J. Varga et Gw. Ehrenstein, HIGH-TEMPERATURE HEDRITIC CRYSTALLIZATION OF THE BETA-MODIFICATION OFISOTACTIC POLYPROPYLENE, Colloid and polymer science, 275(6), 1997, pp. 511-519
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.