Relaxation Spectra of Polymer Associated and Bulk Solvent in Poly(N-isopropylacrylamide) Water / Methanol Solutions

The hydration behavior of the responsive polymer Poly(N-isopropylacrylamide) (PNIPAM) provides a driving force in its demixing transition, and it is crucial for a molecular understanding of the co-nonsolvency effect of PNIPAM, i.e. the depression of the cloud point in water-methanol mixtures. We present an analysis of quasi-elastic neutron scattering (QENS) measurements^1,2 of 25 wt % PNIPAM in water / methanol solvents across the demixing transition in terms of susceptibility spectra which allows for a clear separation of the various relaxation processes in the dynamic structure factor. Through contrast variation by employing deuterated solvent components the contributions from water and methanol can be distinguished. In addition to the bulk solvent processes the susceptibility spectra show two slower relaxation processes that are attributed to polymer associated water and methanol. Across the transition bound solvent is released which influences the effective bulk solvent composition.