Energy Transfer via Ionic Processes in Polymer Films Irradiated by 0.4 MeV Electrons

Abstract

Radiation-induced processes in polymer films have been studied by pulse radiolysis using transient spectroscopic methods. A comparison with low-energy photoexcitation shows that singlet and triplet excited states of aromatic polymers produced by high-energy irradiation are mainly derived from geminate recombination between the polymer radical cations and the excess electrons. A simultaneous production of singlet and triplet excited states, radical cations, and radical anions of aromatic solutes doped in polystyrene was observed on static quenching of the polystyrene excimer. By use of pyrene as the spectroscopic probe, the absolute yields of the pyrene transient species in the polymer right after the pulse were measured as G(1Py*) ≈ 0.8, G(3Py*) ≈ 1.1, G(Py•+) ≈ 1.5, and G(Py•-) ≈ 1.5 at [Py] = 100 mM. The large ion yields indicate an efficient charge generation in the nonpolar polymer matrix. Charge-scavenging experiments further reveal the ionic origin of the solute excited states. In the case of complete electron scavenging in poly(vinylbenzyl chloride), only the strong absorption band characteristic of the pyrene radical cations was observed. Both excited states and radical cations of solutes exhibit resolvable formation rates that increase with the solute concentration. An ionic mechanism of energy transfer is proposed to account for the formation of radical anions, radical cations, and excited states (S1 and T1) of solute molecules, respectively, from the electron scavenging, the positive charge transfer, and the secondary charge recombination reactions. Positive charge migration in polymer matrices is suggested to be responsible for the fast (∼2 ns) charge recombination and charge transfer reactions over large distances. The migration constant of the mobile positive charge is estimated from the charge transfer reaction rates to be Λh ≈ 6.0 × 10-5 cm2/s in polystyrene and poly(vinylbenzyl chloride). Numerical simulation of the experimental data using a single ion pair model shows that on average electrons are ejected ∼60 Å away from the parent cations in solid polystyrene and that ∼87% of the polystyrene excimer is derived from the primary geminate ion recombination. The effects of polymerization on charge separation and transport are discussed from an examination of the radiolytic responses of different polymers compared to those of their model compounds.