Water-Polymer Coupling Induces a Dynamical Transition in Microgels

The long debated protein dynamical transition was recently found also in nonbiological 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 mainly 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.