High Performance Polyimide Oligomers And The Carbon Fibre Reinforced Composites

Aromatic polyimides are well known as polymers with outstanding mechanical properties and high temperature capabilities. They can be utilized for a wide range of applications: as matrices for high performance advanced composite materials, as thin films in electronic applications, as structural adhesives and sealants, as high temperature insulators for aircraft wire coatings, and as membranes for gas separation. In this dissertation, phenethynyl terminated imide oligomers are disscussed to be used as resins in the aerospace field.Matrices for high-performance advanced composite materials (ACM) comprise two types: thermoplastics resins and thermoset resins. They have many advantages and disadvantages, separately. The thermal-plastic resins exhibit good toughness, however, a poor processability limited their widespread adoption as matrix resins for structural composites. To improve the processability, more and more attentions have been focused on the study of various kinds of thermoset resins, such as BMI, PMR-15, epoxy, and so on. These materials have the fatals of low toughness and low CAI values. The aim of the dissertation is to find a newly material to meet the demands.Herein, we synthesized four series of PETI oligomers from the reaction of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) or biphenylenetetracarboxylic dianhydride (s-BPDA) with p-TPEQ and 4-phenylethynylphthalic anhydride (PEPA) or 4-(1-Phenylethynyl) 1, 8-naphthalic anhydride (PENA). It was expected that, for the corresponding cured resins, the incorporation of this unsymmetrical and hindered moiety would disturb the coplannarity of the aromatic unit to reduce packing efficiency and crystalline-ability. This should enhance solubility and maintain high Tg through controlled segmental mobility. In all, sixteen oligomers were prepared and compared the properties. The obtained imide oligomers were molded, and tested by varies method such as DSC, DMA, TGA, melt viscosity and thermal expansion test.All the results proved that the PETIs (phenylethynyl terminated polyimide oligomers) retained good properties. The incorporation of p-TPEQ increased the solubility and processability of the oligomers compared with the other reported PETI oligomers. The uncured oligomers had low Tg values and low minimum melt viscosities at low temperature, which were contributed to the pedant phenyl groups. The Tgs of cured imide oligomers are related not only to crosslink density, but also the oligomer chain conformation. Films obtained from PEPA show higher tensile strength and modulus at break than PENA. The films of cured imide oligomers with Mw 1500 g mol-1 had higher Tg but poorer tensile properties due to higher crosslink densities and branching. Oligo-s-a4 offered the best combination of properties among all the 16 different oligomers. PENA was better than PEPA as the end-capping agent to get more hydrolysis stable polymers in alkali or acid environment.The results proved that the oligomers have reached the high level in the world. Oligo-s-a4 which had the molecular weight of 3000g mol-1, and it is the best of all. Compared with PETI-5(which was exploited by NASA, 5000 g mol-1 ), they had the alike Tg and mechanical properties, however, oligomers in this paper had far lower melt viscosity and good processability. Compared with TriA-PI, (another oligomers which was discovered by JAXA), oligomers in this paper had lower Tg than that of TriA-PI, but they avoid to use the costly monomer (a-BPDA), they has good competition ability.After the deeply evaluation of the oligomers, the selected oligomer resin was used for a long term thermal-stablity test. The oligomer was exposed at a 177℃air atmosphere for 500 hours. The thermal and mechanical properties of the oligomer didn't change, SEM proved that in addition. Then the good long term thermal stability of the oligomer was proved.CFRC (Carbon Fibre Reinforced Composites) were prepared by this oligomer and untreated carbon fibric. The composites were used for primary evaluation and long term thermal stability test. The results prove that the composite has good mechanical behaviors, especially a higher tensile elongation over 20%. After 200 hours of exposure at 177℃, the tensile strength and elongation of the composites showed little change, however, the modulus of the composites is greatly decreased. As the time went by, the modulus didn't show further changes. By SEM analysis, we deduced that it was because the resin on the carbon fibre surface decomposed during the aging treatment time, so the fibre were drawn out of the resin matrices by tensile, and they didn't show the good properties of the fibre. This gave us the suggestion that we should remove the resin on the fibre before the complex, then we can have the good compelx effect. This discovery gives us a newly way to obtain high performance composites.