Phase Behavior of N-(Isopropyl)propionamide in Aqueous Solution and Changes in Hydration Observed by FTIR Spectroscopy

The phase behavior of N-(isopropyl)propionamide (NiPPA), which is the repeat unit of poly(N-isopropyl-acrylamide) (PNiPA), in deuterated aqueous solution was investigated. Temperature induces a phase separation of NiPPA in aqueous solution above the lower critical solution temperature (LCST), as shown by optical microscopy. The phase behavior of NiPPA resembles that of PNiPA, but the demixing domain is much narrower. Monitoring the liquid−liquid phase separation by Fourier transform infrared (FTIR) spectroscopy reveals that a fraction of the NiPPA molecules becomes dehydrated above the LCST. Our findings therefore shed new light on the results of a recent dielectric relaxation experiment in which the behavior of NiPPA was found to be “completely contrary” to that of PNiPA. It is argued that the differences in the spectroscopic results of polymer and repeat unit solutions can be easily understood from the phase behavior of NiPPA and PNiPA.