Molecular dynamics and rheological properties of nematic liquid crystal polymer in electric field

Abstract

Generation of ultra-high modulus fibers from polymeric liquid crystalline solutions has encouraged research into the flow behavior of anisotropic fluids. Doi and Marrucci theoretically predicted the rheological properties of polymeric liquid crystals by generalizing the theory for isotropic and nematic solutions of rigid rods. Previously, the experimental works were restricted to shearing flows and did not make use of the theory to describe the results. In this work, the experimental data were obtained using the response of the polymeric liquid crystal for the flow in an electric field. A capillary rheometer for conventional shearing measurement has also been made. The polarized light microscopy was used to investigate the transition of the isotropic and the anisotropic phases. Polymeric liquid crystals are found to be a class of materials distinct from both Newtonian fluids and flexible polymer solutions. In the steady shear flow, the power law index n of poly (p-phenyleneterephthalamide) varies from one to zero. As the electric field strength increases, viscosity decreases above threshold voltage at which the molecules just begin to change and reaches the saturated state. The threshold voltage decreases with concentration. From these results the dielectric anisotropy is negative. In the saturated state the molecules orient parallel to the wall and perpendicular to the field. The viscosity of this state is Miesowicz viscosity $eta_b$, From $eta_b$, the two parameters; the rotational diffusivity, $D_r$, and the order parameter, S, are obtained. $D_r$ and S increase with concentration. This effect is tube dilation. The results for both the steady shearing flow and the flow in an electric field agree with the theory. As a result, based on the two experimental parameters, rheological properties such as the first and second normal stress differences are easily obtained.