Molecular orientation of a liquid-crystalline polymer solution in mixed shear-extensional flows

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A lyotropic solution of hydroxypropylcellulose in m‐cresol has been studied in a nonhomogeneous shear (plane Poiseuille) flow, and in a set of flows with mixed shear and extension (slit contractions). The average molecular orientation is measured using flow birefringence, while laser‐Doppler velocimetry is used to characterize extensional kinematics in the slit‐contraction flows. In slit flow, we observe very similar behavior to recently published observations in another model lyotropic, PBG in m‐cresol [B. D. Bedford and W. R. Burghardt, J. Rheol. 38, 1657 (1994)]. Steady flow at low rates gives way to an instability characterized by large scale structural heterogeneities and time‐dependent birefringence at higher rates. Throughout the entire flow rate range, however, average birefringence measured in slit flow may be quantitatively predicted from simple shear flow data, assuming that the locally averaged rheological and structural properties in the slit flow are equivalent to those occurring in simple s...

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Experimental/ flow birefringence liquid crystal polymers molecular orientation Poiseuille flow shear flow/ liquid-crystalline polymer solution mixed shear-extensional flows molecular orientation lyotropic solution hydroxypropylcellulose in m-cresol plane Poiseuille flow flow birefringence laser-Doppler velocimetry extensional kinematics slit-contraction flows large scale structural heterogeneities time-dependent birefringence slit flow shear flow/ A6130E Experimental determinations of smectic, nematic, cholesteric, and lyotropic structures A6125H Structure of macromolecular and polymer solutions (solubility, swelling, etc.) polymer melts A4750 Non-Newtonian dynamics