High-temperature hedritic crystallization of the beta-modification of isotactic polypropylene

High-temperature crystallization of the beta-modification of polypropylene (beta-IPP), induced by an active beta-nucleating agent, was studied by polarizing light and scanning electron microscopy (SEM). It was established that, in the early stage of crystallization of beta-iPP, hedrites were formed as precursors of the spherulitic crystallization. Hedrites seen flat-on are characteristic hexagonal formations (hexagonites) with low birefringence. The central core of hedrites had positive birefringence. Hedrites seen edge-on were rod-like formations with strong negative birefringence which would transform into a radially symmetric spherulitic form through sheaf-like and then oval (ovalite) arrangements. According to the SEM micrographs, hedrites are clusters of multilayer lamellar crystallites. On the surfaces of lamellae hexagonal etch pits appear referring to screw dislocations. They are responsible for branching and proliferation of lamellae. Hedrites may reach considerable sizes (several hundred micrometres) presumably due to a coordinated cooperative growth of lamellae. There is an unequivocal correlation between the character of birefringence and the morphological structure shown by SEM of hexagonites, rods, ovalites and spherulites. It was demonstrated that, al temperatures of isothermal crystallization higher than the critical T(beta alpha), mixed polymorphic structures of alpha- and beta-modifications were formed.