Tuning of anisotropic electrical conductivity and enhancement of EMI shielding of polymer composite foam via CO2-assisted delamination and orientation of MXene

Integration of a foam structure into a shielding material not only reduces its mass density but also enhances its electromagnetic interference (EMI) shielding performance. In this study, poly(vinylidene fluoride) (PVDF) composite films and foams consisting of carbon nanotubes (CNTs) and bulk Ti₃C₂T_x MXenes were fabricated. Prominent anisotropy with high in-plane electrical conductivity (σ_∥) and low through-plane electrical conductivity (σ_⊥) was achieved (e.g., σ_∥ = 17.02 S/m and σ_⊥ = 0.42 S/m with 1 wt% MXenes) in the composite films due to the aligned structure of the MXenes in the PVDF matrix. With CO₂-assisted foaming, the intercalation of PVDF chains and cell-induced biaxial stretch facilitated reorganized orientation and delamination of the MXenes. Consequently, σ_∥ decreased and σ_⊥ increased, and exhibiting an isotropic feature. Furthermore, the integration of the foam structure resulted in an increased σ_⊥, which improved the EMI shielding effectiveness (EMI SE) to 65.1 dB at a MXene content of 12 wt%, owing to enhanced multi-reflections, dielectric loss and conduction loss. Moreover, the shielding effectiveness coming from reflection was reduced to lower than 3 dB due to the decrease in σ_∥ and destructive interference in the composite foam, thus leading to an absorption-dominated mechanism. Additionally, the prepared composite foams exhibited outstanding thermal conductivity and mechanical strength, enabling them to be used as lightweight and robust EMI shielding materials that could transform EM wave energy into Joule heat.