ENERGY CALCULATIONS FOR ISOTACTIC POLYPROPYLENE - A COMPARISON BETWEEN MODELS OF THE ALPHA-CRYSTALLINE AND GAMMA-CRYSTALLINE STRUCTURES

Packing energy calculations have been performed on both alpha and gamma forms of crystalline isotactic polypropylene with the following objectives: (a) to compare the stability of the two crystal lattices; (b) to compare the effects of the up-down statistical disorder in the two polymorphs; (c) to investigate the stability of a possible alternative model for the gamma phase; (d) to define a compromise structural model for gamma-iPP satisfying both the crystallographic and energetic requirements. The results indicate very nearly identical values of the packing energies of the two lattices, the gamma form and the alpha form displaying a preference for the up-down inversions to occur at closely related sites. While the alternative model for the gamma structure shows a substantially higher packing energy, the minimum-energy structure deviates only marginally from the one refined from X-ray powder diffraction data. The compromise model shows a disagreement factor nearly as low as the value obtained considering only X-ray data and an energy only 0.4 kcal/mol of trimer higher than the minimum value.