Reactivity of Benzophenone Ketyl Free Radicals in an Elongated Elastomer Film

The effect of polymer film elongation leading to the thinning of the film up to three times on the decay of transients was studied. Kinetics of benzophenone (B) triplet state 3 B* and ketyl free radical BH• in soft rubber poly­(ethylene-co-butylene) (abbreviated as E) was investigated by nanosecond laser flash photolysis. We monitored decay kinetics of the triplet state of 3 B* and of BH• decay in the polymer cage and decay of BH• in the polymer bulk. The fast exponential decay of 3 B*­(lifetime τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with the formation of BH• and a polymer free radical R•. The decay of BH• in the polymer cage occurs during τc ≈ 1 μs. Cage recombination, in turn, was followed by a cross-termination of BH• in the polymer bulk (τb ≈ 100 μs under our conditions) and is characterized by a rate constant k b ≈ 108 M–1 s–1. We studied changes of rates of transients decay upon elongation (thinning) of E. Decay of 3 B* is practically independent of elongation of the film. Recombination of BH• in the solvent bulk occurs with a two times lower k b than in a nonelongated E. The decrease in k b is ascribed mainly to a lower fractional polymer free volume V f in elongated E compared with that in nonelongated E. Dependencies of log10 k b versus l − 1 , where l is the thickness of the film, turned out to be linear with a negative slope. At the same l recombination proceeds slower in the elongated elastomer compared with the nonelongated elastomer. Cage effect increases twice as well due to a lower rate of radicals escape from the polymer cage in the elongated film. We observed relatively large effects of external magnetic field (B = 0.2T) on the kinetics of cage recombination and recombination in the polymer bulk. Magnetic field effect on recombination rates in the cage and in the solvent bulk does not depend on elongation.