WO2016157741A1

WO2016157741A1 - Particle assembly

Info

Publication number: WO2016157741A1
Application number: PCT/JP2016/001295
Authority: WO
Inventors: Norishige Kakegawa; Yuichiro Miyauchi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2015-03-31
Filing date: 2016-03-09
Publication date: 2016-10-06
Anticipated expiration: 2017-09-30
Also published as: JP2016190178A

Abstract

A particle assembly low in angle dependence properties of visual performance and excellent in chroma is provided. A particle assembly in which a surface of an optically isotropic structure in a material developing a structural color is covered with a colloidal crystal in which a particle is regularly arranged, and a void between the particles is packed with a material having a lower refractive index than that of a material of the particle is provided.

Description

PARTICLE ASSEMBLY

The present invention relates to a particle assembly that allows the change in color observed depending on the angle in measurement to be small and that develops a vivid color, and a method for producing the same.

A common material as a color developing material, such as a dye, absorbs light in a specific wavelength range to result in transition of an electron to an excited state, namely, allows light energy to be converted to electron energy. On the other hand, light at a wavelength not absorbed by the material is reflected and is recognized as a color by eyes.
On the other hand, a structural color material is known which develops a color by use of light interference. The structural color material is a material that regularly changes the refractive index of a medium on a nanometer scale, and regularity of the refractive index allows only light in a specific wavelength range to be reflected and allows the remaining light to penetrate, thereby allowing the structural color material to seem to develop a color.
The structural color material includes no heavy metal and thus is good for the environment, and attracts attention as a color developing material that causes no waste of energy (NPL 1). The structural color material is utilized as a material for enhancement in designability in, for example, cosmetics, a car body, an artificial bait as a fishing tool, and a craft.

A colloidal crystal is one kind of the structural color material, and is a material where a particle uniform in refractive index and particle size is regularly arranged in a medium. The colloidal crystal reflects light by regularity of the particle under the Bragg condition to develop a color.
On the other hand, in the colloidal crystal, the particle is regularly arranged, and therefore regularity of the particle arrangement is different depending on the angle in observation. As a result, the wavelength and the strength of light detected are largely changed depending on the angle of incident light or the angle in observation. That is, angle dependence properties of visual performance are high. Therefore, the application of the colloidal crystal is limited.

As the method for reducing angle dependence properties of the colloidal crystal, a method for reducing regularity of a particle arrangement has been proposed in recent years (PTL 1, NPL 2). In such Literatures, the particle arrangement of the colloidal crystal is an amorphous structure to thereby reduce the angle dependence properties.
The amorphous structure is a structure in which, while regularity of the particle arrangement is present for a short period, the regularity is almost absent when viewed for a long period and no anisotropy is present in the particle arrangement even when observed from any angle.

In PTL 1, the amorphous structure is produced by using particles having a different size or by using a method for aggregating a particle by addition of a salt to a suspension so that the particle is not regularly arranged.

In the method proposed in PTL 1, however, while the angle dependence properties of visual performance of a structural color are decreased, the structural regularity of a material is reduced and therefore internal scattering in the material is stronger. As a result, whiteness of the material is increased and it is difficult to produce a material which displays a vivid color to such an extent that such a material can be used as a color material.

In NPL 2, a material has been proposed in which a black color material is added into the amorphous structure to absorb stray light due to internal scattering in the material, thereby displaying a more vivid color.

In the method proposed in PTL 2, however, the black color material absorbs not only stray light but also light at a wavelength required, thereby causing the entire reflection rate of the material to be decreased. Therefore, it is difficult to achieve the effect of essentially increasing the chroma of the color material. The black color material takes lightness away from the color material, and therefore the black color material cannot be mixed in a large amount into the amorphous structure and exerts a limited effect as a method for increasing apparent chroma.

In NPL 3, a structural color particle has been proposed in which a crystalline layer of an ordered structure is disposed on the outermost surface of an amorphous structure by dropping and aggregating in oil a colloid suspension where silica is dispersed in water. The ordered structure is disposed on the particle surface to thereby allow light at a specific wavelength to be selectively strongly reflected as compared with a general amorphous structure, resulting in an increase in the entire chroma.

In the method proposed in PTL 3, however, a large particle of a submillimeter order is mainly generated and the particle size distribution is also broad. If the amorphous layer in the interior of the particle produced is thick, internal scattering is also increased and it is difficult to increase the chroma. Moreover, in NPL 3, a black color material is also added into the structural color particle, but it is difficult to essentially increase the chroma, as described above.

As described above, a problem in the prior art is that even if an amorphous structure having low angle dependence properties of visual performance can be produced, the vividness of a color is decreased as trade-off.

