Thermal conductivity of polycaprolactone/three-dimensional hexagonal boron nitride composites and multi-orientation model investigation

Three-dimensional boron nitride (3D-BN) scaffolds were fabricated by the ice-templating method, and polycaprolactone/3D-BN (PCL/3D-BN) composites were prepared by vacuum impregnation of caprolactone monomers into 3D-BN scaffolds and then polymerization via the microwave-assisted technique. The results reveal that hexagonal boron nitrides (hBNs) in the PCL/3D-BN composites present three orientation patterns, that is, parallel (z-direction) and vertical (x-direction) alignment in the direction of ice-crystal and random orientation. Moreover, the orientation of hBNs plays an essential role in improving the thermal conductivity of the composite. The maximum thermal conductivities of the PCL/3D-BN composite are 1.42 and 1.01 W m(-1) K-1 in the z-direction and x-direction, which are 7.10 and 5.05 times higher than that of the pure PCL, respectively. However, the existing models cannot be used to analyze the orientation of hBNs in the composites quantitatively. In this work, based on two-phase models, a multi-orientation model was developed to explore the volume fractions of hBNs oriented in each pattern. The results indicate that the hBN ratio of random orientation (f(r)/f) enhances with the increase of hBN content in the PCL/3D-BN composites, and especially while increasing the hBN content from 13.41 vol% to 15.48 vol%, the value of f(r)/f has soared from 33.0% to 63.7%, leading to the decrease of thermal conductivities. The predictions from the multi-orientation model are in good agreement with the experimental data for the other composites, implying that the model could be used to analyze the orientation of fillers quantitatively and guide the design of thermally conductive scaffolds.