Kink Bands in Form II of Syndiotactic Polypropylene Finizia Auriemma,* Claudio De Rosa, Odda Ruiz de Ballesteros, and Paolo Corradini Dipartimento di Chimica, Universita ` degli Studi di Napoli Federico II, via Mezzocannone 4, 80134 Napoli, Italy Received March 4, 1997; Revised Manuscript Received June 22, 1997 X ABSTRACT: 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 C2221 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 C2221 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. Introduction Kink bands were firstly described in metals and salt crystals and have been observed in semicrystalline and amorphous oriented polymeric samples as well (see for instance ref 1, pp 497-498 and references therein). The kink bands are a type of deformation band which can be created by compressive stress parallel to the c axis direction in macromolecular crystals, (i.e. the crystal- lographic axis usually taken parallel to the chain axis) and involve a change in neighboring chains of the chain axis direction. They have been observed, for instance, in the electron microphotograph of extended chain crystals of poly(tetrafluoroethylene), 2,3 polyethylene, 4 and R-nylon 5,6 as a result of defective crystal growth or deformation (see also ref 1, p 466). More recently detailed structural X-ray diffraction studies on poly (vinylidene fluoride) have evidenced the presence of kink bands in the polymorphs named form I and form II. 7,8 They consist in the presence in the crystallites of portion of chains assuming a conformation different from the conformation of the limit-ordered form. The defects, which concern groups of neighboring chains, have the property of deviating locally the chain axis direction but still preserving the parallelism among chain axes and extend over wide regions of the crystallites. Kink bands have been also postulated in our preced- ing papers in the case of syndiotactic polypropylene (s- PP) 9,10 in order to explain the solid state 13 C NMR CP MAS spectra of highly disordered s-PP samples (quench precipitated from the solution) showing structural fea- tures close to those calculated for the “ideal” C-centered orthorhombic structural model of s-PP (form II after ref 11). The solid state 13 C NMR CP MAS spectra of such s-PP samples present, indeed, additional resonances besides those expected in the case of the limit-ordered form II with chains in fully helical (TTGG) n conforma- tion, belonging to nuclei placed in well defined confor- mational environments embedded in rigid structures. In ref 9 some possible structural models of s-PP in this disordered form were drawn: the proposed models comprise defective but energetically feasible portions of chains in trans-planar conformation (disturbing the dominating (TTGG) n conformation); the last are imag- ined to be clustered on planes which cross the 3D otherwise ordered portions of the crystallites and form kink bands. We recall that the most stable form of s-PP (form I) found by Lotz, Lovinger, et al. 12 also presents disorder phenomena. They are related to packing rather than to conformational defects 13-16 and indeed, solid state 13 C NMR spectra of s-PP samples crystallized in form I present only resonances typical of (TTGG) n helical conformation. 9,10,17 It is worth noting that another polymorph of s-PP, form IV after ref 11, with chains in a (T 6 G 2 T 2 G 2 ) n 18 conformation, presents spectral features analogous to those of the quench-precipitated sample of s-PP of ref 9, in the 13 C NMR CP MAS spectrum. 19 In refs 9 and 10, we already excluded that the quench-precipitated s-PP samples would be prevailingly in form IV, based on (i) the different positions of the Bragg reflections in form II and form IV and (ii) the different thermal behavior of the two kinds of samples. As described in detail in ref 10, the 13 C NMR CP MAS spectra of the quench-precipitated s-PP sample obtained at different temperatures do not present substantial differences up to 100 °C; the extra resonances disappear completely only upon the melting (i.e. above 140 °C). For form IV, instead, it is known from the literature 18 that it trans- forms to the more common crystalline form with chains in the (TTGG) n fully helical conformation on heating the sample at T g 50 °C, already. As outlined in ref 20, the disordered model structures proposed for the quench-precipitated s-PP samples in ref 9 may be considered intermediate between the limit- ordered models of form II and form IV: corresponding to the kink bands, the chains assume a conformation as well as a packing mode close to those of form IV. This explains the analogy of the 13 C NMR CP MAS spectra of samples in form IV and in the disordered form II containing kink bands. A detailed structural analysis of the aforementioned disordered structure of s-PP involving kink bands has not yet been performed so far. X Abstract published in Advance ACS Abstracts, August 15, 1997. 6586 Macromolecules 1997, 30, 6586-6591 S0024-9297(97)00284-2 CCC: $14.00 © 1997 American Chemical Society