Preparation And Tuning Mechanisms Of Photonic Crystal/Liquid Crystal Optical Functional Materials With Tunable Band Gap

Photonic crystals (PCs), which can control and modulate the propagation of light, are a kind of optical materials. PCs have attracted much attention and been extensively studied during these years. Among this kind of materials, responsive PCs are more attractive due to their important applications in areas such as biological and chemical sensors, inks and paints, color displays and so on. Meanwhile, Liquid crystals (LCs) have also evoked a lot of attention because of their unique optical properties and sensitiveness to external stimuli. Herein, we fabricate a series of PCs/LCs optical functional composite materials with tuning photonic bandgaps (PBGs) based on the external stimulus-responsive LCs. The tuning mechanisms have also been investigated in details. The modulation mechanism also has been researched. The main work includes:1. One-dimensional cholesteric LC PC/polymer network (1D ChLC PC/PN) composite films have been fabricated by the ultraviolet (UV) photopolymerization induced phase separation method. PBG of the composite film is sensitive to pressure and electric field. The effects of the photosensitive monomer concentration and functionality on the optical properties as well as the pressure/electic field responsive propertis of the 1D ChLC PC/PN composite films have been studied. The results demonstrate that reflectivity and the contrast ratio are decreased but the resolution is improved with the increasing photosensitive monomer concentration and functionality. Additionally, the critical pressure that induces the appearance of PBGs is about 85kPa. And then the PBGs disappear when a 20V electric field (50Hz, square wave) is applied.2. Monodispersed poly(styrene-methyl methacrylate-acrylic acid) (P(St-MMA-AA)) microspheres and silica microspheres were synthesized via surfactant-free emulsion polymerization method and the Stober method, respectively. And the microspheres are used to prepare 3D opal PCs by vertical deposition self assembly method. The effects of the addition amount of emulsifying agent and ammonium hydroxide on the partical size have been studied. The results indicate that the increase of the emulsifying agent concentration causes the decrease of the P(St-MMA-AA) spheres size, and the increase of the ammonium hydroxide concentration allows us to synthesize the silica microspheres with larger size. In addition, increasing the microspheres diameter will casuse a red shift of the corresponding opal PC’s PBG.3.3D silica opal PC/liquid crystal elastomer (LCE) composite films were prepared through UV polymerization. The thermal response properties and mechanism of composite films have been investigated. During the heating process, the film has a bending deformation and its PBG shifts towards shorter wavelength at the same time. The deformation process of the composite film is completely reversible. When cooled down, the film regains its original shape (flat) and the PBG also recovers. This reversible deformation thermally-induced by a variation of strain in the thickness direction is attributed to the different molecular orientation of LCE in the silica opal PC/LCE composite film.4. LCE invese opal PC with single structure color and dual structure colors have been prepared by one step and two steps UV photopolymerization method, respectively, using 3D silica opal PCs as templates. The thermal response properties and mechanism of the LCE inverse opal PCs were investigated. The results show that the PBGs for both single and dual structure-colored LCE inverse opal PCs have obvious red shift (approximately 50nm) as tempreture elevates; the PBGs could restore to their original wavelengthes when the PCs are cooled down. This reversible change is caused by the thermally induced molecular orientation change of the LCEs and thus the changes of the{111} plane spacing of the inverse PC.