Pressure Dependence of Water Dynamics in Concentrated Aqueous Poly(N-isopropylacrylamide) Solutions with a Methanol Cosolvent

The water dynamics of a concentrated poly(N-isopropylacrylamide) (PNIPAM) solution in an 80:20 v/v water/methanol mixture are investigated across the cloud point Tcp at atmospheric and high pressure (200 MPa). Quasi-elastic neutron scattering (QENS) reveals the relaxation spectra of bulk and hydration water over a frequency range of 4 decades (GHz to THz) and their changes upon the reversal of co-non-solvency at high pressures. At atmospheric pressure, the susceptibility spectra in the one-phase region provide evidence of polymer-bound water, which is released in part nearly discontinuously at Tcp. Simultaneously, the elastic line strength increases abruptly, which is consistent with a coil-to-globule transition. At high pressures, the fraction of bound water decreases gradually with increasing temperature, while the elastic line strength increases at Tcp. Comparing the bulk diffusion time τd of water in the one-phase region with its values from the neat solvent mixture, dominant methanol adsorption is found at atmospheric pressure, whereas water adsorbs preferentially on the chains at high pressures. At 0.1 MPa, the relaxation time of bound water in the one-phase region near Tcp is smaller than that at 200 MPa, i.e., the binding strength between water and PNIPAM is weakened by the presence of adsorbed methanol. Raman spectroscopy, probing the interaction between the solvent molecules and the methyl groups of PNIPAM, indicates that methanol adsorbs to the hydrophobic groups at atmospheric pressure, while it is diminished at high pressures.