Photoinduced Reactions of Benzophenone in Biaxially Oriented Polypropylene.

The photoinduced reactions of benzophenone (B) in biaxially oriented polypropylene (BOPP) were studied with nanosecond laser photolysis (N2 laser, λ337.1 nm). The first observed transient was a triplet state 3B*. Decay of 3B* led to formation of a radical pair (RP) of BH• and R•, where R• is a radical formed by hydrogen abstraction from BOPP (RH) by 3B*. We studied BOPP after the preheating for a short time in a temperature range 298-423 K, which is essentially lower than its melting point of 453 K. All measurements with not-heated and with preheated (annealed) BOPP were made at 298 K. A radical pair (RP) apparently decays as a contact pair 3[BH•, R•] in nonheated BOPP. A critical phenomenon takes place: dissociation of RP with a formation of free radicals in the polymer bulk is observed at preheating temperature Tcrit ≈ 403 K and at a higher T. The physical process of heating and cooling of BOPP apparently resulted in the restructuring of crystallites, their agglomeration, shrinking of the distribution of crystallites according to their sizes in BOPP. Overall BOPP becomes softer which manifests itself in the radical kinetics. The decay kinetics of 3B* and RP in the cage fits well the first-order law. Rate constants were obtained. Radicals BH•, which exit into the polymer bulk at temperatures of preheating T ≥ 403 K, decay by cross-termination according to the second-order law. A relatively high rate constant ∼108 M-1·s-1 for this reaction was obtained due to diffusion of BH• enclosed in the soft amorphous phase of BOPP. Properties of BOPP containing B were studied with ESR, DSC, IR, and WAXD.

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 kb ≈ 10(8) 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 kb than in a nonelongated E. The decrease in kb is ascribed mainly to a lower fractional polymer free volume Vf in elongated E compared with that in nonelongated E. Dependencies of log10 kb versus [Formula: see text], where [Formula: see text] is the thickness of the film, turned out to be linear with a negative slope. At the same [Formula: see text] 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.

Kinetics of benzophenone ketyl free radicals recombination in a polymer: reactivity in the polymer cage vs. reactivity in the polymer bulk.

The decay kinetics of intermediates produced under photolysis of benzophenone (B) dissolved in soft rubber poly(ethylene-co-butylene) films (abbreviated as E) was studied by ns laser flash photolysis in the temperature range of 263-313 K. We monitored decay kinetics of the triplet state of (3)B* and of benzophenone ketyl free radical BH˙. The fast exponential decay of (3)B* (life-time τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with a formation of BH˙ and a polymer free radical R˙. Decay of (3)B* was followed by decay of BH˙ in the polymer cage (geminate recombination) with τc ≈ 1 µs. Cage recombination in turn was followed by a decay of BH˙ in the polymer bulk (τb ≈ 100 µs). Fortunately, all three processes are separated in time. Both cage and bulk reactions were decelerated by the application of magnetic field (MF) of 0.2 T by approximately 20%. Geminate recombination was fit to the first-order kinetic law, and recombination in the solvent bulk fits well to the second-order law. Both geminate recombination and recombination in the solvent bulk are predominantly a reaction between BH˙ and R˙. It was assumed that the reaction radius ρ12 and a mutual diffusion coefficient D12 of BH˙ and R˙ are the same for the cage and bulk recombination, respectively. This led to an estimation of ρ12 = 3.3 nm and D12 = 1 × 10(-7) cm(2) s(-1). These values are discussed. We obtained activation energy, Eact, equal to 6 kcal mol(-1) and 7 kcal mol(-1) for cage decay and for recombination in the polymer bulk, respectively. These Eact coincide with each other within experimental error of their determination (±0.5 kcal mol(-1)). This indicates the same diffusion character in the cage and in the polymer bulk. It was demonstrated that an exponential model of cage effect sufficiently describes the obtained experimental data in rubber.

Decay kinetics of benzophenone triplets and corresponding free radicals in soft and rigid polymers studied by laser flash photolysis.

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.