[PTL 1] Japanese Patent Application Laid-Open No. 2010-58091

[NPL 1] Shuichi, KINOSHITA, Bionanophotonics, Structural Color Introduction Textbook, published in 2010
[NPL 2] Richard O. Prum. et al, Biomimetic Isotropic Nanostructures for Structural Coloration, Advanced Materials, published in 2010, 22 p.p. 2939-2944
[NPL 3] Yukikazu Takeoka et al, Structurally Coloured Secondary Particles Composed of Black and White Colloidal Particles, Scientific Reports, published in 2013, 3 p.p. 2371-2737

The present invention has been made in view of such background arts, and an object thereof is to provide a structural color material that is low in angle dependence properties of visual performance and that is produced by assembling a particle which allows a color to look vivid.

In order to solve the above problem, the present invention provides a particle assembly in which a particle is assembled and a void is packed with a material having a lower refractive index than that of a material of the particle.

Advantageous Effect of Invention

The particle assembly of the present invention, can be used to thereby provide a structural color material low in angle dependence properties of visual performance and also excellent in chroma.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1 is a view describing a particle assembly in which a particle is assembled on an isotropic structure, of the present invention. Fig. 2 is a view describing a particle assembly in which a particle is assembled, of the present invention. Fig. 3 is a view describing an electrospray method for producing the particle assembly of the present invention.

Hereinafter, suitable embodiments of the present invention are described in detail with reference to the accompanied drawings, but are not intended to limit the scope of the present invention.

The present invention provides a particle assembly in which a particle assembled forms a layer. Fig. 1 is a schematic view of one embodiment of the particle assembly according to the present invention. A particle 1 in the Figure is a unit particle that forms a colloidal crystal 14. No problem is caused as long as the diameter of the particle 1 is 50 nm or more and 1000 nm or less, and the coefficient of variation of the particle size distribution, namely, the numerical value obtained by dividing the average diameter of the particle by the standard deviation is 10% or less. More suitably, a particle having a diameter of 150 nm or more and 300 nm or less and a coefficient of variation of 3% or less can be used. The material of the particle 1 is not particularly limited as long as the refractive index thereof is in a proper range, and a material having a refractive index of light at a wavelength of about 550 nm, of 1.4 or more and 2.0 or less, more suitably a refractive index of 1.46 or more and 1.60 or less, is used. As the particle 1, a particle of a monomer, a dimer, or a trimer or higher organic polymer, a particle of an inorganic polymer obtained by the sol-gel method, or the like can be used. Examples of the organic polymer include a group including polystyrene, an acrylic acid ester, a methacrylic acid ester and derivatives thereof, and an epoxy resin, a polycarbonate resin, a polyamide resin, a polyimide resin and a polyurethane resin. One suitable example of the particle 1 includes a polystyrene particle having a refractive index of about 1.6, in which the particle size distribution is easily uniform by an emulsion polymerization method or the like. Alternatively, the particle 1 may include an inorganic material produced by the sol-gel method. In such a case, a specific material includes silicon oxide. A high refractive index material such as aluminum oxide, titanium oxide or zirconium oxide can also be used, or a material in which such a high refractive index material and a low refractive index material such as silicon oxide or magnesium fluoride is combined can also be used. One suitable example can also include a silicon oxide particle having a refractive index of about 1.46, in which the particle size distribution is uniform by the Stober method or the like.

A colloidal crystal 14 in a particle assembly 15 has a structure present on the surface of an isotropic structure 3, in which the particle 1 is regularly packed due translation symmetry, and the colloidal crystal 14 has a crystal plane 2. The colloidal crystal 14 is required to have two or more layers each including the particle 1 in order to allow particular light interference to strongly occur. The thickness of the colloidal crystal 14 is preferably in the range from about 1 to 5 μm, more suitably in the range from about 1 to 2 μm. A thickness of the colloidal crystal 14 of 5 μm or more cannot be adopted because structural periodicity in the depth direction is longer to cause strong structural anisotropy, resulting in strong exhibition of angle dependence properties of visual performance. The domain size of the crystal plane 2 of the colloidal crystal 14 in the face direction can be about several μm. If a periodic structure having translation symmetry is long-continued, the structural anisotropy becomes stronger to cause angle dependence properties of visual performance to be strongly exhibited. That is, if the surface of the isotropic structure 3 is covered with a single homogeneous crystal plane, the angle dependence properties become higher. On the other hand, if the domain size of the crystal plane 2 of the colloidal crystal 14 in the face direction is decreased for limitation only to a space between adjacent particles, a different crystal plane appears to form a smoothly continuous layer. No spatial margin for absorbing distortion of the structure is, however, present at a boundary between the crystal planes 2, resulting in making it difficult to stack the particle 1 in the depth direction of the colloidal crystal 14, to exert a low interference effect. As a result, it is difficult to develop a color that is vivid and low in angle dependence properties.

