PNNL

Abstract

For free-radical photopolymerization with a photobleaching initiator in an initially uniform medium, we incorporate nonuniform photoinitiation into a simple kinetic model to show how the degree of monomer conversion varies spatially and temporally with incident light intensity, initial photoinitiator concentration, and the propagation and termination rates. We show that the spatio-temporal variation of monomer conversion depends on two dimensionless parameters: the initial absorbance g , and an effective kinetic ratio b . For each value of g and each layer-averaged extent of monomer conversion, there is a minimum value of b beyond which the specified monomer conversion is assured. When the initial photoinitiator concentration is insufficient to ensure complete conversion of monomer throughout the layer, the final degree of monomer conversion is lowest near the optical "front" of the layer, where the light absorption and photoinitiation rates are initially highest. This is because at high values of the initial absorbance and small degrees of photoinitiator consumption, the primary radical concentration reaches a maximum near the front of the layer, corresponding to high rates of second-order termination. This leads to lower final degrees of monomer conversion than at the rear of the layer. The results are discussed in terms of controlling the spatial variation of monomer conversion, as well as in terms of the spatial variation of the radical concentration and other quantities.