Studies On Nanoparticle-based P（MMA-AA）/TiO₂ One-dimensional Photonic Crystal Films With Structural Colors

The color of nature is composed of structural color and pigment color.Compared with pigment color,structure color has the advantages of brightness,high saturation and fade-resistance.Photonic crystal is the main source of the structure color.According to the distribution of photonic bandgap in space,it can be divided into one-dimensional,two-dimensional and three-dimensional photonic crystals.One-dimensional photonic crystal has the advantages of simple preparation,low cost and beautiful color,it is an important species for structural color construction.The organic/inorganic hybrid one-dimensional photonic crystal possesses functions of organic matter and has advantage of large refractive index difference between the media.However,there are problems such as the penetration of organic solution in inorganic nanoparticle layer and the environmental pollution caused by organic solvent evaporation in preparation of one-dimensional photonic crystal.In this paper,poly（methyl methacrylate-acrylic acid）（P（MMA-AA））microemulsion as the low refractive index material,water: ethanol = 1: 1 dispersed TiO₂ sol as high refractive index material,were chosen to fabricate P（MMA-AA）/TiO₂ one-dimensional photonic crystal via spin coating.As these two kinds of media are both nanoparticle morphology,it can avoid the penetration of the organic solution in inorganic nanoparticle layer,so as to obtain a one-dimensional photonic crystal with clear boundary and controllable layer thickness.On the other hand,as the dispersing medium of the microemulsion is water,much environment-friendly process is achieved for stack assembly.The prepared one-dimensional photonic crystals have good solvent vapor response.Firstly,P（MMA-AA）microemulsion was prepared by a modified microemulsion method,and a small amount of acrylic acid（AA）was introduced to improve the hydrophilicity of polymethylmethacrylate（PMMA）for good compatibility with TiO₂ layer.The effects of different AA content on the microemulsion polymerization were discussed.With the increase of AA content,the average particle size,PDI and thermal decomposition temperature of microemulsion increased,while the glass transition temperature decreased a little.P（MMA-AA）microemulsion with average particle size of 37.34 nm,PDI of 0.14 and surface Zeta potential of 27.9 mV was prepared under the optimum microemulsion preparation conditions when the mass ratio of MMA to AA was 10: 1.Secondly,P（MMA-AA）/TiO₂ one-dimensional photonic crystal was assembled with alternate spin-coating of P（MMA-AA）microemulsion and TiO₂ sol.The prepared one-dimensional photonic crystals possess clear boundary and controllable layer thickness,and the environmental pollution problem is avoided during the preparation process.The effects of Bragg stacking number,incident angle,spin-coating time,sol concentration and spin-coating speed on the optical properties of the photonic crystals have been investigated and discussed.The results show that with the increase of the number of Bragg stacks,the intensity of the reflection peak of the one-dimensional photonic crystal increases and the peak width becomes narrow.With the increase of the incident angle,the reflection peak turns blue and the color changes obviously,indicating the one-dimensional photonic crystal has obvious angle dependence.With the increase of the times of spin-coating or the concentration of titania,the thickness of the titania layer increases and the reflection peak redshifts.As the speed of spin-coating increases,the thickness of both the organic layer and the inorganic layer decreases,resulting in blueshift of the reflection peak.Thus,it is possible to prepare one-dimensional photonic crystal with different structural colors by controlling the thickness of the high or low refractive index materials.Thirdly,P（MMA-AA）/TiO₂ one-dimensional photonic crystal shows good solvent vapor responsiveness and reproducibility.When exposed to saturated acetone,chloroform,ethanol and ethyl acetate vapors for 5 seconds,it has a distinguishable color change.When removed from the vapor after 1s,it can restore the original color.Samples were still able to maintain good reproducibility in 10 cycles.