期刊
JOURNAL OF MEMBRANE SCIENCE
卷 489, 期 -, 页码 194-203出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/j.memsci.2015.03.090
关键词
Liquid crystal polymer; Anisotropy; Ionic crosslinking; Ion transport; NMR diffusion
资金
- China Scholarship Council (CSC) [2010634027]
- Teijin Aramid (Arnhem, The Netherlands)
- National Science Foundation, United States [DMR 0844933, CHE 1057797]
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [1507764] Funding Source: National Science Foundation
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.
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