Hydration Dynamics in Perdeuterated Poly(N-isopropyl acrylamide) Aqueous Solution Probed with Quasi-elastic Neutron Scattering

We investigate the hydration dynamics of perdeuterated poly(N-isopropylacrylamide) (PNIPAM-d10) in aqueous solution to elucidate the molecular mechanism of its demixing transition. Quasi-elastic neutron scattering (QENS) and Raman spectroscopy were performed on a 25 wt% PNIPAM-d10 solution in H₂O across the demixing transition. There is a significant upward shift in the cloud point by ~ 4 degrees. Deuteration suppresses incoherent scattering from the polymer, allowing a clear view of hydration water dynamics. The dynamic susceptibilities reveal two relaxation processes for bulk and bound water, with the bound fraction decreasing sharply near the transition. The relaxation times of hydration water are consistently lower than in the protiated polymer, indicating faster local dynamics. Raman spectra show abrupt C–D band downshifts at the transition, reflecting side-group dehydration.