Laser light-scattering studies of soluble high performance fluorine-containing polyimides

Two sets of soluble high performance polyimides synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2′-(trifluoromethyl)-4,4′-diaminobiphenyl diamine (PFMB), and from 2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyl-tetracarboxylic dianhydride (HFBPDA) and 2,2′-(trifluoromethyl)-4,4′-diaminobiphenyl diamine (PFMB) have been investigated by static and dynamic laser light scattering (LLS) in tetrahydrofuran (THF) at 30°C. The calibrations, for 6FDA-PFMB: <R_g> (nm) = 3.87 × 10^?2 <M_w> ^0.568, <R_h> (nm) = 2.38 × 10^?2 <M_w>^0.560 and <D> (cm²/s) = 2.13 × 10^?4 <M_w>^?0.560; for HFBPDA-PFMB: <R_g> (nm) = 2.24 × 10^?2 <M_w>^0.626, <R_h> (nm) = 1.27 × 10^?2 <M_w>^0.621 and <D> (cm²/s) = 3.99 × 10^?4 <M_w>^?0.621, have been established, where <M₂>, <R_g>, <R_h> and <D> are the weight-average molar mass, the root mean square z-average radius of gyration, the z-average hydrodynamic radius and the z-average translational diffusion coefficient, respectively. A combination of <M_w> and the translational diffusion coefficient distribution G(D) leads to the calibrations of D (cm²/s) = 2.41 × 10^?4M^?0.564 and D (cm²/s) = 6.16 × 10^?4M^?0.656 for 6FDA-PFMB and HFBPDA-PFMB, respectively, where D and M correspond to monodisperse species. With these calibrations, we can convert a translational diffusion coefficient distribution G(D) into a corresponding molar mass distribution f_w(M). On the basis of the Kratky-Porod wormlike chain model, the persistence lengths (q) were found to be ? 3.3 nm and ? 4.5 nm, respectively, for 6FDA-PFMB and HFBPDA-PFMB, which indicates that both polyimide chains have an extended conformation. In addition, <R_g> / <R_h> ? (1.7-1.9) shows that they are in coil conformation. Therefore, we conclude that both polyimides have an extended coil conformation.