Mechanism for lamellar branching in isotactic polypropylene

Abstract

Selected-area electron diffraction and x-ray diffraction have been used to study microstructure of spherulites in thin films of isotactic polypropylene crystallized from the melt. Our purpose has been to seek an explanation for a branching habit, unique to this polymer, in which parent and daughter lamellar crystals make an acute dihedral angle of ∼81°. We find that branching of this kind is invariably associated with the presence of a triclinic crystalline phase (γ form) and is sensitive to temperature changes during growth. We follow Khoury's suggestion that epitaxial overgrowth is responsible, but with the important difference that branching is initiated by deposition of blocks of γ-phase material onto (010) lateral surfaces of lathlike lamellar crystals of α phase; we propose that the a and c crystallographic axes in the γ-phase blocks are laid down along the −c and −a axes, respectively, of crystals of α phase. On the basis of this mechanism, hinging upon the presence of the γ phase, variations in the sign of optical birefringence in polypropylene spherulites crystallized over a range of temperatures can be accounted for. We propose that the same mechanism is responsible for similar branching in polypropylene crystallized from solution.