Study On Structure And Property Of Quaternized Poly (aryl Ether Sulfone) Anion Exchange Membranes

Energy is the material basis for human survival.People have always attached great importance to the exploitation of new energy sources,and hydrogen energy is considered the cleanest energy source.Fuel cell technology is one of the methods for people to efficiently obtain hydrogen energy.Because of its small size and energy conversion process which is not affected by the Carnot cycle,it has been widely studied.Anion exchange membrane fuel cells(AEMFC)based on the types of membrane electrolytes can use non-noble metals as catalysts to operate in alkaline environments,which has the potential to lower cost of fuel cells.At present,anion exchange membranes(AEM)reported in the literature are still difficult to meet the application requirements of AEMFC,mainly because of the low ion conductivity and poor alkaline resistance.It has been reported that building an optimized continuous hydrophilic-hydrophobic microphase structure by modifying the polymer can promote ion transport.Increasing local density of ionic groups can promote the aggregation of ionic clusters and the formation of larger hydrophilic domains;the ordered segments in block copolymers can promote the aggregation of hydrophilic-hydrophobic segments and effectively improve the degree of phase separation.There is a lack of detailed research on the relationship between the order of molecular chain arrangement and microscopic morphology.In addition,the alkaline resistance of AEM is affected by polymer backbone and ionic groups.In recent years,researchers believe that ether bonds in the molecular chain of poly(arylene ether)materials are easily attacked by hydroxide radicals,causing degradation of the main chain.In alkaline environment,when benzyl quaternary ammonium salts are used as conductive groups,they are also extremely vulnerable to the attack of hydroxide radicals,resulting in a decrease in membrane performance.Based on the above problems,in order to further improve the ion transport capacity and alkaline resistance of AEM,this article has carried out research from both the structure construction of the segment and the cationic group,and discussed how the arrangement of the segment from random to partial order caused change in microscopic morphology and performances.The performance changes caused by the type and location of conductive groups are also discussed.In order to construct high-density ion regions via molecular design,in the first part of this article,a bisphenol monomer BTP-OH with a bis-tetraphenyl methane structure was first synthesized.And then a series of poly(aryl ether sulfone)s(BTP-x-PES)with a bis-tetraphenyl methane structure was prepared with the monomer,4,4'-dihydroxydiphenylsulfone and 4,4'-difluorodiphenylsulfone via nucleophilic polycondensation.Blanc chloromethylation reaction was adopted to prepare the chloromethylated product BTP-x-CMPES,in which the reaction conditions of chloromethylation were optimized.Accurate structure analysis was carried out by ~1H NMR,which proved that the degree of chloromethylation was over 95%.The BTP-x-CMPES membranes were immersed in trimethylamine aqueous solution to exchange to hydroxide form and obtain BTP-x-QAPES anion exchange membranes,and the structure was characterized.it is shown that BTP-x-QAPES possesses good thermal stability,dimensional stability,hydroxide conductivity and chemical stability,and its morphology is characterized,proving that the molecular design of high-density ionization can effectively improve comprehensive performance of AEM.At present,research on the relationship between the order of chain segment distribution and the microscopic morphology and properties of AEM is not comprehensive,so in the second part of this article,the influence of order of molecular chain on the performance of AEM is explored in detail.Firstly,two bisphenol monomers containing single-tetraphenyl methane and triple-tetraphenyl methane structure were designed and synthesized and combined with 4,4'-dihydroxydiphenylsulfone respectively.Through nucleophilic polycondensation with4,4'-difluorodiphenylsulfone,a series of polymers,STP-x-PES and TTP-x-PES,with the same chemical composition as BTP-x-PES and different molecular chain arrangements,were prepared.Under the optimized chloromethylation conditions,chloromethylated products STP-x-CMPES and TTP-x-CMPES were prepared.The ~1H NMR spectra proved that the degree of chloromethylation of the three polymers is completely consistent.Finally,the STP-x-CMPES membranes and the TTP-x-CMPES membranes were immersed in a trimethylamine aqueous solution to exchange to hydroxide form to obtain STP-x-QAPES and TTP-x-QAPES anion exchange