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Mahdi Hamidinejad , Biao Zhao , Azadeh Zandieh , Nima Moghimian , Tobin Filleter , Chul B. Park
Lightweight high-density polyethylene (HDPE)–graphene nanoplatelet (GnP) composite foams were fabricated via a supercritical-fluid (SCF) treatment and physical foaming in an injection-molding process. We demonstrated that the introduction of a microcellular structure can substantially increase the electrical conductivity and can decrease the percolation threshold of the polymer–GnP composites. The nanocomposite foams had a significantly higher electrical conductivity, a higher dielectric constant, a higher electromagnetic interference (EMI) shielding effectiveness (SE), and a lower percolation threshold compared to their regular injection-molded counterparts. The SCF treatment and foaming exfoliated the GnPs in situ during the fabrication process. This process also changed the GnP’s flow-induced arrangement by reducing the melt viscosity and cellular growth. Moreover, the generation of a cellular structure rearranged the GnPs to be mainly perpendicular to the radial direction of the bubble growth. This enhanced the GnP’s interconnectivity and produced a unique GnP arrangement around the cells. Therefore, the through-plane conductivity increased up to a maximum of 9 orders of magnitude and the percolation threshold decreased by up to 62%. The lightweight injection-molded nanocomposite foams of 9.8 vol % GnP exhibited a real permittivity of ε′ = 106.4, which was superior to that of their regular injection-molded (ε′ = 6.2). A maximum K-band EMI SE of 31.6 dB was achieved in HDPE–19 vol % GnP composite foams, which was 45% higher than that of the solid counterpart. In addition, the physical foaming reduced the density of the HDPE–GnP foams by up to 26%. Therefore, the fabricated polymer–GnP nanocomposite foams in this study pointed toward the further development of lightweight and conductive polymer–GnP composites with tailored properties.