J. Phys. II France
Volume 3, Numéro 6, June 1993
Page(s) 865 - 889
DOI: 10.1051/jp2:1993172
J. Phys. II France 3 (1993) 865-889

Pattern formation in the splay Freedericks transition of a nematic side-group polysiloxane

Norbert Schwenk and Hans Wolfgang Spiess

MPI für Polymerforschung, Postfach 3148, D-55021 Mainz, Germany

(Received 19 November 1992, accepted 28 January 1993)

Periodic director patterns in the magnetic-field induced splay Freedericks transition of a nematic side-group polysiloxane are reported. For this purpose liquid crystal cells (10  $\mu$m-500  $\mu$m) are studied by polarization microscopy as well as by deuteron NMR. Through this combinatin, the optically observed spatial dependence of the director can be quantitatively analyzed in terms of director distributions extracted from the NMR lineshape. In the equilibrium state of the Freedericks transition (static Freedericks effect), the director exhibits a one-dimensional periodicity perpendicular to the initial director orientation n0. 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 the nematic (back-flow) leads to a reduction of the viscosity.

61.30 - 76.60

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