Crystalline-crystalline and crystalline-amorphous syndiotactic polypropylene based di-block copolymers prepared by living olefin polymerization

We report the morphology, structure and mechanical properties of semicrystalline, polyolefin-based block copolymers (BCPs) with the aim of obtaining ordered nanostructures and innovative thermoplastic elastomers. By using living, stereoselective insertion polymerization catalysts, syndiotactic polypropylene-block-poly(ethylene-co-propylene) (sPP-b-EP) and syndiotactic polypropylene-block-polyethylene (sPP-b-PE) were synthetized.1 Thin films of BCPs were prepared from solution by dip-casting. The formation of large-sized, well-oriented lamellar nanostructures of BCPs composed of a crystallizable sPP block linked to an amorphous rubbery block EP (sPP-b-EP) was successfully achieved by using epitaxial crystallization2 with p-terphenyl as substrates. TEM bright field images and electron diffraction patterns of gold decorated nanostructures reveals the presence of well oriented crystalline lamellae ordered over large area alternated to amorphous layers. Crystalline-crystalline block copolymers (sPP-b-PE) epitaxially crystallized onto crystals of p-terphenyl show a more complex lamellar morphology. For BCPs with high volume fraction of sPP block only one lamellar orientation can be observed. In this case sPP crystallize first. For sPP-PE BCPs with similar block lengths, TEM images show regions characterized by double orientation of PE lamellae due to the epitaxial relationship of PE with p-terphenyl. In these regions PE crystallize first. They alternate with regions presenting a single lamellar orientation of sPP according to the epitaxial relationship with p-terphenyl. In these regions sPP crystallize first. In other words the block that crystallize first determines the local morphology. An analysis of the mechanical properties and a detailed structural study of BCPs have been performed in order to clarify the effects of the presence of crystalline (PE) or amorphous (EP) block on the crystallization behavior of sPP, and in particular on the stress-induced transformations.