Separate measurements of the flexoelectric and surface polarization in a model nematic liquid crystal p-methoxybenzylidene-p '-butylaniline: Validityof the quadrupolar approach - art. no. 031707

The temperature dependences of the surface polarization have been measured at the interface of a conductive glass with both the homogeneously and home otropically oriented nematic liquid crystal p-methoxybenzylidene-p'-butylan iline. The polarization was found in the field-off regime from the pyroelec tric response of a cell to a short laser pulse, absorbed in the bulk of the liquid crystal. The temperature increment was calculated from the measurem ents of the birefringence induced by the same light pulse. It has been show n 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 t o an interface, respectively (with a magnitude m(h) similar to -0.3 pC/m an d m(p) approximate to +0.2 pC/m at 25 degreesC). The experimental data may be explained in terms of the quadrupole model of the order-electric polariz ation with account of some additional contribution from molecular dipoles. The same technique also allows for the measurements of the z component of t he flexoelectric polarization using a pyroelectric response of a hybrid (ho meoplanar) aligned nematic cell and proper subtracting of the surface contr ibutions. The flexoelectric polarization has been shown to be opposite to t he sum of the surface terms m(h)+m(p) and directed from the planar to homeo tropic interface. This means that the sum of the flexoelectric coefficients e = (e(1) + e(3)) is positive (e congruent to 1.7 pC/m at 28 degreesC). Th e temperature dependence of e has been shown to involve a combination of bo th the quadrupolar and dipolar contributions.