Solvent Dynamics in Solutions of PNIPAM in Water/Methanol Mixtures-A Quasi-Elastic Neutron Scattering Study

The solvent dynamics of concentrated solutions of poly(N-isopropylacrylamide) (PNIPAM, 25 wt %) in water/methanol mixtures (85:15 v/v) are measured with the aim of shedding light onto the cononsolvency effect. Quasi-elastic neutron scattering (QENS) with contrast variation has been carried out at temperatures below and above the cloud point by using in the first set of experiments the mixture H₂O:d-MeOD (d-MeOD denotes fully deuterated methanol) as a solvent and in the second set of experiments the mixture D₂O:MeOH (MeOH denotes methanol). As a reference, bulk H₂O, bulk MeOH and the mixtures H₂O:d-MeOD and D₂O:MeOH (both 85:15 v/v) have been investigated as well. In the PNIPAM solution in H₂O:d-MeOD, two water populations are identified, namely strongly and less strongly arrested water. At the cloud point, the former is partially released from PNIPAM. The diffusion coefficient of the latter one is similar to the one in the water/methanol mixture, and its residence time decreases at the cloud point. The PNIPAM solution in D₂O:MeOH reveals similar dynamics to the one in H₂O:d-MeOD which may reflect that the dynamics of MeOH near the PNIPAM chain is similar to the one of H₂O. The similarity may, however, partially be due to H/D exchange between D₂O and MeOH. In both PNIPAM solutions, the mean-square displacement of the PNIPAM chain decreases gradually above the cloud point.