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Chapter 1 Introduction
Chapter 1
Introduction
The introductory chapter presents an overview of the developments in the area of
preceramic polymers with a special reference to polysilahydrocarbons and
polyborosiloxanes. A brief account of end-uses of preceramic polymers is given.
The chapter concludes with the objective and scope of the present investigation.
2 Chapter 1
he field of polymer science forms one of the major areas of molecular and
materials science. This field impinges on nearly every aspect of modern
life, from electronics technology, to medicine, to the wide range of fibers,
films, elastomers and structural materials on which everyone depends. Most of
these polymers are organic materials where the long-chain backbones consist
mainly of carbon atoms linked together or separated by heteroatoms such as
oxygen or nitrogen. Organic polymers are derived from petroleum and less
frequently from plants, animals and microorganisms. Hence, they are generally
accessible in large quantities and at moderate cost.
In spite of the widespread importance of organic polymers, attention is
being focused increasingly towards polymers that contain inorganic elements as well
as organic units in their backbone. Such polymers, referred to as inorganic or
organometallic polymers, overcome many of the disadvantages of organic
polymers. Most of the organic polymers react with oxygen or ozone over a long
period of time and lose their advantageous properties. Organic polymers, in
general, have poor thermooxidative stability and undergo degradation when
exposed to ultraviolet or gamma radiation. On the other hand, inorganic and
organometallic polymers possess excellent high temperature properties, thermo-
oxidative stability and radiation resistance. Other advantages include fire resistance,
chemical inertness and resistance to free radical cleavage.1
The polymers which contain only inorganic elements in the polymer
backbone are called “ inorganic polymers”, whereas the ones which contain
metallic elements along with organic units in their backbone are called
“organometallic polymers”. Truly speaking, there appears to be no clear-cut
demarcation between these two categories and quite often they are used
interchangeably.
Inorganic and organometallic polymers find wide range of applications as
high temperature coatings, thermal protective systems, matrix resins, high
temperature elastomers and precursors for ceramics, due to their excellent high
temperature properties and thermooxidative stability. In the last three decades
considerable attention is paid towards using these polymers as precursors for
T
Introduction 3
ceramics. Inorganic and organometallic polymers capable of giving high ceramic
residue (50% or above) on heat treatment in inert atmosphere are called
”preceramic polymers”.
Following the first report by Yajima et al.2 that disclosed the use of polycarbosilane derived from polydimethylsilane as a precursor for SiC fiber, wide variety of inorganic and organometallic polymers have been reported as precursors for ceramics. Polysilanes, polycarbosilanes, polysilahydrocarbons, polycarbosilazanes, polysiloxanes, polyborazines, polycarborane-siloxanes, borosiloxanes etc. have all been explored as precursors for ceramics. The developments in the area of preceramic polymers have been discussed in various reviews.3-9 In view of the voluminous literature available on this topic, it is rather difficult to discuss all the developments in detail. Hence, the introductory chapter of the present thesis provides only an overview of the important developments in the area of preceramic polymers. While presenting the literature, emphasis is laid on silicon, and silicon and boron containing polymers as the thesis deals with polysilahydrocarbons and borosiloxanes.
1.1 Silicon containing polymers 1.1.1 Polysilanes
Polydiorganosilylenes commonly referred to as polysilanes is relatively a
new class of inorganic polymers having an all silicon backbone. The first
polysilane was probably prepared by Kipping10,11 in the early 1920s by the
dechlorination of diphenyldichlorosilane (DPDCS) using sodium. But it was
infusible and insoluble. Bukhard12 synthesized polydimethylsilane (PDMS) from
dimethyldichlorosilane (DMDCS), using a similar procedure, which also suffered
from the same problem as in the case of polydiphenylsilane. This led to the
neglect of this field until Yajima and coworkers2 in 1975 reported the conversion of
intractable PDMS by heat treating it in an autoclave to a processable
polycarbosilane.
West and coworkers3,13-17 prepared soluble copolymers from
methylphenyldichlorosilane (MPDCS) and DMDCS by Wurtz reaction with sodium.
