Preparation And Characterization Of Transparent Benzimidazole Polyimide With High Temperature Resistance

In recent years,with the rapid development of lightweight,flexible and highly integrated optoelectronic devices,it has become imperative to develop high temperature resistant transparent polymer films to replace traditional glass substrates.Among many polymer materials,polyimide(PI)has been widely studied for its high glass transition temperature(T_g>300°C)that can withstand high temperature process requirements,as well as great benefits such as outstanding mechanical properties,dielectric properties,resistance to chemicals and flame retardant properties.However,there are electron donor and acceptor interactions between diamines and dianhydrides fragments in the conventional polyimide molecular structure leads to the formation of intra-and intermolecular charge transfer complexes(CTC),resulting in polyimide generally having strong absorption in the visible region and poor optical transparency,which limits its application in the field of flexible displays.In order to overcome the above problems and prepare colorless transparent polyimide(CPI),besides optimizing the manufacturing process and the imidization method,researchers have conducted a lot of research work from molecular structure design,such as the introduction of strong electronegative groups,alicyclic structures,bulky side groups and asymmetric and rigid non-coplanar structures in the molecular structure of polyimide can significantly improve the optical transparency of PI films.Among them,it is noteworthy that the introduction of asymmetric and non-co-planar structures can improve the optical properties of PI films without losing their thermal properties.Benzimidazole polyimide(PBII)is obtained by the polymerization of benzimidazole diamine and dianhydride and has very good thermal properties,mainly due to the rigid structure of benzimidazole and the presence of intermolecular hydrogen bonds,but this also brings strong coloring to the benzimidazole polyimide.Based on this,this topic creatively starts from the structure of benzimidazole and takes advantage of the naturally rigid asymmetric structure of benzimidazole diamine monomer to introduce non-co-planar structure by structural modification of conventional benzimidazole diamine monomer to prepare colorless and transparent PI films,and the main research works are as follows:Two kinds of novel 2-substituted N-CH₃benzimidazole diamine monomers,e.g.6,4-diamino-1-methyl-2-methyl-2-benzimidazoleand6,4-diamino-1-methyl-2-trifluoromethyl-2-benzimidazole,were prepared.The monomer structures were determined by ~1H NMR,IR spectroscopy and elemental analysis.The two diamine monomers were then polymerized with commercial dianhydrides:1,2,4,5-cyclohexanetetracarboxylic acid dianhydride(HPMDA)and hexafluoro isopropyl phthalic anhydride(6FDA)to prepare the corresponding benzimidazole polyimides(PBIIs)by high-temperature one-step polymerization,respectively.The properties of PI films were systematically investigated by DMA,UV-Vis spectroscopy,TGA,TMA,universal material testing machine and water absorption test.The results are as follows:due to the rigid and non-coplanar structure characteristics of the benzimidazole diamine monomer,the prepared PI films performed excellent thermal as well as optical properties:they were colorless and transparent,and the UV cutoff wavelengths(?_cutoff)were in the range of 330-379 nm,and the transmittance at 400nm(T₄₀₀)was 14-80%.The glass transition temperatures of the PBIIs films ranged from 360 to421°C and the thermal weight loss temperatures of 5%under nitrogen atmosphere were 446-505°C.Besides,these PBIIs films exhibited excellent solubility and very small water absorption:these PIs had favorable solubility in common polar solvents such as NMP,DMSO,and DMF at room temperature,and the water absorption of these PBIIs was 0.6?1.2%,which is mainly attributed to the introduction of N-CH₃to suppress the formation of hydrogen-bonded water molecules.The tensile strength of PBIIs film were 100?135MPa,elongation at break were 7.6?84.3%,tensile modulus was 2.8?3.3GPa.The excellent overall performance of these films has showed tremendous future in the field of optoelectronics such as flexible displays and flexible printed circuit boards.