Atomic scale investigation of the volume phase transition in concentrated PNIPAM microgels

TL;DR

This study combines neutron scattering, calorimetry, and simulations to investigate the volume phase transition in concentrated PNIPAM microgels, revealing how it varies with concentration and confirming the formation of an attractive gel state.

Contribution

It provides a comprehensive multi-technique analysis of the VPT in highly concentrated PNIPAM microgels, including experimental and simulation insights into the transition mechanism.

Findings

VPT appears as a sharp drop in hydrogen atom mean square displacement.

The VPT becomes smoother as dry conditions are approached.

VPT temperature exhibits a minimum at about 43 wt%.

Abstract

Combining elastic incoherent neutron scattering and differential scanning calorimetry, we investigate the occurrence of the volume phase transition (VPT) in very concentrated PNIPAM microgel suspensions, from a polymer weight fraction of 30~wt\% up to dry conditions. Although samples are arrested at the macroscopic scale, atomic degrees of freedom are equilibrated and can be probed in a reproducible way. A clear signature of the VPT is present as a sharp drop of the mean square displacement of PNIPAM hydrogen atoms obtained by neutron scattering. As a function of concentration, the VPT gets smoother as dry conditions are approached whereas the VPT temperature shows a minimum at about 43~wt\%. This behavior is qualitatively confirmed by calorimetry measurements. Molecular dynamics simulations are employed to complement experimental results and gain further insights into the nature of the VPT, confirming that it involves the formation of an attractive gel state between the microgels. Overall, these results provide evidence that the VPT in PNIPAM-based systems can be detected at different time- and length-scales as well as in overcrowded conditions.