Homogeneous reaction rate model for hydrogen production from ion-irradiated polymers

A theoretical model has been constructed to calculate the time or fluence dependence of G-values for H2 production, G(H2), from the ion irradiation of the polymers polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), and Kapton. Measurements of the G(H2) for 1 Mev Ar+ over a fluence range from about 1 x 1011 to about 5 x 1013 /mm2 have been made in order to determine the parameters of the model. The model is based upon rate equations describing the electronic-generation of and the interaction of a uniform distribution of free radicals. Satisfactory fits to the data could be made by adjusting two key parameters - the effective C-H bond energy and the hydrogen-carbon recombination rate constant relative to the hydrogen-hydrogen recombination rate constant. It was found that the effective C-H bond energy varied from the lowest value of ∼8 eV for PE to the highest value of ∼100 eV for Kapton. From the effective bond energy, an average value for hydrogen radical production, G(H.), was deduced. The effects of the parameters on the G-value versus time/fluence curves are shown and the significance of the parameters are discussed. The data was also compared to percolation model predictions, but the deviations between data and this model were seen to be large at high fluence. (orig.)