JP7548532B2 - Dispersion and method for producing same - Google Patents

Description

The present invention relates to a dispersion and a method for producing the same.

It is known that when light with a wavelength comparable to the periodic structure of a medium enters the medium, the interaction between the medium and the light becomes greater. This interaction is caused by the photonic band gap in the photonic crystal. Photonic crystals can localize light based on group velocity anomalies due to quantum optical effects, control the refraction of light paths in microscopic spaces, and even induce stimulated radiation such as laser oscillation, so in recent years, expectations have been high for them as new optical devices. Colloidal photonic crystals are known to change color when external stimuli such as pressure are applied.

A stimuli-responsive color-changing liquid composition is disclosed, which contains a stimuli-responsive color-changing material that includes a liquid and a polymer gel that absorbs or releases the liquid in response to a stimulus, causing a reversible volume change, and is characterized in that the stimuli-responsive color-changing material is dispersed in a binder and is in a liquid state; a color-changing coating film formed by curing the composition; and a color-changing laminate formed by laminating the coating film on a substrate (see, for example, Patent Document 1).

An identification medium has been disclosed that includes, from the observation side, a cholesteric liquid crystal layer in which a hologram is formed, and a layer of polymer gel in which colloidal particles are periodically arranged (see, for example, Patent Document 2).

JP 2004-107601 A JP 2011-253051 A

Photonic crystals are integrated using microfabrication techniques used in semiconductor device fabrication, but due to the complicated process, they are not easily obtained.
The colloidal photonic crystal used in the stimulus-responsive color-changing material described in Patent Document 1 and the discrimination medium described in Patent Document 2 is required to be further improved in terms of color change upon application of an external stimulus.

After extensive research, the inventors discovered that when the particle size of the particles of a hydrophilic polymer compound having a crosslinked structure is controlled within a specific range and the coefficient of variation of the particle size distribution is equal to or less than a certain value, a change in the reflected color of the dispersion can be observed when an external stimulus is applied to the dispersion.

The present invention was made in consideration of the above circumstances, and aims to provide a dispersion whose color tone changes with changes in temperature or ion concentration, and a method for producing the same.

That is, the means for solving the above problems include the following embodiments.
<1> Particles of a hydrophilic polymer compound having a crosslinked structure;
An aqueous solvent;
An electrolyte;
The dispersion has a volume average particle size of the hydrophilic polymer particles at 20° C. of 100 nm to 2000 nm and a coefficient of variation of particle size distribution of 16% or less.
<2> The dispersion according to <1>, wherein the solid content of the hydrophilic polymer particles is 1% by mass to 75% by mass with respect to the total mass of the dispersion.
<3> The dispersion according to <1> or <2>, wherein the hydrophilic polymer compound contains a structural unit derived from at least one of a (meth)acrylamide compound and (meth)acrylic acid.
<4> The dispersion according to <3>, wherein the (meth)acrylamide compound is a compound having a hydrophobic group.
<5> The dispersion according to <4>, wherein the content of the structural units derived from the (meth)acrylamide compound having a hydrophobic group is 50% by mass to 99% by mass based on all structural units of the hydrophilic polymer compound.
<6> The dispersion according to any one of <1> to <5>, wherein the particles of the hydrophilic polymer compound have a volume average particle size at 20° C. of 200 nm to 300 nm.
<7> The dispersion according to any one of <1> to <6>, wherein the lower limit of the content of the electrolyte is 1.85×10 ⁻⁴ mol/L or more.
<8> A step of mixing a polymerizable monomer and a crosslinkable monomer constituting a hydrophilic polymer compound in the presence of an aqueous solvent to prepare a mixed solution;
mixing the mixed solution with a polymerization initiator to obtain a dispersion of particles of a hydrophilic polymer compound having a crosslinked structure;
A step of purifying the particles of the hydrophilic polymer compound having a crosslinked structure;
Including,
The hydrophilic polymer particles have a volume average particle size at 20° C. of 100 nm to 2000 nm and a coefficient of variation of particle size distribution of 16% or less.
<9> The method for producing a dispersion according to <8>, further comprising a step of adding an electrolyte.
<10> The method for producing a dispersion according to <9> or <10>, wherein the polymerizable monomer constituting the hydrophilic polymer compound includes a (meth)acrylamide compound having a hydrophobic group.

The present invention provides a dispersion whose color tone changes with changes in temperature or ion concentration, and a method for producing the same.

