X-ray scattering measurements of molecular orientation in thermotropic liquid crystalline polymers under flow
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In situ synchtrotron x-ray scattering is used to make quantitative measurements of molecular orientation in thermotropic liquid crystalline polymers (TLCPs) in both simple and complex flows. Two different model TLCPs consisting of mesogenic units separated by flexible spacers are studied in simple shear flow as a function of temperature and shear rate. Both samples exhibit significant net molecular orientation under steady shear within the nematic phase. One of the samples undergoes a flipping transition from alignment along the flow direction to alignment along the vorticity direction at low temperatures. This is associated with a subtle phase transition detected by DSC. At low temperatures, it is possible to induce a transition back to orientation along the flow direction by application of high shear rates. A commercial TLCP fully aromatic copolyester was studied in a diverging channel flow. In straight channel sections, significant net orientation along the flow direction is observed. In the expansion region, however, extensional gradients are capaple of inducing alignment transverse to the prevailing flow direction. Both the degree and direction of orientation have been measured as a function of position in this complex flow.
