Water Sorption in Amorphous Poly(ethylene terephthalate)

摘要

The water sorption characteristics of poly(ethylene terephthalate) (PET) amorphous samples of 250 #mu#m thickness have been studied at various temperatures in a saturated atmosphere. Concerning diffusivity, one can distinguish the folowing two domains characterized by distinct values of the activation energy: E_D ≈ 36 kJ mol~(-1) at T > 100deg C, and E_D ≈ 42 kJ mol~(-1) at T < 60deg C, with a relatively wide (60-100deg C) intermediary domain linked to the glass transition of the polymer. The crystallization of this latter occurs in the time scale of diffusion above 80deg C but doesn's change the Fickian character of sorption curves. The equilibrium concentration m_(infinity) is an increasing function of temperature, but the solubility coefficient S decreases sharply with this latter, with hte apparent enthalpy of dissolution #DELTA#H_s being of the order of -28 kJ mol~(-1) at T < 80deg C and -45 kJ mol~(-1) at T > 80deg C. Density measurements in the wet and dry states suggest that water is almost entirely dissolved in the amorphous matrix at T < 80deg C but forms partially a separated phase at T > 80deg C. Microvoiding can be attributed to crystallization-induced demixing.