Temperature dependence of the yield and flow stresses of linear high-molecular-weight polyethylene

Abstract

The yield stress and the flow stress at different strains have been measured as a function of temperature for a linear polyethylene of M_w=2.5×10⁶ (AC1220). Two transitions are found for the flow stress (after the Lüders strain), at about 175 and 265°K, which divide the stress‐temperature plot into three temperature regions. Only one transition (at ∼170°K) is found for the yield stress. By means of cyclic experiments both in strain rate and temperature along the stress‐strain curve, the apparent activation enthalpy for plastic deformation is determined between 180 and 360°K. Previous measurements of the apparent pressure activation volume for deformation indicate that the 265°K transition (for strains greater than the Lüders strain) is shifted to 297°K by a pressure of ∼2.5 kbars. Thermally activated mechanisms for deformation are tentatively assigned to each of the three temperature regions.