Tuning anisotropic properties of epoxy/multi-walled carbon nanotube composites via two-step foaming

Epoxy (EP) composite foams with 1 wt% of multi-walled carbon nanotube (MWCNT) were prepared through a two-step foaming process, which involved first limited foaming the prepreg in a mold and then foaming the resultant foam again. The evolution of nanotubes' arrangement and cell morphology during different second-step foaming (free-foaming or limited-foaming) process and its effect on compressive, electrical and electromagnetic interference (EMI) shielding properties of EP/MWCNT foams in both vertical and horizontal directions were carefully investigated. Differently interesting combination of anisotropy and isotropy in compressive strength, electrical resistivity, permittivity and EMI SE were facilely realized. For example, Limited-Free foam with a density of 0.401 g/cm 3 exhibited a significant anisotropy in electrical resistivity (rho(||)/rho(perpendicular to) = 2.93x10(-3)) but an isotropy in compressive strength (sigma(perpendicular to)/sigma(||) = 1), permittivity (epsilon(||)'/epsilon(perpendicular to)' = 1) and EMI shielding effectiveness (SE||/SE perpendicular to = 1), while Limited-Limited foam with a density of 0.304 g/cm(3) showed an isotropy in electrical resistivity (rho(||)/rho(perpendicular to) = 0.96), permittivity (epsilon(||)'/epsilon(perpendicular to)' = 0.99) and EMI SE (SE||/SE perpendicular to = 0.99) but an obvious anisotropy in compressive strength (sigma(perpendicular to)/sigma(||) = 0.74). In addition, the relationship between microsturcture and properties of composite foams was established and the mechanism of tuning anisotropic properties was also investigated. This study paves a way to the development of conductive composite foams with designed isotropic/anisotropic properties.