Permeation and sorption properties of benzene, cyclohexane, and n-hexane vapors in poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) membranes

The permeability and solubility of benzene, cyclohexane, and n-hexane (C-6 compounds) in poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) membranes were determined experimentally, and the corresponding diffusivity was obtained by analyzing the data with the solution-diffusion model. The permeability of benzene ranged from several hundreds to several thousands Barrers, and that of cyclohexane and n-hexane ranged from several decades to several hundreds Barrers. All the permeabilities increased exponentially with the vapor activity and increased with temperature. The sorption isotherms of those C-6 compounds were well described by the Henry's law relationship, and constant solubility at each temperature could be determined from the linear correlation. The solubility seems to be inversely proportion to the molecular size of the penetrant when there is not much strong polymer-penetrant interaction. The diffusivity of those C-6 compounds ranged from 10−7 to 10−9 cm2/s, and the values of benzene were much larger than those of cyclohexane and n-hexane at the same temperature and vapor activity. The diffusivities of those C-6 compounds also increased exponentially with the vapor activity, and the relative magnitude of the diffusivity is determined by the molecular size (collision diameter) of them. The diffusivities of those C-6 compounds increased with temperature and their activation energies of diffusion were very similar possibly due to the same energy characteristic of polymer main chain movement.