| Abstract: | In previous work we have shown that the stable form of syndiotactic polypropylene (sPP) adopts a fully antichiral packing in its crystalline lattice. At the same time, we had also discovered a number of prohibited reflections from crystallographic planes with a c-axis component, indicating distortions or defects along the molecular-chain direction and possible departures from the ideal Ibca unit-cell structure. Three studies that appeared in 1996 have proposed a number of possible minimized structures to explore the energetics of departure from such an ideal Ibca symmetry and to account for the prohibited reflections. Two of the studies were based upon energy minimization of the unit cell using empirical or spectroscopically derived force fields. The third model was based upon Rietveld refinement of the bulk X-ray diffractogram combined with ad hoc choices of space groups selected to reproduce the prohibited reflections, and results in a structure with minimal departures from Ibca. In the present article we examine the structural implications of all these models on various reciprocal-lattice sections of sPP by comparing calculated electron-diffraction patterns for appropriate zones with those we obtained experimentally from single crystals. We find that none of the proposed models fully accounts for the experimental data. One of the energy-minimized models (involving translation along the molecular axis) and the Rietveld-refined structure reproduce some of the observed reflections but are not in agreement with observed intensities and introduce additional unobserved reflections or other artifacts. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2523–2533, 1997 |