The isotropic structure 3 in the particle assembly 15 is not particularly limited as long as the structure includes a spherical and optically isotropic material. The optically isotropic material means a material that is non-crystalline, that has no birefringence and that achieves the same optical properties even in observation in any direction, and mainly includes a non-crystalline polymer or glass material, or an amorphous structure including a particle. As the material of the isotropic structure 3, a material having a refractive index equal or close to the refractive index of the material of the particle 1 forming the colloidal crystal 14 can be suitably used because of being capable of decreasing light interference at an interface between the crystal plane and the isotropic structure.

As illustrated in Fig. 2, the isotropic structure 3 in the particle assembly 15 may be an aggregate 6 that includes an amorphous structure in which a particle 5 having a size equal to or smaller than a wavelength of light intended (hereinafter, sometimes designated as "structural particle" in order to distinguish the particle forming the isotropic structure from the particle constituting the colloidal crystal) is optically isotopically assembled. In such a case, the structural particle 5 forming the aggregate 6 can be densely packed in order to decrease scattering of light. The structural particle 5 is not particularly limited as long as the particle includes an optically isotropic material, and a material having a refractive index equal or close to the refractive index of the material of the particle 1 forming the colloidal crystal 14 can be suitably used because of being capable of decreasing light interference at an interface between the crystal plane and the isotropic structure. Alternatively, the same particle as the particle 1 may also be used as the structural particle 5.

The diameter of the particle assembly 15 of the present invention is not particularly limited and can be 100 μm or less. When the diameter is increased, the interference effect is larger to increase the effect of reflecting light intended, but the number of internal scattering events of light is also increased. As a result, the lightness and the chroma of the particle assembly 15 in terms of per unit thickness are decreased to result in deterioration in color development efficiency. More suitably, a particle assembly 15 having a diameter of 30 μm or less can be used. In contrast, a particle assembly 15 having a diameter of 5 μm or less may cause the interference effect to be too small, resulting in deterioration in light-emitting efficiency. In terms of the color material, if the diameter of the particle assembly is a submicron order, one particle can be easily visualized. From the foregoing, the diameter of the particle assembly 15 is preferably 5 μm or more and 100 μm or less, more preferably 5 μm or more and 30 μm or less, further preferably 5 μm or more and 10 μm or less.

A material having a lower refractive index than the material of the particle 1 can be used for a packing material 4. The void of the particle 1 constituting the colloidal crystal 14 can be embedded with the packing material 4, to result in a decrease in the difference in refractive index at an interface. As a result, scattering between particles can be dramatically suppressed, and the chroma of the particle assembly 15 can be enhanced. Furthermore, the void of the particle 1 is embedded with the packing material 4 to result in an increase in the refractive index of the colloidal crystal 14 as compared with a case where the void is present. Therefore, when a color having the same hue angle as in the case where the void is present is developed, the size of the particle 1 can be decreased. The particle size of the particle 1 can be decreased to also result in a decrease in the amount of scattering from the particle 1, thereby increasing the chroma of the particle assembly. In addition, the particle size can be decreased because, when the difference in refractive index between the particle 1 and the packing material 4 or the refractive index ratio thereof is increased, the amount of scattering is increased. Specifically, when the difference in refractive index between the particle 1 and the packing material 4 is in the range from about 0.6 to 0.2 and the refractive index ratio thereof is in the range from about 1.4 to 1.1, a particle assembly 15 high in chroma can be produced. On the other hand, if the difference in refractive index is 0.1 or the refractive index ratio is less than 1.1, a particle assembly 15 having a size of 100 μm or less cannot impart sufficient light interference, and cannot allow a vivid color to be developed.

The packing material 4 is required to be a material that binds the particle 1. In addition, when the packing material 4 is utilized as the color material, the particle assembly is required to have physical strength. Accordingly, the void between the particles 1 cannot be packed with a material like a liquid. An organic polymer type material high in binding properties or an inorganic material produced by the sol-gel method can be used. Examples of the organic polymer include a group including polystyrene, an acrylic acid ester, a methacrylic acid ester and derivatives thereof, and an epoxy resin, a polycarbonate resin, a polyamide resin, a polyimide resin, a polyurethane resin and a silicone resin. A silicone resin that is thermally stable can be used as a suitable material example. In addition, a fluororesin that can further decrease the refractive index can also be used. For a specific material as the inorganic material produced by the sol-gel method, silicon oxide or the like is used. Alternatively, in order to decrease the refractive index, a porous material having a mesopore or micropore having no optical influence can also be used.

