Cationic photopolymerizations of thick polymer systems: Active center lifetime and mobility

Cationic photopolymerizations are essentially non-terminating and the long-lived active centers may lead to “dark cure” long after the illumination has ceased. In this contribution, it is shown that long cationic active center lifetimes known to be responsible for dark cure can also lead to “shadow cure” of unilluminated regions of thick systems. “Shadow cure” occurs when the active centers migrate out of the illuminated region, leading to polymerization of unexposed monomer. Photopolymerizations of a cycloaliphatic diepoxide monomer were performed in which active centers were produced in the first 0.7 mm of a thick system by illuminating one end of the sample, and the shadow cure was monitored up to 8 h. A polymerization front (with a highly crosslinked polymer matrix on one side and unreacted monomer on the other) was observed to move from the illuminated region into the shadow region at a rate proportional to the square root of time. The effective shadow cure diffusion coefficient at 50 °C was found to be 1 × 10⁻⁵ cm²/s, and the temperature dependence of the diffusion coefficient was well described by the Arrhenius relationship with an activation energy of 89 kJ/mol. Studies based upon photoinitiator counter-ions of differing size revealed that the system with the larger counter-ion (and therefore a correspondingly higher propagation rate constant) exhibited a significantly higher effective shadow cure diffusion coefficient. All of the experimental observations are consistent with the hypothesis that the active center mobility responsible for shadow cure arises largely from reactive diffusion.