A copolymer obtained with an equimolar ratio of MPDCS and DMDCS is called
4 Chapter 1
“polysilastyrene” (Structure 1) due to its structural resemblance with polystyrene
and it exhibits a bimodal molecular weight distribution with a molecular weight
( WM ) of ~ 50,000.
where x/y =1
Structure 1 Alkali metals such as sodium and potassium are generally employed for
the dechlorination5 whereas lithium favors the formation of cyclic products. Use of
catalytic amount of crown ethers (18-crown-6 or 15-crown-5) along with sodium is
found to enhance the polymer yield and change the molecular weight distribution
from bimodal to monomodal.18-22
Polysilastyrene after crosslinking by UV irradiation gave ~30% ceramic
residue on pyrolysis and the poor ceramic residue was attributed to incomplete
crosslinking.15 Schilling et al.23-25 described the synthesis of vinyl-functionalized
polysilane by reacting methylvinyldichlorosilane (MVDCS), DMDCS and
trimethylchlorosilane (TMCS) with sodium in an inert solvent and also the
formation of polysilanes containing Si-H and Si-vinyl functional groups by
dechlorinating methyldichlorosilane (MDCS) with MVDCS and TMCS in presence
of sodium in toluene/THF mixture.
Functionalization of polysilanes drastically influences their solubility, Tg,
cure kinetics and ceramic residue.26-28 In recent years, polymethylsilane,
synthesized by dechlorination of MDCS has been studied extensively as a
promising precursor to SiC8,29-52 because of its ideal stoichiometry. However,
polymethylsilane gives lower ceramic yield on pyrolysis. Several authors focused
on the role of Si-H bonds in polymethylsilane for improving the final ceramic yield
by promoting crosslinking.8,38-46 In order to improve the ceramic yield,
polymethylsilane-based precursors were used in presence of crosslinking agents
nyx
CH3CH3
C6H5CH3
SiSi
Introduction 5
such as boron containing catalysts31,35,36,41,48,49 and metallocene26 which enhanced
the precursor processability and ceramic yield. When metallocene was used as a
catalyst, long reaction time and high pressure were required.
Iseki et al.52 reported a reflux heat treatment as a simple and effective
procedure to enhance the ceramic yield without using catalysts or high pressure.
The reflux process is an effective way to convert linear Si-Si units into CSiSi3 units
prior to carrying out Kumada’s rearrangement.53 Gon et al.54 reported a method for
enhancing the molecular weight of polymethylsilane at 70°C by carrying out the
synthesis in the presence of polyborazine.
Polymethylsilane is, however, not sufficiently oxygen stable and is often
flammable in air at room temperature.47 In addition, polymethylsilane-derived
ceramics contain considerable amount of silicon. Iseki et al.55,56 had overcome
these problems by using poly(methylsilane-dimethylsilane) copolymers in place of
polymethylsilane. These copolymers serve as matrix resins for ceramic matrix
composites (CMCs).
Radiation curing of polymethylsilane is now considered as an industrially
viable method to produce high heat resistant SiC fibers with low oxygen content.
γ-Ray radiation curing studies of polymethylsilane and poly(methylsilane-
dimethylsilane) precursor with improved ceramic yield were reported by
Iseki et al.56
Apart from the classical Wurtz reaction, several novel electrochemical57-62
and ultrasonic methods21,63-65 have also been reported for the synthesis of
polysilanes. Of late, synthesis of newer polysilanes and their characterization have
been reported.66-76 Recently, a room temperature synthesis of polysilanes with
high yield (50 to 83%) was reported by Holder et al.77
1.1.2 Polycarbosilanes
Polymers with backbone comprising of silicon and carbon are termed
“polycarbosilanes”. Yajima et al.2,78-80 in their pioneering work, prepared
polycarbosilane by heat treatment of dodecamethylcyclohexasilane (Me2Si)6 or
6 Chapter 1
polydimethylsilane in an autoclave in argon atmosphere at temperatures above
400°C under pressure (Scheme 1.1)
Scheme 1.1. Yajima's process
Yajima et al.81 studied the use of a catalyst, viz., borodiphenylsiloxane for
the thermal conversion of PDMS to polycarbosilane at atmospheric pressure at
350°C. Habel et al.82 reported the use of poly(borodiphenylsiloxane) for converting
polysilane containing methyl and phenyl groups to boron containing
polycarbosilane. Polycarbosilanes were also prepared by ring opening
polymerization of 1,3-disilacyclobutane83 and substituted 1,3-disilacyclo-
butanes83,84 using chloroplatinic acid as catalyst. Poly(diethylsilmethylene) having
molecular weight ( nM ) of 2,50,000 was prepared by the ring opening
polymerization of 1,1,3,3-tetramethyl-1,3-disilacyclobutane with platinum
catalyst.85 This polymer is thermally stable up to 600°C in inert atmosphere, but
readily decomposes at 200°C in presence of air.
