Pulse radiolysis study on transient species produced in polydimethylsiloxane

The primary and secondary ionic reactions were investigated in pulse irradiated, crosslinked polydimethylsiloxane, PDMS, matrix. The UV-VIS, structureless spectra of pulse irradiated PDMS, in the temperature range 100-295 K can be attributed to the transient radical species. Some evidences were presented that PDMS itself was able to scavenge the primary charges i.e. electrons and positive holes but the absorption spectra of products of these reactions were not found. The electronic spectra of PDMS doped with pyrene, Py, represented Py radical ions bands (450 nm and 490 nm maxima), Py triplets (410 nm) and Py radical (400 nm) absorptions. The lowering of temperature down to 100 K led to substantial increase of Py radical ions yield detected after the pulse. The decay of Py radical ions was found to be temperature and matrix dynamics dependent. Activation energies for Py radical anion decay were calculated to be equal 19.5 kJ mol(-1) for T>215 K; 61.5 kJ mol(-1) in the temperature range 215-165 K and only 0.6 kJ mol(-1) for T<165 K. The pulse irradiated Py-PDMS system generated at room temperature solute monomer and excimer fluorescence. The lowering of temperature caused the excimer emission to disappear and the Py monomer emission intensity and lifetime to increase. These temperature-dependent phenomena can be explained by photochemical mechanisms. The detection of the delayed Py fluorescence observed after the pulse can be treated as an evidence for the ionic origin of Py emission. The Py radical ions formation in irradiated PDMS and their secondary reactions strongly support the conclusion that ionic mechanism contributes to the radiation induced processes in PDMS. (C) 1998 Elsevier Science S.A. All rights reserved.