Long chain branching indices of low density polyethylenes from size exclusion chromatography and 13C-NMR spectroscopy

Size exclusion chromatography (SEC) and 13C-NMR analyses have been used to measure long chain branching indices in low density, free radical-polymerized polyethylenes. Calculation of long chain branching frequency from SEC data requires the assumption of a molecular model. Generally, a randomly branched polymer is assumed, with branches originating at trifunctional branch points. Long chain branch concentrations determined by SEC analysis alone appear to be valid only in a relative sense. The parameters used in the model assumptions vary from polymer to polymer. Despite these limitations, SEC analyses appear to be internally consistent when applied to polyethylenes made by a single manufacturer and process. Comparisons of absolute values of long branch frequency between products from different reactor types require NMR as well as SEC data.