Determination of molecular weight distribution by melt rheology – a revision of Tuminello's model on isotactic polypropylene

The present work is creating an extended remake of a long-existing modulus model for extracting information about molecular weight distribution ( MWD ) from the melt rheology data of polymers. All key variables are redefined for isotactic polypropylene, iPP. Measurements of high-temperature size exclusion chromatography coupled with multi-angle laser light scattering, HT-SEC-MALLS, of three technical iPP grades differing in terms of their average molecular weight and the width of their distribution both, are projected onto their dynamic mechanical data recorded at T = 190°C. For both scaling laws, η 0 ∼ M w α and 1/ω ∼ M α +1 , it clearly shows that the exponent α is passing a turnover from 3.6 to 3.0 as soon as the reptation molecular weight of M r = 4.9×10 5 g/mol is exceeded. The latter quantity is related to the plateau modulus, which is reevaluated from literature data to yield G N 0 = 5.7×10 5 Pa. With these two different exponents being employed to interpret the relaxation behavior of iPP melts, their overall cumulative molecular weight distribution, CMWD , can be derived correctly. Missing information at the low molar weight flank need to be filled up by using appropriate approximation functions. Additionally to the tanh-function being already in place, another useful model based on the distribution of a Schulz-Zimm type is introduced. Errors in data approximation can be effectively ruled out by the zero-shear viscosity η 0 as an independent consistency criterion. The new model edition hides potentially existing short-chain branches, SCBs, and the quality of the obtained MWD s can compete with those ones from HT-SEC.