No abstract

The crystal structure of the 7 form of isotactic polypropylene (7-iPP) is refined with the Rietveld method on X-ray diffraction data collected at low (-120 °C) temperature. The analysis, leading to the proposal of the totally novel crystal architecture with nonparallel chain axes, is discussed in detail, and the reliability of the proposed structure is assessed, also with reference to alternative models. While the overall structure is best represented in terms of the statistical copresence of anticlined isochiral helices at each crystallographic position, as implied by space group Fddd, local packing modes which cannot retain this feature are satisfactorily described in terms of space groups F2dd or Fd2d. Some relevant implications of the 7isotactic polypropylene crystal structure on the crystalline morphology of this polymer are presented, while issues concerning the development of this novel architecture remain largely open to future contributions.

The structures of the series of oligothienyls (T4-T5-T6) have been determined and refined from X-ray powder diffraction data using the Rietveld full-profile analysis. Unlike polythiophene, all the molecules in the isomorphous series display no internal symmetry, being only approximately planar within 12". Packing is always achieved through a common molecular disposition, which is usually defined as a herringbone arrangement, with an angle between mean planes of adjacent molecules which spans from 60" to 70".

Packing energy calculations have been performed on both a and 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 y phase; (d) to define a compromise structural model for -iPP satisfying both the crystallographic and energetic requirements. The results indicate very nearly identical values of the packing energies of the two lattices, the y form and the a form displaying a preference for the up-down inversions to occur at closely related sites. While the alternative model for the y 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.

The non-parallel chain structure determined for y-phase isotactic polypropylene (yiPP) is confirmed by Rietveld analysis for highly isotactic high molecular mass iPP crystallized at 200 MPa. The new refinement shows that: i) stereoregularity or crystallization pressure do not significantly influence the lattice dimensions; ii) defect inclusion in y-iPP crystals is unlikely. The a and y forms have nearly identical bulk internal energy and density, but a-iPP should be normally kinetically favored over y-iPP which may in turn predominate due to its greater ability to host defects at the crystalline-amorphous interface.