Over the past 25 years, membrane research on perfluoropolymers has revealed surprising transport behavior. At a given throughput, perfluoropolymers are approximately 250% more efficient than their hydrocarbon-based analogues at separating hydrogen from helium. Unfortunately, it is difficult to determine the fundamental mechanism of transport for these systems because hydrogen and helium have characteristically fast diffusion kinetics and extraordinarily low solubility in polymers, thereby precluding the use of traditional transport characterization techniques.
Herein, we seek to identify the fundamental transport characteristics of hydrogen and helium for a series of polymers, both fluorinated and non-fluorinated, glassy and rubbery, that span a wide range of fractional free volume, and contain, in certain instances, complex morphology (e.g., Nafion N117). To accomplish this end, we have used a high precision magnetic suspension balance to determine sorption isotherms for hydrogen and helium. Coupled with permeability experiments and the use of the solution-diffusion model, our results reveal that highly fluorinated polymers have similar diffusion selectivities to hydrocarbon-based polymers but strikingly different solubility selectivities.
Our results are explained by a failure of the geometric mean assumption in the Hildebrand regular solution theory, and support for this interpretation is provided by modeling work using the Non-equilibrium Lattice Fluid (NELF) model. Additionally, supporting examples of hydrogen and helium solubility in liquids are provided to emphasize the universality of this phenomena across both liquids and polymers.