Boron Nitride Nanosheets and Their Applications in Polymeric Composites

Boron nitride sheets, as the isoelectric analogue of graphene, have gradually attracted researchers’ attentions due to their excellent mechanical, electrical and thermal properties. Compared with graphene, the exploration of potential applications of boron nitride sheets for polymeric nanocomposites is limited due to the lack of practical and efficient preparation methods. Current methods like solvent sonication and chemical vapor deposition are not ideal and difficult to scale up for composites applications. The work described in this thesis focuses on the solution of two key problems of fabricating boron nitride/polymer nanocomposites. The first problem is the exfoliation of hexagonal boron nitride. In this work, two approaches are developed and can be used for the preparation of boron nitride nanosheets: chemical functionalization and direct exfoliation by interfacial trapping. By chemically modifying hexagonal boron nitride, hydroxylated boron nitride nanosheets are prepared. Not only are the van der Waals forces between sheets reduced, but functional groups are introduced on the sheets increasing their dispersibility in water and polymer matrices. Further modifications are also feasible based on the attached hydroxyl groups. Unfunctionalized pristine boron nitride sheets can be directly prepared by the second approach: an interfacial trapping technique. Sheets are formed at the interface of the phase-separated water/organic solvent mixture to minimize interfacial energy. When prepared in different conditions, sheets stabilize the interface in two different forms: thin, continuous films or emulsions. Mechanisms of the two exfoliation approaches are studied. The other problem is the preparation of BN sheets/polymer composites with improved properties. To fully take advantage of each exfoliation method, melt mixing is employed for the functionalized boron nitride nanosheets/polymer composites while several more complicated methods are used for the direct use of exfoliated sheets. Those methods include solution casting, heat processing and in-situ polymerization. Obtained nanocomposites show one or more improved properties, such as mechanical properties, thermal stabilities and electrical properties. Potentially, these nanocomposites may be used as fire resistant building materials, dielectric materials or mechanically strong/lightweight materials.