In-situ growth of porous Cu3(BTC)2 on cellulose nanofibrils for ultra-low dielectric films with high flexibility

•Porous copper-based MOFs were introduced to decrease the polarizability of ANF.•In-situ growth of MOFs nanoparticles on cellulose nanofibrils was synthesized.•ANF/CNF@CuBTC films were fabricated by a pressured-extrusion procedure.•The composite films exhibited excellent mechanical, thermal and dielectric properties.•The composite films reached a low dielectric constant lower than 2.2 at 100 MHz.