Correlations of single-point parameters of linear rheology and molecular weight distribution of polypropylene homo- and copolymers

For various purposes, it is required to compress the shape of the molecular weight distribution (MWD) of polymers into a limited set of parameters. With increasing molecular weight and polydispersity, the MWD data obtained from chromatography become increasingly unreliable due to deficiencies in the high molecular weight region, making estimation via melt rheology more preferable. A number of empirical parameters obtained from melt rheology can be related back to MWD parameters. The target of this study is to establish the reliability of such relations for polypropylene homo- and copolymers. It is found that correlations between polydispersity from rheological crossover modulus and polydispersity via chromatography are not always valid. Therefore, the range of applicability must be kept in mind when attempting predictions based on these correlations because rheological measurements are sensitive to molecular characteristics in ways different from chromatography. The use of a modified polydispersity index is shown to be more reliable. This works puts to the test the correlation of parameters derived from melt rheology and chromatography, for the case of polypropylenes with different molecular architectures. While the modified polydispersity index provides solid predictions, for broader molecular weight range the applicability of other empirical parameters is less favorable than predicted by theory.image