Time Constants from Shear Viscosity Data

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Abstract
The process of extracting time constants from shear‐viscosity data was investigated. Fourteen viscosity functions were fitted to 12 diverse sets of experimental data using a Gauss‐Newton optimization technique. It was found that the three‐ and four‐parameter models, while providing excellent fits to the data, did not provide time constants with as low an expected error as the better two‐parameter models. An analysis of the sensitivity of the error in the time constant to error in the viscosity data showed that regions at very low shear rate and at shear rates near the reciprocal of the time constant were critical. Time constants from shear‐viscosity data for a set of low‐density polyethylenes correlated well with Rouse times based on the weight average molecular weight.

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