Rheological Properties of Acetoxypropylcellulose in the Thermotropic Chiral Nematic Phase

Abstract

Experimental data for the rheological behavior of two thermotropic liquid crystalline (LC) samples of acetoxypropylcellulose (APC) with different molecular weights, at 120°C, and in shear rates between 0.01 and 10_s ⁻¹, are presented and analyzed in the framework of the continuum theory for LC polymers recently proposed by Martins¹. The viscosity η(γ.)shows a strong shear thinning in the range of shear rates γ. studied, with an hesitation at shear rates of about 0.1-0.2 s⁻¹, depending on the molecular weight, and the first normal stress difference N_l(γ.) shows only positive values, increasing with shear rate γ., with an hesitation at shear rates of an order of magnitude higher, i.e. about 1-2 s^−l, also depending on the molecular weight. The hesitation points of the flow functions are displaced towards lower values of the shear rate, with increasing molecular weight. For small and intermediate γ. the shear viscosity of the higher molecular weight sample is greater than the corresponding viscosity for the lower molecular weight sample, but this pattern is reversed at higher γ., the crossover point being at γ. = 1.5 s⁻¹. The molecular weight dependence of the first normal stress difference follows a similar pattern. All these observations can be interpreted by Martins' theory. The expressions for η(γ.) and N₁(γ.) derived from this theory fit very well to the experimental data, therefore allowing for some fundamental viscoelastic parameters to be estimated.