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

摘要

The effect of composition and cooling rate on polymorphic occurrence and crystal- lization 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,p and P' polymorphs were observed at cooling rates of 0.5 K/min and less,and the a 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 a and beta' forms.However,when cooling at 1 K/min and higher,the double peaks appear to arise from the crystallization of two alpha 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 alpha form show that the amount of supercooling at the onset of crystallization is virtually independent of com- position.However,peak width information shows that,once started,pure-component samples crystallize faster than mixtures.