Probing Hydration Changes of Perdeuterated Poly(N-isopropylacrylamide) Across the Demixing-Transition with Vibrational Spectroscopy

Replacing hydrogen with deuterium can have a profound effect on the phase behavior of biological and synthetic polymers. We investigate concentrated aqueous solutions of perdeuterated poly(N-isopropylacrylamide) (PNIPAM-d10) employing Raman scattering and infrared absorption spectroscopy. In comparison to PNIPAM the cloud point temperature increases by several degrees in PNIPAM-d10. The C-D vibrational bands of the alkyl groups are shifted to lower frequencies by a factor of 1.4 as expected for the isotope effect. In variable temperature experiments we observe an abrupt red-shift of the peak frequencies of the C-D bands at the demixing transition similar to those of the C-H bands in PNIPAM. These frequency shifts may be attributed to dehydration of the polymer chains at their coil-to-globule transition.