Tough and Elastic Thermoplastic Organogels and Elastomers Made of Semicrystalline Polyolefin-Based Block Copolymers

Development of stereo- and regioselective living alkene polymerization catalysts has led to the capability to produce multiblock copolymers with semicrystalline syndiotactic polypropylene blocks and poly(ethylene-co-propylene) rubbery blocks that have excellent elastomeric properties, both undiluted and as gels with as little as 8 wt % copolymer in mineral oil. During step cycle mechanical processing, the crystals can plastically deform and partially transform from lamellae into rod-like fibrils, which align along the tensile direction, giving rise to very large tangent moduli at large strains. The dimensions of these aligned fibrils can be established from small-angle X-ray scattering and used to construct models of the large strain elasticity, as well as its evolution with plastic strain, based on short fiber composite theory. Wide angle X-ray scattering experiments as well as FTIR measurements suggest that the oriented fibrils have the transplanar (form III) crystal structure. The large toughness of the dilute gels can be attributed to the plastic deformation of the crystalline component, which not only evens out the forces on the rubbery chains running from crystal to crystal but also provides a mechanical hysteresis that is effective in absorbing energy at the tip of a growing crack.