Water and sodium transport and liquid crystalline alignment in a sulfonated aramid membrane

Two sulfonated aramids, poly(2,2'-disulfonylbenzidine terephthalamide) (PBDT) and poly(2,2'-disulfonylbenzidine isophthalamide) (PBDI), were synthesized with the aim to explore their unique morphology for proton exchange membrane applications. Due to the different polymer structures, PBDT forms a nematic liquid crystal, whereas PBDI is isotropic. Both polymers show excellent thermal stabilities (T-d(5%) > 400 degrees C), high storage moduli (E' = 3-15 GPa) and crosslinked films are flexible and easy to handle. Pulsed-held-gradient NMR diffusometry reveals that the in plane water diffusion in the nematic PBDT membrane is as high as 3.3 x 10(-10) m(2)/s, whereas the diffusion in amorphous PBDI is only 2.5 x 10(-10) m(2)/s. Whereas near and crosslinked PBDI shows isotropic diffusion, near PBDT shows a high diffusion anisotropy (D-parallel to/D-perpendicular to = 3.0), which increases as a function of crosslink density (D-parallel to / D-perpendicular to = 4.6 at 80% crosslinking). This diffusion anisotropy is substantially higher than that typically observed for low molecular weight liquid crystals and for oriented polymeric conductors such as Nafion (R) (D-parallel to/D-perpendicular to similar to 2.0). The nematic order in the PBDT membrane also promotes directed ionic conductivity, i.e. Na+ conductivity in PBDT is 224 x 10(-2) S/cm and 1.67 x 10(-2) S/cm for PBDI, respectively. We propose that the rigid-rod PBDT chains form nano-scale hydrophilic channels, which act as pathways for transporting water molecules and ions. (C) 2015 Elsevier B.V. All rights reserved.