Study On The Polymerization And Rheological Behavior Of Polybenzimidazole In Ionic Liquids

Polybenzimidazole(PBI) is a kind of linear polymer with a heterocyclic imidazole structure. With aromatic heterocyclic molecular structure,it has some excellent properties,such as high temperature resistance,strong acid and alkali resistance and excellent flame retardance.Thus, polybenzimidazole has important applications in high-temperature filtering fabrics,anti-flame protective clothing,space suit and chemical resistance materials.Commercialized poly[2,2-(m-phenylene)-5,5-bibenzimidazole]were prepared by melt polymerizationAs a new type of solvent,ionic liquids(ILs) have 'zero' vapor pressure,high stability and catalytic function,thus,it has been widely used in organic synthesis reactions and polymerization reaction.According to our research work,polybenzimidazole has high solubility in ionic liquids compared with conventional solvents.In this paper,ILs were used as solvent to prepare polybenzimidazole in order to achieve better polymerization results.By changing the reaction conditions,we could find a way to get PBI with high molecular weight.Then different cleaning systems were used to purify the PBI and the results were discussed.In the end,PBI was dissolved in ILs and the rheological behavior of PBI/ILs solution was studied.Based on the above research work,conclusions can be summarized as follows.(1) Poly[2,2-(m-phenylene)-5,5-bibenzimidazole]was synthesized by using 3,3',4,4'-diaminobenzidine(TAB) and isophthalic acid(IPA) as monomers,P₂O₅ as dehydrating agent, 1-Butyl-3-methyl-imidazolium chloride as solvent.FTIR and ¹H-NMR results proved that PBI could be synthesized through the polycondensation reaction in ionic liquids.TGA was also used to analyze the thermal stability of PBI and the results also affirmed the synthesis of PBI.(2) The synthesis process of polybenzimidazole and the impact factors of the molecular weight were discussed by changing the reaction conditions.The results showed that the high molecule weight PBI could be synthesized using TAB and IPA as monomers,9.5%P₂O₅ as dehydrating agent,1-Butyl-3-methyl-imidazolium chloride as the solvent at 150℃for 2h and 210℃for 15h.(3) Ethanol system is found to be the best solvent to purify PBI according to the cleaning experiments with different systems made up of distilled water,ethanol and acetone,and the ethanol system could achieve the goal after about 16h.(4) Then,the rheology performance of PBI/[Bmim]Cl was studied.The influences of concentration and temperature on the rheological behavior of solution were discussed.The results indicated that PBI/[Bmim]Cl exhibited a shear-thinning phenomenon.The value of the viscous flow activation energy of PBI/[Bmim]Cl was relatively large according to the calculation.This result showed that the PBI/[Bmim]Cl solution is temperature-sensitive.(5) The non-newtonian index decreases with the increase of the content of PBI,and increases as the temperature rises through the analysis of the relation between shear stress and shear rate.Then we analyzed the curves and obtained the critical concentration(about 8%) and the critical temperature(about 130℃).