Optical And Dielectric Characteristics Study Of Polymer Dispersed Liquid Crystal Doped With Nanomaterials

The integration,miniaturization,low power consumption and high performance of liquid crystal devices are the development trends of basic scientific research and industrial applications.The emergence of polymer dispersed liquid crystals(PDLC)with excellent optoelectronic properties has promoted the development of liquid crystal devices.Among them,nanomaterial doping improves the electro-optical properties of PDLC and expands its related applications.Based on the doping of nanomaterials,this thesis reveals the Maxwell-Wagner interface effect between nanomaterials,liquid crystals and polymers through the characteristics of non-uniform dispersion and uniform dispersion of nanomaterials,as well as the modeling and analysis of microphysical mechanisms.The polymerization diffusion kinetic process and the dielectric relaxation process extend the theory of PDLC related optical and dielectric properties.The first theoretical analysis combined with experiments found that most of the nanomaterials are distributed in the liquid crystal(dark)area,and a small part are distributed in the polymer(bright)area,to achieve low threshold voltage and the best external electric field frequency electronic control performance;and for the first time to establish nanomaterial blending Hybrid holographic PDLC grating diffusion dynamic model;further reduce the threshold voltage and relaxation time of PDLC thin film devices,and expand the application of PDLC materials to ethanol gas sensing.This thesis specifically conducts research from the following three aspects.First,research the electro-optical properties of holographic PDLC doped with nanomaterials.1)Starting from the non-uniform dispersion of nano-silver particles in the holographic PDLC structure to reflect different electro-optical characteristics,a non-uniform medium equivalent loop model is established to study the frequency response characteristics of the grating threshold voltage.Analyzing the Maxwell-Wagner interface effect of nano-silver and polymer and nano-silver and liquid crystal,it is deduced that the threshold voltage of the grating will have a turning point with the increase of the electric field frequency.By changing the content of nano-silver particles in the liquid crystal region in the model,the change of the dielectric relaxation oscillation frequency of the grating was revealed,and the above theoretical analysis results were verified through electro-optical characteristic experiments.Studies have shown that most of the nano-silver particles will be distributed in the liquid crystal region,and a small amount will be distributed in the polymer stripes.When the frequency of the applied electric field is 5 k Hz and the nano-silver doping concentration is 0.05%,the threshold driving voltage of the grating is at least 1.2 V/μm.2)Scanning electron micrographs and transmittance spectra of holographic PDLC doped with multi-walled carbon nanotubes are analyzed.Studies have shown that multi-walled carbon nanotubes can improve the phase separation structure of the grating,can effectively promote the photoinitiator to absorb light energy,and then initiate chain growth to polymerize the prepolymer monomer.At the same time,an equivalent circuit model is established based on the Maxwell-Wagner effect doped with multi-walled carbon nanotubes.The study found that the frequency of the electric field at the turning point decreases as the concentration of multi-walled carbon nanotubes increases.This is obviously different from the behavior observed when doped with nano-silver particles.Through the comparison of simulation and experimental results,the mass distribution ratio of the multi-walled carbon nanotubes in the liquid crystal region is deduced,and most of them diffuse into the liquid crystal region,making the dielectric interaction in the liquid crystal region stronger.Multi-walled carbon nanotubes can effectively adjust the relaxation frequency of the grating,reduce the overall dielectric constant,and increase the size of liquid crystal droplets.Studies have shown that when the frequency of the applied electric field is 5 k Hz and the doping concentration is 0.05%,the diffraction efficiency can reach 91%,which is 40%higher than the undoped diffraction efficiency,and the driving voltage is 0.77 V/μm.The saturation voltage is 2.4 V/μm.Second,the diffusion kinetics of holographic PDLC doped with nanomaterials are studied.1)During the holographic exposure,the one-dimensional non-local diffusion dynamics model is derived through the multi-component interdiffusion method.The model is used to analyze the physical mechanism of micro-diffusion between monomers and liquid crystals,monomers and nanoparticles.It deduces the spatial distribution map of the concentration of each component,and analyzes the plasmon resonance effect of nanometals.Due to the changes in the effective refractive index modulation and absorption modulation caused by the movement of the concentration of each component substance with the exposure time at the bright and dark stripes.Combined with the coupled wave theory to simulate the curve of the diffraction efficiency(632.8 nm wavelength)of the holographic PDLC grating with the exposure time,and measure the diffraction efficiency of the grating in real time.By fitting the corresponding parameters of the model and limiting the boundary conditions,the simulation results are consistent with the experimental data.In addition,comparing the effects of nano silver particles with different doping concentrations and exposure time,the method of preparing holographic PDLC gratings with high diffraction efficiency is discussed.Third,the dielectric properties of PDLC doped with uniformly dispersed nanomaterials are studied.1)Analyzing the scanning electron micrograph and transmittance spectrum of the zinc oxide nanorods doped PDLC.It found that the zinc oxide nanorods can slow down the phase separation process of PDLC polymerization and increase the liquid crystal droplet size to the microscale.Increase the transmittance of the visible light waveband,because the increase in liquid crystal droplets leads to a decrease in the number,which affects the reduction of the overall reflection loss.2)Through the measurement of the dielectric spectrum(The wavelength band is 7×10~7m to300 m)of the PDLC doped with zinc oxide nanorods and the analysis of the Cole-Cole diagram and the HN function equation fitting analysis,it can cause interface polarization,dielectric constant and dielectric loss at low and high frequencies.Increase,and have high dielectric strength characteristics,reduce relaxation time.In the low frequency range,zinc oxide rods can increase the conductivity of PDLC by nearly 10times and affect the phase transition temperature of liquid crystals.Electro-optical characteristics experiments have proved that the zinc oxide nanorods can significantly reduce the PDLC threshold voltage 1.25 V/μm to 0.63 V/μm.3)According to the dielectric analysis of zinc oxide nanorod doped PDLC in ethanol gas,it has high sensitivity and good selectivity to ethanol molecules at a measurement frequency of 100Hz.The structure is equivalently fitted to the circuit components,and the influence of the ethanol polar gas on the equivalent circuit parameters of the PDLC is discussed.The film has the advantages of stable structure,easy manufacturing,high repeatability and high sensitivity.In order to solve the shortcomings of the current use of zinc oxide nanorods for sensing polar gases in terms of preparation cost,sensing structure,repeatability,etc.,the application of PDLC devices in ethanol sensing is expanded.This paper focuses on theoretical modeling and experimental analysis of the optical and dielectric properties of the nanomaterial-doped PDLC system,and proposes an equivalent circuit model,a diffusion dynamics model,and a dielectric relaxation model respectively.This work is suitable for studying nanomaterials to optimize the electro-optical properties of PDLC and reduce energy consumption,and to provide a reference for the preparation of holographic PDLC high-diffraction materials.Zero-dimensional and one-dimensional nanomaterials can be selected as dopants for sensing or electronically controlled holographic devices.Application,and promote its development in nanophotonics and other fields.