Synthesis And Properties Of Benzimidazole-containing Sulfonated Polyimide Proton Exchange Membranes

Proton exchange membrane (PEM) is the key component for fuel cell.Sulfonated polyimide (SPI) is the most thoroughly studied material among non-fluorinated sulfonated proton exchange membranes. Compared with other protonexchange membrane materials, SPI has excellent thermodynamic stability, goodmechanical strength, and outstanding film formability and low gas crossover.However, the chemical stability, especially oxidative stability and hydrolytic stability,limit its application. Previous researches show that sulfonated polyimide whichcontains the benzimidazole group in the main chain could enhance its oxidativestability.However, in the other hand, the existence of benzimidazole group can damagethe hydrolytic stability of sulfonated polyimide.In this article, we focused on the study of benzimidazole-containing sulfonatedpolyimides that have excellent hydrolytic stability as well as oxidative stability. We hadtwo methods to achieve these goals:(1) moving the benzimidazole group from the mainchain to the side chain;(2) enhancing the hydrolytic stability of main chain.We synthesized a single side chain benzimidazole-containing diamine known as5-(2-Benzimidazole)-1,3-bis (4-aminophenoxy) benzene (BIBAz) in the third chapter.A series of single side chain benzimidazole-containing polyimides were synthesized byrandom condensation polymerization of BIBAz,4,4’-bis (1,8-dicarboxylicanhydride-4-naphthaloxy) biphenyl (BPNDA) and4,4’-diaminodiphenyl ether (ODA).Thosepolymers were reacted with sulfonic acid to form benzimidazole-containing sulfonatedpolyimide proton exchange membranes. The further study of these membranes indicatethat benzimidazole on the side chain will not damage their hydrolytic stability. Thesulfonated polyimide proton exchange membrane SBPNDA-ODA/BIBAz（6/1）canmaintained in the100℃water after120h while the SBPNDA-ODA/APABI(4/1)which contains the benzimidazole group in the main chain dissolved completely inthe100℃water after24h. These benzimidazoles which at side chain also can enhance the oxidative stability in certain extent. SBPNDA-ODA/BIBAz（6/1）proton exchangemembrane started to dissolve in the3%H2O23ppm FeSO4reagent after170min whichis twice longer than the non-benzimidazole-containing polyimide SBPNDA-ODA.Since the third chapter cannot give an effective evacuation of oxidative stabilitybetween the main chain benzimidazole-contain sulfonated polyimide and side chainbenzimidazole-contain sulfonated polyimide. Also, sulfonating the polymer coulddamage the mechanical ability of proton exchange membrane. So in the fourth chapterof this article, we synthesized a double side chain benzimidazole-containing diaminenamed Bis (4-(1-benzimidazol-2-yl) phenoxy) biphenyl-4,4’–diamine (BIPOB).A series of double side chain benzimidazole-containing SPIs were synthesized byrandom polymerization of BIPOB,1,4,5,8-naphthalene tetracarboxylic dianhydrideNTDA and4,4’-bis(4-aminophenoxy)biphenyl-3,3’-disulfonic acid BAPBDS. Theresearch of these proton exchange membranes reconfirmed the result of third chapterthat side chain benzimidazoles could not affect the hydrolytic stability of polymer.NTDA-BAPBDS/BIPOB（4/1）stayed in the140℃water after24h only lost4%weightand5%viscosity which is similar to the result of NTDA-BAPBDS/BAPB(3/1),whilemain chain benzimidazole-contain sulfonated polyimide NTDA-BAPBDS/BAPBI(5/1)under same condition lost12%weight and30%viscosity. As for the oxidative stability,the study showed that benzimidazoles on the main chain would enhance the oxidativestability more obviously than side chain. NTDA-BAPBDS/BAPBI (5/1) could sufferlonger in the Fenton reagent before it start to dissolve than NTDA-BAPBDS/BIPOB（4/1）.In the fifth chapter of this article, we focused on enhancing the hydrolytic stabilityof main chain benzimidazole-containing sulfonated polyimide by synthesizing ladderstructure through reaction of quadriamine and dianhydride.A series of sulfonatedpolyimide-polybenzimidazole copolymers (SPIBIs) were synthesized via randomcondensation polymerization of1,4,5,8-naphthalene tetracarboxylic dianhydride(NTDA),4,4’-bis(4-aminophenoxy)biphenyl-3,3’-disulfonic acid (BAPBDS) and anamine-terminated polybenzimidazole oligomer (PBI-NH2). The incorporation ofbenzimidazole significantly improved the oxidative stability, as the result SPIBI-19could survive200min in the Fenton’s reagent which is twice longer than the regularSPI. In the other hand, SPIBI-19maintained its mechanical strength in the140℃deionized water after24h which due to the ladder structure in the main chain. Also, theSPIBI-19showed high perform in the fuel cell test at90℃92%relative humidity. The above research shows that polyimide-polybenzimidazole copolymers(SPIBIs) have excellent oxidative stability and hydrolytic stability. In order to restrainthe swelling ratio of SPIBIs and increase their proton conductivity, in the chapter six,SPIBI-19was synthesized as a hydrophilic block, then reacted with non-fluorinated,partial fluorinated or full fluorinated hydrophobic blocks. These block polymers havesame hydrophilic block and different hydrophobic blocks. The research showed thatthese block polymers can restrain the swelling of membrane in the water, all the blockpolymers’ swelling ratio in-plane direction were lower than15%.Also the result ofAFM showed that block polymers which have fluorinated hydrophilic blocks such asSPIBINF and SPIBIFF, will lead to ion-channel. However, because of low IEC of theseblock polymers, the increasing of proton conductivity is not very successful.