Twist viscosity of side-chain liquid-crystalline polysiloxanes in a nematic solvent

Electric-field-dependent dynamic light scattering (EFDLS) was used to deter mine the twist elastic constant and twist viscosity of miscible nematic sol utions of side-chain liquid crystal polysiloxanes (SCLCP) in 4'-(pentyloxy) -4-cyanobiphenyl (5OCB). The results show that addition of SCLCP leads to a decrease in the twist relaxation rate, which derives mainly from an increa se in twist viscosity. The magnitude of the twist viscosity increment varie s with molecular weight and spacer length, proportional to changes in the m olecular hydrodynamic volume of the polymer. These results are consistent w ith the Brochard model of the viscous loss introduced by the dissolved poly mer, modified by inclusion of an additional dissipation mechanism, postulat ed to arise from an elastic torque between director rotation and LCP orient ation.