ISGS AEROGELS WORKSHOP (St. Petersburg, Russia) August 25, 2019

Transparent and Mechanically Robust Aergoels based on Hybrid Networks

Kazuki NakanishiKazuyoshi Kanamori, Guoqing Zu, Taiyo Shimizu

Institute of Materials and Systems for Sustainability,Nagoya University, Japan

Institute for Integrated Cell-Material Sciences /Department of Chemistry, Graduate School of Science,

Kyoto University, Japan

Institute of Materials and Systems for Sustainability

Low density, transparent, low RI

Low thermal conductivity

What is aerogel?

100 nm

Nano-sized fine porous structure

ρbulk ~ 0.10−0.20 g cm−3

T550nm ~ 90 %/10-mm

λ ~ 12−20 mW m−1 K−1

Excellent thermal insulation of aerogels

Ther

mal

con

duct

ivity

, λ/m

Wm

−1K−

1

Hig

her i

nsul

atio

n

100

90

80

70

60

50

40

30

20

10

0

Superinsulation by aerogels has been expected for more than 80 years !

100 nm

Mesoporous structure consists of weakly connected nanoparticles with 10−20 nm

Mechanically “friable” because of the pore structureHigh-pressure supercritical drying (SCD) is requiredDifficulties in handling and shaping of obtained aerogels

Secondary particles

Primary particles~ 1 nm

10−20 nm

Nanostructure and low mechanical property

Before drying

Duringdrying

Afterdrying

Temporal shrinkage

Aerogel-likeXEROGEL

Spring-back

Gels with high・Strength & flexibility・Hydrophobicity

are needed.

Organogel(e.g. n-hexane)

• Condensation between surface silanols may limit the spring-back.

Si-OH HO-Si → Si-O-Si + H2O

Possibility of ambient pressure drying (APD)

• Gels with low mechanical strength will be damaged.

Si

SiSi

SiSi

OHCH3

O

O

O

O

HO

CH3

CH3

O

CH3

CH3

H3C

Si

Si

Si O

O

O

Si

CH3

R

O

CH3

OO

Hydrophobic methyl groups Enhances “spring-back”

Lower crosslinking density Flexibility

Fewer residual silanols Suppresses irreversible shrinkage

SiCH3

OCH3CH3OOCH3

Methyltrimethoxysilane(MTMS)

Preparation of aerogel-like xerogels without SCD

Polymethylsilsesquioxane (PMSQ)

Polymethylsilsesquioxane(PMSQ)

H+ OH−

Surfactant

H2O, H+ OH−

−H2O, −CH3OH

Hydrolysis at r.t.

Polycondensation at > 60 ˚C

Acetic acidWaterUrea

Surfactant

＋

(NH2)2CO + H2O 2NH3 + CO2

PMSQ gel

SiCH3

OCH3CH3OOCH3

SiCH3

OHHOOH

pH

Methyltrimethoxysilane(MTMS) −CH3OH

Avoiding precipitation by• acid-base 2-step reaction for monolithic gel formation• surfactant for suppression of phase separation

One pot process to transparent PMSQ gels

Starting solution

10 mm

CTAC-1 CTAB-1 F127-1

C16H33N+(CH3)3Cl−0.40 g

CTA+Br−0.40 g

EO106PO70EO1061.0 g

Density ~ 0.14 g cm−3

Pore size ~ 50 nm Light transmittance ~ 90 %

MTMS 5 mL, 5 mM HOAc 10 mL, urea 3.0 g, surfactant

• First transparent PMSQ aerogels!• Properties are similar to silica aerogels, except mechanical property

Kanamori et al., Adv. Mater. 19, 1589 (2007), J. Ceram. Soc. Jpn. 117, 1333 (2009), etc.

Obtained PMSQ aerogels

29Si single pulse MAS NMR

SiCH3

OSiSiOOSi

T3

SiCH3

OSiSiOOH or OCH3

T2

800100012001400Tr

ansm

ittan

ce

Wavenumber/cm–1

Linear, branchedSi-O-Si

Cyclic, cageSi-O-Si

Si−OH

C−HCH3C−Si−O

-90-80-70-60-50-40ppm

FTIR

分子レベルの構造Molecular-level structures

T3:T2 = 87:13Condensation degree = 96 %

Optimized aerogel (CTAB system)

h0 h

F127 systemCTAB system

0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

Stre

ss, σ

/MPa

Strain, ε

Uniaxial Compression Test on PMSQ Aerogel

Max. shrinkage: 40 % in linear, 78 % in volume

FF ×9601 s = 16 min1 min = 16 h

Successful ambient pressure drying (Movie)

