Investigation of the effect of 60Co-γ radiation on single and polycrystalline polyethylene by broad line N.M.R.

The broad line N.M.R. spectra of irradiated and unirradiated high density bulk (HD) and single crystals (SC) of polyethylene were found to give rise to two components, e.g. broad and narrow lines, attributed to crystalline and amorphous regions, respectively. An increase in the line width of the narrow component with radiation dose is more marked in bulk HD specimens, and is ascribed to crosslinking in the radiation-sensitive amorphous region; whereas no such increase is apparent for the SC specimen. Though there seems to be a slight increase in the second moment of the total absorption curve with radiation up to about 200 Mrad, both the second moment and the line width of the broad component was found to decrease with radiation dose for both the samples. This indicates an increased chain mobility in the crystalline regions and is considered to be due to the destruction of regularity in the crystal lattices by the combined processes of crosslinking (or endlinking) in the amorphous regions and the formation of lattice defects and imperfections by the ionizing radiation. A relatively faster decrease of line width with radiation dose above about 1000 Mrad may be attributed to the predominance of the latter process at high doses.