Molecular Design of Precise Network Polymerized Ionic Liquid Membranes for Toluene/Heptane Separations

Precise network polymerized ionic liquid membranes with tethered imidazolium cations, variable anion, and controlled cross-link density were developed to understand how molecular structure impacts solubility and diffusion in the context of toluene/heptane separations. Switching the anion from tetrafluoroborate (BF4) to bis(trifluoromethane sulfonimide) led to a 25 K drop in the glass transition temperature, concomitant increase in penetrant diffusion coefficients, and increased solubility of toluene relative to heptane. Reducing cross-link density led to an increase in toluene swelling, while heptane uptake remained relatively low and constant. Interestingly, differences in toluene and heptane diffusion coefficients exhibited a maximum at intermediate cross-link density. It is hypothesized that a fully cross-linked network has a mesh size small enough to impede transport, while lower cross-link densities allow toluene to swell the system to a greater extent, in contrast to heptane. The interplay of diffusion and solubility effects lead to a nonmonotonic trend in selectivity, which will inform the design of membranes as effective organic liquid separations media.