ESR study of structural changes in high-density polyethylene subjected to cyclic tensile stress

Bulk-crystallized high-density polyethylene is subjected to cyclic tensile stress and then irradiated by γ-rays to “spin-label” polymer molecules. The change of crystalline texture in the fatigue process before initiation of necking is detected by the ESR method. The line-resolution parameter of the ESR spectrum, which reflects the degree of regularity of molecular arrangement, first decreases, then increases somewhat, and finally becomes almost constant during fatigue cycling. The concentration of radicals trapped in a fatigued sample is larger than that in the unfatigued one. The excess radicals disappear during heating by a process of low activation energy in the temperature region of the mechanical γ dispersion. The degree of crystallinity evaluated from x-ray diffraction and density scarcely changed during fatigue cycling. The size of the mosaic block crystals estimated from the radical trapping capacity showed a rapid decrease at the initial stage of fatigue. The x-ray paracrystalline analysis indicates a similar but not so drastic decrease. These results lead to the conclusion that the slight disordering of crystalline texture accompanying the decomposition of mosaic block crystals in the lamellae occurs at the initial stage of the fatigue process, following which crystals reform somewhat and then remain unchanged until the initiation of necking.