The linear viscoelastic behaviour of a polymer melt is strongly influenced by its molar mass distri-bution (MMD), and in particular by the high molar mass end thereof. Even small changes of the MMD curve in this region will therefore have a significant effect on the rheological properties of the polymer. While gel permeation chromatography (GPC) is a common technique for the determination of the MMD, its representation of the longest molecules is often inaccurate. It seems therefore useful to assess the MMD via rheological measurements.
Following the rheological characterisation of several PP grades, a routine based on the BSW spec-trum and the Schausberger mixing rule was applied to calculate the dynamic moduli from the MMD. When only GPC data were used as input for the calculation, the thus obtained moduli deviated strongly from the experimental data. However, an excellent correlation of experimental and calculated moduli could be achieved if the GPC curves were supplemented with log-normal distributions in the high molar mass region.
Besides the MMD, also the phase morphology of a polymer melt has a tremendous influence on its viscoelastic behaviour; if phase separation occurs, the deformation of the interface causes additional long-time relaxation processes that are reflected by a marked increase of the dynamic moduli at low frequencies compared to monophasic melts. This makes oscillatory shear rheometry a powerful tool to detect phase separation in a polymer melt. Several PP/PE blends were prepared and rheologically characterised. A potential error source for such an investigation is phase separation that is induced by crystallisation during sample preparation, which might prevail throughout the experiment. Therefore an experimental set-up was designed where the molten blends are directly injected into the measuring cell of a rheometer, thus avoiding crystallisation between compounding and the rheological characterisation. The relaxation time spectrum of a heterophasic polymer blend differs significantly from the combined spectra of the pure components. This discrepancy may be interpreted as the spectrum of the interface itself, and is mainly governed by the interfacial tension and the size of the inclusions. The Gramespacher-Meissner (GM) analysis offers a useful correlation between these parameters and the rheological behaviour, but its applicability is restricted to near-uniform particle sizes.
Therefore a modification to the GM analysis is introduced that allows describing the viscoelastic behaviour of the investigated blends more accurately.