JP7720089B2 - Aerogel and its manufacturing method - Google Patents

Description

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

Currently, polymer foams widely used for purposes such as thermal insulation, shock absorption, and gas adsorption have large pores exceeding the micrometer size. This leaves much room for improvement in terms of thermal insulation, gas adsorption, and mechanical strength.

In contrast, aerogels with pores of about 100 nanometers generally have excellent heat insulating properties and gas adsorption. Aerogels made of silica gel have long been studied as a typical aerogel (Non-Patent Document 1), but they are brittle materials that crumble with even slight deformation and can withstand extremely small stresses.
Research has been reported on the use of methyltrimethoxysilane instead of the conventionally used tetramethoxysilane and tetraethoxysilane as silica gel raw materials, aiming to reduce crosslink density and improve brittleness (Non-Patent Document 2). However, the mechanical strength is unsatisfactory, and the material can only be compressed to 80% without plastic deformation, with a stress of approximately 9 MPa. Furthermore, the synthesis of aerogel requires multiple steps, prolonged heating, and strict pH control. Furthermore, the raw materials tetramethoxysilane and tetraethoxysilane lack structurally variable sites, making it difficult to tune (adjust and design) the network structure to suit specific needs.

In addition, research has been reported that aims to improve brittleness by using aerogels that are carbonized after cross-linking polymerization of resorcinol and formaldehyde instead of silica gel (Non-Patent Document 3), but the mechanical strength is unsatisfactory. Aerogels with normal density crack when compressed by about 3%, with a stress of about 0.8 MPa. By lowering the density, they can be compressed to about 50%, but the stress at that point is extremely small at 0.08 MPa. Furthermore, the synthesis of aerogels requires multiple steps and prolonged heating. The network structure can be tuned by changing the pH or amount of catalyst used when reacting resorcinol and formaldehyde, but this is not a direct tuning method.

Samuel Stephens Kistler, "Coherent expanded aerogels and jellies," Nature, May 16, 1931, vol. 127, p. 741 Kazuyoshi Kanamori et al., "New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties," Advanced Materials, 2007, vol. 19, p. 1589-1593 R. W. Pekala, "Organic aerogels from the polycondensation of resorcinol with formaldehyde", Journal of Materials Science, 1989, vol. 24, p. 3221-3227

Therefore, the present invention aims to provide an aerogel and a method for producing the same that have excellent mechanical strength, can reduce synthesis costs in terms of raw material prices, number of steps, reaction time, and condition control, and further allows for tuning of the composition.

As a result of intensive research to solve the above problems, the present inventors have found that the following aerogel can solve the above problems, and have completed the present invention.
That is, the present invention provides the following inventions.

<1>
An aerogel having a structural unit represented by general formula (1):
In general formula (1), ^R1 and ^R2 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; ^R3 represents a divalent group selected from alkylene groups having 1 to 10 carbon atoms which may have a (meth)acryloyl group or a (meth)acryloyloxy group and which may have a nitrogen atom, and alkyleneoxy groups represented by _{( CxH2xO} ) _y (x is an integer of 1 to 4, and y is an integer of 1 to 15); each A independently represents a single bond, NH, or O; and * represents a bonding site.
<2>
The aerogel according to <1>, which is made of a polymer having 40 or more structural units represented by general formula (1) in one molecule.
<3>
<1> or <2>, wherein the aerogel has pores of 100 nm or less.
＜4＞
<1> to <3>, the method for producing an aerogel according to any one of the above items <1> to <3>, comprising: a step of reacting a compound represented by general formula (1') in a solvent in the presence of a polymerization initiator to obtain a gel having a structural unit represented by general formula (1); and a step of drying the gel.
In general formula (1'), ^R1 and ^R2 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; ^R3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, and represents a divalent group selected from alkylene groups having 1 to 10 carbon atoms which may have a nitrogen atom, and _alkyleneoxy groups represented by ( _CxH2xO ) _y (x is an integer of 1 to 4, and y is an integer of 1 to 15); and A each independently represent a single bond, NH, or O.
<5>
<4> The method for producing an aerogel according to <4>, wherein the solvent used in the step of obtaining the gel is at least one selected from dimethylformamide, dimethyl sulfoxide, methanol, isopropyl alcohol, and water.

The aerogel of the present invention can be compressed up to 95% without breaking, and has excellent mechanical strength with a maximum stress of 138.7 MPa. Furthermore, the aerogel of the present invention can be synthesized in a short time by radical polymerization at room temperature using inexpensive raw materials with structurally variable sites, reducing synthesis costs and enabling the aerogel network structure to be tuned according to purpose.

1 is a scanning electron microscope image of an aerogel of the present invention. 2A is a stress-strain curve of an aerogel made from a difunctional monomer, FIG. 2B is a stress-strain curve of an aerogel made from a trifunctional monomer, and FIG. 2C is a stress-strain curve of an aerogel made from a tetrafunctional monomer.

