Dispersion and rheology of stabilized graphene colloids and gels

Date

2014-08

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Abstract

Single layer graphite, known as graphene, has evolved as an outstanding nanomaterial in recent years due to its excellent mechanical, thermal, and electrical properties, including the capability for several unusual quantum effects. Graphene holds great promise for novel applications in advanced materials and devices; however, difficulties in dispersion quality and interfacial strength between filler and matrix have been a persistent problem for graphene-based nanocomposites, particularly for pristine, unfunctionalized graphene. Our work focuses on producing graphene from graphite without covalent functionalization of the graphene surface, which preserves the unique properties of pristine graphene. We demonstrate a suite of techniques to obtain high concentration dispersion of pristine graphene in a range of solvents, and evaluate the effect of the aggregation-resistant graphene sheets on the properties of polymer composites and gels. First, we illustrate a unique in situ polymerization technique to develop localized polymer coatings (nylon coating) on the surface of dispersed pristine graphene sheets in solution. These polymer coatings do not disrupt the pristine structure or superior properties of the graphene sheets; instead, these coatings allow for stable, aggregation resistant graphene dispersions, as characterized through rheology, SEM, and AFM. Second, we utilize a triphenylene based molecule (C10) to stabilize pristine graphene in water with a high graphene/stabilizer ratio. C10 molecules π-π stack with the graphene surface and prevent reaggregation. This dispersion can be reversibly destabilized based on pH and is stable against heat and lyophilization. Using these pristine graphene dispersions, variety of polymer nanocomposites which include electrospun nanofibers, graphene-polymer hydrogels and graphene-loaded epoxy composites were fabricated. This is the first report of electrospun nanofibers reinforced with dispersed pristine graphene. We examine the relationship between graphene loading and critical electrospinning parameters revealing that the presence of graphene enhances the mechanical and thermal properties of the parent polymer. Pristine graphene/polyacrylamide (PAM) hydrogels are synthesized via in-situ polymerization of acrylamide monomer in PAM-stabilized graphene dispersion. In-situ polymerization leads to the uniform dispersion of the graphene sheets in the hydrogel. The pristine graphene sheets interact with the elastic chains of the hydrogel through physisorption and permit gelation in the absence of any chemical cross-linker. This study represents the first report of pristine graphene as a physical cross-linker in a hydrogel. The properties of the graphene-polymer hydrogel are characterized by viscoelastic measurements and compressive tests. The physically cross-linked graphene hydrogels also exhibit excellent self healing properties. We also utilized the polymer-stabilized graphene sheets as nanofillers in commercially available thermoset such as epoxy. In this work, we aim to utilize stabilizer molecules to enhance the dispersibility of the nanofiller in the polymer matrix. The presence of the stabilizer affects the property predictions of the final composite based on the interactions between the stabilizer and the polymer chains. Specifically, we focus on the evolution of a connected nanofiller network in the epoxy resin and the onset of rheological percolation with the increase in graphene loading via viscoelastic measurements. We cured these samples and measured the glass transition temperature (Tg) of both the samples. It is observed that with a change in dispersion quality, the Tg increases with well-dispersed graphene samples and decreases in presence of aggregates. This study provides a uniform picture about the effect of pristine graphene sheets on the Tg of the epoxy resin.

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Keywords

Graphene, Nanomaterial, Nanocomposite, Polymer, Hydrogel, Rheology, Dispersion

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