Hydration Dynamics of Perdeuterated PNIPAM in Aqueous Solution

Perdeuterated poly(N-isopropylacrylamide) (PNIPAM-d10) is synthesized by free radical polymerization and characterized by 2H NMR, Fourier-transform infrared, and Raman spectroscopy. A concentrated solution of this polymer in H2O is investigated with respect to the dynamics of the hydration and bulk water using quasi-elastic neutron scattering. Due to the exchange of the protons in the polymer with deuterons, the incoherent neutron scattering from the polymer is suppressed, offsetting apart the hydration water. Deuteration causes a remarkable upward shift of the cloud point temperature by 4 °C. It is found that, when heating through the cloud point, the fraction of hydration water decreases abruptly. The relaxation time of the hydration water is consistently lower than the one observed previously for PNIPAM in H2O. Raman spectroscopy of the CD bands across the demixing transition shows a frequency shift consistent with polymer dehydration.