Effects of Monomer Composition on CO₂−Selective Polymer Brush Membranes

Membranes that contain a high fraction of amorphous poly(ethylene glycol) (PEG) are attractive for selective removal of CO 2 from H 2 streams, but crystallization of PEG chains restricts both flux and selectivity. The high thickness of solution-cast membranes also limits flux. This study demonstrates the formation of composite, PEG-containing membranes through atom transfer radical polymerization of poly(ethylene glycol methyl ether methacrylate) from initiator-modified, porous substrates. The resulting membrane skins are only 50−500 nm thick, and copolymers that contain a mixture of short and long PEG side chains do not readily crystallize. The smaller PEG chains (8−9 ethylene oxide units) prevent crystallization, while the presence of longer side chains (23−24 ethylene oxide units) allows the membranes to maintain a CO 2 /H 2 selectivity of 12 at room temperature. This work examines the effect of side-chain length on polymerization rate as well as the permeability, selectivity, and crystallinity of copolymer films. Reflectance FTIR spectroscopy reveals the fraction of different monomers incorporated into copolymer films and demonstrates when crystallization occurs.