Separate measurements of the flexoelectric and surface polarization in a model nematic liquid crystal p-methoxybenzylidene-p'-butylaniline: validity of the quadrupolar approach.
The temperature dependences of the surface polarization have been measured at the interface of a conductive glass with both the homogeneously and homeotropically oriented nematic liquid crystal p-methoxybenzylidene-p(')-butylaniline. The polarization was found in the field-off regime from the pyroelectric response of a cell to a short laser pulse, absorbed in the bulk of the liquid crystal. The temperature increment was calculated from the measurements of the birefringence induced by the same light pulse. It has been shown that the surface polarization at the homeotropic (m(h)) and planar (m(p)) interfaces is directed from an interface into the bulk and from the bulk to an interface, respectively (with a magnitude m(h) approximately -0.3 pC/m and m(p) approximately +0.2 pC/m at 25 degrees C). The experimental data may be explained in terms of the quadrupole model of the order-electric polarization with account of some additional contribution from molecular dipoles. The same technique also allows for the measurements of the z component of the flexoelectric polarization using a pyroelectric response of a hybrid (homeoplanar) aligned nematic cell and proper subtracting of the surface contributions. The flexoelectric polarization has been shown to be opposite to the sum of the surface terms m(h)+m(p) and directed from the planar to homeotropic interface. This means that the sum of the flexoelectric coefficients e=(e(1)+e(3)) is positive (e congruent with 1.7 pC/m at 28 degrees C). The temperature dependence of e has been shown to involve a combination of both the quadrupolar and dipolar contributions.
Blinov
L M
LM
Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan. lcl@ns.crys.ras.ru
Barnik
M I
MI
Ohoka
H
H
Ozaki
M
M
Yoshino
K
K