As described above, the configuration of the structural color material that is low in angle dependence properties and that looks vivid, according to one embodiment of the present invention, is a particle assembly in which the surface of an optically isotropic structure is covered with a colloidal crystal in which a particle is regularly arranged and the void between the particles is packed with a material having a lower refractive index than the material of the particle, and furthermore, in one example of exemplary embodiments, is a polyhedron-shaped particle assembly in which the surface of an optically isotropic structure is covered with a colloidal crystal in which a particle having a diameter of 50 nm or more and 1000 nm or less is regularly arranged and the void between the particles is embedded with a packing material having a lower refractive index than the material of the particle.

Next, the method for producing the particle assembly according to the present invention is described below, but is not intended to limit the scope of the present invention.

An electrospray method can be suitably used for production of the particle assembly 15 illustrated in Fig. 1 and Fig. 2. When the electrospray method is used, a particle whose size is small and homogeneously uniform can be prepared as compared with other methods. In order to produce the particle assembly 15 that has a diameter of 30 μm or less and a coefficient of variation of the average particle size of within 10%, and that is less uneven in color tone, the electrospray method is an optimal method.

Fig. 3 illustrates a schematic view of the electrospray method. A syringe 7 is packed with a suspension in which the particle 1 and the isotropic structure 3 or the structural particle 5 that serve as raw materials for the particle assembly 15 are dispersed. A voltage of 1 kV to 20 kV or -1 kV to -20 kV is applied to a nozzle 8 by a high-voltage power supply 9. The ground is connected to a substrate 10, and the potential difference applied by the high-voltage power supply 9 is generated between the nozzle 8 and the substrate 10. A solvent 11 is present between the substrate 10 and the nozzle 8, and the potential at the bottom of a container accommodating the solvent 11 is connected to the ground and is 0 V. When the suspension is pushed out by a syringe pump 12 in the state where a proper potential is applied between the bottom of the solvent 11 and the nozzle 8, the suspension charged is shot out as a spray liquid 13. The spray liquid 13 is charged and therefore a droplet thereof is mutually repulsive to be formed into a finer droplet. The droplet is landed in the solvent 11, and the particle 1 and the isotropic structure 3 or the structural particle 5 are aggregated in the solvent 11 to provide the particle assembly 15.

In the electrospray method, when the electric field strength applied and the surface tension of the liquid sprayed are balanced, a Taylor corn in which the liquid trails down in a conical shape at the tip of the nozzle 8 can be made. When the electric field strength exceeds the limiting value and is further increased, a fine droplet jet is ejected from the tip of the Taylor corn towards the extreme opposite. The operation mode is referred to as "corn jet mode", and the jet includes an ion having a large excess of positive or negative charge. The corn jet has a large excess of charge and therefore is unstable, and is split into a fine droplet in a short time.
That is, it is important for producing a fine and homogeneous particle assembly 15 to make the corn jet mode by parameter control of the apparatus illustrated in Fig. 3.

When a fine droplet is produced by the corn jet mode, the liquid sprayed can have a larger surface tension to thereby provide a finer droplet. In addition, the liquid can have a lower dielectric constant and a higher conductivity to thereby provide a finer droplet. In order that the droplet is physically fine, the diameter of the nozzle 8 can be small and can be specifically about 200 μm or less. While a diameter of 100 μm or less can allow a finer droplet to be produced, a too small diameter may cause the liquid to be clogged. A lower flow rate of the liquid fed out by the syringe pump 12 can allow a finer droplet to be made. Specifically, the flow rate can be 20 μL/min or less, but a too low flow rate cannot allow the Taylor corn to be made.

In order that the particle assembly 15 of the present invention is obtained by the electrospray method, an aggregation process of the particle 1, the isotropic structure 3 and the structural particle 5 is also required to be considered. The aggregation process of the particle can be described by the DLVO theory to some extent. The following is described by the DLVO theory: the interaction acting between particles is represented by the sum of the electric repulsive force and the van der Waals attractive force between particles. In the course of drying a particle-containing suspension, when the electric repulsive force between particles is strong, the van der Waals force overcomes the electric repulsive force, and aggregation occurs only when the distance between particles is considerably small. That is, a particle is densely packed until aggregation occurs, and is high in crystallinity. On the other hand, when the electric interaction between particles is weak, a particle is mutually easily aggregated, and is coarsely packed to result in deterioration in regularity of a particle arrangement. An example of the method for controlling the electric interaction between particles is a method involving a treatment of the surfaces of the particle 1, the isotropic structure 3 and the structural particle 5 illustrated in Fig. 1 and Fig. 2, and proper selection of the solvent for control of the dielectric constant of the medium. Furthermore, another example of the method is a method including adding a salt to the particle-containing suspension to control the thickness of an electric double layer on the particle surface.

