Reciprocity law experiments in polymeric photodegradation: a critical review

Accelerating the photodegradation of polymeric materials is of great practical interest in weathering research. Acceleration can be achieved by exposing polymeric materials to a high radiant flux; however, questions have arisen within the weathering community as to whether high radiant flux results can be extrapolated to in-service flux levels. Experiments designed to test this premise are called reciprocity law experiments. An extensive review has been conducted to assess the state-of-the-art of reciprocity law experiments in the photography, photoconductivity, photo-medicine, photobiology, and polymer photodegradation literatures. From this review, the Schwarzschild law (a power law generalization of the reciprocity law) appears to model adequately photoresponse vs. radiant flux for most materials and systems. A band theory model has been presented to explain variations in the Schwarzschild law coefficients and other experimental phenomena commonly associated with reciprocity experiments. Obstacles to the general acceptance of high radiant flux, laboratory-based experiments are discussed.