RHEOLOGICAL PROPERTIES OF ACETOXYPROPYLCELLULOSE IN THE THERMOTROPIC CHIRAL NEMATIC PHASE

Experimental data for the rheological behavior of two thermotropic liq uid crystalline (LC) samples of acetoxypropylcellulose (APC) with diff erent molecular weights, at 120 degrees 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 Martins(1). The viscosity eta(gamma over dot) shows a strong shear thinning in the ran ge of shear rates gamma over dot studied, with an hesitation at shear rates of about 0.1-0.2 s(-1), depending on the molecular weight, and t he first normal stress difference N-1(gamma over dot) shows only posit ive values, increasing with shear rate gamma over dot, with an hesitat ion at shear rates of an order of magnitude higher, i.e. about 1-2, s( -1), also depending on the molecular weight. The hesitation points of the flow functions are displaced towards lower values of the shear rat e, with increasing molecular weight. For small and intermediate gamma over dot the shear viscosity of the higher molecular weight sample is greater than the corresponding viscosity for the lower molecular weigh t sample, but this pattern is reversed at higher gamma over dot, the c rossover point being at gamma approximate to 1.5 s(-1). The molecular weight dependence of the first normal stress difference follows a simi lar pattern. All these observations can be interpreted by Martins' the ory. The expressions for eta(gamma over dot) and N-1(gamma over dot) d erived from this theory fit very well to the experimental data, theref ore allowing for some fundamental viscoelastic parameters to be estima ted.