FROM DYNAMIC MODULI TO MOLECULAR-WEIGHT DISTRIBUTION - A STUDY OF VARIOUS POLYDISPERSE LINEAR-POLYMERS

The linear viscoelastic behavior of various polydisperse linear polyme rs in the melt is used to predict their average molecular weights and polydispersity index. The method is based on simplified molecular dyna mics and has been previously shown to enable a correct description of the dynamic moduli of polypropylenes from the knowledge of their molec ular weight distribution (MWD). This so-called forward calculation onl y requires a few parameters, namely the scaling law for the zero-shear viscosity of narrow fractions eta(0) = f(M), the plateau modulus G(N) (0), and the value of the molecular weight between entanglements M-e. The main goal of the present work is to find a solution to the ''inver se'' problem. To avoid the problem of becoming ill-posed, the shape of the MWD has to be prescribed. Using the assumption of a typical logar ithmic bell-shaped Wesslau MWD, the method has been proven to be succe ssful for the recovery of the weight average molecular weight and of t he polydispersity index of many linear polymers in a large range of mo lecular weights and polydispersity indices. Rough estimates of M-z hav e been obtained for well characterized polypropylenes by the use of a generalized exponential distribution (GEX). Attention has also been fo cused on some requirements for the frequency window which are necessar y for reasonable accuracy of the values of the parameters M-w and I-p. It was found that, because of tube renewal and constraint release, th ree or four decades is generally sufficiently for a wide range which c an be easily achieved with routine rheological dynamic measurements. ( C) 1997 The Society of Rheology.