High intrinsic thermal conductivity in cellulose nanocrystal films through pitch regulation

Polymers are widely used in thermal management systems for modern electronics due to their light weight, electrical insulation abilities, and low cost. However, the low intrinsic thermal conductivity (TC) of polymers restricts their applications. This low intrinsic TC is due to the entanglement of the polymer chains and defects such as voids, which act as phonon scattering sites. Regulating the structure of polymers is an effective way to improve their intrinsic TC. Generally, traditional approaches can achieve only a partially ordered structure, while the construction of an ordered structure on both the microscale and macroscale is beneficial to phonon transportation. Inspired by the excellent crystalline properties of biopolymers of CNCs (cellulose nanocrystals), herein, we develop an approach for the fabrication of a CNC composite film that has a highly ordered structure on both the microscale (crystalline chains of cellulose) and macroscale (highly compacted lamella structure). CNCs combined with Fe3O4/CNC (CNCs loaded with Fe3O4 nanoparticles) can self-assemble into a chiral nematic structure, and the pitch of the ordered structure can be adjusted under an external magnetic field. With an increase in the magnetic field from 0 mT to 32 mT, the pitch decreases from 363 +/- 35 nm to 210 +/- 11 nm, and the TC improves significantly because Fe3O4/CNC compresses the structure uniformly and reduces voids and cracks. At the optimized concentration of 1.10 wt%, the CNC composite film shows a TC of up to 0.798 W m(-1) K-1, which is 2-3 times that reported for a typical polymer. The film has good transparency and is suitable for the application requirements of mobile devices and windows. This work paves an avenue for the design and fabrication of polymer matrices with extremely high intrinsic TC, and it is expected to contribute to theoretical studies.