Concluding Remarks

Results

Experimental Research on Polymer Nanocomposites [1-5]

• Design of Experiments

• Formulation

• Fabrication

• Testing

• Characterization (TEM and SEM)

• Response Surface Modeling

• Statistical Analysis

Computational Research on Polymer Nanocomposites [6,7]

• Molecular Dynamics Simulation

• Materials Studio

• VGCNF/VE Thermoset Nanocomposite

• Implication for Interphase Formation

• Different styrene/VE monomer ratios

• Surface Chemistry Effects

Computational Research on Potential Development [8]

• Reactive Potential for Polymers Based on the Modified Embedded-

Atom Method (MEAM) for Metals

• New bond order formalism in MEAM to handle unsaturation

• Parameterization for CHON system of elements

Current and Prior Research

Objectives and Abstract

Computational Formalism Results

The objective of this work is to extend the computational formalism of the modified embedded-atom method

(MEAM), developed by Baskes in 1992 for metals and metal alloys, to organic molecules, specifically

saturated hydrocarbons. The formalism and parameterization will be extended bin a future work to oxygen-

and nitrogen-containing organic/inorganic compounds, polymers, and multi-component multi-element

material systems involving interfaces between dissimilar materials, such as polymer/metal systems. MEAM

is a modification to the original embedded-atom method (EAM) developed by Daw and Baskes in 1984, that

includes a formalism for covalent materials (directional bonding), such as silicon and silicon-germanium

alloys.

We developed an interatomic potential for saturated hydrocarbons using the modified embedded-atom

method (MEAM), a reactive semi-empirical many-body potential based on density functional theory and pair

potentials. We parameterized the potential by fitting to a large experimental and first-principles (FP)

database consisting of 1) bond distances, bond angles, and atomization energies at 0 K of a homologous

series of alkanes and their select isomers from methane to n-octane, 2) the potential energy curves of H2,

CH, and C2 diatomics, 3) the potential energy curves of hydrogen, methane, ethane, and propane dimers,

i.e., (H2)2, (CH4)2, (C2H6)2, and (C3H8)2, respectively, and 5) pressure-volume-temperature (PVT) data of a

dense high-pressure methane system with the density of 0.5534 g/cc. We compared the atomization

energies and geometries of a range of linear alkanes, cycloalkanes, and free radicals calculated from the

MEAM potential to those calculated by other commonly used reactive potentials for hydrocarbons, i.e.,

second-generation reactive empirical bond order (REBO) and reactive force field (ReaxFF).

MEAM reproduced the experimental and/or FP data with accuracy comparable to or better than REBO or

ReaxFF. The experimental PVT data for a relatively large series of methane, ethane, propane, and butane

systems with different densities were predicted reasonably well by MEAM. Although the MEAM formalism

has been applied to atomic systems with predominantly metallic bonding in the past, the current work

demonstrates the promising extension of the MEAM potential to covalently bonded molecular systems,

specifically saturated hydrocarbons and saturated hydrocarbon-based polymers. The MEAM potential has

already been parameterized for a large number of metallic unary, binary, ternary, carbide, nitride, and

hydride systems, and extending it to saturated hydrocarbons provides a reliable and transferable potential

for atomistic/molecular studies of complex material phenomena involving hydrocarbon-metal or polymer-

metal interfaces, polymer-metal nanocomposites, fracture and failure in hydrocarbon-based polymers, etc.

The latter is especially true since MEAM is a reactive potential that allows for dynamic bond formation and

bond breaking during simulation. Our results show that MEAM predicts the energetics of two major chemical

reactions for saturated hydrocarbons, i.e., breaking a C-C bond or a C-H bond, reasonably well. However,

the current parameterization does not accurately reproduce the energetics and structures of unsaturated

hydrocarbons and, therefore, should not be applied to such systems.

Development of a Reactive Semi-Empirical Potential for the Atomistic/Molecular Simulations of Damage

and Failure in Polymeric and Polymer/Metal Material SystemsSasan NouranianCenter for Advanced Vehicular Systems (CAVS), Mississippi State University, MS 39759, USA

Collaborators

Michael I. Baskes

Mark A. Tschopp

Steven R. Gwaltney

Mark F. Horstemeyer

Funding: Department of Energy

References

1. Abuomar, O., Nouranian, S., King, R., Bouvard, J.-L., Toghiani, H., Lacy, T.E., Pittman, Jr., C.U. “Data

Mining and Knowledge Discovery in Materials Science and Engineering: A Polymer Nanocomposites Case

Study.” Advanced Engineering Informatics, Pre-published online on September 5, DOI:

10.1016/j.aei.2013.08.002 (2013).

