Viscoelastic studies of linear polydimethylsiloxanes

Abstract

The viscoelastic properties of four polydimethylsiloxane liquids of narrow molecular mass distribution have been determined over the frequency range 10 -1 Hz to 450 MHz at a temperature of 296.2 K. The molecular masses of the polymers range from 66 000 to 189 000 and are appreciably greater than the critical value, M c = 21000, above which entanglements occur. At low frequencies (10 -1 to 10 2 Hz) where entanglement effects predominate it is found that for the three liquids of lowest molecular mass the reptation theory of Doi and Edwards, and developments of the Rouse theory are equally satisfactory in describing the viscoelastic behaviour. However, for the polymer of highest molecular mass only the Doi-Edwards theory provides a satisfactory description. These results are regarded as conclusive evidence in support of the concepts of reptation theory. At higher frequencies, where the behaviour is dominated by the motion of the polymer chain between entanglements, reptation theory is inapplicable. Existing developments of the Rouse theory do not provide completely satisfactory descriptions of the experimental results and improvements in the theoretical analysis of chain motion in this region are required.