FIG. 2 is a graph showing the maximum absorption wavelength (nm) and the reflectance (%) when the concentration of hydrophilic polymer particles in a dispersion is changed in the present embodiment. FIG. 4 is a diagram showing the change in volume average particle diameter of particles of a hydrophilic polymer compound at each temperature in the present embodiment.

The dispersion according to the embodiment of the present invention will be described below.
In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
In this specification, the amount of each component in a composition means, when a plurality of substances corresponding to each component are present in the composition, the total amount of the plurality of substances present in the composition, unless otherwise specified.
In this specification, the term "solid content" refers to components other than the aqueous solvent.
In this specification, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

In this specification, "(meth)acrylic acid" is a concept that includes both acrylic acid and methacrylic acid, "(meth)acrylate" is a concept that includes both acrylate and methacrylate, and "(meth)acryloyl group" is a concept that includes both acryloyl group and methacryloyl group.

Dispersion
The dispersion of the present embodiment contains particles of a hydrophilic polymer compound having a crosslinked structure (hereinafter, may also be simply referred to as a "hydrophilic polymer compound"), an aqueous solvent, and an electrolyte, and the particles of the hydrophilic polymer compound have a volume average particle diameter at 20°C of 100 nm to 2000 nm and a coefficient of variation of the particle size distribution of 16% or less.
In the dispersion of this embodiment, the volume average particle size and the coefficient of variation of the particle size distribution of the hydrophilic polymer compound particles are controlled within specific ranges, and therefore the color tone of the dispersion can be changed by changing the temperature or ion concentration.
The mechanism behind this is unclear, but is speculated to be as follows.

In the dispersion of this embodiment, the volume average particle size of the hydrophilic polymer particles is controlled within a specific range. When the temperature or ion concentration of the dispersion containing the hydrophilic polymer particles, electrolyte, and aqueous solvent is changed, the volume average particle size of the particles changes. This is presumably because the volume phase transition occurs when the three-dimensional structure changes from a hydrophilic water-swelled state to a hydrophobic shrunken thread-like state near the lower critical solution temperature of the hydrophilic polymer compound.
It is presumed that under conditions lower than the lower critical solution temperature, the particles of the aqueous resin dispersed in the aqueous solvent are arranged in a swollen state and therefore exhibit Bragg reflection (colored state), whereas under conditions higher than the lower critical solution temperature, the particles of the aqueous resin shrink, causing the arrangement of the particles in the aqueous solvent to become disordered, resulting in the disappearance of Bragg reflection (white state).
In this manner, the color tone of the dispersion of this embodiment can be changed by changing the temperature of the dispersion of this embodiment or the ion concentration in the dispersion.
Each component of the dispersion of this embodiment will be described in detail below.

<Hydrophilic polymer compound>
The dispersion of the present embodiment contains particles of a hydrophilic polymer compound having a crosslinked structure.
The hydrophilic polymer compound is not particularly limited as long as it can change the color tone of the dispersion of the present embodiment by changing the temperature or ion concentration when dispersed in an aqueous solvent containing an electrolyte as described below. .
Examples of the hydrophilic polymer compound include a hydroxy group, a hydroxyethyl group, a hydroxypropyl group, an amino group, an amido group, an aminoethyl group, an aminopropyl group, a carboxy group, a carboxylate group, a sulfo group, a sulfonato group, and a phosphate group. It is preferable that the polymer compound has a hydrophilic group such as the above.

The hydrophilic polymer compound may be a homopolymer obtained by homopolymerizing a monomer having a hydrophilic group, a copolymer obtained by polymerizing two or more monomers having a hydrophilic group, or a copolymer obtained by polymerizing a monomer having a hydrophilic group with a monomer not having a hydrophilic group.

Specific examples of hydrophilic polymer compounds include gum arabic, casein, gelatin, starch derivatives, carboxymethyl cellulose and its sodium salt, sodium alginate, poly(meth)acrylates, polyhydroxyethyl(meth)acrylate, polyhydroxypropyl(meth)acrylate, copolymers and homopolymers of hydroxybutyl methacrylate, alkyl(meth)acrylates having a hydroxy group such as hydroxybutyl(meth), polyvinyl alcohols, polyvinyl formal, polyvinyl butyral, polyvinylpyrrolidone, poly(meth)acrylamide, and poly(N-methylol(meth)acrylamide).
From the viewpoint of availability, the hydrophilic polymer compound preferably has a structural unit derived from at least one of a (meth)acrylamide compound and a (meth)acrylic compound, and more preferably has a structural unit derived from a (meth)acrylamide compound.

