Rheology of pharmaceutical systems: oscillatory and steady shear of non-Newtonian viscoelastic liquids.

A comparative analysis of oscillatory and steady shear rate measurements was made on carboxymethylcellulose solutions of two concentrations and two viscosity grades. In the oscillatory methods, the material is examined under nearly quiescent equilibrium conditions. Steady shear, conversely, produces large deformations and may yield false results, often interpreted as thixotropy, if the shear rate experiment is not conducted properly. Solutions of carboxymethylcellulose at concentrations ordinarily used in drug product formulations were examined by oscillatory and steady shear methods at low shear. Viscoelastic properties of pharmaceutical materials were measured using a newly developed oscillometric instrument. Mathematical expressions, formulated on the basis of a generalized Maxwell model for viscoelasticity and viscosity in steady shear, were correlated using these two rheological test methods. The results showed large increases in viscosity and relaxation time with increasing carboxymethylcellulose concentrations as well as with increasing molecular weights of the polymeric solute. The behavior of carboxymethylcellulose under both oscillatory and steady shear agreed with theory, linking the two methods of testing. Applications in pharmacy to this rheological analysis are presented. The present investigation attempted to bridge the gap between oscillatory and steady shear methods, demonstrating how both can find appropriate use in the analysis of non-Newtonian materials of pharmaceutical importance.