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 Mc', those above Mc' or up to Mc, and those above Mc. (Mc' 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 Mc is the critical molecular weight at which the entanglement between the polymers begins.)The polymer below Mc' has a Newtonian behaviour throughout the whole shear rate range. The polymer above Mc' up to Mc is non-Newtonian fluid, and its upper Newtonian viscosity depends on the molecular weight. In the case of its being above Mc, the flow curves of the polymers join in the upper Newtonian region, and η∞ is almost independent of the molecular weight.