SiClCH3
ClCH3
NaNa
(C6H5)2SiCl2Na
Si
CH3
CH3
n
Si
CH3
CH3
n
Si Si SiSi
CH3 CH3 CH3
CH3CH3CH3 C6H5
C6H5
n
Si C
CH3 H
H H n
Si C Si C n n
autoclaveautoclave (C6H5)2SiOBO2 n
PCS-I PCS-I PCS-I I II
Introduction 7
Seyferth and Lang86 synthesized polycarbosilanes with regularly
substituted pendant Si-vinyl groups by the ring opening polymerization of 1-vinyl-
1-silacyclobutane in tetrahydrofuran (THF) and hexamethyl phosphoramide using
butyllithium as a reducing agent. Polymers were obtained in 60-88% yield and the
molecular weight ranges from 6,000 to 11,000. They are stable up to 400°C and
undergo decomposition above 450°C.
Pillot et al.87 explored the reductive coupling of dichlorodisilylmethane with
alkali metal as a means of preparing polycarbosilane. Polycarbosilanes were also
obtained by the dehalogenation of diorganodichlorosilanes with dihalomethanes
using alkali metals.88
Whitmarsh and Interrante89 studied the Grignard coupling of
chloromethyltrichlorosilane (ClCH2SiCl3) by reacting with magnesium in
diethylether. A branched polycarbosilane was formed when the chloroderivative
was reduced with LiAlH4. Habel et al.90,91 reported the reduction of halogenated
poly(methylphenylsilmethylene) and poly(diphenylsilmethylene) with LiAlH4 to
obtain poly(hydridopolycarbosilane)s.
Schilling and co-workers29,92,93 synthesized branched polycarbosilanes
from a vinyl-containing chlorosilane monomer. When dechlorination of MVDCS
was carried out in presence of TMCS using potassium in THF,
tetra(trimethylsilylated) compound was obtained as the major product.92 This
suggests that vinyl group takes part in the polymerization. Schilling et al.29,93
synthesized branched polycarbosilanes by dechlorinating MVDCS with potassium
in THF in presence of DMDCS or MDCS and TMCS.
Polycarbosilanes with ethylenic and acetylinic functionality were prepared
from poly(dichlorosilmethylene)s.94,95 Poly(silaacetylene)s were prepared by
reacting dilithioacetylene with diorganodichlorosilanes (RR'SiCl2) [where R = Me
or Ph] or with 1,2-dichlorodisilane (ClMe2Si-SiMe2Cl).95 A silylenediacetylene
polymer of high molecular weight was prepared by Maghsoodi and Barton96
starting from hexachlorobutadiene. Habel et al.97 reported the synthesis of
poly(alkynylsilmethylene) containing the repeating unit, -SiR2CH2-, where
8 Chapter 1
R= -C≡CH, -C≡C(CH2)nCH3 or -C≡C-C6H5. Uhlig98 reviewed the synthesis of new
organosilicon polymers using silyl triflate intermediates. Proto-dephenylation of
phenylated polysilanes as well as poly(silylenemethylene)s by triflic acid gave new
functionalized compounds. Thermally stable polysilylenemethylenes with siloxane
crosslinking moieties were successfully synthesized by Ogawa et al.99 with Tg
ranging from 15 to 20°C.
A wide range of new poly(silylenemethylene)s100-102 of the type
[SiRR’CH2]m were prepared and their conversion to ceramic material was studied
in detail. Kwak and Masuda103 found that the polymerization of p-(dimethylsilyl)-
phenylacetylene in toluene at 25°C and 80°C using Rh1(PPh3) catalyst afforded
highly regio- and stereoregular poly(dimethylsilylenephenylene-vinylene)s.
