| Polyimides are well known as typical high-performance polymers with high strength and modules, outstanding thermal stability, resistance to chemical corrosion and radiation properties, which are widely applied for aerospace, electrical and electronic applications, chemical industry etc. With the fast development of high technology industries, ever-increasing requirements to polyimides have been raised, which attract more and more attentions on the preparation of novel polyimides with improved properties. More work focuses on the improvement of their mechanical and thermal properties.In this paper, 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 benzimidazole rings, a diamine monomer containing bis-benzimidazole,2,2'-p-phenylenebis(5-aminobenzimidazole) (PBABI) was designed and synthesized. Derived from the resulted monomer PBABI, a series of polyimides containing bis-benzimidazole were prepared through polycondensation with the aromatic dianhydrides. Copolyimides containing bis-benzimidazole were also obtained from the polycondensation of PBABI 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 property and optical property were investigated and discussed in detail.To prepare PBABI, the nucleophilic substitution reaction of 4-nitro-1,2-phenylenediamine and terephthalyl chloride in the presence of lithium chloride was carried out, followed by catalytic reduction with hydrazine hydrate and Pd/C. FTIR, 1H-NMR, 13C-NMR, elemental analysis and DSC were employed to characterize the structures and the properties of the resulting PBABI. It was indicated that we had obtained the expected product PBABI through the above reaction with a high purity and the melting point of 418-419℃measured by DSC. A series of polyimides containing bis-benzimidazoles were prepared from the resulting diamine PBABI 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 from 1.12 to 2.67 dL/g and exhibited good film-forming properties. The structures of the polyimides were investigated by means of FTIR, elemental analysis and XRD, showing that the resulting polyimides were the designed polymers and exhibited amorphous morphology. The thermal stability of these polyimides was investigated using TGA and DMA. The polyimides displayed outstanding thermal stability with only slight weight loss up to 600℃in nitrogen and 560℃in air, as well as the amount of the carbonized residue at 900℃in nitrogen was 64.8-71.4 wt%. These polyimides had high glass transition temperature (Tg) in the range of 446-475℃, which displayed the outstanding resistance to high temperature. The tensile strength and modulus of these polyimides was in the range of 120-180 MPa and 4.83-5.79 GPa, respectively, which indicated that they exhibited good mechanical properties.Copolyimides containing bis-benzimidazole units were produced from PBABI and 4,4'-diamino diphenylether (ODA) as the diamine monomers and 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) as the dianhydride monomer. The structures, thermal stability, mechanical and optical properties of the polyimides were studied. As a result, the introduction of the PBABI monomer in the main chains has a little effect on the decomposition temperature, but greatly increases the glass transition temperatures of the polyimides. The addition of PBABI enhanced the mechanical properties of the polyimides, and the tensile strength was up to 200 MPa as the molar ratio of ODA to PBABI was 9:1. |
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