Sulfonated poly(arylene ether ketone)s prepared by direct copolymerization as proton exchange membranes: Synthesis and comparative investigation on transport properties

Sulfonated poly(aryl ether ketone)s (SPAEK) copolymers were synthesized by aromatic nucleophilic polycondensation from 3,3', 5,5'-tetramethyl-4, 4'-biphenol, 1,4-bis(4-fluorobenzoyl) benzene, and disulfonated difluorobenzophenone. The SPAEK membranes did not exhibit excessive swelling in hot water and at the same time show the proton conductivities in the range of 0.030 S/cm to 0.099 S/cm at 80 degrees C. The methanol diffusion coefficients of the SPAEK membranes were in the range of 4.7 X 10(-7) to 8.1 x 10(-7) cm(2)/S measured at 25 degrees C. The transport properties of this series of SPAEK copolymers were compared to poly(aryl ether ether ketone)s (SPEEK), poly(aryl ether ether ketone ketone)s (SPEEKK), and Nafion (R) membranes. It was found that the transport properties (including proton conductivity and methanol permeability) follows the trend of SPEEKK-60 < SPAEK-60 < SPEEK-60 < Nafion (R) 117, the order of which is also attributed to the differences in the chemical structure of the polymers and the membrane morphology. In general, this novel series of SPAEK membranes possess various advantages, such as low cost of the initial monomers, high thermal and mechanical stability, and low methanol permeability while simultaneously possessing sufficient proton conductivity, which makes them notably promising as proton exchange membrane (PEM) materials in direct methanol fuel cell (DMFC) applications. (C) 2008 Wiley Periodicals, Inc.