Amoros et al.104 studied the reaction of poly(dimethylsilane) and
poly(dimethylcarbosilane) with bis(cyclopentadienyl)M dichloride Cp2MCl2 (M= Ti,
Zr, Hf) complexes to obtain precursors for SiC ceramics. Tsumura and Iwahara105
developed crosslinked polycarbosilanes with excellent mechanical properties by
hydrosilylation curing of multi-functional vinylsilanes with hydrosilanes. Chemically
modified polycarbosilanes containing organofluoric groups106 and Si-Al-C-O
linkages107 with improved ceramic yield, were synthesized by Okuzaki et al.106,107
by reacting polycarbosilane with fluoric acid and aluminium triisopropoxide
respectively. Cao et al.108 obtained dry-spinnable polycarbosilane by reacting the
polycarbosilane, prepared by Yajima’s process, with 1 wt% of polyborazine at
300°C for 1 h. A hyperbranched polycarbosilane of the type [R3SiCH2–]x-
[–SiR2CH2–]y [–SiR(CH2–)1.5]z [–Si(CH2–)2] was prepared by Interrante et al.109,110
for end-use as an inorganic/organic hybrid material.
1.1.3 Polysilahydrocarbons
Polymers containing two or more carbon atoms between silicon atoms are
referred111 to as “polysilahydrocarbons”. They were synthesized through the
dechlorination of diorganodichlorosilanes in presence of vinyl or diene
monomers111-115 (Scheme 1.2). Schilling et al.111 synthesized polysilahydro-
carbons by dechlorinating diorganodichlorosilane in presence of styrene or
isoprene.
Introduction 9
Scheme 1.2. Synthesis of polysilahydrocarbons
Van Aefferden et al.112 and Packirisamy et al.113-115 synthesized
polysilahydrocarbons by reacting DMDCS and styrene under dechlorination
conditions. They observed that the copolymers contain alternating units of disilyl
and styryl in the polymer backbone. Packirisamy et al.113-121 studied in detail the
effect of monomer feed ratio and reactivity of diorganodichlorosilanes on the
microstructure and mechanism of formation of polysilahydrocarbons through the
dechlorination of diorganodichlorosilanes in presence of styrene. In the
copolymers synthesized from styrene and DMDCS, disilyl and styryl/oligostyryl
units alternate with each other for the monomer feed ratios (styrene:DMDCS)
0.5:1, 1:1 and 1.5:1. With further increase in concentration of styrene, i.e., for the
monomer feed ratio of 3:1, no disilyl linkage was observed and the repeating units
mainly consisted of monsilyl linkages separated by oligostyryl units. However, for
styrene-MPDCS system, disilyl linkages separated by oligostyryl units were
observed even when the monomer feed ratio (styrene:MPDCS) was 5:1. This is
attributed to the higher reactivity of MPDCS compared to that of DMDCS.
Microstructural analysis of polysilahydrocarbons supported the conclusion that
disilyl reaction intermediates mediate the polymerization. When styrene is present
in large excess, styryl reaction intermediates mediate the polymerization resulting
in the formation of monosilyl linkages separated by oligostyryl units. A generalized
structure of the polysilahydrocarbon synthesized from MPDCS and styrene is
given in structure 2.
MeSiHCl2
K/THF
Isoprene
K/THF
Si CH2 CH
PhMe
H n
Si CHCCH2 CH2
MeH
Me
n
PhCH2 CH
10 Chapter 1
Structure 2
Ambadas et al.122,123 synthesized a novel polysilahydrocarbon from a
single source precursor. Dechlorination of 1,2-bis(chlorodimethylsilyl)ethane
[BCDMSE] in presence of potassium resulted in the formation of a polymer,
poly(tetramethyldisilyleneethylene) (Structure 3) whereas no polymer was
obtained when sodium was used for the dechlorination. This was attributed to the
low reactivity of this monomer under dechlorination conditions. Interestingly, this
monomer underwent dechlorination in presence of styrene.
Structure 3
Thus, it is obvious that depending on the reactivity of
diorganodichlorosilane, either disilyl or styryl reaction intermediates mediate the
polymerization. A typical structure of the polysilahydrocarbon synthesized from
BCDMSE and styrene in presence of Na/18-crown-6 is given below (Strucure 4).
