POLYMORPHIC TRANSITIONS IN CRYSTALLINE E-ALPHA AND Z-ALPHA-PHENYLCINNAMIC ACIDS FOLLOWED BY DSC

Polymorphic transitions of alpha-phenylcinnamic acid stereoisomers cry stallized from various solvents (CHCl3, ethanol, diethyl ether), preci pitated with different acids (acetic acid or HCl) or sublimed have bee n studied by differential scanning calorimetry (DSC). Measurements wer e also supplemented by powder X-ray diffractometry (XRD). Polymorphic transitions were not found for any of the E isomer samples irrespectiv e of the method of treatment. However, in heating-cooling cycles rever sible transitions could be detected for the Z isomer. On heating an ex othermic peak was registered on samples sublimed, crystallized from et hanol or diethyl ether before melting, while samples precipitated by H Cl or crystallized from chloroform exhibited more complex transitions. Here, endothermic effects were also observed. Enthalpy values for tra nsformations (either for melting or polymorphic and other transitions) are also given.