Synthesis, Structure And Properties Of Polyimides Containing Bis-benzoxazole Moieties

Aromatic polyimides have been extensively applied in many areas such as aerospace, microelectronics 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 temperature 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.In this thesis, the development process of polyimides and the structural characteristics of high-performance polyimides were introduced. Based on the viewpoint of molecular design and combination of the characteristics of benzoxazole rings, a diamine monomer containing bis-benzoxazole,2,6-di(p-aminophenyl)benzo-[1,2d;5,4d’]-bisoxazole (DAPBBO) was designed and synthesized. Derived from the monomer DAPBBO, a series of polyimides containing bis-benzoxazole were prepared through polycondensation with the aromatic dianhydrides. Copolyimides containing bis-benzoxazole were also obtained from the polycondensation of DAPBBO and ODA as the diamine monomers and BPDA as the dianhydride monomer. FTIR, elemental analysis and XRD was employed to investigate the two classes of polyimides. Their solubility, thermal stability, mechanical and optical property were measured and discussed in detail.The synthesis of2,6-di(p-aminophenyl)benzo-[1,2d;5,4d’]-bisoxazole (DAPBBO) was studied by designing orthogonal experiments, and the factors influencing the reaction were analyzed. FTIR,’H-NMR, elemental analysis, DSC and TGA were employed to characterize the structures and the properties of the resulting DAPBBO. It was indicated that we had obtained the expected product DAPBBO through the above reaction with a high purity and a good heat-durability and thermal stability.A series of polyimides containing bis-benzoxazole were prepared from the resulting diamine DAPBBO and various commercially aromatic dianhydrides such as BPDA, BTDA, ODPA and PMDA through a conventional, two-stage polymerization method. The inherent viscosities of the polyamic acid (PAA) solutions ranged from1.18-2.44dL/g. The structures of the polyimides were investigated by means of FTIR, elemental analysis and XRD, showing that the resulting polyimides were the designed polymers. The thermal stability of these polyimides was investigated using TGA and DMA. The polyimides displayed outstanding thermal stability. These polyimides had high glass transition temperature (Tg) which displayed the outstanding resistance to high temperature. The tensile modulus of WAXD a polyimids was20GPa, respectively.Copolyimides containing bis-benzoxazole units were produced from DAPBBO and4,4’-diamino diphenylether (ODA) as the diamine monomers and3,3’,4,4’-biphenyl tetracarboxylic dianhydride (BPDA) as the dianhydride monomer. The structure, thermal stability, mechanical and optical properties of the polyimides were studied. As a result, the introduction of the DAPBBO monomer in the main chains greatly increases the glass transition temperatures of the polyimides. The addition of DAPBBO enhanced the mechanical properties of the polyimides, and the tensile strength and modulus was in the range of462-827MPa and6.51-11.59GPa.