Rheological Characterization of Long Chain Branching in a Melt of Evolving Molecular Architecture

A simple rheological method is presented for estimating the degree of long chain branching, x (fraction of branched chains or, equivalently, the average number of branches per chain), of a polymer melt undergoing the early stages of a cross-link inducing reactive modification. The model is tested on a series of polypropylenes of increasing x, all produced from the same linear precursor in the presence of varied peroxide concentrations. The branching indices predicted from analyzing the zero shear viscosity of the samples are in good agreement with those evaluated by size exclusion chromatography combined with intrinsic viscosity measurements. Monitoring and interpreting the relatively modest, yet readily measurable, increase of the zero shear rate viscosity which accompanies branching is a more appealing alternative, on which our method is based.