Strain-induced β-α polymorphic transition in iPP as revealed by microhardness

5 pags., 4 figs.The microhardness (H) technique was used for characterization of the β-α polymorphic transition in isotactic polypropylene (iPP). For this purpose the microhardness in the damage zone of a tensile loaded deeply edge-notched (DEN-T) β-iPP specimen was mapped. Mapping of H was performed, both along the loading direction (central) and close to the shorter fracture edge. Around half-length of the plastic zone a sharp increase of the H values in both cases was observed. The H increase is related to the β→α polymorphic transition. Microvoid formation in the central part results in lower H values. However for the edge zone close to the top of the fracture surface unusually high H values (around 200 MPa) are obtained. The latter are explained in terms of the formation of microfibrils due to crazing during deformation which are characterized by very high molecular orientation as reported from X-ray analysis.Grateful acknowledgement is due to DGICYT (Grant PB94-0049), Spain, for the support of this investigation. The partial support of German Science Foundation (Grant No DFG Ka 1202/4-1) is also highly appreciated. It is a pleasure for one of us (S.F.) to acknowledge the tenure of a sabbatical grant from DGICYT, Spai