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Abstract
A detailed thermotropic study of 1-deoxy-1-(N-methyloctanamido)-D-glucitol (MEGA-8) revealed three crystalline forms of MEGA-8, formed at different conditions of solvent and temperature. Thermal polarizing microscopy was used to visualize birefringent solvent crystals (CS ), needle crystals (CN ), and terraced crystals (CT ) and to study their nucleation requirements. Differential scanning calorimetry (DSC) determined the melting temperatures (CT = 78.4°C, CS = 81.0°C, CN = 87.5°C) and enthalpies of each crystalline phase transition to the isotropic phase. DSC was also used to study the kinetics of the phase transformations, and a Gibbs free diagram was constructed that shows CN and the isotropic phase as the lowest free energy forms over the temperature range 25–100°C. Powder X-ray diffraction confirmed three crystalline forms that melt to the isotropic phase at their characteristic temperatures. When the isotropic melt was cooled to room temperature, a smectic liquid crystalline intermediate appeared before recrystallization of CT , as observed by both X-ray diffraction and polarizing microscopy.