When a hydrophilic polymer compound has a structural unit derived from a compound having a hydrophobic group, it tends to be easier to prepare a polymer compound that is responsive to stimuli.
From the above viewpoint, the hydrophilic polymer compound preferably has a structural unit derived from a (meth)acrylamide compound or a (meth)acrylic compound having a hydrophobic group, and more preferably has a structural unit derived from a (meth)acrylamide compound having a hydrophobic group. The hydrophobic group is not particularly limited, but is preferably an alkyl group having 1 to 10 carbon atoms, more preferably a linear or branched alkyl group having 1 to 8 carbon atoms, even more preferably a linear or branched alkyl group having 1 to 5 carbon atoms, and particularly preferably a branched alkyl group having 3 to 5 carbon atoms.

From the viewpoint of stimuli responsiveness, the hydrophilic polymer compound preferably has a structural unit derived from a (meth)acrylamide compound having a linear or branched alkyl group with 1 to 5 carbon atoms, more preferably has a structural unit derived from an acrylamide compound having a branched alkyl group with 3 to 5 carbon atoms, and even more preferably is poly(N-isopropylacrylamide) (NIPA).

The content ratio of structural units derived from (meth)acrylamide having a hydrophobic group (preferably (meth)acrylamide having a linear or branched alkyl group with 1 to 5 carbon atoms, more preferably acrylamide having a branched alkyl group with 3 to 5 carbon atoms, and even more preferably N-isopropylacrylamide) in the hydrophilic polymer compound is preferably 50% by mass to 99% by mass, more preferably 70% by mass to 99% by mass, based on the total structural units of the hydrophilic polymer compound.

The hydrophilic polymer compound may further contain other structural units.
Examples of other structural units include styrenes and alkyl (meth)acrylates.
When the hydrophilic polymer compound contains other structural units, the content of the other structural units is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably substantially none, based on the total structural units of the hydrophilic polymer compound.

The solid content of the hydrophilic polymer particles is preferably 1% by mass to 75% by mass, more preferably 1% by mass to 50% by mass, even more preferably 1% by mass to 20% by mass, and particularly preferably 2% by mass to 15% by mass, based on the total mass of the dispersion, since wavelength reflection can be confirmed in the visible range.

(Particle size)
The hydrophilic polymer particles have a volume average particle diameter of 100 nm to 2000 nm at 20° C. For the dispersion of the present embodiment, any particles adjusted to have a volume average particle diameter in the range of 100 nm to 2000 nm at 20° C. can be selected and used.
From the viewpoint of easily controlling the switching of Bragg reflection due to changes in temperature or ion concentration, the particles of the hydrophilic polymer compound preferably have a volume average particle size at 20° C. of 100 nm to 300 nm, and more preferably 200 nm to 300 nm.

The volume average particle size of the hydrophilic polymer particles can be measured, for example, using a dynamic light scattering (DLS) particle size distribution measuring device (product number: UPA-UT151, manufactured by Nikkiso Co., Ltd.) with the particle size temperature adjusted to 20°C.

(Coefficient of variation of particle size distribution)
The coefficient of variation of the particle size distribution of the hydrophilic polymer particles is 16% or less. When the coefficient of variation of the particle size distribution is 16% or less, the color tone of the Bragg reflection can be changed by changing the temperature or ion concentration. From the above viewpoint, the coefficient of variation of the particle size distribution is preferably 14% or less, and more preferably 12% or less.

The coefficient of variation of the particle size distribution of hydrophilic polymer particles can be determined, for example, by measuring the particles (diameter) of the hydrophilic polymer particles under the following measurement conditions using a dynamic light scattering (DLS) particle size distribution measuring device (product number: UPA-UT151, manufactured by Nikkiso Co., Ltd.), calculating the standard deviation using the following formula, and dividing the standard deviation by the volume average particle diameter.

Standard deviation = (d(84%)-d(16%))/2
In the above formula, d(n%) means the volume average particle diameter at the point where the cumulative curve is n% when the total volume of the particles of the hydrophilic polymer compound is taken as 100%.

Coefficient of variation of particle size distribution (CV value) (%) = standard deviation / volume average particle size x 100

The particle shape of the hydrophilic polymer compound is not particularly limited as long as the particle size can be measured, and examples of the shape include spheres, ellipsoids, polyhedrons, stars, and hollow shapes. Among these, from the viewpoint of stimuli responsiveness, the particle shape is preferably a sphere.