In order that the particle assembly 15 illustrated in Fig. 1 is obtained by the electrospray method, the particle 1 aggregated on the surface of the isotropic structure 3 is required to be densely packed and assembled as an ordered structure. For example, when a suspension in which a particle whose surface is modified by sulfonic acid, carboxylic acid, amine or the like is used for the particle 1 and dispersed in a water solvent is used for the spray liquid 13, the particle is hydrophilic and the electric repulsive force between the particles becomes stronger. When a long chain alcohol or the like slightly soluble in water is used for a solvent 11 in the spray liquid, water is diffused in the solvent 11 from the liquid landed, and the hydrophilic particle 1 is aggregated. The electric repulsive force between the particles in the aggregation process is strong, and therefore the particle is densely packed. As a result, the plane 2 is formed on the surface of the isotropic structure 3. Here, the isotropic structure 3 is required to be incorporated in the spray liquid 13, and the isotropic structure 3 can also be subjected to the same surface treatment as in the particle 1.

In order that the particle assembly 15, in which the isotropic structure includes the structural particle 5, illustrated in Fig. 2 is obtained by the electrospray method, it is necessary to control the aggregation states on the surface and in the interior of the particle assembly 15. For example, when a suspension, in which a particle whose surface is modified by sulfonic acid, carboxylic acid, amine or the like is used for each of the particle 1 and the structural particle 5 and dispersed in a water solvent, is used for the spray liquid 13, the particle is hydrophilic and the electric repulsive force between the particles is strong. When a long chain alcohol or the like slightly soluble in water is used for a solvent 11 in the spray liquid, water is diffused in the solvent 11 from the liquid landed, and the hydrophilic particle 1 is aggregated. When the aggregation process of the particle 1 occurs near an interface between the droplet landed and the solvent 11, spatial limitation in packing of the particle 1 is less and therefore the particle 1 is regularly packed. As a result, the colloidal crystal 14 having the crystal plane 2 is formed. On the other hand, the particle 1 and the structural particle 5 present in the droplet undergo spatial limitation in aggregation, and are irregularly packed. As a result, the aggregate 6 has an optically isotropic, amorphous or random particle arrangement.

When the material of the particle 1 is different from the material of the structural particle 5 in production of the particle assembly 15 in which the isotropic structure includes the structural particle 5, as illustrated in Fig. 2, the difference in electric repulsive force may be made between the material of the structural particle 5 and the material of the particle 1 to allow the material of the particle 1 to be preferentially aggregated.

The process for embedding the void of the particle 1 with the packing material 4 includes a method including mixing a precursor of the packing material 4 into the spray liquid 13 in advance, and forming the crystal plane 2 and at the same time performing packing during electrospray. For example, when a particle including polystyrene as a main component is used for the particle 1 and silica is used for the packing material 4, a silica sol or a particle having a diameter of about several nm can be used for the precursor of the packing material 4. A procedure may also be used which includes producing a particle assembly 15 having no packing agent and thereafter impregnating the assembly with the packing material 4.

The particle assembly 15 of the present invention, which serves as the structural color material, can be used in various color materials. The particle assembly 15 obtained by electrospray is recovered as it is and the solvent is evaporated to dryness to thereby provide a powder. The resulting powder can be used in the drying state as it is, and thus utilized in a pigment such as toner, a dry electrodeposition coating material or the like. The powder of the particle assembly can be re-dispersed in a proper solvent prepared, and thus utilized as a coating material such as a paint. The particle assembly of the present invention uses no dye, and therefore, when used in various color materials, the particle assembly is considered to be excellent in weather resistance. Furthermore, the particle assembly can also be used for an optical member such as a color filter.

Hereinafter, the present invention is specifically described with reference to Examples. The present invention, however, is not intended to be limited to such Examples.

