Molecular Design, Preparation And Properties Of Polyimides With Excellent Solubility

Aromatic polyimides, a class of polymers with excellent performances, have been widely employed in many high-tech areas. However, the conflicts between poor processability and excellent thermal stability are the most factor which restricts their further development. In an attempt to overcome these conflicts, researchers generally improve the solubilities of the polyimides while maintain their outstanding thermal properties by introducing special structures, such as pyridine ring, branched structures and huge pendants. Therefore, we synthesized a novel triamine monomer, 2,6-bis(4-aminophenyl)-4-(4-(4-aminophenoxy) phenyl) pyridine(BAAP), and prepared a series of polyimides by terminating BAAP with phthalic anhydride(PA). In the meanwhile, we prepared several series polyimides with different structures, and studied the influences of pyridine ring, branched structures, phthalimide pendant groups and copolymer structures on the aggregation states, solubilities and thermal properties of polyimides.BAAP was synthesized through combining the nucleophilic aromatic substitution with modified Chichibabin reaction and hydrazine hydrate reduction. Two diamine monomers, 2,6-bis(4-(4-aminophenoxy) phenyl)-4-phenyl pyridine(BAPP) and 1,3-bis(4-aminophenoxy) benzene(TPER), were prepared for comparison. The chemical structures of each amino monomer and its intermediate products were identified by Fourier transform infrared spectroscopy(FT-IR), 1H nuclear magnetic resonance spectroscopy(1H-NMR) and elemental analysis.A series of polyimides with phthalimide pendants were prepared by a three-step route, which consisted of terminating the triamine by incorporating phthalic amic acid as pendant, ring-opening polymerization to form poly(amic acid)s and further chemical imidization to acquire polyimides. All the three polyimides held completely amorphous structures, excellent solubility and thermal properties.A series of high temperature resistant adhesives were prepared from PA-BAAP-6FDA type polyimide(PI-3) and E-51 epoxy resin. The moderate amount of PI-3 could effectively improve the tensile shear strength of the adhesive at high temperature. At 180℃, when the PI-3 content is 30%, the maximum tensile shear strength of 15.93 MPa was obtained.The influences of the related structures on polyimides were studied through preparing BAAP-type hyperbranched polyimides, BAPP-type polyimides and TPER-type polyimides. The results suggested that the introduction of branched structures and pyridine ring would have a minor effect on the glass transition temperature of polyimides. Introducing branched structures could reduce the regularity of polyimide molecular chain, so that the crystallinity was deteriorated. Meanwhile, the introduction of branched structures could improve the solubilities of polyimides significantly, and strengthen their thermal stabilities. Introducing pyridine ring could also improve the solubilities, but reduce the initial thermal decomposition temperature significantly. Meanwhile, the introduction of pyridine ring could decrease the thermal decomposition rate while polyimide pyrolyzed, and improve their char yields at 750℃.The experimental results indicated that the method of preparing polyimides with phthalimide pendants through terminating BAAP by PA could reduce the d-spacing of the resulting polyimides, but wouldn’t have major adverse effects on their solubilities and thermal properties. Meanwhile, the process could be carried out under mild conditions.Two series of pyrimidine-containing hyperbranched polyimides were prepared from 2,4,6-triaminopyrimidine(TAP), 4,4’-diaminodiphenyl ether(ODA) and 3,3’,4,4’-benzophenonetetracarboxylic dianhydride(BTDA). For the noncopolyimides, with the increasing content of BTDA, the degree of branching(DB) values increased. For the copolyimides, with the increase of the content of ODA, the DB values decreased. All the non-copolyimides held completely amorphous structures. For the copolyimides, the crystallinity of polyimide improved while the content of ODA increased. The solubilities of non-copolyimides were relevant to the content of BTDA, the higher the content of BTDA, the better the solubility. Moderate amount of ODA could improve the solubility of polyimide, but excessive amount would lead to the opposite results. When the molar ratio of BTDA/TAP/ODA was 4:2:1, the obtained polyimide would have the most excellent thermal stability.Terminating BAAP by PA, and introducing pyridine ring, branched structures would have a large influence on the pyrolysis process of polyimides, and make their kinetic models changed. Introducing pyridine ring into the backbone of polyimides would reduce the apparent activation energy required for the initial thermal degradation of the polyimides, and make the polyimides more prone to pyrolysis. The introduction of branched structures would increase the apparent activation energy for the initial thermal degradation, while reducing the energy for the later stage, indicating that the introduction of branched structures into polyimides would make it harder to pyrolysis for the initial stage, but more prone to pyrolysis for the later stage. Terminating BAAP by PA, the apparent activation energy of the second stage of thermal decomposition process for the obtained polyimide was much higher than the corresponding hyperbranched polyimide, but these of the first stages were close. Terminating BAAP by PA has slight impact on the first stage of thermal degradation, and makes the polyimides more difficult to pyrolysis for the second stage.In this paper, several series of polyimides were prepared from different amine monomers. The effects of pyridine ring, branched structures, copolymer structures and terminating BAAP by PA on the aggregation states, solubilities, thermal properties and thermal decomposition processes of polyimides were studied systematically. It is hoped that this research could provide reference for the subsequent research of polyimide.