The method for producing the hydrophilic polymer particles is not particularly limited, and the particles can be produced by a general particle forming method. The particle forming method can be appropriately selected from various methods depending on the intended use.
Examples of the particle forming method include a method of forming particles from a hydrophilic polymer compound by physical pulverization or a particle forming polymerization method such as emulsion polymerization, suspension polymerization, or dispersion polymerization.
Alternatively, the hydrophilic polymer compound may be produced by a method in which a hydrophilic polymer before crosslinking is extruded through a nozzle nozzle or the like to form fibers, which are then crosslinked and pulverized, or by a method in which the fibers are pulverized to form particles and then crosslinked.
From the viewpoint of easy control of particle size, the particles of the hydrophilic polymer compound of this embodiment are preferably prepared by adding and dispersing a polymerizable monomer constituting the hydrophilic polymer compound described above and a crosslinkable monomer described below in an aqueous solvent described below, and then polymerizing the resulting particles of the hydrophilic polymer compound using a polymerization initiator described below, and purifying the resulting particles.

The crosslinking of the hydrophilic polymer compound is not particularly limited, and can be carried out by a publicly known method.
For example, a hydrophilic polymer compound having a crosslinked structure in which the hydrophilic polymer compound is crosslinked to have a three-dimensional network structure can be produced by a method in which a polymerizable monomer constituting the hydrophilic polymer compound described above and a crosslinkable monomer are allowed to coexist in an aqueous solvent described below and polymerized, or a method in which the hydrophilic polymer compound described above is reacted with a crosslinkable monomer and then dispersed in an aqueous solvent described below.

<Crosslinkable Monomer>
The crosslinkable monomer is not particularly limited, but a polyfunctional monomer can be suitably used.
As the crosslinkable monomer, methylene bisacrylamide, ethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, etc. are preferred, and from the viewpoints of solubility, stability, cost, etc., methylene bisacrylamide is more preferred.

The temperature at which the particles of the hydrophilic polymer exhibit a volume change (phase transition temperature) can be appropriately designed depending on the structure and composition of the hydrophilic polymer.
The phase transition temperature is selected from the range of 0°C to 100°C, preferably from 10°C to 80°C, and more preferably from 10°C to 50°C.

<Aqueous Solvent>
The dispersion of the present embodiment contains an aqueous solvent. The aqueous solvent is not particularly limited, and may be, for example, water, a mixed solvent of a medium miscible with water, an aqueous solution of a polyol compound such as ethylene glycol or glycerin, or an aqueous solution of an oligosaccharide, a monosaccharide, a sugar alcohol, or the like.
The mixed solvent may be, for example, a mixed solvent of water and a lower alcohol having 1 to 3 carbon atoms.
In this specification, an aqueous solvent means a solvent containing at least 70% by mass of water.

The content of the aqueous solvent is preferably 25% by mass to 99% by mass, more preferably 50% by mass to 99% by mass, even more preferably 80% by mass to 99% by mass, and particularly preferably 85% by mass to 98% by mass, relative to the total mass of the dispersion.

<Electrolytes>
The dispersion of the present embodiment contains an electrolyte. By containing an electrolyte, the dispersion can be imparted with switching of Bragg reflection.
The electrolyte is not particularly limited as long as it is a compound that dissociates in the aqueous medium to form ions.
Examples of the electrolyte include inorganic acids and their salts, inorganic alkali salts, organic salts such as surfactants, etc. The electrolyte may be used alone or in combination of two or more kinds.

Examples of inorganic acids include phosphoric acid, hydrochloric acid, nitric acid, nitrous acid, sulfuric acid, boric acid, carbonic acid, bicarbonate, carboxylic acid, and sulfonic acid.
Examples of inorganic acid salts include inorganic acid salts of alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs); and inorganic acid salts of alkaline earth metals such as magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).
Examples of inorganic alkali salts include alkali metal hydroxides such as sodium hydroxide, ammonium hydroxide, and potassium hydroxide; ammonium salts such as ammonia, ammonium acetate, and ammonium sulfate; and alkylamine salts.

Examples of organic salts include surfactants such as cationic surfactants, anionic surfactants, and betaine surfactants. Among these, anionic surfactants are preferably used because they supply ions.

