Coupled chemical reactions in dynamic nanometric confinement: V. The influence of Li⁺ and F^- ions on etching of nuclear tracks in polymers

Etching of continuous nuclear tracks in thin polymer foils from both sides is known to lead to the formation of double-conical nanopores. In this work and related ones we try to find out how this etching kinetics is modified when materials are added which react with each other upon their contact towards some new product that influences the etching. For that purpose we have chosen here Li⁺ and F^- ions as the additions, which react with each other to form LiF precipitations. The coupled etching and precipitation kinetics is recorded by measuring the electrical current that is transmitted through the foils upon application of a low-frequency alternating sinusoidal voltage. Depending on the etchant concentrations, the etching temperature and the time of Li⁺ and F^- addition, different effects are found that range from (a) no alteration of the transmitted current at all, via (b) the emergence of an alternating current with a temperature-dependent amplitude, and (c) the complete vanishing of any transmitted current at all, towards (d) chaotic transmitted current histories with phases with strong current spike emission and (e) rather quiet phases, alternating with each other in a rather unsystematic way. The observed effects are ascribed to (a) the enhanced penetration efficiency of both the Li⁺ and F^- ions through the polymeric bulk and/or latent ion tracks after the removal of the polymer's protective surface layer by the etchant, (b) the high mobility of preferentially the F^- ions within the polymer, (c) the LiF precipitation within the polymer or on its surface upon encounter of Li⁺ and F^- ions, (d) the nanofluidic properties of narrow etched tracks covered with Li⁺ ions on the wall surfaces and F^- ions beyond, and/or (e) the formation of LiF membranes within the etched tracks.