Optical characterization of photopolymers materials: theoretical and experimental examination of primary radical generation

Abstract

Several studies of the time varying photon absorption effects, which occur during the photo-initiation process in photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) the dye absorption, (ii) recovery, and (iii) bleaching. Based on an analysis of these mechanisms, the production of primary radicals can be physically described and modelled. In free radical photo-polymerization systems, the excited dye molecules induce the production of the primary radicals, R •, which is a key factor in determining how much monomer is polymerized. This, in turn, is closely related to the refractive index modulation formed during holographic recording. In this article, by modifying the composition of a polyvinylalcohol/acrylamide based photopolymer material, i.e. excluding any co-initiator, the photo-kinetic behaviour of the material is greatly simplified. In this way, the rate constant of intersystem crossing, k st, in going from the excited singlet state dye to the excited triple state dye can be determined. k st is then available to be applied in a full model of the photo-initiation process making it possible to accurately predict the time varying concentration of primary radicals generated during exposure.