Tie Molecule Formation in High Density Polyethylene

The mechanical toughness of a semicrystalline polymer, such as polyethylene (PE), originates from tie molecules: polymer chains that span two or more crystalline lamellae. During rapid crystallization, polymers cannot disentangle, so tie molecules form when the size of the polymer coil in the melt, controlled by its molecular weight (M), exceeds the intercrystalline spacing (L) in the solid state, governed by the specimen’s crystallization history. Prior studies have yielded some insight into the brittle-to-ductile transition in PE as M is increased, but often for broadly-distributed PE with no measure of L. Here we synthesize a series of narrowly-distributed (dispersity < 1.2), perfectly linear (high density) PEs with targeted M using ring-opening metathesis polymerization of cyclopentene followed by hydrogenation. PEs were either quenched or slow-cooled to vary crystallinity (0.65 - 0.85) and L, measured by differential scanning calorimetry and small-angle x-ray scattering. We investigate the brittle-to-ductile transition of PE by uniaxial tensile testing in relation to its M, end-to-end distance in the melt, and L, and find a relatively abrupt transition in these narrow-distribution PEs.