Structure 4
Packirisamy et al.117 synthesized crosslinked polysilahydrocarbons by
reacting MVDCS with styrene under dechlorination conditions. During
dechlorination, vinyl group of MVDCS participates in the polymerization resulting
a
CH CH2
CH CH
b n
2
Ph
Si Si
Me
Me
Ph
Si Si
Me
MePh
Ph Ph
Ph
Ph
C H2C
H
Me
MeMe
SiCH2CH2Si
Men
Me
Me Me
Si CH 2 CH 2 Si
Me n
Introduction 11
in the formation of crosslinks. A typical structure of poly(methylvinylsilylene-co-
styrene) is given below (Structure 5).
Structure 5
Polysilahydrocarbons have poor thermal stability and give very poor
ceramic residue (<20%). Sartori et al.124,125 explored the possibility of converting
polysilahydrocarbon synthesized from DMDCS and styrene to thermally stable
polycarbosilane by heat treating the polysilahydrocarbons at 385°C in an
unconfined atmosphere under the flow of argon.
Packirisamy et al.115,126 studied the microstructure, thermal stability and
ceramic conversion of polycarbosilanes obtained by heat treatment of
polysilahydrocarbons in a partially confined environment. It was observed that the
incorporation of small amount (<10 mole%) of methylphenylsilyl units in the
copolymer containing dimethylsilyl and styryl units drastically improved the thermal
stability and ceramic residue of polycarbosilane. This has been attributed to the
phenylene insertion reaction taking place during heat treatment. A typical structure
of the heat-treated polysilahydrocarbon is given below (Structure 6).
Structure 6
nx
Si Si
Me
Me
Me
Me
Si Si
Me
Si
Me Me
H H
CH 2
CH 2
Ph
CHSi
nMeCH
CH2
Si
Me
a
CH
Si
Me
CH
CH2
CH2
xSi
Meb
CH
CH
Ph
CH2
CH2
12 Chapter 1
In the present investigation, thermal degradation kinetics of
polysilahydrocarbons, synthesized by Ambadas122 and the polycarbosilanes
derived from them,126 and the end-use of polysilahydrocarbons as atomic oxygen
resistant coatings for low earth orbit space structures have been studied and they
form the basis for the chapters 3.6 and 3.7.
Polysilahydrocarbons serve as potential candidates for liquid
lubricants.127 This advanced space lubrication systems replace the currently
employed perfluoropolyalkylethers, which promote metal corrosion and undergo
degradation under boundary lubrication conditions. The properties of these
polysilahydrocarbons can be tailored by varying the alkyl substituent.
Corriu et al.128-130 described the preparation of linear poly(silylethylene)
from vinyl hydrogenosilane using platinum in charcoal as a catalyst. Catalytic
hydrosilylation of ethynylsilanes [(R2HSiC≡CH), where R=Me, Et or Ph] with
chloroplatinic acid was studied as a means of preparing polymers with ethylenic
functional moieties in the backbone.
Wu and Interrante131 prepared poly(silethylene)s by the ring opening
polymerization of 1,1,3,3-tetrachloro-1,3-dimethyl-1,3-disilacyclobutane using
chloroplatinic acid or platinumdivinyltetramethyldisiloxane complex as a catalyst.
A new polysilahydrocarbon, viz., chemically crosslinked polysilastyrene
was prepared by Krishnan et al.132 through the dechlorination of a monomer
mixture containing DMDCS and MPDCS in presence of divinylbenzene. The
crosslinked polymer on pyrolysis at 1500°C gave β-SiC.
In recent years, polysilarylenes and polycarbosilarylenes with excellent
thermal stability have been reported.133-135
1.1.4 Polysilazanes and polycarbosilazanes
Polymers with silicon-nitrogen linkages in their backbone are termed
”polysilazanes”.8,136,137 They are generally prepared either by ammonolysis or
aminolysis of organochlorosilanes.
Introduction 13
Verbeek,138,139 in his pioneering work, synthesized polysilazanes by
aminolysis of chlorosilane monomers. Polysilazanes were prepared in a two step
process involving amination of trichloromethylsilane followed by the thermal
condensation of the product tris(N-methylamino)methylsilane.138-140 Aminolysis of
DMDCS and methylamine following a similar procedure as above was carried out
by Winter et al.141 Seyferth and Wisemann142 reported the synthesis of
N-methylpolysilazane from methylamine and dichlorosilanes.
A perhydropolysilazane was synthesized by passing ammonia gas
through a pyridine-dichlorosilane adduct.143 Polysilazanes were also prepared by
reacting trichlorosilane with hexamethyldisilazane [(Me3Si)2NH].4
Seyferth et al.144,145 described in detail the ammonolysis of organodichlorosilanes.
