The Effects of Molecular Structure on the Melt Rheology of Low Density Polyethylene

Abstract

A rapid method for obtaining information on average molecular weights, molecular weight distribution, molecular size, and long chain branching (LCB) characteristics of low density polyethylene (LDPE) from gel-permeation chromatography (GPC) and intrinsic viscosity (IV) was recently reported. This investigation is primarily concerned with the effect of molecular weight and molecular structure, obtained by the use of the GPC-IV method, on the melt rheology of whole LDPE. Some of the assumptions involved in the GPC-IV method are also examined. It is found that the melt viscosity of whole LDPE depends not only on weight average molecular weight, but also strongly on the LCB characteristics. The combined influence of these two molecular parameters on melt viscosity can be described through the effect which each has on the weight-average mean square radius of gyration of the polymer coil, (S2)w. The experimental data indicate that the dependence of zero shear viscosity on (S2)w is substantially greater for LDPE than for linear polymers. The apparent flow activation energy at zero shear is found to be about 12 kcal/mole for all whole LDPE samples of differing LCB characteristics studied in this investigation. Our calculations based on various assumptions in the GPC-IV method suggest that the polydisperse model of the Zimm and Stockmayer equation, which relates molecular weight and branching frequency to the branching parameter (g), and the exponent of b = 1/2 in the equation, gb = [ƞ]B/[ƞ]L are the best choices for whole LDPE. [ƞ]B is the IV of branched molecule and [ƞ]L is the IV of linear molecule of the same molecular weight.