Fabrication and characterization of multifunctional nanocomposites filled with two-dimensional hexagonal boron nitride nanoplatelets

Abstract

The low-dimensional Boron nitride (BN) materials are one of the most promising inorganic nanomaterials reported so far. When compared to multi-dimensional structures of elemental carbon (i.e. fullerenes, carbon nanotubes, and graphene), hexagonal boron nitride has structural similarities resulting from sharing the same total number of electrons between the neighboring atoms. While boron nitride also has various dimensional forms (0-D, 1-D, 2-D structures) with excellent properties, including low density, high mechanical strength, superior thermal and chemical stabilities, elemental differences result in different intrinsic properties, such as electrical insulation and potential biocompatibility, from its carbon analogue. In fact, h-BN also may attractive material in use as biomedical applications, due to its biocompatibility resulting from the chemical stability and inertness that other nanomaterials, such as graphene or carbon nanotube, are unable to provide. However, while biocompatibility of BN nanotube (BNNT) has been reported, those of BNNP need to be further verified. Therefore, biological properties of BNNP nanocomposite are investigated to evaluate the potential of using BNNPs in biomedical fields, such as tissue regeneration material. Regardless of these unique combinations of properties, the applications of BN nanoplatelets have been limited due tendencies to cause irreversible aggregation and agglomeration, because of their high surface area and strong van der Waals interaction between them. In this research, we developed two-dimensional hexagonal boron nitride nanoplatelets (BNNPs) reinforced nanocomposite systems for new applications. By utilizing and modifying previously developed scalable liquid exfoliation method, bulk h-BN crystals can be exfoliated into mono- and few-layer nanoplatelets with hydroxyl functional groups. Manipulation of functional groups is effective methods of further modifying the physical and chemical properties of nanofillers to improve the compatibility, dispersion, and interfacial interaction of 2D nanofillers with a matrix. Herein, we propose fabrication process of multifunctional metal and polymer nanocomposite systems using BNNP as nanofiller. For polymer nanocomposites, vacuum filtration-assisted self-assembly process was utilized to fabricate bioinspired BNNP/polymer nanocomposites. The proposed approach is capable of producing highly strong bioinspired nanostructure by aligning BNNP and inducing electrostatic interaction between functional groups. Further chemical modification by covalent functionalization using hyperbranched polyglycerol molecules and non-covalent functionalization using melamine were designed in order to enhance the self-assembly and interfacial bond strength between these entities. The mechanical properties of the resulting HPG-g-BNNP/Gelatin and MBNNP/PVA nanocomposites can be tailored to closely match those of human bone and tendon, respectively. These results suggest that nanostructured BNNP/polymer nanocomposites can be used in biomedical applications, such as orthopedic substitute or implant materials. For metal matrix nanocomposites, molecular-level mixing process and spark plasma sintering (SPS) process was utilized to fabricate BNNP/metal nanocomposites. The proposed approaches resulted in BNNP/Cu nanocomposites with excellent mechanical properties, suggesting that the molecular-level mixing was an effective method to avoid the issues of inhomogeneous dispersion and interfacial bonding by inducing chemical bonds. This strengthening behavior was attributed to combination of grain boundary strengthening and load transfer strengthening by BNNPs. In order to determine and compare the effectiveness of BNNP as reinforcement material in metallic matrix, both room temperature and high temperature (300 $^\circ C$) mechanical properties were compared with those of Graphene/Cu nanocomposite analogue. By comparing the mechanical behaviors of BNNP with graphene in nanocomposite system, unique strengthening mechanisms of BNNP has been realized. In this study, the BNNP reinforced nanocomposites were successfully fabricated and characterized. In both metal matrix nanocomposite and nanostructured nanocomposite, BNNPs were homogenously dispersed throughout the matrix and formed strong interfacial bonding with matrix phase. In both cases, the increase in the amount of BNNP resulted in a significant enhancement in mechanical properties. The BNNP reinforced nanocomposites presented in this study can broaden the possibility of using BNNP as a reinforcement for various matrix composites and the developed BNNP reinforced nanocomposite could be applicable for high strength structural and multi-functional materials.