Development of Organic-inorganic Hybrid Thermal Insulation Materials
Based-on Polysilsesquioxane（ポリシルセスキオキサンを利⽤した有機–無機ハイブリッド型断熱材料の開発）

March 2023
SAKINO TAKASE

Contents
Chapter 1 General Introduction

1

1.1 Energy use and CO2 emissions

1

1.2 Thermal insulation materials

1

1.3 Organic-inorganic hybrid materials

5

1.4 Polysilsesquioxane

61.4.1 PSQ-based thermal insulation materials
1.5 Objectives and outline of this dissertation

8
10

Chapter 2 Structure–Thermal Property Relationships of Polysilsesquioxanes for Thermal
Insulation Materials

13

2.1 Introduction

13

2.2 Results and discussion

14

2.2.1 Synthesis of PSQ via nitrogen flow method

14

2.2.2 Synthesis of ethoxy-free PSQ

192.2.3 Preparation of freestanding gel films

21

2.2.4 Thermal diffusivity of the PSQ films

22

2.2.5 Thermal stability of the PSQ films

28

2.3 Conclusions31

2.4 Experimental

32

Chapter 3 Organic-Inorganic Hybrid Thermal Insulation Materials Prepared via Hydrosilylationof Polysilsesquioxane Having Hydrosilyl Groups and Triallylisocyanurate

36

3.1 Introduction

36

3.2 Results and discussion

37

3.2.1 Preparation of hybrid films by hydrosilylation reaction37

3.2.2 Thermal insulation property of hybrid films prepared by hydrosilylation reaction 42
3.2.3 Thermal stability of hybrid films prepared by hydrosilylation reaction

49

3.3 Conclusions

51

3.4 Experimental

51

Chapter 4 Double-Decker Silsesquioxane-Grafted Polysilsesquioxane Hybrid Films as Thermal
Insulation Materials

55

4.1 Introduction

55

4.2 Results and discussion

56

4.2.1 Preparation of DDSQ-grafted PSQ films via a two-step reaction

56

4.2.2 Thermal diffusivity of the DDSQ-grafted PSQ films

62

4.2.3 Thermal stability of the DDSQ-grafted PSQ films

67

4.3 Conclusions71

4.4 Experimental

72

Chapter 5 Conclusions

74

List of publications

76

Bibliography

77

Acknowledgments

85

Chapter 1 General Introduction
1.1 Energy use and CO2 emissions
The increase in fossil fuel use over the past decades raises concerns about the future depletionof fossil resources, as well as serious environmental issues leading to the problem of climatechange [2, 3]. The Paris Agreement sets a common long-term threshold for global warming below
2 ºC, preferably 1.5 ºC, in order to minimize the risks from climate change compared to preindustrial levels [4]. The main reason for global warming is the accumulation of various
greenhouse gases (GHGs) [5]. In the past few decades, the increase in worldwide fossil fuelconsumption has increased GHG emissions [2, 5]. Globally, CO2 is the most abundant
anthropogenic GHG, accounting for 76% of total anthropogenic GHG emissions in 2008, andCO2 emissions from fossil fuel use reaches 62% of total GHGs. Global emissions of CO2 from1850 to 2020 are summarized in Figure 1.1. Since the mid-20th century, CO2 emissions havecontinued to increase rapidly and global CO2 emissions have increased about sevenfold over the
past 70 years [1, 6, 7].

CO2 global emission / billion ton

40
35
30
25
20
15
10
5
0
1850

1870

1890

1910

1930 1950
Year

1970

1990

2010

Figure 1.1 Trend in CO2 emissions from fossil fuel combustion[1]1.2 Thermal insulation materials
Efficient energy use in buildings is an important factor contributing to the reduction of GHG
emissions. The building and construction business accounts for 30-40% of global energy

1consumption, mostly due to heating, ventilation, and air conditioning systems to keep people
comfortable in buildings [8]. Thermal insulator is a material that slows the rate of heat flow byconduction. Other benefits of using insulators sometimes include reduced noise levels and fire
protection [9]. In addition, insulators have many applications, for instance, in oil and liquefied
natural gas pipelines to keep the temperature low preventing evaporation of the transported
materials [8, 10]. Recent applications are in particular for automotive and aircraft engines, spacetechnology, and heat-generating equipment in industries.
Insulation materials that have been conventionally used can be roughly classified into organicand inorganic materials. Inorganic materials including glass and rock wool account for 60% of
the insulation market, while organic insulation materials such as polyurethane (PUR),
polyisocyanurate (PIR), extruded polystyrene (XPS), and expanded polystyrene (EPS) accountfor 27% [11]. Examples of organic and inorganic insulation materials are provided below.

Organic materials
Polystyrene
Polystyrene is an organic cellular resin that is used in two types of forms, expanded polystyrene(EPS) and extruded polystyrene (XPS) [12]. EPS and XPS have a partially porous structure withexcellent engineering properties, such as high energy efficiency and cost-effectiveness as athermal insulator, soundproofing properties, and packaging versatility, and is often used forbuilding insulation. It can be cast as a board or molded into many shapes for different purposesin the production line. EPS is made from small polystyrene beads that contain an expanding agent,e.g., pentane. These beads expand when heated with steam and are glued together at their contact
points. Whereas, XPS is produced by an extrusion process of melted polystyrene resin containing
a form-generating reagent or supercritical CO2. Usually, EPS and XPS can be used only below
100 °C [13]. XPS containing flame retardants are widely used as building materials.

Polyurethane Foam
Polyurethane foam (PUR) is produced by the reaction of polyisocyanates with polyols (Figure1.2) [14]. Primarily, isocyanates react with alcohols to produce urethane bonds in the polymer. In

2

addition, PUR contains a variety of functional groups such as urethanes, ureas, isocyanurates,
carbodiimides and uretdiones, which are formed by reactions with amines, water, ureas,
urethanes, and even other isocyanates generated in the polymerization process [15-17]. Heating
the PUR in an inert atmosphere, the urea and urethane decompose at 160-200 °C. PUR is also
used as an expanded foam in construction, for example, as window and door seals or as a filler tofill various gaps [18]. The maximum service temperature of PUR is 120 ºC [12].
O
O C N

R

N C O

+

polyisocyanate

HO

R’

OH

O

R

R’
N
N
O
O
H
H n
polyurethane (PUR)

polyolFigure 1.2 The main reaction of polyisocyanates with polyols

Polyisocyanurate Foam
Polyisocyanurate foam (PIR) is a kind of PUR foam and obtained from polyisocyanate andpolyol similarly to PUR with using a higher percentage of isocyanate to form isocyanurate ringsby cyclotrimerization of isocyanate (Figure 1.3). Isocyanurate is thermally stable cross-linking
group and decomposes at 270-300 °C [16]. PIR is characterized by high fire resistance as well as
high thermal stability. Among conventional foamed plastics, PIR has the best fire resistance. The
maximum operating temperature for PIR is typically 150 ºC [14].

O
R

N
N
R
N

O
O
O C N

R

N C O

polyisocyanate

O
R
O

N

N
N
R

R

N

N

R
O
O

N

O
R

O
O

O

N

N
N
R

polyisocyanurate (PIR)

Figure 1.3 Cyclotrimerization of isocyanate

3

RO

Melamine Foam
Melamine foam is produced by the reaction of melamine and formaldehyde (Figure 1.4). It
exhibits a three-dimensional network skeleton with a highly porous structure. In recent years,
melamine form is used as a thermal insulator for exhaust gas pipes and ducts, because of its
excellent flame retardant properties due to the high nitrogen content [13, 19]. The maximum
operation temperature of melamine form is approximately 200 °C, much higher than those ofother polymer foams such as PUR, polystyrene, polyethylene, and polypropylene forms [13, 20,
21].

H 2N

N
N

NH2

H
N

CH2O

N

N

H
N

N

H
N

N

N

NH

NH2

H
N

N
N
NH

melamine

Figure 1.4 Melamine-formaldehyde resin

Cellulose Fiber
Cellulose fiber insulators are made by pulverizing paper fibers, mainly unsold newspapers andused paper [8]. It is usually treated with inorganic additives (borax: sodium borate) that inhibit
flame retardants and mold growth. Cellulose is highly suitable as a filler for buildings andconstructions to fill various gaps and spaces. The maximum service temperature for cellulose
fiber is 80 ºC [13].

Inorganic materials
Foam glass
Most glass foams are obtained by sintering glass powder with a blowing agent [22]. Glass foam
is a porous, lightweight foam material with very low thermal conductivity and desirable fire
resistance. It is a sustainable insulation material that can be also manufactured from recycled
waste glass. Typically, the maximum service temperature of glass foam is 500-600 °C, andpotentially suitable lining material for thermal power plants [23, 24].

4

Rock wool and glass wool
Rock wool is typical mineral wool that is produced as boards, mats, and loose fillers. Rockwool is obtained by melting diatomaceous earth, dolerite, or other stones at temperatures around1500 °C. The final product is manufactured by spinning at a temperature of 1400 ℃ in the formof fibers. Glass wool is produced using borosilicate glass in a similar manner. Commercial glass
wool and rock wool usually contain dust-proof oil and/or phenolic resin added to improve theproperties of the product and bind the fibers together, respectively. The maximum service
temperature for rock wool and glass wool are approximately 800 ºC and 500ºC, respectively [12].
The above mentioned materials are used in many applications such as for building and thermalenergy storage due to their low thermal conductivity. Most organic insulation materials are easy
to process and provide high thermal insulation properties, but are combustible and easily melt at
high temperatures, limiting their use. In contrast, inorganic insulation materials are resistant toflame and heat and can be used at high temperatures without burning or melting. However, thethermal insulation properties of inorganic insulation materials are generally lower than those oforganic insulation materials. In addition, their poor processability and heavy weight make themdifficult to accommodate uneven shapes and locations. From the perspective of climate change,
legal obligations regarding energy consumption and related CO2 emissions are expected to
become stricter in the future. As an effective means of responding to this paradigm shift, thedevelopment of new thermal insulation materials with high heat resistance and processability isdesired to explore their use in new areas of applications.

1.3 Organic-inorganic hybrid materials
Many studies have been conducted for organic-inorganic hybrid materials that have not only
the properties of both organic and inorganic components, but also new functionalities arising fromthe synergistic effects of both components. Generally, an organic-inorganic hybrid material isdefined as that with a combination of organic and inorganic components with submicron or
nanoscale domain sizes, as opposed to a simple mixture and a composite material [25]. In hybridmaterials, inorganic components play various roles in improving mechanical and thermal
stability, modulating refractive index, or contributing to specific magnetic, electronic, redox,
5electrochemical, or chemical properties, while organic components provide unique functions suchas stimulus responsiveness, selective binding to a guest molecule for sensing, and fine tuning of
optical and electrochemical functions. For example, Qui et al. found a material that exhibits highoptical transparency and high mechanical hardness by the hybrid formation of a copolymer ofpoly(methyl methacrylate) (PMMA) and poly(3-trimethoxysilyl propyl methacrylate) (PMSMA)
with nano-sized glass particles (SiO2) [26]. Similar hybrid films of PMMA, and SiO2 or zirconia(ZrO2) were prepared by Zhong and Du et al. Because of the formation of nano-sized organicinorganic networks via chemical bonding, good compatibility of PMMA and inorganic particles
was realized resulting in good optical transparency and thermal stability [27].

1.4 Polysilsesquioxane
Polysilsesquioxane (PSQ) is a type of organic-inorganic hybrid material consisting of a Si-OSi inorganic siloxane network and organic substituents on silicon atoms. PSQ is represented by
the general formula of (RSiO1.5)n where R is an organic group such as an alkyl or aryl group,
typically a methyl, phenyl, or vinyl group. The organic groups (R) are selected to adjust thematerial properties, such as solubility, compatibility, flexibility, and surface properties according
to the desired applications. The siloxane network, on the other hand, provides thermal andmechanical properties, basically improved compared to organic polymers [28, 29]. Figure 1.5 (a)shows the acid- or base-catalyzed hydrolysis/polycondensation reaction of a trialkoxysilane
precursor leading to PSQ.

6

(a)
R

OEt
Si OEtOEt

Acid or Base + Water
Hydrolysis

R

OH
Si OH
OH

Trialkoxysilane
R = CH3, Ph, etc.

R

Si

O

O O
R Si

R
R
R
R
R
R
O
O
Si
Si
Si O Si O Si O Si O
O
O
O
O
O
O
Si O Si O Si O Si O Si O Si O
O
R
R
R
R
R
R
O

ladder structure

random structureR
O(RSiO1.5)nCondensationR
Si O
OH Si R OO O Si OO
Si O Si
Si O R
R
O
R
R O
O
O
Si
Si R
Si R
O R HOHOOOSi O SiO R R O

(b)

R

OH
Si OH
OH

Si
O

O

O

R
Si
Si O

O

O

Si
O
O Si
R
O
R
Si
Si
O
R
R

Ph
Si
HO
HO

Si
Ph

Ph Ph
O Ph
Si
O Si
Si
OH
O
Si O
O
O Si OHO Si O
Ph
Ph Ph

O
O

R
Si
R

Ph
Si
O
O

Si
Ph

Ph Ph
O Ph
Si
O Si
Si
O
O
R
Si
Si O
R
O
O Si OO Si O
Ph
Ph Ph

O
O

double-decker structure(DDSQ)

cage-shaped structure(POSS)

Figure 1.5 (a) Schematic diagram of the sol-gel method; (b) silsesquioxane oligomer frameworks

PSQs can be classified into categories of random, ladder, cage, and double-decker PSQs
(Figure 1.5 (b)) with respect to the structural skeletal. Random PSQs are often liquid compounds
when the molecular weights are low that are soluble and easy to process. In the so-called sol-gel
process, random PSQ oligomer sols are calcined at high temperature after processing to form
polymeric gel materials by accelerating further network formation. Random PSQs are easy to
prepare and most widely studied among the PSQs [30]. Recent examples include poly(3aminopropyl)silsesquioxane and poly(3-(2-aminoethylaminopropyl)silsesquioxane) that show
excellent antifogging properties in films [31]. Applications as gas or water separation membranes[32-34], optical materials [35], low-dielectric materials [36], insulating materials [37], etc. were7

also investigated. Random PSQs are also studied as base materials for further hybrid formation
with inorganic nano particles and as modifiers of inorganic surfaces.
Ladder PSQs are considered as one of the most regular structures among the isomers. The
pseudo one-dimensional structures composed of two polysiloxane chains connected regularly
without branches provides high solubility of the polymer materials. Micrometer-sized ladder-typesilsesquioxanes are known to be thermally stable and processible, and high-performance selfhealing and scratch resistant ladder PSQ materials have been reported [38, 39]. Phenyl andcarbazole groups are known as substituents of ladder-type PSQ. Cage-shaped polyhedraloligomeric silsesquioxanes are called as POSS. In particular, POSS molecules with a highly
symmetric T8 (R8Si8O12) cubic inorganic core with eight silicon atoms at the vertices have been
most widely studied as the smallest and simplest silica particles with 1-3 nm in diameter,
including the vertex groups [40]. Recently, double-decker silsesquioxane (DDSQ) structures havealso attracted attention, featuring open or closed backbones with two or four reactive moieties;
Zheng et al. reported that the inclusion of DDSQ in the main chain of polyimides improved thethermal stability and surface hydrophobicity (Figure 1.6) [25].

O
O

N

O

O
N
m

O

O

CH CH
Si
Me

Ph
Si
O
O

Si
Ph

Ph Ph
O Ph
Si
O Si
Si
O
O
Me
Si
O
Si
CH CH
O
O
O Si
O Si O
Ph
Ph Ph

O
O

O

O

O

N

N

O

O

n

Figure 1.6 Structure of DDSQ in the main chain of polyimides

1.4.1 PSQ-based thermal insulation materials
As described above, PSQ materials are expected to have properties derived from both theorganic and inorganic moieties, and therefore expected to be used as excellent thermal insulating
materials with high processability, mechanical strength, thermal stability, and flame resistance.
Aerogels, defined as porous materials in which the liquid in the wet gel is replaced by gas withlittle or no shrinkage were first prepared by S. S. Kistler of Stanford University in the 1930s
through supercritical drying methods. Since then, aerogels have been extensively investigated asfunctional materials such as thermal insulating materials. However, the structure fragility and the8high production cost due to the use of supercritical drying have been major barriers to thedevelopment of practical applications of silica aerogels [11]. PSQ aerogels are ultra-low density
nanoporous solids with excellent properties such as high surface area and porosity, low dielectric
constant and thermal conductivity, and good optical transparency. In general, these PSQ aerogels
are prepared using organotrialkoxysilanes as starting substrates in the two-step sol-gel methodwith acid or base-catalyzed reaction, followed by liquid evacuation from the wet gel byatmospheric or supercritical drying methods [41]. Rao et al. prepared highly flexible silica
aerogels obtained using methyltrimethoxysilane (Figure 1.7) [42]. However, the capillarypressure caused cracking due to evaporation of excess water and ethanol.

