Structure and properties of the interphase in coated carbon fiber epoxy systems

When reinforcement and resin components come into contact during processing, interactions between them cause the formation of an interfacial region adjacent to the fibers. The methodology followed in this work focused on identifying and describing the diffusional and chemical processes taking place during the formation of the interphase. Two distinct cases of uncoated and coated fibers embedded in an epoxy matrix were analyzed. Preferential absorption was shown to lead to the formation of microscopic interphases in both uncoated and coated systems. The application of fiber coatings was shown to lead to the formation of a macroscopic property variable interphase as well.; The Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) technique was used to measure the diffusivity of an amine curing agent inside a solid epoxy layer over a wide temperature range. The same FTIR-ATR method was used to examine epoxy-amine kinetics and the effect of initial stoichiometry on the rate. Epoxy resin structure-property relationships were investigated both experimentally and theoretically. Properties such as glass transition temperature, storage modulus and thermal expansion coefficients as a function of prepolymer structure and stoichiometry were examined by means of thermal analysis. Tensile tests were conducted to evaluate tensile modulus and strength as a function of stoichiometry and temperature. A statistical model was used for the correlation of these properties with the epoxy network structure.; Stoichiometric concentration gradients and stoichiometry-property relationships were combined in order to predict property gradients in a variety of different epoxy coated fiber reinforced systems. The modulus and glass transition temperature gradients predicted thus were verified with the use of the single fiber fragmentation test performed at a range of temperatures. An elasticity analysis based on a three concentric cylinder shear lag model was used to predict fragmentation length as a function of interfacial and bulk matrix properties.