Designing the disorder: the kinetics of nonisothermal crystallization of the orientationally disordered crystalline phase in a nematic mesogen

Abstract

This article presents the molecular dynamics and solidification behavior of a 2,3-difluoro-4-propylphenyl 2,3-difluoro-4-(4-pentylcyclohexyl)benzoate nematic liquid crystal (5C4FPB3) observed by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Polarized optical microscopy (POM) is also performed to confirm the phase transition temperatures. Our investigation reveals rare crystallization of the orientationally disordered crystal (ODIC) phase from the nematic phase and a glass transition of the crystal at cooling rates higher than 1 K min⁻¹. The deconvolution of the dielectric spectra with derivative techniques is necessary because of the complex molecular dynamics in the crystalline phase. The BDS method enables us to capture the relaxation processes reflecting pre-crystallization molecular movements. The kinetics of nonisothermal crystallization is studied using the Ozawa, Mo, and isoconversional methods. The present studies suggest that the dominant factor of the crystal growth mechanism depends on the cooling rate. Two types of crystallization mechanisms are identified at cooling rates lower and higher than 5 K min⁻¹. We design a diagram with crystallization and glass transition borders against the cooling rates. Estimations show that crystallization of the present compound can be bypassed at cooling rates higher than 78 kK min⁻¹, at which a glass transition of the nematic phase occurs. We show various scenarios of the molecular order and the crystallization mechanism designed based on the process rate.