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 C222(1) form II of s-PP. The solid state C-13 NMR CP MAS spectrum shows, however, evidences of the presence of a significant amount (approximate to 20%) of long portions of chains in trans planar conformation (Tr)(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.