Preparation And Properties Of High Brightness Magnetic Photonic Crystal Inks

One-dimensional magnetically responsive photonic crystals（MRPCs）are capable of swiftly and reversibly tuning structural color in a wide visible spectrum by changing magnetic field strength.They show promising potential applications in the fields of sensing,anti-counterfeiting,self-adaptive camouflage,information encryption and fluorescence enhancement.However,the developed MRPCs still have following challenges:Firstly,the colloidal particles which work as“building blocks”of MRPCs generally have coating shells of hydrophilic substances,such as silica（SiO₂）,carbon（C）,polyvinyl pyrrolidone（PVP）and polyelectrolytes.They cannot be dispersed in oily resins such as oily acrylate resins,oily epoxy resins and oily polyurethane resins,which are commonly used in coatings,inks,adhesives and related fields.Thus,the application of MRPCs in the above fields are severely hindered.Secondly,the colloidal particles of MRPCs suffer from very dark and even close to black intrinsic color,resulting in the strong absorption upon visible light.As a result,the MRPCs demonstrate low reflectivity,reducing the discernibility of structural colors by naked eyes under natural illumination.In order to address the above problems,this thesis firstly explored selective concentration polymerization of lipophilic monomers on the surface of Fe₃O₄@PVP magnetic colloidal particles in microheterogeneous dimethyl sulfoxide–water（DMSO–H₂O）binary solvents to prepare Fe₃O₄@PVP@Polymer magnetic colloidal nanoparticles.The as-prepared Fe₃O₄@PVP@Polymer magnetic colloidal nanoparticles can be well dispersed in oily epoxy resins and exhibit a remarkable structural color under an external magnetic field.Secondly,a strategy to enhance the brightness of MRPCs based on Fe₃O₄@PVP and Fe₃O₄@PVP@Polymer particles was developed via an oxidation with a hydrogen peroxide solution.It was found that after the oxidation of Fe₃O₄@PVP and Fe₃O₄@PVP@Polymer particles with a hydrogen peroxide,the intrinsic chemical color of the particles became light-colored.Correspondingly,their assembled MRPCs show significantly enhanced reflectivity as well as brightness of structural colors.Consequently,a high-performance photonic crystal ink was obtained by dispersing the Fe₃O₄@PVP@Polymer magnetic colloidal nanoparticles after hydrogen peroxide treatment into an oily resin.The ink can be used to screen-print a magnetic photonic crystal film with a specific pattern and heterochromatic color by using a magnetic-field-assisted light curing technology,which has important potential applications in the field of anti-counterfeiting.The main work of the thesis are as follows:（1）Fe₃O₄@PVP@Polymer magnetic nanoparticles with regulated lipophilic shells were prepared in a DMSO–H₂O binary solvent.By this strategy,lipophilic diacrylate monomers tend to be concentrated in the vicinity of Fe₃O₄@PVP particles in DMSO–H₂O binary solvents with volume ratio of 870:130.Subsequently,redox initiators can initiate the interface polymerization of monomers around Fe₃O₄@PVP particles,resulting in the formation of lipophilic Fe₃O₄@PVP@Polymer magnetic colloidal particles.The parameters influencing the lipophilic shell of Fe₃O₄@PVP@Polymer were thoroughly investigated including the solvent ratio,temperature,and the content of monomers or initiators.（2）The brightness of structural colors for the photonic crystal based on magnetic colloidal nanoparticles was significantly improved by oxidation with hydrogen peroxide.In our protocol,when Fe₃O₄@PVP and Fe₃O₄@PVP@Polymer colloidal particles were treated with a certain concentration of hydrogen peroxide-ethanol mixed solution,the Fe₃O₄could catalyze the decomposition of hydrogen peroxide into the oxygen and hydroxyl radicals,which converted Fe²⁺ions in Fe₃O₄into Fe³⁺ions.As a result,magnetic particles containingγ-Fe₂O₃were obtained,which show weakened absorption to light.At the same time,the produced hydroxyl radicals also attacked the pyrrole rings in the PVP to form carboxyl groups.This enhances the electrostatic repulsion between the colloidal particles.The factors influencing the optical properties of the colloidal particles were investigated in detail such as the amount of hydrogen peroxide,the amount of Fe₃O₄@PVP colloidal particles,the reaction time and the sizes of particles.Based on the above results,a magnetic photonic crystal ink with high-brightness was obtained based on the oxidation of Fe₃O₄@PVP@Polymer magnetic particles with hydrogen peroxide,which promoted the application of magnetic photonic crystals in the field of anti-counterfeiting.