Audio-Frequency Measurements of the Viscoelastic Properties of Polydimethylsiloxane Liquids

Abstract

Results of previous measurements of the viscoelastic behavior of a range of polydimethyl siloxane liquids at frequencies of alternating shear stress above 10 kcps have been shown to be in agreement with theoretical predictions based upon modifications of the Rouse theory. The higher‐molecular‐weight members of this series exhibit entanglement coupling, giving rise to a plateau in the curves of G′ and G″ (the components of the complex rigidity modulus) versus frequency, centered around a frequency of 1000 cps. The existence of this plateau region is a factor of considerable importance in the theoretical analysis and interpretation. Previous measurements did not extend into this region, for the lack of a suitable experimental technique and the shear properties at these lower frequencies, in the audio range, were deduced from the measured behavior at higher frequencies. The evidence in support of this extrapolation procedure was good and self‐consistent but lacked experimental verification. This has now been provided by shear‐wave measurements obtained over the frequency range 20–1500 cps on four of the polydimethylsiloxane liquids used previously having viscosity grades from 1000 to 100 000 cS. Excellent agreement is found between the results now obtained and the previous theoretical predictions, which have not been modified in any way. The design, construction, and performance of the new experimental system is described. It is based upon torsional shearing of an annulus of liquid placed between an inner rigid cylindrical plug and an outer cup. Separate electromagnetic excitation and detection systems are used, and the theory of the measuring procedure is described. It can be used to measure liquids with steady flow viscosities exceeding 1 P.