Modeling molecular weight and degree of branching distribution of low-density polyethylene

A new strategy for the simultaneous modeling of molecular weight distribution (MWD) and degree of branching distribution (DBD) for such branched polymers as bimodal low-density polyethylene is presented, based on the Galerkin h-p finite element package PREDICI, a commercial code. The key problem of how to address a bidimensional distribution is successfully solved by using so-called reduced or pseudo distributions. The branching distribution per chain length is modeled by moment equations, thus yielding distributions of branching moments over chain length. No closure relationships are required. The MWD/DBD curves obtained are the most probable ones for the given reaction mechanisms and kinetic data. Simulated MWD and DBD curves are compared to experimental data from gel permeation chromatography and light scattering; the agreement found is good in general and excellent in one case. The bimodal MWD of the autoclave low-density polyethylene (ldPE) IUPAC Alpha could be reproduced well. It is finally shown that the shapes of MWD and DBD are highly sensitive, quantitative measures for random scission.