(93a) Photopolymerization Of Thick Systems And Elimination Of Oxygen Inhibition

The use of light to initiate a polymerization reaction offers many advantages including spatial and temporal control of the initiation process. In this presentation, two recent advances in photopolymerization systems will be discussed. Oxygen inhibition is perhaps the most widespread and important problem in free radical polymerizations. The presence of oxygen in free-radical polymerization system known to be a primary cause of an inhibition period and will ultimately affect the attainable properties of polymer and lower the rate of polymerization. A strategy for eliminating oxygen inhibition will be presented which involves the inclusion of two specially selected components in the reactive formulation: 1) a light-absorbing molecule (the singlet oxygen generator), which produces an excited singlet state of oxygen, and 2) a second compound (the singlet oxygen trapper) which reacts with the singlet oxygen thereby removing it from the system. A variety of photo-induced singlet oxygen generators with different light absorption spectra may be used in this method, however porphyrins were found to be especially attractive because they allowed singlet oxygen to be produced using light in the red or the near IR regions of the spectrum. The experimental results illustrate that this method can be used to reduce or eliminate the inhibition period, and enhance the photopolymerization rate for reactions carried out in air. To effectively cure thick systems the initiating light must penetrate through the depth of the sample. A mathematical model will be presented to describe the spatial and temporal evolution of the light intensity gradient, the initiator concentration gradient, and initiation rate profiles for photobleaching initiator systems. The model that includes the effects of absorption by the initiator fragments, absorption by the monomer, and diffusion of the initiator. Simulation results confirmed that the initiation rate profile resembles a wave front that propagates through the sample, and that there is an optimum initiator concentration for efficient photopolymerization of thick samples.