Analysis of physical properties of nanomaterials-polymer dispersed liquid crystal composites

Abstract

In the modern era, dispersed liquid crystals particularly polymer dispersed liquid crystal materials play an important role in flexible displays, electronic glasses, sen sors and smart windows due to its unique features and technological importance. In this thesis, we have investigated the impact of various nanostructured materials on the morphological behaviour, electro- optic, optical and dielectric properties of pol ymer dispersed liquid crystals prepared using FLC/NLC, polymer and nanomaterials. The thesis is divided into seven chapters which include the introduction, literature survey, experimental techniques, results and discussion and finally the summary and conclusion. Chapter 1 provides a comprehensive overview of LCs, its features and characteristic properties. It delves into their classification, spanning lyotropic, thermotropic, and polymeric types and its phases. Focusing on the core of the thesis, attention turns toward polymer-dispersed liquid crystals (PDLCs), comprising low-molecular weight liquid crystals dispersed within an isotropic polymer matrix. The chapter in vestigates diverse preparation methods for PDLCs while considering factors influ encing phase separation and mesomorphic behaviour. Emphasis is also placed on the understanding of LC droplet configurations in different conditions and factors. In last practical applications of liquid crystal dispersions are also presented and discussed. Chapter 2 encapsulates a comprehensive summary of prior research work carried out by various research groups in dispersed liquid crystals. It offers insights into his torical aspects, portraying the evolution of this field. The chapter highlights diverse approaches adopted to enhance conventional PDLCs properties. It provides a de tailed review of nanoparticle-doped PDLC composites, doped with SiO2, TiO2, Sb2O5, Au, Ag, ZnO, MgO, CuO, BTO, BaTiO3 and Fe3O4. Emphasis is placed on the extensive work conducted to improve the physical properties of these compo sites. Despite significant advancements, remaining questions persist regarding the precise role and impact of nanoparticle dispersion on PDLCs properties. The aim of thesis is to address these unanswered questions, directing its focus in Chapter 4-7 towards unveiling insights into the relationship between nanoparticle dispersion and PDLC properties. Chapter 3, describes the physical properties of LCs, polymers and other material's used in the work. The detailed fabrication process of PDLC and nanoparticle-PDLC composites are also described. The specific procedures employed for preparing these composites and constructing LC sample cells are presented. The chapter elaborates on the instrumental aspects, providing comprehensive details regarding the equip ment and setups utilized throughout the experimentation. It sheds light on the charac terization techniques employed for diverse analyses, including textural investigation, electro-optical measurements, investigation of switching behavior, and dielectric characterization. Chapter 4 investigates citrate buffer-stabilized gold nanoparticles (AuNPs) doped PDLC composites prepared by polymerization-induced phase separation techniques. Study examines the influence of voltage and temperature on liquid crystal droplet configurations, transmission, permittivity, conductivity, and transition temperatures in AuNPs-doped PDLCs. The inclusion of AuNPs in PDLC composites resulted in a notable enhancement, showing a 22% increase in relative transmission and a 54% improvement in contrast compared to undoped PDLC. The threshold voltage (Vth) in the doped composite decreased by 13.34% relative to the undoped, and the effect on Vth and transmission was minimal with increasing AuNP concentrations. Interesting ly, lower AuNP concentrations demonstrated better optical responses. LC droplet sizes, ranging from 35 to 53 μm, appeared to be unaffected by voltage or temperature changes. Beyond the threshold voltage, a distinct Maltese-type LC droplet formation occurred. Photoluminescence spectra indicated higher intensity in AuNPs (1.5 μl)- doped sample, with a blue shift observed in the 3.0 μl AuNPs-doped sample com pared to others. Furthermore, an increase in AuNPs concentration led to an increase energy bandgap, reaching approximately 3.4 eV at higher concentrations. Chapter 5 proposes smart windows based on PDLCs, employing a thiol-ene-based UV-curable photopolymer and nematic liquid crystals (E7) and doped with varying concentration of alumina nanowires (ANWs) to assess their impact on physical properties. The inclusion of ANWs in PDLC nanocomposites notably increased the nematic to isotropic transition temperature. Doping with ANWs led to a substantial reduction in threshold and saturation voltages at specific concentrations in PDLC. Dielectric anisotropy decreased with temperature and ANWs doping across all sam ples. Electric field-induced outcomes highlight the potential of the proposed ANWs-doped PDLC composite film, operating at low voltage and improved transparency. These findings offer insights for the development of rapid-switching smart windows and energy-efficient devices, leveraging the advantageous properties of ANWs doped PDLC composites. Chapter 6 In this chapter porous carbon nanoparticles (PCNPs) derived from mag nolia champaca seed pods were synthesized and investigated for their impact on PDFLC properties. The study explores the influence of varying PCNP concentrations (≤0.75 wt. %) on PDFLC mesomorphic properties, polarization, and permittivity within thin sample cells. The temperature-dependent electro-optic and dielectric properties, in SmC* phase and around the transition temperature of SmC*-SmA* are given. Increasing PCNP doping show an effect on spontaneous polarization and an choring energy coefficients without affecting phase transition temperatures across all samples. The enhanced electro-optic and dielectric properties observed in these composites suggest promising applications in displays, sensors, and optical devices. These findings stimulate further exploration and utilization of these functional mate rials in advanced electronic and photonic technologies. Chapter 7 provides a comprehensive summary of the work done, on study the ef fects of citrate buffer-stabilized gold nanoparticles, alumina nanowires, and porous carbon nanoparticles on the morphological, phases, phase transitions, electro-optic, and dielectric parameters of PDLCs. A significant effort is devoted to understanding the roles and interaction mechanisms within dispersed LCs systems. The inclusion of NMs demonstrates a substantial impact on customizing, altering, and fine-tuning the physical properties of PDLCs. Lastly, this chapter and the thesis conclude by shedding light on potential avenues for future research and exploration in this field.