Carbon Molecular Sieve Gas Separation Membranes Derived From An Intrinsically Microporous Polyimide Precursor

Thermal treatment is a classical technique to induce microporosity in polymers that has gained strong attention in the development of materials with higher permeabilities and selectivities for gas separation applications. However, as demonstrated through work on disubstituted polyacetylenes in the 1980s, intrinsic microporosity in polymers can also be achieved by inefficient packing of chains in the solid state. Recently, it has been shown that incorporation of sites of contortion into a rigid, ladder-type backbone is another successful strategy in the design of microporous polymers. A polyimide (PIM-6FDA-OH) exhibiting significant microporosity was recently synthesized using a novel diamine containing a spiro center. The effect of thermal treatment on this already microporous precursor was studied using a series of pyrolyzed membranes derived via a step-wise heat treatment to 440, 530, 600, 630 and 800^oC, respectively.  The physical characteristics, including porous texture, and pure- and mixed-gas transport properties were studied as the polymer transitioned through a polybenzoxazole to a series of carbon molecular sieves (CMS). CMS membranes based on this intrinsically microporous precursor rank among the best reported to date for CO₂/CH₄ separation.