| Fiber-reinforced polymer composite is the most important composite material with high specific strength and high specific stiffness. The property of fiber reinforced polymer composite depends on the fiber reinforcement and polymer matrix, and largely on the quality of the interface adhesion. In this thesis, the most advanced three dimensional preforms woven from high performance carbon fiber and basalt fiber were used as reinforcement, the most promising high performance polyimide was used as polymer matrix, and atmospheric plasma treatment was used to improve the fiber/matrix interface adhesion. Polyimide (PI) is promising high temperature polymer matrix for fiber-reinforced composite. Three-dimensional (3D) woven fabric is a type of advanced textile preform and reinforcement of polymer composite with 3D fiber architecture is an effective method to overcome the delamination problem encountered in composites reinforced with a two-dimensional (2D) fiber structure. In this thesis, a PMR polyimide composites reinforced with 3D woven fabric was fabricated by means of an impregnation/hot-compression two step method and thermal, mechanical, and dielectric property of the composites were characterized; The dielectric constants and dielectric losses of four samples of 3D woven fabric reinforced polymer composites were measured, and the dielectric constants of two samples of 3D woven glass fiber fabric reinforced polymer composites were compared with the dielectric constants from calculation through a theoretical model for dielectric constant of 3D orthogonal woven fabric reinforced polymer composite.The performance of fiber reinforced polymer composite depends largely on the quality of the fiber/matrix interface, which determines the way loads can be transferred from the polymers to the fiber. Atmospheric plasma surface treatment of carbon fiber is an efficient method to improve the boding strength of the fiber/matrix interface. In this thesis, PAN-based carbon fibers were treated by atmospheric pressure plasma for different time and the treatment effect on the tensile strength, morphology, wetting property, surface chemistry of the carbon fibers were investigated, and the treatment effect on the interfacial shear strength (IFSS) of the carbon fiber/polyimide interface was evaluated by single-fiber fragmentation test.1. PMR polyimide matrix resin was derived from 4,4’-methylenediamine (MDA), diethyl ester of 3,3’,4,4’-oxydiphthalic (ODPE) and monoethyl ester of cis-5-norbornene-endo-2,3-dicarboxylic acid (NE). The rheological properties of the powders of PMR polyimide prepolymer were investigated. Based on the cure reaction of the PMR type polyimide and the results of the rheological studies, an optimum two step fabricating method was established. In the first step, the three dimensional fabric preforms were impregnated with polyimide resin in a vacuum oven at 70℃for 1 hour followed by removing the solvent and pre-imidization. In the second step, composites were compressed by an optimized molding procedure. Three samples of PMR polyimide composites reinforced with 3D woven basalt fiber fabric,3D woven E-glass fiber fabric and 3D woven carbon fiber fabric were fabricated by the designed impregnation/hot-compression two step method.2. The viscosity of the monomer solution was measured and a 65wt% monomer solution was used to impregnate the 3D woven preforms. The differential scanning calorimetry (DSC) analysis was conducted for the powder of PMR polyimide prepolymer. The Fourier transform infrared (FTIR) spectra of PMR polyimide prepolymer thermally treated by different temperature procedure were also investigated. The glass transition temperature (Tg) of the cured PMR polyimide was 366℃, which determined by the dynamic mechanical analysis (DMA). The internal structures of the composites were observed with a scanning electron microscope (SEM) and the void volume fractions of the three composites were calculated by an ASTM standard method. Thermal property of the composites was investigated by Thermogravimetric analysis (TGA), three composites all showed good thermal resistance. No obvious weight loss was observed when the temperature is lower than 450℃and the weight loss is relatively low until the temperature reaches 850℃. The tensile strength and the tensile modulus of the PMR polyimide composite reinforced with 3D woven basalt fabric were 436MPa and 22.7GPa, respectively. The flexural properties of the three composites were also tested and a multi-stage layer-by-layer rupture mode was observed in the three-point bending test.3. Four samples of polymer composites reinforced with 3D woven basalt fiber fabric and 3D woven E-glass fiber fabric were fabricated, and PMR polyimide and vinyl ester polymer were used as the polymer matrix. The dielectric constants and the electric losses of four samples were tested by a cylindrical cavity resonance method. The dielectric constants of the two samples of polymer composites reinforced with 3D woven E-glass fabric were calculated using a theoretical model for 3D orthogonal woven fabric reinforced polymer composite. The calculated dielectric constant of the vinyl ester polymer composites was agreed well with the tested value, but the PMR polyimide composites was not so good and it was believed which was caused by the distorsion of the fiber architecture.4. Atmospheric plasma treatment was used to modify the surfaces of the PAN-based carbon fibers, and the treatment durations were 16s,32s and 64s. The effect of atmospheric pressure plasma treatment on the strength of carbon fibers was determined using single fiber tensile tests at the gauge length of 5mm,10mm,20mm, and 50mm. The plasma treatment caused no significant decrease to the tensile strength of the carbon fibers at a long gauge length, which was evaluated by statistical analysis using the Weibull distribution. However, the tensile strength of the carbon fibers slightly increased for the gauge length of 5mm. Scanning electron microscopy (SEM) and the atomic force microscopy (AFM) studies were performed in order to determine the changes in the surface morphology. Observations on the SEM microphotographs lead to the conclusion that the plasma treatment "cleaned" the original surfaces of carbonaceous impurities. The surface was rougher after the plasma treatment, which was concluded by the AFM test. Wetting studies were conducted to evaluate the solid surface tension. Treating carbon fibers in He/O2 atmospheric plasma resulted in an increased surface polarity and an improved wetting behavior versus water. The surface chemistry of the control and plasma treated carbon fibers were investigated by X-ray photoelectron spectroscopy (XPS) and XPS analysis revealed an obvious increase of oxygen content and oxygen containing polar functional groups.5. Single-fiber polyimide composites were prepared from control and plasma treated carbon fibers, and single fiber fragmentation tests were performed in order to characterize fiber/matrix interfacial adhesion. A polarized microscope with a polarizer attachment was used to observe the fragmentation and the patterns were recorded by a digital camera. The data of fragment lengths was fitted to a Weibull distribution. The quality of the interface improved after the plasma surface treatment, supporting the ability of atmospheric plasma oxidation to enhance the adhesion of carbon fiber to polyimide matrix. The interfacial shear strength (IFSS) of the fiber/matrix interface increase by 12%,21%, and 5% after plasma treatment of 16s,32s, and 64s. It could be concluded that suitable treatment is effective to enhance the interface bonding strength and too long treatment time had a negative effect on the enhancement. |
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