Determination of potentially homogeneous-nucleation-based crystallization in<i>o</i>-terphenyl and an interpretation of the nucleation-enhancement mechanism

Abstract

A homogeneous-nucleation-based crystallization was found in o-terphenyl in the glass-transition temperature region with an adiabatic calorimeter, and the crystallization process was investigated by direct microscopic observation. The crystallization showed its maximum rate at 248 and ceased at 250 K, while the ordinary crystal-growth process was observed to proceed only above 255 K. The two crystals formed at 248 and 297 K showed the same x-ray-diffraction patterns, indicating that they were in the same crystalline phase. The nucleation-based crystallization was observed in the temperature range of 225 to 250 K as advance of the crystal front into the liquid phase under the microscope, and the crystalline phase exhibited the appearance of the aggregation of fine crystallites which was consistent with the presence of residual entropy and the premelting property. From these results, the crystallization process was interpreted to proceed through the coalescence of crystal embryos into the crystalline phase on the liquid-crystal interface. It was concluded that the decrease in the effective interfacial energy of the embryo due to the coalescence produced an enhancement of the crystal nucleation, and that the enhancement mechanism must have played an essential role for the macroscopically observable crystallization below 250 K.