(1) Production of particle 1
Two polystyrene particles (hereinafter, sometimes abbreviated as "PS particle") including polystyrene as a main component were produced by an emulsion polymerization method. Pure water, a styrene monomer and sodium p-styrenesulfonate were added to a four-necked round bottom separable flask, and stirred using a mechanical stirrer under nitrogen bubbling for 30 minutes. After a sample was heated to 70°C with being stirred in an oil bath, potassium persulfate as a catalyst was added thereto and a polymerization reaction of styrene was performed in a nitrogen atmosphere for 8 hours. After the sample was cooled, a precipitate was recovered by centrifugation, and the product was washed using pure water. The resulting sample was dispersed in pure water to provide a PS particle suspension. The PS particle was subjected to particle size and particle size distribution measurements by an electron microscope, and as a result, the diameter was 205 nm and the coefficient of variation of the particle size distribution was 2.8%, or the diameter was 195 nm and the coefficient of variation of the particle size distribution was 2.7%.

(2) Production of isotropic structure 3
Silica beads (produced by Duke Scientific Corporation) having average particle diameters of 40 μm and 80 μm were dispersed in a solution in toluene, to which mercaptopropyltrimethoxysilane was added, and the solution was stirred at 60°C for 10 hours. The product was washed with an alcohol and immersed in an aqueous 10% by weight nitric acid solution overnight. After the immersion, glass beads were recovered from the aqueous solution, and the resultant was evaporated to dryness to provide an isotropic structure 3. Silica beads: Sicastar (produced by Micromod Partikeltechnologie GmbH) having an average diameter of 20 μm, the surface of which was modified by sulfonic acid, were also used as the isotropic structure 3 in the same manner.

(3) Production of particle assembly by electrospray method
A syringe 7 having a volume of 1 mL was packed with a spray liquid 13 prepared by dispersing the particle 1, the isotropic structure 3, a structural particle 5 and the like in pure water. A metallic syringe needle having an inner diameter of 70 μm or 120 μm was used for a nozzle 8. The spray liquid was fed at a flow rate of 10 μL/min to 20 μL/min, and sprayed to a substrate 10 located below by 14 cm and filled with n-butanol. During spraying, the potential difference between the syringe needle and the substrate 10 was adjusted between 6 kV and 10 kV to maintain the potential difference at which a corn jet was formed on the tip of the nozzle 8. After spraying for about 20 minutes to 60 minutes, the apparatus was stopped and the product was removed together with n-butanol to provide a particle assembly 15.

(4) Film formation of particle assembly
A dispersion liquid of the particle assembly obtained by the electrospray method was filtered, and a precipitate was recovered on filter paper. Here, the amount of the dispersion liquid filtered was adjusted so that the particle assembly was attached as an almost monolayer to a fiber on the filter paper. A silicone elastomer was interfused in the filter paper to which the particle assembly was attached, and solidified to thereby provide a thin film of the particle assembly.

(5) Observation of form of particle assembly
An optical microscope (manufactured by ZEISS) was used to observe the form of the particle assembly at a magnification of 500 times. Alternatively, an electron microscope: S-5500 (manufactured by Hitachi High-Technologies Corporation) was used for the observation. The diameter was measured from the resulting image to determine the average diameter. The surface shape of the particle assembly obtained was also observed by the electron microscope. Furthermore, FIB-SEM: Nova600 (manufactured by FEI Company) was used to cut the particle assembly, and the shape of the cross section was observed.

(6) Optical measurement
In order to quantitatively evaluate angle dependence properties of visual performance, a liquid crystal view angle measurement apparatus: EZLite Micro (manufactured by Eldim S.A.) was used. The apparatus is an apparatus that can perform optical evaluation at any angle by using a unique optical system called a Fourier lens. In particular, the apparatus can perform measurement where the incident light is over a wide angle, and therefore can perform measurement of a color tone close to the color tone observed under a living space in which various light sources are present. In the present Example, when the angle of incident light relative to a line perpendicular to the surface of the sample observed was assumed to be 0 degrees, the angle was widened to ±30 degrees to perform measurement. The measurement was also performed in the detection range of ±30 degrees.
The measurement results were quantified based on color assessment: L^*C^*h color system with the sensitivity of human eyes. The L^*C^*h color system is a display system made based on the CIE 1976L^*a^*b^* color space according to the Japanese Industrial Standards (JIS Z 8781-4). In the L^*C^*h display system, L^* represents the lightness, C^* represents the chroma, and h represents the hue angle. In particular, C^* representing the chroma is a square root of the value obtained by squaring a^* and b^* of the L^*a^*b^* color space, respectively, and adding the resultants, and h representing the hue angle is represented by the arc tangent of the value obtained by dividing b^* by a^*. In the display system, merits are as follows: the chroma C^* exhibiting vividness of a color can be directly determined, and the difference in color tone between different points, namely, color difference: ΔE can be represented by the magnitude of a vector in the color space. That is, in order to determine the ΔE, the values obtained by squaring the respective differences in L^*, a^* and b^* between different points may be summed to determine the square root. In the present Example, the chromaticity at a detection angle of 0 degrees was defined as the base, and the color difference at each angle in measurement up to ±30 degrees was determined.

