Carbon fiber-reinforced epoxy composites currently play a significant role in many different industries. Due to their high cross-link density, aromatic epoxy polymers used as the matrix in composite materials are very strong and stiff however they lack toughness. This dissertation investigates three areas of the carbon fiber-reinforced composite, which have the potential to increase toughness: the carbon fiber surface; the fiber/matrix interphase; and the matrix material. Approaches to improving each area are presented which lead to enhancing the overall composite toughness without reducing other composite mechanical properties. The toughening of the base matrix material, DGEBA/mPDA, was accomplished by two methods: first, using low concentrations of aliphatic copolymers to enhance energy absorption and second by adding graphene nano-platelets (GnP) to act as crack deflection agents. 1wt% copolymer concentration was determined to substantially increase the notched Izod impact strength without reducing other static-mechanical properties. Toughening of DGEBA/mPDA using 3wt% GnP was found to be dependent on the aspect ratio of GnP and treatment of GnP with tetraethylenepentamine (TEPA). GnP C750 enhanced flexural properties but not fracture toughness because the small aspect ratio cannot effectively deflect cracks. TEPA-grafting enhanced GnP/matrix bonding. Larger aspect ratio GnP M5 and M25 showed significant increases in fracture toughness due to better crack deflection but also decreased flexural strength based on limited GnP/matrix bonding. TEPA-grafting mitigated some of the flexural strength reductions for GnP M5, due to enhanced GnP/matrix adhesion. In the high-fiber volume fraction composite, the fiber/matrix bonding was enhanced with UV-ozone surface treatment by reducing a weak fiber surface boundary layer and increasing the concentration of reactive oxygen groups on the fiber surface. Further increases in Mode I fracture toughness were seen with the addition of an epoxy fiber sizing: aromatic epoxy increased the modulus and aliphatic epoxy increase the shear-strain to failure and toughness at the fiber/matrix interphase. All improvements were made without reducing other static-mechanical properties. Combining the above surface treatments with a 1wt% aliphatically toughened matrix increase Mode I fracture toughness without reducing other static-mechanical properties. The fracture toughness enhancement was more pronounced in composites with low fiber/matrix adhesion. Due to reduced diffusion of the aliphatic epoxy away from the fiber surface with the addition of 1wt% aliphatic to the matrix, leading to a more compliant interphase, the aliphatic fiber sizing composite showed reductions in flexural properties and Mode I fracture toughness. The addition of GnP M5 to the aromatic fiber sizing enhanced the Mode I fracture toughness substantially by defecting cracks away from the fiber/matrix interphase. All flexural properties were similar with heat-treated GnP and better with TEPA-grafted GnP. Aliphatic fiber sizing with TEPA-grafted GnP reduced flexural properties with no change in fracture toughness by an enrichment of aliphatic epoxy at the fiber surface leading to a more compliant interphase.
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