Crystal morphology of the γ (triclinic) phase of isotactic polypropylene and its relation to the α phase

Abstract γ‐phase crystals of isotactic polypropylene (iPP) obtained from low‐molecular‐weight extracts of pyrolyzed polymers are examined by electron microscopy and electron diffraction. γ‐phase crystals differ from α‐phase crystals in three important respects: (i) they are elongated along the b* rather than the a* axis, (ii) the chain axis is inclined at 50° to the lamellar surface (indexed as 101) rather than normal to it, and (iii) they show screw dislocations, while α crystals do not. γ crystals are nucleated on the lateral (010) faces of a α crystals; the bα and b axes are parallel. Virtually no nucleation of the α phase takes place on the γ phase, which is therefore not involved in the repetitive lamellar branching leading to iPP quadrites. Crystallization of the γ phase appears to be favored by or linked to the absence of chain folds and may be involved in the macroscopic curvature of iPP branches.