Distribution of chain defects and microstructure of melt-crystallized polyethylene. III. An investigation of point defects and amorphous defects by selective degradation

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Abstract
An exploratory study on the distribution and location of chain defects (mainly butyl and longer branches) in melt-crystallized polyethylene by means of selective nitric acid degradation is reported. The data presented here complement earlier studies of Holdsworth and Keller on copolymers and extend our previous results regarding the evaluation of the level of defect inclusion within the lattice in the light of a model involving the penetration of chain defects (branches) at interstitial crystal sites by means of generation of step-chain defects (kinks). The concentration of chain defects incorporated as point defects in the crystal lattice is compared with the total concentration of defects, as derived from the methyl infrared absorption band throughout the stages of the degradation process. The reported experiments reveal, in consonance With preceding studies, that a large fraction of chain defects (60–95%, depending on the degree of branching) accumulate in the amorphous layer and consequently are removed in the first stage of degradation (t ≤ 50 hr). An interesting feature of this study is that a fraction of branches which do not expand the lattice, but are nevertheless still detectable after removal of the amorphous phase, are most probably occluded as “amorphous” and/or cooperative defects (longer branches filling voids created by missing chains) within the crystals. After sufficiently long exposure to the acid (t ∼ 180 hr), the suggested amorphous defects are finally removed. In contrast, those chain defects efficiently incorporated at interstitial lattice sites, making nearly perfect space-filling point defects, persist for the more advanced stage of degradation.