Synthesis And Characterization Of Sulfonated Polybenzoxazoles By Direct Polycondensation For Proton Exchange Membranes

Proton exchange membrane (PEM) is one of the key components of proton exchange membrane fuel cell. So far, defects of the commercialized Nafion membrane (Dupont) such as expensive cost and the transmission of H+relies heavily on the presence of water (complex water/heat management and poisoning of catalyst was caused at low temperatures) limited its further application. It is very important to develop a kind of membrane which can operate at high temperature. Polybenzoxazoles (PBO) are regarded as a promising candidate for the matrix of PEM because of their good thermal stability, mechanical properties and chemical stability.In this paper, hexafluoroisopropylidene moieties and sulfonic acid group were successfully incorporated to the Polybenzoxazole backbone to prepare a kind of high temperature PEM. Detailed research contents are as follows:1. A new monomer,2,2’-sodium sulfonate-4,-4’oxydibenzoic acid (SODBA),was firstly synthesized from2,2’sodium sulfonate-4,4’oxydibenzoic acid (ODBA). Sulfonated polybenzoxazoles (SPBO) were synthesized from4,6-diaminoresorcinol dihydrochloride(DAR), terephthalic acid (TPA),5-sulfoisophthalic acid monosodium salt (SIPA) by direct polycondensation in polyphosphoric acid (PPA). The structures of SPBO were verified by FTIR and elemental analysis. SPBO/PPA polymerization solution were hot pressed directly to form PPA doped SPBO membranes. The values of inherent viscosity ranged from2.56to1.12dL/g, implying that SPBO possessed low molecular weight. TGA results showed that SPBO had good thermal stability with5%weight loss in the rage of246～351℃in nitrogen. At high temperature, SPBO/PPA membranes showed excellent proton conductivity. For instance, SPBO/PPA membrane with a sulfonation degree of10%exhibited a proton conductivity of0.123S/cm at170℃. Meanwhile the mechanical property and oxidation stability of SPBO/PPA was not good enough for application as PEMs.2. Sulfonated polybenzoxazoles containing hexafluoroisopropylidene moieties (SPBO-6F) were synthesized from DAR, TPA, SIPA and4’4-(hexafluoroisopropylidene)bis(benzoic acid)(BIS-B-AF) by direct polycondensation in PPA. The structures of SPBO-6F were verified by FTIR and elemental analysis. The values of inherent viscosity ranged from9.7to1.44dL/g, implying that SPBO-6F possessed high molecular weight. TGA results showed that SPBO6F had good thermal stability with5%weight loss in the rage of368～557℃in nitrogen. The SPBO15-6F3/PPA membrane exhibited a proton conductivity of0.153S/cm at170℃. The tensile strength of PPA doped SPBO-6F membranes ranged from34.5to3.4MPa and decreased with the increasing of sulfonation degree. SPBO-6F/PPA membranes showed improved mechanical properties and oxidation stability compared to SPBO/PPA membranes.3. A series of fluorinated benzoxazole polymers (PBO-6F) were synthesized by introducing4’4-(hexafluoroisopropylidene) bis (benzoic acid)(BIS-B-AF) into PBO macromolecular chains. The structures of PBO-6F were verified by FTIR and elemental analysis. The values of inherent viscosity ranged from15.53to6.19dL/g, implying that PBO-6F possessed high molecular weight. TGA results showed that PBO-6F had excellent thermal stability with5%weight loss in the rage of581～596℃in nitrogen. The PBO-6F/PPA membranes exhibited a proton conductivity of0.125S/cm at170℃. The tensile strength of PPA doped PBO-6F membranes ranged from36.9to48.5MPa, which was nearly closed to that of Nafion. The oxidation stability of PBO-6F/PPA membranes was comparable with Nafion too. The results indicated PBO-6F/PPA membrane could be expected to be used in high temperature PEMs.