ON THE GAMMA-PHASE OF ISOTACTIC POLYPROPYLENE

The crystalline morphology of a relatively high molecular weight isota ctic polypropylene (i-PP) (M(w) = 18000 g/mol) with well-defined regio - and stereoirregularities is studied by means of atomic force microsc opy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scatt ering (SAXS), and light microscopy. This i-PP crystallizes under certa in crystallization conditions nearly exclusively in the gamma-modifica tion. The growth of the gamma-phase is often influenced by the initial growth of the alpha-phase, but also the neat gamma-phase is observabl e. The formation of the gamma-phase can be related to stereo- and regi oirregularities in the polymer chain caused by the polymerization mech anism using metallocene catalysts. WAXS measurements show that at larg e supercoolings the formation of alpha-phase i-PP is preferred, wherea s at low supercoolings this sample crystallizes nearly exclusively in the gamma-modification. Light microscopy and AFM reveal for the gamma- phase i-PP morphologies different from the well-known spherulites form ed by the alpha- or beta-modification. However, it can be shown that t he morphology development of gamma-phase i-PP can be widely influenced by very small amounts of alpha-phase i-PP. A lamella thickness of abo ut 4 nm and a long period of 12 nm of gamma-phase i-PP isothermally cr ystallized at 100 degrees C are obtained from SAXS measurements. At ve ry low supercoolings, supermolecular structures of the neat gamma-modi fication are formed in competition with mixed (alpha and gamma) morpho logies. In thin films the neat gamma-modification shows network struct ures, which are formed by triangular entities. Etched bulk samples rev eal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extende d chain lamellae with periodic distances of rows of methyl groups of 3 .6 Angstrom 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 Bruckner and Meil le.