Aerogel The aerogel of the present invention has a structural unit represented by general formula (1).

In general formula (1), * represents a binding site.

In general formula (1), ^R1 and ^R2 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms. Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, an s-butyl group, a t-butyl group, a pentyl group, and a hexyl group.
R ¹ and R ² are preferably a hydrogen atom or a methyl group.

In general formula (1), ^R3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, and is a divalent group selected from alkylene groups having 1 to 10 carbon atoms and optionally having a nitrogen atom, and _alkyleneoxy groups represented by ( _CxH2xO ) _y (x is an integer of 1 to 4, and y is an integer of 1 to 15).
The alkylene group having 1 to 10 carbon atoms may be linear or branched, and specific examples thereof include a methylene group, an ethylene group, a propylene group, a trimethylene group, a tetramethylene group, a pentamethylene group, a neopentylene group, a hexamethylene group, a heptamethylene group, an octamethylene group, a nonamethylene group, and a decamethylene group.
Some of the hydrogen atoms in the alkylene group having 1 to 10 carbon atoms may be substituted with (meth)acryloyl groups or (meth)acryloyloxy groups. Furthermore, some of the carbon atoms in the alkylene group having 1 to 10 carbon atoms may be substituted with nitrogen atoms. When the alkylene group has a nitrogen atom, it is preferable that one or two carbon atoms in the alkylene group are substituted with nitrogen atoms, and further it is preferable that a hydrogen atom bonded to the nitrogen atom is substituted with a (meth)acryloyl group.

The alkyleneoxy group represented _{by (CxH2xO)y may be} linear or branched. Specific examples of the repeating unit enclosed in parentheses in the alkyleneoxy group include the following units (groups):
_{CH2O
CH2CH2O ​
CH2CH2CH2O ​ ​}
C( _CH3 ) _HCH2O
CH2C ( _CH3 )HO
_{CH2CH2CH2CH2O ​ ​ ​}
When y is 2 or more, the repeating unit may be composed of a plurality of types.

Preferred examples of ^R3 include a methylene group, a pentamethylene group, a neopentylene group, a 2-ethyl-2-methyl-trimethylene group, a hexamethylene group, and an octamethylene group, as well as groups in which some of the hydrogen atoms in these groups have been substituted with (meth)acryloyl groups or (meth)acryloyloxy groups and/or groups in which some of the carbon atoms in these groups have been substituted with nitrogen atoms.
Specific examples of ^R3 that are preferred include divalent groups represented by the following structural formulas:
In the above structural formula, R ⁴ each independently represents a (meth)acryloyl group or a (meth)acryloyloxy group, and "-" indicates the bonding point with A in general formula (1).

In general formula (1), A's each independently represent a single bond, NH, or O.

The aerogel of the present invention has a structural unit represented by the general formula (1), and it can be estimated that the number of repeating units in one molecule must be 40 or more to form an aerogel. However, it is difficult to directly measure the molecular weight of the polymer in the aerogel. Therefore, it is impossible or impractical to directly determine the number of structural units represented by the general formula (1) in the aerogel of the present invention.
The aerogel of the present invention may be a polymer in which the structural unit represented by general formula (1) in one molecule is the same unit, or may be a polymer in which a plurality of types of units are present.

Production Method The aerogel of the present invention can be produced, for example, by a method having the following two steps.
Process 1
Step 2: A step of reacting a compound represented by general formula (1') in a solvent in the presence of a polymerization initiator to obtain a gel having a structural unit represented by general formula (1).
drying the gel

Process 1
As a source of the structural units of the aerogel having the structural units represented by the general formula (1) of the present invention, a compound (monomer) represented by the following general formula (1') is used.

In the general formula (1'), R ¹ and R ² each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.
In general formula (1'), ^R3 represents a divalent group selected from alkylene groups having 1 to 10 carbon atoms and _alkyleneoxy groups represented by ( _CxH2xO ) _y (x is an integer of 1 to 4, and y is an integer of 1 to 15).
In the general formula (1'), A's each independently represent a single bond, NH or O.

Specific groups of R ¹ , R ² and R ³ in general formula (1′) include the same groups as those exemplified as R ¹ , R ² and R ³ in general formula (1), respectively.

