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 dilut e 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 periodi cities 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 t ransients of the LCP/8CB solution. In both cases, analysis of the transient responses indicates that the change in Leslie viscosity coefficients on di ssolving the LCP are delta alpha(2) < 0 and delta alpha(3) > 0. The amplitu de of the ER response, defined as the viscosity difference between the on a nd off states, Delta eta = eta(on) - eta(off) is only weakly affected by th e dissolution of LCP. These rheological results can be interpreted consiste ntly using a modified version of a hydrodynamic theory by Brochard, provide d an additional dissipation mechanism is included, which derives from the p resence of an elastic torque between director rotation and LCP orientation.