OCH3
Hydrolysis:

H3C Si OCH3

+ 3H2O

OCH3

C2H2O4MeOH

OH
H3C Si OH+ 3CH3OH

OH

Trimethylmethoxysilane
OH
OH

OH

NH4OH

Condensation: 2 H3C Si OH + HO Si CH3
OH

OH

OH

H3C Si O Si CH3
OO+ 2H2OH3C Si O Si CH3OHOH

Figure 1.7 Preparation of PSQ aerogel precursor.

To obtain PSQ-based insulation materials that rigid and easy to handle, Hamada et al. haveproposed several strategies. For example, PSQ-based thermal insulating films with an ethylene
crosslinked structure were prepared by hydrosilylation of hydrosilyl- and vinylsilyl-substituted
oligomethylsilisesquioxanes (MSQ-SiH and MSQ-SiVi) to exhibit high thermal stability [43]. In
further efforts to improve the adiabatic performance of PSQ, POSS was embedded into the PSQ
network by hydrosilylation, and it was found that the void formation around the rigid POSS
structure was particularly favorable for improving the adiabatic performance (Figure 1.8) [44].

9

O
Si
O

O
Si

O

O O
O
O
Si
Si
O Si
O
O
Si
H

Si
O
Si
O
O Si
Si
O
O
Si
O

O

Si O
Si O
O
O
O
Si
Si
O

Pt

Si

O

Si O
O

Si

O

O
Si
O

Si

O Si OOSiO O

MSQ-SiVi+
O

MSQ-SiH
O
Si

O
Si
O

O

Si
Si O

Pt

O

O Si
Si

O
O

Si

Si

O
Si
O

O

O Si

Si
O
O

Si

O
O

O
Si

O

O

O
O Si
Si

Vinyl-POSS

O

Si
Si O

O

Si
O

Si

O
Si

O

O

O

O

O
Si
O Si

O
O

Si

Si
O
Si
O
O Si O

Figure 1.8 Schematic diagram of cross-linking formation by hydrosilylation between MSQ-SiH
and MSQ-SiVi or Vinyl-POSS [43, 44].

1.5 Objectives and outline of this dissertation
As mentioned above, the development of high-performance thermal insulation materials is ofcurrent necessity as a solution to the problems caused by global warming and increasing energy
demand. In particular, those usable in various conditions, like harsh conditions at hightemperature are desired. In this dissertation, the efforts to develop PSQ-based next-generation
thermal insulating materials for a wide range of applications are described. The relationshipbetween PSQ structures and the thermal insulating properties and new approaches for the
suppression of heat transfer are discussed. The contents of the chapters that follow this chapter
are described below.
Chapter 2 describes the investigation of structure-thermophysical property correlation of PSQbased gel film. PSQ with different molecular weights and residual alkoxy group contents weresynthesized by changing the molar ratio of water to trialkoxysilane for the hydrolysis toinvestigate their effects on the adiabatic properties. To understand the effect of substituents

10

attached to the silicon atoms of PSQs, PSQ films with different substituents attached to the silicon
atoms were synthesized and the adiabatic and thermal stability of the gel films were investigated(Figure 1.9).

Figure 1.9 Structure of PSQ gel films for thermal insulator

In Chapter 3, alkoxy- and silanol-free oligo(methylsilsesquioxanes) (MSQ) were synthesizedand hybrid films containing triallylisocyanurate (TAIC) as cross-linking units were prepared(Figure 1.10). The obtained MSQ/TAIC hybrid films were colorless, transparent, and stiff,
regardless of the amount of TAIC in the PSQ network. The thermal diffusivity of the MSQ/TAIC
hybrid films decreased from 1.14 × 10–7 to 1.01 × 10–7 m2/s with an increasing amount of TAICin the molar ratio range of 1/0.25–1/1.15 and was lower than that of a TAIC-free MSQ film
prepared by the coupling reaction of MSQ-SiH (1.44 × 10–7 m2/s) and a PSQ/tris(3triethoxysilylpropyl)isocyanurate (TESIC) film prepared by polycondensation (1.25 × 10–7 m2/s).
A positron annihilation lifetime analysis indicated that the nature of the isocyanurate ring, rather
than the formation of intramolecular void spaces, was responsible for the decrease in thermal
diffusivity.

O
Me Si O
Me

O
Si
Me

O Me
O Si
Si
O
Me
Me

N

O
N

O
N

O

Me
O
Si
Me O Si O
MeFigure 1.10 Structure of MSQ/TAIC hybrid film

11

In

Chapter4,thehydrosilylation

reaction

betweenhydrodimethylsilylated

oligomethylsilsesquioxane and vinyl-functionalized double-decker silsesquioxane (vinylDDSQ), which is a highly thermally stable unit, was conducted to obtain aggregation-free DDSQgrafted PSQ films (Figure 1.11). Unlike POSS-containing hybrid films, the high miscibility of
the DDSQ molecule allowed the introduction of a high amount of vinyl-DDSQ as a crosslinker,
giving colorless and transparent DDSQ-grafted PSQ films without aggregation of the DDSQ
molecules. The thermal diffusivities of the DDSQ-grafted PSQ films decreased from 1.21 × 10–7
to 1.02 × 10–7 m2/s with increasing the DDSQ amount from 5 to 25 mol%, as was similarly
observed for POSS-containing hybrid films. These results indicate that the low thermal diffusivityof the DDSQ-grafted PSQ films did not stem from the formation of intramolecular void spaces
but from a decrease in the crosslinking density upon the introduction of the DDSQ structure.

O
O Si

O Si O
Si
O

O

Si

O
O

O

Si
O

Si

Si

O
O

Si

Si O
Si

O
O Si

O
Si

O
O

Si

Si

O
O Si OOSiO OFigure 1.11 Structure of DDSQ-grafted PSQ film

In Chapter 5, the author concludes the works described in Chapters 2-4 and describes thepotential of PSQ-based heat insulating materials in the future.

12Chapter 2 Structure–Thermal Property Relationships ofPolysilsesquioxanes for Thermal Insulation Materials
2.1 Introduction
PSQs exhibit good processability, which enables the easy preparation of their films via solution
processes such as spin coating and dip coating. Hamada et al. reported a strategy for the
preparation of crack- and shrinkage-free ethylene-bridged PSQ (EBPSQ) films as thermal
insulation materials [43]. The introduction of SiCCSi bonds as an ethylene-bridge structure
decreased the density of the PSQ film by forming intramolecular void spaces, enhancing thethermal insulation property. Furthermore, the EBPSQ films exhibited high thermal stability up toaround 300 °C, which demonstrates the suitability of PSQ-based films as practical thermal
insulation materials. However, the relationship between the PSQ structure and the thermal
insulation property of the corresponding gel films is still not well understood. Unveiling thestructure–thermal property relationship can be expected to provide a platform for the design andsynthesis of thermal insulation materials.
With this aim, this chapter describes the investigation of the structure–thermal property
relationship in gel films prepared from PSQs. To investigate the influence of residual alkoxygroups in PSQs on the thermal insulation property, PSQs with different molecular weights andamounts of residual alkoxy groups were synthesized by changing the molar ratio of water andtrialkoxysilane for hydrolysis/condensation. To understand the influence of the substituents
bonded to the silicon atom in PSQs on the thermal insulation property of their gel films,
copolysilsesquioxanes (co-PSQs) with different substituents bonded to the silicon atom weresynthesized by changing the monomers and the monomer ratio. Furthermore, alkoxy-free PSQs
with different substituents bonded to the silicon atom were prepared via hydrolysis andpolycondensation reaction of alkoxysilane under excess acid and water to investigate theinfluence of the substituents on the thermal insulation property and the thermal stability of the gel
films.

13

2.2 Results and discussion
2.2.1 Synthesis of PSQ via nitrogen flow method
For the preparation of PSQs, we conducted the sol–gel reaction of triethoxymethylsilane using
the nitrogen flow method according to a previously reported procedure, which allows controllingthe molecular weight of PSQ and the amount of residual alkoxy groups [45]. In this method,water, catalyst, and solvent are volatilized by passing a nitrogen flow during thepolycondensation, providing a mild polycondensation condition that prevents the formation of an
insoluble gel. Because our purpose was to investigate the influence of residual alkoxy groups on
the thermal insulation property of PSQ gel films, we synthesized polymethylsilsesquioxanes
containing ethoxy groups (PSQ-Me (OEt)) with different molecular weights and amounts ofresidual ethoxy groups by changing the molar ratio of water and monomer, as shown in Table
2.1. As described in Experimental Section in this chapter, we used the following conditions forthe synthesis: 0 °C for 10 min and room temperature for another 10 min for the hydrolysis and
90 °C for 3 h for the polycondensation reactions. In the sol–gel reaction of triethoxymethylsilane,
when the molar ratio of water/monomer increased from 1.1 to 1.5, the weight-average molecularweight (Mw) of PSQ-Me (OEt) increased from 1000 to 13000 and the amount of residual ethoxy
groups decreased from 22% to 7% estimated by gel permeation chromatography (GPC) and 1H
NMR, respectively (Table 2.1, entries 1–5). Although the obtained high-molecular-weight PSQMe (OEt) was highly viscous, it showed good solubility in ordinary organic solvents such as THF,
acetone, and chloroform. For a water/monomer molar ratio of 1.6, a gel was formed during
heating at 90 °C (Table 2.1, entry 6).

14

Table 2.1. Preparation of PSQ-Me (OEt) and PSQ-Me (OBui) via the sol–gel methoda,b

Temp.

Time

Molar ratio of

Residual

(°C)

(h)

H2O/silane

OR (%)

1

90

3

1.1

2

90

3

3

90

4

Entry

a

Silane

Mw

Mw/Mn

22

1000

1.8

1.2

14

1800

1.9

3

1.3

10

4600

4.1

90

3

1.4

7

8000

3.1

5

90

3

1.5

7

13000

7.3

6

90

3

1.6

7

100

5

1.8

19

1200

1.7

8

100

5

2.0

16

1500

1.8

9

100

5

2.5

14

1800

2.0

10

100

5

3.0

12

3000

2.7

Molar ratio of monomer/HCl = 0.105.

b

Molar ratio of solvent/monomer = 2.

15

GelationIn the case of the sol–gel reaction of triisobutoxymethylsilane, we conducted thepolycondensation reactions at 100 °C for 5 h to completely remove the isobutyl alcohol because
the boiling point of isobutyl alcohol is higher than that of ethanol. As shown in Table 2.1,
the Mw of polymethylsilsesquioxane having isobutoxy groups (PSQ-Me (OBui)) increased from1200 to 3000 with increasing the water/monomer molar ratio (1.8–3.0), and the amount of residual
isobutoxy groups in PSQ-Me (OBui) also decreased from 19% to 12% (Table 2.1, entries 7–10).
The hydrolysis of the isobutoxy group was slower than that of the ethoxy group because of thelarger steric hindrance of the former, as was similarly described in a previous report. (18) At asmall water/monomer molar ratio, PSQ-Me (OBui) with low molecular weight and a high amountof isobutoxy groups was obtained (Table 2.1, entry 7). Although increasing the water/monomer
molar ratio up to 3 (Table 2.1, entry 10) increased the Mw to 3000 and decreased the residual
isobutoxy group to 12%, these changes were not as significant as those of the sol–gel reaction oftriethoxymethylsilane, particularly regarding the molecular weight.
Figure 2.1 shows the plots of Mw and residual alkoxy groups versus the water/monomer molar
ratio, respectively. The Mw value of PSQ-Me (OEt) and PSQ-Me (OBui) increased linearly withincreasing the amount of water, which indicates that the sol–gel reaction performed via thenitrogen flow method allowed controlling the molecular weight of PSQs by changing thewater/monomer molar ratio. The slope of the Mw plots of PSQ-Me (OBui) was gentler than thatof PSQ-Me (OEt). This is due to the steric hindrance of the isobutoxy group, which limits thereactivity of the residual isobutoxy group in the PSQ for the hydrolysis and the polycondensation
reaction. Consequently, the sol–gel reaction of triisobutoxymethylsilane afforded a lowmolecular-weight PSQ-Me (OBui). Nevertheless, this is no drawback to our purpose, that is, toinvestigate the effect of the alkoxy group on the thermal insulation property of the gel film.

16

5

10

40

Residual alkoxy group (%)

(a)

Mw

104

103

2

(b)

35
30
25
20
15
10
5
0

10

0

0.5

1

1.5

2

2.5

3

0

3.5

0.5

1

1.5

2

2.5

3

3.5

Molar ratio of water/monomer

Molar ratio of water/monomer

Figure 2.1 Influence of the water/monomer molar ratio on (a) the molecular weight of the polymer(Mw) and (b) the residual alkoxy group content in polymethylsilsesquioxanes prepared withtriethoxymethylsilane (pink closed circles) and triisobutoxymethylsilane (blue closed circles).

Next, to investigate the influence of substituents bonded to the silicon atom on the thermal
insulation property, we attempted the synthesis of PSQs using bis(triethoxysilyl)ethane andtriethoxyphenylsilane via the nitrogen flow method. When we conducted the sol–gel reaction ofbis(triethoxysilyl)ethane at 90 °C for 3 h with a water/monomer molar ratio of 2.2, Mw and Mw/Mn(Mn = number-average molecular weight) of the bridged PSQ were 4000 and 4.9, respectively.
Unfortunately, bridged PSQ prepared from bis(triethoxysilyl)ethane contained 40%
unhydrolyzed ethoxy groups. When the molar ratio of water/monomer was over 2.3, bridged PSQ
formed a gel during the polycondensation reaction. Similarly, when we conducted the sol–gel
reaction of triethoxyphenylsilane at 90 °C for 3 h with a water/monomer molar ratio of 2.0,
Mw and Mw/Mn of the resulting polyphenylsilsesquioxane having the ethoxy group (PSQ-Ph(OEt)) were 700 and 1.4, respectively, even though the amount of residual ethoxy groups was
16%. However, PSQ-Ph (OEt) was not obtained as a soluble sol by using a water/monomer molar
ratio over 2.1 due to gel formation during the polycondensation reaction, which hindered a precise
comparison of the effect of substituents on the silicon atom.
To examine the influence of structure of alkoxysilane monomer on the thermal insulation
property, we synthesized co-PSQ with a similar amount of residual ethoxy groups using17

triethoxymethysilane/bis(triethoxysilyl)ethane and triethoxymethylsilane/triethoxyphenylsilane(Table 2.2). The copolycondensation of triethoxymethylsilane and triethoxyphenylsilane (feedmolar ratio = 2/1) at 90 °C for 3 h afforded PSQ-Me/Ph (2/1) (OEt) with Mw of 7900 and Mw/Mn of
1.5 (Table 2.2, entry 1). Upon increasing the feed ratio of triethoxyphenylsilane (feed molar ratio= 1/1), Mw and Mw/Mn of the corresponding co-PSQ decreased to 1900 and 1.4, respectively, most
likely due to the steric hindrance of the phenyl group on the silicon atom (Table 2.2, entry 2). The
amount of residual ethoxy groups in co-PSQs was similar, which is an advantage to investigate
the effect of the phenyl group on the thermal insulation property. Similarly, thecopolycondensations of triethoxymethylsilane and bis(triethoxysilyl)ethane at feed molar ratiosof 2/1 and 1/1, respectively, at 90 °C for 3 h gave PSQ-Me/EB (2/1) (OEt) and PSQ-Me/EB (1/1)(OEt) with similar amounts of residual ethoxy groups (Table 2.2, entries 3 and 4). As in the
copolymerization of triethoxymethylsilane and triethoxyphenylsilane, the molecular weight also
decreased with increasing the amount of bis(triethoxysilyl)ethane because of the larger sterichindrance of the ethylene group on the silicon atom compared with that of the methyl group.

