Effect of Molecular Weight Distribution on Viscosity

The effect of molecular weight distribution, MWD, is somewhat more subtle but still very important. In general, commercial polymers have a rather broad molecular weight distribution, although materials produced using metallocene catalysts can have polydispersities 

A very broad molecular weight distribution hampers the use of the viscosity for molecular characterization, because the longest molecules can be stretched while contributing little to the stress as a result of the dominant viscous contribution of the shorter molecules. 

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Middlemans. Effect of molecular weight distribution on viscosity of polymeric fluids. J. Appl. Polymer. Sci. 11,417-424 (1967). 

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