Temperature Dependence of Strain-Hardening Behavior of Polyethylene Solids Evaluated by SAXS, WAXD, and Raman Spectroscopy

We investigated the effect of temperature on the strain-hardening behavior using Raman spectroscopy and small- and wide-angle X-ray scattering during uniaxial tensile tests. To accurately measure strain hardening, predrawn specimens elongated until the onset of the strain-hardening region were prepared. The temperature dependence of the strain-hardening modulus showed a maximum value at the crystalline-relaxation temperature (∼50 °C). For low-temperature stretching, the crystalline structure was disordered because of the strong stretching load applied to the crystalline chains. Moreover, the formation of a large number of highly loaded molecular chains, which we attributed to taut-tie chains, was revealed by Raman spectroscopy. However, for high-temperature stretching, the crystalline structure was stable in the strain-hardening region, and the stretching load was applied to the crystalline chains more homogeneously than under low-temperature stretching. Moreover, the contribution of plastic deformation was much greater for high-temperature than low-temperature stretching. This suggests that crystalline fragmentation occurred above the crystalline-relaxation temperature. Our results demonstrate that strain hardening proceeds with an increase in the entropy elasticity of taut-tie chains below the crystalline-relaxation temperature, resulting in a negative temperature dependence of the strain-hardening modulus. However, above the crystalline-relaxation temperature, plastic flow due to crystal fragmentation dominates strain hardening, resulting in a decrease in the strain-hardening modulus with increasing temperature.