STRUCTURE AND DEFECTS IN FULLY SYNDIOTACTIC POLYPROPYLENE

We have studied the structure and defects of fully syndiotactic polypr opylene [sPP] in its dominant (t2g2)2 phase using X-ray and electron d iffraction and molecular modeling. In agreement with our previous prop osal from much less syndioperfect sPP (77%), we find the chains to pac k in a fully antichiral cell along both the a and b axes, which is thu s 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. Crystalli zation at temperatures below ca. 130-degrees-C causes incorporation of packing defects along the b axis, which we have simulated using molec ular modeling. At high crystallization temperatures, the defects consi st of individual rows of chains displaced by b/4 and identified by nar row 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 do mains and leading to streaking of the individual h20 spots. No evidenc e for large-scale row or point vacancies is found. In addition, we fin d evidence for new intermolecular packing defects along the chain dire ction, which may arise from conformational defects and may involve sma ll-scale (ca. 5-10% of c) axial displacements of the macromolecules.