On the γ-Phase of Isotactic Polypropylene

The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) ( M w = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.