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Support from the Petroleum Research Fund, administered by the American Chemical Society has been instrumental for conducting a number of simulation studies focused on graphene sheets nanocomposites. The entire award has been used to support one Ph.D. student. Two papers have been resulted from our work, published in highly recognized journals (Nano Letters and Applied Physics Letters). The results have been instrumental for securing continuous funding from the Department of Defense, and for preparing one other proposal that has been submitted to the Department of Energy.

 Graphene sheets, one-atom-thick layers of carbon atoms, are receiving enormous scientific attention because of extraordinary electronic and mechanical properties. These intrinsic properties will lead to innovative nano-composite materials that could be used to produce novel transistors and thermally-conductive polymeric materials. Such applications are currently hindered by the propensity of these nanoparticles to agglomerate when dispersed in aqueous and/or organic matrixes.

 In November 2006 we stated that ‘We are here interested in composites in which graphene sheets are dispersed within organic oils. Our goal is to control the composite thermal transport properties by manipulating the assembly of the graphene sheets. (…) We will assess the effective interactions between graphene sheets in oils, the structure of the fluid surrounding the graphene sheets, and the resistance to heat transfer from a graphene sheet to the surrounding matrix (Kapitza resistance)’. To reach those objectives one graduate student, Deepthi Konatham, has been supported by the ACS PRF grant starting in the summer of 2007, through the spring of 2009. The entire amount awarded ($40,000) has been used to support Ms. Konatham.

 During the first year of the project we published one paper, titled ‘Molecular design of stable graphene nano-sheets dispersions’. Deepthi and I reported the results of molecular dynamics simulations for pristine and functionalized graphene sheets of 54 and 96 carbon atoms each dispersed in liquid organic linear alkanes (oils) at room conditions. For the first time, our results showed that, although pristine graphene sheets agglomerate in the oils considered, graphene sheets functionalized at their edges with short branched alkanes yield stable dispersions. We characterized the simulated systems by computing radial distribution functions between the graphene sheets centers of mass, pair potentials of mean force between the graphene sheets in solution, and site-site radial distribution functions. The latter were used to determine the preferential orientation between approaching graphene sheets and the packing of the organic oils on the graphene sheets. Our results are useful not only for designing practical recipes for stabilizing graphene sheets in organic systems, but also for comparing the molecular mechanisms responsible for the graphene sheets aggregation to those that stabilize graphene sheets – containing dispersions, and for controlling the coupling between organic oils and graphene sheets used as fillers. We demonstrated that excluded-volume effects, generated by the branched architecture of the grafted functional groups, are responsible for the graphene sheets stabilization. Our results have inspired the experimental work of Fang et al. [M. Fang, H. Lu, Y. Yang, S. Nutt, Single Layer Graphene Nanosheets with Covalently Grafted Polymer Chains, submitted to JACS], who recently produced stable GS dispersions.

 During the second year of the project we have quantified the barriers to heat transfer between the graphene sheets and the surrounding oils. We conducted non-equilibrium molecular dynamics simulations for such purposes. The simulations have been performed as follows: the system composed by one graphene sheet immersed in n-octane molecules is first equilibrated at room conditions, and then the temperature of the graphene sheet is instantaneously increased to 500K. At this point the simulations are conducted in the NVE ensemble and the difference in temperature between the nanoparticle and the surrounding fluid, ΔT, is monitored as a function of time. As heat flows from the graphene sheet to the surrounding oil ΔT decreases to 0. The faster the decay, the lower the Kapitza resistance is. Our results have been summarized in the manuscript ‘Thermal Boundary Resistance at the Graphene-Oil Interface’. We demonstrated that it is possible to significantly reduce the Kapitza resistance at the graphene sheet – liquid octane interface by appropriately functionalizing the graphene sheets. The key concept is that the functional groups, to be effective, must show vibrational modes compatible with those of the organic matrix. Because functionalizing graphene sheets at their edges should not compromise their exceptional intrinsic thermal-transport properties, our results suggest a practical recipe for manufacturing high-thermal-transport polymeric nanocomposites. We are now taking advantage of these results to design, using multi-scale simulation techniques, graphene sheet – nylon 6/6 polymeric nanocomposites to replace metals in modern, energy-efficient engines.

 The results have been instrumental for securing support from the US Department of Defense. The proposal ‘’ has been recently awarded to a group of 7 faculty members at the University of Oklahoma, including Striolo. Within this project, Striolo’s responsibility will be the assessment of the Kapitza resistance at various carbon-based nanoparticles-polymer matrix interfaces. We have also recently submitted the proposal titled ‘’ to the Department of Energy to continue our studies on the subject.

 Two papers have resulted from this award:
D. Konatham and A. Striolo, Molecular Design of Stable Graphene Nano-Sheets Dispersions, Nano Letters 8 (2008) 4630.
D. Konatham and A. Striolo, Thermal Boundary Resistance at the Graphene-Oil Interface, Applied Physics Letters (2009) in Press.

 We have presented our results at several conferences:

 A. Striolo, Modeling Graphene Sheets Composites, NanoFocus and Bioenergy – Oklahoma EPSCoR Annual State Conference, Oklahoma City, OK, March 31^st-April 1^st, 2009.
D. Konatham and A. Striolo, Graphene Sheets-Oil Nanocomposites: Equilibrium and Transport Properties from Molecular Simulation, AIChE Annual Meeting, Nashville, TN, November 8^th-13^th, 2009.
D. Konatham and A. Striolo, Graphene Sheets-Oil Nanocomposites: Equilibrium and Transport Properties from Molecular Simulation, 17^th Symposium on Thermophysical Properties, Boulder, CO, June 21^st-26^th, 2009.
A. Striolo and D. Konatham, Graphene Sheets-Oil Nanocomposites: Equilibrium and Transport Properties from Molecular Simulation, MRS Fall Meeting, Boston, MA, December 1^st-5^th, 2008.
D. Konatham and A. Striolo, Graphene Sheets-Oil Nanocomposites: Equilibrium and Transport Properties from Molecular Simulation, AIChE Annual Meeting, Philadelphia, PA, November 16^th-21^st, 2008.