Spectroscopic investigation of three-component initiator systems
In this contribution, photodiffrential scanning calorimetry (DSC) and in situ time-resolved steady state fluorescence spectroscopy are used to study the initiation mechanism of the three-component systems: 1) methylene blu-methyldiethanolamine/diphenyliodonium chloride (MB/MDEA/DPI), 2) eosin Y/MDEA/DPI (EY/MDEA/DPI), and 3) eosin Y, spirit soluble/MDEA/DPI (EYss/MDEA/DPI). Kinetic studies reveal that the photopolymerization behavior for eosin dyes is quite similar to that observed for methylene blue. The fastest polymerization rate is obtained when all three components are present, the next fastest with the dye/amine pair, and the slowest with the dye/iodonium pair. In the MB/MDEA/DPI system, it is concluded that the primary photochemical reaction involves electron transfer from the amine to the dye. We suggest that the iodonium salt reacts with the resulting dye-based radical (which is active only for termination) to generate the original dye and simultaneously produce a phenyl radical that is active in initiation. Moreover, oxygen is found to quench the triplet state of the dye leading to retardation of the reaction. In the eosin/MDEA/DPI system, experimental results showed that the two eosin dyes function quite similarly in the three component system eosin/MDEA/DPI. We propose a sequence of steps in which the dye is rapidly oxidized by DPI in a reaction that is not highly efficient at producing initiating radicals. However, the dye may also be reduced by MDEA producing active initiating amine radicals. Another active radical may be produced if the reduced dye radical is further reduced to its leuco form by an additional molecule of amine. Additionally, the reduction of the bleached dye radical (formed via reaction with DPI) by the amine has a two fold effect. First it regenerates the original eosin dye, and second, it produces an active amine radical in place of the less active dye radical. Dual cell UV-visible absorption spectroscopy yielded no evidence for the formation of a ground state complex between the eosin dyes and the iodoniurn salt.
Published Article/Book Citation
Photoinitiated Polymerization, 847 (2003) pp.15-26.