Evidence of a low-temperature dynamical transition in concentrated microgels

TL;DR

This study reports the first observation of a low-temperature dynamical transition in nonbiological aqueous systems, specifically in concentrated PNIPAM microgels, indicating a universal feature in complex macromolecular systems.

Contribution

It demonstrates the existence of a protein-like dynamical transition in nonbiological microgels at high concentrations, extending understanding of such transitions beyond biological systems.

Findings

Dynamical transition occurs at ~250 K in concentrated microgels

Transition is independent of PNIPAM mass fraction

Water and polymer dynamics transition simultaneously

Abstract

A low-temperature dynamical transition has been reported in several proteins. We provide the first observation of a `protein-like' dynamical transition in nonbiological aqueous environments. To this aim we exploit the popular colloidal system of poly-N-isopropylacrylamide (PNIPAM) microgels, extending their investigation to unprecedentedly high concentrations. Owing to the heterogeneous architecture of the microgels, water crystallization is avoided in concentrated samples, allowing us to monitor atomic dynamics at low temperatures. By elastic incoherent neutron scattering and molecular dynamics simulations, we find that a dynamical transition occurs at a temperature $T_d\sim250$~K, independently from PNIPAM mass fraction. However, the transition is smeared out on approaching dry conditions. The quantitative agreement between experiments and simulations provides evidence that the transition occurs simultaneously for PNIPAM and water dynamics. The similarity of these results with hydrated protein powders suggests that the dynamical transition is a generic feature in complex macromolecular systems, independently from their biological function.