Thickness = 5 mm

400 500 600 700

20

40

60

80

100

Tran

smitt

ance

(%)

Wavelength/nm

PMSQ xerogelPMSQ aerogelSilica aerogel

PMSQ aerogel Xerogel

Bulk density/g cm−3

Transmittance at 550 nm/%

PMSQ xerogel 0.14 84

PMSQ aerogel 0.13 89

A silica aerogel 0.17 77

Comparable properties between aerogel and xerogel

10-2 10-1 100 101 102 103 104 1050

0.005

0.010

0.015

0.020

0.025

0.030

Nitrogen gas pressure/Pa

Ther

mal

con

duct

ivity

/W m

–1K–1

• Thermal conductivity comparable with silica aerogel• A problem remains: Low bending strength

Thermal conductivity of PMSQ xerogel

PMSQ xerogelSilica aerogelNitrogen gas

(Review) Kanamori, J. Mater. Res. 29, 2773 (2014)

Our new PMSQ aerogels and xerogels

PMSQ aerogel

Xerogel Monolith(Ambient Pressure Drying)

First transparent, superinsulating PMSQ aerogel

“Spring-back” behavior

Granules Composite with fibrous material (Blanket)

Kanamori, Nakanishi, et al., Adv. Mater. 19, 1589 (2007)Hayase, Kanamori, Nakanishi, et al. ACS Appl. Mater. Interfaces 6, 9466 (2014), etc.

Today’s MenuApplications

http://www.emg-pr.com/en/prfitem.aspx?id=3024Transparent insulating windows

Daylighting insulating windows/wallsMonolithGranules

Composites House/buildingwall insulation

Next objective of our research

We’ve got cool materials … But…what about

bending strength?

Chemical design of the network for better

mechanical properties

Bridged polymethylsiloxanes: 2nd generation

1,2-Bis(MethylDiEthoxysilyl)ethane(BMDE-ethy)

Si

CH3

H5C2OH5C2O

Si

CH3

CH2−CH2

OC2H5

OC2H5

Bridged PolyMethylSiloxane(Ethy-BPMS and Ethe-BPMS)

Shimizu, Kanamori, Nakanishi, et al., Langmuir 32, 13427 (2016) and Langmuir 33, 4543 (2017).

1,2-Bis(MethylDiEthoxysilyl)ethene(BMDE-ethe)

Si

CH3

H5C2OH5C2O

Si

CH3

CH=CHOC2H5

OC2H5

=CH=CHCH2−CH2O

Ethe-BPMSEthy-BPMSPMSQ

Experimental for Ethe-BPMS

Polyoxyethylene 2-ethylhexyl ether (EH-208), n ~ 8

Tetramethylammoniumhydroxide (TMAOH)

BMDE-ethe EH-208 5 mM Nitric acid

10 min at r.t.

TMAOH aq.

Gelation & aging at 60 °C or 80 °C

30 s

Solv. exch. with methanol, IPA

CO2 SCD14 MPa, 80 °C, 10 h

BMDE-ethe

This synthetic process was first developed for VTMS system:Shimizu, Kanamori, Nakanishi, et al. Chem. Mater. 28, 6860 (2016).

Almost the same for Ethy-BPMS.

0.10 M 0.30 M 0.60 M 0.90 M 1.2 M 1.5 MCTMAOH = 0.050 M

Tgel = 80 °C

BMDE-ethe EH-208 5 mM NA TMAOH aq.

0.50 mL0.50 mL0.50 mL0.50 mL

Transparency of aerogels is changed depending on CTMAOH.

Shrinkage does not change.

Ethe-BPMS aerogels (base concentration varied)

[H2O]/[BMDE-ethe] ~ 35

ρbulk = 0.14 g cm−3

T550nm = 83 %/10 mm

Ethe-BPMS

Ethe-BPMS Ethy-BPMS PMSQρbulk/g cm−3 0.14 0.15 0.14

T550nm/% 83 78 86

PMSQ

Ethy-BPMS

Mechanical comparison among similar networks

=CH=CHCH2−CH2O

Ethe-BPMSEthy-BPMSPMSQ

Three-point bending Uniaxial compression

Modulus and bending strength are higher for Ethe-BPMS and Ethy-BPMS.

Bending strain at break is higher for PMSQ.

