| Carbon fiber-epoxy composite utilized as aerospace materials should possess modulus as high as possible on the basis of satisfying the strength. The epoxy matrix possessed high modulus can decrease the transverse deformation of carbon fiber composite. High modulus carbon fibers need high modulus epoxy matrix to form excellent composite. The cure reactions are directly control the composite processing and the final epoxy network structure and mechanical response. Hence, it is important to understand the cure reactions. The interface properties is critical to the composite performance and it has been referred to as the’heart of the composite. In view of the above issues, the main works in this study are as follows:(1) Curing reaction pathways were examined for the rate-determining step, i.e., the initial conversion using model compounds in this work to study how a slight modification of diamine structure influenced the curing characteristics of the epoxy-amine systems:MTB/TDE85, TFMB/TDE85, DDM/TDE85and DDS/TDE85. The kinetic analysis results were corresponding to the simulation results that the curing reaction in the prior is catalytic. Transition state parameters for the catalyzed reaction of the amine-epoxy systems were studied. The curing kinetics of epoxy resin was studied through the revised nth-order model and the rules of reacting activation of curing agent obtained by kinetics analysis were corresponding to the simulation results. Therefore, this simulated method is of great help in curing properties prediction.(2) To investigate the Tg of the cured epoxy resins, MTB/TDE85, TFMB/TDE85, DDM/TDE85and DDS/TDE85of unit cells were prepared, respectively. The variation of the MSD curves and density-temperature relationship for the epoxy systems are used to obtain the Tg. The simulated results were corresponding to the experimental results. The structural analysis results show that a slight modification of amine structures results in a visible change in the flexibility of molecular chains and the cohesive energy density, which obviously affect the Tg of the resulting specimens. To understand the mechanism of modulus improvement of epoxy resin matrices at molecular level, the flexibility of molecular chains, packing abilities of cross-linked molecular, the free volume of epoxy resins and the cohesive energy density were investigated. It was found that small free volume and low cohesive energy density will significantly result in low modulus. We can believe that such an approach will be especially useful for future structure-property studies in epoxy based composites.(3) Commercially carboxylic acid functionalized multi-walled carbon nanotubes were introduced in carbon fiber/epoxy composites interface to establish modulus intermediate layer. The CNTs presents an graded distribution state in the interface layer. After the CNTs coated the fiber, the interfacial shear strengths (IFSS) of the carbon fiber/epoxy composite are improved. The relationship between surface treatment of the carbon fiber and the interfacial improvement was discussed. The interfacial enhancement mechanism was explored by analyzing the surface morphology of the carbon fibers, CNT-deposition state, fractographic observation of cross-sections, and fracture surfaces of the composites. These results indicate that the CNTs on the fiber surface cause the increasing strength and toughness of the interfacial region surrounding the carbon fiber. Therefore, the fracture failure occurs because of resin cracking and deformation within the matrix nearby the carbon fiber surface. Results indicate that an alternative method for improving the interfacial properties of carbon fiber composites by controlling the fiber-matrix interface was developed. |
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