Effect of cation exchange on dimethyl methylphosphonate permeation kinetics in a pentablock hydrocarbon ionomer and a perfluorocarbon ionomer

The present work is concerned with the effect of cation exchange, with calcium, aluminum and cupric cations, on the permeation kinetics of dimethyl methylphosphonate (DMMP) in a pentablock ionomer (Nexar™) and a perfluoroionomer (Nafion™). The diffusion constant was determined by matching a Fickian solution to early time data, with a zero-time shift, initially attributed to the effect of immobilization. The diffusion time-lag relation was applied in an expanded form to include contributions of immobilization and relaxation. The primary effect of cation exchange was a marked reduction in diffusion constant and introduction of significant relaxation. The effects were explored in a two step procedure in which the sample was first exposed to DMMP vapor at activity 0.6 and then at activity 0.8. In cation modified Nexar at the lower activity the delay in onset of permeation was due solely to immobilization. The delay was absent at the higher activity, indicating that extent of immobilization was unchanged with increased concentration, but approach to steady state was still relaxation controlled. In cation modified Nafion delay in onset of permeation was dominated by relaxation, but kinetics at the higher activity was essentially Fickian. The strong relaxation effects appear to be a consequence of cation modification, which results in a network of ionic crosslinks formed by multivalent cation–sulfonate interactions. The combined analytical and experimental procedure developed in this study provided a systematic set of values for the effect of cation exchange and ionomer characteristics on parameters controlling DMMP permeation kinetics.