18

Table 2.2. Preparation of copolysilsesquioxanes (co-PSQs)a,b

Temp.

Molar ratio of

Residual(°C)

H2O/silane

OEt (%)PSQ-Me/Ph (2/1) (OEt)

90

1.5

2

PSQ-Me/Ph (1/1) (OEt)

90

3

PSQ-Me/EB (2/1) (OEt)

4

PSQ-Me/EB (1/1) (OEt)

Entry

Copolysilsesquioxane

1

a

Mw

Mw/Mn

8

7900

1.5

1.5

10

1900

1.4

90

1.5

25

4200

3.9

90

1.5

30

2700

3.2

Molar ratio of monomer/HCl = 0.105.

b

Molar ratio of solvent/monomer = 2.

2.2.2 Synthesis of ethoxy-free PSQ
Having synthesized the series of PSQ-Me (OEt) and PSQ-Me (OBui) with different amountsof residual alkoxy groups with diverse sizes and PSQ-Me/Ph (OEt) and PSQ-Me/EB (OEt) fromtwo kinds of monomers to investigate the influence of the residual alkoxy group and thesubstituents on the thermal insulation property, respectively, we considered it important to
prepare an ethoxy-free PSQ to investigate the influence of the substituents on the silicon atom in
more detail. In a previous paper, we reported the synthesis of oligomethylsilsesquioxane and the19subsequent capping reaction with chlorodimethylsilane. Intermediate oligomethylsilsesquioxane,
which was synthesized at 0 °C by using excess water and HCl, had almost no ethoxy groups [43].
On the basis of this result, we attempted the synthesis of ethoxy-free PSQ frombis(triethoxysilyl)ethane following the same procedure. However, the product was not isolated
upon extraction after hydrolysis of bis(triethoxysilyl)ethane. Meanwhile, ethoxy-free PSQ-Ph
was easily synthesized under reflux for 3 h by using excess water and HCl, but the content ofresidual ethoxy groups was only 2%, which was sufficient to investigate the influence of thesubstituents on the thermal insulation property. Similarly, the residual ethoxy group content in
ethoxy-free polyphenethylsilsesquixane (PSQ-Phe) synthesized from triethoxyphenethylsilane
was also negligible (4%). As shown in Table 2.3, the molecular weight of PSQ-Ph and PSQ-Phe
synthesized under reflux condition was considerably low because ethanol as the reaction solvent
was not volatilized during the polycondensation reaction, which contrasts with the result of thesol–gel reaction conducted via the nitrogen flow method [45]. Nevertheless, their molecularweight was sufficient to investigate the influence of the substituted group on the silicon atom.

20

Table 2.3. Preparation of PSQ-Me, PSQ-Ph, and PSQ-Phe via the sol–gel methoda,b

Entry

a

Solvent

Temp.

Molar ratio of

Residual(°C)

H2O/silane

OEt (%)

Mw

Mw/Mn

1c

THF

0

15

2

450

1.3

2

ethanol

reflux

15

2

700

1.3

3

ethanol

reflux

15

4

1800

1.6

Molar ratio of monomer/HCl = 6.67.

bcX

Molar ratio of solvent/monomer = 5.

These data were obtained from Reference [43].

2.2.3 Preparation of freestanding gel films
The conditions for the preparation of freestanding gel films are summarized in Table 2.4. We
successfully prepared freestanding gel films of PSQ-Me (OEt) in a poly(tetrafluoroethylene-coperfluoroalkylvinyl ether) (PFA) vial with an inner diameter of 18.4 mm. First, we placed a 20
wt% THF solution of PSQ-Me (OEt) in the vial and heated the polymer solution at 80 °C for 1 h
to remove the solvent and then at 160 °C for 1 h to slowly start the cross-linking reaction. Finally,
we heated the obtained product at 200 °C for 2 h to complete the cross-linking reaction and took
the final product from the PFA vial. The freestanding gel film prepared from PSQ-Me (OEt) wastransparent and stiff after heating at 200 °C for 2 h, whereas PSQ-Me (OBui) required 4 h at
200 °C to complete the cross-linking reaction and produce a transparent and stiff freestanding gel
film. Therefore, we fixed the temperature for the cross-linking reaction at 200 °C and changed
the heating time for the preparation of each freestanding film. Similarly, PSQ-Ph and PSQ-Phe

21required 3–6 h at 200 °C to produce freestanding gel films because of the steric hindrance of thesubstituents on the silicon atom. A freestanding gel film of PSQ-Ph was not obtained due todewetting of PSQ-Ph on the PFA vial during heating of the polymer solution. Therefore, the PSQPh film was prepared onto a Kapton film by using an applicator. In contrast, PSQ-Me/EB (OEt)
easily formed gel films within 1 h at 200 °C because of the high degree of cross-linking sites inthe ethylene-bridged structure. As shown in Table 2.4, partial cracks were only observed in thegel film of PSQ-Me/EB (OEt) probably due to its high cross-linking ratio affording a too stiff
film that shrunk upon elimination of unhydrolyzed ethoxy groups. In fact, a gel film of ethylenebridged polysilsesquioxane prepared from bis(triethoxysilyl)ethane was reported to contain many
cracks upon heating at 200 °C for 20 min [43].
Table 2.4. Results and conditions for the preparation of freestanding gel films
Temperature

Time

Film preparation

Film state

(°C)

(h)

methodPSQ-Me (OEt)200

2drop castingtransparent

PSQ-Me (OBui)200

4drop castingtransparent

PSQ-Me/Ph (OEt)200

4drop castingtransparent

PSQ-Me/EB (OEt)200

1drop casting

partially crack

PSQ-Ph200

3applicator coatingtransparent

PSQ-Phe200

6drop castingtransparent

2.2.4 Thermal diffusivity of the PSQ films
To investigate the influence of the residual alkoxy group in PSQ on the thermal insulation
property, we measured the thermal diffusivities of the freestanding gel films with thickness of
300–700 μm prepared from PSQ-Me (OEt) and PSQ-Me (OBui) and plotted them as a functionof the residual alkoxy group content (Figure 2.2 and Table 2.5). In this experiment, we assumed
that the trend in the amount of residual alkoxy group was not changed between polymer and gel

22

film. Unfortunately, the low amount of residual ethoxy groups in the gel film hindered its
quantitative determination by Fourier transform infrared (FT-IR) spectroscopy.

Figure 2.2 Influence of the residual alkoxy group content in the polymer on the thermal diffusivityof gel films prepared from polymethylsilsesquioxanes containing ethoxy groups (pink closed
circles) and triisobutoxy groups (blue closed circles).

23

Table 2.5. Thermal diffusivities of the gel films
Residual

Thermal diffusivity

OEt (%)(m2/s)

1.8

22

1.43 × 10−7

1800

1.9

14

1.24 × 10−7

PSQ-Me (OEt)

4600

4.1

10

1.31 × 10−7

4

PSQ-Me (OEt)

8000

3.1

7

1.20 × 10−7

5

PSQ-Me (OEt)

13000

7.3

7

1.22 × 10−7

6

PSQ-Me (OBui)

1200

1.7

19

1.23 × 10−7

7

PSQ-Me (OBui)

1500

1.8

16

1.20 × 10−7

8

PSQ-Me (OBui)

1800

2.0

14

1.18 × 10−7

9

PSQ-Me (OBui)

3000

2.7

12

1.14× 10−7

10

PSQ-Me/Ph (2/1) (OEt)

7900

1.5

8

1.17 × 10−7

11

PSQ-Me/Ph (1/1) (OEt)

1900

1.4

10

1.07 × 10−7

12

PSQ-Me/EB (2/1) (OEt)

4200

3.9

25

1.42 × 10−7

13

PSQ-Me/EB (1/1) (OEt)

2700

3.2

30

1.47 × 10−7

14

PSQ-Mea

450

1.3

2

1.36 × 10−7

15

PSQ-Ph

700

1.3

2

0.94 × 10−7

16

PSQ-Phe

1800

1.6

4

1.17 × 10−7

Entry

Polysilsesquioxane

Mw

Mw/Mn

1

PSQ-Me (OEt)

1000

2

PSQ-Me (OEt)

3aThese data were obtained from Reference [43].

24The thermal diffusivities of the freestanding gel films prepared from PSQ-Me (OEt) decreased
from 1.43 × 10–7 to 1.20 × 10–7 m2/s with decreasing the residual ethoxy group content from 22%
to 7%. Similarly, the thermal diffusivities of the freestanding gel films prepared from PSQ-Me(OBui) decreased from 1.23 × 10–7 to 1.14 × 10–7 m2/s with decreasing the residual isobutoxygroup content from 19% to 12%, although this sample showed lower thermal diffusivities in thewhole range of the residual isobutoxy group content compared with the PSQ-Me (OEt)-derived
gel films. Interestingly, a clear influence of the alkoxy group on the thermal insulation property
was observed: the presence of isobutoxy groups in PSQ enhanced the thermal insulation property
compared with the residual ethoxy group, whereas the increase in the residual ethoxy groupcontent decreased the thermal insulation property. The thermal diffusivity of gel films might bealso relevant with the molecular weight of PSQ-Me (OEt) and PSQ-Me (OBui). The PSQ withthe high molecular weight forms a high-level network structure, and the heat conduction occurs
via the siloxane network. However, the thermal diffusivity of PSQ-Me (OEt)- and PSQ-Me(OBui)-derived gel films unexpectedly decreased with increasing the molecular weight. In
addition, PSQ-Me (OBui) forms gel films with low thermal diffusivity even though the molecularweight of PSQ-Me (OBui) was lower than PSQ-Me (OEt). On the freestanding film preparation,
treatment of PSQs at high temperature enhanced the polymer network by the cross-linking andintermolecular condensation with respect to residual silanol units to yield insoluble gels,regardless of the molecular weight of pristine polymer. Therefore, it is concluded that the thermal
diffusivity of gel film was affected primarily by the alkoxy group in PSQ.
Considering the bulkiness of the isobutoxy group, we attributed the decrease in the thermal
diffusivity of the freestanding gel film prepared from PSQ-Me (OBui) to the residual isobutoxygroups forming large intramolecular void spaces. This hypothesis was supported by the results ofa positron annihilation lifetime analysis. We measured the lifetime spectra of PSQ-Me (OEt) and
PSQ-Me (OBui) having similar amounts of alkoxy groups (Table 2.5, entries 2 and 8), separated
them into four components, and determined the longest lifetime corresponding to the pick-off
annihilation of ortho-positronium. Using a numerical Laplace inversion technique, we obtained
the probability density function of the annihilation rate. As shown in Figure 2.3, the peak positionof PSQ-Me (OBui) was shifted to a longer time region compared with that of PSQ-Me (OEt). The

25

τ3 value of PSQ-Me (OBui) (4.81 ± 0.01 ns) was larger than that of PSQ-Me (OEt) (3.74 ± 0.01ns), which indicates that the free-volume spaces of PSQ-Me (OBui) were larger than those in the
PSQ-Me (OEt) film. We estimated the radius of free-volume holes (R) for PSQ-Me (OEt) and
PSQ-Me (OBui) to be 0.41 and 0.47 nm, respectively, using the Tao–Eldrup equation [46, 47].
The calculated hole volume of PSQ-Me (OBui) was 0.42 nm3, which was apparently larger thanthat of PSQ-Me (OEt) (0.29 nm3). This explains the lower thermal diffusivities of the gel films
prepared from PSQ-Me (OBui) in the whole range of residual alkoxy group content comparedwith that of the PSQ-Me (OEt) gel films. Considering the formation of intramolecular void spacesin the freestanding gel films as a result of the presence of the residual alkoxy groups, the thermal
diffusivity could be expected to decrease with increasing the amount of residual alkoxy groups.
However, the thermal diffusivity of the freestanding gel films increased with increasing theresidual alkoxy group content (Figure 2.2). This increase in the thermal diffusivity might be dueto the polar alkoxy group enhancing the thermal conductivity of the film.

Figure 2.3 Probability density function (PDF) of the lifetimes of ortho-positronium in PSQ-Me(OEt) and PSQ-ME (OBui) and PSQ-Ph.

26

Next, to investigate the influence of substituents bonded to the silicon atom on the thermal
insulation property, we measured the thermal diffusivities of freestanding gel films prepared fromco-PSQs, and the results are shown in Table 2.5. The thermal diffusivities of the gel films of PSQMe/Ph (2/1) (OEt) and PSQ-Me/Ph (1/1) (OEt) were 1.17 × 10–7 and 1.07 × 10–7 m2/s,
respectively, which showed a dependence on the phenyl group content and were clearly lower
than that of PSQ-Me (OEt) having a similar amount of ethoxy groups (1.31 × 10–7 m2/s) (Table
2.5, entries 3, 10, and 11). Considering that the amount of ethoxy group was similar (Table 2.5,
entries 10 and 11), we attributed the improvement of the thermal insulation property to the bulky
phenyl group on the silicon atom creating intramolecular voids spaces. In contrast, the gel filmsof PSQ-Me/EB (2/1) (OEt) and PSQ-Me/EB (1/1) (OEt) showed thermal diffusivities of 1.42 ×
10–7 and 1.47 × 10–7 m2/s (Table 2.5, entries 12 and 13), respectively, which were similar to thatof PSQ-Me (OEt) with an ethoxy group content of 22%. The ethylene-bridged structure in PSQ
increased the thermal diffusivity of its gel film. However, we reported in a previous paper thatthe ethylene-bridged structure decreased the thermal diffusivity of an EBPSQ film due to theformation of intramolecular void spaces [43]. For example, an EBPSQ film prepared via
hydrosilylation reaction exhibited a low thermal diffusivity of 1.15 × 10–7 m2/s and a density of
1.13 g/cm3, which was inconsistent with the high thermal diffusivities in the gel films of PSQMe/EB (2/1) (OEt) and PSQ-Me/EB (1/1) (OEt) [43]. This difference could be due to the presenceof unhydrolyzed ethoxy groups in the EBPSQ synthesized from bis(triethoxysilyl)ethane via thenitrogen flow method, which increased the thermal diffusivity as mentioned above.
Unfortunately, the influence of the ethoxy group on the heat conduction in the PSQ films was
not clear. Recently, Naka et al reported beads-on-string-shaped polyurea with high thermal
conductivity as a result of the formation of an extended hydrogen-bond network promoted by thehydrophobic cage-type silsesquioxane in the main chain of polyurea [48].Accordingly, in thepresent PSQ films, the polar ethoxy groups could form hydrogen bonds with silanol, leading to arefinement film with enhanced thermal diffusivity.
We also measured the thermal diffusivities of the gel films prepared from ethoxy-free PSQ togain a deeper understanding on the influence of substituents bonded to the silicon atom (Table
2.5, entries 14–16). As reported in a previous paper, despite not containing the polar ethoxy group,27

a PSQ-Me film prepared from oligomethylsilsesquioxane exhibited a high thermal diffusivity of
1.36 × 10–7 m2/s because oligomethylsilsesquioxane formed a highly cross-linked structure [43].
Meanwhile, the gel film prepared from PSQ-Ph exhibited a low thermal diffusivity of 0.94 × 10–
7m2/s. As discussed for the thermal diffusivity of the PSQ films, we expected that the phenylgroup on the silicon atom decreased the thermal diffusivity of the PSQ film due to the formationof intramolecular void spaces. However, the lifetime spectrum of PSQ-Ph contradicted thishypothesis, as shown in Figure 2.3. The obtained τ3value was 2.96 ± 0.01 ns, which corresponds
to the lowest lifetime among the films evaluated. For PSQ-Ph, the Tao–Eldrup equation afforded
an R value of 0.36 nm, and the hole volume was 0.20 nm3. Therefore, the thermal diffusivity of
the PSQ films containing an aromatic group might decrease due not only to the formation of
intramolecular void spaces but also to the influence of the substituent bonded to the silicon atomon the cross-linking density; namely, PSQ-Ph might form a low density network structure, eventhough the effect of the phenyl group on the silicon atom is still unclear.
Similarly, the PSQ-Phe film showed a low thermal diffusivity of 1.17 × 10–7 m2/s because of
the presence of the bulky phenyl group (Table 2.5, entry 16). We expected that the ethylenebridged structure between the phenyl group and the silicon atom would create intramolecular void
spaces, further decreasing the thermal diffusivity. However, the thermal diffusivity of the PSQPhe film was higher than that of the PSQ-Ph film. This might be due to the low steric hindranceof the phenethyl group around the silicon atom.
Overall, although the heat conduction in PSQ materials is complicated and the detailed
mechanism is not clear, the residual alkoxy groups and the substituents on the silicon atom
significantly influenced the thermal insulation property of the gel films.

