Post-sulfonation for Precisely Controllable Preparation of Main-chain Type Sulfonated Poly(phenylquinoxaline)s and Their Properties for Proton Exchange Membrane

A series of main-chain type sulfonated poly(phenylquinoxaline) (SPPQ) were prepared by post-sulfonation of PPQs, which were synthesized from copolymerization of 4,4'-bis(4-(2-phenylethylenedione) phenoxybiphenyl and 4,4'-bis(2-phenylethylenedione)diphenylether with 3,3',4,4'-tetraaminobiphenyl under different molar ratios. They were confirmed by the model compounds that sulfonic acid groups were precisely introduced to the 2,2'-position of the biphenyl fragment with high electron cloud density on SPPQ backbone. Therefore, sulfonic acid groups can be predicablely introduced to the polymer main-chain under mild conditions by the combination of monomer molecular structure design and post-sulfonation proceeding. Relative viscosity of these SPPQs was higher than 3.8 dL/g, indicating their high molecular weight. SPPQ-based proton exchange membranes (PEMs) were prepared by solution casting method. Their properties such as ion exchange capacity (IEC), water uptake, swell ratio, oxidative stability, mechanical properties and proton conductivity were investigated. The TGA results indicated that SPPQ PEMs had good thermal stability with the desulfonic acid groups temperature at about 320 degrees C and the decompose temperature at about 550 degrees C. All SPPQ PEMs showed water uptake less than 39% and in-plane swelling ratio linearly increased with increasing IEC and temperature, with the values ranging from 2.1%-13%. For example, SPPQ-5 with the IEC value up to 2.21 meq/g showed excellent dimensional stability with only 11% and 13% in-plane direction and thickness direction of the swelling ratios at 80 degrees C, respectively. Free radical oxidative stability test in Fenton reagent showed that the breaking time of SPPQ PEMs decreased with increasing IEC. For example, the SPPQ-1 (1.29 meq/g) has breaking time of over 150 h at 20 degrees C, whereas the value decreased to 81 h for SPPQ-5 (2.21 meq/g). Proton conductivity of SPPQ PEMs were increased obviously with the increase of temperature and IEC, and the maximum proton conductivity reached 64 mS/cm. The proton conductivities are much lower than that of Nafion NR212, due to the formation of acid-base groups between sulfonic acid groups and quinoxaline groups and the obviously low water uptake of the PEM.