Enhanced beta to alpha recrystallization in beta isotactic polypropylene with different thermal histories

A small but meaningful increase in the content of beta-crystals was observed in beta-nucleated isotactic polypropylene (iPP) samples isothermally crystallized at high temperature and then cooled to room temperature. The secondary crystallization of these crystals could also be linked to a distinct exothermic effect revealed by differential scanning calorimetry upon cooling. These crystals constituted of thinner lamellae certainly show earlier melting upon reheating, with respect to the isothermally crystallized fraction. As such, the enhancement in beta alpha recrystallization in iPP cooled below 100 degrees C is attributed to the formation of a self-nucleated or partially ordered melt upon melting of these crystals, which induces the fast nucleation and growth of the alpha-phase, because of larger undercooling with respect to the beta-phase at elevated temperatures. On the other hand, the recrystallization of the beta-phase from partially molten beta-iPP samples is tentatively assigned to homo-epitaxial nucleation on the surface of thicker beta-phase lamellae.