ENERGY CALCULATIONS FOR ISOTACTIC POLYPROPYLENE - A CONTRIBUTION TO CLARIFY THE BETA-CRYSTALLINE STRUCTURE

An analysis of the crystal structure of the beta polymorph of isotacti c polypropylene (beta-iPP) is carried out with the methods of molecula r mechanics, taking into account simultaneously intra- and intermolecu lar interactions. Results indicate that efficient crystal packing may be achieved in a number of different, nearly isoenergetic arrangements , most of them showing the features of frustration. This fact may well be due to the low density of beta-iPP as compared to its other crysta lline polymorphs. The most stable beta-iPP structure is found in space group P3(1). Up-down disordering modes, found also in a and gamma-iPP , have marginally higher energy and give rise to P3(1)21 symmetry, wit hout superposition of methyl groups belonging to anticlined chains. Th is structure, involving random up-clown disordering, is thus proposed as the best average structure for individual beta-iPP domains. Mechani sms for twinning among different domains are also investigated in orde r to explain the observed hexagonal symmetry of electron diffraction p atterns.