A SANS study on uniaxial alignment of columnar discotic liquid crystals using a rotating magnetic field

Columnar DLCs have attracted great attention for their anisotropic charge carrier mobilities and photoconductivities along the columnar axis. In order to develop new molecular devices employing the anisotropic properties, the uniaxial alignment of columnar domains of DLCs over a bulk scale is crucial. The uniaxial alignment of columnar discotic liquid crystals, cobalt octa(n-alkylthio)-porphyrazine (CoSn, n is substituted alkyl chain length), using a rotating magnetic field has been investigated by SANS measurements. The magnetic alignment of CoSn (n=10, 12, 14) under a static magnetic field was measured by SANS. The results show that when the samples are cooled from isotropic phase to columnar liquid crystal phase under 1.OT, the columns of CoSn align their directors perpendicular to the applied magnetic field. In the plane perpendicular to the magnetic field. however, the directors point random directions. The alignment is induced by both of the diamagnetic interaction of aromatic core and the paramagnetic interaction of central cobalt with the applied magnetic field. The saturation magnetic field for alignment is higher for the longer alkyl chain length, which may be caused by the increased entropy with chain length. To uniaxially align the columns of CoSn (n=10, 12) in a bulk scale, the samples were rotated at various speed (2, 5, 50, 125, 250rpm) under a static magnetic field 1.OT. SANS measurements clearly showed that the columns of CoSn uniaxially aligned parallel to the axis of sample rotation. Among the various rotation speed used, the order parameter S which defines the degree of uniaxial alignment was peaked at 5 rpm (S=0.552 at 2 rpm, 0.697 at 5 rpm, 0.596 at 50 rpm, 0.590 at 125 rpm, 0.540 at 250 rpm). To achieve the uniaxial alignment the rotation speed should be higher than the inverse of reorientational relaxation time of the columns under an applied magnetic field. Therefore, at low rotation speed the order parameter was increased as increasing the rotation speed. However at high rotation speed above 50 rpm due to the disturbance induced by centrifugal forces the order parameter slightly decreased as increasing the rotation speed