Crystallinity variations in the double yield region of polyethylene

Abstract

Numerous specimens of a linear low density polyethylene sample were uniaxially deformed up to different elongations to study the double yield phenomenon. Extruded samples were analyzed to calculate the crystallinity and to estimate the mean crystal size, under stressed state and released state (after removal of the stress), using the wide angle X-ray scattering technique. The crystallinity degree and the mean crystal dimension associated to the (110) orthorhombic reflection of the specimen without deformation were of 55% and 16 nm. These parameters in the stressed state, as functions of the elongation, presented a multi-step behavior. A decrement after the first yield point (48%, 13 nm), then another decrement, and an abrupt increment followed by a decrement at higher strain values around the second yield point (28, 40, and 30%; 12, 14.5, and 11 nm). The behavior was more notorious in the stressed state than in the released state. The latter results were interpreted in terms of a partial melting followed by a recrystallization process. These experimental findings show that the second yield is not only associated with the deformation of the crystalline region. This partial meltingrecrystallization process is one of the main mechanisms of the double yield phenomenon.

Strain rate effect on crystallinity variations in the double yield region of polyethylene

“…When the deformation takes place rapidly, the effect of the stress relaxation is small; however, when the deformation is applied slowly, the stress value is smaller because the relaxation has enough time to reduce the stress. Visual observations of the deformation process also detected the onset of necking around the second yield, as reported 6–8, 12, 27…”

“…This work complements a previous study that had a similar objective 27. The connections of the two studies will become clear from the results of this study.…”

“…After the applied stress is removed, the intensity of (110) peak becomes higher for all elongations, while the intensity for the (200) peak decreases for elongations below the second yield but increases for higher elongations. In a recent report27 for 150/70 sample, it was confirmed that in the RS the intensity of the (110) reflection at the first yield point almost restores its value for the unstretched specimen, indicating that the initial properties are almost recoverable.…”

“…Respect to this interpretation, a recent study was reported on the variations of the degree of crystallinity and mean crystal size as functions of uniaxial elongation in the double yield behavior of a linear low‐density polyethylene (LLDPE). Wide‐angle X‐ray scattering (WAXS) was utilized and the results showed a decrease and then an increase of the degree of crystallinity around the second yield point, suggesting that indeed melting and recrystallization occurs in this region 27. The mean size of crystallites also detected the partial melting.…”

Solid-state microcellular foaming of PE/PE composite systems, investigation on cellular structure and crystalline morphology

“…Classically, transformation from original spherulitic structure to fibrillar one, is a typical mechanism for cold drawing and yielding process in semi-crystalline polymers [16][17][18][19][20][21][22]. Peterlin in the 1960s proposed that drawing of polyethylene leads to permanent deformation of the lamellae causing the lamellae to tilt and slip, thereby rupturing into crystalline blocks.…”