Sunnam Kim

Photochemical switching behavior of azofunctionalized polymer liquid crystal/SiO2 composite photonic crystal

A photochemically tunable photonic crystal was prepared by infiltrating azopolymer liquid crystal in a SiO2 inverse opal structure. The SiO2 inverse opal film obtained reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation shifted the reflection band to longer wavelength more than 15nm. This is caused by the formation of anisotropic molecular orientation of the azopolymer. The switched state was stable in the dark, and the reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiations.

Manipulation and assembly of small objects in liquid crystals by dynamical disorganizing effect of push-pull-azobenzene-dye

The phase transition of a nematic liquid crystal containing a push-pull azobenzene dye could be induced efficiently during irradiation with visible light. The dynamical disorganizing effect of the push-pull azobenzene dye on the liquid crystalline order through its trans-cis-trans photoisomerizaion cycle under visible light was contributed to the efficient phase transition. Then, the effects of light irradiation on the motion of small objects dispersed in the liquid crystals containing the push-pull azobenzene were explored, and the manipulation and assembly of those objects were successfully achieved in the nematic phase but also in the smectic phase. The combination of the photo-controlled dynamical change in the liquid crystalline order and the intrinsic self-assembly property of a liquid crystal is promising for use in technologies that require not only the organization of small objects but also the photo-driving of nano- and micro-sized mechanical materials.

On/off switching of structural color by using multi-bilayered films containing copolymers having azobenzene and biphenyl side groups

In this paper, the change in structural color of multi-bilayered films fabricated by stacking copolymers, PMAz–PMBP (m : n), consisting of azobenzene and biphenyl side chain groups, and polyvinyl alcohol (PVA) alternately, was investigated. The multi-bilayered films consisting of polyacrylates having an azobenzene side chain group and PVA were found to reflect a light of specific wavelength, and to show reversible change in the reflection intensity by thermal annealing and irradiation with non-polarized ultra violet (UV) light. However, a long irradiation time was required for the change in reflection intensity of the multi-bilayered films. Therefore, we synthesized PMAz–PMBP (m : n), consisting of azobenzene and biphenyl side chain group having no absorbance over 300 nm, and the PMAz–PMBP (m : n)/PVA multi-bilayered films were fabricated. All the multi-bilayered films showed reversible change in the reflection intensity by thermal annealing and irradiation with non-polarized UV light. The re-coloration speed of PMAz–PMBP (75 : 25)/PVA 20-multibilayered film was faster than others fabricated in this study. It is related to penetration depth of light into the films. The effect of the introduction of the biphenyl side groups in the copolymers on the response behavior of the change in the structural color will be discussed.

Synthesis and Characterization of Photochromic Liquid Crystalline Polymer Beads

ABSTRACT Sub-micron sized liquid crystalline polymer beads containing azo groups and mesogenic groups were synthesized in order to develop 3D array showing photoinduced stimulus response properties. The size of beads was dependent on the polymerization temperature and the molar ratio of the azo and mesogenic groups. Films of the polymer beads were prepared by casting the suspension liquid on a glass plate. By irradiation of polarized light (488 nm) on the film of the polymer bead assembly, the photoisomerization effect of azo groups on the shape of the polymer beads as well as the molecular orientation of the mesogenic groups in the beads was investigated.

Fabrication of wide angle structural color with the patchy multi-bilayered films

ABSTRACT Multi-bilayered films with precisely controlled layer thickness were constructed for structural color and their reflection angular dependences were investigated. For wide angle structural color, it was intended to the macro irregularity of multi-bilayered films by randomly dispersed micron size multi-bilayered films in PDMS matrix. In addition, on-off switching of the reflection was examined with respect to the molecular orientation behavior of azobenzene chromophores by annealing and light irradiation.

Photo-Controlled Manipulation of Micrometer-Scale Objects on Polyethyleneglycol Thin Films with Azobenzene Compounds

We demonstrated manipulation of micrometer-sized objects (micro-objects) by scanning light irradiation on polyethyleneglycol (PEG) thin films doped with azobenzene derivatives. 4-Ethoxy-4′-methoxyazobenzene (EMAB)-/ Disperse Red 1(DR1)-doped polyethyleneglycol films were selected as matrix. The motion of micro-objects was caused by irradiation upon UV (λ = 375 nm) and visible (λ = 488 nm) light, which suggested that the photo-responsive motion was affected by the following three factors: light intensity, the concentration of azobenzene compound in PEG, and the distance between the irradiation position and the micro-objects. Moreover, the travelling speed of micro-objects on the DR1-doped PEG film was more effective than that on the EMAB-doped PEG film under the visible light.

Photo-Response Properties of Inverse Opal Infiltrated with Push-Pull Type Azobenzene Functionalized Polymer Liquid Crystals

Azo-containing polymers, poly(DR1Mc), poly(Dc2AzMc) and their copolymers, having push-pull type of azo-group were prepared and their photo-orientation behaviors were investigated. They were infiltrated into silica inverse opal, and reversible Bragg reflection shift was induced by irradiation of light in a range of 500 nm to 600 nm. With linearly polarized light irradiation, Bragg reflection of inverse opal was shifted to longer wavelength region, because azobenzene compounds oriented perpendicular to the axis of incident polarized light and the refractive index contrast was changed. On the other hand, with unpolarized light, Bragg reflection was shifted to shorter wavelength region. This blue shift was due to the molecular orientation preferring parallel to the incident light direction.

Photo-response properties of PBG composite material consisting of inverse opal structure and azo-polymer liquid crystal

Photo-tunable PBG composite material was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO2 inverse opal structure. The SiO2 inverse opal film infiltrated with the polymer LC reflected a light, which is called Bragg diffraction, corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the the Bragg diffraction band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for various optical materials such as memory, display so on.

Photo-induced orientation behaviors of azobenzene liquid crystal copolymers for photonic crystals

A certain wavelength of incident light is forbidden based on the optical periodicity related to the intervals and refractive indices of the comprising materials. For multi-bilayered films with periodicity, on–off switching of reflection is possible due to the presence or absence of a refractive index difference between two consisting films. Changes in the refractive index can be induced by stimuli-responsive orientation behaviors of liquid crystal (LC) molecules with birefringence. Azobenzene (Az) groups are employed for photo response and copolymerized with non-photo-responsive LC mesogen groups for cooperative orientation to improve light penetration by the dilution of Az concentration. LC mesogen groups are designed to have a high refractive index to improve the reflection contrast. The refractive indices and the orientation behaviors of the copolymer films are investigated. On–off switching of reflection for multi-bilayered films consisting of alternate Az-containing LC copolymers and poly vinyl alcohol is conducted using ultraviolet and visible light irradiation, and their reflection intensity and response speed are compared between the different LC mesogen types and molar ratios.

Light-driven motion of micro-particles by means of photoisomerization of azobenzene groups as molecular motor on a liquid crystalline film (Presentation Recording)

Light-driven micro-particles (MPs) were prepared by modification of photo-responsive azobenzene(Azo) groups as molecular motors. In a nematic liquid crystal, 5CB, we have achieved remote control of the Azo-modified MPs (Azo/MPs), which constructed with glass spheres (GSs) or rod-like glass particles (GRs) as MPs, by light stimuli. As results of photo-induced isomerization from UV-visible extinction spectroscopy and from phase evaluation under microscope observation, the motion behavior suggest to be correlated with the generation of micro-sized isotropic phase (I-phase) regions, which could be caused by disorganization of the 5CB molecules in the nematic phase (N-phase) due to photo-induced continuous isomerization of Azo groups.