HIGH-TEMPERATURE HEDRITIC CRYSTALLIZATION OF THE BETA-MODIFICATION OFISOTACTIC POLYPROPYLENE

High-temperature crystallization of the beta-modification of polypropy lene (beta-IPP), induced by an active beta-nucleating agent, was studi ed by polarizing light and scanning electron microscopy (SEM). It was established that, in the early stage of crystallization of beta-iPP, h edrites were formed as precursors of the spherulitic crystallization. Hedrites seen flat-on are characteristic hexagonal formations (hexagon ites) with low birefringence. The central core of hedrites had positiv e birefringence. Hedrites seen edge-on were rod-like formations with s trong negative birefringence which would transform into a radially sym metric spherulitic form through sheaf-like and then oval (ovalite) arr angements. According to the SEM micrographs, hedrites are clusters of multilayer lamellar crystallites. On the surfaces of lamellae hexagona l etch pits appear referring to screw dislocations. They are responsib le for branching and proliferation of lamellae. Hedrites may reach con siderable sizes (several hundred micrometres) presumably due to a coor dinated cooperative growth of lamellae. There is an unequivocal correl ation between the character of birefringence and the morphological str ucture 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 o f alpha- and beta-modifications were formed.