PULSE-RADIOLYSIS STUDY ON TRANSIENT SPECIES PRODUCED IN POLYDIMETHYLSILOXANE

The primary and secondary ionic reactions were investigated in pulse i rradiated, crosslinked polydimethylsiloxane, PDMS, matrix. The UV-VIS, structureless spectra of pulse irradiated PDMS, in the temperature ra nge 100-295 K can be attributed to the transient radical species. Some evidences were presented that PDMS itself was able to scavenge the pr imary charges i.e. electrons and positive holes but the absorption spe ctra of products of these reactions were not found. The electronic spe ctra 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 n m) absorptions. The lowering of temperature down to 100 K led to subst antial increase of Py radical ions yield detected after the pulse. The decay of Py radical ions was found to be temperature and matrix dynam ics dependent. Activation energies for Py radical anion decay were cal culated to be equal 19.5 kJ mol(-1) for T>215 K; 61.5 kJ mol(-1) in th e 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 t he excimer emission to disappear and the Py monomer emission intensity and lifetime to increase. These temperature-dependent phenomena can b e 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 co nclusion that ionic mechanism contributes to the radiation induced pro cesses in PDMS. (C) 1998 Elsevier Science S.A. All rights reserved.