Science Concentrates BIOTECHNOLOGY
The structure of PIM-TMN-Trip’s molecular chains (line drawing at left and 3-D model in center) cause the polymer to form membranes with permanent pores (blue regions in simulated cross section at right), which enhance gas permeability.
POLYMERS
Highly permeable polymer separates gases Advance may broaden application of low-cost polymer membranes for gas separation 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 a polymer with molecular branches that pack inefficiently, researchers have 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), or PTMSP, films whose 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 designed a polymer to have PTMSP-like
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C&EN | CEN.ACS.ORG | AUGUST 7, 2017
permeability but have greater 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-TMNTrip, 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 (
