Viscoelastic Properties of Monodisperse Polymers. I. Tensile Creep Studies of Poly(α-methylstyrene)

Abstract

Tensile creep measurements were made on seven monodisperse polymer samples of poly(α-methylstyrene) ranging in molecular weight from 69000 to 1740000. The temperature dependence of the tensile creep compliance was described by the method of reduced variables, and the shift factors were identical for all samples, except for those on the level of glassy hardness. The steady-state compliance and tensile viscosity were determined by using the extrapolation method of Ninomiya. It was shown that the dependence of these quantities on molecular weight is much different from that predicted by the Rouse–Bueche theory or by the semi-empirical Ferry–Landel–Williams theory. That is, in the region of molecular weight somewhat above the entanglement region the steady-state compliance is independent of molecular weight. At lower molecular weights the values of the steady-state compliance increase with decreasing molecular weight. The dependence of tensile viscosity on molecular weight follows the familiar 3.4 power law up to molecular weight of about 2×105, but there is a distinct decrease in the viscosity dependence upon molecular weight at higher molecular weights. The value of the average molecular weight between entanglements was derived from tensile creep compliance versus temperature curves by use of a reference value of time equal to 1000 sec. The value evaluated decreased with increasing molecular weight up to about 2×105 and then leveled off to a certain constant value.