Using rheological data to determine the branching level in metallocene polyethylenes

A technique for using rheological information to determine the level of long chain branching in polyethylenes produced using a constrained geometry catalyst and having low levels of branching is described. The complex viscosity as a function of frequency is used, along with a molecular weight distribution (MWD) determined by gel permeation chromatography (GPC). A procedure previously proposed to infer the MWD of linear polymers from complex viscosity data is used to determine a "viscosity MWD" for the branched materials, and this is compared with the GPC MWD. The difference in the location of the peaks in the two curves is then correlated with the level of branching. Advice is provided regarding the range and density of experimental viscosity data needed for a reliable determination of long chain branching.