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

The temperature of crystallization and of melting of water in poly(vin yl alcohol) (PVA) membranes with various degrees of acetylation have b een measured at different cooling and heating rates, and compared to t he glass transition temperature (T-g) of the swollen materials. Above a certain critical concentration of water, c, when water begins to cr ystallize 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 segregatio n of water in the amorphous phase during the process of crystallizatio n. Finally, the effects of melting temperature depression and of broad ening of the melting peak of ice in PVA-water systems, very similar to those observed in saccharose-water and alcohol-water mixtures, are ex plained by the phenomenon of dissolution, and not by the confinement e ffect.