Molecular Design And Properties Of Quaternized Poly(Arylene Ether)s Containing High Density Quaternary Ammonium Groups

Nowadays, power sources and environment are attracting the attention of the whole world, new highly efficient and clean power sources need to be developed to solve environmental problems. A fuel cell is a device that could convert the chemical energy of fuel into electricity. As a new kind of fuel cells, anion exchange membrane fuel cells absorb the advantages of alkaline fuel cells and proton exchange membrane fuel cells. Anion exchange membrane fuel cells can employ non-noble metal such as nickel as a catalyst, and greatly reduce the production cost. At the same time, polymer membranes are exploited as the electrolyte, to avoid the alkaline electrolyte carbonation. These advantages make AEMFCs become a research highlight.As the key component of anion exchange membrane fuel cell, there aren’t any commercial anion exchange membranes that can satisfy the working requirements of the cell. Because of a series of excellent advantages such as big rigidity and density, no corrosion, and strong fracture resistance and creep ability, poly（arylene ether）s become a kind of excellent candidates of AEM substrate materials. Large amounts of anion exchange membranes are obtained directly from functionalization of poly（arylene ether）. However, if too much conductive functional groups are modified, the resulted membranes will be weakened by too much unexpected swelling. Neverthless if the mount of conduction functional groups is much more lacking, the conductivity will be obviously inadequate to provide sufficient currency. Hence, starting from the perspective of molecular design, we tried to prepare anion exchange membrane materials with excellent properties to break the dilemma between ionic conductivity and dimensional stability.In the first chapter, to acquire excellent dimensional stability and mechanical properties, the ionomers should contain enough hydrophobic portions and relatively fewer hydrophilic portions, which could ensure adequate conductivity at low levels of IEC. Moreover, the conspicuous difference in polarity between the hydrophilic sections and hydrophobic sections leads to a larger extent of microphase separation to form welldeveloped hydrophilic channels that exist in the membranes for ease of ion conduction. A series of poly（arylene ether ketone）s copolymers（MePAEK-x） carrying four benzyl groups on the side chain were prepared by 1,5-bis（3,5-dimethylbenzoyl）-2,6-dihydroxynaphthalene, 4,4’-（hexafluoroisopropylidene） bisphenol and 4,4’-difluorobenzophenone. After the bromination reaction and the quaternization, anion exchange membranes（QPAEK-x） were prepared. These anion exchange membranes show excellent dimensional stability, adequate thermal stability and high hydroxide conductivity, owing to the pendent quaternary ammonium structures concentrated on the side chain. The highest hydroxide conductivity(0.0653 S cm^-1) appears in QPAEK-0.8（IEC = 1.69 mmol g-1）, and the swelling ratio of length is 11.7% at 80 ?C. However, the alkaline stability is unsatisfactory, it is because that there is a strong electronwithdrawing carbonyl connected with the benzene ring as well as trimethylamine benzyl quaternary ammonium, which makes trimethylamine benzyl quaternary ammonium become easy to be attracted by hydroxid in strong alkali environment.In the next chapter, quaternary ammonium groups were denser, and were grafted onto main chain, with the spacer of flexible long alkyl chain. It is beneficial for the quaternary ammonium groups as ionic clusters to form well-developed hydrophilic channels that exist in the membranes for ease of ion conduction. Meanwhile, the hydrophobic aromatic main chain stay away from hydrophilic quaternary ammonium groups with alkyl interstitial spacers, which is helpful for dimensional stability. In this part, alkaline stability is another factor that should be considered at the same time. We designed and synthesized a new type of graftable molecule with tetraphenyl moiety, and connected it onto poly（arylene ether sulfone）s main chain. After chloromethylation, and quaternization, a series of anion exchange membranes（PAES-QDTPM-x） were prepared. PAES-QDTPM-x have pretty high conductivity(0.0394 S cm^-1 at 20 ?C, and 0.0761 S cm^-1 at 80 ?C) and excellent dimensional stability（11.9% for length and 11.2% for thickness at 80 ?C）. As was shown in SAXS profile, nano-scale phase separation was formed by the ionic cluster. Due to the electron donating ether linkage connected to the aromatic ring, the trimethylamine benzyl quaternary ammonium become more stable, and the alkaline stability get better than QPAEK-x.In summary, two series of anion exchange membranes with quaternary ammonium groups concentrated on the side chain were prepared. Through structure and various performance characterizations, it is proved that the application of densely quaternized poly（arylene ether） membranes in fuel cells is promising.