Viscoelastic relaxation in poly-1-butenes of low molecular weight

Abstract

Measurements have been made of the viscoelastic properties of a range of poly-1-butene liquids of different molecular weights under cyclic shearing stress. The five liquids studied range in steady-flow viscosity at 20 °C from 5.5 to 9330 P corresponding to number average molecular weights from 448 to 2700. Measurements over the temperature range – 60 to +90 °C were made at frequencies of alternating shear of 64 kc/s, 6, 18 and 30 Mc/s. The liquid of lowest molecular weight (448) was nominally pure, having eight repeat units, while the remaining four each had a distribution of molecular weights. In all cases, the dependence of steady flow viscosity upon temperature follows the equation In η = A + B /(T - T 0 ), (1) which is derived from the free-volume equation with a linear dependence of density upon temperature. Recent measurements on a wide range of pure liquids which have viscosities described by equation (1) have been interpreted in terms of a simple phenomenological model for viscoelastic relaxation which allows the behaviour to be predicted (Barlow, Erginsav & Lamb 1967 b ). Analysis of the present results on the liquid of lowest molecular weight shows that the measured behaviour can also be described by this model. For the four liquids of higher molecular weight a second relaxation process is found at lower frequencies. This is attributed to the increased chain length of the molecules giving rise to 'quasi-Rouse’ modes of motion. At low frequencies the results for these four liquids show a behaviour intermediate between that of a simple liquid and that exhibited by a long chain polymer which conforms to the extended form of the Rouse theory.