Mechanism of Thermal Phase Transition of a Ferroelectric Liquid Crystal with Monotropic Transition Temperature Studied by Infrared Spectroscopy Combined with Principal Component Analysis and Sample—Sample Two-Dimensional Correlation Spectroscopy

Infrared (IR) spectra of FLC-154 (FLC: ferroelectric liquid crystal) with monotropic phase transition under a nonalignment state with a sample layer thickness of 24.5 μm were measured for heating process from 55 to 90 °C and a cooling process from 90 to 55 °C in increments of 1 °C. The thermal dynamics of FLC-154 were investigated by use of IR spectroscopy combined with principal component analysis (PCA) and sample–sample two-dimensional (2D) correlation spectroscopy. During the cooling, the FLC-154 molecule passes through the monotropic smectic- C* (Sm- C*) phase, which is transformed from the Sm- A phase. The results from PCA suggest that during the heating process, the thermal dynamics of the alkyl chains, core moiety, and C=O groups are similar to each other. Furthermore, PCA and sample–sample 2D correlation spectroscopy indicate that the alkyl chains and C=O groups in the chiral and core moieties are responsible for the emergence of the Sm- C* phase. This conclusion is very important because the IR data have given more evident cause for the emergence of the Sm- C* phase than the theoretical models such as the molecular-statistical theory of ferroelectric ordering and the indigenous polarization theory. Moreover, it has been found that some of the trans conformations of the alkyl chains of FLC-154 change partly to the gauche conformation when the phase transition from the crystalline phase to the Sm- A phase occurs. It has also been found that the intermolecular interactions of the C=O group in the core moiety in the Sm- A phase are weaker than those in the crystalline phase and that the conformational change occurs on the C–O–C bonds in the core moiety upon going from the crystalline to the Sm- A phase.