Structure and defects in fully syndiotactic polypropylene

The structure and defects of fully syndiotactic polypropylene \sPP\ in its dominant (t sub 2 g sub 2 ) sub 2 phase were studied using X-ray and electron diffraction and molecular modeling. In agreement with the previous proposal from much less syndioperfect sPP (77%), the chains are found to pack in a fully antichiral cell along both the a and b axes, which is thus doubled from its currently accepted dimensions. Cell parameters are a = 1.45 nm, b = 1.12 nm, c = 0.74 nm, and space group Ibca. Crystallization at temperatures < approx 130 deg C causes incorporation of packing defects along the b axis, which was simulated using molecular modeling. At high crystallization temperatures, the defects consist of individual rows of chains displaced by b/4 and identified by narrow streaks superimposed upon sharp h20 spots in electron-diffraction patterns from tilted crystals. At lower crystallization temperatures, entire groups of such chains are displaced by b/4, forming discrete domains and leading to streaking of the individual h20 spots. No evidence for large-scale row or point vacancies is found. In addition, evidence is found for new intermolecular packing defects along the chain direction, which may arise from conformational defects and may involve small-scale ( approx 5-10% of c) axial displacements of the macromolecules.