Viscoelastic relaxation of polydimethylsiloxane liquids

Abstract

The shear mechanical impedance of six polydimethylsiloxane liquids of viscosity grades ranging from 100 to 100000 cS has been measured over the frequency range 10 kc/s to 78 Mc/s, and at temperatures from –50 to +50 °C. It was found possible to reduce the experimental data for each liquid to give a single curve for each component of the mechanical impedance by using the method of reduced variables. From these curves the components of the shear modulus and the dynamic viscosity have been determined as a function of frequency, normalized to a temperature of 30 °C. The theory of Rouse, as developed by Ferry and others, describes adequately the viscoelastic relaxation of those siloxane liquids of comparatively low molecular weight in which entanglement coupling between molecules is absent. Quantitative predictions from the theory require a knowledge only of the variation of viscosity as a function of molecular weight and of the molecular weight distribution. In order to account for the viscoelastic behaviour of the higher molecular weight siloxanes the theory has been further developed to describe the properties of polydisperse polymers in which considerable entanglement coupling is present. Good agreement between the experimental and the theoretical results has been obtained. It has thus proved possible to predict the viscoelastic behaviour of polydimethylsiloxane liquids from a knowledge only of the molecular weight distribution and of the variation of viscosity with molecular weight.