Reverse-selective membranes allow larger molecules to cross in preference to smaller ones. Their applications include removing higher hydrocarbons from methane during the purification of natural gas, separating organic monomers from nitrogen in polyolefin production and removing hydrocarbons from hydrogen in refineries. Membranes are an attractive way to carry out molecular separations because, compared with techniques such as distillation, absorption and adsorption, they are often low-cost, energy-efficient and relatively green.

Typically, reverse-selective membranes are made of high-glass transition temperature, amorphous, low-density substituted polyacetylenes. Mixing nanoscale inorganic particles into these polymers can tune the membrane's selectivity.

In this study the researchers added nonporous, nanoscale fumed silica particles to glassy amorphous poly(4-methyl-2-pentyne) (PMP). The particles, which had hydrophobic trimethylsilyl surface groups, had a primary diameter of 13 nm.

As the proportion of fumed silica particles increased, the nanocomposite became more permeable to methane. At 50wt% fumed silica - the largest concentration used - the permeability of the PMP/fumed silica nanocomposite was more than 240% greater than that of pure PMP.

The researchers reckon that the fumed silica particles disrupt the packing of the rigid, bulky PMP chains, "increasing the accessible free volume in the polymer matrix without introducing cavities large enough to promote weakly selective or nonselective free-phase flow mechanisms".

The scientists also added other silicas of varying size, carbon black and alpha-alumina powder to PMP. All of the particles increased the PMP's permeability, although significant increases were only seen for particles smaller than 50 nm. Adding fumed silica to other high-free-volume glassy polymers produced similar effects.

The US participants were from the Research Triangle Institute, the University of Texas at Austin, North Carolina State University, and Membrane Technology and Research, while the Australian scientists came from the Commonwealth Scientific and Industrial Research Organisation - Division of Manufacturing Science and Technology, and Monash University.