Synthesis And Performance Study Of High Performance Polyimide Nanofiber Containing Benzimidazole Unit

As a special engineering material, polyimide(PI) has excellent performance. Because of its good mechanical property, high heat resistance, well radiation resistance and chemical resistance, PI has been used widely in many fields such as aerospace, national defense and military, biology and medicine, microelectronics and so on. In recent years, it has been getting more and more attention that introducing aromatic heterocyclic structure into the main chain of polymer molecular. Introducing aromatic heterocyclic structure into PI can not only improve the solubility, electrical property and some other properties, but also significantly enhance the mechanical property and heat resistance. Benzimidazole unit is a kind of typical rigid aromatic heterocyclic units. If been introduced into the PI’s main chain, it could achieve the above purpose.In this paper, 2-(4- amino phenyl)- 5- amino benzimidazole was used as the diamine monomer. A series of polyamic acids(PAA) containing benzimidazole unit were synthesized through solution polymerization at low temperature, then electrospun into PAA nanofibers. PAA nanofibers were translated into PI nanofibers through thermal imidization. The PI nanofibers were characterized by SEM, FT-TR, DMA, TGA and some other instruments. Otherwise, the PI nanofibers containing benzimidazole unit were made into short fibers to improve the mechanical property of thermoplastic polyurethane(PU) films. The following is three main parts of my work:1 The synthesized and characterized of BPDA-PBIDA PI fibers: The PAA containing benzimidazole unit was prepared from biphenyl tetracarboxylic dianhydride(BPDA) and 2-(4- amino phenyl)- 5- amino benzimidazole(PBIDA). Then through electrospinning and thermal imidization, the PI nanofibers were gotten successfully. The influence of imidization temperature on the performance of PI was studied.The tensile test demonstrated, when imidization temperature was 370 ℃,the mechanical property of PI nanofibers were best: the strength was 620.97 MPa,the modulus was 4.78 GPa and the strain was 22.57 %.These were because that, the molecular oriention and degree of crystallinity of nanofiber were in the best state in 370 ℃.Dynamic mechanical analysis(DMA) demonstrated, when the PAA nanofibers were treated in 330-400 ℃, the glass-transition temperature(Tg) were similar above 390 ℃.Thermogravimetric analysis(TGA) showed that, the 5 % weight loss temperature of PI nanofibers treated in different imidization temperatures were similar in 580 ℃under N2 atmosphere, and the residual rate of carbon were above 65 % in 800 ℃,they had high heat resistance.2 The synthesized and characterized of BPDA-PBIDA-BPA PI fibers: The Co-PAAs containing benzimidazole group were prepared from biphenyl tetracarboxylic dianhydride(BPDA), 2-(4- amino phenyl)- 5- amino benzimidazole(PBIDA) and biphenyl diamine(BPA).The mole ratio of PBIDA and BPA was different. Then through electrospinning and thermal imidization in 370 ℃, the Co-PI nanofibers were prepared. The test demonstrated that, when the ratio was 4:6, the strength of Co-PI nanofibers was best: 871.35 MPa,and the modulus was 7.94 GPa, the strain was 15.18 %,the strong hydrogen bond between molecular chain made the chain well-organized, so that the nanofiber’s mechanical property was best; at the same time, the Co-PI nanofibers had good heat resistance, Tg was 360.4 ℃, the 5 % weight loss temperature of Co-PI nanofibers was 585.1 ℃, and the residual rate of carbon were 62 % in 800 ℃ under N2 atmosphere.3 Study of BPDA-PBIDA PI short nanofibers improving the mechanical property of thermoplastic polyurethane(PU) films: The BPDA-PBIDA PI nanofibers were made into short fibers through the method of mechanical pulverization. The short fibers were dispersed in N, N-Dimethylformamide(DMF) and then added into thermoplastic polyurethane(PU) solutions in different mass ratios. A series of mixed PU films were prepared by the traditional casting film method. The testing results showed that, when the added short fibers mass ratio was 5 %, the mechanical property of the mixed films was best: the strength was 72.24 MPa, compared with the pure PU film, it increased by about 75 %;the modulus was 0.02 GPa, doubling from original; meanwhile, the mixed film kept well flexibility,the strain was 526.67 %. With the increasing of short fibers, agglomeration was formed, and the film’s interface stability was broken. So the strength of films begun to decrease.