RESUMO
N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules.
RESUMO
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.
