THE BANDED MICROSTRUCTURE OF SHEARED LIQUID-CRYSTALLINE POLYMERS

It has been widely documented that nematic and cholesteric liquid-crys talline materials can develop a one-dimensionally periodic microstruct ure subsequent to uniaxial draw, unidirectional shear, or injection mo ulding, This microstructure gives rise to a banded texture between cro ssed polars. A review of the literature, which contains many conflicti ng studies, identified several molecular and processing variables that may affect the rate of banded texture evolution. Our research focused on banded textures in three lyotropic systems of hydroxypropyl cellul ose (HPC), a semiflexible polymer: HPC-water, HPC-methanol and HPC-ace tone. Solutions were sheared unidirectionally with a motorized paralle l-plate apparatus. The rate of band evolution after shear was quantifi ed consistently in terms of the time taken for the average band spacin g to reach a minimum, and was found to depend on a combination of prio r shear rate, specimen thickness, solvent type and concentration. The dependence on solvent type is especially sensitive. The qualitative wa y in which these variables control the rate of microstructural change after shear is discussed in terms of (1) their effect on the ability o f a sheared microstructure to store the energy that drives subsequent reordering and (2) their effect on the mobility of the microstructure.