Effect of molecular architecture on the electrorheological behavior of liquid crystal polymers in nematic solvents

Abstract

Low molar mass liquid crystal solvents with positive dielectric anisotropy exhibit an electro-rheological (ER) effect such that the viscosity, η_on, in the presence of a strong electric field, applied transverse to the flow, is larger than that, η_off, in the absence of such a field. Dissolution of a liquid crystal polymer (LCP) enhances the magnitude of the ER effect (η_on - η_off) by an amount, ση_on - ση_off which is an increasing function of LCP concentration and depends on the molecular architecture of the LCP Specifically, we show that two main-chain LCPs, with different chemical structures, strongly increase the ER response, a side-on side-chain LCP moderately increases the response, and an end-on side-chain LCP weakly increases the response. These diverse behaviors can be interpreted using theoretical arguments which assume that the LCP conformation is an ellipsoid of revolution whose orientation relative to the flow direction is determined by the balance between the hydrodynamic and electric torques on the fluid. The different ER responses are consistent with the idea that main-chain LCPs are highly prolate, the side-on side chain LCP is moderately prolate, and the end-on side chain LCP is quasi-spherical. A molecular description is obtained by equating η_on, and η_off, respectively, to the Miesowicz viscosities η_c and η_b, and using a hydrodynamical model developed by Brochard which deduces that ση_c/ση_b = R ⁴_‖ /R ⁴_⊥ , where R _‖ and R _⊥ are the end-to-end distances of the chain, respectively, parallel and perpendicular to the director.