Surface and flexoelectric polarization in nematic and smectic A phases of liquid crystal 4-octyloxy-4 '-cyanobiphenyl

The temperature dependencies Of Surface polarization in the nematic (N) and smectic A (SmA) phases have been measured for both the planar and homeotro pic orientations of 4-octyloxy-4 ' -cyanobiphenyl (8OCB). Polarization was determined in the field- off regime from the pyroelectric response of the c ell to a short laser pulse, and the temperature increment was calculated fr om the in situ measurements of the light induced birefringence. The surface polarization in the N-phase for both orientations is directed from an inte rface into the bulk at similar magnitudes (about 0.5-0.8 pC/m) but its chan ge at the N-SmA transition was qualitatively different for the two cases. T he experimental data are consistent with the quadrupolar model of the order -electric polarization. The field-off measurement technique also allowed fo r the measurements of the z-component of the flexoelectric polarization of 80CB in both the N and SmA phases using a hybrid aligned nematic cell. The sum of the flexoelectric coefficients (e(1) + e(3)) in both phases was show n to be positive (e(1) + e(3) similar or equal to 7.4 pC/m at 40 degreesC), which is opposite to the case of 4-pentyl-4 ' -cyanobiphenyl (5CB). On the molecular scale, the difference is accounted for by the different distribu tion of electric charges in molecular quadrupoles forming the corresponding liquid crystalline phases.