Specific examples of the compound represented by formula (1') include:
N,N'-methylenebisacrylamide,
N,N'-methylenebismethacrylamide,
N,N'-ethylenebisacrylamide,
N,N'-ethylenebismethacrylamide,
N,N'-propylene bismethacrylamide,
Bisacrylamides such as N,N'-butylenebismethacrylamide;
ethylene glycol diacrylate,
ethylene glycol dimethacrylate,
1,3-propanediol diacrylate,
1,3-propanediol dimethacrylate,
1,4-butanediol diacrylate,
1,4-butanediol dimethacrylate,
Neopentyl glycol diacrylate,
Neopentyl glycol dimethacrylate,
3-methyl-1,5-pentanediol diacrylate,
3-methyl-1,5-pentanediol dimethacrylate,
1,6-hexanediol diacrylate,
1,6-hexanediol dimethacrylate,
1,9-nonanediol diacrylate,
1,9-nonanediol dimethacrylate,
1,10-decanediol diacrylate,
1,10-decanediol dimethacrylate,
diethylene glycol diacrylate,
diethylene glycol dimethacrylate,
triethylene glycol diacrylate,
triethylene glycol dimethacrylate,
polyethylene glycol diacrylate (number of ethylene glycol repeating units: 4 to 15);
polyethylene glycol dimethacrylate (number of repeating ethylene glycol units: 4 to 15);
Polypropylene glycol diacrylate (number of propylene glycol repeating units: 4 to 15),
Polypropylene glycol dimethacrylate (number of propylene glycol repeating units: 4 to 15)
and polyfunctional acrylamides or (meth)acrylates represented by the following formulas.

To promote the polymerization reaction of the compound represented by general formula (1'), a polymerization initiator is used in step 1. The polymerization initiator used in the production of the aerogel of the present invention is not particularly limited as long as it is an initiator that can polymerize the above-mentioned monomers, but photopolymerization initiators are preferred, such as 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methylpropiophenone.

The polymerization reaction of the compound represented by general formula (1') is carried out in a solvent that can dissolve the compound represented by general formula (1') but does not react with the compound represented by general formula (1'). Examples of such solvents include dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone, and isopropyl alcohol. These solvents may be used alone or as a mixture of two or more. Dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone are particularly preferred solvents.

Additives may be added to the reaction system in Step 1 to improve the mechanical strength of the aerogel or to impart functionality, provided that the effects of the present invention are not impaired. Examples of additives include nanoparticles, nanorods, or nanosheets of metals, metal oxides, metal hydroxides, alloys, etc.; various nanofibers; and nanocarbon materials such as carbon nanodots, carbon nanotubes, graphene, and graphene oxide.

If the concentration of the compound represented by general formula (1') in the reaction system in step 1 is 40 mg/mL or less, the polymer will be too soft to form an aerogel, so the concentration is preferably more than 40 mg/mL. On the other hand, if the concentration is 120 mg/mL or more, the polymer will become hard like a resin and will not easily form an aerogel, so the concentration is preferably less than 120 mg/mL.
The amount of the polymerization initiator in the reaction system is appropriately determined depending on the amount of the compound represented by general formula (1') and the type of polymerization initiator, etc. For example, when the concentration of the compound represented by general formula (1') in the reaction system is 60 mg/mL, the concentration of the polymerization initiator is preferably 1 to 5 mg/mL, particularly 2 mg/mL.

Furthermore, the reaction conditions for the polymerization reaction of the compound represented by general formula (1') are not particularly limited as long as a gel can be obtained, and the reaction temperature, reaction time, etc. can be in the same range as for conventional gels.
The polymerization reaction is preferably carried out in an atmosphere of an inert gas such as nitrogen or argon.

The above reaction produces a gel (hydrogel or organogel) in which the reaction solvent is the dispersion medium. Furthermore, before step 2, if necessary, solvent substitution of the dispersion medium of the gel obtained in step 1 may be carried out. Solvent substitution may facilitate the drying process in the next step 2. When solvent substitution is carried out, examples of the solvent used include ethanol, methanol, and 2-propanol. Solvent substitution methods include immersing the sample in the substitution solvent for a certain period of time, either once or repeatedly several times.

Process 2
The gel obtained in step 1 is dried to obtain an aerogel. As the drying method, any known method for drying an aerogel can be used. Examples include atmospheric drying, supercritical drying, and freeze drying. However, supercritical drying is preferred because it does not cause destruction or shrinkage of the gel skeleton.
When the gel obtained in step 1 is dried by supercritical drying, the dispersion medium of the gel is removed at a temperature and pressure equal to or higher than the critical point of the dispersion medium of the gel. In the case of supercritical drying, it is preferable to perform the solvent substitution described above, whereby the dispersion medium of the gel is substituted from the reaction solvent of step 1 with ethanol, and further with liquefied carbon dioxide.

The aerogel of the present invention has pores of 1 nm or more and 100 nm or less. The pore size is measured by scanning electron microscopy of the aerogel.

The density of the aerogel of the present invention can be changed depending on the application, and can be set in the range of, for example, 0.1 to 0.2 g/cm ³ .

Applications of the Aerogel The aerogel of the present invention is expected to be used as a heat insulating material, shock absorbing material or gas adsorbing material, and also as a construction material, helmet material, blanket, space material including spacesuit, and the like.

