Mechanism of refractive index increase in photosensitized poly(methyl methacrylate)

Poly(methyl methacrylate) prepared under special circumstances exhibits a substantial increase in refractive index after irradiation with ultraviolet light. The essential step in the preparation is peroxidation of the monomer prior to polymerization. This increase in refractive index results from a photo-induced polymerization of unreacted monomer (1-2 percent) within the film which produces an increase in density (and hence refractive index) in the irradiated region. It is believed that peroxides, both polymeric and monomeric, act as photoinitiators. Sensitivity depends on the concentration of photoinitiator but the absolute value of Δn depends on the amount of unreacted monomer.