The stress relaxation behaviour of liquid crystal‐forming ethyl celllulose (EC) solutions in m‐cresol was determined by means of a cone‐plate type viscometer at 30°C. The effect of molecular weight (MW) on the behaviour was also determined. The relaxation behaviour could be fitted with the following equation: where σi and σf are steady‐state shear stresses at shear rate \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm i}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm f}$\end{document}, σ(t) is time‐ dependent stress, A1 and A2 are constants, τ1 and τ2 are relaxation times, t is time, and tc is a characteristic time. When log σ* was plotted against time, one straight line was obtained for isotropic solutions, whereas anisotropic solutions yielded two straight lines. This suggests that the liquid crystalline solutions have two separate relaxation processes: Process 1 has a relatively short relaxation time, and process 2 has a long one. The parameters τ1, τ2, and A2 were greatly dependent on polymer concentration, combination of \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm i}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm f}$\end{document}, and MW, whereas A1 was independent thereof and was close to unity. The process 1 was supposed to be valid for individual molecules, and process 2 for liquid crystalline domains or randomly aggregated or entangled molecules.
SYNOPSISThe stress growth and relaxation behavior of liquid crystalline hydroxypropyl cellulose solutions in dimethylacetamide was characterized by using exponential functions. The parameters evaluated for the stress growth and relaxation processes were compared and the shear history effect on the parameters was determined. The exponential functions proposed were valid for our system. The isotropic solutions had one retardation time and one relaxation time, whereas the liquid crystalline solutions had plural retardation and relaxation times. The concentration dependence of the parameters for the stress growth process was similar to that for the stress relaxation process and to that for the steady-state shear viscosity. The stress growth and relaxation behavior for the liquid crystalline solutions was originated from the change (deformation or decrease) in liquid crystalline domains. The deformation of liquid crystalline domains with shear seemed to be slower than the recovering of the domains to original shape. The stress growth process was a progressive event, whereas the relaxation process was a sudden event. Stress relaxation behavior for the liquid crystalline solutions was sensitive to the shear history. 0 1994
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