Elasticity of Polydimethylsiloxane Melts

Abstract

THE observation of Mieras and Van Rijn¹ that the steady shear flow compliance of polymer melts is dependent only on the width of the molecular weight distribution, characterized by M̄z/M̄w, is in accord with our similar measurements on polydimethylsiloxane melts. The shear stress p₁₂ and first normal stress difference p₁₁–p₂₂ were measured with a Weissenberg cone and plate viscometer as a function of shear rate ɣ̇ for fractions and blends of polydimethylsiloxane. (With the usual convention 1 is the flow direction, and 2 is normal to the measuring surface.) The Newtonian viscosity η and the limiting value of the normal stress coefficient θ (defined by p₁₁–p₂₂/ɣ̇²) were found, for molecular weights greater than the “entanglement” molecular weight Me (55,000), to vary according to the equations where G and L are constants. The steady shear compliance J was therefore given by The experimental values of J are shown in Fig. 1, together with data from the tensile stress relaxation of anionically polymerized polystyrenes^2,3. There are different constants G for the two polymers, and the values of G correlate well with the shear modulus of a rubber network with a molecular weight between cross-links equal to Me where ρ is the polymer density, R the gas constant and T the absolute temperature.