Embrittlement induced by oxidation in polyethylene: Role of initial bimodality

•Oxidation changes the chain length distribution from bimodal to unimodal.•Oxidation leads to the embrittlement of the polymer.•An embrittlement criterion can be defined for polyethylene.•Critical amorphous layer thickness can describe embrittlement with a value of 7.5 ± 0.5 nm. High density polyethylene films (200 µm) with an initial bimodal chain length distribution were aged in ovens at 60 °C, 70 °C, and 80 °C. Oxidation effects were characterized at the macromolecular scale to measure the chain length distribution, the amount of crystalline phase, the type of crystallites, and the thickness of the amorphous layer. Additionally, mechanical properties were measured using tensile tests. Regardless of the aging temperature, the same behavior was observed: bimodal chain length distribution became unimodal, the crystallinity ratio increased, the amorphous layer thickness decreased, and the polyethylene became brittle. The embrittlement of the polymer is discussed, and two criteria are proposed: a critical molar mass and a critical amorphous layer thickness. The latter appears to be independent of the initial chain length distribution of the polyethylene.