A water spray liquid containing 0.5% by weight of the PS particle produced, having a diameter of 205 nm, was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water spray liquid containing 2.0% by weight of the PS particle produced, having a diameter of 205 nm, was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water spray liquid containing 10.0% by weight of the PS particle produced, having a diameter of 205 nm, was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water dispersion liquid containing 2.0% by weight of the PS particle produced, having a diameter of 205 nm, was dropped by a dropper in n-butanol, and thereafter the solution was shaken to provide a particle dispersion. The resulting product was subjected to film formation to provide particle dispersion film.

A water spray liquid in which an isotropic structure 3 including glass beads having a diameter of 20 μm, and the PS particle produced, having a diameter of 205 nm, were mixed in a weight ratio of 1 : 1 was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water spray liquid in which an isotropic structure 3 including glass beads having a diameter of 40 μm, and the PS particle produced, having a diameter of 205 nm, were mixed in a weight ratio of 1 : 1 was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water spray liquid in which an isotropic structure 3 including glass beads having a diameter of 80 μm, and the PS particle produced, having a diameter of 205 nm, were mixed in a weight ratio of 2 : 1 was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

A water spray liquid containing 0.5% by weight of the PS particle produced, having a diameter of 195 nm, was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

Comparative Example 1

A water spray liquid containing 1.0% by weight of the PS particle produced, having a diameter of 205 nm, and 0.1% by weight of polyvinylpyrrolidone was used to perform electrospray. The resulting product was subjected to film formation to provide a particle assembly film.

Comparative Example 2

A glass substrate was coated with a water dispersion slurry containing 20.0% by weight of the PS particle produced, having a diameter of 205 nm, by use of an applicator. The resultant was dried at an ordinary temperature to provide a photonic crystalline film.

Comparative Example 3

The particle assembly produced in the same manner as in Example 1 was attached to a fiber on filter paper and solidified without being impregnated with a silicone elastomer, to provide a particle assembly film.

Comparative Example 4

The particle assembly prepared in the same manner as in Example 8 was attached to a fiber on filter paper and solidified without being impregnated with a silicone elastomer, to provide a particle assembly film.

(Performance evaluation)
The structure and optical properties of the particle assembly in each of Examples and Comparative Examples are shown in Table 1.

The material produced was observed in SEM, and a large particle of several tens μm order was observed except for Example 4 and Comparative Example 2. The sample in Example 4 had a particle diameter of 114 μm. The sample in Comparative Example 2 was a thin film of the colloidal crystal, and the film thickness was about 2 μm.

The surface of the sample in each of all Examples and Comparative Examples 3 and 4 was covered with the crystal plane in which the polystyrene particle was regularly assembled. The domain size of the crystal plane was 5 μm or less, and the shape of the particle was a polyhedron shape. The sample in Comparative Example 1 had no crystalline structure on the surface thereof, and the polystyrene particle is randomly aggregated.

The sample produced was cut by FIB-SEM and the cross section shape thereof was observed, and as a result, the thickness of the crystal plane present on the surface of the sample in each of all Examples and Comparative Examples 3 and 4 was several μm. It was also confirmed that the polystyrene particle was aggregated in the amorphous state in the interior of the sample in each of Examples 1, 2, 3, 4 and 8 and Comparative Examples 1, 3 and 4, and an amorphous structure was taken. Herein, the amorphous structure was confirmed by determining the distance between the centers of particles to create a radial distribution function, and finding the absence of a long periodic regularity. It was confirmed that the interior of the film of the sample in Comparative Example 2 was also crystalline. It was confirmed that the sample in each of Example 5, 6 and 7 had a glass sphere present in the interior thereof.

The average color difference in the measurement range of ±30 degrees was determined by optical measurement, and was found to be about 8 or less in the Examples and the Comparative Examples except for Comparative Example 2. The average color difference in each of the Examples and the Comparative Examples except for Comparative Example 2 was a color difference so that the almost same color was seen by a visual impression, and low angle dependence was achieved. On the other hand, the crystal structure in Comparative Example 2 had an average color difference of more than 15. The numerical value indicates that the color tone is varied depending on the angle in observation.

