| Abstract: | When polymeric materials doped with nitroxides of the 2,2,6,6-tetramethylpiperidine type are exposed to light, the nitroxide concentration decreases. The two mechanisms for the decrease of the nitroxide are the reaction of nitroxide with free radicals produced during photolysis of the polymer to form amino ethers and the abstraction of hydrogen atoms by excited-state nitroxides to form hydroxyl amines. Excited-state nitroxides can be formed in two ways: by direct absorption and by energy transfer. In this paper, the effect of energy transfer on the rate of decay of the nitroxide signal is studied, and measurements of nitroxide decay are used to probe energy transfer in crosslinked polymeric coatings. A simple kinetic scheme is used to interpret nitroxide decay during photolysis of both solutions and polymers containing benzophenone. These results are used to show that the slope of the line relating nitroxide decay rate to nitroxide concentration is essentially determined by energy transfer from a coating-based chromophore to nitroxide. The nitroxide decay assay is also used to study the effectiveness of a benzotriazole ultraviolet light absorber as a quencher. |
