• A directional freezing technique with polyimide assistance enables vertical alignment of graphene nanosheets, achieving a through-plane thermal conductivity of 14.56 W·m−1·K−1 in PDMS composites, 81 times that of pure PDMS.
• The method reuses scraps of highly thermally conductive graphene films, converting waste into valuable anisotropic thermal interface materials.
• The polyimide/graphite nanosheet (PG) foam exhibits enhanced order and density, improving both mechanical strength and heat transfer performance of the composite.
• This work provides a simple and scalable approach for fabricating anisotropic TIMs with high through-plane thermal conductivity, addressing a critical limitation of conventional graphene-polymer composites.