Ultrathin, Gas-Selective Polyimide Membranes Prepared from Multilayer Polyelectrolyte Films

Alternating electrostatic deposition of poly(amic acid) salts and poly (allylamine hydrochloride) (PAH) followed by heat-induced imidization yields ultrathin, gas-selective polyimide membranes on porous alumina supports. These membranes are appealing because of the stability and selectivity of polyimides and the fact that alternating polyelectrolyte adsorption allows formation of ultrathin (as low as 35−40 nm), defect-free films that allow high flux. FTIR spectroscopy shows that heating of poly(amic acid)/PAH membranes at 250 °C for 2 h completely converts the poly(amic acid) to the corresponding polyimide, and scanning electron microscopy reveals uniform films with minimal deposition in substrate pores. Permeability coefficients and selectivities (O 2 /N 2 up to 6.9 and CO 2 /CH 4 up to 68) of three different imidized poly(amic acid)/PAH membranes are comparable to literature values for the corresponding bulk polyimides, provided that the ratio of poly(amic acid) to PAH in the film is high (9:1). Electrostatic deposition of poly(amic acids) thus provides a convenient way to prepare high-flux, gas-selective films on porous supports.