PATTERN-FORMATION IN THE SPLAY FREDERICKS TRANSITION OF A NEMATIC SIDE-GROUP POLYSILOXANE

Periodic director patterns in the magnetic-field induced splay Freeder icks transition of a nematic side-group polysiloxane are reported. For this purpose liquid crystal cells (10 mum-500 mum) are studied by pol arization microscopy as well as by deuteron NMR. Through this combinat ion, the optically observed spatial dependence of the director can be quantitatively analyzed in terms of director distributions extracted f rom the NMR lineshape. In the equilibrium state of the Freedericks tra nsition (static Freedericks effect), the director exhibits a one-dimen sional periodicity perpendicular to the initial director orientation n 0. The dynamics of the Freedericks transition involves a transient two -dimensional director pattern, representing convection rolls in which the nonlinear coupling between director rotation and viscous flow of t he nematic (back-flow) leads to a reduction of the viscosity.