Ammonolysis of MeHSiCl2 resulted in a cyclic silazane (MeSiHNH)3-4 and a linear
polymer (Scheme 1.3).
Scheme 1.3. Synthesis of polysilazane Seyferth et al.144 also reported the preparation of polysilazane free from
carbon by reacting dichlorosilane (SiH2Cl2) with gaseous ammonia in polar
solvents like dichloromethane or diethyl ether.
Burns et al.146 prepared polysilazanes by the ring opening polymerization
of cyclic silazanes prepared by ammonolysis and co-ammonolysis method. In the
first route silacyclobutasilazane was equilibrated with chlorosilane to give a
mixture of chloro end-blocked silazanes, which was ammonolyzed to obtain the
desired substituted silacyclobutasilazane. In the second route, a mixture of
1,1-dichloro-1-silacyclobutane and a chlorosilane was co-ammonolyzed directly.
Ammonolysis of a mixture of MeHSiCl2 and MVDCS or allyldichlorosilane afforded
vinyl- or allyl-functionalized polysilazane.145 The advantage of having a vinyl or
allyl group is that it provides -C=C- functionality which can be crosslinked via.
hydrosilylation reaction.
(MeSiHNH)x MeSiHCl 2 NH 3 Et 2 O/0°C
(ii)CH I 3 (i) KH THF/RT
Polymer
14 Chapter 1
Xu et al.147 synthesized a new kind of polysilazane precursor containing
linear-cyclic structure from the dilithium chloride condensation reaction of
hexamethylcyclotrisilazane lithium salts and 1,3-dichlorotetraorganodisilazanes
[(ClR1R2Si)2NH]. Bao et al.148 prepared a copolysilane-cyclodisilazane precursor
which was then heated to 1650°C to obtain SiC and Si3N4. Kleebe et al.149
prepared different liquid polysilazanes with tailored molecular structures by
ammonolysis of functionalized chlorosilanes. Pyrolysis of the polysilazanes
resulted in the formation of amorphous SiCN ceramics.
Hydrosilylation between 1,1,3,3-tetramethyldisilazane and 1,3-divinyl-
1,1,3,3-tetramethyldisilazane occurs rapidly at 90°C in the presence of Karstedt's
catalyst (Pt-divinyltetramethyldisiloxane) to give large macrocyclic products.
Similar observation was made by Son and Rasul150 when trimethylsilylated or
methylated derivative of 1,3-divinyl-1,1,3,3-tetramethyldisilazane was reacted with
1,1,3,3-tetramethyldisilazane.
Trassl et al.151 prepared polyvinylsilazane, a precursor for SiCN ceramics,
by ammonolysis of functionalized chlorosilanes. Pyrolysis under inert atmosphere
at 1000°C led to the formation of an amorphous SiCN ceramic. A polymeric
precursor for the Si3N4-SiC-Y2O3 ceramic system was synthesized by
Sawai et al.152 by block copolymerization of perhydropolysilazane with hydroxy-
polycarbosilane, followed by chemical modification with yttrium methoxide.
Polycarbosilazanes have alternating silicon, carbon and nitrogen units in
their backbone. A polycarbosilazane was synthesized by heating a polysilazane
obtained from the sodium reduction of a mixture of DMDCS and l,3-dichloro-l,3-
dimethyldisilazane.153 This polymer on pyrolysis gave SiC-Si3N4 mixed non-oxide
ceramics.
1.1.5 Polysilylcarbodiimides
Kienzle et al.154 developed a new class of silicon and nitrogen containing
polymers called “polysilylcarbodiimides”. These polymers were prepared by
reacting organochlorosilane with cyanamide. Polysilylcarbodiimides were also
Introduction 15
prepared by reacting chlorosilanes with bis(trimethylsilyl)carbodiimide.155 These
polysilylcarbodiimides can be transformed into amorphous silicon carbonitride
ceramics with about 65% ceramic yield, by pyrolyzing them in an inert atmosphere
at 1100°C. The advantage with these polymers is that no prior crosslinking is
required before pyrolysis.
