The Relation between Molecular Weight and Viscosity of Poly(Dimethyl Siloxane) in the High Shear Rate Region

Abstract

The upper (or second) Newtonian, viscosities (η_∞) of poly (dimethyl siloxanes) of different molecular weights were determined with a capillary viscometer driven by means of N₂ gas pressure. In order to diminish an experimental error, the reservoir of the viscometer was planned so as to have its much greater radius than that of the capillaries used. By the introduction of such a combination of capillaries and a reservoir, the upper limit of measurable shear rate was increased to 4.3×10⁶sec^-1.
These polymers η_∞ can be sorted into three groups according to their molecular weights, those below M_c', those above M_c' or up to M_c, and those above M_c. (M_c' is the critical molecular weight at which the increases in the density and in the activation energy of flow with increasing molecular weight saturate, and M_c is the critical molecular weight at which the entanglement between the polymers begins.)
The polymer below M_c' has a Newtonian behaviour throughout the whole shear rate range. The polymer above M_c' up to M_c is non-Newtonian fluid, and its upper Newtonian viscosity depends on the molecular weight. In the case of its being above M_c, the flow curves of the polymers join in the upper Newtonian region, and η_∞ is almost independent of the molecular weight.