Orthogonal and parallel superposition measurements on lyotropic liquid crystalline polymers |
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Authors: | Lynn M Walker Jan Vermant Paula Moldenaers Jan Mewis |
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Institution: | (1) Department of Chemical Engineering Colloids, Polymers and Surfaces Program Carnegie Mellon University Pittsburgh, PA 15213, USA, US;(2) Department of Chemical Engineering Katholieke Universiteit Leuven 3001 Leuven (Heverlee), Belgium e-mail: Jan.mewis@cit.kuleuven.ac.be Tel.: +32-16-322361, Fax: +32-16-322991, BE |
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Abstract: | Mechanical spectroscopy is used to probe the structure of lyotropic liquid crystalline polymers during flow and after the
cessation of flow. The oscillatory flow is either parallel or perpendicular to the steady-state flow. The resulting moduli
provide information about the time- and shear-dependent microstructure, including anisotropy. Two different concentrations
of poly(benzylglutamate) (PBG) in m-cresol and a concentrated hydroxypropylcellulose (HPC) solution, also in m-cresol, are investigated. In all cases, the orthogonal superposition moduli evolve differently from the parallel ones. The
former are less sensitive to the flow-induced changes in structure than the latter ones. Together with the lack of sensitivity
of the superposition moduli to texture refinement during flow, this suggests a strong relation between director orientation
and superposition moduli. After the cessation of flow the parallel moduli decrease for the PBG solutions, whereas the opposite
is observed in the HPC solutions. A comparison with the orthogonal moduli provides a direct measure of anisotropy. At rest,
the PBG solutions tend toward a higher degree of anisotropy while the HPC solutions become more isotropic. In the latter systems,
all moduli are much larger, reflecting a larger contribution from the texture.
Received: 8 July 1999/Accepted: 1 October 1999 |
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Keywords: | Liquid crystalline polymers Superposition rheometry Flow-induced structures Anisotropic viscoelasticity |
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