Simultaneous in-situ WAXS/SAXS and d.s.c. study of the recrystallization and melting behaviour of the α and β form of iPP

Abstract

The behaviour of the α and β form of isotactic polypropylene (iPP) was studied during heating. The β form is usually found in iPP specimens that have been subjected to mechanical deformation, e.g. in injection moulded and extruded products. A sample containing a large fraction of the β form was obtained by doping with small amounts of quinacridone. To enable the study of recrystallization and melting of α and β iPP, simultaneous wide-angle X-ray scattering and small-angle X-ray scattering were used. Differential scanning calorimetry was used to facilitate correlations with previous work. Using this combination of techniques, several partial processes could be distinguished. Melting behaviour depended on the formation history. In samples formed at high cooling rates, best comparable to processing conditions, the melting of β crystals is followed by an absolute increase in α crystallinity. In the slowly cooled samples, α and β crystal lamellae melt independently on heating. Also the recrystallization of α crystals, resulting in lamellar thickening, was detected.