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

Color change is an important problem in commercial applications involv ing both the radiation-processing of polymeric materials and the use o f polymers in certain optical devices in radiation environments. We re port on absorption changes in optical polymers exposed to gamma radiat ion. Discoloration arises due to formation of two different types of c olor centers:'permanent' and 'annealable'. Annealable color centers ar e associated with radical species trapped within glassy polymer matric es. They disappear with time through two different mechanisms: one inv olves reaction with oxygen diffusing into the sample from the surround ing atmosphere, while the other involves a nonoxidative, highly temper ature-dependent process that can be attributed to recombination reacti ons. 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 structure s, under the influence of the free-radical chemistry induced by irradi ation, is an important factor in the extent to which the material disc olors.