Local field effects and the field-dependent dielectric response of polymer dispersed liquid crystals

An anisotropic version of the Maxwell Garnett approximation is applied for studying the dielectric properties of polymer dispersed liquid crystals containing bipolar liquid crystal droplets. This approach provides an explicit link between the droplet orientation distribution and the macroscopic response of the material. The electrostatic energy of the droplets is balanced with a strong anchoring elastic energy term for different initial orientation distributions. For aligned droplets we find a switching process whose sharpness depends on the initial orientation and a concentration dependent threshold field. For a planar distribution we find sharp transitions with a hysteresis loop whose width depends on the droplet concentration. For a random distribution the droplet reorientation is more gradual. The theory is also applied to the negligible elastic energy limit, recently observed at temperatures near the nematic-isotropic phase transition, where the droplets consist of bipolar nematic cores coated by isotropic liquid shells. This structural change within the droplets causes a considerable modification of the electro-optical properties. The Maxwell Garnett approach is used to calculate the dielectric response of this structure and reproduces all the main features of the experimental results.