Synthesis And Characterization Of Polyimides In Novel Solvents

Aromatic polyimides(PIs) are one type of high performance polymers.Due to their excellent chemical stability,thermal stability and mechanical properties, aromatic polyimides have been extensively used in many areas,such as space navigation,electrics,microelectronics,and chemical engineering.Many previous researchs in polyimide focus on the development of structural and functional high performance polyimides with new properties without addressing the limitations of the current synthetic methods.In this thesis,ionic liquids(ILs) and polyphosphoric acid(PPA) were used as the novel reaction solvents for the polyimides synthesization.Ionic liquids(ILs) are now becoming the novel reaction media because of its strong dissolving ability,extremely low volatility and excellent thermal stability.In this thesis ILs were synthesized and used as solvents to produce polyimides.The chemical structures of ILs had an influence on the inherent viscosities of the high performance polymers.FTIR and ¹³C-NMR were employed to confirm that the structures of polyimides synthesized in ILs,which were similar to one synthesized by the traditional method in DMAc.The effects of ILs' nature on the crystalline property of the polyimides were investigated by WXRD.As a result,the polyimides with high inherent viscosities was synthesized in the solvent of[BMIM]BF₄ at the reaction temperature of 80℃,and the ratio of the reaction monomers was 1.02.The determination of TGA showed that the polyimides which obtained in the reaction media of[BMIM]BF₄ has better thermal stability than other polymers.Due to the good properties of solubilization and dehydration for many organic compounds,Polyphosphoric acid(PPA) has been used as solvent to synthesize high performance polymers with heterocycles such as polybenzimidazoles(PBI), polybenzothiazoles(PBT),and polybenzoxazoles(PBO),and poly(ether ketone)s.Polyphosphoric acid(PPA) possesses good properties of solubilization and dehydration for many organic compounds,thus the solvent has been used to synthesize high performance polymers with heterocycles such as polybenzimidazoles (PBI),polybenzothiazoles(PBT),and polybenzoxazoles(PBO),and other polymers such as poly(ether ketone)s.In this thesis,Polyphosphoric acid(PPA) played a role of an excellent media for one-step synthesis of completely cyclicized polyimides from the corresponding diamines and dianhydrides.The polymerization was carried out in the solid content of 10 wt/v%at 220℃,and the products exhibited a homogeneous solution in the non-volatilizable PPA instead of precipitation form of the polyimides.The influence of the PPA composition on the inherent viscosity was studied by using various PPAs with the P₂O₅ content of 80,82,84 and 86 wt%,and a kinetic study on the formation of polyimide in PPA was further carried out by measuring the inherent viscosity at different polymerization time.FTIR and Carbon-13 NMR were employed to analyze the compound structures and TGA was used to evaluate its thermal stability.The results indicated that the polyimides prepared by one-step route gave a relative high thermal stability compared to that prepared with conventional polymerization methods in the reaction media of DMAc and m-cresol.The decomposition kinetics of the polyimides was studied by Flynn-Wall-Ozawa method and Kissinger method.As a result,the thermal decomposition activation energy of polyimides which synthesized in PPA was 409 kJ/mol was higher than that obtained by the traditional method in the DMAc(300 kJ/mol) or m-cresol(304 kJ/mol).