Reorientation instabilities and viscoelastic measurements in a main chain thermotropic nematic polymer Optical and NMR studies

The reorientational behaviour of a nematic monodomain, in a static magnetic field, following the rotation of the mean director through an angle α ≈ π/2 to the magnetic field direction, is investigated by optical polarizing microscopy and proton NMR spectroscopy. Optical evidence supporting a reorientation process through a pure bend transient distortion of the director field is reported for poly (4,4′-dioxy-2,2′-dimethylazoxybenzene nonanediyl), a main-chain nematic thermotropic polymer. The time evolution of the distortion and flow patterns are deduced from a detailed analysis of the NMR lineshapes recorded during the reorientational process. The viscosity coefficients γ1, α1, α2, ηc and the ratio K 3/K 1 of the bend to splay Frank elastic constants are determined and discussed in terms of the existing theories.