Polyimides Containing Phosphaphenanthrene Skeleton: Gas-Transport Properties and Molecular Dynamics Simulations

A series of new semifluorinated polyimide (PI) films with phosphaphenanthrene skeleton were prepared by thermal imidization of poly(amic acid)s derived from a diamine monomer: 1,1-bis[2'-trifluoromethyl-4'-(4″-aminophenyl)phenoxy]-1-(6-oxido-6H-dibenz⟨c,e⟩⟨1,2⟩oxaphosphorin-6-yl)ethane on reaction with four structurally different aromatic dianhydrides. The chemical structures of the polymers were established by Fourier transform infrared and ¹H NMR spectroscopy techniques. The polymers showed a good combination of thermal and mechanical properties (T _d10 up to 416 °C under synthetic air and tensile strength up to 91 MPa), low dielectric constant (2.10-2.55 at 1 MHz), and T _g values as high as 261 °C. Gas permeabilities of these films were investigated for four different gases CO₂, O₂, N₂, and CH₄. The PI films showed high gas permeability (P _CO up to 175 and P _O up to 64 barrer) with high permselectivity (P _CO /P _CH up to 51 and P _O /P _N up to 7.1), and the values are better than those of many other similar polymers reported earlier. For the O₂/N₂ gas pair, the PIs (PI A) surpassed the present upper boundary limit drawn by Robeson. A detailed molecular dynamics (MD) simulation study has been conducted to understand better the gas-transport properties. The effect of phosphaphenanthrene skeleton, its spatial arrangement, and size distribution function of the free volume were studied using molecular dynamics (MD) simulation and the results are correlated with the experimental data.