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 nematic layers under an in-plane electric field. Longitu dinal domains were observed with a period inversely proportional to th e applied d.c. electric field. These domains were considered as a firs t experiment manifestation of the theoretical prediction of R.B. Meyer (1969) and their study has permitted the evaluation of the difference in flexo-coefficients e(1z) - e(3x). Their appearance can also be fol lowed for the non-complexed HOAB, using instant video-microscopy frame s. Mixtures of Azpac, up to 10 wt%, and MBBA were oriented homeotropic ally and band flexoelectric deformations were observed, both in d.c. a nd in a.c. (1 to 1000 Hz) electric fields normal to the director. The dynamics of director reorientation were studied by the method of flexo electric light modulation. A system comprising a He-Ne laser arid a lo ck-in amplifier interfaced by a PC was developed. By operating this in a frequency sweep regime, viscoelastic spectra of director dynamics w ere recorded. These spectra were excited using a linear flexoelectric coupling mechanism. Breaks in the spectra were observed in the range 2 00 to 600 Hz, indicating a cross-over from bulk to surface dissipation of energy. For the first time, a surface viscosity of 2 . 6 x 10(-8) J s m(-2) was determined for MBBA homeotropically anchored on a DMOAP- coated glass surface.In concentrations at low as 2 . 5 wt%, Azpac was found to cancel the bend flexo-coefficient of MBBA and at higher conce ntrations, a steep rise in the flexo-coefficient of the mixture was ob served, but with an opposite sign. Thus, the application of Azpac as a n effective additive for adjustment of the value and sign of the flexo -coefficient in flexoelectro-optic displays or light modulators could be suggested.