Synthesis and characterization of 3,3 '-disulfonated-4,4 '-dichlorodiphenyl sulfone (SDCDPS) monomer for proton exchange membranes (PEM) in fuel cell applications
Sankir, M; Bhanu, VA; Harrison, WL; Ghassemi, H; Wiles, KB; Glass, TE; Brink, AE; Brink, MH; Mcgrath, JE
A systematic study of the synthesis and characterization of 3,3'-disulfonated-4,4'-dichlorodiphenyl sulfone (SDCDPS) monomer was conducted by varying reactant stoichiometries (molar ratios of 4,4'-dichiorodiphenyl sulfone (DCDPS) to SO3 1 : 2.2, 1 : 2.9, and 1 : 3.3), reaction temperature (90-120 degrees C), and reaction time (4-6 h). The optimum synthesis batch process variables were I : 3.3 reactant molar ratio (DCDPS: SO3) at 110 degrees C for 6 h. In earlier studies, recrystallization of the "crude" disulfonated monomer from alcohol-water mixture was necessary to remove the monosulfonated and DCDPS impurities that lowered yield. However, in the current research, SDCDPS was successfully synthesized at nearly 100% conversion, which effectively eliminated the need for recrystallization. Recrystallization of SDCDPS from several alcohol-water mixtures (methanol-, ethanol-, and isoproponal-water mixtures) was investigated to compare product purities. Several characterization methods including proton NMR, H-PLC, UV-visible, and fast atom bombardment mass spectroscopy confirmed that the crude SDCDPS was completely disulfonated and identical to recrystallized SDCDPS, without having any monosulfonated or starting material DCDPS impurities. Hence, it was demonstrated that the crude SDCDPS monomer by the current one-step process and the recrystallized SDCDPS monomer were identical. This optimized monomer synthesis has been used to scale up the SDCDPS and poly(arylene ether sulfone) random and statistical copolymers at controlled disulfonation (35 and 45 mol %) levels, which were then used to fabricate proton exchange membranes for fuel cell applications. The intrinsic viscosity data confirmed that high molecular weight film forming copolymers were synthesized. The calculated degree of disulfonations by proton NMR was in close agreement with target disulfonations. It may be concluded that this optimized SDCDPS synthesis eliminates the need for recrystallization, which would be expected to improve process economics. (c) 2006 Wiley Periodicals, Inc.