Structural transformations of isotactic polypropylene induced by heating and UV light

Samples of commercial-grade isotactic polypropylene, both neat and nucleated with a specific β-nucleating agent (N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide), were exposed to annealing and UV irradiation. Structural parameters were assessed by X-ray diffraction. Tensile mechanical characteristics were derived from stress–strain curves. The specific nucleation caused more than 70% of the crystalline phase to occur in the trigonal β-modification. The content of the β-phase introduced by the specific nucleation distinctly decreased after a certain induction period, different for annealing at 140 °C and UV exposure. At the same time, the size of the α-crystallites increased while the size of the β-crystallites remained virtually unchanged. These results indicate the β → α recrystallization. On the other hand, the strain-at-break values continuously and monotonically decreased with increasing exposure time reflecting defect formation, both at the surface and in the bulk of the specimens. The differences between the effects of thermal ageing and UV degradation are discussed.