Abstract: The rapid miniaturization and a consequent increase in electromagnetic wave (EM) and heat emission in integrated electronic devices are now urgently desiring for multifunctional polymer composites that simultaneously possess high mechanical properties, excellent electromagnetic interference (EMI) shielding ability and heat dissipation capability. Herein, a series of novel polyimide-based composite films were constructed by rationally assembling the carbon nanofiber-grafted carbon fiber (NCF) and reduced graphene oxide (rGO) into a highly electrically and thermally conductive pathway within polyimide matrix via a sequential bidirectional freezing casting and hot-pressing strategy. The combination of high tensile strength and toughness was obtained by incorporating rGO into the NCF-reinforced PI composite, giving rise to a hierarchical reinforcing structure of NCF and crack deflection induced by rGO nanosheets. The high electrical conductivity (7.0 × 10³ S m^-1) endows the PI/NCF30/rGO3.5 composite film with a good EMI shielding effectiveness of 45 dB. Moreover, the well-aligned thermally conductive NCF and rGO and strong interfacial bonding between the polymer matrix and reinforcing fibers give rise to improved thermal conductivity (λ), particularly along the in-plane direction. Typically, the PI/NCF30/rGO3.5 exhibits an in-plane thermal conductivity of 5.18 W m^-1 K^-1, ∼4.7 times increment compared to the pure PI (0.91 W m^-1 K^-1). Besides, the resultant PI/NCF30/rGO3.5 composite film presents a superior Joule heating performance with some features of fast thermal response, ease of regulation and sufficient reliability. Accordingly, the developed multifunctional polyimide-based composite films demonstrate high potential as advanced EMI shielding materials with excellent heat dissipation.

Key words: Polyimide nanocomposites, Mechanical performance, Emi shielding, Heat dissipation