The anionic surfactant can be appropriately selected from known surfactants, and examples thereof include sulfate ester salts, sulfonate salts, phosphate salts, fatty acid salts, and formalin condensates.
Examples of the cation that forms the salt include an ammonium ion, a triethanolamine ion, and a metal cation.
The anionic surfactant preferably contains at least one selected from the group consisting of sulfonates and alkyl sulfates, in view of easy availability.
Examples of sulfonates and alkyl sulfates include sodium lauryl sulfate, linear sodium dodecylbenzenesulfonate, branched sodium dodecylbenzenesulfonate, di-2-ethylhexyl sodium sulfosuccinate, and sodium dodecyl sulfate, with sodium dodecyl sulfate being preferred.

From the viewpoint of switching the Bragg reflection, the electrolyte is preferably at least one selected from the group consisting of an alkali metal salt of chloride, bromide or iodide, an inorganic acid, an inorganic alkali salt and an anionic surfactant, more preferably at least one selected from the group consisting of an alkali metal salt of chloride, an alkali metal hydroxide, hydrochloric acid and sodium dodecyl sulfate, and even more preferably at least one selected from the group consisting of sodium chloride, sodium hydroxide, hydrochloric acid and sodium dodecyl sulfate.

The content of the electrolyte can be appropriately selected.
From the viewpoint of stimuli responsiveness, the lower limit of the electrolyte content is preferably 1.85×10 ⁻⁴ mol/L or more, and more preferably 1.85×10 ⁻⁴ mol/L or more and 1.85×10 ⁻¹ mol/L or less.
The electrolyte content can be determined by capillary electrophoresis.

The dispersion of the present embodiment may contain components other than the hydrophilic polymer particles, the aqueous solvent, and the electrolyte (hereinafter, also referred to as "other components"), as necessary.
Examples of other components include various additives such as antioxidants, pH adjusters, antifoaming agents, ultraviolet absorbers, and light stabilizers.

<<Method for Producing Dispersion>>
The method for producing the dispersion of the present embodiment includes a step of mixing a polymerizable monomer and a crosslinkable monomer constituting a hydrophilic polymer compound in the presence of an aqueous solvent to prepare a mixed solution (hereinafter also referred to as a "mixed solution preparation step"); a step of mixing the mixed solution with a polymerization initiator to obtain a dispersion of particles of a hydrophilic polymer compound having a crosslinked structure (hereinafter also referred to as a "polymer gel synthesis step"); and a step of purifying the particles of a hydrophilic polymer compound having a crosslinked structure (hereinafter also referred to as a "purification step"). The particles of the hydrophilic polymer compound have a volume average particle size at 20°C of 100 nm to 2000 nm and a coefficient of variation of the particle size distribution of 16% or less.
In the method for producing a dispersion of the present embodiment, by including the above steps, the particles of the hydrophilic polymer compound can have a volume average particle size at 20° C. of 100 nm to 2000 nm and a coefficient of variation of the particle size distribution of 16% or less. A dispersion containing particles of such a specific particle size can exhibit a color change (i.e., Bragg reflection) due to a change in temperature or ion concentration.
Each step of the method for producing the dispersion of the present embodiment will be described below, but the present invention is not limited thereto.

(Mixed solution preparation process)
The mixed solution preparation step is a step of preparing a mixed solution by mixing a polymerizable monomer and a crosslinkable monomer that constitute a hydrophilic polymer compound in the presence of an aqueous solvent.
Detailed description of the components that are the same as those contained in the dispersions described above will be omitted.

In the method for producing the dispersion of the present embodiment, a non-reactive surfactant may be included in the mixed solution preparation step. When a non-reactive surfactant is added in addition to the polymerizable monomer and crosslinkable monomer constituting the hydrophilic polymer compound, the emulsion stability tends to be good when the polymerizable monomer constituting the hydrophilic polymer compound is, for example, emulsion-polymerized in the polymer gel synthesis step described below, and the dispersion state of the hydrophilic polymer compound particles tends to be uniform.
In this specification, the term "non-reactive surfactant" refers to a surfactant that does not have an ethylenically unsaturated group.

The non-reactive surfactant is not particularly limited as long as it is a surfactant that does not have an ethylenically unsaturated group, and any known surfactant can be used.
As the non-reactive surfactant, for example, the anionic surfactant that does not have an ethylenically unsaturated group, the nonionic surfactant that does not have an ethylenically unsaturated group, the cationic surfactant that does not have an ethylenically unsaturated group, and the amphoteric surfactant that does not have an ethylenically unsaturated group can be mentioned.Among these, from the viewpoint of particle stability, it is preferable to use at least one selected from the group consisting of the anionic surfactant that does not have an ethylenically unsaturated group and the nonionic surfactant that does not have an ethylenically unsaturated group as the non-reactive surfactant, and it is more preferable to use the anionic surfactant that does not have an ethylenically unsaturated group.
The non-reactive surfactant may be used alone or in combination of two or more kinds.

