Charge transfer processes in pulse-irradiated polymers

Mobile electron-positive hole pairs were found to form in pulse-irradiated polyethylene(PE) and poly(methyl methacrylate) (PMMA) polymer matrices. Negative charges can be stabilized in PE as trapped electrons, e(t)(-), evident in near IR spectra (< 2000 nm) or, in PMMA, as radical-anions evident in the UV-VIS range (Fig. 1). Positive holes can be trapped in PE as the unsaturation centers in the polymer chain. In PMMA, positive (mobile) holes are transformed into radical centers by a fast proton transfer process. In the aromatic [pyrene (Pi); dimethyldiphenyl (DMD)]-doped polymer matrices, short-lived radical-ions are formed (Fig. 2) which recombine to produce excited singlet monomer and excimer states [Eqn. (6)]. After an irradiation pulse, both monomer and excimer emission is observed as delayed luminescence (Fig. 4). A second-order kinetic equation including a time-dependent constant can describe the kinetics of recombination (decay) of the solute radical-ions in the PE matrix. The Arrhenius-type equation describes the effect of temperature on the solute recombination rate constant. A two-segment linear plot was obtained and, with the PE matrix, the intersection point occurred at the glass transition temperature (Tg) (Fig. 3).