Synthesis of hyperbranched copolyimides and their application as selective layers in composite membranes

Self-standing dense membranes from hyperbranched copolyimides based on (hexafluorolsopropylidene)diphthalic anhydride (6FDA), 1,3,5-tris(4-aminophenoxy)benzene (TAPOB) and 4,4'-(hexafluoroisopropylidene)dianiline (6FpDA) were successfully prepared and characterized for physicochemical properties. The synthesis involved the formation of hyperbranched poly(amic acid) precursors in the first step and their thermal imidization in the second step. Molar ratio of comonomers was 0.95:1 for 6FDA-TAPOB polyimide and 0.95:0.5:0.5 for 6FDA-6FpDA-TAPOB copolyimide. No gelation occurred during polymerizations if the concentration of resulting polymer was kept below 4 wt.%. Prepared polyimides showed moderate molecular weights (M(w) similar to 3 7,000), 6FDA-6FpDA-TAPOB copolyimides showed significantly broader polydispersity index (4.3) then for 6FDA-TAPOB poylimide (1.9). Both polymers exhibited similar glass transition temperatures (Tg similar to 230 degrees C) and 10 wt.% loss temperatures T(10) similar to 505 degrees C). Gas transport measurements showed significantly higher permeability coefficients for 6FDA-6FpDA-TAPOB [P(H(2)) = 33 Barrer; P(N(2)) = 0.59 Barrer] copolyimide than for 6FDA-TAPOB [P(H(2)) = 6.8 Barrer; P(N(2)) = 0.06 Barrer]. Ideal selectivities were significantly higher for 6FDA-TAPOB [alpha(H(2)/CH(4)) = 228, alpha(O(2)/N(2)) = 6.4] than for 6FDA-6FpDA-TAPOB [alpha(H(2)/CH(4)) = 112, alpha(O(2)/N(2)) = 5.9]. Novel composite membranes based on these hyperbranched (co)polyimides (applied as selective layer) were successfully prepared. Preparation was made by dip-coating of poly(acrylonitrile) porous supports. Selective layers were covered with additional PDMS layer to seal possible microdefects and for protection. Both composite membranes showed moderate single gas fluxes [J(H(2)) = 0.28 m(3) (STP) m(-2) h(-1) bar(-1) for 6FDA-6FpDA-TAPOB] and selectivities were about 65-75% lower than corresponding dense membranes. Composite membrane with 6FDA-6FpDA-TAPOB selective layer exhibited slightly better separation performance than that one based on 6FDA-TAPOB. In further work we plan to optimize the parameters of composite membrane preparation process to achieve better separation performance.