Semicrystalline polymers of low glass transition temperature, such as polyethylene (PE), can be either brittle or ductile depending on their content of intercrystallite stress transmitters—such as tie molecules (TMs), chains that directly bridge the intercrystalline amorphous layer. TM content will increase with increasing molecular weight (M), or with the fraction of high-M chains in a disperse polymer, and with decreasing intercrystallite repeat spacing d, which can be manipulated through thermal history and the incorporation of comonomer. Yet despite decades of work on the structure and properties of PE, the minimum molecular weight to achieve ductility is known only empirically, and only approximately (30-50 kg/mol in quenched specimens). To understand the microstructural aspects required for ductility, we examine the failure mode of model narrow-distribution linear PEs (LPEs, high-density PEs) of high crystallinity, where d is varied through crystallization history (either quenching or slowly cooling), and of ethylene-butene copolymers (hydrogenated polybutadienes, hPBs) of moderate crystallinity, where d is limited by the short-branch content. In both cases, d is directly measured via small-angle x-ray scattering (SAXS). For each series (LPEs with different thermal histories and quenched hPBs), a rather sharp brittle-to-ductile transition (BDT) is observed with increasing M, at a value MBDT. However, across the three series, the value of MBDT does not depend solely on the value of d; indeed, a higher M is required to achieve ductility in quenched samples of hPB than in LPE, despite the much lower values of d for hPB. Consequently, the calculated value of TM fraction at the BDT (PBDT, calculated via the classic Huang-Brown approach) increases strongly as crystallinity decreases, by a factor of approximately 50 from slow-cooled LPE to quenched hPB. This strong dependence is explained by considering the influence of TMs on the brittle fracture stress (σb), with the BDT occurring when there are sufficient TMs for σb to exceed the yield stress (σy), which is strongly dependent on crystallinity but independent of TM content.

Molecular weight and tie molecule content required for ductility in polyethylenes of varying crystallinity

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