Molecular orientation in channel flows of main-chain thermotropic liquid crystalline polymers

In situ wide angle X-ray scattering is employed for quantitative measurements of flow-induced molecular orientation in channel flows of thermotropic hydroxypropyl-cellulose (HPC). An extrusion die, constructed to allow X-ray access, generates slit flow, slit flow with superimposed contractions and expansions in cross-section, and slit flow past an obstruction. In slit flow, weak molecular orientation develops slowly with downstream position. Superimposed extension associated with contraction flows leads to a strong enhancement in orientation, which persists with distance further downstream of the contraction. Conversely, transverse extension present in expansion flows generates a bimodal orientation state and substantial reductions in average molecular orientation. These results are compared to earlier measurements on a commercial fully aromatic thermotrope. HPC is found to respond more strongly to superimposed extension, and more weakly to the prevailing inhomogeneous shear flow than the commercial material.