Electrorheological Behavior of Side-Chain Liquid-Crystalline Polysiloxanes in Nematic Solvents

The electrorheological properties of well-characterized side-chain liquid-crystalline polysiloxanes with different backbone and spacer lengths were studied in two nematic solvents: 4‘-(pentyloxy)-4-biphenylcarbonitrile (5OCB), which has positive dielectric anisotropy, and N -(4-methoxybenzylidene)-4-butylaniline (MBBA), which has negative dielectric anisotropy. Specifically, we measured the steady-shear viscosities, η off and η on, in the absence and presence, respectively, of a saturation electric field. For 5OCB solutions, the electrorheological (ER) effect is positive, i.e., η on > η off . For the intrinsic viscosities, we find [η off ] ≫ [η on ], and [η off ] shows a strong dependence on molecular weight ([η off ] ∼ M 0.4 ) and spacer length, whereas [η on ] is insensitive to the change of molecular weight and spacer length. In contrast, the ER effect of MBBA solutions is very small and slightly negative, i.e., η on < η off . In addition, [η off ] and [η on ] are of comparable magnitude and each shows a strong dependence on molecular weight ([η off ] ∼ [η on ] ∼ M 0.3 ). These distinctive patterns of ER behavior can be explained by a hydrodynamic model which assumes the side-chain liquid-crystalline polysiloxane has an asymmetric conformation, such that the root-mean-square end-to-end distance parallel to the director, R ∥, is different from that perpendicular to the director, R ⊥ . R ∥ is aligned along the shear gradient in 5OCB when the field is on but is tilted toward the flow direction with the field off. In MBBA, R ∥ is oriented perpendicular to the shear gradient both with the field on and with the field off.