Preparation and Characterization of Sulfonated Polyphenylquinoxalines

Poly(phenylquinoxaline)s.(PPQs) are a family of aromatic condensation polymers known for their outstanding thermal and chemical stability. The pendant phenyl groups and chains isomerism improve the solubility and processing characteristics of these polymers. PPQs have also been shown to possess excellent thermo-oxidative stability and thermohydrolytic stability. This stability makes these polymers candidates for development as proton exchange membranes (PEMs) to be used in fuel cells. In addition to thermohydrolytic stability, PEMs require high protonic conductivity and, in order to achieve this they also require high water uptake. Aromatic condensation polymers do not possess these properties, but ionomers derived from them may. The usual method to derivatiziting these polymers is through sulfonation. In the frames of the present investigation we have carried out sulfonation of two PPQs using an H2SO4-oleum mixture (4 : 1) as sulfonating agent at 125 degrees C. As a quinoxaline ring is readily formed in acidic medium synthesis of sulfonated PPQs (SPPQs) was also carried out directly from monomers using an H2SO4-oleum mixture as solvent, catalyst and as sulfonating agent. Depending on the conditions of the reaction (temperature, duration, and the ratio of components in a sulfonating mixture) the polymers containing 0.2-6.7% S were prepared. SPPQs are soluble in polar organic solvents; from the solutions of SPPQs high strength films (sigma = 80-100 MPa) were cast. On the basis of sulfonated PPQs new cation-exchange membranes were prepared and investigated. Among the cation-exchange membranes developed those of the greatest interest are proton-exchanging membranes for fuel cells. Proton conductivity of the membranes prepared strongly depends on relative humidity and comparable with the conductivity of Nafion 117.