INVESTIGATION OF FLEXOELECTRIC PROPERTIES OF A PALLADIUM-CONTAINING NEMATIC LIQUID-CRYSTAL, AZPAC, AND ITS MIXTURES WITH MBBA

Flexoelectricity of pure Azpac (an HOAB-palladium complex) was studied using planar nematiclayers under an in-plane electric field. Longitudinal domains were observed with a periodinversely proportional to the applied d.c. electric field. These domains were considered as a firstexperiment manifestation of the theoretical prediction of R. B. Meyer (1969) and their study haspermitted the evaluation of the difference in flexo-coefficients el, - e3x. Their appearance canalso be followed for the non-complexed HOAB, using instant video-microscopy frames.Mixtures of Azpac, up to IOwt%, and MBBA were oriented homeotropically and bandflexoelectric deformations were observed, both in d.c. and in a.c. (1 to 1OOOHz) electric fieldsnormal to the director. The dynamics of director reorientation were studied by the method offlexoelectric light modulation. A system comprising a He-Ne laser and a lock-in amplifierinterfaced by a PC was developed. By operating this in a frequency sweep regime, viscoelasticspectra of director dynamics were recorded. These spectra were excited using a linearflexoelectric coupling mechanism. Breaks in the spectra were observed in the range 200 to600 Hz, indicating a cross-over from bulk to surface dissipation of energy. For the first time, asurface viscosity of 2.6 X 10 - * J s m - * was determined for MBBA homeotropically anchoredon a DMOAP-coated glass surface.In concentrations at low as 2.5 wt %, Azpac was found to cancel the bend flexo-coefficientof MBBA and at higher concentrations, a steep rise in the flexo-coefficient of the mixture wasobserved, but with an opposite sign. Thus, the application of Azpac as an effective additive foradjustment of the value and sign of the flexo-coefficient in flexoelectro-optic displays or lightmodulators could be suggested.