New aspects of the β-α Polymorphic transition in plastically deformed isotactic polypropylene studied by microindentation hardness

6 pags., 5 figs.The β-α polymorphic transition in plastically deformed isotactic polypropylene (iPP) was characterized by means of microindentation hardness. For this purpose microindentations were mapped onto the surface of the necking zone of a tensile loaded injection molded β-iPP "dumb-bell" specimen. Results evidence a sharp decrease of the H-values instead of the expected H-increase due to the β-α polymorphic transition. Far away from the necking zone an H-increase is detected. It is shown that the destruction of the starting isotropic spherulitic structure and the decrease of crystallinity in the necking zone gives rise to lower H-values. However, at larger distances from the neck, the emerging fibre structure induces a better chain orientation that results in a slight H-increase. Analysis of the isotropic and necked samples before and after their annealing using DSC and WAXS supports the assumption regarding the role of the microvoids in decreasing the hardness value.Grateful acknowledgement is due to the MEC, Spain (Grant FIS2004-013314) for the generous support of this investigation. One of us (S.F.) deeply appreciates the financial support of NATO—Spain for the award of a fellowship making possible his stay at Instituto de Estructura de la Materia, CSIC, Madrid, where this study was carried out. S.F. and M.B. acknowledge also the hospitality of the same Institute