Crystal Structure of the Trigonal Form of Isotactic Polypropylene as an Example of Density-Driven Polymer Structure

Abstract

Propene-hexene copolymers crystallize in a new polymorphic form of isotactic polypropylene when the concentration of hexene is higher than nearly 10-15 mol %. The hexene units are included in the crystals, inducing an increase of density that allows crystallization of 3-fold helical chains in a trigonal unit cell according to the space group R3c or Rc, where the helical symmetry of the chains is maintained in the crystal lattice. The structure of this new form is similar to those of isotactic polybutene and polystyrene and does not crystallize in polypropylene homopolymer because it would have too low density. The crystal structure of isotactic polypropylene is therefore no longer an exception to the principles of polymer crystallography, but the new structure represents the fulfillment of these principles and indicates that the packing of polymer molecules is mainly driven by density.

Enhancing the formation of the new trigonal polymorph in isotactic propene-1-pentene copolymers: Determination of the X-ray crystallinity

“…Nevertheless, the crystalline diffractions are characteristic of the new trigonal δ form of iPP, appearing at 2θ angles of 10.8, 18.5, and 21.3 degrees, and corresponding to the planes (110), (300), and (220+211), respectively. [23][24][25][26] They are, however, rather wide peaks, indicative of crystals with small lateral size (in the directions of the corresponding crystallographic planes).…”

Thermal Behavior, Structure and Morphology of Propene/Higher 1-Olefin Copolymers

“…Recently a reflection at 10.28 (2u) has been found [51,52] in the WAXD of propene-hexene copolymers with a hexene content higher than around 10 mol-%. According to Corradini and coworkers, [52] the increased density due to the inclusion of the bulky hexene stabilizes a new modification of iPP with a trigonal unit cell, which can not form in the homopolymer due to its very low density.…”

“…According to Corradini and coworkers, [52] the increased density due to the inclusion of the bulky hexene stabilizes a new modification of iPP with a trigonal unit cell, which can not form in the homopolymer due to its very low density. Contrary to predictions, [52] we have found this new reflection in propenebutene copolymers containing a relative small amount of comonomer. It must also be pointed out that we have not found evidences of the reflection at 10.28 (2u) in the copolymer E-PBu6.8, which has a similar composition and a slightly higher isotacticity.…”

Synthesis, molecular characterization, evaluation of polymorphic behavior and indentation response in isotactic poly(propylene-co-1-heptene) copolymers

“…(principle of entropy-density driven phase formation in polymers) [24]. At contents lower than 14 mol%, this polymorph competes with the other ones that can be developed (a and c forms as well as the mesomorphic phase), depending on rate applied during the crystallization process and/or on supercooling from the isotropic melt before ordering.…”