APPRAISAL OF A MOLECULAR-WEIGHT DISTRIBUTION-TO-RHEOLOGY CONVERSION SCHEME FOR LINEAR POLYETHYLENES

A theoretical scheme relating quantitatively the rheology of linear po lymer melts to molecular weight distribution (MWD) would be of great s ignificance in that it can help interpret polymer rheology and process ing behavior in terms of molecular weight characteristics. The present paper is concerned with the appraisal of such a theory-the partition model of Bersted. The theory involves four parameters that are fitted to experimental data on a variety of linear polyethylenes. Although th e model does fit the data with better than 10% accuracy, the parameter s show some variability that points to theory limitations. An error an alysis also shows that this variability is partly accounted for by exp erimental errors in MWD determination that are amplified in the predic ted rheological properties. The rheological properties are shown to be heavily influenced by the high molecular weight tail of the MWD. Erro rs in MWD determination are detrimental for a MWD-to-rheology conversi on, even if a conversion scheme were perfectly accurate. Low levels of long-chain branching that may possibly be present are also detrimenta l to an MWD-to-rheology conversion. The inverse problem of determining the complete MWD from rheology appears to be practically infeasible f or broad MWD polymers.