Appraisal of a molecular weight distribution‐to‐rheology conversion scheme for linear polyethylenes

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Abstract
A theoretical scheme relating quantitatively the rheology of linear polymer melts to molecular weight distribution (MWD) would be of great significance in that it can help interpret polymer rheology and processing 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 the model does fit the data with better than 10% accuracy, the parameters show some variability that points to theory limitations. An error analysis also shows that this variability is partly accounted for by experimental errors in MWD determination that are amplified in the predicted rheological properties. The rheological properties are shown to be heavily influenced by the high molecular weight tail of the MWD. Errors in MWD determination are detrimental for a MWD‐to‐rheology conversion, even if a conversion scheme were perfectly accurate. Low levels of long‐chain branching that may possibly be present are also detrimental to an MWD‐to‐rheology conversion. The inverse problem of determining the complete MWD from rheology appears to be practically infeasible for broad MWD polymers. © 1993 John Wiley & Sons, Inc.

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