Molecular weight distribution, impact on morphology, crystallinity and mechanical properties

Abstract: This master thesis is the result of cooperation between Lund University and Tetra Pak, performed by a student at Engineering Nanoscience, LTH. The aim is to explain how different molecular weights and molecular weight distribution of isotactic homo-polypropylene affect resulting properties. Examined properties are mechanical properties by tensile testing and tensile impact, shear viscosity by capillary rheometry and crystallinity by Differential Scanning Caliometry. Aim was also to find good rheological measures for polydispersity, by comparing these measures of molecular weight distribution by Size Exclusion Chromatography. These rheological examinations were executed by use of oscillating rheometry sweeps at different temperatures and calculations build upon equations found in literature. The different types of polypropylene examined for mechanical properties were injection moulded. In injection moulding shear forces create an anisotropic resulting material, with higher orientation in the direction of shear. This was examined by testing the mechanical properties in different directions of injection moulded plates. The plates were stored after processing to examine if the properties vary during storage, up to 6 weeks. Scanning Electron Microscopy was utilised to analyse the different zones created by injection moulding in the cross section of the plates. Findings were among other that molecular weight distribution strongly influence the mechanical properties, and that the mechanical properties vary widely with storage time after injection moulding. This variations over time is assumed to arise from post crystallisation and physical ageing.