Structure and properties of semicrystalline-rubbery multiblock copolymers

Linear (EP) _n multiblock copolymers, with n = 2, 4, 6, 8, 10, and 12, containing semicrystalline poly(ethylene) (E) and rubbery poly(ethylene-alt-propylene) (P) blocks were prepared by catalytic hydrogenation of poly(1,4-butadiene-b-1,4-isoprene) _n block copolymers. These materials were investigated by small- and wide-angle X-ray scattering (SAXS and WAXS), transmission electron microscopy (TEM), and tensile mechanical testing. Microstructure was created during cooling in two ways: melt state ordering followed by E crystallization and crystallization-induced segregation from the disordered melt. Two categories of mechanical performance were identified. A relatively low true stress at break (σ _T,B < 85 MPa) always was correlated with crystallization-induced segregation. A high true stress at break (σ _T,B ≈ 270 MPa) was recorded when crystallization occurred within microphase-separated lamellae or when n ≥ 10 notwithstanding crystallization-induced segregation. These results are interpreted on the basis of the degree of coupling between E crystals, which depends on the mode of ordering. This model is supported by SAXS and WAXS data obtained from shear oriented (EP) ₂ triblock copolymer and differential scanning calorimetry results collected from all the multiblock copolymers.