In situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope

Abstract

The α- and β-spherulites in polypropylene (PP) were identified by direct observation in the scanning electron microscope after appropriate etching. The α-phase has a dark contrast while the β-phase is brighter. Results concerning the individual behaviour of α- and β-spherulites in polypropylene samples which have been subjected to tensile and shear loading are reported. Under tensile loading, the α-spherulites exhibit a brittle behaviour, while the β-phase deforms plastically up to high deformations. The brittle behaviour of the monoclinic structure is characterized by cavitation at an early stage of deformation at the spherulites boundaries or at their equatorial region perpendicular to the tensile axis. Under shear loading, the α-phase cavitation disappears and both phases are then capable of undergoing large strains. However, quantitative characterization of the local deformation in each spherulite species showed that the α-structure deforms less than the global deformation while the β-phase compensates for this lack of plastic deformation of the other phase. The interlocked structure of the α-spherulites is discussed as being the leading contributing factor towards their brittleness, since it makes the plastic glide of this phase very difficult. In contrast, the radial lamellae of the β-spherulites allow the initiation and propagation of plastic glide more easily. The presence of a β-phase in PP with coarse spherulites considerably improves the mechanical properties at room temperature.