Synthesis, Characterization And Fluorescence Property Of Novel Polyimides Containing Oxadiazole

Aromatic polyimides (PI) are well known for their outstanding thermal resistance and thermal stability, low dielectric constant, and excellent mechanical strength, which are widely used for adhesives, coatings, and composites in advanced aerospace and electrical and electronic applications. 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 functional properties. Their various characteristics such as good adhesion to metal, high thermal stability, excellent film formability and especially high glass transition temperature make them to be potential thermostable materials for organic light-emitting diode (OLED).Based on the viewpoint of molecular design and combined with low-cost synthesis routes, a series of novel polyimides containing oxadiazole, hereafter named as PI-3(3), PI-4(4), PI-mix (1:1), PI-mix (2:3) and PI-mix (3:2) respectively, were designed and synthesized. These polyimides were synthesized by a one-step condensation polymerization with isomeric bis-chlorophthalimido diphenyl ether and fluorescent 2,5-bis(p-hydroxybiphenyl)-1,3,4-oxadiazole. They were expected to combine the high fluorescent efficiency of oxadiazole with the excellent characteristics of polyimide. While put the flexible ether linkages into the rigid backbone of PI, the polyimides exhibited good solubility.The fluorescent monomer of 2,5-bis(p-hydroxybiphenyl)-1,3,4-oxadiazole and five isomers of bis-chlorophthalimido diphenyl ether monomers were successfully synthesized. The monomers'structure was confirmed by elementary analysis, IR, and 1H NMR. Through DSC, TGA, UV-vis spectra and photoluminescence (PL) spectra analysis, it is comfirmed that 2,5-bis(p-hydroxybiphenyl)-1,3,4-oxadiazole monomer has excellent thermostability and high fluorescent intensity.Five isomeric PI's were synthesized by bis-chlorophthalimido diphenyl ether monomers and 2,5-bis(p-hydroxybiphenyl)-1,3,4-oxadiazole monomer. The results showed that, under the condition of 170 oC, 21.8% concentration in DMSO as solvent, in the presence of potassium hydroxide, the polycondensation went smoothly and high molecular weight polyimides were obtained after 15 h reaction time.The polymers were characterized by IR, 1H NMR and inherent viscosities. Their thermal stability was evaluated by TGA and DSC. The glass transition temperatures (Tg) were observed up to 200 oC,and 10 wt.-% weight loss temperatures were above 437 oC. Because of the existence of flexible ether linkages and unsymmetrical structure units in the backbone,the PIs exhibited good solubility in polar aprotic solvents such as NMP, DMSO etc. Their thin films could thus be prepared by directly spin coating from their solutions onto quartz.The UV-vis spectra and Photoluminescence (PL) spectra characteristic analysis indicated that the series of PIs have high fluorescent stability in solid and film state. Compared with the solution PL spectra of the same PI series, the films on quartz have a new emission peak in 290 nm, which may originate from the presence of intra- or inter-chain charge transfer complexes. The peaks of oxadiazole moieties still existed with a slightly red shift and intensity changes.