Pressure-Dependence of Water Dynamics in Concentrated Aqueous Poly(<i>N</i>-isopropylacrylamide) Solutions with a Methanol Co-solvent

The application of high pressure has a substantial effect on the phase behavior of the thermoresponsive polymer poly(N-isopropyl acrylamide) (PNIPAM). In a water-methanol mixture the one phase region is hugely expanded along the pressure axis in comparison with purely aqueous solutions. The water dynamics of a 25 wt % PNIPAM 80:20 v/v H₂O/CD₃OD solution are probed over wide temperature ranges around the respective cloud points at both 0.1 and 200 MPa. Quasi-elastic neutron scattering reveals the relative fractions and relaxation rates of bulk and hydration water during the reversal of co-nonsolvency at high pressure. At atmospheric pressure, the susceptibility spectra are in agreement with preferential adsorption of methanol on the chains far below the cloud point temperature, while bound water is released at the transition. On the other hand, at 200 MPa the polymer chains are more hydrated on the cost of methanol while dehydration sets in gradually at the cloud point. The pressure dependence of the relaxation time of the bulk water diffusive mode reflects the release of methanol from the polymer chains altering the effective solvent composition.