Mechanical consequences of molecular composition on failure in polyolefin composites containing glassy, elastomeric, and semicrystalline components

In order to gain insights into the mechanisms of deformation and ultimate failure in a homologous series of lamellae-forming polyolefin block copolymers comprised of glassy poly(cyclohexylethylene) (C), elastomeric poly(ethylene-alt-propylene) (P), and semicrystalline poly(ethylene) (E), the anisotropic tensile properties of samples in which the microphase separate structure is oriented on a macroscopic length scale were probed. Reciprocating shear processing of monodisperse CPCPC and CPEPC-ξ polymers having mass fraction w_C ∼ 0.39-0.44 and 0 ≤ ξ ≤ 1, where ξ = w_E/(w_E + w_P), produces "single-grain" polymer samples with perpendicular-oriented lamellae. Tensile deformation studies in which the strain axis coincides with the lamellar normal direction yield varied mechanical responses ranging from brittle fracture for CEC (ξ = 0) to ductile behavior for CPEPC (ξ > 0) and CPCPC. Tandem small- and wide-angle X-ray scattering analysis of samples undergoing deformation shows that application of strain along the lamellar normal in the CPEPC materials results in formation of a folded lamellar structure or "chevron" morpohology within which the E crystals cant relative to the strain direction. Since the ultimate failure mechanism for materials strained in this direction is chain pullout in the glassy domains, a simple mechanical model applied to the data enables quantitation of the stress required for chain pullout at ∼4 MPa. Additionally, the mechanical properties of miscible blends of CEC and CPC polymers with matched segregation strengths are shown to mimic those of the covalently linked CPEPC pentablock copolymer.