Polyimide-assisted fabrication of highly oriented graphene-based all-carbon foams for enhancing thermal conductivity in polymer composites

Graphene often tends to be horizontally oriented during processing owing to its two-dimensional layer structure with a high aspect ratio. Consequently, polymer/graphene thermal interface materials (TIM) typically exhibit high in-plane (IP) thermal conductivity (K), while the limited through-plane (TP) K is not as desirable in practical applications. Herein, vertically aligned high-quality polyimide/graphite nanosheets (PG) skeletons were developed to improve the TP K of polymer-based composites by a simple directional freezing strategy. Notably, the graphene-based graphite nanosheets (GNs) are obtained from highly thermally conductive graphene film scraps by crushing. Water-soluble polyamic acid salt solution is used for direct dispersion of hydrophobic GNs fillers to achieve directional freezing. The polyimide has assisted the directional alignment of GNs and has been converted into graphite after graphitization. Moreover, the introduction of GNs enhances the orderliness and density of the PG, thus further improving the strength and heat performance of its polydimethylsiloxane (PDMS) composite. The obtained PDMS/PG composite (PG: 21.1 wt%) exhibits an impressive TP K of 14.56 W·m-1·K-1, 81 times that of pure PDMS. This facile polyimide-assisted graphene alignment method provides ideas for the widespread fabrication of anisotropic TIM and enables the reuse of graphene film scraps.