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

The double yield phenomenon was studied using numerous specimens uniaxially deformed up to different elongations of linear low‐density polyethylene samples. Extruded samples prepared under different conditions were deformed at 1, 10, and 50 mm/min. The crystallinity under stressed state was calculated using the wide‐angle X‐ray scattering technique. The crystallinity degrees of the samples without deformation were less than 55%. This parameter, as a function of the elongation, presented a multistep behavior. An increment before the first yield point and a decrement after this point; then, at higher elongation values around the second yield point, another decrement and an abrupt increment. The behavior was more notorious at intermediate and lower strain rates. The results around the second yield point were interpreted in terms of melting of the less perfect crystallites followed by a recrystallization process. These experimental findings show that the partial melting–recrystallization process is one of the main mechanisms of the double yield phenomenon. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008