Construction And Optical Response Behavior Of Flexible And Free-standing PEGDA-based Photonic Crystal Films

Photonic crystal（PC）is an artificial microstructure formed by two or more constituent materials with different refractive indices.PCs have unique photonic band gap properties and distinct structural colors,thereby showing a wide range of potential applications in the optical waveguide,laser,optoelectronic communication,color display,printing,textile,color encryption and anti-counterfeiting fields.However,the practical application ranges of PCs have been severely restricted due to some problems,such as strong dependence on the substrate,high rigidity,no bright color and poor durability.Therefore,it is of great significance for promoting the long-term development of PCs to prepare PC materials with good flexibility,free-standing function,bright structural color and high durability.In this dissertation,through the selection of the material system and the design of the process scheme,the idea of using monodisperse SiO₂ microspheres and polymer polyethylene glycol diacrylate（PEGDA）composite was finally determined,and the new strategy of three steps was adopted to prepare flexible and self-supporting PC films with opal structure.In the first step,SiO₂ microspheres self-assemble into ordered structures in PEGDA precursor solution via an evaporation-induced self-assembly method,forming the SiO₂-PEGDA concentrate with bright structural color.In the second step,the SiO₂-PEGDA concentrate with structural color is transferred to the tiny gap formed by two glass slides via a shear-induced molding method.In the third step,the PEGDA prepolymer is cured by UV light to obtain a solid SiO₂-PEGDA composite PC film.After removing the glass slide substrates,the flexible and self-supporting PC film material is obtained.The good adhesion of the PEGDA polymer network to SiO₂ microspheres endows the SiO₂-PEGDA PC film with good durability and self-supporting characteristic.The high-volume fraction（60%-75%）of PEGDA polymer gives the SiO₂-PEGDA PC film high flexibility.In addition,the inverse opal（IO）structured PEGDA PC film with higher flexibility can be obtained by removing the SiO₂ microspheres from the SiO₂-PEGDA PC system using HF solution etching.By studying the microstructure and optical properties of the SiO₂-PEGDA PC film,it can be found that the PC film possesses a coexistence structure of ordered polycrystalline regions and disordered grain boundary regions.The arrangement of SiO₂microspheres in the ordered crystalline region is consistent with that of the（111）plane in a face-centered cubic structure,while the SiO₂ microspheres in the amorphous region show random distribution.The formation mechanism of the polycrystalline structure can be explained by the nucleation-growth mechanism.This polycrystalline structure endows the SiO₂-PEGDA PC film with angle-dependent and bending-independent structural color.A similar refractive index between SiO₂ and PEGDA materials makes the film high transparency.The optical properties and structure of the SiO₂-PEGDA PC film will remain unchanged after 10,000 bending tests,which proves its good flexibility.The IO structured PEGDA PC films have similar structures and optical properties to opal structured SiO₂-PEGDA PC film,and they show good optical responsiveness to water molecules.The anti-counterfeiting application of the above two flexible and free-standing PC materials has been studied in this dissertation.According to the characteristics of SiO₂-PEGDA PC film with high transparency,angle-dependent and bending-independent structural color,an anti-counterfeiting label was produced.The anti-counterfeiting label is invisible under a low light intensity,but shows the angle-dependent structural color under high light intensity.According to the good optical responsiveness to water molecules of IO structured PEGDA PC films,a photonic pattern that was invisible under the microscope and displayed under a water vapor stimulation can be obtained,where the pattern was fabricated by the hydrophilic and hydrophobic modification method.The invisible PCpatterns reported in the literature are identified by single central reflection wavelength（structural color）information,while the patterns prepared in this dissertation are identified by dual optical information of brightness and central reflection wavelength.Simultaneous adjustment of brightness and central reflection wavelength through water vapor stimulation has not been reported.In addition,the invisible PC patterns also have extremely fast appearance speeds（1.0 s）and recovery speeds（2.2 s）,which can be used as an ideal candidate material in the structural color-based anti-counterfeiting field.