Morphology of ultradrawn polyethylene strands. II. Nitric acid etching and melting behavior

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Abstract
A transparent, ultraoriented, high‐density polyethylene morphology has been produced by solid‐state (ultradraw) extrusion in a capillary rheometer. From the perspective of modulus and nitric acid etching behavior, the uniquely high draw ratios (<325) experienced by the polyethylene during extrusion result in a morphology with a high level of chain extension. The effect of nitric acid etching on strand thermal behavior has been determined by DSC. The observed melting points of unetched strands were sensitive to the thermal contact between sample and sample pan. Under conditions ensuring improved contact, strand superheating is reduced to one‐third of previously reported values. The negligible shrinkage evidenced by these strands up to 130° is consistent with the presence of a thermally stable component such as extended chain crystals or crystallized tie chains. The single, high‐melting peak is gradually replaced by a nonsuperheating, lower melting peak during the initial stages of acid etch. The resultant peak melting temperature is consistent with the value predicted for the peak crystal thickness of the etched polymer. No evidence is found for a higher melting peak attributable to the extended chain crystalline component. A highly constrained morphology produced by the large tie chain content is believed responsible for strand melting behavior. The melting point of the extended chain crystalline component is reduced by defects and a large ratio of lateral to basal surface area.

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