Energy Calculations for Isotactic Polypropylene:  A Contribution To Clarify the β Crystalline Structure

An analysis of the crystal structure of the β polymorph of isotactic polypropylene (β-iPP) is carried out with the methods of molecular mechanics, taking into account simultaneously intra- and intermolecular 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 β-iPP as compared to its other crystalline polymorphs. The most stable β-iPP structure is found in space group P31. Up−down disordering modes, found also in α- and γ-iPP, have marginally higher energy and give rise to P3121 symmetry, without superposition of methyl groups belonging to anticlined chains. This structure, involving random up−down disordering, is thus proposed as the best average structure for individual β-iPP domains. Mechanisms for twinning among different domains are also investigated in order to explain the observed hexagonal symmetry of electron diffraction patterns.