Rheo‐optical studies on the deformation mechanism of semicrystalline polymers. X. Orientation retardation spectrum of polyethylene

In order to explore the correlation between rheo‐optical and mechanical dispersions, orientation and lattice‐deformation retardation spectra are introduced in the phenomenological treatment of the stress‐orientation coefficient and crystal lattice compliance functions in dynamic x‐ray diffraction. A similar complementary treatment of the stress‐optical retardation spectrum is also carried out for the dynamic birefringence. To determine the mechanical and rheo‐optical retardation spectra, a deconvolution method based on the Fourier transform is developed. By this method, the mechanical and rheo‐optical retardation spectra of a low‐density polyethylene are computed from the imaginary components of the corresponding experimental functions; and subsequently, the real and imaginary components of the original functions are recalculated numerically from their respective retardation spectra and compared with the observed ones. The similarity of the normalized mechanical and rheo‐optical retardation spectra demonstrates that the α mechanical dispersion is related to orientation retardation of the crystal grains at their boundaries. Furthermore, separation of crystalline and amorphous contributions to the stress‐optical retardation spectrum is made on the basis of the two‐phase hypothesis for polyethylene. The amorphous contribution to the stress‐optical retardation spectrum is further resolved into two components corresponding to the α and β mechanical dispersions under the assumption that the resolved component for the α dispersion is symmetric in shape.