Unexpected Ductility Enhancement in Crystalline-Crystalline Polyolefin Diblock Copolymers without Introducing Soft Segments.

Combining polyethylene and polypropylene (isotactic or syndiotactic) crystalline blocks within a single macromolecule offers a powerful framework to elucidate how the molecular architecture governs deformation and phase transformations during stretching in polyolefins. In this study, polyethylene-block-isotactic-polypropylene (PE-b-iPP) and polyethylene-block-syndiotactic-polypropylene (PE-b-sPP) copolymers with well-defined block lengths, synthesized using single-site catalysts, were investigated to elucidate the relationship between molecular architecture, crystalline structure, and mechanical response. X-ray diffraction and tensile analyses revealed that despite the absence of amorphous soft segments, both block copolymers exhibit remarkable ductility enhancement compared to their corresponding homopolymers when a long iPP or sPP block is linked to a PE block. The mechanical performance strongly depends on the relative block lengths and the polymorphic transformations that occur during deformation. In PE-b-iPP samples, the α-form of iPP progressively transforms into the mesomorphic form under deformation, while in PE-b-sPP copolymers, the helical form I of sPP transforms into the trans-planar form III. These stress-induced transitions promote energy dissipation and delay fracture, enabling large deformations with pronounced strain hardening. The results demonstrate that high ductility in crystalline polyolefin block copolymers can be achieved in hard-hard systems through deformation-assisted polymorphic transitions, offering an alternative molecular design strategy without introducing soft segments for tough, extensible crystalline materials.