Existing viscosity data on polyethylenes were studied as a function of temperature and molecular weight. New measurements were made on low density polyethylenes which had molecular weights useful for the test of theory. Viscosities of normal alkanes, considered as a lower extension of polyethylenes, were also correlated. The equation, log η = A log (M) − B (where η is absolute viscosity, M is molecular weight, and A and B are constants), is applicable over a wide range of Newtonian viscosities. Bueche's theory postulates that A approaches unity for chain lengths up to a critical molecular weight, Mc, where molecular entanglement abruptly becomes important. Above Mc, the theory requires A to be 3.4. Values of A for normal alkanes are shown to approach unity below Mc. The entanglement point, Mc, is temperature dependent and occurs at relatively low molecular weights. Above Mc, A for linear polyethylenes is near 3.4. However, certain polyethylene data give higher values for A. Activation energies for viscous flow, ΔE*, were obtained for polyethylenes and normal alkanes. They were found to be internally consistent and to vary as a linear function of the log of molecular weight. The change of ΔE* with less general functions of molecular weights shows a maximum curvature near Mc. Results are considered in terms of flow theory.
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Porter et al. (1960) studied this question.