The effect of temperature and concentration on N-1 and tumbling in a liquid crystal polymer

The shear rate min, at the relative minimum in the N-1 flow curve is studie d as a function of temperature and concentration for liquid crystalline (hy droxypropyl)cellulose (HPC). For lyotropes, at least, gamma(min) is the she ar rate necessary to halt director "tumbling" and align the molecules. HPC is a convenient polymer for studying the relationship between lyotropic and thermotropic liquid crystalline polymers because it exhibits a pure thermo tropic phase at elevated temperatures, and room temperature lyotropic phase s at moderate concentrations in m-cresol. At the highest possible polymer c oncentration at which reliable rheology data can be obtained (around 70 wt % polymer), indirect evidence for director tumbling is observed, in that N- 1 retains a local minimum versus shear rate. For the highest concentrations this minimum N1 value is positive, rather than negative, as is the case at lower concentrations and as is predicted by the Doi theory. Empirical time -temperature and time-concentration shifting can be used to estimate gamma min from measured values of the shear viscosity. (C) 1999 The Society of Rh eology. [S0148-6055(99)00401-0].