# Water-polymer coupling induces a dynamical transition in microgels

## TL;DR

This study uses molecular dynamics simulations to reveal that water-polymer hydrogen bonding drives the dynamical transition in PNIPAM microgels, highlighting the importance of water-macromolecule coupling in hydrated systems.

## Contribution

It provides a molecular-level understanding of the water-polymer coupling responsible for the dynamical transition in non-biological macromolecules.

## Key findings

- Hydrogen bonding between water and PNIPAM is key to the transition.
- Below the transition temperature, methyl rotations and hydrogen bonds dominate dynamics.
- Water-macromolecule coupling is crucial across hydrated systems.

## Abstract

The long debated protein dynamical transition was recently found also in non-biological macromolecules, such as poly-N-isopropylacrylamide (PNIPAM) microgels. Here, by using atomistic molecular dynamics simulations, we report a description of the molecular origin of the dynamical transition in these systems. We show that PNIPAM and water dynamics below the dynamical transition temperature Td are dominated by methyl group rotations and hydrogen bonding, respectively. By comparing with bulk water, we unambiguously identify PNIPAM-water hydrogen bonding as the main responsible for the occurrence of the transition. The observed phenomenology thus crucially depends on the water-macromolecule coupling, being relevant to a wide class of hydrated systems, independently from the biological function.