Yield Stress Enhancement in Polyethylene-Glassy Diblock Copolymers

The low-strain mechanical properties of linear polyethylene (PE) can be substantially altered by the incorporation of a short block of a polymer with a high glass transition temperature (T_g) into a majority-PE block copolymer. In particular, the yield stress and the tensile modulus can be sharply increased with the addition of a relatively small fraction of glassy block, especially when combined with a thermal history that promotes high crystallinity and crystal thickness of the PE block. For example, the incorporation of 15 wt % of a hydrogenated poly(norbornyl norbornene) block (T_g = 115 °C) into a PE diblock copolymer, cooled from the melt at ∼1 °C/min, doubles the yield stress (from ∼30 to ∼60 MPa) and tensile modulus (from ∼1.5 to ∼3.5 GPa) relative to the values for a PE homopolymer treated with the same thermal history. These property enhancements are closely associated with the composition of the amorphous layer between crystal lamellae and the spatial distribution of the glassy block within this layer. Finally, the ductility of these polymers at high strains is governed by the presence or absence of tie molecules, which can be correlated with the chain dimensions of the PE block in the melt and the distance between crystal lamellae.