(190v) Computer Simulation of Carbon Nanotubes In Liquid Crystalline Solvents

Shivkumar Bale and Francisco R. Hung

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803

Liquid crystals (LCs) have been used in the past to control the orientation of carbon nanotubes (CNTs). If the CNTs are adequately dispersed in the LC, they will tend to align with their long axes parallel to the director field n(r) of the LC. Furthermore, and since the direction of n(r) can be easily modified by using electric and magnetic fields, flows and/or external surfaces, dispersion of CNTs in LCs provides an excellent way to manipulate and align nanotubes. However, one of the main challenges in using LCs to manipulate and align CNTs is on how to efficiently disperse the nanotubes in the LC. Suspensions of CNTs in LCs tend to be very unstable, and most of the nanotubes promptly aggregate. The key point to obtain high-quality dispersions of CNTs in LCs is to understand the interactions between the nanotubes and the LC molecules. Molecular simulations are uniquely positioned to investigate interactions at the molecular level of detail, as well as their effects on the macroscopic properties of the systems. Results from this research are relevant for potential applications of these systems in displays, nanoscale electronics, optical sensors, and in the formulations of composites with unique mechanical, thermal and/or electronic properties.

We performed molecular dynamics simulations of systems of CNTs in different LCs and determined potentials of mean force (PMF) in order to understand the molecular-level interactions between the CNTs and the LCs. The PMF measures the free energy required to keep the two nanotubes separated by a distance d, when they are immersed in a given LC solvent. In our research, we considered several representative LC molecules, such as two members of the 4-cyano-4’-n-alkylbiphenyl series, 5CB and 8CB, as well as 5CF (4’-(3,4-difluoro-phenyl)-4-pentyl-bicyclohexyl). We used single walled CNTs with armchair (n, n) configuration in our calculations. Other properties, such as local densities, radial distribution functions and scalar order parameters, are also reported. Our results indicate that the PMF minima for all systems is at the state where the two CNTs are very close (i.e., no LC molecules in between the CNTs). It is also observed that the features of the PMF strongly depend on the molecular structure of the LC molecules.