Control of mechanical behavior in polyolefin composites: Integration of glassy, rubbery, and semicrystalline components

The mechanical properties of a homologous series of polyolefin block copolymers comprised of glassy poly(cyclohexylethylene) (C), elastomeric poly(ethylene-alt-propylene) (P), and semicrystalline poly-(ethylene) (E) are documented. Monodisperse CPEPC, CPE, and CEPEC with mass fractions w_c ∼ 0.39-0.44 and 0 < ζ < 1, where ζ = W_E /(W _E + W_P), were synthesized by sequential anionic polymerization of styrene, isoprene, and butadiene followed by catalytic hydrogenation. These materials hierarchically microphase separate into lamellae, within which templated crystallization-induced segregation occurs. As ζ increases, the unoriented, polydomain CPEPC materials exhibit monotonically increasing elastic moduli and yield stresses, comparable ultimate tensile strengths, and decreasing failure strains. Cold drawing the CPEPC polymers yields high-strength materials, the structures of which are examined by small- and wide-angle X-ray scattering. Drawn CPEPC-70 (ζ = 0.70) exhibits improved toughness with an ultimate tensile strength σ_fail = 75 ± 10 MPa and elongation at break of ε_fai; = 1.22 ± 0.22, as compared to σ_fail = 92 ± 21 MPa and ε_failfaii = 0.86 ± 0.18 for drawn CEC (ζ = 1.00). Elasticity measurements on the drawn samples demonstrate that materials with xzeta; > 0 exhibit low degrees of stress softening, while smaller permanent sets and higher failure strains are observed as ζ decreases.