A modulated dsc study on the strain-induced βα-transformation in a β-form isotactic polypropylene

The mechanical strain-induced βα-transition of a β-phase isotactic polypropylene (β-iPP) was studied by modulated differential scanning calorimetry (MDSC). Samples were taken after tensile fracture of a double notched specimen from its process and plastic zones, respectively, and the related calorimetric response was compared to that of the bulk material. In contrast to conventional DSC results, it was found that the βα-transformation was not completed in the process zone. Furthermore, the melting of the α-iPP showed both non-reversing and reversing characteristics, whereas the melting of the β-phase proved to be a reversing process. Therefore, it was recommended to consider the conversion grade of the βα-transformation by the relative change in the melt flux of the reversing β-melting peak. This work was supported by the German Science Foundation (DFG-Ka 1202/4-1).