Electrorheology of a nematic poly(n-hexyl isocyanate) solution

The shear flow behavior of a unidomain nematic solution of poly (n-hexyl iso-cyanate) (PHIC) subjected to an electric field was studied experimentally in a parallel plate rotational rheometer. At a shear rate of 0.4 s-l, the Miesowicz viscosity r]c (with the director oriented along the velocity gradient) was found to be 35 times greater than the steady shear viscosity in the absence of an electric field. The mechanism for this electrorheological (ER) effect is the ori-entation of the permanent dipole moment of the PHIC molecules; the ER effect is an order of magnitude larger than that for low molecular weight liquid crystals. Director tumbling is postulated to occur at a shear rate of 0.4 s- I for the PHIC solution if the applied dc electric field is lower than approximately 0.4 MV/m. At higher electric fields, tlow alignment is regained. In this case, the transient stress undershoot is suppressed. Steady state viscosity vs electric field data were fit to Carlsson and Skarp’s two-dimensional approximation of the Ericksen, Leslie, and Parodi theory, yielding values for the Leslie coefficients a 1, ~2, and a3 that compare favorably with those predicted by Doi theory. I