MORE ON THE PREDICTION OF MOLECULAR-WEIGHT DISTRIBUTIONS OF LINEAR-POLYMERS FROM THEIR RHEOLOGY

Two methods have been extended (those due to Tuminello and to Shaw), a mong the several currently available, for calculating the molecular we ight distribution function W(M) of linear polymers from their rheology data. An extension of Tuminello's method is proposed by using the ter minal Rouse relaxation time as a scaling factor. The 79th percentile o f the zero-shear viscosity normalized flow curve is used to evaluate t he terminal Rouse relaxation time. Using this technique, it is possibl e to convert frequency to molecular weight, without the use of an exte rnal standard. An extension to Shaw's numerical technique is also prop osed leading to an analytical expression for the polydispersity ratio. For this purpose the Elbirli-Yasuda-Carreau viscosity model is used t o describe the flow curve. The development is carried out for both the logarithmic and the power law mixing rules considered by Shaw. Due to the approximate nature of both the original methods that Tuminello an d Shaw started out with and the subsequent approximations, the resulti ng distribution should only serve as a first order approximation to th e actual W(M) and the calculated moments of W(M) may be off by as much as about 50% from their true values. Since the more rigorous inverse problem techniques that are also currently available can offer greater accuracy, justification for the use of the approximate methods discus sed in this work rests solely on their relative simplicity and ease of implementation. (C) 1997 The Society of Rheology.