Evolution of Thermal Properties of Polyethylene in the Double Yield Region After Uniaxial Deformation

Abstract

The double yield phenomenon was studied using specimens of linear low density polyethylene (LLDPE) uniaxially deformed up to different elongations. Extruded samples prepared at 150°C and 70 rpm were deformed at 10 mm/min. The crystallinity in the released state (after elimination of stress) was calculated using the differential scanning calorimetric technique. The crystallinity degree and melting temperature of the samples, as a function of the elongation to which they were subjected, presented a multi-step behavior. A decrement before the first yield point and an increment after this point were observed. Then, at higher elongation values, before the appearance of the second yield point, a significant decrement followed by an abrupt increment were detected. These results in the second yield region were interpreted in terms of partial melting followed by recrystallization. These experimental findings show that the partial melting-recrystallization process is one of the main mechanisms of the double yield phenomenon.