Lamellar morphology of random metallocene propylene copolymers studied by atomic force microscopy

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Description/Abstract

Four sets of propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene and 1-octene, and a total defect content up to similar to 9 mol% (including co-unit and other defects), were studied after rapid and isothermal crystallization. Etched film surfaces and ultrarnicrotomed plaques were imaged so as to enhance contrast and minimize catalyst and co-catalyst residues. While increasing concentration of structural irregularities breaks down spherulitic habits, the formation of the gamma polymorph has a profound effect on the lamellar morphology. Lamellae grown in the radial axis of the spherulite and branches hereon are replaced in gamma-rich copolymers with a dense array of short lamellae transverse or tilted to the main structural growth axis. This is the expected orientation for gamma iPP branching from a seeds. Spherulites are formed in copolymers with non-crystallizable units ( 1-hexene and 1-octene) up to similar to 3 mol% total defect content and were observed up to similar to 6 mol% in those with partially crystallizable comonorners (ethylene and 1-butene). However, lamellae were observed in all the copolymers analyzed, even in the most defective ones, highlighting the important role of the gamma polymorph in propagating lamellar crystallites in poly(propylenes) with a high concentration of defects. Long periods measured from AFM and SAXS are comparatively analyzed.