The superior mechanical properties of multi-walled carbon nanotubes (MWCNTs) play a significant role in the improvement of the mechanical and thermal stability of an epoxy matrix. However, the agglomeration of carbon nanotubes (CNTs) in the epoxy is a common challenge and should be resolved to achieve the desired enhancement effect. The present paper investigated the thermal, mechanical, and water uptake properties of epoxy nanocomposites with surfactant-modified MWCNTs. The nanocomposites were prepared through the incorporation of different weight concentrations of MWCNTs into the epoxy matrix. Comparative analysis of neat epoxy and epoxy/CNT nanocomposites were conducted through thermal, mechanical, microscopic, and water uptake tests to reveal the improvement mechanism. The homogenous distribution of the CNTs in the epoxy was achieved by wrapping the surfactant onto the CNTs. The addition of surfactant-modified CNTs into the epoxy caused an obvious increase in the mechanical and thermal properties. This improvement mechanism could be attributed to the uniform dispersion of the CNTs in the epoxy matrix reducing the free volume between the polymer chains and restricting the chain segmental mobility, leading to strong interfacial bonding and an efficient load transfer capability between the CNTs and the epoxy matrix. However, the mechanical and thermal properties of the epoxy/CNT nanocomposite decreased owing to the agglomeration effect when the concentration of the CNTs exceeded the optimal percentage of 1.5%. Additionally, the CNTs could impart a reduction in the wettability of the surface of the epoxy/CNT nanocomposite, leading to the increase in the contact angle and a reduction in the water uptake, which was significant to improve the durability of the epoxy. Moreover, the higher weight concentration (2%) of the CNTs showed a greater water uptake owing to agglomeration, which may cause the formation of plenty of microcracks and microvoids in the nanocomposite.

In order to balance the cost performance of carbon and glass fibers, the unidirectional carbon/glass hybrid fiber reinforced polymer composite rods were developed through the pultrusion technology in the present paper. Two kinds of fiber hybridization types were proposed including intra-yarn hybrid (uniform dispersed hybrid, UDH) and inter-layer hybrid (core-shell hybrid, CSH). Furthermore, the effects of fiber hybridization types on the mechanical properties (interface/short beam shear strength, three-point bending strength and tensile strength) were investigated experimentally. The full-field strain was obtained through the digital image correlation method to reveal the hybrid mechanism. It was found that the effects of fiber hybridization types on mechanical properties were remarkable. Through uniformly dispersing the carbon fiber into the glass fiber, the maximum increase percentages of available strength were up to 10.9% for short beam shear, 60.3% for three-point bending and 58.7% for tensile strength, respectively. The results of full-field strain analysis revealed that the mechanical failure of CSH rods was dominant by the interface debonding of shell/core, which significantly decreased the mechanical properties. In comparison, UDH rods had the intact interface bonding of carbon/glass fiber/resin, the mechanical failure was derived from the fracture of carbon fiber. The tensile strength of carbon fiber was fully utilized, which contributed to the higher mechanical properties. The larger scale carbon fiber tow may form the weaker interface layer of carbon/glass fiber/resin, which led to uneven interface stress distribution under the external loading and the decrease of interface shear strength.