Since graphene-based materials are finding increasingly more applications in various technological fields, it is of paramount importance to design these materials with useful and tunable properties by modification of the structural features of their building blocks. In this research, the mechanical and thermal properties of graphene-based materials, including graphene sheets, graphene oxide (GO) papers and graphene/polymer composites, are evaluated using molecular dynamics simulation (MDS) and analytical modeling in an effort to tailor these properties for useful applications. The modeling results are also compared and verified by experimental findings.

The effects of GO size and wrinkling on the Young’s modulus of GO papers are specifically studied based on the MDS. Molecular models o...[ Read more ]

Since graphene-based materials are finding increasingly more applications in various technological fields, it is of paramount importance to design these materials with useful and tunable properties by modification of the structural features of their building blocks. In this research, the mechanical and thermal properties of graphene-based materials, including graphene sheets, graphene oxide (GO) papers and graphene/polymer composites, are evaluated using molecular dynamics simulation (MDS) and analytical modeling in an effort to tailor these properties for useful applications. The modeling results are also compared and verified by experimental findings.

The effects of GO size and wrinkling on the Young’s modulus of GO papers are specifically studied based on the MDS. Molecular models of GO papers are built using monolayer GO sheets containing oxygenated functional groups according to the Lerf-Klinowski model. The Young’s moduli of GO papers both with and without water increase with increasing GO sheet size. The shear interactions between the adjacent GO sheets in different planes play a key role in resisting the deformation of GO papers on the nanoscale. The stretching of GO sheets themselves in tension also become increasingly important when GO size increases. GO sheets initially placed in the edge-to-edge configuration tend to form a more stable face-to-face configuration by buckling in the presence of edge carboxyl groups. Therefore, ‘Peak and valley’ wrinkles at the edge-to-edge interaction sites are formed in GO papers. The Young’s moduli of graphene sheets and GO papers are affected to different extents by the presence of wrinkles. There is a negligible reduction in modulus due to wrinkles in single-layer graphene sheets while the modulus obtained at high strains in the GO papers with wrinkles is only 40% that of the GO papers without.

Thermal conductivities (TCs) of GO, functionalized GO and their epoxy composites are studied by a combined multi-scale modeling and experimental approach. By using MDS, we show that the TCs of GO sheets can be tuned by uniaxial tensile loads. The TC of GO unexpectedly increases in response to an external tensile load, a completely opposite trend to those shown by other nanostructured materials, including pristine graphene. The unique structure of GO suppresses the phonon scattering under tension, effectively ameliorating thermal conduction. The chemical modifications of graphene with different functional groups much reduce its intrinsic TC, while they improve the interface thermal conductance between the functionalized graphene and epoxy. Such improved interface conductance dominates in determining the TCs of composites when the sizes of fillers are smaller than a few micrometers, as revealed by the analytical model. When the sizes further increase, the high in-plane TC of pristine graphene becomes predominant, making it more efficient than functionalized ones in improving the TCs of composites. The effects of graphene layer thickness and the corresponding aspect ratio on TCs of graphene/epoxy composites are also investigated. MDSs show that, with the increasing number of layers of graphene sheets embedded in epoxy, the in-plane TCs of graphene sheets and TCs across the graphene/epoxy interfaces increase simultaneously. However, as revealed by the analytical model, the TCs of composites decreased with increasing number of layers when the sizes of graphene sheets are small due to small aspect ratios. Therefore, the large lateral size that yields high aspect ratio of the filler is the key to achieve higher TCs of composites with multi-layer graphene than the single-layer one. Based on the modeling results, high-aspect-ratio graphite nanoplatelets with tens of layers are produced by simple sonication and show a higher efficiency in improving TCs of composites compared to those containing single or a few layers reported in literatures.