Water dynamics in poly(vinyl pyrrolidone)-water solution before and after isothermal crystallization

In this work we have studied the low-temperature dynamics of water molecules after isothermal crystallization at different temperatures (Tc) and times (tc) of an aqueous solution of poly(vinyl pyrrolidone) (PVP) by broadband dielectric spectroscopy (BDS) and optical microscopy. Three different water concentrations (cw = 45, 50 and 55 wt.%) were explored. Two major different phases were observed by optical microscopy during crystallization: one related with the core ice formed during crystallization and the other one related with the amorphous phase. Between them a rich interface was also seen. By BDS, two dielectric processes (I and II) are observed in the amorphous samples whereas, in the partially crystallized samples a third relaxation (process III) emerges in the dielectric spectra. Process II is related with the rotational dynamics of water molecules in the amorphous phase with similar characteristics to that found in the complete amorphous solutions whereas the process III is attributed to the rotation of water molecules at the interface between the ice cores and the amorphous water-PVP phase. Typical characteristics of these processes on the relaxation map are discussed.