Yield Stress Enhancement in Polyethylene-Glassy Block Copolymers

Polyethylene (PE) has the highest annual production volume of all synthetic polymers, and is broadly employed due to its toughness, processability, chemical resistance, and low cost. However, PE is not suited to certain applications due to its modest yield stress and Young’s modulus, ~30 MPa and ~1 GPa respectively for high-density PE. Irreversible deformation results from dislocation of crystal stems and crystal fragmentation under stress. At room temperature, the liquid-like amorphous layer provides no mechanical support to the crystal surface. The mechanical properties of PE can be modified via incorporation of a short block with a high glass transition temperature into a majority-PE block copolymer. We investigate glassy-PE block copolymers prepared by ring-opening metathesis polymerization of norbornylnorbornene and cyclopentene, followed by hydrogenation. A large change in mechanical behavior can be achieved even at low glassy block content, e.g. doubling the yield stress and Young’s modulus with the addition of ~15 weight percent glassy block (T_g = 115 °C). Property enhancement is closely associated with the composition of the amorphous layer and the spatial distribution of the glassy block. Furthermore, we relate ductility with the presence or absence of tie molecules.