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The effects of thermal oxidation on the‐ molecular structure and on the morphology of a series of high‐density polyethylene pipes have been studied “as received” or after service. The existence of a 40 to 170 μm thick oxidation skin was established, the structure and morphology of which were characterized by gloss measurements, scanning electron microscopy, polarized microscopy, infrared spectroscopy, gel measurements, and differential scanning calorimetry. The skin surface frequent contained craterlike structures, 25‐50 μm in diameter, whereas small‐scale structures occurred much less frequently than in non‐oxidized surfaces, and the gloss increased significantly. The presence of a carbonyl concentration gradient through the skin was established, and the gel measurements also indicated a crosslink gradient. The skin was found to be composed of a top layer with a mainly non‐spherulitic structure of very low crystallinity and melting point and a large‐scale spherulitic layer with a crystallinity and melting point slightly below the corresponding bulk values. The formation of this brittle surface layer and its effect on the long‐term fracture performance of the pipe are discussed.

SynopsisThe limit of linear viscoelasticity is determined for poly(methy1 methacrylate) (PMMA) and poly(ethy1 methacrylate) (PEMA) in uniaxial tension creep over the temperature range of 20" to T, -10°C. The linear limit is definedas the point at which the creep compliance deviates by more than 1% from its mean value in the linear viscoelastic range. For both materials, the stress limit of linear viscoelasticity falls to a minimum or plateau level at a temperature below T,. It is suggested that the 8-mechanism plays animportant role in the existence of this minimum.The time span covered is from 10 to 1000 sec.

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