Cooperative dehydration is a major driving force in the demixing transition of thermoresponsive polymers, causing chain collapse followed by aggregation. For the co-nonsolvency effect preferential polymer solvation is another key factor. Employing quasi-elastic neutron scattering (QENS) over a frequency range of four decades, we investigate the water dynamics in concentrated poly(N-isopropylacrylamide) (PNIPAM) solutions and the influence of a co-solvent across the cloud point Tcp at pressures of 0.1 and 200 MPa [1,2]. 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. At high pressure, the fraction of bound water decreases gradually with increasing temperature. For PNIPAM in a 80% H2O / 20% methanol mixture the one-phase region is hugely expanded along the pressure axis. QENS experiments point to dominant methanol adsorption at atmospheric pressure, whereas water adsorbs preferentially on the chains at high pressure. Release and adsorption of solutes by the polymer chains correlate with a change in effective solvent composition as evidenced by the diffusive properties of bulk water.
 B.-J. Niebuur, W. Lohstroh, M.-S. Appavou, A. Schulte, C. M. Papadakis, Macromolecules 52, 1942 (2019)
 B.-J. Niebuur, W. Lohstroh, C.-H. Ko, M.-S. Appavou, A. Schulte, C. M. Papadakis, Macromolecules 54, 4387 (2021)
Dr. Jitae Park
Dr. Dominic Hayward