Permeable polymer is choosy about what gases it passes

As college students prepare for the journey to school later this month, they’ll likely be packing comfy chairs and other precious finds tightly into their cars. In some situations, such as designing polymer membranes, tight packing is not the way to go, a study shows.

By creating an organic polymer with molecular branches that pack inefficiently, an international team of researchers has made a potentially highly useful membrane (Nat. Mater. 2017, DOI: 10.1038/nmat4939). The exceptionally permeable material may lead to new types of membranes for large-scale separation of air gases, biogas purification, and carbon capture.

Separating gases by passing them through membranes could be a low-cost, energy-saving alternative to other gas separation methods. For instance, cryogenic distillation chills air to ultralow temperatures to liquefy it, which is energy intensive, then separates the components by their boiling points. A few membrane-based techniques have been commercialized. But in general, polymer films are not very gas permeable, which leads to slow separations.

A notable exception is poly(trimethylsilylpropyne) (PTMSP) films, for which gas permeability is several orders of magnitude higher than other polymers. But PTMSP barely discriminates between different types of gases, and its internal pores, required for gas molecules to permeate, tend to collapse with age.

So Ian Rose and Neil B. McKeown of the University of Edinburgh and coworkers went back to the molecular drawing board to try to come up with a membrane material that would exhibit PTMSP-like permeability but have greater chemical selectivity. The team reasoned that combining rigid bicyclic triptycene (Trip) units, which are known to form porous polymers, with bulky tetramethyltetrahydronaphthalene (TMN) moieties, should lead to a polymer with two-dimensional ribbon-shaped chains that cannot form a compact 3-D network. The team refers to such materials as polymers of intrinsic microporosity (PIMs).

The researchers prepared membranes from that polymer, dubbed PIM-TMN-Trip, and a structurally similar one, called PIM-TMN-SBI, that was designed to pack a little more efficiently. Then they conducted molecular simulations and permeability and selectivity tests on the membranes with CO₂/CH₄, CO₂/N₂, and other gas pairs.

The results show that the Trip membrane’s permeability is roughly twice as high as SBI’s and comparable to that of PTMSP, yet the new material is better at separating gases and more durable. The study also shows that awkward packing of PIM-TMN-Trip chains results in a high concentration of small pores (<0.7 nm), which aid selectivity, and larger ones (0.7–1.0 nm), which enhance permeability.

“Higher gas permeability often comes with lower selectivity,” says Tianjin University’s Michael D. Guiver, a porous-polymer specialist. But given the advances reported in this study, Guiver anticipates that this design approach to ultrapermeable membranes “will set new directions for further improvements and stimulate the competitiveness of membrane technology.”