Evolution of anisotropic bubbles and transition of the mechanical and electrical properties during a non-continuous two-step foaming of epoxy/carbon nanofiber composites

Abstract A non-continuous two-step foaming process was reported to prepare microcellular epoxy/carbon nanofiber (EP/CNF) composite foams. This approach involved firstly limited-foaming epoxy tablets in a mold with a fixed cavity to obtain preformed foams with designed anisotropic cellular structure and following by free-foaming them in a heating oven. The evolution of cell morphology, rearrangement of carbon nanofibers, and changes of the mechanical and electrical properties in epoxy composite foams during the two-step foaming process were systematically investigated. Although the anisotropic cellular structure tended to become isotropic after reheating, the final composite foams (Two-step foam) still exhibited an anisotropy in the electrically conductive and mechanical properties. This result was attributed to that the further growth of anisotropic bubbles cannot completely disrupt the vertical alignment of CNF. The oriented CNF, in turn, significantly hindered the expansion in that direction, leading to an anisotropic expansion of the preformed foams during reheating, which could inevitably affect the evolution of cell morphology. In addition, the two-step foaming process effectively eliminated the negative effect of compressed gas occurring during the limited-foaming process, enhancing the glass transition temperature nearly to that of the foams prepared through the free-foaming process. This work opens a potential avenue to realign the nanofillers and prepare composite foams with tunable properties.