Effects of stereochemistry, mesogenic core and spacer length on crystallization from nematic and isotropic melts of cyclohexane-based glass-forming liquid crystals

Abstract

The crystallization velocity of a series of glass-forming, nematic liquid crystals derived from cyclohexane was determined as a function of temperature between T _g and T _m via the observation of linear spherulitic growth from a nematic as well as an isotropic melt using a polarizing optical microscope. The results suggest that stereochemistry plays a predominant role in maximum crystallization velocity (CV), which was found to occur at (0.93 ± 0.01) T _m for all glass-forming liquid crystals presently investigated. Specifically, the observed maximum CV values follow the order: all-axial > all-equatorial > axial-equatorial mixed modes. Furthermore, the axial-equatorial mixed modes exhibited maximum CV values comparable to that of a typical ‘slowly crystallizing’ polymer, for example, isotactic polystyrene. In addition, it was found that a stronger ncmatogenic core with a higher T _m and T _c and a broader T _m to T _c range tends to depress morphological stability of the resultant GLMLC system by showing a greater maximum CV value. However, the flexible spacer length was found to play a negligible role in affecting the maximum CV value, although both T _g and T _m were found to be depressed whereas T _c significantly elevated with a longer spacer.