Highly thermally conductive and electrically insulating polymer nanocomposites with boron nitride nanosheet/ionic liquid complexes†
Highly thermally conductive and electrically insulating polymer materials are eagerly anticipated for thermal management of various applications including next-generation power electronic devices. Herein, boron nitride nanosheet (BNNS)/ionic liquid (IL)/polymer composites with high thermal conductivity (TC) and high electrical insulation were fabricated. BNNSs were exfoliated and noncovalently functionalized with ILs by one-step route using liquid-phase exfoliation of hexagonal boron nitrides in ILs. ILs improved exfoliation by physical adsorption on BNNS surfaces, forming highly soluble few-layered BNNS/IL complexes with high yields. The use of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) gave sufficient amounts of BNNS/[bmim][PF6] complexes for fabrication of BNNS/IL/polymer composites. Then BNNS/[bmim][PF6]/poly(methyl methacrylate) (PMMA) composite films were prepared using a simple wet-process, significantly enhancing both through-plane and in-plane TCs. The through-plane and in-plane TCs of the BNNS/[bmim][PF6]/PMMA composite films containing 50 wt% (≈34 vol%) BNNS reached approx. 5.4 W m−1 K−1 and approx. 7.3 W m−1 K−1, respectively. The through-plane TC is superior to those of previously reported BNNS/thermoplastic (TP) polymer composites with similar BNNS loadings. This high through-plane TC derives from randomly dispersed BNNSs and good affinity between PMMA and [bmim][PF6] on the BNNS surface. The optimum functionalization ratio (FR, [bmim][PF6]/BNNS mass ratio) found for enhancing the TC represents a balance of increased compatibility of BNNS/PMMA and a decrease of TC caused by extra amorphous [bmim][PF6]. Furthermore, the combination of IL and polymer matrix species is important. The through-plane TC of BNNS/[bmim][PF6]/polybutylene terephthalate (PBT) composite films containing 50 wt% BNNS was extremely high (approx. 5.8 W m−1 K−1), although that of BNNS/[bmim][PF6]/polycarbonate (PC) composite films was very low (approx. 1.2 W m−1 K−1) because of the lower affinity of [bmim][PF6] with PC. Moreover, the volume resistivity of the BNNS/[bmim][PF6]/TP polymer composites was improved compared with that of h-BN/TP polymer composites. The BNNS/IL/polymer composites are extremely promising for various applications requiring highly TC and electrical insulation.