Expanded graphite has attracted considerable attention as a nanoscale filler in composite materials. This interest stems from a combination of factors including high aspect ratio, nanometer scale, organic compatibility, and low cost. In addition, the conjugated structure of the graphene sheet imparts both thermal and electrical conductivity to the matrix resin.
However, although numerous publications cite improvements in the modulus and electrical conductivity of expanded graphite-filled composites, there are often reports of poor mechanical properties. This primarily results from incomplete exfoliation of expanded graphite in a polymer matrix and, thus, poor dispersion of the filler. Improved results were observed by using graphite that had been oxidized to produce graphite oxide (GO). The GO was then split into individual graphene sheets through a rapid expansion process(ref. 1). Most of the particles were smaller than 1 μm in the lateral dimensions. Because of the presence of residual epoxide and hydroxide sites, this material was referred to as a functionalized graphene sheet (FGS). The FGS used in this study was prepared by researchers at Princeton University.
The FGS dispersed well in an epoxy matrix without the need for additional functionalization, as evidenced by the uniform dispersion displayed in the transmission electron microscopy image. This was attributed to the presence of residual epoxy and hydroxy sites on the graphene sheets.
Transmission electron microscopy image of epoxy composite with 0.50-wt% as-received FGS.
Despite some of the mechanical property improvements expected by the incorporation of nanoscale fillers, dispersion of a rigid nanoparticle in a resin matrix often reduces the toughness of the system, as has been observed with phyllosilicate-reinforced nanocomposites (ref. 2). The toughness of the FGS-epoxy nanocomposites was measured as the energy required to break the tensile specimens (see the table). This value was calculated by the area under the load displacement curve following tensile tests.
| FGS sample loading, wt% |
Energy to break, N-m | ||
|---|---|---|---|
| Unmodified FGS |
Carbon 18 (18C -modified FGS |
10-percent excess amine in epoxy formulation |
|
| 0 | 0.875±0.27 | 0.875±0.27 | 0.314±0.11 |
| 0.10 | 0.914±0.18 | 0.724±0.11 | 0.981±0.02 |
| 0.25 | 0.898±0.10 | 0.725±0.06 | 0.804±0.11 |
| 0.50 | 1.000±0.10 | 0.395±0.19 | 1.015±0.56 |
Nanocomposites prepared with 10-percent excess amine in the epoxy displayed a significant increase in toughness over that of the comparable neat resin, also containing excess amine. This is due to two possible mechanisms: better interfacial bonding and optimized epoxy amine ratios in the composite formulation. Covalent bonding between excess amine and the graphene would strengthen the filler-matrix interface. In addition, without excess amine the surface epoxides on the FGS unbalance the optimum stoichiometry of the epoxy reactions. Addition of excess amines actually contributes to returning the epoxy matrix properties to their optimal values; however, 10-percent excess amine may not be the appropriate amount.
The FGS-reinforced resin nanocomposites displayed up to a 40-percent reduction in the coefficient of thermal expansion. Addition of the rigid particles resulted in restricted polymer chain motion near the particle interface and,therefore, an enhanced dimensional stability (ref. 3).
These results suggest that FGS-epoxy nanocomposites could be suitable materials for use in cryogenic propellant tanks. Reduced resin CTE and improved toughness are desirable for this application since they can help to mitigate thermal-cycling-induced microcracking, which has been a major impediment to the use of composites in cryotanks.
This work was performed by the NASA Glenn Research Center in collaboration with Professors Ilhan Aksay and Robert Prud’homme, as well as Dr. Douglas Adamson at Princeton University, supported from the NASA University Research, Engineering, and Technology Institute on Bio Inspired Materials (BIMat) under Award No. NCC-1-02037.
Last updated: December 14, 2007
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