Examples of anionic surfactants having no ethylenically unsaturated group include alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, alkyl sulfates such as sodium lauryl sulfate, polyoxyethylene alkyl ether sulfates such as sodium polyoxyethylene lauryl ether sulfate and polyoxyethylene polycyclic phenyl ether, fatty acid salts such as sodium stearate soap, and cellulose derivatives such as carboxymethylcellulose.
From the viewpoint of polymerizability, the anionic surfactant having no ethylenically unsaturated group is preferably an alkyl sulfate, more preferably a sodium salt of an alkyl sulfate, and even more preferably sodium dodecyl sulfate (SDS).

From the viewpoint of particle stability, the content of the non-reactive surfactant is preferably 1.0×10 ⁻³ parts by mass to 8.0 parts by mass, more preferably 1.0×10 ⁻² parts by mass to 4.0 parts by mass, and even more preferably 3.0×10 ⁻² parts by mass to 2.0 parts by mass, relative to 100 parts by mass of the total of the polymerizable monomers constituting the hydrophilic polymer compound.

In the mixed solution preparation process, the mixing method is not particularly limited as long as it is possible to mix the components, and mixing can be performed by any known method.

The temperature in the mixed solution preparation process is, for example, preferably 40°C to 100°C, and more preferably 50°C to 100°C.

(Polymer gel synthesis process)
The polymer gel synthesis step is a step of mixing the mixed solution with a polymerization initiator to obtain a dispersion of hydrophilic polymer compound particles.

The temperature in the polymer gel synthesis step is, for example, preferably 40°C to 100°C, and more preferably 50°C to 100°C.
The mixing time for mixing the mixed solution and the polymerization initiator in the polymer gel synthesis step is, for example, preferably 1 hour to 8 hours, and more preferably 2 hours to 6 hours.

The polymerization initiator is not particularly limited, and any known initiator that can be used in a normal polymerization method can be used.
Examples of the polymerization initiator include peroxides having a sulfonyl group, such as ammonium persulfate, sodium persulfate, and potassium persulfate; peroxides not having a sulfonyl group, such as hydrogen peroxide; benzoyl peroxide, lauroyl peroxide, di-t-butylperoxyhexahydroterephthalate, t-butylperoxy-2-ethylhexanoate, 1,1-t-butylperoxy-3,3,5-trimethylcyclohexane, t-butylperoxyisopropylcarbonate, t-butyl hydroperoxide, cumene hydroperoxide, t-butylperoxypivalate, and organic peroxides not having a sulfonyl group, such as t-hexylperoxypivalate; and azo compounds, such as azobisisobutyronitrile, azobis-4-methoxy-2,4-dimethylvaleronitrile, azobiscyclohexanone-1-carbonitrile, azodibenzoyl, and 4,4'-azobis(4-cyanovaleric acid).
Among these, the polymerization initiator preferably contains at least one selected from the group consisting of ammonium persulfate, sodium persulfate, and potassium persulfate, and more preferably sodium persulfate.

The polymerization initiator may be used alone or in combination of two or more kinds.
The polymerization initiator may be added in two or more portions.

(Refining process)
The purification step is a step of removing unreacted raw materials and by-products by purification. The purification method is not particularly limited as long as it can purify particles of a hydrophilic polymer compound.
An example of the purification step includes suction filtering a dispersion of hydrophilic polymer compound particles using a plurality of membrane filters with different mesh sizes, centrifuging the filtrate, and then purifying the resultant by desalting using an ion exchange resin.
As an example of the purification step, a dispersion of hydrophilic polymer particles is subjected to suction filtration using a plurality of membrane filters having different mesh sizes, and the filtrate is purified by dialysis.

The method for producing the dispersion of this embodiment may further include a step of adding an electrolyte. The step of adding the electrolyte is preferably performed after the polymer gel synthesis step.

(Other processes)
The method for producing the dispersion of the present embodiment may include other steps as necessary in addition to the mixed solution preparation step, the polymer gel synthesis step, the purification step, and the electrolyte addition step.

