Insight into cold- and melt crystallization phenomena of a smectogenic liquid crystal

Abstract

This article presents the insight into the complex crystallization phenomena of the (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy) pentyl-1-oxy]-benzoate chiral smectogenic liquid crystal. The kinetics of cold crystallization under non-isothermal conditions is described by Ozawa, Mo, Kissinger and Augis–Bennett models, and under isothermal conditions it is treated as a two-route process in terms of a modified Avrami model. Our research reveals two different mechanisms of the non-isothermal cold crystallization: driven by the diffusion rate for slow heating and by the nucleation for fast heating. The dimensionality of the crystallites is different depending on the applied heating rate of the sample. The isothermal cold crystallization is a two-route process, in which the mechanism of the first route corresponds to three-dimensional crystal growth and takes place faster than the second route with two-dimensional crystal phase growth. The kinetics of both routes depends primarily on diffusion rates. On the other hand, the slower heating rate of the sample to the cold crystallization temperature causes the isothermal cold crystallization to become a one-route process, and its kinetics is well described by the traditional Avrami model. Additionally, the melt crystallization under isothermal conditions is also discussed. The obtained results indicate isotropic three-dimensional crystal growth and provide nucleation-controlled crystallization kinetics. In the infrared spectra, the bands associated with CO and C–O–C stretching as well as C–H bending vibrations are susceptible to the structural changes occurring during isothermal melt crystallization. The correlation between the characteristic crystallization times and relaxation times of the α-process shows which factors (thermodynamic driving force or molecular mobility) determined the crystal growth.