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

An anisotropic version of the Maxwell Garnett approximation is applied for studying the dielectric properties of polymer dispersed liquid crystals con taining bipolar liquid crystal droplets. This approach provides an explicit link between the droplet orientation distribution and the macroscopic resp onse of the material. The electrostatic energy of the droplets is balanced with a strong anchoring elastic energy term for different initial orientati on distributions. For aligned droplets we find a switching process whose sh arpness depends on the initial orientation and a concentration dependent th reshold field. For a planar distribution we find sharp transitions with a h ysteresis loop whose width depends on the droplet concentration. For a rand om distribution the droplet reorientation is more gradual. The theory is al so applied to the negligible elastic energy limit, recently observed at tem peratures near the nematic-isotropic phase transition, where the droplets c onsist of bipolar nematic cores coated by isotropic liquid shells. This str uctural change within the droplets causes a considerable modification of th e electro-optical properties. The Maxwell Garnett approach is used to calcu late the dielectric response of this structure and reproduces all the main features of the experimental results.