Threshold Tie Molecule Concentration for the Brittle-Ductile Transition in Linear Polyethylene

Linear polyethylene (PE) can be either brittle or ductile, depending on molecular weight and crystallization history. The brittle-ductile transition (BDT) occurs when the polymer is able to form sufficient tie molecules (TM), polymer chains that connect different crystallites across the amorphous layer. The Huang-Brown model predicts the concentration of TM by calculating the fraction (P) of polymer chains whose end-to-end distance in the melt is equal to or greater than a critical distance in the semicrystalline solid, defined by the sum of two crystal and one amorphous layer thicknesses. The threshold TM concentration needed for the BDT, P_BDT, is experimentally evaluated by observing an abrupt change in the breaking strain of narrowly-distributed (D < 1.2) PEs with varying molecular weights and their bimodal blends, synthesized by ring-opening metathesis polymerization followed by catalytic hydrogenation. TM concentrations are further varied by either quenching (Q) or slow-cooling (SC) the PE from the melt. The results show that the P_BDT of SC-PE is lower than that of Q-PE, and a further comparison with literature data for hydrogenated polybutadiene suggests that increased crystallinity lowers the P_BDT of PE, favoring ductility.