2. Lee, J., Nouranian, S., Torres, G.W., Lacy, T.E., Toghiani, H., Pittman, Jr., C.U., DuBien, J. L.

“Characterization, Prediction, and Optimization of Flexural Properties of Vapor-Grown Carbon

Nanofiber/Vinyl Ester Nanocomposites by Response Surface Modeling.” Journal of Applied Polymer Science

130(3) (2013): 2087-2099.

3. Nouranian, S., Toghiani, H., Lacy, T.E., Pittman, Jr., C.U., DuBien, J. L. “Response Surface Predictions of

the Viscoelastic Properties of Vapor-Grown Carbon Nanofiber/Vinyl Ester Nanocomposites.” Journal of

Applied Polymer Science 130(1) (2013): 234-247.

4. Torres, G.W., Nouranian, S., Lacy, T.E., Toghiani, H., DuBien, J., Pittman, Jr., C.U. “Statistical

Characterization of the Impact Strengths of Vapor-Grown Carbon Nanofiber/Vinyl Ester Nanocomposites

Using a Central Composite Design.” Journal of Applied Polymer Science 128(2) (2013): 1070-1080.

5. Nouranian, S., Toghiani, H., Lacy, T.E., Pittman, Jr., C.U., DuBien, J. “Dynamic Mechanical Analysis and

Optimization of Vapor-Grown Carbon Nanofiber/Vinyl Ester Nanocomposites Using Design of Experiments.”

Journal of Composite Materials 45(16) (2011): 1647-1657.

6. Jang, C., Nouranian, S., Lacy, T.E., Gwaltney, S.R., Toghiani, H., Pittman, Jr., C.U. “Molecular Dynamics

Simulations of Oxidized Vapor-Grown Carbon Nanofiber Surface Interactions with Vinyl Ester Resin

Monomers.” Carbon 50(3) (2012): 748-760.

7. Nouranian, S., Jang, C., Lacy, T.E., Gwaltney, S.R., Toghiani, H., Pittman, Jr., C.U. “Molecular Dynamics

Simulations of Vinyl Ester Resin Monomer Interactions with a Pristine Vapor-Grown Carbon Nanofiber and

Their Implications for Composite Interphase Formation.” Carbon 49(10) (2011): 3219-3232.

8. Nouranian, S., Tschopp, M.A., Gwaltney, S.R., Baskes, M.I., Horstemeyer, M.F. “An Interatomic Potential

for Saturated Hydrocarbons Based on the Modified Embedded-Atom Method.” arXiv Preprint:1305.2759v3

[physics.chem-ph] (August 30, 2013).

We have successfully developed a new semi-empirical many-body potential for saturated hydrocarbons

based on the modified embedded-atom method (MEAM). The potential parameterization was performed

with respect to a large database of atomization energies, bond distances, and bond angles of a homologous

series of alkanes and their isomers up to n-octane, the potential energy curves of H2, CH, and C2, (H2)2,

(CH4)2, (C2H6)2, and (C3H8)2 and the pressure-volume-temperature (PVT) relationship of a dense

methane system. The new potential successfully predicts the PVT behavior of representative alkane

systems at different densities and temperatures. Furthermore, MEAM predicts the energetics and

geometries of the methane and ethane molecules undergoing bond-breaking reactions reasonably well. The

significance of this work is in the extension of the classical MEAM formalism for metals and metal hydride,

carbide, and nitride systems to saturated hydrocarbons. This is the first step toward its universality for all

atomic and molecular systems. The main benefit of using this potential versus other potentials for various

atomic and molecular dynamics simulation studies is its vast parameter database for metals. This makes it

possible, for example, to study complex polymer-metal systems using the same formalism for both metals

and organic molecules. In addition, MEAM is inherently linear scaling, making possible simulations on very

large systems. Since MEAM is a reactive potential, numerous possible simulation studies of reactive

organic/metal systems as well as void and crack formation and growth in polymer systems are envisioned.

Future Work

Total Energy of a system of atoms:

Embedding function:

Background electron density decomposed into spherically symmetric and angular partial electron densities:

Combination of partial electron densities to give to total background electron density:

Atomic electron density:

Pair interaction term:

Universal equation of state:

Screening factor:

MEAM parameters for C and H;

MEMA parameters for the CH diatomic:

Atomization energies at 0 K:

C-H and C-C bond distances in select alkanes:

H-C-H, H-C-C, and C-C-C bond angles in select alkanes:

Potential energy/interaction energy curves:

Bond breaking reactions (damage and failure):

Molecular dynamics simulations (PVT behavior) of select alkanes:

1. The current computational formalism of the MEAM potential will be modified further to account for bond

order and, hence, unsaturation in organic/inorganic molecules.

2. The modified MEAM potential will be parameterized for the CHON system of elements as well as

halogens.

3. The modified MEAM potential will be parameterized for select metal/organic material systems, such as

titanium/polyethylene, and the resulting potential will be used in the molecular dynamics simulations to

elucidate certain interfacial properties of these hybrid materials.