CRYSTALLIZATION ONSET AND POLYMORPHIC OCCURRENCE IN PURE SATURATED TRIACYLGLYCEROLS DURING COOLING FROM THE MELT

Abstract

The effect of cooling rate on crystallization onset temperature and polymorphic occurrence in four pure saturated triacylglycerols, tridodecanoate (LLL), trimyristate (MMM), tripalmitate (PPP) and tristearate (SSS) has been studied using differential scanning calorimetry (DSC), and X-ray diffraction under constant cooling rates. In this study, differential scanning calorimetry (DSC) experiments were conducted to detect the crystallization onset temperatures under 9 different cooling rates between 2.5 C˚ /min and 25 C˚ /min in a random sequence. A MATLAB program was developed to automatically process the bulky DSC data files and calculate the onset, which is the first point that deviated from the heat flow baseline. Time-resolved small and wide-angle X-ray diffraction (SAXD, WAXD) measurements at a synchrotron allowed the identification of polymorphic forms at the onset temperature. X-ray data were fitted to a Voigt distribution to calculate mass fractions  and '. By analyzing an abundance of DSC thermograms, a first onset of crystallization was observed above the melting point of the  polymorph under 1-15 C˚ /min for LLL and 1-5 C˚ /min for MMM. Double peaks were observed in the DSC exotherms of LLL and MMM under slow cooling rates. As evidenced by the X-ray data, the ' polymorph crystallized first, followed by the second peak, corresponding to the formation of the  polymorph. However, this phenomenon was not observed at high cooling rates. The second onset (' onset) of the heat flow signal was identified by fitting an extended Avrami (Weibull) model to the thermograms. A MATALB program was developed to process the data from the first onset until the end of the crystallization. The values of the eight parameters of the extended Avrami model were obtained for the 162 thermograms.