Electrically and thermally graded microcellular polymer/graphene nanoplatelet composite foams and their EMI shielding properties

Polymer composites can offer a striking combination of properties when a gradient in electrical and thermal properties is generated. Functionally graded composites have shown great promise in electromagnetic interference (EMI) shielding, energy storage materials and sensors. This work presents a simple manufacturing route to develop graded microcellular structures, and thereby graded functionality, within polymer composite foams containing graphene nanoplatelets. The polymer/graphene composite foams were fabricated via supercritical fluid treatment in an injection molding machine followed by foaming through rapid depressurization in the mold cavity. The microstructural gradient developed within the composite foams, ranged from shear-induced elongated cells to more isotropic cellular structures over the length of the molded composites. This distinct microstructure offered graded electrical and thermal properties in the composites. The electrical conductivity, permittivity and thermal conductivity of the nanocomposite foams increased, respectively, up to 7 orders of magnitude, 1340% and 143% over the length of the composites. The specific EMI shielding raised up to 45% over the length of the nanocomposite foams. This study shows that foaming can pave the way for manufacture of functionally graded polymer composites for existing and emerging applications such as electromagnetic shielding, energy storage materials and sensors.