2.2.5 Thermal stability of the PSQ films
As mentioned in Introduction of this thesis (Chapter 1), the thermal stability of thermal
insulation materials is also important for their application under severe conditions such as hightemperature. To study the influence of the substituents bonded to the silicon atom on the thermal
stability of the gel films, we performed a TGA analysis in air on the ethoxy-free PSQ-derived gel
films because ethoxy groups are usually unstable and decompose easily at lower temperature than28

other organic groups. As previously reported, a gel film prepared from PSQ-Me showed highthermal stability and was stable up to 400 °C (Figure 2.4) [43]. In this study, the PSQ-Me film
showed a 5% weight loss temperature (Td5) of 436 °C and a 10% weight loss temperature (Td10)
of 481 °C (Table 2.6). In this temperature region, some products such as methane, hydrogen,carbon monoxide, and carbon dioxide might be generated by thermal decomposition [49].

Table 2.6. Thermal properties of gel films prepared from PSQ-Me, PSQ-Ph, and PSQ-PheaaResidual OEt (%)

Td5 (°C)

Td10 (°C)

PSQ-Meb

2

436

480

PSQ-Ph

2

485

560

PSQ-Phe

4

280

327

Measured at a heating rate of 10 °C/min under an air flow of 100 mL/min.

b

These data were obtained from Reference [43].

Figure 2.4 Thermogravimetric analysis traces of gel films prepared from PSQ-Me, PSQ-Ph, PSQPhe measured at a heating rate of 10 °C/min under an air flow of 100 mL/min. The data for PSQMe were obtained from ref [43].

29Polyphenylsilsesquioxane typically has high thermal stability, and the main degradation
mechanism is the formation of a phenyl radical [49]. Indeed, the thermal stability of the PSQ-Ph
film was considerably higher, with Td5 and Td10 values of 485 and 560 °C, respectively. A similar
enhancement of the thermal property of a polymer by the phenyl group was observed for a phenylmodified MQ resin, which was more resistant to thermal oxidation than a related methyl MQ resin[50]. According to the literature, this phenomenon was attributed to the strengthening of thesiloxane bond by an increased participation of the lone pair electrons on the oxygen atom[50]. Very recently, Kickelbick’s group also reported that polyphenylsilsesquioxane gel preparedfrom phenyltrimethoxysilane show high Td5 and Td10over 500 °C [51]. However, the thermal
stability of the PSQ-Phe film was lower than that of the gel films of PSQ-Me and PSQ-Ph eventhough PSQ-Phe also contained a phenyl ring. Thus, the Td5 and Td10 of the PSQ-Phe film
decreased to 280 and 327 °C, respectively. According to the literature, poly(benzyl acrylate) isthermally more unstable than poly(methyl acrylate) and decomposes at 270 °C into carbon
dioxide, benzyl alcohol, and low-molecular-weight polymer [52]. This is due to the nature of the
benzyl group, in which the carbon at the benzyl position oxidizes around 250 °C. The glasstransition temperatures (Tg) of PSQ-Me, PSQ-Ph, and PSQ-Phe were investigated by DSC.
However, the gel films exhibited no distinct Tg over the temperature range from −60 to 120 °C.
Organoalkoxysilanes are known to form melting gels through hydrolysis and polycondensation
reactions under certain conditions [51, 53, 54]. In previous studies, the Tg of a melting gel
prepared from phenyl-substituted trialkoxysilane was below 100 °C [51, 53, 54]. However, PSQPh showed no distinct Tg because it was prepared at 200 °C to form the cross-linked network.
Indeed, a glass-like material prepared from phenyltrimethoxysilane at 200 °C exhibitedno Tg because the formation of a glass-like material from a melting gel above 200 °C is an
irreversible consolidation process [51].This indicates that PSQ-Ph formed a well-grown crosslinked network and was thermally stable. Therefore, it is a more suitable thermal insulationmaterial than organic materials. It can be concluded that the substituents bonded to the silicon
atom have a significant influence on the thermal stability of the films. Particularly, the phenylgroup as a substituent confers the polymer films with stability, rendering them suitable as heatresistant materials.

30

2.3 Conclusions
In this study, we prepared a series of PSQs via the sol–gel reaction to investigate the influenceof residual alkoxy groups and substituents on the thermal insulator property of the corresponding
freestanding gel films.
The thermal diffusivities of freestanding gel films prepared from PSQ-Me (OEt) and PSQ-Me(OBui) decreased with decreasing the amount of residual alkoxy groups. Furthermore, the thermal
diffusivity of the PSQ-Me (OBui) freestanding gel film was lower than that of freestanding PSQMe (OEt) gel films. Interestingly, a clear influence of the alkoxy group on the thermal insulator
property was observed; that is, the presence of the isobutoxy group in the polymer enhanced thethermal insulation property, whereas increasing the residual alkoxy group content decreased thethermal insulation property.
The substituents on the silicon atom also affected the thermal insulator property. The gel films
prepared from PSQ-Me/Ph (2/1) (OEt) and PSQ-Me/Ph (1/1) (OEt) exhibited thermal diffusivitiesof 1.17 × 10–7 and 1.07 × 10–7 m2/s, respectively, which were clearly lower than that of the PSQMe (OEt) gel film having a similar residual ethoxy group content (1.31 × 10–7m2/s). The bulky
phenyl group on the silicon atom apparently decreased the thermal diffusivity depending on theamount of phenyl group (PSQ-Me/Ph (1/1) (OEt) < PSQ-Me/Ph (2/1) (OEt) < PSQ-Me (OEt)).
To precisely understand the influence of substituents, we synthesized ethoxy-free PSQs such as
PSQ-Me, PSQ-Ph, and PSQ-Phe and measured the thermal diffusivities of their gel films. The gel
films prepared from PSQ-Ph and PSQ-Phe showed low thermal diffusivities of 0.94 × 10–7 and1.17 × 10–7 m2/s, respectively, compared with that of the PSQ-Me gel film (1.36 × 10–7 m2/s).
This indicates that the phenyl and phenethyl groups on the silicon atom clearly decreased thethermal diffusivity of the films by the formation of intramolecular void spaces and the change inthe cross-linking density.
The thermal stability of the PSQ-Ph film was considerably higher than that of the PSQ-Me and
PSQ-Phe films, and its Td5 and Td10 values were 485 and 560 °C, respectively. However, despite
the presence of a phenyl ring in PSQ-Phe, its thermal stability was lower than that of PSQ-Meand PSQ-Ph, exhibiting Td5 and Td10 values of 280 and 327 °C, respectively. We attribute thisresult to the inherent instability of the benzyl group at high temperature in air. Overall, this study
31

demonstrates that the thermal stability of gel films prepared from PSQ was considerably affectedby the substituents on the silicon atom.

2.4 Experimental
Materials
Triethoxymethylsilane, triethoxyphenylsilane, and trichloromethylsilane were purchased from
Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Trichlorophenethylsilane was purchased from
Shinetsu Chemical Co., Ltd. (Tokyo, Japan). Bis(triethoxysilyl)ethane was purchased from
Oakwood Products, Inc. Ethanol (super dehydrated) was purchased from FUJIFILM Wako Pure
Chemical Co., Ltd. (Osaka, Japan). All these chemicals were used as received. Tetrahydrofuran(THF), toluene, 2-methyl-1-propanol, triethylamine, and 6 mol/L hydrochloric acid (HCl) werepurchased from FUJIFILM Wako Pure Chemical Co., Ltd. (Osaka, Japan). THF, toluene, and 2methyl-1-propanol were distilled over calcium hydride before use. Triethylamine was distilled
and dried over potassium hydroxide, and 6 mol/L HCl was used as received without purification.
Water was purified by using a Millipore Mill-Q UV system and had a resistance of 18.2 MΩ cm
and a total organic carbon content of <10 ppb.

Characterization
1

H nuclear magnetic resonance (NMR) measurements were performed on a Varian 400 MHz

spectrometer in deuterium chloroform (CDCl3) by using residual chloroform as an internal
standard (7.26 ppm) for chemical shift referencing. 13C NMR measurements were performed ona Varian 400 MHz spectrometer and a Varian 500 MHz spectrometer. 29Si NMR spectra were
measured by using a Varian 500 MHz spectrometer, and tetramethylsilane was used as an internal
standard. GPC was performed on a Shimadzu LC-20AD system equipped with a RID-10A
detector and three directly connected TSKgel G6000H/G4000H/G2000H columns. THF was used
as an eluent at 40 °C and a flow rate of 1 mL/min, and polystyrene standards were used forcalibration. Thermogravimetric analysis (TGA) data were obtained with a SII EXSTAR TGDTA6200 thermal analyzer at a heating rate of 10 °C/min under an air flow of 100 mL/min.
Differential scanning calorimetry (DSC) measurements were performed on an SII EXSTAR32

DSC6200 under a nitrogen flow of 100 mL/min. Heating and cooling were performed at
10 °C/min over the temperature range −60 to 120 °C. Thermal diffusivity measurements of thefilms were performed on an ai-phase Mobile M3 type 1 system. A plate of zirconium oxide witha thickness of 500 μm was used for calibration. The measurements were repeated nine times, andthe averaged thermal diffusivity was calculated. Positron lifetime measurements were performedby using a system based on a digital oscilloscope. Positron lifetime measurements were performed
using a 22Na source (400 kBq). The lifetime spectrum SLT (t) is expressed by the following
equation (2.1)

𝑆!" (t) = ' 𝜆# 𝐼# exp(– 𝜆# 𝑡)(2.1)

where λi and Ii are the annihilation rate and the relative intensity of the positron of the ith
component, respectively (∑Ii = 1). The lifetime of positron λi is given by 1/λi. The observed
spectra were analyzed with a time resolution of 185 ps (full width at half-maximum) using the
RESOLUTION computer program. The radius of free-volume holes, R (nm), was calculated
according to the following Tao–Eldrup equation (2.2) [46, 47]
1
𝑅
1
2𝜋𝑅
= 2 × 31 –
+
sin :
;<
𝜏$
𝑅%
2𝜋
𝑅%

(2.2)

where R0 is a fitted empirical electron-layer thickness (=R + 0.1656 nm). The holevolume V(nm3) was calculated according to the following equation (2.3)

𝑉 =

4 $
𝜋𝑅
3

(2.3)

Synthesis of triethoxyphenethylsilane
In a 200 mL flask, triethylamine (25.6 g, 252 mmol), ethanol (12.3 g, 268 mmol), and 105 mLof toluene were added and cooled in an ice/water bath. Trichlorophenethylsilane (17.2 g, 72.0mmol) was slowly added dropwise at 0 °C under an argon atmosphere, and the reaction mixture

33

was then refluxed for 3 h. The resulting triethylamine salt was removed by filtration, and thesolution was evaporated under reduced pressure. The residue was distilled under reduced pressure(114.0–118.5 °C/6 mmHg) to give triethoxyphenethylsilane (11.1 g, 58%) as a colorless
liquid. 1H NMR (400 MHz, CDCl3, ppm): δ = 7.19 (5H, m), 3.83 (6H, q, J = 7 Hz), 2.78 (2H, m),
1.25 (9H, t, J = 7 Hz), 1.02 (2H, m). 13C NMR (125 MHz, CDCl3, ppm): δ = 144.9, 128.2, 125.0,
57.5, 28.0, 17.5, 12.5. 29Si NMR (100 MHz, CDCl3, ppm): δ = −45.8.

Synthesis of triisobutoxymethylsilane
In a 200 mL flask, triethylamine (14.6 g, 144 mmol), isobutyl alcohol (11.3 g, 153 mmol), and60 mL of toluene were added and cooled in an ice/water bath. Trichloromethylsilane (6.1 g, 41mmol) was slowly added dropwise at 0 °C under an argon atmosphere. After refluxing for 3 h,
the reaction mixture was filtrated to remove the resulting triethylamine salt, and the solution was
evaporated under reduced pressure. The residue was distilled under reduced pressure (80–
83 °C/17 mmHg) to give triisobutoxymethylsilane (9.63 g, 90%) as a colorless liquid. 1H NMR(400 MHz, CDCl3, ppm): δ = 3.59 (6H, d, J = 6.4 Hz), 1.83 (3H, nonet, J = 6.4 Hz), 0.93 (18H,d, J = 6.8 Hz), 0.21 (3H, s). 13C NMR (100 MHz, CDCl3, ppm): δ = 69.5, 31.1, 19.2, −7.5. 29Si
NMR (100 MHz, CDCl3, ppm): δ = −42.9.

Synthesis of PSQ via the nitrogen flow method
Typically, in a 100 mL four-necked flask equipped with a mechanical stirrer and nitrogen inlet
and outlet tubes, triethoxymethylsilane (3.566 g, 20.00 mmol) and ethanol (1.843 g, 40.00 mmol)
were stirred at a rotation rate of 150 rpm in an ice/water bath for 10 min under a nitrogen flowrate of 360 mL/min. For the hydrolysis, a solution of 0.3828 g of 6 mol/L HCl and 0.2343 g ofwater (HCl/monomer molar ratio = 0.105 and water/monomer molar ratio = 1.5) was slowly
added dropwise and stirred at 0 °C for 10 min and then at room temperature for an additional 10
min. For the subsequent polycondensation reaction, the reaction mixture was heated for 3 h at
90 °C to afford polymethylsilsesquioxane having ethoxy groups (PSQ-Me (OEt)) as a colorlessviscous liquid. The synthesized PSQs were labeled as PSQ-X (OR), where X denotes the organic
substituent and OR denotes the residual alkoxy group. PSQ-Me (OEt) was dissolved in THF to34

prepare a concentration of 20 wt % and stored in a refrigerator. A series of PSQ-Me (OEt) samples
with various molecular weights were prepared by changing the amount of 6 mol/L HCl and water(HCl/monomer molar ratio = 0.105 and water/monomer molar ratio = 1.1–1.6). Several co-PSQs
were synthesized in a similar manner.

Synthesis of ethoxy-free PSQ
In a 100 mL flask, triethoxyphenylsilane (4.607 g, 20.00 mmol) and ethanol (4.607 g, 100.0mmol) were added and stirred in an ice/water bath for 10 min. A solution of 0.5834 g of 6 mol/L
HCl and 4.968 g of water (3 mmol of HCl and 300.0 mmol of water) was added dropwise andstirred at 0 °C for 10 min and then at room temperature for another 10 min. The reaction mixture
was refluxed for 3 h and evaporated under reduced pressure to give PSQ-Ph as a glass-like solid.
PSQ-Ph was dissolved in THF to prepare a concentration of 20 wt % and stored in a refrigerator.
PSQ-Phe was also synthesized in a similar manner.

Preparation of freestanding gel films
Freestanding films were typically prepared as follows: 1.5 mL of a 20 wt % THF solution of
PSQ-Me (OEt) was poured into a PFA vial and heated at 80 and 160 °C for 1 h, respectively. The
obtained film was heated at 200 °C for 2 h to give a freestanding gel film.

