Tie Molecules and the Brittle-to-Ductile Transition in Near-Monodisperse and Bidisperse Linear Polyethylene

The Huang-Brown (HB) model predicts that tie molecules (TM), polymer chains that connect different crystalline lamellae across the amorphous layer, form when the end-to-end distance of the coil in the melt (R) exceeds the intercrystalline spacing (d) in the solid state, i.e., the TM probability should scale with R/d. To test the HB model, we synthesize narrowly-distributed (dispersity < 1.2) linear PEs of varying molecular weight (M) using ring-opening metathesis polymerization of cyclopentene followed by hydrogenation. Each PE is either quenched or slowly-cooled from the melt to vary the thermal history, giving two different d values. Uniaxial tensile tests of these PEs show a rather abrupt brittle-to-ductile transition (BDT) with increasing M, but the transition occurs at different R/d for the different crystallization conditions, in the contrast to the HB prediction. Quenched blends with variable contents of high-M PE in a low-M PE matrix show a BDT at the same HB tie chain probability as the quenched PEs. The differences among the values of R/d at the BDT for PEs with different thermal histories and hydrogenated polybutadiene (literature data) suggest that the degree of crystallinity has a significant influence on the value of R/d at the BDT.