Kink Bands in Form II of Syndiotactic Polypropylene

Samples of highly syndiotactic polypropylene (s-PP), when quench precipitated from solution, have an X-ray diffraction spectrum close to that calculated for the limit-ordered C 222 1 form II of s-PP. The solid state 13 C NMR CP MAS spectrum shows, however, evidences of the presence of a significant amount (≈20%) of long portions of chains in trans planar conformation (TT) n immobilized between crystal regions with chains in (TTGG) n conformation. It is shown that form II of s-PP is geometrically suitable for the formation of kink bands, in which portions of chains in trans planar conformation are embedded: atomic coordinates can be deduced for a simplified statistical model of disordered crystal assumed to be periodic in the two directions perpendicular to the (TTGG) helical stretches. All gauche bonds have the same sign (G + or G - ) in the disordered crystal. Formulas may be written for the calculation of the X-ray diffraction of such crystals. The X-ray diffraction pattern calculated for a set of small, partially disordered C222 1 crystals comprising portions of chains which have statistically approximately six consecutive trans bonds in the kink bands every 20−30 monomeric units, as in the ratio indicated by the NMR spectra, is in good agreement with the experimental pattern.