Water Dynamics in Poly(N-isopropyl acrylamide) Solutions at Ambient and High Pressure Probed with Quasi-elastic Neutron Scattering

We investigate the dynamic behavior of hydration water in a 25 wt% aqueous Poly(N-isopropyl acrylamide) (PNIPAM) solution in dependence on temperature (25 – 50 ^oC) and pressure (0.1 – 130 MPa) employing quasi-elastic neutron scattering (QENS). The susceptibility spectra span the frequency range from 2 GHz to 2 THz at momentum transfers between 0.7 to 1.7 A^-1 and reveal the relaxation peak of the hydration water near 10 GHz, in addition to the known diffusive and effective local and vibrational processes of bulk water. Evaluating the temperature dependence, we find that, at atmospheric pressure, the relative population of (bound) hydration water sharply decreases upon heating from the one-phase to the two-phase state, i.e. the chains dehydrate strongly. In contrast, at 130 MPa, no sharp decrease is observed, i.e. the dehydration takes place over a much broader temperature range. This suggests an enhanced hydrophobic hydration at high pressure.