Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases, pc and Pa, were calculated after direct determination of the 9 9 2 ...... mean square denszty fluctuatzon < 7? >. pa increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.
Branched polyethylene irradiated (0-400 Mrad) with a Co 6~ 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 Q, crystallinity (we or vc) and dl00 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 0c and 0a 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 crystallinity vc and the measured WAXS crystallinity we. Without consideration of the effects of crosslinks on 0c and Qa one would have obtained a divergence of the two crystallinities, especially at radiation doses greater than 100 Mrads.
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