Polymorphism of POS. II. kinetics of melt crystallization

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Abstract
Melt crystallization of four polymorphs of POS, α,δ, pseudo‐β′ andβ, was examined with pure samples (>99.9%). Induction time, τ, for newly occurring crystals was measured with a polarizing microscope equipped with a temperature‐controlled growth cell. Rate of crystallization, 1/τ, was obtained for each polymorph, whose identification was done with x‐ray diffraction (XRD) and differential scanning calorimetry (DSC). Two modes of crystallization, melt cooling and melt mediation, were applied. From these experiments, the following conclusions were obtained: (i) The rate of melt‐mediated crystallization was always higher than of simple melt cooling; (ii) the pseudo‐β′ form was crystallized in a wider range of temperature than the less stable δ form; (iii) the occurrence behavior of the polymorphs differed between simple melt cooling and melt mediation; (iv) the δ form was crystallized only by simple melt cooling in a narrow range of temperature, 25.5°C∼28.3°C. This means that there is a possibility that δ may result from racemic compounds that are crystallized in a specific manner. The experimental results are discussed in comparison to 1,3‐dipalmitoyl2‐oleoylglycerol (POP), 1,3‐distearoyl‐2‐oleoylglycerol (SOS) and cocoa butter.

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