Abstract
Controlling the structure of graphene and graphene oxide (GO) phases is vitally important for any of its widespread intended applications: highly ordered arrangements of nanoparticles are needed for thin-film or membrane applications of GO, dispersed nanoparticles for composite materials, and 3D porous arrangements for hydrogels. By combining coarse-grained molecular dynamics and newly developed accurate models of GO, the driving forces that lead to the various morphologies are resolved. Two hydrophilic polymers, poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), are used to illustrate the thermodynamically stable morphologies of GO and relevant dispersion mechanisms. GO self-assembly can be controlled by changing the degree of oxidation, varying from fully aggregated over graphitic domains to intercalated assemblies with polymer bilayers between sheets. The long-term stability of a dispersion is extremely important for many commercial applications of GO composites. For any degree of oxidation, GO does not disperse in PVA as a thermodynamic equilibrium product, whereas in PEG dispersions are only thermodynamically stable for highly oxidized GO. These findings-validated against the extensive literature on GO systems in organic solvents-furnish quantitative explanations for the empirically unpredictable aggregation characteristics of GO and provide computational methods to design directed synthesis routes for diverse self-assemblies and applications.
Highlights
Controlling the structure of graphene and graphene oxide (GO) phases is the characterization of GO so uncertain, relating the structural or compositional vitally important for any of its widespread intended applications: highly properties of GO flakes to their final morordered arrangements of nanoparticles are needed for thin-film or memphology in composites is a challenging brane applications of GO, dispersed nanoparticles for composite materials, and 3D porous arrangements for hydrogels
Coveney Centre for Computational Science University College London in this article is based on the GraFF forcefield, which we introduced for graphene 2 years ago.[8]
For low oxidation state GO in poly(ethylene glycol) (PEG) polymer, aggregation via the graphitic domains occurs for the system of C:O ratio 5.0 leading to partially aggregated morphologies, while in system of C:O ratio 10.0, fully aggregated morphologies arise, the oxidized regions of the flake aggregating along with the graphitic domains. These findings demonstrate the greater interaction of poly(vinyl alcohol) (PVA) polymer molecules with the hydroxyl groups on the flakes’ surfaces compared to PEG molecules, thereby making the intercalated morphology much more favorable than the aggregated one for PVA GO systems, even via graphitic domains
Summary
Controlling the structure of graphene and graphene oxide (GO) phases is the characterization of GO so uncertain, relating the structural or compositional vitally important for any of its widespread intended applications: highly properties of GO flakes to their final morordered arrangements of nanoparticles are needed for thin-film or memphology in composites is a challenging brane applications of GO, dispersed nanoparticles for composite materials, and 3D porous arrangements for hydrogels. GO sheets have aggregated via the graphitic domains on the surface of the flake in some replicas, while in other replicas, an intercalated structure has self-assembled (see Figure 2-i,ii).
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