CPS K 843 Radiation-induced crosslinking: Effect on structure of polyethylene

Branched polyethylene irradiated (0–400 Mrad) with a Co⁶⁰ source at room temperature under vacuum was studied by density, wide- and small-angle X-ray scattering (WAXS and SAXS) measurements. The radiation effects on the structure of bulk, branched polyethylene are quite similar to those observed by others on single crystals or oriented preparations. These effects include changes in bulk density, crystallinity(w_c orv_c) and¯d₁₀₀ and¯d₂₀₀ spacings as a function of irradiation. A decrease in crystallinity is seen to begin at radiation dose 100 Mrad whereas lattice expansion indicating onset of an orthorhombic-hexagonal transition can begin as low as 10 Mrads. The decrease in crystallinity can be attributed to additional lattice distortions primarily introduced by the crosslinks occurring at the lateral grain boundaries, while lattice expansion can be associated with the same crosslinking mechanism which begins at the defects both within the crystals as well as those outside the crystals at the lateral grain boundaries. Strong evidence for a primary crosslinking-at-the-defects mechanism has also come from_c and_a data obtained in this study as a function of radiation dose. The same data have also led to an excellent correspondence between the measured density crystallinityv_c and the measured WAXS crystallinityw_c. Without consideration of the effects of crosslinks on_c and_a one would have obtained a divergence of the two crystallinities, especially at radiation doses greater than 100 Mrads.Dedicated Prof. Dr. R. Bonart on the occasion of his 60^th birthday