Twist viscosity of mixtures of low molar mass nematics |
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Authors: | Pei-Yuan Liu Alex M Jamieson |
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Institution: | (1) Department of Macromolecular Science Case Western Reserve University Cleveland, OH 44106-7202, USA e-mail: amj@po.cwru.edu, US |
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Abstract: | Measurements of the twist viscosity, γ1(DLS) and twist elastic coefficient, K22(DLS) by electric-field-dependent dynamic light scattering (EFDLS) are reported for low molar mass nematics (LMMNs) 4′-heptyl-4-cyanobiphenyl
(7CB) and 4′-octyl-4-cyanobiphenyl (8CB), and their binary mixtures at several temperatures in the nematic state. The results
are compared with values (γ1(Rheol)=α3–α2) computed from rheological measurements of the Leslie viscosities α2 and α3. For the binary mixtures, at each temperature, the measured twist viscosity γ1(DLS) and corresponding twist elastic constant K22(DLS) show approximately a linear additive dependence on concentration. The calculated twist viscosity, γ1(Rheol), agrees with γ1(DLS) for the pure components, but is significantly smaller for the binary mixtures. Our observations appear to be consistent
with a recent report of a discrepancy between values of the tumbling parameter λ, determined using a small-strain oscillatory
optical technique, vs those measured by a rheological method. These results suggest that, in the rheological measurements
at large strains, the rate of director rotation for mixtures may be affected by a flow-induced change in structure, e.g.,
shear-induced biaxiality.
Received: 17 March 2000 Accepted: 17 July 2000 |
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Keywords: | Twist viscosity Nematic mixtures Director rotation |
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