The phase behavior of rigid rods in an anisotropic mean field with applications to carbon nanotubes in nematic liquid crystals

Flexoelectric coefficients enhancement via doping carbon nanotubes in nematic liquid crystal host

Flexoelectric coefficients of carbon nanotube (CNT) doped nematic liquid crystals (NLCs) are studied based on the Helfrich theory. Weak and hard anchoring conditions between the NLC molecules and CNTs are considered. The volume fraction of the CNTs in nematic host is assumed to be low, which makes nanotubes aggregation phenomena negligible. Also, the length of doped CNTs is assumed to be lower than 10μm, so these rigid rods with low concentration cannot possess any flexoelectric polarization by themselves, only their presence modifies the flexoelectric coefficients of the NLC system. The Landau-de Gennes theory is used to calculate the order parameter changes in the medium. Also, the numerical density definition is renewed in the presence of nanotubes. It is shown that in the nematic phase the flexoelectric coefficients increase along with the increase of the coupling strength and temperature. The enhancement in flexoelectric coefficients is more significant in hard anchoring conditions than in the weak anchoring case. The flexoelectric coefficients increase up to 5-fold is calculated near the phase transition temperature, which is in good accordance with the experimental reported data.

Liquid crystallinity of carbon nanotubes

In this review, we first briefly recapitulate the orientation characteristics of liquid crystalline carbon nanotubes (CNTs), emphasizing their inherent properties. Both the high Young's modulus and the strong attractive interaction between them make the liquid crystallinity apt to show splay deformations (splay defects). It is these defects that often produce apparent low-order structures for long and deformed nanotubes. However, the application of doping, shearing, magnetic or electric fields will be efficient routes toward highly ordered CNT assemblies from such defects. Then, we describe the electrical behavior of CNTs in the electric field, which combines desirable features of the CNTS with those of classical liquid crystals (LCs). An electric field will generate an induced dipole moment on CNTs and align them in the field direction, minimizing the dipolar energy. Finally, we review the potential application of CNTs in the area of liquid crystal displays (LCD). In the LC cell unit, CNTs as dopants in LC layers can have compatible stability with LCs, with the orientation consistent and with surprising complementary advantages. And also CNT films as nanostructured electrodes can substitute ITO electrodes in the LC cell unit, exhibiting a strong electrical anisotropy due to their excellent axial conductivity. Furthermore, CNT films as an alignment layer have the potential to replace the traditional PI film, aligning LC molecules effectively along the direction of the nanotubes. Besides, CNTs acting as polarizers can absorb or transmit incident light when the electric vector propagates parallel or perpendicular to the nanotube axis. All of these applications demonstrate that CNTs in LC ordering will effectively improve the performance of materials and their related devices. Thus, we should improve the ordering of CNT assemblies as far as possible, which is critical to make full use of their exceptional axial properties and further to develop novel materials and applications successfully.

Magnetic field induced orientational transitions in liquid crystals doped with carbon nanotubes

We propose a continuum theory of orientational phase transitions induced by an external magnetic field in a suspension of carbon nanotubes in a nematic liquid crystal. It is shown that in a magnetic field a non-uniform and two different uniform phases are possible in the suspension. The uniform phases of the suspension differ by the type of orientational coupling of nanotubes with the liquid crystal matrix (the planar type when the nanotubes are oriented along the matrix director, and the homeotropic type when the nanotubes are perpendicular to the director). The possibility of a redistribution of the nanotube concentration (segregation effect) is shown. The fields of orientational transitions between uniform and non-uniform phases of the suspension are found analytically. It is shown that, when the nanotubes are weakly coupled to the matrix, the magnetic field induces reentrant transitions (uniform planar phase-non-uniform phase-uniform homeotropic phase-non-uniform phase). These transitions can be of first or of second order depending on the carbon nanotubes segregation intensity.