Synthesis And Characterization Of Soluble Polybenzimidazole Containing Diazafluorene Groups

Aromatic polybenzimidazole(PBI)has good thermal chemical stability and mechanical properties,which was widely used as refractory fiber,anti-corrosion coating,aerospace and fuel cell and other fields.In the structure of PBI,weak alkaline imidazole ring can adsorb phosphoric acid,which has the ability of proton conduction,so phosphoric acid doped polybenzimidazole membrane is regarded as the most potential high temperature proton exchange membrane.However,the traditional polybenzimidazole has poor solubility of processing performance due to its rigid structure,which greatly limits for practical application.Therefore,it is of great significance to study the preparation of easily soluble polybenzimidazole.Based on the molecular structure design,pyridine,ether,large volume of pendant groups and twisted,non-coplanar diazofluorene with Cardo structures were successfully introduced into polybenzimidazole main chain by synthesizing novel aromatic tetramine and diacid monomers.A series of readily soluble polybenzimidazoles with good mechanical properties and good thermochemical stability were obtained.The proton exchange membrane had good proton conductivity and stability after phosphoric acid doping.The main research contents are as follows:1.Prepare a twisted non-coplanar structure of the Cardo type aromatic acid monomer: 9,9-bis(4-carboxyphenyl)-4,5-diazofluorene.And the soluble polybenzimidazole polymer was prepared by the programmed temperature method with 9,9-bis(4-carboxyphenyl)-4,5-diazofluorene and 2,6-bis(3,4-diaminophenyl)-4-phenylpyridine.Two different structures of tetraamine monomer 3,3',4,4'-biphenyltetramine and 2,6-bis(3,4-diaminophenyl)-4-phenylpyridine were used for research the properties of polybenzimidazole.The effect of non-coplanar structure on the solubility of polybenzimidazole was clarified.The results show that the new polybenzimidazole has excellent solubility in polar aprotic organic solvents(such as DMF,DMAc,DMSO,NMP)at room temperature.And the introduction of the aromatic heterocycle did not weaken the thermal and chemical stability of the polymer.The two polymer films were prepared with 5% weight loss temperature at 318 ° C and 10% weight loss temperature at 540 ° C.In addition,after doping with the 5 mol/L phosphoric acid solution,the phosphoric acid doping level of pyridine tetramine structure polybenzimidazole(PyTAB-PBI)reached 8.12,wich was higher than 6.40 of biphenyl tetramine structure polybenzimidazole(TAB-PBI),The proton conductivity increased from 0.046 S cm-1 to 0.068 S cm-1.2.Based on the molecular structure design,prepared a aromatic acid monomer with ether and diazofluorene structure: 9,9-bis(4-carboxyphenoxy)-4,5-diazofluorene.The introduction of flexible ether to polybenzimidazole molecular chain,it can improve the flexibility of the molecular chain,lead to the molecular chain easy to flip,increase the distance between the molecular chain,thereby reducing the distance between the molecular chain to improve its solubility.The presence of the ether bond resulted in a slight decrease in the thermal stability of the polymer,with a 5% weight loss temperature at 204 oC.In addition,the phosphoric acid doping test of polybenzimidazole membranes with different concentrations of phosphoric acid solution(5,7,9,11 mol/L),the result showed that the phosphate phosphoric acid doping level up to 12.37 in 9 mol/L phosphoric acid solution.In the dry state,and the proton conductivity of this membrane at 180 oC was 0.072 S cm-1.3.Based on the molecular structure design,synthesis of a tetramine monomer containing hydroxyl in side chain: 2,6-bis(3,4-diaminophenyl)-4-hydroxyphenyl pyridine.Introducing the hydroxyl into polybenzimidazole side chain,the hydroxyl can form an ion cross-linked structure with the imidazole ring of the main chain.The presence of hydrogen bonds in the molecule maybe reduces the solubility of the membrane.The solubility test shows that the polymer can be completely dissolved after heating to 100 °C when the hydroxyl content exceeds 60%.The cross-linked structure enhances the dimensional stability of the membrane,and the swelling ratio of side-chain hydroxyl-containing PBI membrane is about 30% lower than that of the non-hydroxyl-containing PBI membrane.The hydroxyl in side chains can form a bridge structure with phosphate molecules,and the phosphate molecules are immobilized in the membrane,the phosphoric acid doping levels are slightly higher than that of non-hydroxyl-containing PBI membrane.