Structure and morphology of poly(propylenes): A molecular analysis

The progress made in the understanding of the crystal polymorphism and morphology of isotactic and syndiotactic poly(propylene) (iPP and sPP) is reviewed. In 1960, the crystal structure of the stable form of IPP, the alpha phase, was already solved and the general principles of polyolefin chain conformation and crystallography established. The molecular analysis of the lamellar branching of alpha iPP-a case of homoepitaxy specific to this polymer and this crystal form-was given in 1986. The gamma phase of iPP, solved in 1989, is the first and so far unique example of a polymer structure with non-parallel chain stems. The beta phase is the first example of a frustrated structure in polymers, and rests on packing of isochiral helices in the unit-cell. The stable, high temperature form of sPP described in 1988 is based on full antichiral packing of helices, whereas the original structure corresponds to another limiting (but not experimentally observed in pure form) case based on packing of isochiral helices. Details of the molecular structure of IPP and sPP, such as the hand of individual helical stems and setting of chains, are accessible by crystallographic considerations which make use of molecular markers provided by the alpha-alpha and alpha-gamma epitaxies in iPP or by direct visualization by atomic force microscopy. Copyright (C) 1996 Elsevier Science Ltd.