Twist viscosity of a side-chain liquid-crystalline polysiloxane dissolved in a tumbling nematic solvent

The dynamics of the twist distortion was characterized by electric-field-dependent dynamic light scattering (EFDLS) for a dilute solution of a side-chain liquid crystalline polysiloxane (SCLCP) dissolved in the tumbling nematic solvent, 4'-octyl-4-cyanobiphenyl (8CB). The results show a decrease in twist relaxation rate on dissolution of the SCLCP due to a large increase in twist viscosity delta gamma(1) with negligible change in twist elastic constant K-22 By comparing the twist viscosity values, gamma(1) = alpha(3) - alpha(2), with previous measurements of the strain periodicity of oscillations in the shear stress transients due to director tumbling, gamma(p) = (pi/delta(p))(1 + delta(p)(2)), where delta(p) = (-alpha(3)/alpha(2))(1/2), we further investigate the corresponding changes in Leslie viscosities, delta alpha(2) and delta alpha(3). The results indicate that delta alpha(2) and delta alpha(3) have opposite signs. These observations, together with Literature data on the electrorheological behavior of these solutions, can be self-consistently described by a hydrodynamic model of the viscous dissipation by Brochard, modified through inclusion of an additional dissipation process which derives from an elastic torque between director rotation and LCP orientation.