Dynamical Behavior of a Ferroelectric Liquid Crystal with Monotropic Transition Temperatures during Electric-Field-Induced Switching Studied by Time-resolved Infrared Spectroscopy Combined with Singular Value Decomposition

Abstract

The switching dynamics of a ferroelectric liquid crystal (FLC-154) with monotropic transition temperatures has been explored under the surface-stabilized states by polarized infrared (IR) absorption and time-resolved IR spectroscopy. The polarized IR and time-resolved IR studies have revealed that FLC-154 shows significantly different dynamics from FLCs with non-monotropic thermotropic transition temperatures. The analysis of the relative orientation of the transition moments of the CH2 symmetric and antisymmetric stretching modes has suggested that the alkyl chains of FLC-154 take a gauche-trans-gauche conformation. It has also been revealed from the same analysis that the alkyl chains take different alignment under dc applied voltages of 20 V of both polarities at 65°C. Time-resolved IR spectra of FLC-154 in a planar-aligned cell were measured as a function of the polarization angle ranging from 0° to 180° under a rectangular electric field of ±20 V with a 5 kHz repetition rate in the smectic-C* (Sm-C*) phase at 65°C. These spectra have enabled us to explore the reorientation processes of the alkyl chains, the core and the large C=O dipole moments of FLC-154 at all the delay times. The singular value decomposition (SVD) was applied to the time-resolved IR spectra in the 3020–1000 cm-1 region of FLC-154. The results of SVD have provided information about the profile of the principal component (PC) score versus the delay time, enhancing the ability of time-resolved IR in probing the switching dynamic. It has been found from the SVD analysis that the average orientation of a large C=Obiphenyl dipole moment (it arises from the C=O group adjoining the biphenyl ring) of FLC-154 is unchanged during the whole electric-field-induced switching. It seems likely that the core of FLC-154 straddles the polyimide molecules on the surface of the FLC layer. This configuration of FLC molecule forces the transverse C=Obiphenyl dipole moment to be parallel with the rubbing direction, and the molecules of FLC-154 revolve around the average orientation of the C=Obiphenyl dipole moment. These results have suggested that the FLC-154 molecules revolve around the large C=Obiphenyl dipole moment during the electric-induced-field switching. The dynamic SVD profile for the alkyl chains, a large C=Ochiral dipole moment (it arises from the C=O group in the chiral moiety) and the core moiety have shown that all the moieties require an induction period of 10 µs before responding to the electric field.