The average chroma in the measurement range of ±30 degrees was determined by optical measurement, and was found to be about 20 or more in each Example. On the other hand, the lightness in each of Comparative Examples 1, 3 and 4 was 10 or less. The result in Comparative Example 2 was as high as 44. A chroma of 10 or less is close to the chroma of a scatterer, and a material having such chroma is strong in whiteness as the color material, and develops a non-vivid color. A chroma of 20 or more means that a material having such a chroma develops a vivid color which can be clearly recognized as a color. The result was highly affected by the presence of the packing material 4. It was considered that the packing material 4 was introduced to thereby suppress internal scattering of the particle assembly, dramatically enhancing the chroma. On the other hand, the colloidal crystal film in Comparative Example 2 had a high chroma of 44, but was high in angle dependence of visual performance and was not suitable for use as a material that developed a stable color.

The sample in each of Comparative Examples 1 and 2 was easily pulverized by scraping by a spatula, and formed into a white powder. The SEM image of the powder was observed, and it was confirmed that the particle assembly or the colloidal crystal film was broken. On the other hand, the sample of each of all Examples and Comparative Examples 3 and 4 was not changed in color tone even by scraping by a spatula and pushing by a fingertip. It was also confirmed that the particle assembly of each of all Examples and Comparative Examples 3 and 4 was not changed in the shape thereof by observation by SEM. It was considered that the packing material 4 was introduced to result in enhancement in physical strength.

It has been found from the foregoing that the particle assembly according to the present invention is small in the change in color observed, depending on the angle in measurement, and develops a vivid color.

The particle assembly of the present invention can be used to provide a structural color material low in angle dependence properties of visual performance and also excellent in chroma. Specifically, the particle assembly can be utilized in a color material like an ink or toner for a printer, a coating material such as paint, a color material for electrostatic coating, a pigment that is kneaded with a plastic or glass material for coloration, and the like. Various color tones can be produced by the same material configuration, and therefore industrially large merits in terms of cost and uniformity of a coloration process are imparted. The particle assembly can also be used as an optical member for a color filter, and the like.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2015-070588, filed March 31, 2015, which is hereby incorporated by reference herein in its entirety.

1 particle
2 crystal plane
3 isotropic structure
4 packing material
5 structural particle
6 aggregate
7 syringe
8 nozzle
9 high-voltage power supply
10 substrate
11 solvent
12 syringe pump
13 spray liquid
14 colloidal crystal
15 particle assembly

Claims

A particle assembly, wherein
a surface of an optically isotropic structure is covered with a colloidal crystal in which a particle is regularly arranged, and
a void between the particles is packed with a material having a lower refractive index than that of a material of the particle.
The particle assembly according to claim 1, wherein a surface is configured by a polyhedron of the colloidal crystal.
The particle assembly according to claim 1, wherein the colloidal crystal comprises a particle having a diameter of 50 nm or more and 1000 nm or less.
The particle assembly according to claim 3, wherein a coefficient of variation of the colloidal crystal is 10% or less.
The particle assembly according to claim 3, wherein the colloidal crystal comprises a particle having a diameter of 150 nm or more and 300 nm or less.
The particle assembly according to claim 5, wherein a coefficient of variation of the colloidal crystal is 3% or less.
The particle assembly according to claim 1, wherein the colloidal crystal is configured by a material having a refractive index of light at a wavelength of 550 nm, of 1.4 or more and 2.0 or less.
The particle assembly according to claim 7, wherein the refractive index is 1.46 or more and 1.60 or less.
The particle assembly according to claim 1, wherein the particle is configured by an organic polymer material.
The particle assembly according to claim 1, wherein the particle is configured by an inorganic material.
The particle assembly according to claim 1, wherein a thickness of the colloidal crystal is in the range from 1 to 5 μm.
The particle assembly according to claim 1, wherein a difference in refractive index between the particle and the material having a lower refractive index for packing is in the range from 0.6 to 0.2.
The particle assembly according to claim 1, wherein a refractive index ratio of the particle and the material having a lower refractive index for packing is in the range from 1.4 to 1.1.
The particle assembly according to any one of claims 1 to 13, which develops a structural color.
The particle assembly according to any one of claims 1 to 14, wherein the optically isotropic structure is an amorphous structure.
The particle assembly according to any one of claims 1 to 15, wherein the particle is configured by a material comprising polystyrene as a main component.
The particle assembly according to any one of claims 1 to 16, wherein the material having a lower refractive index than that of a material of the particle comprises silicone as a main component.
A method for producing the particle assembly according to claim 1, comprising
spraying a liquid in which a particle is dispersed, to a solvent under a potential difference.
A color material comprising the particle assembly according to any one of claims 1 to 17.
An optical member comprising the particle assembly according to any one of claims 1 to 17.