Normal stress, shear recovery and viscosity in polydimethyl siloxanes

Abstract

The Weissenberg-Roberts rheometer and a cone and plate viscometer have been used to study the viscous and elastic properties of some polydimethyl siloxane fluids as a function of shear rate and of temperature. Similar experiments with molten polythene provide a comparison between the two types of polymer. It is found that the normal stress is small compared to the tangential stress at low rates of shear, but at high rates the normal stress is the larger; that is, the curves representing the shear rate variation of these stresses cross. The value of the stresses at the cross-over is found to be independent, to a good approximation, of molecular weight and of temperature. The elastic properties determined from normal stresses and recovery measurements are compared in the light of theoretical studies by other workers. It is shown that the so-called ‘Hooke's law in shear’, i.e. the proportionality between total recoverable strain and shear stress, is not applicable to these materials. Consideration is also given to the criterion required for a material to show normal stresses, and the structural features involved are discussed.