[Applications of dispersions]
The dispersion of this embodiment may be used, for example, in a form encapsulated in a capsule membrane, in a sensor that detects changes in temperature or ion concentration by reflected wavelength (preferably reflected color).
The method for encapsulating the dispersion in the capsule membrane is not particularly limited, and the capsule membrane can be produced by a known encapsulation method or a method for producing a microcapsule.

For example, the dispersion of this embodiment can provide reflected light in response to changes in temperature or ion concentration, so toys, emergency light sources, interior items (figurines, shelves, etc.), building materials (floors, walls, stairs, etc.), tableware, containers, etc. may be manufactured using microcapsules encapsulating the dispersion of this embodiment.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the following description, all "%" is by weight unless otherwise specified.

Example 1
<Production of Hydrophilic Polymer Particles>
600 mL of ultrapure water was added to a 1 L separable flask, and while stirring at 250 rpm (revolutions per minute), sodium dodecyl sulfate (SDS), 30 g of N-isopropylacrylamide (NIPA), and 2 g of N,N'-methylenebisacrylamide (BIS) as a crosslinkable monomer were dissolved in the amounts shown in Table 1, and the mixture was stirred for 30 minutes while bubbling nitrogen gas. Thereafter, the mixture was heated to 70°C and stirred for 1 hour, and 0.3 g of potassium peroxodisulfate (KPS) was dissolved in 15 mL of ultrapure water, added little by little with a syringe, and reacted for 4 hours. The obtained dispersion (suspension) was suction-filtered using membrane filters with mesh sizes of 5.0 μm, 1.0 μm, and 0.45 μm in this order. The filtrate was then centrifuged, and this procedure was repeated three times for purification, after which a desalting treatment was carried out with an ion exchange resin to obtain poly(N-isopropylacrylamide) particles (polymer gel particles 1).

The volume average particle size and particle size distribution of the obtained polymer gel particles 1 were measured at room temperature (20°C) under the following conditions using a dynamic light scattering (DLS) particle size distribution measuring device (model number: UPA-UT151, manufactured by Nikkiso Co., Ltd.). The results are shown in Table 1.

(Measurement conditions)
Shape of measurement target substance: True sphere Dispersion medium: Water Refractive index of dispersion medium: 1.33
Refractive index of the measurement target substance: 1.508

Furthermore, the standard deviation was calculated using the formula below, and the coefficient of variation (CV value) of the particle size distribution was calculated by dividing the standard deviation by the volume average particle diameter. The results are shown in Table 1.

Standard deviation = (d(84%)-d(16%))/2
In the above formula, d (n%) means the volume average particle diameter at the point where the cumulative curve is n% when the total volume of the polymer gel particles 1 is taken as 100%.

Coefficient of variation of particle size distribution (CV value) (%) = standard deviation / volume average particle size x 100

Polymer gel particles 1 were mixed with water to prepare a dispersion (suspension) so that the solid content (mass%) was as shown in Table 2 below. Each dispersion was placed in a screw tube, irradiated from the side with a white light source (HALOGEN LITHSOURCE HL-2000; Ocean Optics), and the reflection spectrum was measured with a reflection spectrum device (model number: USB2000+; Ocean Optics). The temperature condition for the reflection spectrum measurement was room temperature (approximately 25°C). The results are shown in Table 2 and Figure 1.

As shown in Table 2, when the solid content of polymer gel particles 1 is in the range of 3% by mass to 12% by mass relative to the total mass of the dispersion, a reflectance spectrum in the visible range can be confirmed.

<Measurement of Reflection Spectrum>
Water and sodium chloride (NaCl), sodium dodecyl sulfate (SDS), sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to polymer gel particles 1 to give the concentrations (mol/L) shown in Tables 3 to 6 to prepare dispersions (suspensions) of polymer gel particles 1 with a solid content of 10 mass %. Reflection spectra were measured for these dispersions at room temperature (about 25°C) and in a water bath (about 60°C). The results are shown in Tables 3 to 6.

As shown in Tables 3 to 6, when the electrolyte concentration (mol/L) of sodium chloride, sodium dodecyl sulfate (SDS), sodium hydroxide (NaOH), or hydrochloric acid (HCl) was 1.85×10 ⁻⁵ mol/L or less, no change in the reflection spectrum due to temperature change was observed, and the color was green before and after the temperature change. Dispersions with electrolyte concentrations of 1.85×10 ⁻⁴ mol/L or more showed Bragg reflection and showed green color at room temperature (25°C), and showed no Bragg reflection and showed white color when heated to 60°C. This shows that switching of Bragg reflection occurs when the electrolyte concentration in the dispersion is 1.85×10 ⁻⁴ mol/L or more.

