Directional Thermal Conductivity Enhancement in Plasma-Surface-Modified hBN and Polyrotaxane Composites by Electric Field

Recently, flexible materials with high thermal conductivity have been extensively studied, e.g. for applying to flexible devices. We have developed hexagonal boron nitride particles (hBN)/polyrotaxane¹ (PR) tough composite, which is a highly thermally conductive flexible insulator, by applying plasma-surface modification of hBN resulting in their higher dispersibility in the composite and enhancing toughness via using PR as a slide-ring material². hBN is known to have anisotropic thermal conductivity, as in-plane direction shows about 20 times higher conductivity than out-of-plane direction. Therefore, thermal conductivity of such composites can be increased in one direction, if in-plane directions of hBN are oriented parallelly. Such demonstration has been achieved with hBN/polysiloxane composites, by applying electric field³. Electric field can not only orient hBN, but also form pillar-like structures of the oriented hBN, which could further increase the directional thermal conductivity. The purpose of this study is to synthesize tough composite sheets with high thermal conductivity in out-of-plane direction, by combining above-mentioned two approaches: (i) composites of plasma-surface-modified hBN and PR, and (ii) applying electric field for increasing thermal conductivity in the applied filed direction.<br/>hBN with various sizes (0.2-23 µm) were pre-treated via plasma in liquid and the mixture, including hBN and PR, were exposed to an electric field during the polymerization. The results show that the thermal conductivity in the field direction is enhanced comparing with the case applying no electric field. It was also confirmed that blending different-sized hBN can improve the thermal conductivity by keeping same hBN content in the composites. For example, the higher thermal conductivity is achieved with 0.2-µm (10%) and 7-µm (90%) hBN, comparing with that with 100% 7-µm hBN. We attribute this improvement to stronger heat pass formation by smaller hBN bridging larger hBN in pillar-like structures.<br/>The synthesized electric-field-applied composites show anisotropic properties in the thermal conductivity as well as their Young’s modulus. Such composites show higher thermal conductivity in the electric field direction (out-of-plane direction of composite sheets) and lower Young’s modulus in the direction perpendicular to the field (in-plane direction of composite sheets), comparing with those of composites without electric field application. These directional changes of properties are sometimes ideal for a thermal-conductive flexible materials. By optimizing the conditions, a flexible composite with elastomer-class flexibility (Young’s modulus, perpendicular to the field direction: 58 MPa) and metal-class thermal conductivity (parallel to the field direction: 11 W/mK) has been achieved so far.<br/>Further details, including hBN contents dependence and their blending-ratio dependence, will be presented at the symposium.<br/><br/><br/>1. Okumura, Y. & Ito, K. The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-links. <i>Adv. Mater.</i> <b>13,</b> 485–487 (2001).<br/>2. Inoue, K., Goto, T., Ito, T., Shimizu, Y., Hakuta, Y., Ito, K. & Terashima, K. Boron nitride with high zeta potential via plasma processing in solution for preparation of polyrotaxane composite. <i>J. Phys. Appl. Phys.</i> <b>54,</b> 425202 (2021).<br/>3. Cho, H.-B., Nakayama, T., Suematsu, H., Suzuki, T., Jiang, W., Niihara, K., Song, E., Eom, N. S. A., Kim, S. & Choa, Y.-H. Insulating polymer nanocomposites with high-thermal-conduction routes via linear densely packed boron nitride nanosheets. <i>Compos. Sci. Technol.</i> <b>129,</b> 205–213 (2016).