Ladder‐Type Polymer Enables Dual‐Directional Phonon Bridging in Thermally Conductive h‐BN/TOCNF Film

ABSTRACT The persistent trade‐off between preserving the intrinsic thermal conductivity of fillers and minimizing interfacial thermal resistance remains a critical bottleneck in polymer‐based thermal interface materials (TIMs). Herein, we address this challenge through a “dual‐directional bridging” strategy using poly(benzimidazobenzophenanthroline) (BBL), a rigid ladder‐type conjugated polymer, as an interfacial mediator. Unlike conventional modifiers, BBL features an extended superplane architecture that concurrently establishes maximized π – π interlocking with the hexagonal boron nitride (h‐BN) basal plane and weaves dense hydrogen‐bonding networks with the TEMPO‐oxidized cellulose nanofiber (TOCNF) matrix. Combined spectroscopic analyses and solvent‐corrected DFT calculations support π ‐associated coupling between BBL and h‐BN together with hydrogen‐bonding interactions between BBL and TOCNF, increasing the calculated interfacial binding energy from 0.85 to 2.86 eV under an aqueous environment. Crucially, this creates a continuous “dual‐directional phonon bridge” that substantially mitigates interfacial thermal resistance and localized phonon scattering, all without disrupting the intrinsic h‐BN lattice. Consequently, the BBL@h‐BN/TOCNF film achieves in‐plane and through‐plane thermal conductivities of 8.78 and 1.18 W m −1 K −1 , outperforming standard commercial thermal silicone grease and demonstrating superior heat extraction in simulated high‐power CPU cooling. Overall, this work establishes a rational structural paradigm to unlock the macroscopic thermo‐mechanical potential of two‐dimensional (2D) composites for next‐generation electronics.