, giving good experimental agreement with macroscopic measurements of the osmotic pressure. DLS measurements in gels are, however, tributary to depolarised light scattering from the network as well as to multiple light scattering. The paper examines these effects, as well as the instrumental corrections required to determine the osmotic modulus. For guest polymers trapped in a hydrogel the measured intensity, extrapolated to zero concentration, is identical to that found by static light scattering from the same polymers in solution. The gel environment modifies the second and third virial coefficients, providing a means of evaluating the interaction between the polymers and the gel.