Polymeric membranes composed of polystyrenes tethering amino acids for preferential CO₂ separation over H₂

Various N-tert-butoxycarbonyl (N-Boc) amino acids, such as glycine, valine, and serine, were combined to 4-chloromethylstyrene. The resulted monomers were readily polymerized to give polystyrenes tethering an amino acid by a conventional free radical polymerization technique. After deprotection of the N-Boc group with trifluoroacetic acid, the obtained polystyrene derivatives were soluble in ethanol and spin-coated on a commercial poly(ether sulfone) porous membrane to fabricate a composite membrane for CO₂ separation over H₂. The CO₂ selective layer of synthesized PSt was ca. 200 nm thick, and the composite membranes showed preferential CO₂ separation over H₂, while parent polystyrene passed through H₂ rather than CO₂. Especially, the separation factor \( \alpha_{{{\text{CO}}_{ 2} /{\text{H}}_{ 2} }} \) was raised after alkaline treatment to form free primary amine on the terminus of graft chain. Serine tethered polystyrene showed the highest \( \alpha_{{{\text{CO}}_{ 2} /{\text{H}}_{ 2} }} \) (16) among them, which suggested the hydroxyl group would enhance the CO₂ affinity of primary amine.