► Similar behaviors between Ethe-BPMS and PMSQ

► Spring-back of Ethy-BPMS is slower (more viscoelastic)

Mechanical comparison among similar networks

Bending strain, ε/%

Stre

ss, σ

/MPa

Stre

ss, σ

/MPa

Compressive strain, ε/%

Stress relaxation for 1 h after 50 % uniaxial compression

Higher stress relaxation in Ethy-BPMS Similar behaviors in Ethe-BPMS and PMSQ

Stress relaxation upon compression

Rel

ativ

e st

ress

, σ/σ

0

Time, t/s

Preparation of Ethy-BPMS xerogels by APD

ρbulk 0.18 g cm−3

T550nm 77 %/10 mm0.19 g cm−3

76 %/10 mm

Aerogel Xerogel

Research on APD of Ethe-BPMS is ongoing. No thermal conductivity data yet. High precursor cost is an important issue for

industrialization (for most bridged precursors).

• PVSQ aerogels have been prepared by two-step acid base in an amphiphilic solvent

• Vinyl groups in the network are available for functionalization and strengthening.

SiCH=CH2

OCH3CH3OOCH3

VTMS

Liq. surfactant as solvent

H+ OH−

Uniform pore structure

Polyvinylsilsesquioxane (PVSQ)

Shimizu, Kanamori, Nakanishi, et al., Chem. Mater. 28, 6860 (2016)

EH-208n ρbulk = 0.17 g cm−3

T550nm = 62 %/10 mm

300 nm

SiCH=CH2

OCH3CH3OOCH3

VTMS

Liq. surfactant as solvent

H+ OH−

Wet Gel

“Vulcanization” with radical initiator

SiO

O OSiSi

O

OSi

Si

OSiO

Si

O

O

O O Si

Si

OO

Si

OSi

O

Si

OO

O

SiO

O

O

Si O

O

With AIBN in 2-propanol

at 60 ˚C

SiO

O OSiSi

O

OSi

Si

OSiO

Si

O

O

O O Si

Si

OO

Si

OSi

O

Si

OO

O

SiO

O

O

Si O

O

Post-gelation curing (“vulcanization”) for improvement in mechanical properties.

VulcanizedPVSQ gel

29Si MAS NMR

New peak appears around −65 ppm and increases with increasing [AIBN].

This can be assigned to T3 Si bonded with elongated alkyl chains.

Structural changes by vulcanization

SiO

OO

SiO

OO

n

[AIBN]= 48.7 mM24.4 mM12.2 mM6.09 mM

Pristine PVSQ

Chemical shift, δ/ppm

Ambient pressure drying

[AIBN]/mM0.1457

0.1459

Bulk density/g cm−3

Transmittance/%

6.090.1458

12.2 24.4 36.50.1558

48.70.1651

Stress and resilience increase with increasing [AIBN].

Nearly perfect spring-back in aerogels treated with AIBN with high concentration.

No obvious changes in bulk density and transparency.

0.1664

0

Uniaxial compression

Changes in compressive behavior

PVSQ Vulcanized(highest [AIBN])

Aerogel

Xerogel

Successful ambient pressure drying

0.14 g cm−3

0.81 g cm−3

0.16 g cm−3

0.17 g cm−3

T550nm = 60 %/10-mmλ = 15.3 mW m−1 K−1

or or

Radical polymerization

Vinylalkoxysilane orAllylalkoxysilane Alkoxysilane polymer

Doubly crosslinked network

or

R1,R2,R3 = OCH3, OCH3, OCH3

= OCH3, OCH3, CH3

= OCH3, CH3, CH3

Polysiloxane

Vinyl polymer

HydrolysisPolycondensation

Aerogels Xerogel

Zu, Kanamori, Nakanishi et al. ACS Nano 12, 521 (2018)Zu, Kanamori, Nakanishi et al., Chem. Mater. 30, 2759 (2018)

JP Patent 2017-162308

Doubly crosslinked system: 3rd generation

A detailed example in PVPMS

Polyvinyl-polymethylsiloxane(PVPMS) network

APD without any solvent-exchange

Di-tert-butyl peroxide

(1-5 mol%)

120 ºC, 48 h

BzOH or IPA,H2O, Catalyst (TMAOH)

Polyvinyl-methyldimethoxysilane

PVMDMS(n ~ 40-70)

Vinylmethyl-dimethoxysilane

100 ºC, 4 d

200 nm

Transparent to translucent

Homogeneous pores High BET surface area Scalable High hydrophobicity Compressive flexibility Bending flexibility Low-cost processBut lower thermal durability(~ 200 ºC)

Xerogel

80 % uniaxial compression and perfect recovery

Immersed in n-

hexane

APD againNot damaged!

