Surface and Flexoelectric Polarization in Nematic and Smectic A Phases of Liquid Crystal 4-octyloxy-4′-cyanobiphenyl

Abstract

The temperature dependencies of surface polarization in the nematic (N) and smectic A (SmA) phases have been measured for both the planar and homeotropic orientations of 4-octyloxy-4′-cyanobiphenyl (8OCB). Polarization was determined in the field- off regime from the pyroelectric response of the cell to a short laser pulse, and the temperature increment was calculated from the in situ measurements of the light induced birefringence. The surface polarization in the N-phase for both orientations is directed from an interface into the bulk at similar magnitudes (about 0.5–0.8 pC/m) but its change at the N–SmA transition was qualitatively different for the two cases. The experimental data are consistent with the quadrupolar model of the order-electric polarization. The field-off measurement technique also allowed for the measurements of the z-component of the flexoelectric polarization of 8OCB in both the N and SmA phases using a hybrid aligned nematic cell. The sum of the flexoelectric coefficients (e1+e3) in both phases was shown to be positive (e1+e3 ≃7.4 pC/m at 40°C), which is opposite to the case of 4-pentyl-4′-cyanobiphenyl (5CB). On the molecular scale, the difference is accounted for by the different distribution of electric charges in molecular quadrupoles forming the corresponding liquid crystalline phases.