Preparation And Properties Of Multi-block Ionomers Anion Exchange Membranes With High-density Quaternary Ammonium Groups

Anion exchange membrane(AEM) is a critical component of anion exchange membrane fuel cells(AEMFCs). To a large extent, the properties of AEMs can directly determine the performance of the AEMFCs. Up to date, AEMs still have some urgent problems that need to be addressed, such as low ionic conductivity, poor mechanical property, insufficient chemical stability, etc. In this paper, from the perspective of polymeric design of ionomers, we designed and prepared a series of multi-block ionomers AEMs with high-density quaternary ammonium groups. Their properties were investigated, including ionic conductivity, water uptake, swelling ratio, mechanical property, thermal and alkaline stability. The relationship between the chemical structures and the properties of the AEMs was explored, providing some beneficial lessons for the subsequent development of novel AEMs with better performance. Specific content is as follows.3,3’-Di(3’’,5’’-dimethylphenyl)-4,4’-difluorodiphenyl sulfone(DDMPDFDPS) was synthesized by Suzuki coupling reaction, which has four pendant benzyl groups. The hydrophilic oligomers(oligomer-F) were syhthesized via polycondensation of 3,3’-di(3’’,5’’-dimethylphenyl)-4,4’-difluorodiphenyl sulfone and 9,9-bis(4-hydroxyphenyl)fluorine, and the hydroxyl-terminated hydrophobic oligomers(oligomer-OH) were obtained from 4,4’-difluorodiphenyl sulfone(DFDPS) and bisphenol A(BPA). A series of multi-block AEMs with fluorene-containing hydrophilic segments densely functionalized by aromatic conjugated side-chain quaternary ammonium groups were fabricated after the processes of copolymerization, bromination, membrane preparation, quaternization and alkalization. The performance parameters of these AEMs were investigated, including ion exchange capacity, ionic conductivity, water uptake, swelling ratio, mechanical property, thermal and alkaline stability. The morphology of the AEM was investigated by atomic force microscopy(AFM), and the microphase separation was ananyzed by small angle X-ray scattering(SAXS). The results indicate the formation of well-defined phase segregation and ion-aggregating structure. The designed AEMs show rather high ionic conductivity(85.0 m S/cm) with relatively low IECm values(1.28 meq/g), which is a prominent performance. Meanwhile, the values of water uptake and swelling ratio are moderate, and the mechanical properties and alkaline stability are good. We speculate the good performances may be attributed to the following reasons. First, the sequential hydrophilic/hydrophobic structures of multi-block copolymers can further strengthen the phase-separation behavior of side-chain AEMs, which facilitates the formation of well-defined phase separations. Second, quaternary ammonium groups of high local density favor the formation of aggregated ion clusters and hydrophilic domians. Third, the sulfone-co-fluorene structure of the hydrophilic segments presents a helical chain shape, resulting in a higher volumetric density of quaternary ammonium groups. Moreover, the bulky and rigid fluorene groups can force each polymer chain to be apart, affording large free volume to confine water molecules, which can reduce the temperature dependence of the properties of the as-made AEMs.Monomer with pendant benzyl group, 4’-methylbiphenyl-3,5-diol(MBPD), was prepared by substitution reaction of phloroglucinol and toluene. The fluorine-terminated hydrophilic oligomers were synthesized via polycondensation of 3,3’-di(4’’-methyl-phenyl)-4,4’-difluorodiphenyl sulfone(DMPDFDPS) and 4’-methylbiphenyl-3,5-diol(MBPD). Multi-block AEMs that have hydrophilic blocks with high-density aromatic conjugated side-chain quaternary ammonium groups were synthesized by coupling the hydrophilic oligomer and the above-mentioned hydrophobic oligomer. Aromatic conjugated side-chain quaternary ammonium groups were all attached to the hydrophilic segments by means of polymeric design. The obtained QMPAESs AEMs have high ionic conductivity, especially, the ionic conductivity of QMPAES-X7Y10(IECm = 1.20 meq/g) reaches 67.0 m S/cm at 80 °C, which is superior to our previously reported random PAES-Q and some other AEMs with similar or higher IECm values. By comparing the chemical structures and performances of different AEMs, we may find that combining some favorable factors by polymeric design, such as multi-block structure, aromatic conjugated side-chain cationic groups and aggregated ion clusters, can effectively enhance the properties of AEM.