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

TL;DR

This study shows that combining crystalline blocks of polyethylene and polypropylene can significantly improve ductility without using soft segments.

Contribution

The paper introduces a new strategy for enhancing ductility in crystalline materials through polymorphic transitions in hard–hard block copolymers.

Findings

PE-b-iPP and PE-b-sPP copolymers show enhanced ductility compared to homopolymers without amorphous segments.

Deformation induces polymorphic transitions in iPP and sPP blocks, promoting energy dissipation and strain hardening.

Ductility in hard–hard systems is achievable through deformation-assisted structural changes.

Abstract

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.

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