Measurements have been made of the recovery of polyethylene samples from bending deformations of varying severity. The amount of spring-back from a severe bend is quite different for different densities (degrees of crystallinity), especially at short times after release, being much greater for lower-density polyethylenes. Short-term spring-back measurements thus afford a simple and rapid means of determining the approximate density. Immediate recovery is small, 5 to 15 percent, and is in rough accord with expectations for elastic-plastic solids. A major protracted recovery process follows. It is accelerated by raising the temperature, in reasonable agreement with the WLF relation for segmental mobility. It is therefore attributed to retarded molecular motions in the amorphous fraction. Final recovery is greater for lower-density materials, but it is incomplete even after very long times or for mild degrees of bending. Thus, some irreversible processes appear to take place, even when bending strains as low as 3 percent are imposed.

Some bending properties of films

Citations

Recovery (spring-back) of polyethylene from an imposed bend

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