<Measurement of Volume Average Particle Size of Polymer Gel Particles 1 with Temperature Change>
Using a dynamic light scattering (DLS) particle size distribution measuring device (model number: UPA-UT151, manufactured by Nikkiso Co., Ltd.), the volume average particle size (nm) of polymer gel particles 1 was measured at each temperature from 20° C. to 60° C. The results are shown in Table 7 and FIG.

(Examples 2 to 8)
Polymer gel particles 2 to 8 were synthesized in the same manner as in Example 1, except that the amount of sodium dodecyl sulfate (SDS) added in Example 1 was changed to the amount shown in Table 1. Using these polymer gel particles 2 to 8, the volume average particle size (nm) and the coefficient of variation (CV value) of particle size distribution were determined in the same manner as in Example 1. The results are shown in Table 1.
Moreover, the volume average particle diameters of polymer gel particles 2 to 8 were measured at each temperature from 20° C. to 60° C. in the same manner as in Example 1. The results are shown in Table 7 and FIG.

(Example 9 and Comparative Examples 1 and 2)
The polymer gel particles synthesized above were mixed in the ratio (mass ratio) shown in Table 8 to prepare a sample containing mixed polymer gel particles (hereinafter also referred to as "mixed polymer gel particles"). The mixed polymer gel particles were treated in the same manner as in Example 1 to measure the volume average particle size (nm) and standard deviation of the mixed polymer gel particles at 20°C and calculate the CV value. In addition, the mixed polymer gel particles were mixed with water so that the solid content of the mixed polymer gel particles was 10 mass%, to prepare a dispersion (suspension) of mixed polymer gel particles. The reflectance spectrum of this suspension was measured in the same manner as in Example 1. The results are shown in Table 8.

As shown in Table 8, Comparative Example 1 and Comparative Example 2, in which the CV value exceeds 16%, did not exhibit Bragg reflection. On the other hand, Example 1 and Example 9, in which the CV value is 16% or less, exhibited Bragg reflection.

Claims (7)

Particles of a hydrophilic polymer compound having a crosslinked structure;
An aqueous solvent;
an electrolyte selected from the group consisting of an inorganic acid salt of an alkali metal, an alkali metal hydroxide, an inorganic acid, and an anionic surfactant;
The content of the electrolyte is 1.85×10 ^mol /L or more,
The hydrophilic polymer particles have a volume average particle size at 20° C. of 200 nm to 300 nm and a coefficient of variation of particle size distribution of 11% or less.
A dispersion that reflects light in response to changes in temperature or ion concentration (excluding those containing polymers with an average molecular weight of 10,000 or more). The dispersion according to claim 1, wherein the solid content of the particles of the hydrophilic polymer compound is 1% by mass to 75% by mass with respect to the total mass of the dispersion. The dispersion according to claim 1 or 2, wherein the hydrophilic polymer compound contains a structural unit derived from at least one of a (meth)acrylamide compound or (meth)acrylic acid. The dispersion according to claim 3, wherein the (meth)acrylamide compound is a compound having a hydrophobic group. The dispersion according to claim 4, wherein the content ratio of the structural units derived from the (meth)acrylamide compound having a hydrophobic group is 50% by mass to 99% by mass relative to the total structural units of the hydrophilic polymer compound. A step of mixing a polymerizable monomer and a crosslinkable monomer constituting a hydrophilic polymer compound in the presence of an aqueous solvent to prepare a mixed solution;
mixing the mixed solution with a polymerization initiator to obtain a dispersion of particles of a hydrophilic polymer compound having a crosslinked structure;
A step of purifying the particles of the hydrophilic polymer compound having a crosslinked structure;
adding an electrolyte selected from the group consisting of an inorganic acid salt of an alkali metal, an alkali metal hydroxide, an inorganic acid, and an anionic surfactant;
Including,
The content of the electrolyte is 1.85×10 ^mol /L or more,
The hydrophilic polymer particles have a volume average particle size at 20° C. of 200 nm to 300 nm and a coefficient of variation of particle size distribution of 11% or less.
A method for producing a dispersion (excluding those containing polymers with an average molecular weight of 10,000 or more) that provides reflected light in response to changes in temperature or ion concentration . The method for producing a dispersion according to claim 6 , wherein the polymerizable monomer constituting the hydrophilic polymer compound includes a (meth)acrylamide compound having a hydrophobic group.