1.1.6 Polysiloxanes and polysilsesquioxanes
Polysiloxanes are unique among inorganic polymers.1 They are the most
studied and the most important ones due to their commercial significance. The
Si-O backbone of this class of polymers endows it with a variety of fascinating
properties. The high strength of Si-O bonds is responsible for high thermal stability
of polysiloxanes.
Diorganodichlorosilanes obtained by Rochow’s process, that starts from
elemental silicon, are used for the synthesis of polysiloxanes. This compound with
the general formula R2SiCl2, can be used for the preparation of a wide variety of
polymers having both organic and inorganic character. On hydrolysis, R2SiCl2
gives dihydroxy structures, which readily condense to give basic repeating units
(Scheme 1.4). The nature of the product mostly depends on reaction conditions.
where R = alkyl or aryl Scheme 1.4. Synthesis of polysiloxane
Polysilsesquioxane is an example for highly crosslinked organic-inorganic
hybrid material. Its inorganic cage frame work is made up of silicon and has the
formula of (SiO1.5)n where n = 8, 10 or 12.156,157 This inorganic cage is externally
covered by organic substituents, which enhance their solubility in organic solvents.
They are prepared by hydrolytic condensation (sol-gel process) of trifunctional
organosilicon monomers.158
Si + R2SiCl22RClH2O
Si
R
nO
R
16 Chapter 1
Among various silsesquioxanes, the octaphenylsilsesquioxane (PhSiO1.5)8
(Structure 7), decaphenylsilsesquioxane (PhSiO1.5)10 and dodecaphenyl-
silsesquioxane (PhSiO1.5)12 systems are the most interesting ones because of
their high temperature stability.
Structure 7
Polysiloxanes and silsesquioxane sol-gels were reported as precursors to
ceramics such as siliconoxynitride,159 siliconoxycarbide160-162 and SiC163,164 and
black glass matrices for composite materials.164-166 Since 1987 attempts have
been made to use siloxane polymers as precursors for ceramics and as binders
for sintering of SiC monoliths.167 The properties of these resins which make them
attractive as binders are the following: i) facile synthesis, ii) thermal and aerobic
stability for subsequent processing, iii) compatibility with the ceramic powder, iv)
decomposition into stable ceramics comprising of SiC and C at temperatures
below those required for sintering of SiC (~ 2000°C) and v) sufficient char yield
thereby minimizing the amount of volatiles evolved in the pyrolysis.163,168
White et al.160 reported that heating (PhSiO1.5)x gels to 1500°C produced a
partially crystalline mixture of β-SiC, SiO2 and C as detected by XRD and IR.
Babonneau et al.169 reported NMR evidence for the formation of β-SiC from
(Me2SiO)x (SiO2)1-x.
Porous ceramics with open or closed porosity combine various interesting
properties, like low density, low thermal conductivity and high specific strength.170
Therefore, they are of interest for an increasing number of applications
Si
O
SiOSi
O
OSi
Si O
O
SiO
Si
O
SiO
O
O
O
R
R
R
R
R
R
R
R
where R=Ph
Introduction 17
(e.g., thermal insulation, catalyst support, lightweight structures). Siliconoxy-
carbide ceramics are favorable candidates to meet these requirements. They are
commonly prepared by two techniques. One technique is based on the sol-gel
process171 and the other method uses preceramic polymers such as polysiloxanes
as starting materials. When polysiloxane is used the shrinkage in the 'green body',
the undesired feature of the polymers from the sol-gel process, is avoided. Yet
another advantage is that polysiloxanes are easily mouldable.172
A series of trimethylsilyl end-functionalized aliphatic hyperbranched
polymers were studied to template porosity in polymethylsilsesquioxane fi lms
prepared by heat treatment of a spin cast methylsilsesquioxane precursor.173 By
varying the extent of the functionalization, ceramic foams with controlled pore
sizes were obtained.
A number of polysiloxane resins filled with different oxide, carbide or
metallic powders were investigated as precursors for ceramics.174-180 Thin-walled
(wall thickness: 100–2000 µm) Si-O-C-N mono- and bilayered ceramic tubes were
obtained by centrifugal casting of a suspension of Si and SiC powders in
polyorganosiloxane/triethoxysilane solution.181 After centrifugal casting in a Teflon
tube with a rotational speed of 2000 rpm and subsequent crosslinking at 130°C
and 60 rpm, the tubes were pyrolyzed in argon or in nitrogen at 1400–1600°C.
