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 approp riate etching. The alpha-phase has a dark contrast while the beta-phas e is brighter. Results concerning the individual behaviour of alpha- a nd 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 d eforms plastically up to high deformations. The brittle behaviour of t he monoclinic structure is characterized by cavitation at an early sta ge of deformation at the spherulites boundaries or at their equatorial region perpendicular to the tensile axis. Under shear loading, the al pha-phase cavitation disappears and both phases are then capable of un dergoing large strains. However, quantitative characterization of the local deformation in each spherulite species showed that the alpha-str ucture deforms less than the global deformation while the beta-phase c ompensates for this lack of plastic deformation of the other phase. Th e interlocked structure of the alpha-spherulites is discussed as being the leading contributing factor towards their brittleness, since it m akes the plastic glide of this phase very difficult. In contrast, the radial lamellae of the beta-spherulites allow the initiation and propa gation of plastic glide more easily. The presence of a beta-phase in P P with coarse spherulites considerably improves the mechanical propert ies at room temperature.