Permeability of Gases and Ammonia for Phosphorus-Containing Copolyimides

New copolyimides based on phosphorus-containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), oxidianiline (ODA), and 4,4'-(4,4'-isopropylidenediphenoxy)bis-(phthalic anhydride) (BPADA) containing 50, 60, and 70 mol % DOPO were synthesized. The chemical structure of the obtained copolyimides has been confirmed by FTIR and 1H NMR. A comprehensive physico-chemical investigation of the polymers has shown that they are characterized by high molecular weights (Mw = 40–60 kDa), polydispersity indices (PDI = 2.8–3.6), and have close glass transition temperatures (Tg = 223–227°C), however, they differ significantly in their physico-mechanical behavior: with an increase in the proportion of phosphorus-containing diamine in copolyimide sharply decreases the tensile strength (from 193 to 27 MPa), elasticity modulus (from 0.18 to 0.90 GPa) and elongation at break (from 1.8 to 0.6%). The gas permeation parameters for a standard set of gases (He, H2, O2, N2, CO2, CH4) and ammonia measured by the barometric method were found to slightly depend on the phosphorus-containing diamine content, are close to each other, and to the corresponding parameters for a commercial polyimide Ultem based on BPADA. An increase in the upper membrane pressure of ammonia leads to an increase in its permeability and diffusion coefficient, which is caused by plasticization of the polyimide matrix. The synthesized new phosphorus-containing copolyimides and commercial Ultem polyimide have advantageous gas separation characteristics for He–CH4 (α = 100–120) and NH3–N2 (α = 170–220) gas pairs. Significant deviations of the data for ammonia from the linear correlation of the diffusion coefficient with the square of the effective kinetic diameter (deff) by means of the literature data for deff(NH3) were found. An attempt was made to estimate the new effective kinetic diameter of ammonia, which was 2.85 ± 0.17 Å, based on diffusion data for the three studied copolyimides.