Preparation And Properties Of Polymer Dispersion Liquid Crystal Systems Doped By Sulfhydryl-functionalized Micro And Nano Particles

Polymer dispersed liquid crystal（PDLC）is a smart composite film that the liquid crytal（LC）droplets randomly dispersed and embedded in the polymer matrix that acted as a continuous phase.The combination of the two gives PDLC both the unique optical properties of LC and the mechanical strength of polymer.In consideration of their advantages,such as alignment layer-free and simplification of device fabrication,these composites have been found the applications in three-dimensional displays,sensors,and smart windows,just name a few.Unfortunately,higher voltages are generally needed to drive these devices owing to the large content of polymers and the strong anchoring effect on mesogenic molecules,which not only induces the danger of use but also increases the energy consumption of the devices.Therefore,the development of PDLC films with a low driving voltage has been a prerequisite for their applications.Introducing nanoparticles that have special physicochemical properties into PDLC is a simpe and low-cost technique to enhance the properties of the film.However,inorganic or metal nanopartilces are usually less compatibile with liquid crystal/polymer mixture and thus are highly susceptible to aggregation in thin films,which causes damage to the texture of liquid crystal and the electro-optical performance of PDLCs.Therefore,how to enhance the compatibility of micro-and nanoparticles in organic systems and their dispersion in PDLC films is an urgent issue to improve the electro-optical properties of PDLC films doped with nanoparticles.Based on the above-mentioned research background,herein we developed a highly compatible micro-nanoparticle/liquid crystal/polymer composite system where the nanoparticles could be chemically bond to polymer matrix via photo induced thiol-ene polymerization between thiol groups modified on the surface of nanoparticles and acrylate monomers.Moreover,a series of low-energy-consumption flexible films are achieved after through formulation design.More detailed research contents are provided in the followings.First,a thiol polyhedral oligomeric silsesquioxane（POSS-SH）was prepared and introduced into the PDLC film.The effects of the concentration of POSS-SH and the preparation conditions on the electro-optical performance and the polymer morphology of the PDLC samples were systemically studied.The results indicated that the driving voltage was decreased dramatically by improving the content of POSS in the polymer matrix.The dramatic regulation effect on the electro-optical properties of PDLC samples was contributed to the weakened anchoring effect of the polymer matrix containing POSS microstructure because of the lower surface energy as well as the increased steric repulsions between LC molecules and polymer surfaces.The PDLC sample with a low driving voltage(threshold voltage(V_th)23.8 V,saturation voltage(V_sat)32.9 V)below the safe voltage（<36 V）can be achieved by controlling the content of POSS in the polymer matrix preparation conditions,such as UV light intensity and polymerization temperatures.Subsequently a low driving voltage but high contrast ratio flexible film was fabricated by using zwitterion-treated ITO substrates via photoinitiated polymerisation-induced phase separation method.Zwitterion-treated LC devices were also fabricated using different processing methods.Both the addition of zwitterionic layers and the processing techniques have profound effects on the electric-optical performance of the resultant devices.The introduction of the zwitterionic moiety into the polymer matrix not only reduced the resistivity of the polymer matrix,but also had an obvious effect on the molecular orientation of LCs.Besides,not only the driving force from the electric field but also the dipole-dipole force from zwitterionic moieties help the LC molecules to overcome the anchoring force from the polymer matrix Under a voltage.We inferred that all the factors mentioned above may lead to the dramatic evolution in electric-optical performance for the zwitterion-treated PDLC samples.The experimental results confirm that the driving voltage of the devices is optimized to 14 V and the contrast ratio is still above 200.Owing to the low driving voltage and large contrast ratio,the flexible smart film not only can be applied in lowenergy-consumption smart windows but is also used for wearable devices.On the other hand,to enhance the mechanical properties of the trans-liquid crstal/polymer composite films,a flexible reverse mode PDLC film with low driving voltage was fabricated based on the POSS-SH/liquid crstyal/epoxy monomers system.Taking advantage of the different curing rate of the click chemistry between thiol and alkene group and the cationic polymerization between epoxy groups,we designed a star-shaped polymer consisting of s giant POSS core and tough epoxy chains linked by small bifunctional molecules with acrylate and epoxy groups.The introduction of the POSS with a cage-like nanostructure could optimize the electro-optical properties of the liquid crystal/polymer composite film,and the epoxy monomers were selected to obtain satisfactory mechanical performances of the polymer network.The results indicated that the flexible film could be fully driven at an electric field of 25.0 V.In addition,the reliability test results showed that the as-made device could be reversibly switched more than 500 times between the on-state and off-state.Especially,flexible films can be obtained by replacing the substrates with flexible ones,such as polyethylene terephthalate.And the electro-optical properties can be maintained even at bending state owing to the excellent mechanical properties.To extend the method of improving system compatibility to functional nanoparticles and further enhance the optical modulation performance of the PDLC films,Si O₂-SH nanoparticles with thiol groups on the end were fabricated and then was used to fabricate Cs_xWO₃/Si O₂-SH nanoparticles with infrared shielding properties.The optical modulation effect of PDLC film covers the solar irradiation in the range of 400-2500 nm after doped with the Cs_xWO₃/Si O₂-SH nanoparticles.In this part,the dispersibility of Si O₂and Si O₂-SH nanoparticles in the film was analyzed,and the effects of the two nanoparticles on the microstructure and electro-optical properties of PDLC films were systematically investigated.In addition,the dispersibility of Cs_xWO₃and Cs_xWO₃/Si O₂-SH nanoparticles in the film and the effects of the two nanoparticles on the microstructural and electro-optical properties of PDLC were also systematically investigated.The film doped with Cs_xWO₃/Si O₂-SH nanoparticles could effectively reject about 90%of the NIR irradiation in the light scattering state and 70%in the transparent state.Moreover,the indoor and outdoor simulation experiment results demonstrated the thermal insulation properties of the as-made film.