The epoxy resin was modified by hyperbranched structure of hydroxyl terminated to obtain waterborne epoxy resin for using as the carbon fiber (CF) sizing agent. By mixing method of waterborne epoxy resin and water, the waterborne epoxy sizing agent can be fabricated directly. In the sizing process, free of volatile organic solvents ensure the safety of operators and environment. Moreover, the introduction of tertiary amine group in hyperbranched structure will promote the reaction between epoxy and hydroxyl groups, and thus enhance the bonding strength between CF and epoxy resin. Results showed that the introduction of sizing agent does not sacrifice the CF single fiber tensile strength. The structure of epoxy resin retained in sizing agent is beneficial to the good wettability between CF and epoxy resin according to surface energy results. Interfacial shear strength value increases from 51.75 to 72.34 MPa, and interlaminar shear strength value increases from 80.32 to 90.34 MPa. These mechanical properties show that hyperbranched epoxy resin sizing agent with a large amount of hydroxyl groups can effectively improving the interface adhesion between CF and epoxy resin, showing a broad application prospect in composite materials field.

Multiwalled carbon nanotubes (MWCNTs) were modified by oxidizing agent to obtain O-MWCNTs, and the surface of it was coated with active group. 4,4′-Diaminodiphenylmethane bismaleimide (MBMI) was used as matrix, 3,3′-diallyl bisphenol A (BBA) and bisphenol-A diallyl ether (BBE) were used as reactive diluent, polyethersulfone (PES) as toughening agent, and O-MWCNTs as modifier; OMWCNT/PES-MBAE composite was prepared through in situ sol-gel method. The effect of PES and OMWCNTs on the mechanical and dielectric properties of composite was analyzed, and the microstructure was examined by transmission electron microscope (TEM) and scanning electron microscope (SEM). The mechanism of composite toughened by PES and OMWCNTs was observed and analyzed. The results showed that Diels-Alder reaction between MBMI and allyl compounds occurred completely and unsaturated double bond disappeared. O-MWCNTs and PES resin dispersed smoothly in polymer matrix and were used as reinforcement, and PES resin and O-MWCNTs could synergistically improve the properties of the composite and exhibited a typical ductile fracture. The impact and bending strengths were 16.09 kJ/m2 and 153.57 MPa, which were 74.32% and 53.08% higher than those of the MBAE matrix, respectively, and the dielectric constant and the dielectric loss were 3.76 (100 Hz) and 2.79 × 10−3 (100 Hz), when the content of PES was 2 wt% and O-MWCNTs was 0.02 wt%. The outstanding properties of the material made it play an important role in high-performance insulating material applications.

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