Structure and defects in fully syndiotactic polypropylene

Abstract

We have studied the structure and defects of fully syndiotactic polypropylene [sPP] in its dominant (t^h phase using X-ray and electron diffraction and molecular modeling. In agreement with our previous proposal from much less syndioperfect sPP (77 %), we find the chains to pack in a fully antichiral cell along both the a and b axes, which is thus 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. Crystallization at temperatures below ca. 130 °C causes incorporation of packing defects along the b axis, which we have simulated using molecular modeling. At high crystallization temperatures, the defects consist of individual rows of chains displaced by b/4 and identified by narrow 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 domains and leading to streaking of the individual h20 spots. No evidence for large-scale row or point vacancies is found. In addition, we find evidence for new intermolecular packing defects along the chain direction, which may arise from conformational defects and may involve small-scale (ca. 5-10% of c) axial displacements of the macromolecules.

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

“…In powder of unoriented s-PP samples, only disordered modifications of form II, characterized by conformationally disordered chains containing transplanar sequences (kink band defects) [213][214][215], can be obtained at atmospheric pressure, for instance, by precipitation from solutions of low stereoregular samples and in copolymers of s-PP with ethylene as the co-monomeric units. The metastable polymorphic forms II, III, and IV generally transform into the stable form I by annealing.…”

Crystallization and melting behavior in syndiotactic polypropylene: Origin of multiple melting phenomenon

“…To date, four limit-ordered crystalline modifications of s-PP were proposed and described in the literature. [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] Of the four crystalline forms, only the molecular chain packing models of the limitordered form I 56 -59,64,66 and the limit-disordered form I 59,63,66 (after the most recent nomenclature given by De Rosa et al 70 ) are appropriate for characterizing s-PP samples that are crystallized under quiescent conditions from the melt state (or from solution).…”

“…64 ). The existence of form I is mainly controlled by the amounts of defects present in the packing of the chains, 59,63,66 which is revealed by either the weakness or the absence of the (211) reflection at 2 ϭ 18.9°in samples crystallized at low T c values. The greater the amount of defects present, the higher the deviation from the fully antichiral packing is.…”

Effect of Nucleating Agents on Crystallization and Melting Behavior and Mechanical Properties of Nucleated Syndiotactic Poly(propylene)

“…12.1, 15.8, 20.5 and 24.58, corresponding to a limit-disordered orthorhombic unit cell with axes a ¼ 14.5 Å , b ¼ 5.6 Å and c ¼ 7.4 Å . [17][18][19][20] According to this unit cell, the characteristic X-ray peaks should be observed at a 2y of ca. 12.…”