Two-dimensional clay and graphene nanosheets for polymer nanocomposites and energy storage applications

Abstract

Clay and graphene nanosheets are attractive to materials scientists due to their unique structural and physical properties and potentially low cost. This thesis focuses on the surface modification and structure design of clay and graphene nanosheets, targeting special requirements in polymer nanocomposites and energy storage applications. The high aspect ratio and stiffness of clay and graphene nanosheets make them promising candidates to reinforce polymers. However, it is challenging to achieve a good dispersion of the nanosheets in a polymer matrix. It is demonstrated in this study that organic modifications of clay and graphene nanosheets lead to better filler dispersion in polymer matrices. A prepolymer route was developed to achieve clay exfoliation in a polyurethane-vermiculite system. However, the phase-separated structure of the polyurethane matrix was disrupted. Intragallery catalysis was adopted to promote the clay exfoliation during polymerization. With both catalytic and reactive groups on the clay modifier, the polyurethane-vermiculite nanocomposites showed a significant increase in modulus and improved barrier performance, compared to neat polyurethane. The toughening effect of graphene on thermosetting epoxies and unsaturated polyesters (UPs) was also investigated. Various types of graphene with different structures and surface functionalities were incorporated into the thermosetting resin by in situ polymerization. The toughening effect was observed for epoxy nanocomposites at loading levels of less than 0.1 wt%, and a peak of fracture toughness was observed at 0.02 or 0.04 wt% of graphene loadings for all epoxy-graphene systems. A microcrack-crazing mechanism was proposed to explain the fracture behavior of epoxy-graphene systems based on fractography observations. Similar peak behavior of fracture toughness was not observed in UP system. UP nanocomposites with modified graphene oxide showed better mechanical performance than those with unmodified graphene oxide, which was attributed to better graphene dispersion and a stronger UP-graphene interface. Graphene has also been extensively studied in energy storage applications, due to its high conductivity and surface area. In order to utilize the benefits of graphene, macroscopic graphene/V2O5 films and graphene aerogels were fabricated from the self-assembly of graphene materials. The unique 2D structure of graphene helped to maintain the integrated film morphology in graphene/V2O5 composites and the monolithic macroporous structure in graphene aerogels. Good conductivity was obtained by incorporation of graphene sheets in the structure, which results in good electrochemical performance as electrode materials for batteries or supercapacitors. The facile preparation methods allow good control of the composition and thus the properties of the macroscopic graphene nanostructures.