Water Dynamics and Poly(N-isopropyl acrylamide) Co-nonsolvency in Water-Methanol Solutions at Variable Temperature and Pressure

The phase behavior of 25 % poly(N-isopropyl acrylamide) (PNIPAM) in a 80:20 v/v water methanol mixture is investigated by quasi-elastic neutron scattering (QENS) and Raman spectroscopy at variable temperature and pressure with focus on the co-nonsolvency effect. The susceptibility spectra span the frequency range from 2 GHz to 2 THz at momentum transfers between 0.2 to 1.7 A^-1 and reveal the relaxation peak of the hydration water near 10 GHz, in addition to the known processes of bulk water. The solvent phase is enriched with methanol at high pressure, implying that water is preferentially adsorbed at the chains. The exchange of methanol with water takes place mainly at the hydrophilic Amide groups of PNIPAM. The hydrophobic hydration of alkyl groups in the side chain and the backbone of the polymer in the presence of methanol is reduced at ambient and high pressure. In the two-phase region the preferential adsorption of water at the chains is diminished.