35

Chapter 3 Organic–Inorganic Hybrid Thermal Insulation Materials
Prepared via Hydrosilylation of Polysilsesquioxane Having HydrosilylGroups and Triallylisocyanurate
3.1 Introduction
Among the PSQs, polyhedral oligomeric silsesquioxane (POSS) consisting of a siloxane cage
framework and eight organic groups bonded to silicon is widely used as a building block toprepare organic–inorganic hybrid materials because of its excellent thermal and mechanicalstability. In this context, POSS has attracted increasing attention in materials chemistry.
Recently, Hamada et al. reported that the formation of intramolecular void spaces in an
ethylene-bridged PSQ film prepared by the hydrosilylation reaction contributed to theenhancement of its thermal insulation property [43]. Then, with the aim of further improving thethermal insulation property, octavinyl POSS (vinyl-POSS) was used as a crosslinker to prepare a
hybrid film containing intramolecular void spaces, finding that increasing the content of
intramolecular void spaces around the POSS molecules by increasing the amount of vinyl-POSS
enhanced the thermal insulation property [44]. However, the aggregation of POSS molecules due
to strong intermolecular interactions hindered the preparation of hybrid films with a high amountof vinyl-POSS (over 20 wt %). In addition, the aggregation of POSS molecules leads to a decreasein the thermal insulation property due to an increase in the thermal conductivity of the hybrid film
as a result of the formation of heat conduction pathways. Therefore, we were interested in
exploring other crosslinkers with high miscibility to increase the content of intramolecular void
spaces in the PSQ film.
Owing to their rigid structure, high polarity, solubility, and miscibility, isocyanurate derivatives
are used as building blocks for the preparation of high-performance polymers. For example,
crosslinked polymers containing an isocyanurate core exhibit good thermal and mechanicalproperties [55-57]. In this context, we expected that triallylisocyanurate (TAIC) would be a goodcandidate as a crosslinker in the PSQ network because of its high miscibility. Furthermore, TAIC
could enhance the thermal insulation property and thermal stability of the polymer due to theintroduction of a rigid isocyanurate ring and the formation of intramolecular void spaces aroundit.
36

In this chapter, with the aim of preventing the formation of aggregates to obtain a highly
uniform film with abundant intramolecular void spaces, we designed and prepared hybrid films
by the hydrosilylation reaction of hydrodimethylsilylated oligomethylsilsesquioxane (MSQ-SiH)
and TAIC. We investigated in detail the effect of the introduction of TAIC in the PSQ networkon the thermal insulation property and thermal stability of hybrid films.

3.2 Results and discussion
3.2.1 Preparation of hybrid films by hydrosilylation reaction
In a previous paper, Hamada et al. reported the preparation of ethylene-bridged PSQ films via
the hydrosilylation reaction of MSQ-SiH and dimethylvinylsilylated oligomethylsilsesquioxane[43]. Compared with polymethylsilsesquioxane films prepared by polycondensation, theethylene-bridged PSQ films showed low thermal diffusivity and density, which decreased withincreasing SiCCSi bond content. This was due to the formation of intramolecular void spaces.
Subsequently, we conducted the hydrosilylation reaction of MSQ-SiH and vinyl-POSS, in which
the formation of intramolecular void spaces around the POSS molecules enhanced the thermal
insulation property [44]. As mentioned in the introduction section of this chapter, the presence ofexcess POSS units in the hybrid film decreased its transparency; the hybrid film containing 20
wt % POSS units was turbid [44]. The aggregation of POSS molecules leads not only to thedecrease in transparency but also to the increase in thermal conductivity due to the formation of
POSS-aggregated heat-conducting pathways.
To introduce intramolecular void spaces in the film while preventing the aggregation ofcrosslinker molecules as described in the Introduction section, we used TAIC as a crosslinker forthe hydrosilylation reaction of MSQ-SiH, as shown in Scheme 3.1.

37

Scheme 3.1 Preparation of MSQ/TAIC hybrid films by the hydrosilylation of MSQ-SiH andTAIC

For the preparation of hybrid films by hydrosilylation, MSQ-SiH was synthesized several times
following our previous two-step method to ensure the reproducibility of the reaction [43]. The
Mw and Mw/Mn values of MSQ-SiH were 800–1000 and 1.29–1.59, respectively. On the basis of
the integral ratio of hydrosilyl group and silylmethyl group in the 1H NMR spectrum of MSQSiH, its structure was determined to be [(MeSiO1.5)2(MeSiO1.5SiMe2H)1]n, indicating the presenceof one hydrosilyl group per three units of (MeSiO1.5)n framework. The hydrosilylation reaction
was performed using MSQ-SiH and TAIC according to the procedure described in
the Experimental Section. The turbid liquid mixture obtained upon heating up to 140 °C wastransformed into a stiff film after heating at 160 °C for 4 h. The hybrid films prepared by thehydrosilylation reaction of MSQ-SiH and TAIC were named as MSQ/TAIC (x/y),
where x/y indicates the hydrosilyl group/allyl group molar ratio.
Figure 3.1 shows the ATR–FTIR spectra of MSQ-SiH, TAIC, and MSQ/TAIC (1/1). The
spectrum of MSQ-SiH shows two characteristic peaks at 2129 and 899 cm–1 corresponding to thehydrosilyl group. Absorption peaks at around 1270 and 1100 cm–1 ascribable to Si–CH3 and Si–
O–Si bonds were also observed. Meanwhile, the spectrum of TAIC shows strong absorption peaks
at 1678 cm–1 and at 1643, 989, and 926 cm–1 due to the carbonyl group and the allyl group,
respectively [58]. In the spectrum of MSQ/TAIC (1/1), the peaks corresponding to the hydrosilylgroup at 2129 and 899 cm–1 and to the allyl group at 1643, 989, and 926 cm–1 almost disappeared.
Unfortunately, the formation of SiCH2 could not be confirmed by ATR–FTIR due to theoverlapping of the SiCH2 peak with those of the Si–CH3 group or the C–N bond. Nevertheless,38the 13C solid-state NMR spectrum of MSQ/TAIC (1/1) supported the hydrosilylation reaction
between MSQ-SiH and TAIC. As shown in Figure 3.2, the signals of the allyl group of TAIC
completely disappeared and new signals attributed to the SiCH2CH2CH2N bond were observed
around 14–21 and 43 ppm after the hydrosilylation reaction. In addition, a signal attributable tothe carbonyl group in the isocyanurate structure was observed at 148 ppm. These findings clearly
indicated that TAIC was introduced into the PSQ network via the formation of Si–CH2CH2CH2N
bonds. To confirm the formation of the SiCH2CH2CH2N bond as a crosslink, we evaluated thestability of MSQ/TAIC (1/1) by immersing it in THF. Interestingly, the weight of the hybrid film
remained unaltered after 5 days, indicating that TAIC worked as a crosslinker, providing highly
networked PSQ structures.

Figure 3.1 ATR-FTIR spectra of (a) MSQ-SiH, (b) TAIC, and (c) MSQ/TAIC (1/1).

39

Figure 3.2 13C solid-state NMR spectrum of an MSQ/TAIC (1/1)) prepared by the hydrosilylation
reaction. The asterisks (*) indicate the spinning sidebands.

The 29Si solid-state NMR spectrum of MSQ/TAIC (1/1) provided further structural
information. Broad signals were observed at −63.8 and 10.4 ppm, which can be assigned to theT3 structure and the Si–CH2CH2CH2N unit (Figure 3.3), confirming the successful formation of
MSQ/TAIC (1/1) as a crosslinked gel film. However, an unknown signal was observed at −19.4
ppm after the hydrosilylation reaction. The hydrosilyl group in MSQ-SiH has been previously
reported to undergo a side reaction with ambient moisture [43, 44]. In this case, the hydrosilylgroup could also react with moisture in the presence of the Pt catalyst, forming an Si–O–Si bondby the dehydrogenation reaction of the resultant Si–OH group. This side reaction would furnish
an −Me2Si–O–SiMe2– (D2) structure, which could give rise to the signal at −19.4 ppm. In fact,
MSQ-SiH formed a gel film upon heating in the presence of the Pt catalyst even in the absenceof TAIC. Thus, we obtained a stiff and colorless film, which was named as MSQ film, bysubjecting a mixture of MSQ-SiH and Karstedt’s catalyst without TAIC to the same heat
treatment described for the MSQ/TAIC films. As shown in Figure 3.3, the 29Si solid-state NMR
spectrum of the as-prepared MSQ film showed broad signals at −64.4 and −19.4 ppm
corresponding to the T3 and D2 structures. Besides, a small signal for an unreacted hydrosilyl

40

group was observed at 8.0 ppm. Overall, MSQ-SiH formed the crosslinked gel film via thehydrolysis/dehydrogenation reaction in the presence of the Pt catalyst.

Figure 3.3 29Si solid-state NMR spectra of (a) MSQ/TAIC (1/1) and (b) MSQ film.

In addition, to investigate the effect of TAIC on the thermal property of the films, we preparedhybrid films having different compositions of MSQ-SiH and TAIC using various MSQSiH/TAIC feed molar ratios, i.e., 1:0.25, 1:0.5, 1:0.75, and 1:1.5. All of the obtained MSQ/TAIC(1/0.25–1.15) hybrid films were colorless and transparent.
In our previous paper, a hybrid film containing POSS units showed high surface hardness
stemming from the effect of POSS as a nanofiller; however, the amount of POSS molecules thatcan be introduced was limited by aggregation [44]. In contrast, the high miscibility of TAIC
allowed introducing a high amount of TAIC molecules. Furthermore, MSQ/TAIC exhibited highflexibility. Interestingly, the flexibility of hybrid films increased with increasing amount of TAIC,
which indicates that the property of hybrid films can be tuned by controlling the TAIC content.
Meanwhile, the surface property of the hybrid films did not change upon the introduction of
TAIC, and the contact angles of the MSQ film, MSQ/TAIC (1/1), and MSQ/TAIC (1/0.5) were41

89–90°. This is owing to the hydrophobic nature of the methyl group of thepolymethylsilsesquioxane backbone on the surface of the films.

3.2.2 Thermal insulation property of hybrid films prepared by hydrosilylation reaction
As we previously reported, the crosslinked structure of ethylene-bridged PSQ films createdvoid spaces and reduced the thermal diffusivity of the films [43, 44]. To investigate the effect of
the intramolecular void spaces formed around the TAIC molecules, we prepared hybrid films
having different amounts of TAIC and measured their thermal diffusivities. The thermal
diffusivity of the MSQ film prepared by the coupling reaction of the hydrosilyl group was 1.44 ×
10–7 m2/s (Table 3.1), which was considerably higher than that of the PSQ films prepared via thesol–gel reaction. The previously reported PSQ film prepared using oligomethylsilsesquioxane
containing no ethoxy groups exhibited a high thermal diffusivity of 1.36 × 10–7 m2/s due to theformation of a highly crosslinked network structure [43, 44]. As shown in Figure 3.3, the coupling
reaction of the hydrosilyl group in MSQ-SiH effectively proceeded to form a highly crosslinked
network structure, enabling the formation of the heat conduction pathway. However, theintroduction of TAIC into the PSQ network greatly decreased the thermal diffusivity. Comparedwith the MSQ film, the thermal diffusivity decreased significantly by adding TAIC into the PSQ
network. Thus, MSQ/TAIC (1/0.25) showed a thermal diffusivity of 1.14 × 10–7 m2/s. Moreover,decreasing the MSQ-SiH/TAIC molar ratio from 1:0.25 to 1:1.5 led to a further decrease in thethermal diffusivity of the hybrid film down to 1.01 × 10–7 m2/s (Table 3.1 and Figure 3.4). Thismay be due to the formation of voids around TAIC molecules.

42

Figure 3.4 Influence of the amount of TAIC in the polysilsesquioxane film on the thermal
diffusivities of isocyanurate ring-containing hybrid films prepared from MSQ-SiH and TAIC.

Table 3.1. Thermal diffusivity and density of PSQ films with and without isocyanurate rings
Thermal diffusivity (m2/s)aDensity (g/cm3)

MSQ film

1.44 × 10−7

1.18

MSQ/TAIC (1/0.25) film

1.14 × 10−7

1.18

MSQ/TAIC (1/0.5) film

1.13 × 10−7

1.20

MSQ/TAIC (1/0.75) film

1.10 × 10−7

1.19

MSQ/TAIC (1/1) film

1.08 × 10−7

1.19

MSQ/TAIC (1/1.5) film

1.01 × 10−7

1.18

PSQ/TESIC film

1.25 × 10−7

1.28

a

The thermal diffusivity values are the average of 21 independent measurements. TESIC =tris(triethoxysilylpropyl)isocyanurate.

43

To demonstrate the convenience of using MSQ-SiH to prepare the MSQ/TAIC hybrid films byhydrosilylation reaction, we prepared a PSQ film containing the isocyanurate ring by thepolycondensation reaction of a polymethylsilsesquioxane having a molecular weight similar tothat of MSQ-SiH (Mw = 1200) and TESIC as a control experiment. First, we synthesized thepolymethylsilsesquioxane by the sol–gel reaction of triethoxymethylsilane according to Chapter
2 and TESIC by the hydrosilylation reaction of TAIC with triethoxysilane (Scheme 3.2). The 1H
NMR and HRMS of the as-prepared TESIC confirmed that it was obtained in high purity bysimply removing the solvent from the reaction mixture under high vacuum. To obtain a molar
ratio of polymethylsilsesquioxane and TESIC similar to that of MSQ/TAIC (1/1), we heated a
mixture of polymethylsilsesquioxane and 30 wt % tris(triethoxysilylpropyl)isocyanurate at 80,
100, 120, and 140 °C for 1 h, respectively, and at 160 °C for 4 h in a PFA vial. However, incontrast with the MSQ/TAIC films prepared by hydrosilylation, the film was still not cured at
160 °C and required further heating at 200 °C for 3 h to form a colorless and transparent
PSQ/TESIC film. The chemical structure of PSQ/TESIC was investigated using ATR–FTIR
and 13C solid-state NMR spectroscopic analyses. Although the ATR–FTIR spectra of PSQ/TESIC
provided only limited information, useful information was extracted from the 13C solid-state
NMR of PSQ/TESIC prepared via polycondensation. Specifically, strong signals attributable to
the remaining ethoxy group were observed at 18 and 58 ppm, along with the signal owing to the
SiCH3 group and small signals ascribable to the SiCH2CH2CH2N bond and the carbonyl group in
the isocyanurate structure (Figure 3.5). The PSQ/TESIC film prepared by polycondensation
showed a thermal diffusivity of 1.25 × 10–7 m2/s, which was higher than that of the MSQ/TAIC
hybrid films regardless of the amount of TAIC (Table 3.1).

Scheme 3.2 Preparation of PSQ/TESIC by Polycondensation Reaction

44

Figure 3.5 13C solid-state NMR spectrum of PSQ/TESIC prepared by the polycondensation
reaction of a polymethylsilsesquioxane (Mw = 1200) and TESIC.

To elucidate the reason for the decrease in the thermal diffusivity of hybrid films with theaddition of TAIC, we performed a positron annihilation lifetime analysis of the MSQ film andthe MSQ/TAIC (1/1) hybrid film because the small size of the expected intramolecular void
spaces hindered the analysis via N2 adsorption–desorption isotherms. Three components wereseparated, and the longest lifetime was set as the pick-off annihilation of ortho-positronium.
Then, we obtained the probability density function of the annihilation rate using a numerical
Laplace inversion technique (Figure 3.6) [59]. Surprisingly, MSQ/TAIC (1/1) exhibited a lower
peak position and a shorter lifetime (2.58 ns) than the MSQ film (3.29 ns), as shown in Table 3.2.
The volume of free-volume holes (R) for MSQ and MSQ/TAIC (1/1) estimated using the Tao–
Eldrup equation was 0.23 and 0.15 nm3, respectively [46, 47]. Although we expected that thethermal diffusivities of the MSQ/TAIC hybrid films decreased as a result of the formation of
intramolecular void spaces around the TAIC molecules, the positron annihilation lifetime analysis
showed the opposite results. In fact, the intramolecular void spaces of MSQ/TAIC (1/1) were
smaller than those of the MSQ film, as shown in Table 3.2. In the hybrid film containing POSS

45

units, the three-dimensional structure of the POSS molecules favored the formation of
intramolecular void spaces, which enhanced the thermal insulation property. The density of thehybrid film containing POSS molecules decreased with the increase in intramolecular void spaces[44]. In the case of MSQ/TAIC (1/1), the isocyanurate ring has a planar structure that could hinder
the formation of intramolecular void spaces around the TAIC molecules. Furthermore, unlike theethylene-bridged structure, the longer and more flexible propylene chain is less conducive to theformation of intramolecular void spaces. Similarly, a fully atomistic molecular dynamics (MD)simulation suggested that the number of intramolecular void spaces did not increase by the
introduction of TAIC molecules into the PSQ network (Figure 3.7). Indeed, the densities of the
MSQ/TAIC films were similar regardless of the amount of TAIC molecules, as shown in Table
3.1, even though the reason for this is unclear. Therefore, the decrease in the thermal diffusivityof the MSQ/TAIC hybrid films can be attributed to the nature of the TAIC units rather than to theformation of intramolecular void spaces. In addition, we ruled out the formation of cracks anddimples as a possible reason for the decrease in thermal diffusivity by performing HR-TEM
measurements (Figure 3.8). A similar behavior was observed in Chapter 2 for apolyphenylsilsesquioxane film; namely, its thermal diffusivity was lower than that of other PSQ
films. Although the origin of this phenomenon is unclear at this stage and it is difficult to explain
the decrease in the thermal diffusivity as a result of the nature of the TAIC unit, the TAIC
molecule might also have a role in preventing the heat conduction on the basis of the following
speculations: (i) organic crosslinking units such as C–C and C–N bonds are less prone to conduct
heat than Si–O–Si bonds, (ii) the unreactive allyl group blocks the heat conduction because of thepresence of noncovalent bonds between the polymer chains, and (iii) the cyclic structure of theisocyanurate ring holds the heat. Further studies are required to gain more insight into thisphenomenon.

