Decay Kinetics of Benzophenone Triplets and Corresponding Free Radicals in Soft and Rigid Polymers Studied by Laser Flash Photolysis

Abstract

The kinetics of transients formed under photoexcitation of benzophenone (B) dissolved in three different polymers was studied by ns laser flash photolysis. These polymers were the soft rubbers poly (ethylene-co-butylene) (EB), polystyrene block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) and hard polystyrene (PS). We monitored the decay kinetics of triplet state (3)B(*) and of ketyl radicals BH(●). We observed exponential decay of (3)B(*) and two-stage decay kinetics of BH(●) in EB. The first stage is a fast cage recombination of a radical pair (BH(●), radical of polymer R(●)). The second slow stage of BH(●) decay follows the second-order law with a relatively high rate constant, which corresponds to recombination of BH(●) in a homogeneous liquid with a viscosity of only ~0.1 P (about five times of 2-propanol viscosity). Application of a magnetic field (MF) of 0.2 T leads to deceleration of both stages of BH(●) decay in EB by approximately 20%. Decay kinetics of both transients were observed in SEBS. There was no MF effect on BH(●) decay in SEBS. We only observed (3)B(*) in PS. Decay kinetics of (3)B(*) in this case were described as polychromatic dispersive first-order kinetics. We discuss the effects of polymer structure on transient kinetics and the MF effect.

Cage Effect under Photolysis in Polymer Matrices

“…Experimental data on the measured and calculated rate constants and on ϕ in the Earth's and in the external MF of 0.2 T are presented in Refs. [4,[22][23][24]. In particular, in an elastomer (E), a decrease of ϕ more than two times and a decrease of k b (Equation (7)) by~20% was observed in MF [24].…”

“…A series of works [4,[19][20][21][22][23][24][25] was dedicated to the kinetics of cage effect in polymers or to kinetics of G-pairs decay. Benzophenone (B) was selected as a photoprobe.…”

“…Recombination in the polymer bulk proceeds through F-pairs In publications [20][21][22][23][24][25], the focus was on a quantitative analysis of kinetic traces of transients, comparison of the kinetics of recombination in the cage and in the polymer bulk (in G-and F-pairs), as well as a study of magnetic field effects (MFEs) on G-and also on F-pairs. Figure 1 below presents kinetic traces of 3 B* decay and of geminate recombination at two different temperatures: The contribution of 3 B* into the observed geminate recombination becomes negligible after 300 µs; see Figure 1.…”

“…BH•, which is the product of the cage dissociation, decays relatively slowly in the bulk of a polymer (Scheme 3, Figures 2 and 3): Figure 2 presents a good fit of the kinetics of BH• decay as the second-order kinetics. Second-order rate constants k b for several polymers were reported [22][23][24][25].…”

“…The most probable mechanism of the observed MFE in E [22][23][24] is the hyperfine coupling (HFC) mechanism [29]. Briefly, the HFC mechanism acts the following way.…”