Non-Newtonian viscosity and non-linear flow birefringence of cellulose tricarbanilate

The shear rate dependence of the intrinsic viscosity and of the intrinsic property, which can be derived from the extinction angle, was investigated for a sample of cellulose tricarbanilate. Its number-average molecular weight was 4.4 × 10⁵; the solvents used were dioxane at 25°C and phenyl benzoate at 75°C, the latter being known to break most intramolecular hydrogen bonds. The onset of non-Newtonian viscosity was found at a reduced shear rate of about 0.1, whereas non-linear behaviour in the extinction angle was noticed already at 0.02. The obtained data were compared with the theory by Noda and Hearst¹⁰, which deals with the influence of chain stiffness on the viscoelastic properties of polymer solutions. The qualitative outcome of this theory agrees fairly well with the experimental results. However, the onset of non-linear behaviour, especially in the extinction angle, occurs at lower values of the reduced shear rate than predicted. The stress—optical law remains surprisingly valid for this stiff polymer throughout almost the whole accessible range of reduced shear rates. However, some experiments with the highly viscous solvent tri-o-cresyl phosphate at 30°C show appreciable deviations at reduced shear rates larger than 1.0.