Example 1
[Preparation of aerogel from bifunctional monomers]
180 mg of N,N'-methylenebisacrylamide and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and the solution was bubbled with nitrogen and then sealed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel.
This gel (3.0 ^cm3 ) was immersed in ethanol (100 mL) at room temperature for 6 hours, and then the ethanol was replaced with the same amount of pure ethanol. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel A1.
Aerogel A2 was also obtained in the same manner as in the preparation of Aerogel A1, except that the amount of N,N'-methylenebisacrylamide was changed to 240 mg.

Example 2
[Preparation of aerogel from trifunctional monomers]
180 mg of N,N',N''-triacryloyldiethylenetriamine and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and after bubbling with nitrogen, the solution was placed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel.
This gel (3.0 ^cm3 ) was immersed in ethanol (100 mL) at room temperature for 6 hours, and then the ethanol was replaced with the same amount of pure ethanol. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel B1.
Aerogel B2 was also obtained in the same manner as in the preparation of aerogel B1, except that the amount of N,N',N''-triacryloyldiethylenetriamine was changed to 240 mg.

Example 3
[Preparation of aerogel from tetrafunctional monomers]
180 mg of N,N',N'',N'''-tetraacryloyltriethylenetetramine and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and after bubbling with nitrogen, the solution was placed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel.
This gel (3.0 ^cm3 ) was immersed in ethanol (100 mL) at room temperature for 6 hours, and then the ethanol was replaced with the same amount of pure ethanol. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel C1.
Aerogel C2 was also obtained in the same manner as in the preparation of aerogel C1, except that the amount of N,N',N'',N'''-tetraacryloyltriethylenetetramine was changed to 240 mg.

[Evaluation of aerogel properties]
The obtained aerogel was evaluated for the following physical properties.

Density The density of the aerogels was measured by dimensional and gravimetric measurements, and the results are shown in Table 1.

Scanning electron microscope (SEM)
An arbitrary cross section of the aerogel A1 was observed with a scanning electron microscope.
Method for preparing the sample to be measured: The sample was cut into thin slices using a razor, and platinum was vacuum-deposited on the slices.
Measuring device: Hitachi High-Technologies Corporation SU8010
Measurement conditions: vacuum, room temperature. The results are shown in Figure 1. A three-dimensional network of N,N'-methylenebisacrylamide polymers and pores of several tens to several hundred nanometers were confirmed.

Mechanical property evaluation The mechanical strength of the aerogel was evaluated by a compression test.
Method for preparing the sample to be measured: Cut with a razor and process into a cube with a side length of 10 mm.
Measurement equipment: Shimadzu AGS-10kNS precision universal testing machine Measurement conditions: Room temperature, compression speed 10 mm min ^-1
The results are shown in Figure 2. All aerogels exhibited excellent mechanical strength, and aerogel A1 of Example 1 had a compressive modulus of 2.12 MPa calculated from a strain range of 0 to 10%, and could be compressed up to 95% without fracture, with a stress of 130 MPa or more.

The aerogel of the present invention is expected to be used as an insulating material, shock absorbing material, or gas adsorption material in construction materials, helmet materials, blankets, space materials including space suits, and more.

Claims (2)

A method for producing an aerogel, comprising: a step of reacting a compound represented by general formula (1') by irradiating the compound with light in a solvent at room temperature under an inert gas atmosphere in the presence of at least one photopolymerization initiator selected from the group consisting of 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methylpropiophenone to obtain a gel having a structural unit represented by general formula (1); and a step of drying the gel , wherein the aerogel is
The pores are in the range of 1 to 100 nm,
and
A material that satisfies at least one of the following requirements: density of 0.1 to 0.31 g/cm3 ^and compressibility up to 95% without breaking
A method for producing an aerogel .
In general formula (1'), ^R1 and ^R2 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; ^R3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, and represents a divalent group selected from alkylene groups having 1 to 10 carbon atoms which may have a nitrogen atom, and _alkyleneoxy groups represented by ( _CxH2xO ) _y (x is an integer of 1 to 4, and y is an integer of 1 to 15); and A each independently represent a single bond, NH, or O.
In general formula (1), R1 ^and R2 ^each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; R3 represents a divalent group selected from ^alkylene groups having 1 to 10 carbon atoms which may have a (meth)acryloyl group or a (meth)acryloyloxy group and which may have a nitrogen atom, and alkyleneoxy groups represented by (CxH2xO ) _y ( _x is an integer of 1 to 4, and y is an integer of 1 to 15); each A independently represents a single bond, NH, or O; and * represents a bonding site _. 2. The method for producing an aerogel according to claim 1 , wherein the solvent used in the step of obtaining the gel is at least one selected from the group consisting of dimethylformamide, dimethyl sulfoxide, methanol, isopropyl alcohol, and water.