membranes.The ion exchange capacity of STP-40-QAPES,BTP-25-QAPES and TTP-18.2-QAPES were measured via back titration and calculated from ~1H NMR.It was proved that the purpose of obtaining polymers with similar ion exchange capacity but different segment constructions was achieved.Morphology of the three materials has been characterized to prove that increasing the order of chain segment effectively improves the degree of phase separation.A comprehensive investigation of the performance differences of the three materials due to different microphase morphologies.TTP-x-QAPES with the highest degree of segment order has the lowest water uptake and swelling ratio.The random structure material shows the poorest performance.The TTP-x-QAPES membranes with the highest degree of chain segment distribution exhibits the lowest water uptake and swelling ratio,indicating the best dimensional stability.The water absorption capacity of TTP-x-QAPES is lower than the other two materials,but it shows the highest electrochemical performance,while the random structure of STP-x-QAPES is completely opposite.The results above show that while orderly distribution of chain segments leads to an increase in the degree of phase separation,various membrane properties have been improved.In addition,the three materials all undergo S_N2 nucleophilic substitution reaction under high temperature and alkaline conditions.Compared with the other two materials,the degradation rate of TTP-x-QAPES is slower.To solve the problem of insufficient stability of benzyl quaternary ammonium under alkaline conditions,the third part studies the effect of different quaternary ammonium groups on the performance of membrane materials.poly(aryl ether sulfone)s(TTP-x-PIPPES)containing triple-tetraphenyl methane structure were synthesized by the reaction of N-methylpyridine and TTP-x-CMPES.Phase morphology was characterized,and it was proved that piperidine groups with larger size will influence phase separation.In membrane performance tests,TTP-x-PIPPES showed lower water uptake and swelling ratio compared with TTP-x-QAPES.TTP-x-PIPPES also exhibit better alkaline stability under low temperature,but worse alkaline stability at high temperature.TTP-x-PIPPES whose conducting group is benzyl piperidinium is more susceptible to hydroxide attack due to the presence of benzyl group,which easily causes the ionic group to fall off and degrade.In the third part,benzyl piperidinium was introduced into the polymer branch chain as the conductive group.Although the water management ability of the material and the alkali stability at low temperature were improved,this result was incompatible with the theoretical ability of piperidine group.In the fourth part,a bisphenol monomer containing piperidine was synthesized and homopolymerized with 4,4'-difluorodiphenylsulfone in equal proportions to prepare a main-chain type poly(aryl ether sulfone)s containing piperidine,PIPPES.Then,PIPPES reacted with methyl iodide,butane bromide,hexane bromide and 1-bromooctane respectively to prepare quaternized products with alkyl chains of different lengths(QAPIPPES-x).The quaternization of these polymers was confirmed by ~1H NMR.Compared with the piperidine-containing TTP-x-PIPPES,QAPIPPES-x exhibits higher thermal stability,which proves that the benzyl group and molecular structure may influence the stability of piperidine groups.The phase morphology characterization confirmed that phase separation of QAPIPPES-x was more significant after introducing long alkyl chains.In comprehensive performance tests,water uptake and swelling ratio of QAPIPPES-x was extremely low,which is beneficial to control the dimensional stability of the material.Water absorption and microphase morphology of QAPIPPES-6 reach the best state,and the electrochemical performance is also the best.QAPIPPES-x shows superior performances than TTP-x-PIPPES.Alkaline stability of QAPIPPES-x with piperidine in the main chain is much better than that of TTP-x-PIPPES with piperidine with benzyl group.The main degradation pathway for QAPIPPES-x is?-elimination under 80?alkaline conditions.While the polymer containing longer side chains degraded more serious.In summary,in this paper,several series of poly(aryl ether sulfone)containing quaternized tetraphenyl methane units or piperidinium salts were synthesized.We studied the influence of the degree of ion aggregation,the order distribution of quaternary ammonium groups,the structure and introduction position of quaternary ammonium groups on the performance of anion exchange membranes based on poly(aryl ether sulfone).And this work has improved the ion transport and alkaline stability of anion exchange membranes based on poly(aryl ether sulfone)to a certain extent.