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 microm were measured for heating process from 55 to 90 degrees C and a cooling process from 90 to 55 degrees C in increments of 1 degrees 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.