Swelling of responsive-microgels: experiments versus models

TL;DR

This study compares experimental swelling data of IPN microgels with Flory-Rehner theoretical models, revealing that higher-order approximations are needed for sharper volume-phase transitions and demonstrating control of swelling via environmental conditions.

Contribution

It provides a detailed comparison between experimental swelling behavior of IPN microgels and advanced Flory-Rehner models, highlighting the importance of higher-order terms.

Findings

Swelling behavior of PNIPAM microgels aligns with second-order Flory-Rehner theory.

Third-order approximation is necessary to describe sharp volume-phase transitions.

Swelling can be optimized in water at acidic pH.

Abstract

Interpenetrated Polymer Network (IPN) microgels of PNIPAM and PAAc have been investigated and the experimental data have been compared with theoretical models from the Flory-Rehner theory. We confirm that the swelling behavior of PNIPAM microgels is well described by this theory by considering the second order approximation for the volume fraction $\phi$ dependence of the Flory parameter $\chi(\phi)$. Indeed the Volume-Phase Transition (VPT) of the PNIPAM-PAAc IPN microgel at neutral conditions and in D$_2$O solvents can be well described only considering a third-order approximation. Interestingly we empirically find that sharper is the transition higher is the order of the $\chi(\phi)$ relation which has to be considered. Moreover the VPT can be experimentally controlled by tuning the polymer/solvent interactions through pH and solvent allowing to directly modify the delicate balance between energetic and entropic contributions and to explore the swelling behavior in a wide range of environmental conditions. In particular we find that the most advantageous condition for swelling is in water at acidic pH.