Kinetic study on polymorphic solid-state transformation of organic compounds: Bisphenol A and DL-methionine

Solid-state polymorphic transformation of bisphenol A (BPA) and DL-methionine (DL-Met) crystals was carried out experimentally with XRD and DSC. The kinetics and the governing steps of transformation were discussed on the basis of a model proposed previously by the authors. For BPA, the transformation was found to be controlled by nucleation and growth of the stable polymorph in the early period followed by the growth step after sufficient nucleation had occurred, while the transformation of DL-Met was solely controlled by the growth step. The difference in the governing steps between BPA and DL-Met was attributed to the numbers of nuclei of the stable phase present in the initial crystals. This was supported by additional experiments with mechanically stressed BPA crystals in which the governing step of the transformation was changed to be growth-Iimited. These results showed the validity of the proposed model of solid-state transformation to describe different types of transformation behaviors.