Synthesis of Polyimides Containing Benzoxazole Units and Its Properties
|School||Harbin Institute of Technology|
|Course||Chemical Engineering and Technology|
|Keywords||polyimide benzobisoxazole units imidization kinetics thermal degradation kinetics thermal degradation mechanism|
Aromatic polyimides have been found in many applications such as aerospace, microelectroics and information industries due to their excellent thermal stability, outstanding mechanical, dielectrical and solvent-resistance properties. The aerocraft materials are required to have high resistance to elevated temperatures by the development of spaceflight techniques. Polymer with benzoxazole units is a kind of novel polyimides to enhance comprehensive properties of polyimides, such as mechanical properties and heat-resistant.The synthesis of 2,6-Di（p-aminophenyl）benzo-[1,2d;5,4d′]-bisoxazole （diamine I） and 4-（5-aminebenzoxazole） aniline （diamine II） were studied by designing orthogonal experiments, and the influence factors on the reaction were analysed, then optimal synthesis conditions of diamines were confirmed. Two diamines were characterized by melting point, FT-IR, element analysis, 1H NMR and UV-Vis spectrum. The results of chemical structure and fluorescence spectra characterizations indicate that the structure of two diamines are plane and rigidity. And the results of DSC and TG show that they have good thermal properties.PIs were synthesized by two-step method. Synthesis of the polyimide precursor polyamic acids was studied by viscosity. In the reaction of 2,6-Di（p-aminophenyl）benzo-[1,2d;5,4d′]-bisoxazole with HQDPA, the effects of time, temperature, mole ratio and water percentage in the system on the polyamic acids viscosity were studied and the optimal conditions were determined. Diamine and dianhydride were dissolved into DMAC with the mol ratio of 1:1.02 and reacted for 35h under 5℃. After removing water in reactant and solvent, polyamic acids solution was obtained with major viscosity being 1.7dL/g. The imidization process of polyamic acid was studied by FTIR, and the imidization temperature can reach 300℃. The heating proceed of imidization process was 50℃/1h, 80℃/1h, 100℃/1h, 150℃/1h, 200℃/1h, 250℃/1h, 300℃/1h. It is obtained by DSC that the imidization temperatures were mainly between 100℃and 200℃. The imidization kinetics were studied by TGA method, which illustrates that thermal cyclization experiences two stages: fast and slow stage, and the kinetic parameters were also calculated. Mechanical property of PIs was also tested. Young’s modulus of PI（I）- HQDPA、PI（II）-BPDA、PI（II）-HQDPA were 13722MPa、8266.7MPa和5306.2MPa, respectively, which showed good mechanical property.The kinetic parameters of polyimide degradation were determined by using Flynn-Wall-Ozawa and Friedman methods. According to the value of activation energy that had been obtained, Coats-Redfern and Master-Curves methods were used to discuss the solid-state decomposition mechanism. It was found that the solid-state decomposition mechanism of PI（I）-HQDPA follows A3 mechanism, and the integral form is g（X）=[-ln（1-X）]3. Decomposition mechanism of PI（II）-PMDA follows D3 mechanism, and the integral form is [1-（1-X）1/3]2。Decomposition mechanism of PI（II）-BPDA is D1, and the integral form is X2。Decomposition mechanism of PI（II）-HQDPA is R1, and the integral form is X.