Nematic liquid crystals doped with nanoparticles : phase behavior and dielectric properties

Thermodynamics and dielectric properties of nematic liquid crystals doped with various nanoparticles have been studied in the framework of a molecular mean-field theory. It is shown that spherically isotropic nanoparticles efectively dilute the liquid crystal material and cause a de- crease of the nematic-isotropic transition temperature, while anisotropic nanoparticles are aligned by the nematic host and, in turn, may sig- nifcantly improve the liquid crystal alignment. In the case of strong interaction between spherical nanoparticles and mesogenic molecules, the nanocomposite possesses a number of unexpected properties: The nematic-isotropic co-existence region appears to be very broad, and the system either undergoes a direct transition from the isotropic phase into the phase-separated state, or undergoes first a transition into the ho- mogeneous nematic phase and then phase-separates at a lower tempera- ture. The phase separation does not occur for sufficiently low nanopar- ticle concentrations, and, in certain cases, the separation takes place only within a finite region of the nanoparticle concentration. For ne- matics doped with strongly polar nanoparticles, the theory predicts the nanoparticle aggregation in linear chains that make a substantial contri- bution to the static dielectric anisotropy and optical birefringence of the nematic composite. The theory clarifies the microscopic origin of im- portant phenomena observed in nematic composites including a shift of the isotropic-nematic phase transition and improvement of the nematic order; a considerable softening of the first order nematic-isotropic tran- sition; a complex phase-separation behavior; and a significant increase of the dielectric anisotropy and the birefringence