Rheological behavior of dilute solutions of a side-chain liquid-crystalline polysiloxane in 4,4′-n-octylcyanobiphenyl

The electrorheological (ER) behavior and stress transient response of dilute solutions of a sidechain liquid-crystalline polysiloxane (LCP) in 4,4'-n-octylcyanobiphenyl (8CB) is studied. III the flow-tumbling regime of 8CB, i.e. from T = 34 38 degrees C, the stress transients of both 8CB and LCP/8CB solutions show oscillatory responses, but with shorter oscillation periodicities for the solution. In the how-aligning regime of 8CB, i.e. at 39 and 40 degrees C, a transformation to flow-tumbling is observed in the stress transients of the LCP/8CB solution. In both cases, analysis of the transient responses indicates that the change in Leslie viscosity coefficients on dissolving the LCP are delta alpha(2) < 0 and delta alpha(3) > 0. The amplitude of the ER response, defined as the viscosity difference between the on and off states, Delta eta = eta(on) - eta(off) is only weakly affected by the dissolution of LCP. These rheological results can be interpreted consistently using a modified version of a hydrodynamic theory by Brochard, provided an additional dissipation mechanism is included, which derives from the presence of an elastic torque between director rotation and LCP orientation.