Electro-optic and dielectric responses in PDLC composite systems

Abstract

Due to the inherent optical and dielectric anisotropy of liquid crystals (LCs), there have been enormous advances in both our understanding and our ability to use them in applications. The property of liquid crystals that cause the director to respond to an applied electric field is the basis for most of the display related applications. When such LCs are incorporated/dispersed in a polymer matrix, the advantages of polymers such as processability, flexibility, mechanical and thermal stability, ease of handling are obtained. Polymer Dispersed Liquid Crystals (PDLC) have attracted the attention of scientists and technologists as they exhibit interesting electro-optical, thermo-optical, dielectric, physical and chemical properties. The electro-optical properties of such PDLC films depend upon several parameters such as viscosity of the components, refractive indices, elastic constants, anchoring energy, dielectric anisotropy etc.; wherein aggregation structure of the liquid crystal (LC) droplets in polymer matrix is one the important parameters. There have been continuous efforts to manipulate the LC droplet shape, size and distribution. To enhance their electro-optical properties, various methods of synthesis of PDLC films such as SIPS, PIPS and TIPS techniques have been explored to control LC droplet morphology along with incorporation of nano particles, dye molecules, CNT’s, etc. In the present chapter, the effect of addition of dichroic dye molecules on the morphology, shape, size and distribution of LC droplets and their correlation with the performance (contrast ratio, response time, threshold voltage etc.) of PDLC devices have been discussed. The theory and kinetics of droplet morphology and its impact on the mechanism of electro-optic and hysteresis is narrated. PDLC is a heterogeneous composite system in which LC droplets are embedded in polymer matrix and these components have different electrical conductivities. To understand the intermolecular interaction and co-operative process between the LC molecules and polymer matrix, dielectric relaxation spectroscopic technique has been employed. To depict the nature of the relaxation behaviour dielectric data was modelled using Debye and Cole-Cole theory.