BAND FORMATION UPON CESSATION OF FLOW IN LIQUID-CRYSTALLINE POLYMERS

A banded texture develops upon cessation of flow in polymeric liquid crystals. In addition to the well-known lower critical shear rate, an upper limit for band formation is observed. The kinetics of the band evolution is investigated on a series of PBG solutions as a function of previous shear rate, concentration, temperature, equilibrium structure (nematic versus cholesteric), and gap size. It is concluded that the band texture occurs along two different routes depending on the shear rate range under investigation. At relatively low shear rates both the lateral and the width dimensions increase with time. The shear, however, affects the kinetics in these two directions differently, which explains the existence of a lower critical shear rate. At higher shear rates, on the other hand, the bands immediately reach their final length when the flow stops. Hence the evolution can be considered as one dimensional. After long rest periods the banded structure disappears. This phenomenon depends on the material under consideration. In PBG solutions the bands widen before they disappear, while for HPC solutions the parallelism of the bands is lost without considerable widening. Existing models for band formation are compared with the present data.