ION TRACK EFFECTS ON GEL FORMATION OF POLYSTYRENE

Crosslinking in monodisperse polystyrene induced by energetic ion bombardment in the electronic stopping power regime is studied over a wide range of ion energies (7.5 keV/amu-0.8 MeV/amu). The chemical yield of crosslinks is obtained by solubility measurements of irradiated polymer and the measured values are compared using stopping powers (eV/cm). Experimental results show that the chemical yield is higher(similar to a factor 5-6) for low velocity than for high velocity ions over a wide range of stopping powers (6 X 10(8)-80 x 10(8) eV/cm). All these results are quantitatively explained in terms of the difference in the spatial distribution of microscopic energy deposition, which is more localized for low velocity ions than for high velocity ones. The process efficiency increases with the mean energy deposited inside the ion track and approaches saturation when the mean energy becomes about 1.5 eV/atom. Other data, in the literature, can be explained similarly. This work clearly indicates that the key parameter for crosslink formation in polymers is not the electronic stopping power, but the lateral distribution of the energy deposition.