CRYSTAL-STRUCTURE OF SYNDIOTACTIC POLYPROPYLENE

An analysis of the crystal structure on unoriented and oriented samples of syndiotactic polypropylene is presented, on the basis of a comparison between calculated structure factors and experimental X-ray diffraction intensities. According to the present analysis, syndiotactic polypropylene can exist in different orthorhombic modifications having chains in helical conformation. Stretched samples may contain, at least in part, the C-centered structure proposed by Corradini et al. (limit space group C222(1)), while single crystals and polycrystalline samples are generally in the structure proposed by Lotz, Lovinger, Cais, and Davis (limit space group Ibca). Calculations of structure factors indicate that the two polymorphic forms of s-PP may be characterized by various degrees of disorder depending on the chemical and physical history of the sample. A possible disorder present is related to the statistical substitution of right and left handed helices along the same lattice axes. The space groups for the two limit statistical structures are Cmcm and Bmcm.