Effects of initial domain state on the rheological properties of liquid crystalline polymers

Abstract

In a polydomain model for textured liquid crystalline polymers (LCPs), domain shrinkage and texture coarsening at low shear rates are considered to be driven by the deviations of defect density L from the initial ($L_o$) and steady ($L_s$) states, respectively. The densities of $L_o$ and $L_s$ correspond to the average domain sizes of $a_o$ and $a_s$ at each state. The introduction of $L_o$ into the Larson-Doi defect density evolution equation provides a limit of texture length scale, eliminating the indefinite coarsening of texture. Then, the defect density evolution is refreshed in each case of the steady state, shear inception, and shear cessation. With the inclusion of $L_o$ the shear thinning behavior of a steady state viscosity curve can be developed in the Onogi-Asada``s region I as well as the Newtonian plateau behavior in the region II. Moreover, another Newtonian plateau region over very low shear rates, called "Region 0", can be predicted when smaller domains are considered to be isotropically distributed at the initial state. Transient stresses of LCP after abrupt changes of shear flow condition, responded as damped oscillatory patterns, are examined for three cases of shear flow change: start-up, step-up, and flow reversal. The constitutive equation used is the modified Larson-Doi polydomain equation set including the idea of initial domain state. In the stress expression, Hinch-Leal closure approximations are also adopted to avoid splitting of damped peaks, possibly occurred by the imposition of low shear rate. In the start-up process, the amplitude of first overshoot increases with increasing shear rate imposed at quiescent state. This behavior is possibly predicted for the first time in the present study, which can be including the initial domain state of LCP. The effect of initial domain state can be enhanced in step-up process because the two steady states themselves before and after the step-up are readily considered to be under the influenc...