Study of the polymorphism and the crystallization kinetics of tripalmitin: A microscopic approach

The morphology and kinetics of crystallization of tripalmitin have been examined in detail by optical microscopy. The α‐crystallization process is characterized by a fast heterogeneous nucleation and spherulitic growth, even at low undercooling, resulting in intense birefringence and smooth spherulitic entities. Four different β’‐microstructures have been found—grainy, fibrous, feathery and lamellar. Around 47°C, a clear change from a grainy to a fibrous β’‐microstructure is observed. This transition seems to take place without a drastic change in nucleation or in crystal growth. At 50°C, both nucleation and crystal growth exhibit a clear discontinuity, indicating interference from β‐crystallization. Around 52°C, the β’‐form changes again from a fibrous to a more feathery microstructure; the transition is accompanied by a distinct decrease in crystal growth rate. The lamellar β’‐structure exhibits the highest stability and can be obtained only via an accelerated nucleation at low temperature, followed by further growth at elevated temperature near the melting temperature of the β’‐form. Determination of the β‐form on the basis of its microstructure is not always precise, because the microstructure strongly depends on whether the β‐crystals are obtained from a transformation of α or β’, or whether β‐crystallization occurs directly from the melt. Clear confirmation of the polymorphic nature of the solid state can be obtained from melting point determination.