The toughness of semicrystalline polymers with low T<sub>g</sub>, such as polyethylene (PE), is believed to originate from tie molecules (TM), polymer chains that connect different crystalline lamellae across the amorphous layer. Based on the observation that the chain dimension remains unchanged from the melt upon rapid crystallization, the Huang-Brown (HB) model predicts that TMs form when the end-to-end distance of the coil in the melt (R), controlled by its molecular weight (M), exceeds the intercrystalline spacing (d) in the solid-state, governed by the specimen’s crystallization history, i.e., the TM probability should scale with R/d. Here we synthesize a series of narrowly-distributed (dispersity &lt; 1.2), perfectly linear PEs using ring-opening metathesis polymerization of cyclopentene followed by hydrogenation to critically test the HB model. Each PE is either quenched or slowly-cooled from the melt to vary the crystallization conditions, thus giving two different values of d, and the breaking strain is measured by uniaxial tensile testing. These narrow-distribution PEs show a relatively abrupt brittle-to-ductile transition, and the transition occurs at different values of R/d for the different crystallization histories. Further comparison with literature data on the brittle-to-ductile transition of hydrogenated polybutadiene suggests that the degree of crystallinity has a significant influence on the value of R/d at which the transition occurs.

Brittle-to-ductile transition in high-density polyethylene and Huang-Brown model prediction of tie molecule formation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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