Radiation-Induced Fluorescence from Doped Polyolefins on a Nanosecond Time Scale: Kinetics of the Processes Involving Geminate Radical Ions

Abstract

The delayed radiation-induced fluorescence from polyethylene and its alkyl-and fluorine-substituted analogues doped with aromatic luminophores was studied in the time range of 1−1000 ns. Qualitative analysis of the effects of a magnetic field on the fluorescence decay indicated that, in all polyolefins studied, the main portion of the fluorescence observed arose from the recombination of geminate spincorrelated radical ion pairs (RIPs). In the case of polyethylene, this conclusion was supported by observing the effect of an external electric field on the fluorescence decay. It was shown by comparison with the computer simulation of intratrack recombination that the tunneling character of the RIP recombination, which had an asymptotic time dependence of the geminate recombination rate close to t −1 , was typical of most studied polyolefins at temperatures below 273 K in the time range studied. The increase to room temperature and above caused a gradual transition to a regime where the geminate recombination rate was mainly determined by the migration of RIP partners with time dependence close to t −3/2 . The low estimate of the electron transfer distance upon the ion recombination in this regime was about 2 nm. In polyethylenes, exposed to an irradiation of 0.3−0.4 MGy, the role of charge carrier diffusion became hardly noticeable because of the cross-linking of polyethylene chains and the increase in polymer matrix stiffness. Oxygen, dissolved in a polymer doped with aromatic molecules, caused quenching of the recombination luminescence due to electron transfer from the dopant radical anion to the oxygen molecules. At room temperature, typical distances for such electron transfer were estimated to be ∼1.5 nm.

Primary Radical Cations in Irradiated Poly(isobutylene)

“…Such dependence is a characteristic of nonhomogeneous intratrack recombination of ion pairs in condensed medium, during which the recombination rate is high within a very short time but decreases very quickly because of the disappearance of closely spaced pairs. It was shown previously 13 that in PIB, at temperatures below 333 K, the recombination in a submicrosecond time range proceeds mainly via electron tunneling, while the diffusion of the charge carriers contributes slightly to the recombination rate. fluorescence intensity can be enhanced by an external magnetic field.…”

“…fluorescence intensity can be enhanced by an external magnetic field. 13 In the present work, however, we focused only on the initial part, until ca. 30 ns, of the fluorescent response, to analyze the effects originating from the phase coherence in the spin-correlated RIPs.…”

“…20 Note that the distance of electron transfer during the RIP recombination is of fundamental importance only if, for a sufficiently long time, it is so short that the exchange or dipole−dipole interactions distort the spin evolution. However, a recent study 13 showed that such a situation hardly happens during intratrack recombination in polyolefins in the vicinity of room temperature. As the recombination kinetics is not known with the accuracy that would allow investigating the evolution of the spin state of the RIP ensemble directly, it is more effective to analyze the ratio of fluorescence intensities in strong and zero magnetic fields.…”

“…Previously, this was used to detect, for the first time, the formation of RCs of some branched alkanes in a liquid solution, 11,12 as well as RCs in irradiated polyolefins. 13 2. METHODS 2.1.…”

“…No effects of atmospheric oxygen were found in the studied time range for this work, which is consistent with the very small permeability of oxygen to PIB. 13 2.2. Details of Quantum Chemical Calculations.…”