DISCOLORATION AND SUBSEQUENT RECOVERY OF OPTICAL POLYMERS EXPOSED TO IONIZING-RADIATION

Color change is an important problem in commercial applications involving both the radiation-processing of polymeric materials and the use of polymers in certain optical devices in radiation environments. We report on absorption changes in optical polymers exposed to gamma radiation. Discoloration arises due to formation of two different types of color centers:'permanent' and 'annealable'. Annealable color centers are associated with radical species trapped within glassy polymer matrices. They disappear with time through two different mechanisms: one involves reaction with oxygen diffusing into the sample from the surrounding atmosphere, while the other involves a nonoxidative, highly temperature-dependent process that can be attributed to recombination reactions. The ratio of permanent and annealable color centers and the rate of post-irradiation annealing vary greatly in different polymer types. The ability of a macromolecular material to form conjugated structures, under the influence of the free-radical chemistry induced by irradiation, is an important factor in the extent to which the material discolors.