Bending flexibility of a thick film

Unusual properties in PVPMS xerogels

Durability against solvents

Outstanding machinability

Zu, Kanamori, Nakanishi, et al. ACS Nano 2018, 12, 521.

DCエアロゲルの低熱伝導性と物性比較Thermal conductivity and other properties

DCエアロゲルの低熱伝導性と物性比較Aerogels based on decreased siloxane density

µm

Morphology

Particle size: 1.5-3.0 μm

Pore size: 2.0-20.0 μm

a

b

PA1-1

PA1-2

c PA2-1

d PA2-2

Particle size: 200-400 nm

Pore size: 200 nm-6 μm

PA1

PA2

Superhydrophobic

Superhydrophobic

低架橋DCエアロゲルの多孔構造Coarse pore structures of PA1,2 (less D units)

PA1-1

Mechanical properties High compression flexibility

High bending flexibility

PA1-1

PA2-1

PA1-1

1 cmshaping

PA2-2

ρ = 25 mg cm-3

Excellent machinability

低架橋DCエアロゲルの機械的物性Mechanical properties of PA1,2

Adsorption performance: efficient oil-water separation

PA1-1

Absorption capacity of PA1and PA2 for various organic solvents and oils

n-hexane/water

Absorption/drying cycle performance of PA1-1 for n-hexane (it is dried via evaporation at 60 ℃)

n-hexane

water

aerogel

低架橋DCエアロゲルの油吸収特性Oil absorption performanc of PA1

Porous structure

200 nm

PA3

SBET=438 m2 g-1

SBET=605 m2 g-1

200 nm

PA4

PA3 PA4

With the molar ratio of VDMMS to VMDMS decreasing to 1:1 and 1:2, the macroscopic phase separation is further suppressed by the network with lower hydrophobicity and higher cross-linking density, leading to a microstructure with smaller particle and pore sizes

Particle size: 35-100 nm

Pore size: 30-180 nm

Particle size: 20-80 nm

Pore size: 20-100 nm

the hydrophobicity of PA3 and PA4 becomes lower. In spite of this, the contact angles of water of PA3 and PA4 are still above 140°

Hydrophobicity

D単位の比較的多い系における微細な多孔構造の形成Fine pore structures of PA3,4 (more D units)

High compression flexibility

PA4

PA3

5.1 MPa4.2 MPa

PA4

bend release

shaping

PA4

High bending flexibility

Excellent machinability

air PA3 PA4

17.6 16.2

~26

Thermal superinsulation performance

微細な多孔構造をもつDCエアロゲルの機械的物性と低熱伝導性Mechanical/thermal properties of PA3,4

A simple hybridizing method

PA2-G

Graphene

PVPDMS/PVPMS network

The graphene nanopaltes with 2–10 nm thickness and 5–15 μm width are well distributed in the highly porous aerogel matrix

It is also superhydrophobic with a contact angle of water of ~157°

Morphology and superhydrophobicity

導電性物質 (グラフェン) の導入Introduction of graphene

PA2-GLEDreleasecompress

High compression flexibility Strain-sensitive conductivity

brightness fluctuates with compression and decompression of the aerogels

A strain sensor of PA2-G adhered to a finger

PA2-G

More contactsamong

graphene

less contactsamong

graphene

PA2-G

Normalized electrical resistance versus compressive strain

Zu, Kanamori, Nakanishi, et al. Angew. Chem. Int. Ed. 2018, 57, 9722.

Possibility as strain sensors

Conclusions

H2O, H+/OH−

−H2O, −ROH

Hydrolysispolycondensation

Aerogel-like

XEROGELS!

APD

n-Hexane

Wet Drying Low-densityTransparent

RSi(OR’)3

Si RR’OOR’

CH3

Si OR’OR’

CH3

Si RR’OOR’

OR’Si OR’OR’

OR’R1R2Si(OR’)2

Marshmallow gel

Other flexible aerogels/xerogels Macro-mesoporous RF

Chem. Mater. 29, 2122 (2017)Angew. Chem. Int. Ed. 52, 1986 (2013), etc

Resiliencefor Bendable, Stretchable, Machinable Aerogels

EnthalpyHardBrittle

EntropySoft

Large deformation

Organic-Inorganic Double Crosslink is the best current solution !!

Elasticity

Funds

Excellent Co-workers

Acknowledgements

Dr. Guoqing ZU (Kyoto Univ.)

Dr. Kazuyoshi KANAMORI(Kyoto Univ.)

Dr. Mamoru AIZAWA (tiem factory, Inc)

Dr. Taiyo SHIMIZU(AIST)

Incubation Program of Kyoto University