Evaluation of MolecularWeight Distribution of Isotactic Polypropylene from Linear Viscoelasticity Data

The molecular weight distribution of 4 commercial isotactic polypropylene (iPP) samples was estimated using rheological data. The dynamic moduli data, obtained from rheological measurements in linear viscoelastic region was used for calculation of relaxation time spectrum, h(τ) and then shear relaxation modulus, G(t). Finally, applying Thimm Kernel function and relaxation modulus data in generalized mixing rule, the molecular weight distribution was calculated and compared to the experimental results obtained by gel permeation chromatography (GPC). The results showed that normalized relaxation time spectrum is a good measure for qualitative comparison of molecular weight distributions of various samples. Furthermore, using Thimm Kernel function, it is possible to calculate the molecular weight averages of iPP samples with good accuracy. However, good agreement between the molecular weight distributions determined by GPC and by rheological data evaluation can be achieved only by changing the β mixing parameter value from 3.84 to about 1 and subtraction of Rouse part of relaxation time spectrum.