Quasi‐Isotropically Thermal Conductive, Highly Transparent, Insulating and Super‐Flexible Polymer Films Achieved by Cross Linked 2D Hexagonal Boron Nitride Nanosheets

Abstract Polymer‐based thermal management materials (TIMs) show great potentials as TIMs due to their excellent properties, such as high insulation, easy processing, and good flexibility. However, the limited thermal conductivity seriously hinders their practical applications in high heat generation devices. Herein, highly transparent, insulating, and super‐flexible cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet (PVA/(CNC/PA‐BNNS)) films with quasi‐isotropic thermal conductivity are successfully fabricated through a vacuum filtration and subsequent self‐assembly process. A special structure composed of horizontal stacked hexagonal boron nitride nanosheets ( h ‐BNNSs) connected by their warping edges in longitudinal direction, which is strengthened by cellulose nanocrystals, is formed in PVA matrix during self‐assembly process. This special structure makes the PVA/(CNC/PA‐BNNS) films show excellent thermal conductivity with an in‐plane thermal conductivity of 14.21 W m −1 K −1 and a through‐plane thermal conductivity of 7.29 W m −1 K −1 . Additionally, the thermal conductive anisotropic constants of the as‐obtained PVA/(CNC/PA‐BNNS) films are in the range of 1 to 4 when the h ‐BNNS contents change from 0 to 60 wt%, exhibiting quasi‐isotropic thermal conductivity. More importantly, the PVA/(CNC/PA‐BNNS) films exhibit excellent transparency, super flexibility, outstanding mechanical strength, and electric insulation, making them very promising as TIMs for highly efficient heat dissipation of diverse electronic devices.