Rheo-Optical Studies on the Deformation Mechanism of Semicrystalline Polymers. V. Quantitative Analysis of Grain-Boundary Relaxation in Polyethylene

Abstract

The dynamic mechanical behavior of a low-density polyethylene above room temperature was analyzed in terms of the α and β mechanical dispersions, using a mechanical model reflecting its spherulitic crystalline texture. The mechanical α dispersion, assigned to a crystal grain-boundary relaxation within the crystal lamellae, is quantified in terms of the dynamic orientation of the crystal grains on the basis that the dynamic orientation is proportional to fractional elongation of the grain-boundary region. The mechanical β dispersion, though only a part of its entire spectrum, is also quantified by constructing its master curve; the activation energy of the relaxation process is found to be around 27 kcal/ mol (113 kjmol⁻¹). The significance of the vertical shift factors for dynamic mechanical and optical data is also discussed in terms of temperature dependences of the structural and mechanical parameters of the model. It is found that the mechanical shift factor is related to both the temperature dependences of the crystal elasticity as well as to its crystallinity, especially, to the former, whereas the optical shift factor is mostly related to the latter.