IN-SITU OBSERVATION OF THE PLASTIC-DEFORMATION OF POLYPROPYLENE SPHERULITES UNDER UNIAXIAL TENSION AND SIMPLE SHEAR IN THE SCANNING ELECTRON-MICROSCOPE

The alpha- and beta-spherulites in polypropylene (PP) were identified by direct observation in the scanning electron microscope after appropriate etching. The alpha-phase has a dark contrast while the beta-phase is brighter. Results concerning the individual behaviour of alpha- and beta-spherulites in polypropylene samples which have been subjected to tensile and shear loading are reported. Under tensile loading, the alpha-spherulites exhibit a brittle behaviour, while the beta-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 alpha-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 alpha-structure deforms less than the global deformation while the beta-phase compensates for this lack of plastic deformation of the other phase. The interlocked structure of the alpha-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 beta-spherulites allow the initiation and propagation of plastic glide more easily. The presence of a beta-phase in PP with coarse spherulites considerably improves the mechanical properties at room temperature.