GLASS-TRANSITION TEMPERATURE REGULATION EFFECT IN A POLY(VINYL ALCOHOL)-WATER SYSTEM

The temperature of crystallization and of melting of water in poly(vinyl alcohol) (PVA) membranes with various degrees of acetylation have been measured at different cooling and heating rates, and compared to the glass transition temperature (T-g) of the swollen materials. Above a certain critical concentration of water, c*, when water begins to crystallize upon cooling, the T-g does not decrease according to the Fox equation, but remains constant. This phenomenon of regulation of the T-g and the origin of c* are explained by the phenomenon of segregation of water in the amorphous phase during the process of crystallization. Finally, the effects of melting temperature depression and of broadening of the melting peak of ice in PVA-water systems, very similar to those observed in saccharose-water and alcohol-water mixtures, are explained by the phenomenon of dissolution, and not by the confinement effect.