Bound and Bulk Methanol Relaxation of Poly(N-isopropylacrylamide) in a Water-Methanol Mixture

The competition between water and methanol for binding to a thermoresponsive polymer constitutes a fundamental aspect of the co-nonsolvency effect, where the addition of a cosolvent such as methanol to an aqueous polymer solution may lead to a strong reduction of the cloud point and reentrant behavior. The dynamics of water bound to the polymer has been thoroughly studied, however the association of a methanol cosolvent with the polymer chain has been far less characterized. We present an analysis of quasi-elastic neutron scattering (QENS) measurements conducted on a 25 wt% poly(N-isopropylacrylamide) (PNIPAM) solution in water/methanol solvent across the demixing transition. By measuring the signal in solvents differing only by their isotopic composition, the contribution of methanol relative to that of water, i.e. their mutual dynamics, was determined. Analysis of QENS spectra of PNIPAM in a D₂O-methanol mixture over a wide frequency range reveals an additional contribution at lower frequencies that may be attributed to methanol associated with the polymer chain, in addition to the bulk diffusion peak.