Ultrafast chemistry of nanoenergetic materials studied by time-resolved infrared spectroscopy: Aluminum nanoparticles in teflon

Ultrafast mid-infrared (IR) spectroscopy is used to monitor chemical reactions initiated by flash-heating a nanoenergetic material consisting of 30 nm diameter Al nanoparticle fuel in a Teflon(AF) oxidizer. Teflon(AF) is a copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (dioxole), so Al can react with several different moieties. Transitions associated with CF2 stretching of TFE or CFO stretching or CF3 stretching of dioxole were monitored. The reactions of Al with CFO occurred with time constant (k(1))(-1) = 50 (+/- 20 ps); reactions of Al with CF2 or CF3 were more than 10 times slower, (k(2))(-1) = 0.7 (+/- 0.05 ns). An interesting frequency oscillation is seen in the 1148 cm(-1) band, where the peak frequency undergoes a time-dependent shift from 1148 to 1155 cm(-1) and then back to 1148 cm(-1). Due to the coincidence of CFO and CF2 stretching transitions, this band in the copolymer represents an amalgamated vibration with amplitude on both TFE and dioxole. As concentration is varied from pure dioxole to pure TFE, the band blue-shifts. A kinetic scheme and a model for the concentration dependence of the amalgamated vibration frequency are developed, which show that the frequency oscillation is a consequence of the arrangement of reactants on the nanoscale, which creates two types of oxidizer. The type adjacent to the Al nanoparticle is in a region of high local fuel concentration, while the other type is too distant from the fuel to react.