Takuya Morishita

A novel morphological model for carbon nanotube/polymer composites having high thermal conductivity and electrical insulation

Carbon nanotubes (CNTs), owing to their extremely high thermal conductivity (∼3000 W m−1 K−1), have recently attracted attention as notable nanofiller candidates to improve the thermal conductivity of polymers. However, CNTs cannot practically be used for highly electrically insulating polymers because even a few CNTs impart a high electrical conductivity to the polymers. Here, we design and fabricate CNT/polymer composites having a novel morphology, which achieves both enhanced thermal conductivity and high electrical insulation. This morphology comprises a matrix polymer and a CNT-localizing domain polymer encapsulated by a shell-forming component, which contributes to the selective localization of the CNTs into the dispersed domains. Such a controlled morphology is formed by the self-organization of the CNTs and the constituent polymers. A tailor-made morphology of CNT/polymer composite in accordance with our proposed model represents a promising route to a wide variety of applications of CNTs in materials requiring high electrical insulation.

Optimised exfoliation conditions enhance isolation and solubility of grafted graphenes from graphite intercalation compounds

In bulk applications, it is essential that graphene sheets disperse individually in solvents or matrices, and therefore, suitable functionalisation regimes are crucially important. Here, isolated, highly soluble, alkyl-grafted graphenes were synthesised by reacting exfoliated Na-reduced graphite intercalation compounds (GIC) with alkyl halides. In this reaction, efficient exfoliation of the Na-reduced GICs into individually-dispersed negatively-charged graphenes provides accessible surface area for grafting. Increasing the alkyl chain length leads to large decrease of the grafting ratio (GR), demonstrating that steric factors also play an important role. However, optimising the Na concentration (C/Na ratio) in the reaction was very effective for improved exfoliation and increased GR. The X-ray diffraction measurements suggest that particular C/Na ratios (C/Na = ∼12) led to full exfoliation, by balancing total charge and charge condensation effects and that the GR can be significantly increased even in the case of long alkyl chains (eicosyl chains), corresponding to a high solubility of 37 μg ml−1 and high yield in o-dichlorobenzene. Moreover, the absolute Na concentration is the critical parameter, with the same optimum (∼0.01 M) for exfoliation and grafting of GIC at all graphite concentrations; it was possible to graft even at high graphite concentration (0.3 M (3.6 mg ml−1)) successfully.

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.