Preparation,Functionalization Of Hexagonal Boron Nitride Nanosheets And Their Application In Thermally Conductive Polymer

With the development of the polymer industry, polymer materials have been applied in many aspects of production and life. But the low thermal conductivity of polymer has been the main limitation for its application. Now, thermally conductive polymers have become an important research area of polymers. As we all know, most polymers are poor conductors of heat. Only by the addition of fillers with good heat conductivity in the polymer, the thermal conductivity can be improved. So choosing a good filler is very important for thermal conductivity of polymers. With the development of nanomaterials, nanofillers have been applied to polymers. The hexagonal boron nitride (h-BN) has very good thermal stability, oxidation resistance, corrosion resistance and other excellent properties. Particularly, it has extreme high thermal conductivity in plane. As the first choice of our experiment, h-BN can play a key role in the improvement of thermal conductivity of polymers.In this thesis, we prepared the h-BN nanosheets (h-BNNSs), the functionalization of h-BNNSs, and the composite of h-BNNSs and polymer and analyzed the properties of h-BNNSs and the functionalized h-BNNSs systematically. Furthermore, we studied the thermal and dielectric properties of the composite of h-BNNSs and polymer. The mainly research achievements are as follows:1. We have exploited a method named water freezing expansion exfoliation to produce h-BNNSs from h-BN. The water volume increased in the process of freezing, this force can overcome the van der Waals force between layers and then h-BNNSs were obtained. The sonication can play an auxiliary role in our experiment for reducing time. And the process of water freezing expansion plays the leading role. As the appearance of bumps at the h-BN surface, we proposed the exfoliation mechanism. The morphology feature, size and thickness of h-BNNSs have been characterized by SEM, AFM and TEM. According to the UV-Vis spectra, the band gap of h-BN will be increased with the decrease of number of layers. This method is facile, eco-friendly and universal. It has been proved to be useful for preparation of other two-dimensional nanosheets, such as graphene, WS2 nanosheets and MoS2 nanosheets.2. After the h-BN being exfoliated, there is lots of hydroxyl at periphery and surface of h-BNNSs connected with element boron. We choose vinyltrimethoxysilane (VTMO) as a functional reagent. The hydrolysis of methoxyl groups made it react with hydroxyl groups of h-BNNSs in toluene. According to the result of EDS, FTIR and XRD, the h-BNNSs have been functionalized by VTMOs. This method made h-BNNSs to be connected with many polymers covalently.3. H-BNNSs can be connected with polysiloxane covalently relying on the role of VTMOs. A series of nanocomposite with different loading amount of h-BNNSs were prepared and the properties were investigated systematically. As the mediator, VTMOs not only react with h-BNNSs, but also react with polysiloxane in the polymerization process. Because of the presence of covalent bonds between h-BNNSs and polysiloxane, ensure the uniform dispersion of h-BNNSs in the polymer. Thermal conductivity and thermal stabilities of h-BNNSs/polysiloxane films are increased highly at low h-BNNSs content. In addition, the dielectric constant is reduced and the dielectric loss tangent is basically the same low while increasing the content of h-BNNSs. Especially, the dielectric loss tangent of polysiloxane is decreased at low frequency with the embedding of h-BNNSs. These excellent thermal conductivity and dielectric properties of h-BNNSs/polysiloxane films make them good thermally conductive insulating composite materials with adjustable dielectric properties and nanocomposites for application in electronic devices and energy systems.