Structure and morphology of isotactic polypropylene-based block copolymers prepared by living olefin polymerization

A structural and morphological characterization at different length scales of two novel classes of semicrystalline polyolefin-based block copolymers (BCPs) obtained through metallorganic catalysis is reported. The first class consists of cystalline-crystalline BCPs formed by isotactic polypropylene (iPP) block linked to polyethylene (PE) block while in the second class of BCPs a iPP block is linked to different amorphous or slightly crystalline blocks made of ethylene-based or propylene-based random copolymers. These systems allow studying of the effect of confinement on the crystallization behavior under different crystallization conditions. The final morphology of these systems is path dependent, being affected by the competition between crystallization and phase separation. The properties of this class of materials can be tailored by combining the wide range of morphologies that develop in crystalline BCPs with the polymorphism of iPP, and choosing the proper crystallization conditions. To this aim, a detailed structural characterization has been performed for samples crystallized in the bulk, thin films and stretched fibers and stress-induced phase transformations have been also analyzed. Moreover, simultaneous time and temperature-resolved WAXS and SAXS experiments have been performed with synchrotron radiation to clarify the melting and crystallization behavior. The morphological characterization of bulk crystallized samples confirm that crystallization dominates the morphology, and superstructures like spherulites or bundle-like entities are observed by polarized optical microscopy. Finally, thin films of iPP-block-PE have been epitaxially crystallized onto crystals of organic substrates to achieve a control over the crystallization of both crystalline blocks. TEM images confirm the obtainment of ordered nanostructures where the two blocks are organized in separated alternating lamellar domains guided by the orientation of the crystalline lamellae. The final morphology depends on which polymer block crystallizes first, a sequence that depends on the block copolymer composition and block lengths.