46

Figure 3.6 Probability density function (PDF) of the ortho-positronium lifetime in MSQ film and
MSQ/TAIC (1/1).

Table 3.2. Lifetime (t3), relative intensity (I3), and radius of free-volume holes for theannihilation of ortho-positronium in MSQ film and MSQ/TAIC (1/1)
Lifetime (t3)I3 (%)V (nm3)a

MSQ film

3.29 ± 0.01

38

0.23

MSQ/TAIC (1/1) film

2.58 ± 0.01

35

0.15

a

The radius and volume of free-volume holes were estimated using the Tao–Eldrup equation.

47

Figure 3.7 Intramolecular voids (free volume) in (a) a polysilsesquioxane film without
isocyanurate and (b) an isocyanurate ring-containing polysilsesquioxane film calculated byfull atomic molecular dynamics. J-OCTA/VSOP was used to calculate the crosslinked
structure of the polymers. We divided the cell lattice of the molecular models into subcells
in 0.4 Å increments and placed a test particle having a diameter of 0.4 Å in the center of each
subcell to identify those which contained a void, considering only the Lennard–Jones
interaction between the polymer molecules. We also filled the subcells containing voids with
virtual spheres and evaluated the size of the free volume. Only the virtual spheres with freevolume are shown in the figure.

100 nm
200kV 426nm x300k

Figure 3.8 High-resolution transmittance electron microscopy image of a cross-section of
MSQ/TAIC (1/1).

48

3.2.3 Thermal stability of hybrid films prepared by hydrosilylation reaction
To investigate the effect of the rigid isocyanurate structure on the thermal stability of the films,
we evaluated the thermal stabilities of MSQ/TAIC (1/1) and MSQ/TAIC (1/0.5) by TG/DTA,
along with those of the MSQ film and PSQ/TESIC for comparison (Figure 3.9). As shownin Table 3.3, the MSQ film exhibited high thermal stability with a 5% weight loss temperature(Td5) and 10% weight loss temperature (Td10) of 472 and 724 °C, respectively. Surprisingly, theMSQ film exhibited an extremely high ceramic yield of 89% at 800 °C. In contrast, MSQ/TAIC(1/1) and MSQ/TAIC (1/0.5) showed lower Td5 and Td10 (380–444 and 427–473 °C, respectively).
Unlike the siloxane-based cyanate ester elastomer prepared via the hydrosilylation reaction of
hydride-terminated siloxane and 2,4,6-tris(allyloxy)-1,3,5-triazine [60], the introduction of TAIC
into the PSQ network decreased the thermal stability. However, the Td5 and Td10 of MSQ/TAIC(1/1) and MSQ/TAIC (1/0.5) were considerably higher than those of organic polymer-based
thermal insulation materials, such as polyurethane and expanded polystyrene [61]. In addition,
the thermal stability of the MSQ/TAIC hybrid films was higher than that of TAIC-containing
polyacrylate and polymethyacrylate, which exhibited Td5 values of only 230–280 °C [57], andhybridmaterials

composedofpoly(N-allyimaleimide-co-N-(2-ethylhexyl)maleimide-co-diisobutylene) containing TAIC and the PSQ structure reported by Matsumoto et al. (Td5 =320 °C–370 °C) [30].
Meanwhile, the thermal stability of PSQ/TESIC (Td5 = 246 °C and Td10 = 282 °C) was
significantly lower than that of MSQ/TAIC (1/1) because of the decomposition of the remaining
ethoxy groups (Figure 3.9 and Table 3.3). Taken together, these results demonstrate that MSQSiH without ethoxy groups is a suitable oligomer for preparing hybrid materials with high thermal
stability.

49

Figure 3.9 Thermogravimetric profiles of MSQ, MSQ/TAIC (1/0.5) and MSQ/TAIC (1/1), and
PSQ/TESIC measured at a heating rate of 10 °C/min under nitrogen.

Table 3.3 Thermal stability of MSQ film, MSQ/TAIC (1/0.5) and MSQ/TAIC (1/1), and
PSQ/TESIC
Td5 (°C)a

Td10 (°C)aresidue at 800 °C (%)a

MSQ film

472

724

89

MSQ/TAIC (1/0.5)

444

473

74

MSQ/TAIC (1/1)

380

427

57

PSQ/TESIC

246

282

65

a

Measured at a heating rate of 10 °C/min under a nitrogen flow of 100 mL/min.

50

3.3 Conclusions
MSQ/TAIC hybrid films containing isocyanurate rings were prepared by the hydrosilylation
reaction of MSQ-SiH and TAIC in the presence of Karstedt’s catalyst. ATR–FTIR and 13C
and 29Si solid-state NMR spectra confirmed that the hydrosilylation reaction of MSQ-SiH withTAIC proceeded to form MSQ/TAIC hybrid films, along with the side reaction of the hydrosilylgroup with moisture. When MSQ/TAIC (1/1) was immersed in THF, the film weight remained
unchanged even after 5 days, indicating the formation of a crosslinked structure. The obtained
MSQ/TAIC hybrid films were colorless, transparent, and stiff irrespective of the amount of TAICin the PSQ network. The thermal diffusivity of the MSQ/TAIC hybrid films decreased from 1.14
× 10–7 to 1.01 × 10–7 m2/s with increasing amount of TAIC in the molar ratio range of 1/0.25–
1/1.15 and was lower than that of a TAIC-free MSQ film prepared by the coupling reaction of
MSQ-SiH (1.44 × 10–7 m2/s) and a PSQ/TESIC film prepared by polycondensation (1.25 × 10–7m2/s). A positron annihilation lifetime analysis indicated that the nature of the isocyanurate ring,rather than the formation of intramolecular void spaces, was responsible for the decrease inthermal diffusivity. Although the Td5 and Td10 values of the MSQ film (472 and 724 °C,
respectively) were higher than those of the MSQ/TAIC films, the Td5 and Td10 values of
MSQ/TAIC (1/1) and MSQ/TAIC (1/0.5) (380–444 and 427–473 °C, respectively) were higher
than those of PSQ/TESIC (Td5 = 246 °C and Td10 = 282 °C) and previously reported TAICcontaining polymer films, which demonstrates that using MSQ-SiH without ethoxy groups as anoligomer is a promising strategy toward hybrid materials with high thermal stability.

3.4 Experimental
Materials
Triethoxymethylsilane, triethoxysilane, and TAIC were purchased from Tokyo Chemical
Industry Co., Ltd. (Tokyo, Japan) and used as received. Toluene (super dehydrated), THF (superdehydrated), ethanol (super dehydrated), and 6 mol/L hydrochloric acid (HCl) were purchasedfrom FUJIFILM Wako Pure Chemical Co., Ltd. (Osaka, Japan). Platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution in xylene (Karstedt’s catalyst) was purchased from51

Sigma-Aldrich and used as received. Water was purified using a Millipore Milli-Q UV systemand had a resistance of 18.2 MΩ·cm and a total organic carbon content of <10 ppb.

Characterizations
1

H, 13C, and 29Si NMR measurements were performed on a Varian 500 MHz spectrometer. The

samples were dissolved in CDCl3, and residual chloroform or tetramethylsilane was used as aninternal standard for chemical shift referencing. 13C and 29Si solid-state NMR measurements were
performed on a Varian 600 MHz spectrometer. Attenuated total reflection Fourier transform
infrared (ATR–FTIR) spectra were recorded on a Shimadzu IR Affinity-1 spectrometer equippedwith an ATR unit. High-resolution mass spectra (HRMS) were recorded on a Thermo Scientific
LTQ Orbitrap XL hybrid Fourier transform mass spectrometer using electrospray ionization. The
contact angle of the film was measured using a KYOWA DM 300 contact angle meter; themeasurements were repeated five times, and the average contact angle was calculated.
Thermogravimetry/differential thermal analysis (TG/DTA) was performed on an SII EXSTARTG-DTA 6200 thermal analyzer at a heating rate of 10 °C/min under a nitrogen flow of 100
mL/min. Thermal diffusivity measurements were conducted on an ai-phase Mobile M3 type 1
system. The measurements were repeated 21 times and the averaged thermal diffusivity wascalculated excluding extreme maximum and minimum values. The density of the films wasdetermined via the Archimedes method by measuring the film weight in air and water. Positron
lifetime measurements were performed using a system based on a digital oscilloscope according
to Chapter 2. High-resolution transmittance electron microscopy (HR-TEM) was performed on a
JEOL JEM 2011 microscope operating at 200 kV. For the TEM sample preparation, the film wasembedded in an epoxy resin and the cross-section of the film was sliced with a microtome.

Synthesis of MSQ-SiH
MSQ-SiH was synthesized via a two-step reaction. First, the sol–gel reaction oftriethoxymethylsilane was conducted using excess HCl and water, and the capping reaction wasthen performed using chlorodimethylsilane. The detailed synthesis procedure for MSQ-SiH wasdescribed in previous chapter

52

Synthesis of Polymethylsilsesquioxane
Polymethylsilsesquioxane was prepared as in Chapter 2. Briefly, triethoxymethylsilane (3.57
g, 20.0 mmol) and ethanol (1.84 g, 40.0 mmol) were added into a 100 mL four-necked flask
equipped with a mechanical stirrer and a nitrogen inlet and outlet. The resulting solution wascooled in an ice bath for 10 min under a nitrogen stream of 360 mL/min. After adding 6 mol/L
HCl (0.383 g) and water (0.0902 g) to achieve a water/triethoxymethylsilane ratio of 1.1 whilestirring (150 rpm), the reaction mixture was stirred at 0 °C and room temperature for 10 min.
Finally, the reaction mixture was heated at 90 °C for 3 h to conduct the polycondensation reaction,
which afforded polymethylsilsesquioxane as a slightly viscous liquid (1.55 g).

Synthesis of Tris(3-triethoxysilylpropyl)isocyanurate (TESIC)TAIC (2.23 g, 8.95 mmol) and triethoxysilane (5.12 g, 31.2 mmol) were dissolved in 5 mL of
toluene. After adding a few drops of Karstedt’s catalyst in xylene, the reaction mixture was stirred
for 2 h at 110 °C. Toluene and excess triethoxysilane were evaporated under vacuum to afford
TESIC (6.55 g, 99%) as a colorless liquid, which was used for the polycondensation reaction
without any purification. 1H NMR (500 MHz, CDCl3, ppm): δ = 0.63 (6H, t, CH2Si, J = 8.5 Hz),1.21 (27H, t, CH2CH3, J = 7.0 Hz), 1.69–1.76 (6H, m, CH2CH2CH2), 3.81 (18H, q, OCH2CH3, J =7.0 Hz); 13C NMR (125 MHz, CDCl3, ppm): δ = 7.6, 18.2, 21.3, 45.3, 58.4, 148.8; 29Si NMR (100MHz, CDCl3, ppm): δ = −46.2; and HRMS (ESI, positive): calcd for C30H63O12N3NaSi3m/z =764.36117 [M + Na+], found m/z = 764,36102.

Preparation of Hybrid Films by the Hydrosilylation Reaction of MSQ-SiH and TAICThe hybrid films were prepared according to our previous paper. (17,18) Typically, MSQ-SiH(0.401 g) and TAIC (0.139 g) were added into a plastic cup and mixed using a rotation/revolution
vacuum mixer at 1600 rpm for 10 min. Then, 10 wt % Karstedt’s catalyst solution (10-times
diluted with toluene) was added and mixed again at the same condition. The obtained mixture
was either transferred to a PFA vial with an inner diameter of 18.4 mm or coated on a Tefloncoated substrate using an applicator with a gap size of 300–500 μm, and then heated at 80, 100,53

120, and 140 °C for 1 h at each temperature and at 160 °C for 4 h. Depending on the method used,
the obtained gel film was taken from the PFA vial or peeled off from the Teflon-coated substrate.

Preparation of Hybrid Films by the Polycondensation Reaction of Polymethylsilsesquioxanewith TESICInto a PFA vial with an inner diameter of 18.4 mm, 0.604 g of 50 wt %polymethylsilsesquioxane solution in THF and 0.101 g of TESIC were added and heated at 80,
120, and 140 °C for 1 h, respectively. Finally, to obtain the hybrid film, the highly viscous liquid
was heated in the PFA vial at 160 °C for 4 h and at 200 °C for 3 h.

54

Chapter 4 Double-Decker Silsesquioxane-Grafted Polysilsesquioxane
Hybrid Films as Thermal Insulation Materials
4.1 Introduction
In Chapter 3, we attempted to create intramolecular void spaces in the PSQ film by using TAIC
as a cross-linking agent. Moreover, the high miscibility of TAIC with the PSQ network allowed
introducing a high amount of TAIC. Unfortunately, although the introduction of TAIC enhanced
the thermal insulating property of the PSQ film via another heat conduction mechanism not
attributed to the formation of intramolecular void spaces, its thermal stability was reduced.
In our quest for an ideal crosslinker that creates intramolecular void spaces without aggregation
while maintaining high thermal stability and transparency, we turned our attention to doubledecker silsesquioxane (DDSQ), which exhibits a unique structure composed of two
tetraphenylcyclotetrasiloxanes linked via siloxane bonds bearing two reactive functional groups
such as hydrosilyl and vinyl groups [62, 63]. DDSQ is used as the starting monomer for the
preparation of hybrid polymers because it exhibits high thermal stability and mechanicalproperties similar to those of POSS molecules. For example, polyimide-, polyurethane-, andpolysiloxane-based materials containing DDSQ structures with enhanced thermal stability andmechanical property have been synthesized [64-66]. Additionally, DDSQ-containing polymers
exhibit relatively good solubility in common organic solvents and generally form transparent
films due to their high miscibility with the base polymers.
In this study, hybrid polymer films containing DDSQ were prepared via the hydrosilylation
reaction of MSQ-SiH and DDSQ with two vinyl groups to prevent aggregation and improve thethermal insulation property and thermal stability. The effect of the DDSQ structure in the PSQ
film on the thermal insulation property and the thermal stability compared with POSS-containing
PSQ films was investigated.

55

4.2 Results and discussion
4.2.1 Preparation of DDSQ-grafted PSQ films via a two-step reaction
As reported in our previous paper, POSS-containing hybrid films were successfully prepared
via the hydrosilylation reaction of MSQ-SiH and vinyl-POSS in a vacuum mixer [44]. Owing to
the formation of Si–CH2CH2–Si bonds, the obtained hybrid films containing 5–10 wt% of POSS
molecules were colorless and transparent stiff films. Interestingly, the thermal diffusivities of thehybrid films decreased with an increase the POSS amount in the range of 5–20 wt % as a resultof the formation of intramolecular void spaces around the POSS molecules. However, wepreviously found that under the reaction conditions, the dehydrogenation reaction between thehydrosilyl group in MSQ-SiH and trace moisture also occurred in the presence of the Platinum(Pt) catalyst, competing with the hydrosilylation reaction and forming Si–O–Si bonds . Therefore,
to avoid the undesired dehydrogenation reaction, the hydrosilylation reaction between MSQ-SiH
and vinyl-functionalized DDSQ (vinyl-DDSQ) was conducted according to the two-step process
shown in Scheme 4.1. First, the hydrosilylation reaction of MSQ-SiH and vinyl-DDSQ was
conducted in solution under a nitrogen atmosphere to induce the formation of Si–CH2CH2–Si
bonds in the absence of moisture, thereby creating the desired intramolecular void spaces. Then,the dehydrogenation reaction was conducted in the film state to afford a cross-linked gel film via
the formation of Si–O–Si bonds.

