INCREASE IN THE OPTICAL DAMAGE THRESHOLD OF POLYMETHYL METHACRYLATE.

Polymethyl methacrylate with an improved optical damage threshold has been produced in the laboratory by pursuing possibilities suggested by a micromechanical model for the destruction of polymers by electromagnetic radiation. Experiments with natural amber confirm the conclusions of the micromechanical model, in which it is shown that microscopic regions of local stress are crucial in determining the optical damage threshold of polymers. The optical damage threshold of amber has been reduced by an order of magnitude by creating microscopic regions of local stress (annealing). An ion-sputtering method yields polymethyl methacrylate samples in which there are essentially no microscopic regions of local stress, with the result that the optical damage threshold is increased by a factor of about 20-30.