Preparation And Properties Of TiO₂ Nanoparticles/Liquid Crystal/polymer Composite Electrically Controlled Dimming Film

Liquid crystal/polymer composite systems can combine the properties of liquid crystal molecules and polymer materials,and thus have unique external field response characteristics,excellent mechanical properties and good processability,making them widely used in architectural or automotive dimming films,large-area flexible displays and sensing.Polymer Dispersed Liquid Crystal（PDLC）and Polymer Stabilized Liquid Crystal（PSLC）are two common liquid crystal/polymer composite systems.The PDLC system has a high polymer matrix content and the liquid crystals are dispersed in a porous polymer matrix in the form of microdroplets.The continuous distribution of the polymer matrix provides better mechanical properties and the system has better stability and can be processed in large areas using flexible substrates.The other PSLC system has a lower content of polymer matrix and is formed by polymerization of liquid crystal polymerizable monomers in liquid crystals after orientation.The polymer network formed by polymerization retains that orientation and plays a role in stabilizing the orientation of liquid crystal molecules in the film,and the liquid crystal molecules in the system can be reoriented through an external field to achieve the function of dimming.Since the liquid crystal in PSLC system has a variety of initial phase and orientation options,it can achieve a variety of different switching effects of the dimming function.Based on the above research background,this thesis prepares and investigates a series of composite dimming films with UV shielding function and excellent electro-optical performance for large area flexible processing by introducing TiO₂ nanoparticles after surface modification on the basis of two systems,PDLC and PSLC.First,two different silane coupling agents,APTS and MPTS,were used to modify the surface of TiO₂ nanoparticles,and the unmodified TiO₂ nanoparticles were introduced into the PDLC system respectively to prepare a series of PDLC dimming films with UV shielding function.The different dispersion effects of the unmodified,APTS and MPTS modified TiO₂ nanoparticles in the PDLC inner polymer matrix were compared and analyzed by SEM.On this basis,its influence on the electro-optical properties of PDLC dimming film was studied,and finally a composite PDLC dimming film with UV shielding function and excellent electro-optical performance was obtained.After testing,in the ultraviolet wavelength range of 200～400 nm,the ultraviolet shielding rate of the composite film can reach 95.94%and 99.16%when the electric field is applied and the electric field is not applied,respectively.In order to reduce the driving voltage of PDLC composite dimming films,this thesis proposes a two-step UV polymerization method to prepare polymer dispersion and polymer stabilization coexistence system（PD&SLC）dimming films by successively initiating the free radical and cationic photopolymerization reactions using the difference in the speed of the free radical photopolymerization and cationic photopolymerization reactions.Firstly,a liquid crystal vinyl ether monomer C4V was designed and synthesized for cationic polymerization,based on which a composite system containing cholesteric liquid crystal,acrylate monomer and liquid crystal vinyl ether monomer was designed to prepare PD&SLC dimming films with low driving voltage,simple process,good mechanical properties and fast response time by two-step UV polymerization method.In addition,the effect of liquid crystal vinyl ether monomer content on the electro-optical properties of PDS&LC films was verified and discussed.Compared with PDLC dimming films,the threshold voltage(V_th)and saturation voltage(V_sat)of PDS&LC dimming films were reduced from 43.3 V/88.7 V to 21.2 V/35.5 V,respectively.In order to obtain a composite dimming film with UV shielding function and excellent electro-optical properties,we prepared a TiO₂/PD&SLC composite dimming film using TiO₂ nanoparticles surface-modified by MPTS.Firstly,the coexistence structure of vertically oriented polymer network existing in the pores of the polymer network was constructed by two-step UV polymerization,and then the mechanical properties of PSLC,PDLC and PD&SLC composite films were comparatively studied.Subsequently,the dispersion behavior of TiO₂ nanoparticles in the polymer matrix of the coexisting system after surface modification by MPTS was characterized and analyzed.Finally,a series of PD&SLC composite dimming films with different contents of TiO₂ nanoparticles were also prepared and studied,and the UV-shielding composite dimming films with V_th and V_sat of13.22 V and 19.12 V,respectively,and a contrast ratio（CR）of 148.3 were obtained.On the other hand,in order to expand the application field of the composite dimming film,this paper proposes to use PD&SLC system to improve the light outcoupling efficiency of organic light-emitting diode（Organic Light-Emitting Diode,OLED）devices.In the experiment,the similar refractive index constant of PD&SLC film and OLED substrate material is designed to increase the critical angle at which the light inside the OLED device can exit the substrate surface,thereby improving the light outcoupling efficiency of the OLED device.Further,by utilizing the vertical orientation of liquid crystal molecules in the PD&SLC film,the emitted light with a large incident angle in the OLED device is selectively scattered,but the light with a small incident angle is not scattered,which further improves the light extraction efficiency of the OLED device.Finally,through the integrated design of the device,the PD&SLC film can improve the light output coupling efficiency of the OLED by 48.9%.