COMPARISON OF ISOTHERMAL AND NONISOTHERMAL TTT-CURVES IN THE SYSTEM 2,4-PENTANEDIOL IN WATER

The system 2,4-pentanediol-water is shown to form glasses for concentr ations above 30% by weight for cooling rates above 80 degrees C min(-1 ). Below this concentration, ice crystallizes during the initial cooli ng. Ice crystallization is calorimetrically recorded in isothermal and nonisothermal conditions during either the initial cooling from the m elt or the subsequent warming from the glassy state for glass-forming concentrations. Time-Temperature-Transformation (TTT) curves for a 0.1 crystallization fraction have been constructed. For 35% w/w 2,4-penta nediol in water, two ''noses'' are recorded for isothermal conditions. They are identified with the thermal ranges as being the consequences of homogeneous and heterogeneous ice nucleation. A comparison of isot hermal and non-isothermal conditions is also presented to underline th e difficulty of applying the usual ''nose method'' for the determinati on of the critical cooling and warming rates. The Johnson-Mehl-Avrami model is used for the analysis of the isothermal ice crystallization. comparison of the parameters calculated after cooling or after warming from the glassy state shows that the homogeneous nucleation occurs at a constant rate and that the heterogeneous nucleation occurs as appro ximately the inverse square root of the time rate.