Phase Behavior of cis–trans Mixtures of Double-Decker Shaped Silsesquioxanes for Processability Enhancement

Closed double-decker shaped silsesquioxanes (DDSQ-(Ph)8-2((Me)(R))) with R as phenyl, para-phenylamine, and para-phenylethynyl phenyl were synthesized. Isolation of nearly pure trans and cis isomers was obtained by fractional recrystallization. Crystallographic and thermal characteristics of these isolated isomers were obtained by X-ray diffraction of a single crystal and differential scanning calorimetry (DSC). It was observed that melting temperature increases as the size of the R group decreases from para-phenylethynyl phenyl to phenyl and the magnitude of the entropy difference at melting increases as the size of the R group increases. Isolated isomers were then mixed to different cis-to-trans ratios, and their thermal characteristics investigated by DSC. The upper portion of the phase diagram was constructed for these DDSQ compounds using results from DSC traces. Interestingly, cis and trans isomers of these DDSQ compounds form a binary eutectic. Experimentally observed eutectic composition and temperature were found to be close to the calculated values based on the ideal eutectic mixing rule. These data allow users to broaden the thermal processability window by reducing the system melting temperature without affecting the reaction onset temperature of the functional groups.