CRYSTAL POLYMORPHISM, TRANSFORMATIONS AND GROWTH TRANSITIONS IN ISOTACTIC POLYPROPYLENE AND POLY(l-BUTENE)

摘要

The crystal polymorphism of isotactac polypropylene, (iPP) and poly(l-butcne) (iPBu) illustrates two limiting cases of polymorphism of helical polymers. Indeed, the three polymorphs of iPP rest on the same helix geometry (left and/or right-handed three fold) and differ only in the mutual, often original, arrangement of these helices in the unit-cell. The three crystal forms of iPBu rest on different helix geometries: the polymorphism of iPBu has a conformational root. The existence of polymorphs raises several issues of general or more specific character on polymer crystals and crystallization. First among them is of course the elucidation of the crystal structures themselves: let us recall that the iPP y and P structures have been solved only recently, i.e. have resisted elucidation for over thirty years-a full scientific generation. Other issues relate to the specific growth conditions and nucleating additives required for any specific form, to possible phase transformations from one form to another (either by a solid-solid phase transformation or by melting-recrystallization) or to growth transitions from one phase to another (spontaneous, or triggered by changes in growth conditions).