Processing, Morphology, and Mechanical Properties of Liquid Pool Polypropylene with Different Molecular Weights

Abstract

AbstractSummary: The processability, morphology, and resulting mechanical properties of novel polypropylene (PP) samples of varying molecular weight ($\overline M _{\rm w}$) were studied. A series of homopolymer PP in a wide $\overline M _{\rm w}$ range from 101 000 to 1 600 000 g · mol−1 was polymerised in a liquid pool (LP) under defined conditions. The LP‐PP with a well‐known polymerisation history was manufactured into micro dumbbell specimens by means of a micro injection‐moulding process. The morphology and mechanical properties of the samples were studied by light microscopy, transmission and scanning electron microscopy, and a quasi‐static tensile test. Simulation of the filling behaviour of the molten polymer inside the mould shows that the shear rate increases as the molecular weight increases, up to a maximum shear rate of 750 000 s−1. In addition, the present crystallisation time of the high‐molecular‐weight PP samples is clearly lower than their retardation time; the long macromolecules do not have sufficient time to retard while cooling. As a result of the shear‐induced crystallisation, a highly oriented crystalline structure is formed as a function of the acting shear rate. SEM and TEM investigations show the existence of an oriented shish kebab structure. The density of the shish kebab increases as the molecular weight increases. Evaluations of the shear rate and the morphological structure indicate a critical shear rate of about 300 000 s−1. Above this shear rate level, shish kebab structures are favourably formed. The shear‐induced crystallisation and, therefore, the preferred formation of a highly oriented shish kebab structure lead, obviously, to unusual solid‐state properties of the analysed LP‐PP samples. With a tensile strength up to 100 N · mm−2 and an attainable strain at break of more than 30%, the mechanical performance is much higher than results ever reported in literature.True strain–stress behaviour of moulded the LP‐PP samples of different molecular weight.magnified imageTrue strain–stress behaviour of moulded the LP‐PP samples of different molecular weight.