Dynamic viscoelastic behaviour of low-molecular-mass polystyrene melts

Abstract

Measurements have been made of the viscoelastic properties of a range of low-molecular-mass polystyrenes each having a narrow molecular mass distribution. Several experimental techniques using alternating shear have been employed covering the frequency range 10 -2 Hz to 450 MHz and at temperatures from the glass transition temperature to T g + 80 K. The equilibrium (limiting low-frequency) compliance, J e , and the limiting high-frequency compliance, », have each been determined as a function of temperature. Jw is found to be independent of molecular mass at any given value of T— T g . The sample of molecular mass 580 shows a behaviour closely similar to that of non-polymeric supercooled liquids. Samples of molecular mass above 1100 show distinct polymeric behaviour as an additional lowfrequency contribution to the complex shear modulus which can be accounted for by summation over a limited number of Rouse modes of the unentangled polymer molecule. This, combined with the ‘liquid-like’ behaviour of short elements of the polymer chain, gives calculated curves which are in good agreement with experimental results. An alternative analysis describes the complete behaviour by a single equation for the complex compliance of the form J*(jw) = J∞+1/1wn+Jr/(1+jwTr)B, where J r is the retardational compliance. It is not possible to distinguish between these two approaches on the basis of the present data. At temperatures above T g + 40 K measured values of J e are in agreement with calculations based upon the Rouse equations, which can therefore be used as a basis for predicting the observed behaviour both as regards the equilibrium properties and over the relaxation region.