56

CH3

CH3

Si O

Si O
m

O

Ph
Si
O

O

+

H 3C

n

Si
O

Si
Ph

H3C Si CH3
H

CH3

CH3

Si O
O

O

dehydrogenation reaction

CH3

Si O
m

Ph Ph
hydrosilylation reaction
O Ph
Si
O Si
Si
O
Karstedt’s catalyst
O
Si
THF
Si O
O CH3
O
O
Si
O Si O
Ph
Ph Ph

O
O

DDSQ-grafted MSQ film

Si O
n

O

l

H3C Si CH3 H3C Si CH3
H

Ph
Ph
O
Ph
Si
Si
O Ph
O
O
O Si
Si
Si
O
H 3C
O
O
O
Si
O Si
Si CH3OO SiPhOPh Ph O Si
Ph

Scheme 4.1 Preparation of a double-decker silsesquioxane (DDSQ)-grafted polysilsesquioxane(PSQ) film via a two-step reaction between MSQ-SiH and vinyl-DDSQ.

To prepare DDSQ-grafted PSQ, MSQ-SiH with Mw = 1100 and Mw/Mn = 1.30 was reacted with
15 mol% vinyl-DDSQ in THF in the presence of Karstedt’s catalyst, and the reaction progress
was monitored by 1H NMR spectroscopy. In the 1H NMR spectrum of a mixture of MSQ-SiH andvinyl-DDSQ before adding the Pt catalyst, distinct signals of the vinyl groups of vinyl-DDSQ and
the hydrosilyl groups of MSQ-SiH were observed at 5.91–6.19 ppm and 4.72 ppm, respectively(Figure 4.1). The vinyl group of vinyl-DDSQ was almost consumed within 1 h, whereas a signal
corresponding to the hydrosilyl group was still observed at 4.72 ppm. New peaks attributed to the
Si–CH2CH2–Si bonds formed via hydrosilylation were observed at 0.53 and 0.62 ppm. This
indicates that the hydrosilylation reaction between the vinyl group of vinyl-DDSQ and thehydrosilyl group of MSQ-SiH occurred preferentially over the dehydrogenation reaction in theabsence of moisture. The hydrosilyl group remained even after 3–5 h and gel formation was notobserved in the solution. Finally, to ensure the grafting of DDSQ molecules into the PSQ
backbone, the reaction was allowed to continue overnight. Then, the solvent was removed in an57

evaporator, affording DDSQ-grafted PSQ as a viscous and colorless oil. Additionally, 29Si NMR
spectrum of DDSQ-grafted PSQ containing 15 mol% DDSQ molecules provided useful
information concerning the grafting of DDSQ (Figure 4.2). The peaks at around −64.6 ppm and−78.3~−79.5 ppm, corresponding to the T3 structure in MSQ-SiH and the main backbone of
DDSQ, were mainly observed, along with the remaining hydrosilyl group at −5.4 ppm and theformed the Si–CH2CH2–Si bonds at −31.36 and −31.44 ppm. Therefore, the grafting degree of
DDSQ was controlled by the feed of vinyl-DDSQ and the remaining hydrosilyl group makes it
possible to perform the dehydrogenation reaction for the preparation of film. However, the small
peak was also observed at −17.0 ppm. This might be due to the side reaction, namely, thedehydrogenation reaction of a partial hydrosilyl groups with the ambient moisture underwent theformation of Si–O–Si bond during the process of removal of solvent.
The Mw and Mw/Mn values were estimated to be 4,000 and 2.39 by GPC after the hydrosilylation
reaction of MSQ-SiH with 15 mol% vinyl-DDSQ. The molecular weight of DDSQ-grafted PSQ
increased with increasing the amount of vinyl-DDSQ, and the Mw and Mw/Mn of DDSQ-grafted
PSQ containing 25 mol% DDSQ increased to 13,700 and 3.27, respectively. The same result was
obtained upon adding further vinyl-DDSQ to the overnight reaction.

Figure 4.1 1H NMR spectra of the time course of the hydrosilylation reaction between MSQ-SiH
and vinyl-DDSQ in solution under a nitrogen atmosphere. The spectra were recorded by diluting
CDCl3 aliquots taken from the reaction mixture at the specified time points.

58

Figure 4.2 29Si NMR spectrum of DDSQ-grafted PSQ containing 15 mol% DDSQ in CDCl3.

Next, to prepare the hybrid film, the obtained DDSQ-grafted PSQ was poured into a poly PFA
vial with an inner diameter of 18.4 mm and heated in an oven under air atmosphere at 80, 100,
120, and 140 °C for 1 h at each temperature and finally at 160 °C for 4 h for the dehydrogenation
reaction to proceed. The FT-IR spectrum of the product obtained after this treatment revealed thecomplete disappearance of the characteristic peaks of the hydrosilyl group at 2100 and 900 cm−1,
which were observed in the FT-IR spectrum of DDSQ-grafted PSQ before the hydrogenation
reaction (Figure 4.3). Although the newly formed Si–O–Si bonds could not be identified in the
FT-IR spectrum because their signal overlapped with the peaks of Si–O–Si bonds in the PSQ
backbone, the progress of the dehydrogenation reaction was supported by the change in theappearance of DDSQ-grafted PSQ from a viscous product to a stiff gel, as described in ourprevious report [44].
To investigate the influence of the DDSQ structure on the thermal properties of the hybrid film,
DDSQ-grafted PSQ with different DDSQ contents in the range of 5–25 mol% were prepared via
the hydrosilylation reaction between MSQ-SiH and vinyl-DDSQ. Colorless and transparent
DDSQ-grafted PSQ films were obtained in all cases, regardless of the DDSQ content. In the caseof POSS-containing hybrid films, when the POSS amount exceeded 20 wt%, the hybrid film
became turbid even after the formation of Si–CH2CH2–Si bonds via hydrosilylation, as reported59

in our previous study [44], because POSS molecules tend to aggregate in the film state. In
contrast, the eight phenyl groups on the silicon atoms of the DDSQ molecule enhanced its
solubility, and their steric hindrance prevents the aggregation of DDSQ in the PSQ network. As
a result, the obtained DDSQ-grafted PSQ films showed high transparency even for a DDSQ
amount of 25 mol%. To obtain the morphological information of hybrid film containing the POSS
molecules, DDSQ-grafted PSQ containing 25 mol% DDSQ molecules was casted on the siliconwafer, and it was heated at the same conditions for the atomic force microscope (AFM)
measurement. DDSQ-grafted PSQ film containing 25 mol% DDSQ molecules showed thesmooth surface morphology (Figure 4.4). This means that DDSQ molecules were uniformly
introduced into PSQ network via the covalent bond without aggregation. Furthermore, the pencil
hardness of the POSS-containing hybrid films exhibited an excellent hardness of 6H and washigher than that of a PSQ film (2H) without POSS prepared via the polycondensation reaction ofoligomethylsilsesquioxane (MSQ-OH) [44]. The rigid POSS improved the hardness of the hybrid
film as a filler via the formation of covalent bonds. However, contrary to expectations, thehardness of the hybrid films containing DDSQ decreased with an increase the vinyl-DDSQ
content from 5 to 25 mol%.

60

Figure 4.3 FT-IR spectra of (a) DDSQ-grafted PSQ containing 15 mol% vinyl-DDSQ (before thedehydrogenation reaction) and (b) DDSQ-grafted PSQ film containing 15 mol% vinyl-DDSQ(after the dehydrogenation reaction).

Figure 4.4 Atomic force microscope (AFM) image of DDSQ-grafted PSQ film containing 25
mol% DDSQ on the silicon wafer.

61

4.2.2 Thermal diffusivity of the DDSQ-grafted PSQ films
We previously reported that the ethylene-bridged structure prepared via hydrosilylation
reaction provided intramolecular void spaces around the POSS molecule that enhanced thethermal insulation property [43, 44]. Thus, a POSS-free PSQ film prepared by polycondensing
MSQ-OH exhibited a thermal diffusivity of 1.36 × 10−7 m2/s [43, 44], whereas that of an ethylenebridged PSQ film prepared via the hydrosilylation reaction between MSQ-SiH and dimethylvinylsilylated MSQ (MSQ-SiVi) was 1.15 × 10−7 m2/s [43]. Surprisingly, the thermal diffusivity
decreased from 1.21 × 10−7 to 1.01 × 10−7 m2/s on introducing POSS molecules (5–20 wt%) [44],indicating that the introduction of POSS improved the thermal insulating properties of the PSQ
film. Therefore, the thermal diffusivity of DDSQ-grafted PSQ films prepared in the study withdifferent amounts of DDSQ was measured and compared with that of hybrid films containing
POSS molecules.
The thermal diffusivities of DDSQ-grafted PSQ films containing 5–25 mol% vinyl-DDSQ are
shown in Figure 4.5 and Table 4.1, along with those of an MSQ film prepared via thedehydrogenation reaction of MSQ-SiH and POSS-containing hybrid films prepared via
hydrosilylation [44]. As we previously reported, the MSQ film exhibits a high thermal diffusivityof 1.44 × 10−7 m2/s due to the formation of a highly cross-linked network structure. Interestingly,
the thermal diffusivity of the DDSQ-grafted PSQ film containing 5 mol% DDSQ decreased to1.21 × 10−7 m2/s, and further decreased to 1.02 × 10−7 m2/s proportionally with increasing the
DDSQ amounts in the range of 5–25 mol% (Figure 4.5 and Table 4.1). The introduction of the
DDSQ structure apparently enhances the thermal insulating properties similarly to that observed
for the POSS-containing hybrid films, although the latter exhibited lower thermal diffusivity than
the former, as shown in Figure 4.5 and Table 4.1. These results indicate that intramolecular void
spaces were formed around the DDSQ structure because of the formation of Si–CH2CH2–Si
bonds.
The influence of the amount of Pt catalyst on the thermal diffusivity of the DDSQ-grafted PSQ
films was also investigated. In this experiment, the same Pt catalyst amount as used in thehydrosilylation reaction was added to DDSQ-grafted PSQ, and the dehydrogenation reaction was
conducted. Light-brown DDSQ-grafted PSQ films containing 5–25 mol% DDSQ were obtained.

62

Although the thermal diffusivities of these films also decreased with increasing amounts of DDSQ
(Figure 4.6), considerable differences in the values were observed, most likely because theadditional Pt catalyst increased the reaction rate of the dehydrogenation, which became
uncontrolled.

Figure 4.5 Thermal diffusivities of (a) an MSQ film (black circle), (b) DDSQ-grafted PSQ films
prepared via the hydrosilylation reaction between MSQ-SiH and vinyl-DDSQ (pink circles), and(c) POSS-containing PSQ films prepared via the hydrosilylation reaction between MSQ-SiH andvinyl-POSS (blue circles).

63Table 4.1. Thermal diffusivities of an MSQ film, DDSQ-grafted PSQ films, and POSScontaining hybrid films
oligosilsesquioxane

Vinyl-DDSQ

Vinyl-POSS(mol%)(wt%)athermal diffusivityb (m2/s)

MSQ-SiH01.44 × 10−7c

MSQ-SiH51.21 × 10−7

MSQ-SiH10

1.15 × 10−7

MSQ-SiH

15

1.06 × 10−7

MSQ-SiH20

1.07 × 10−7

MSQ-SiH25

1.02 × 10−7

MSQ-SiH

5 (3.3 mol%)

1.21 × 10−7

MSQ-SiH

10 (6.5 mol%)

1.10 × 10−7

MSQ-SiH

20 (13 mol%)

1.01 × 10−7aThe data were obtained from ref. [43].

bcThe measurements were repeated nine times, and the average value was calculated.

The data were obtained from ref. [44].

Figure 4.6 Thermal diffusivities of DDSQ-grafted PSQ films prepared by adding Platinum
catalyst before the dehydrogenation reaction.

64

To gain insight into the formation of intramolecular void spaces around the DDSQ moleculesin the PSQ film, the lifetime spectra of positron annihilation were measured for DDSQ-grafted
PSQ films containing 10 and 20 mol% DDSQ. The probability density function (PDF) of theannihilation rate was obtained using a numerical Laplace inversion technique (Figure 4.7). The
spectra were decomposed into four components, and the lifetime corresponding to the pick-off
annihilation of ortho-positronium (τps) and the corresponding intensity (IPs) were derived. Table
4.2 summarizes the obtained parameters and the values of free volume (V) estimated using τpsaccording to the Tao–Eldrup model [46, 47]. Compared with a DDSQ-free MSQ film, the DDSQgrafted PSQ film containing 10 mol% DDSQ exhibited a slightly higher τps value of 0.35 ns,
which is consistent with the formation of void spaces in the ethylene-bridged structure producing
a subtle increase in the lifetime. The V value for the DDSQ-grafted PSQ film containing 10 mol%
DDSQ was 0.24 nm3, which was larger than that of the MSQ film (0.23 nm3) [44]. However, theV value for the DDSQ-grafted PSQ film containing 20 mol% DDSQ was estimated to be 0.22
nm3, which was smaller than that of the MSQ film and the DDSQ-grafted PSQ film containing
10 mol% DDSQ. Moreover, the peak top of PDF for the DDSQ-grafted PSQ film containing 20
mol% DDSQ shifts slightly to a shorter time region (Figure 4.7), and a clear difference in the Vvalue was not observed. These results suggest that the decrease in the thermal diffusivity might
not stem from the intramolecular void spaces formed by the hydrosilylation reaction.
The highly cross-linked network of the MSQ film provided a heat conduction pathway thatenhanced the thermal diffusivity. In contrast, since vinyl-DDSQ has only two vinyl groups forthe hydrosilylation reaction, the crosslinking degree should be lower in the DDSQ-grafted PSQ
films containing 10 mol% and 20 mol% DDSQ. This would be detrimental to heat conduction
pathways, resulting in lower thermal diffusivity than that of the MSQ film. Meanwhile, although
the POSS-containing hybrid films exhibited highly cross-linked networks because of the eight
vinyl groups, the decrease in the thermal diffusivity can be attributed to the formation of
intramolecular void spaces because the rigidity of the hybrid films would prevent collapse of thevoid spaces. In the case of the DDSQ-grafted PSQ films, the hardness of the DDSQ-grafted PSQ
films decreased with increasing the vinyl-DDSQ content from 5 to 25 mol%, namely, thecrosslinking density decreased, which could disrupt the intramolecular void spaces, especially for65the DDSQ-grafted PSQ film containing 25 mol% DDSQ. Therefore, the decrease in the thermal
diffusivity of the DDSQ-grafted PSQ films could be due to a decrease in the crosslinking density.
To gain the information about the cross-linking density, the storage modulus (E′) and the loss
modulus (E′′) of DDSQ-grafted PSQ film containing 10 mol% DDSQ molecules were measured
between −150 to 250 °C by a heating rate of 5 °C/min at a frequency of 10 kHz (Figure 4.8). In
the DDSQ-grafted PSQ film containing 10 mol% DDSQ molecules, the distinct peaks wereobserved in the E′′ in the glass-relaxation region. DDSQ-grafted PSQ film apparently exhibitedrubber-like property. The cross-linking density of DDSQ-grafted PSQ film containing 10 mol%
DDSQ molecules could be estimated to be 7.27 × 10−4 mol/cm3 by the following equation (4.1):
𝑐𝑟𝑜𝑠𝑠𝑙𝑖𝑛𝑘𝑖𝑛𝑔 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = 𝐸′P3 ∙ 𝑅 ∙ 𝑇(4.1)

where E′ is the storage modulus at 150 °C. R and T are the gas constant and absolutetemperature, respectively. The further studies are underway to clarify the effect of DDSQ
molecules in PSQ network on the relationship between the crosslinking density and themechanical and thermal insulating property.

Figure 4.7 PDF of the lifetime of ortho-positronium in DDSQ-grated PSQ films prepared from
MSQ-SiH and 5 mol % (pink circles) and 10 mol% (green circles) of vinyl-DDSQ by thehydrosilylation reaction.

66

Table 4.2. Lifetime (tps), relative intensity (Ips), and free volume (V) for the annihilation ofortho-positronium in an MSQ film prepared via hydrogenation of MSQ-SiH and DDSQgrafted PSQ films prepared by the hydrosilylation reaction between MSQ-SiH and 5 and 10
mol% vinyl-DDSQ
silsesquioxane Vinyl-DDSQ (mol%)tpso (ns)b

Ips (%)bV (nm3)a,b

MSQ-SiH03.29 ± 0.01c

38c

0.23c

MSQ-SiH10

3.35 ± 0.01

35

0.24

MSQ-SiH20

3.20 ± 0.01

36

0.22

a

The values of free volume were estimated using the Tao–Eldrup equation ref. [46, 47].

b

Lifetime (tps), relative intensity (Ips), and free volume (V).

