Viscoelastic studies of reptational motion of linear polydimethylsiloxanes

Abstract

Viscoelastic measurements are reported on linear polydimethylsiloxanes with molecular weights greater than the critical value for entanglement (M_c). Data covering a frequency range from 10^–2 to 10⁸ Hz are reported for five samples and the frequency dependence of the modulus and shear viscosity in the terminal region are compared with the predictions of the shifted Rouse and Doi–Edwards models. Provided that allowance is made for the molecular-weight distribution of the polymer sample being studied, it is found that for broad-molecular-weight-distribution samples both approaches are adequate. However, for high-molecular-weight narrow-fraction samples it is found that the Doi–Edwards theory provides a better fit than the shifted Rouse model. Comparison of the data at high frequency with experiment indicates that the use of a Rouse model to describe the intermediate region of the viscoelastic spectrum is inappropriate. Suggestions are put forward as to the origins of the difference between experiment and theory.