Functionalization of Polyaniline Membranes to Enhance Separation of Carbon Dioxide From Methane

Polyaniline (PANI) is formed from the readily available monomer using a simple preparation technique and exhibits excellent stability as well as ability to form thin homogeneous layers on the surface of other polymers. We prepared composite membranes combining polyaniline film with a thickness in the range of 90 - 200 nm as an active layer with a polypropylene support using an in situ deposition technique. PANI surface of this composite membrane was functionalized via photografting of a mixture of 2-hydroxyethyl methacrylate and glycidyl methacrylate to afford both hydrophilicity and reactivity. Epoxy functionalities of the latter were allowed to react with different diamines providing for an increase in basicity.  Further enhancement in basicity was achieved through reaction of the primary amine groups present in the diamine modified surface with 2-ethyl-2-thiopseudourea to form quanidine functionalities. The modified PANI films were examined by X-ray photoelectron spectroscopy and the water droplet contact angles were measured. The surface of functionalized PANI membranes was hydrophilic and allowed for accommodation of significant amounts of water that together with the amine functionalities serving as a fixed carrier enabled permeation of CO₂ via the facilitated transport mechanism. These membranes were used for the separation of carbon dioxide from its mixture with methane. The best membrane with quanidine functionalities featured a notable permeability of 3,466 Barrers and a selectivity of 540 for CO₂, i.e. values that are significantly better than those exemplified by the upper bond in the trade-off plot.