Polymorphic occurrence and crystallization rates of tristearin/tripalmitin mixtures under non‐isothermal conditions

The effect of composition and cooling rate on polymorphic occurrence and crystallization rates in binary mixtures of tripalmitoylglycerol (PPP) and tristearoylglycerol (SSS) has been studied using differential scanning calorimetry (DSC), light microscopy and X-ray diffraction under constant cooling rates. In general, β and β' polymorphs were observed at cooling rates of 0.5 K/min and less, and the α form at higher cooling rates. Double peaks were observed in the DSC exotherms of mixtures containing more than 30% SSS. The X-ray data showed that, when cooling at 0.5 K/min, the double peaks correspond to the crystallization of the α and β' forms. However, when cooling at 1 K/min and higher, the double peaks appear to arise from the crystallization of two a phases - one PPP rich and the other SSS rich. Light microscopy experiments also show two distinct morphologies, which corroborates this view. The apparent SSS-rich phase crystallizes as irregular spherulites whereas the apparent PPP-rich phase forms a fine crystal mass. Analyses of thermograms relating to the α form show that the amount of supercooling at the onset of crystallization is virtually independent of composition. However, peak width information shows that, once started, pure-component samples crystallize faster than mixtures.