Stretching temperature dependence of the critical strain in the tensile deformation of polyethylene copolymer

Abstract

The deformation behavior and structural evolution of the quenched ethylene-butene copolymer under uniaxial loading and unloading were investigated as a function of stretching temperature using step-cycle tests, in situ small angle X-ray scattering (SAXS), and wide angle X-ray diffraction (WAXD) measurements. The true stress-strain curves together with detailed analysis of SAXS results indicated that the irreversible plastic strain set in quite early at small strains as the drawing temperature was increased. The critical strain, at which the recoverable elastic strain reached the plateau region and the long period started to keep essentially constant upon stretching, was found to increase distinctly from about 0.65 at room temperature to 1.00 at 100 °C. This dependence of critical strain on the stretching temperature was directly related to the effective entanglement density of the amorphous network as derived from the Gaussian model of Haward-Thackray. Despite the considerable changes of the long period and the crystallite dimensions at identical strains upon stretching and unloading, there existed a one-to-one correspondence between the strain and the orientational order parameter of polymeric chains in the crystalline phase, irrespective of mechanical stress state. This work yielded new insight into the relationship between mechanical stability and operation temperature in industrial application of crystalline polymers.