Phenomenological transient shear behavior of liquid crystalline polymers after the start-up of shear flow

Transient stresses of liquid crystalline polymers appear as damped oscillatory patterns after the start-up of shear flow. They are examined by using the constitutive equations which were modified to include the idea of an initial domain state in the Larson-Doi polydomain equation set. In order to be consistent with the phenomenological transient shear stress after the start-up in the region of low shear rates, the Hinch-Leal closure approximations were adopted for the domain-averaged (mesoscopic) fourth-order tensors of the director field. It is predicted as an important result that the amplitude of the first overshoot in the transient stress increases with an increase in the shear rate imposed at the start-up, which was found to be largely in agreement with phenomenology. In addition, the effects of parameters such as molecular concentration, initial domain size, and intensities of viscous and elastic contributions on the transient stress under the start-up process were also evaluated.