Figure 4.8 Temperature dependence of the storage modulus (E′), loss modulus (E′′), and loss
tangent (tan δ) for DDSQ-grafted PSQ film containing 10 mol% DDSQ molecules.

4.2.3 Thermal stability of the DDSQ-grafted PSQ films
As mentioned in the Introduction, high thermal stability is required for next-generation thermal
insulation materials. We previously reported that an MSQ film prepared via dehydrogenation of67

the hydrosilyl groups exhibited high thermal stability over 450 °C.25 In this study, the influenceof the DDSQ structure having a rigid core composed of siloxane bonds on the thermal stability of
the DDSQ-grafted PSQ films was investigated by performing a thermogravimetry/differential
thermal analysis (TG/DTA), and the results are summarized in Figure 4.9 and Table 4.3.

Figure 4.9 Thermogravimetric analysis traces of DDSQ-grafted PSQ films containing DDSQ
molecules (5–25 mol%) measured at a heating rate of 10 °C/min under an air flow of 100 mL/min.

Compared with an ethylene-bridged PSQ film prepared via the hydrosilylation reaction of
MSQ-SiH with MSQ-SiVi, which exhibited a 5% mass loss temperature (Td5) of 302 °C and a
10% weight loss temperature (Td10) of 285 °C [43], the DDSQ-grafted PSQ films showed higher
thermal stability, with Td5 and Td10 values of 438 °C–463 °C and 495 °C–523 °C, respectively(Table 4.3). This result is in agreement with previous studies on polyurethane, polysiloxane, andpolyimide containing DDSQ in the main chain, which exhibited enhanced thermal stability witha Td5 of 300 °C–550 °C compared with the corresponding DDSQ-free polymers due to the rigid
structure of DDSQ [64-66], and confirms the enhancing effect of DDSQ in the thermal stabilityof the DDSQ-grafted PSQ films. Differential scanning calorimetry (DSC) of DDSQ-grafted PSQ

68

films containing 5–25 mol% DDSQ was also performed as additional information regarding thethermal property. The endothermic peaks or glass transition temperature like peaks were observedat around 42 °C not depending on the amount of DDSQ molecules (Figure 4.10). The origin of
these peaks was unclear, but it might be derived from the nature of PSQ network.
As shown in Table 4.3, although POSS-containing hybrid films also exhibited high thermal
stability compared with the organic polymer [44], their Td5 and Td10 values decreased with anincrease in the amount of POSS because the remaining vinyl group and the formed Si–CH2CH2–
Si bonds, which are less thermally stable than Si–O–Si bonds, reduce the thermal stability of thehybrid films. However, the vinyl group was completely consumed in the two-step reaction leading
to DDSQ-grafted PSQ films, resulting in higher thermal stability. To prevent the side reaction
during the preparation of the POSS-containing films, a similar two-step reaction was conductedto obtain a hybrid film containing 10 wt% POSS.
A 1H NMR analysis revealed that the vinyl group of vinyl-POSS completely disappeared after
conducting the hydrosilylation reaction of MSQ-SiH and vinyl-POSS overnight. The obtained
freestanding film containing 10 wt% POSS was colorless after the dehydrogenation reaction;
however, for unknown reasons, the surface was considerably rough even though the aggregationof POSS molecules was prevented. In addition, the Td5 and Td10 of the hybrid film containing 10
wt% POSS were 360 °C and 422 °C, respectively. Overall, the two-step reaction did not improve
the thermal stability of hybrid films containing POSS. Since the vinyl group disappeared after thehydrosilylation reaction, the lower thermal stability of the POSS-containing hybrid film could be
attributed to the unstable Si–CH2CH2–Si bonds. Furthermore, the DDSQ-grafted PSQ films
showed excellent thermal stability compared with an MSQ/TAIC hybrid film, in which the TAIC
molecules decreased the thermal diffusivity but the thermal stability was reduced with increasing
the amount of TAIC; an MSQ/TAIC film with a hydrosilyl group/allyl group molar ratio of 1/1
exhibited a Td5 of 380 °C and a Td10 of 427 °C (Chapter 3). In the DDSQ-grafted PSQ films, thethermal diffusivity decreased upon incorporating the DDSQ structure in the siloxane network,
while a high thermal stability over 400 °C was maintained.

69

Table 4.3. Thermal stability of an ethylene-bridged PSQ film, DDSQ-grafted PSQ films, andPOSS-containing hybrid filmsa
oligosilsesquioxane

Vinyl-DDSQ

Vinyl-POSSa(mol%)(wt%)

Td5 (%)

Td10 (%)

MSQ-SiH/MSQ-SiVi

0

302b

285b

MSQ-SiH

5

445

510

MSQ-SiH

10

448

495

MSQ-SiH

15

463

523

MSQ-SiH

20

456

509

MSQ-SiH

25

438

512

MSQ-SiH

5 (3.3 mol%)

382b

453b

MSQ-SiH

10 (6.5 mol%)

370b

448b

MSQ-SiH

20 (13 mol%)

304b

397b

a

Measured at a heating rate of 10 °C under an air flow of 50–100 mL/min.

b

The data were obtained from ref. [43, 44].

70

Figure 4.10 DSC curves of DDSQ-grafted PSQ films containing 5–25 mol% of DDSQ.

4.3 Conclusions
DDSQ-grafted PSQ films containing 5–25 mol% of DDSQ were prepared following a two-stepprocess. First, the hydrosilylation reaction between MSQ-SiH and vinyl-DDSQ forming Si–
CH2CH2–Si bonds was conducted in solution under a nitrogen atmosphere to prevent thedehydrogenation side reaction induced by moisture, and the successive dehydrogenation reactionof the remaining SiH groups was then conducted in the film state to promote the crosslinking
reaction via the formation of Si–O–Si bonds. By conducting the two-step reaction, thedehydrogenation reaction of the SiH groups during the hydrosilylation reaction in solution wasprevented and the Si–CH2CH2–Si bonds were formed quantitatively, according to 1H NMR and
FT-IR analyses.
Unlike POSS-containing hybrid films, the high miscibility of the DDSQ molecule allowed
introducing a high amount of vinyl-DDSQ as a crosslinker, obtaining colorless and transparent
DDSQ-grafted PSQ films without aggregation of the DDSQ molecules. The thermal diffusivitiesof the DDSQ-grafted PSQ films decreased from 1.21 × 10−7 to 1.02 × 10−7 m2/s with increasing
the DDSQ amount from 5 to 25 mol%, as was similarly observed for POSS-containing hybrid
films. However, the hardness of the DDSQ-grafted PSQ films decreased with an increase in the71vinyl-DDSQ content. The V value for DDSQ-grafted PSQ films containing 5 and 10 mol% DDSQ
estimated from a positron annihilation lifetime analysis was 0.24 and 0.22 nm3, respectively, anda clear relationship between the thermal diffusivity and the volume size was not observed. These
results indicate that the low thermal diffusivity of the DDSQ-grafted PSQ films did not stem fromthe formation of intramolecular void spaces but from a decrease in the crosslinking density upon
the introduction of the DDSQ structure. The DDSQ-grafted PSQ films maintained high thermal
stability over 400 °C with Td5 and Td10 values of 438 °C–463 °C and 495 °C–523 °C, respectively,
outperforming POSS-containing hybrid films. According to their high thermal insulating
properties and thermal stability, the DDSQ-grafted PSQ films can be considered promisingmaterials for next-generation thermal insulators.

4.4 Experimental
Materials
MSQ-SiH was prepared according to our previous paper.19 Vinyl-DDSQ was provided by JNC
Corporation and used as received. THF (super dehydrated) and toluene (super dehydrated) werepurchased from FUJIFILM Wako Pure Chemical Co., Ltd. Platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution (Karstedt’s catalyst) in xylene was purchased from
Sigma-Aldrich.

Characterization
1

H NMR measurements were performed on a Varian 400 MHz spectrometer, and 29Si NMR

measurement was performed on a Varian 500 MHz spectrometer. GPC was performed on a
Shimadzu LC-20AD HPLC systems using triply connected TSKgel SW columns. THF was usedas the eluent at a flow rate of 1 mL/min and the molecular weight was calibrated using polystyrene
standards. ATR-FT-IR spectra were recorded on a Shimadzu IR Affinity-1 spectrometer equippedwith an ATR unit. TG/DTA was performed on an SII EXSTAR TG-DTA 6200 thermal analyzer
at a heating rate of 10 °C/min under an air flow of 50 mL/min. DSC was performed on HITACHIDSC7000X thermal analyzer at a heating rate of 10 °C/min under a nitrogen. AFM measurement
was conducted using AFM-5500M HITACHI microscope. Thermal diffusivity measurements72were performed on an ai-phase Mobile M3 type 1 system. The measurements were repeated ninetimes and the averaged thermal diffusivity was calculated. Positron lifetime measurements were
performed using a system based on a digital oscilloscope. The detailed procedure was describedin our previous paper.20 Dynamic mechanical analysis was performed on Hitachi HighTech
DMA6110. The measurements of specimens with width of 9.1 mm and thickness of 0.4 mm wereconducted from −150 to 250 °C at a heating rate of 5 °C/min and a frequency of 10 Hz.

Preparation of DDSQ-grafted PSQ films via the hydrosilylation reaction of MSQ-SiH withvinyl-DDSQ and successive dehydrogenation reaction
A typical preparation procedure was as follows: In a 25 mL flask, MSQ-SiH (0.500 g, 1.21mmol) and vinyl-DDSQ (0.219 g, 0.182 mmol) were dissolved in 5- mL THF and stirred at room
temperature for 10 min under a nitrogen atmosphere. Then, 5 wt% Karstedt’s catalyst solution in
THF (0.0250 g) was added and stirred at 50 °C overnight. The solvent was removed using anevaporator, affording a colorless and viscous liquid. Then, the hybrid films were preparedaccording to the following procedure: A portion of the viscous liquid was added to a PFA vialwith an inner diameter of 18.4 mm and heated in an oven under air atmosphere at 80, 100, 120,
and 140 °C for 1 h at each temperature and at 160 °C for 4 h to obtain freestanding gel films. For
the experiment of the dehydrogenation reaction with an additional amount of Karstedt’s catalyst,
a half amount of the polymer obtained after the hydrosilylation and 0.0125 g of 5 wt% Karstedt’s
catalyst solution in toluene were stirred for 10 min. Then, the mixture was transferred to a PFA
vial with an inner diameter of 18.4 mm and heated in an oven under air atmosphere at the same
conditions described above for the DDSQ-grafted PSQ films.

73

Chapter 5 Conclusions
In conclusion, the relationship between structure and thermal properties is investigated for thecreation of new PSQ-based insulating materials. Furthermore, we proposed a strategy to introduce
crosslinkers into PSQs to improve their adiabatic properties and thermal stability.
In Chapter 2, a series of PSQs were prepared by sol-gel reaction and the effects of residual
alkoxy groups and substituents on the thermal insulating properties of the corresponding
freestanding gel films were investigated. The thermal diffusivity of freestanding gel films
prepared from PSQ-Me (OEt) and PSQ-Me (OBui) was found to decrease as the amount ofresidual alkoxy groups decreased. The substituents on the silicon atoms also affected the thermal
insulator properties. The thermal stability of PSQ-Ph films was considerably higher than that of
PSQ-Me and PSQ-Phe films. However, despite the presence of a phenyl ring in PSQ-Phe, its
thermal stability was lower than that of PSQ-Me and PSQ-Ph, possibly due to the instability ofthe benzyl group at high temperatures in air.
In Chapter 3, MSQ/TAIC hybrid films with isocyanurate rings were prepared by hybridizing
MSQ and TAIC by hydrosilylation. TAIC exhibited excellent solubility and miscibility as a crosslinker. The adiabatic performance of the MSQ/TAIC hybrid film was improved with increasing
TAIC content, and was higher than that of the TAIC-free MSQ film prepared by MSQ-SiH
coupling reaction and the PSQ/TESIC film prepared by polycondensation of the ethoxysilane
precursors. Positron annihilation lifetime analysis revealed that the improved adiabatic
performance is not due to the formation of intramolecular voids but to the nature of theisocyanurate ring.
In Chapter 4, a homogeneous PSQ gel film without aggregation was successfully prepared by
performing a hydrosilylation reaction between MSQ-SiH and vinyl-DDSQ with phenyl groups
on silicon atoms, followed by successive dehydrogenation reactions of the remaining SiH groups
in the film state with moisture. It was found that the adiabatic performance was improved when
the DDSQ content was increased from 5 to 25 mol%. The DDSQ-containing PSQ films showed
lower thermal diffusivity than those prepared from similar reactions of vinyl-POSS reported
previously, which was not due to the formation of intramolecular voids but to the reduction of74

crosslink density associated with the introduction of DDSQ structure. Furthermore, the DDSQgrafted PSQ films maintained excellent thermal stability above 400 °C.
In summary, a variety of PSQ-based hybrid gel films were prepared and their adiabatic
performance was significantly improved by tuning the PSQ chemical structure during the courseof the studies. These findings are expected to lead to the development of next-generation
insulation materials with higher performance.

75List of publications
Chapter 2. Structure–Thermal Property Relationships of Polysilsesquioxanes for Thermal
Insulation Materials
Takashi Hamada, Taku Goto, Sakino Takase, Kenta Okada, Akira Uedono, and Joji Ohshita
ACS Applied Polymer Materials 2022, 4, 4, 2851-2859.

Chapter 3. Organic–Inorganic Hybrid Thermal Insulation Materials Prepared via Hydrosilylationof Polysilsesquioxane Having Hydrosilyl Groups and Triallylisocyanurate
Sakino Takase, Takashi Hamada, Kenta Okada, Susumu Mineoi, Akira Uedono, and Joji Ohshita
ACS Applied Polymer Materials 2022, 4, 5, 3726–3733.

Chapter 4. Double-Decker Silsesquioxane-Grafted Polysilsesquioxane Hybrid Films as Thermal
Insulation Materials
Takashi Hamada, Sakino Takase, Arata Tanaka, Kenta Okada, Susumu Mineoi, Akira Uedono,
and Joji Ohshita
ACS Applied Polymer Materials 2023, 5, 1, 743–750.

This dissertation includes contents reproduced from the papers listed above with permission.
Copyright 2022 ACS.

Other papers not included in this thesis
Polysilsesquioxane-Containing Thermally Degradable Groups for Potential Application as
Thermal Insulation Materials
Sakino Takase, Takashi Hamada, Kenta Okada, Susumu Mineoi, and Joji Ohshita
ACS Applied Polymer Materials 2023, 5, 2, 1390–1397.

76

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Acknowledgments

This thesis is a summary of my studies from 2020 to 2023 under the direction ofProfessor Joji Ohshita at Hiroshima University.
First and foremost, I would like to express my sincere appreciation to Professor
Joji Ohshita, Graduate School of Advanced Science and Engineering, Hiroshima
University, for his mentorship and helpful advice and encouragement.
I would like to express my deepest gratitude to Lecturer Takashi Hamada forsupporting my research activities and providing guidance, advice, and wisdom,
since the beginning.
I would like to thank Professor Atsushi Ikeda, Professor Masakoto Kanezashi,
Professor Akinobu Naka, Professor Akira Uedono, Assistant Professor Yohei
Adachi, Mr. Kenta Okada, Mr. Susumu Mineoi and Associate Professor Hiroyuki
Koga for their useful advice and helps.
I am grateful to Mr. Katsuhiro Horata, Mr. Daiji Katsura, Mr. Tetsuya Maeda, Mr.
Sun Weipeng, Ms. Wang Conghuan, Mr. Takashi Kai, Mr. Kohei Yamada, Mr.
Keigo Kawakami, Mr. Kei Oshima, Ms. Maho Kurihara, Mr. Mitsuru Sakabe, Mr.
Tetsuya Sugimoto, Mr. Arata Tanaka, Mr. Takumi Hasegawa, Mr. Tsubasa
Yoshio, Mr. Lin Zhixin, Mr. Shota Terao, Mr. Ryuji Matsuura, Mr. Ryuto
Miyazaki, and all other members of the past members of Element-Based Organic
Materials Chemistry laboratory for their help and kindness.
Finally, I would like to express my appreciation to my mother Mutsumi Takase
and my father Yasuhide Takase for their support and affection, without which it
would have been very difficult to persevere through my many years of school.

Sakino Takase
January 2023

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