Polyimide-Based Membrane Materials for CO Separation: A Comparison of Segmented and Aromatic (Co)polyimides.

A series of new poly(ethylene oxide) (PEO)-based copolyimides varying in hard segment structure are reported in this work as CO selective separation membranes. Their structural diversity was achieved by using different aromatic dianhydrides (4,4'-oxydiphthalic anhydride (ODPA), 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)) and diamines (4,4'-oxydianiline (ODA), 4,4'-(4,4'-isopropylidene-diphenyl-1,1'- diyldioxy)dianiline (IPrDA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD)), while keeping the content of PEO (2000 g/mol) constant (around 50%). To get a better insight into the effects of hard segment structure on gas transport properties, a series of aromatic polyimides with the same chemistry was also studied. Both series of polymers were characterized by HNMR, FTIR, WAXD, DSC, TGA, and AFM. Permeabilities for pure He, O, N, and CO were determined at 6 bar and at 30 °C, and for CO for pressures ranging from 1 to 10 bar. The results show that OPDA-ODA-PEO is the most permeable copolyimide, with CO permeability of 52 Barrer and CO/N selectivity of 63, in contrast to its fully aromatic analogue, which was the least permeable among polyimides. 6FDA-4MPD-PEO ranks second, with a two times lower CO permeability and slightly lower selectivity, although 6FDA-4MPD was over 900 times more permeable than OPDA-ODA. As an explanation, partial filling of hard domain free voids by PEO segments and imperfect phase separation were proposed.