Liquid Crystal-Anisotropic Nanoparticles Mixtures

Abstract

Carbon nanotubes (Iijima, 1991) are one of the most interesting new materials which emerges during the last twenty years. They either consist of a single sheet of carbon atoms covalently bonded in hexagonal arrays rolled up into a cylinder with a diameter of about one nanometer (single-walled nanotubes SWNTs), or are built up of multiple carbon sheets producing rods with diameters ranging from a few to tens or even hundreds of nanometer (multi-walled nanotubes MWNTs). The aspect ratio of these objects can vary from hundred to many thousands. Most of CNTs extraordinary properties of potential use in various applications could be realized in relatively well aligned samples. One method of alignment consists in dispersing the CNTs into a nematic phase of a LC (either thermotropic or lyotropic). For recent reviews see e.g., (Lagerwall & Scalia, 2008; Rahman & Lee, 2009; Zakri, 2007; Zhang & Kumar, 2008). Theoretically, equilibrium orientation of a single elongated particle immersed in a nematic LC phase is rather well explored (Andrienko et al., 2002; 2003; Brochard & de Gennes, 1970; Burylov & Raikher, 1990; 1994; Hung et. al., 2006). Continuum theory predicts alignment of the particle’s longer axis along the nematic director −→n for different types of boundary conditions in the strong anchoring limit case (Brochard & de Gennes, 1970; Burylov & Raikher, 1990; 1994). On the contrary, in the weak anchoring limit the particle may orient either along or perpendicular to the nematic director depending on the boundary conditions (Burylov & Raikher, 1990). The collective behavior of CNTs dispersed in isotropic solvents or in LC is theoretically relatively weakly explored. Due to their structure and behavior, the CNTs can be consider essentially as rigid-rod polymers with a large aspect ratio (Green et. al., 2009). The steric theory for the electrostatic repulsion of long rigid rods has been used to investigate the SWNT phase behavior in their suspensions (Sabba & Thomas, 2004). Calculations have shown that SWNTs in a good solvent is analogous to the classic rigid-rod system if the van der Waals force between CNTs is overcome by strong repulsive interrods potentials. When the solvent is not good, the van der Waals attractive interactions between the rods are still strong and as a result, only extremely dilute solutions of SWNTs are thermodynamically stable and no LC 29