Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

TL;DR

This study reveals that thermoresponsive microgels undergo a two-step deswelling process driven by electrostatic effects, with experiments and simulations showing a universal behavior that enhances understanding of their volume phase transition.

Contribution

The paper uncovers a two-step deswelling mechanism in microgels caused by electrostatic effects, supported by experiments and simulations, and introduces a predictive master-curve.

Findings

Identification of a minimum in the ratio of gyration to hydrodynamic radii at VPT

Development of a unifying master-curve for two-step deswelling

Electrostatic effects are key to the deswelling mechanism

Abstract

Thermoresponsive microgels are one of the most investigated class of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, particularly regarding the presence and the role of charges in the deswelling process. Here we fill this gap by combining experiments and simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of several microgels with different cross-linker concentrations, charge contents and charge distributions, we build a unifying master-curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on microgel applications ranging from materials to biomedical technologies.