Bilayered tubes with controlled variation of porosity were obtained by overcasting
the monolayer green tubes with a modified slurry composition.
Philipp182 evaluated polysiloxane formed by the controlled hydrolysis of
tetraethoxysilane (TEOS) as a binder for the formation of an oxide ceramic
compact. At high temperature, the oxide particles are held together via reaction
bonded silicates formed from SiO2 obtained from the pyrolysis of the precursor.
1.2 Boron containing polymers
The key developments in organosilicon chemistry led researchers to work
on novel synthetic approaches leading to the development of innumerable boron
containing polymers and to explore their end-uses in terms of technological
advantages offered by them. Boron forms a number of polymeric materials with
18 Chapter 1
many other elements involving particularly bonds of B with H, C, O, N and P. The
high bond energy of some of these bonds is responsible for their high temperature
stability. Majority of boron containing polymers are derived from borazine,
carborane and decaborane.9,183,184 A few polymers have also been synthesized
from vinylpentaborane and boric acid.
1.2.1 Polymers derived from borazines
A tractable polyborazine which can be used as a precursor for boron
nitride ceramic was first reported by Taniguchi et al.185 Later, Sneddon et al.186,187
synthesized a processable polyborazine (also known as polyborazylene) by the
thermolysis of liquid borazine in vacuum at 70°C (Scheme 1.5).
Scheme 1. 5. Synthesis of Polyborazylene
This polymer was obtained as a white powder in high yield (~90%). It has
molecular weight ( wM ) of 7,600 and has a branched structure. Polyborazylenes
serve as excellent precursors for boron nitride ceramics.188 Boron nitride was
obtained by heating this polymer in either argon or ammonia. Narula et al.189
synthesized poly(borazinylamine), by adding excess ammonia to cold toluene
solution of B-tris(dimethylamino)borazine [(Me2N)BNH]3. Kimura et al.190 studied the ammonolysis of B-tris(diethylamino)borazine in
liquid ammonia at room temperature and obtained a crystalline solid
B,B’,B”-triaminoborazine [(H2N.BNH)3]. This solid undergoes rapid deamination at
120°C forming a slightly soluble polymer. Synthesis of substituted borazines such as B-triamino-N-triphenyl-
borazine, B-triamino-N-trimethylborazine and B-trianiloborazine has also been
reported.191,192
B
N
B N
B
NH
H
H
H
H
H
B
N
B N
B
NH
H
H
H
70°C
n
Introduction 19
Paine et al.193,194 and Paciorek et al.195,196 prepared poly(borazinylamine)
by reacting B,B’,B’’-trichloroborazine with hexamethyldisilazane in ether medium
or in a hydrocarbon solvent. Pyrolysis of this polymer in vacuum at 1200°C
resulted in the formation of turbostatic hexagonal boron nitride [(h)-BN].
Maya197,198 synthesized polyaminoborazines through the condensation of
triethylborane or tris(dimethylamino)borane with polyfunctional amines such as
4,4'-methylenedipyrazole, 1,2-phenylenediamine, 3,3-diaminobenzidine and
diethylenetriamine. These polymers were used as precursor for boroncarbonitride
(BCN) ceramics.
Sneddon et al.199-201 prepared poly(β-vinylborazine) by solution
polymerization of β-vinylborazine with a free radical initiator azobisisobutyronitrile
(AIBN). β-Vinylborazine was obtained through the reaction of borazine with
acetylene in presence of Wilkinson’s catalyst [RhH(CO)PPh3]. Pyrolytic
decomposition of poly(vinylborazine) in ammonia at 1200-1400°C produced white
crystalline h-BN. Preparation of poly(styrene-co-β-vinylborazine) by reacting
β-vinylborazine with styrene in presence of AIBN has also been reported.201
Perdigonmelon et al.202 prepared BN catalyst supports from molecular
precursors and demonstrated the influence of the precursor on the properties of
BN ceramic. Fine powders and foams of boron nitride have been prepared from
various molecular precursors for use as noble metal supports in the catalytic
conversion of methane. A molecular precursor derived from trichloroborazine
(TCB), after reaction with ammonia at room temperature and then thermolysis
up to 1800°C, gave BN. The aminoborazine-derived precursors when thermolyzed
gave BN, expanded like foam.
Toury et al.203 prepared boron nitride fibers from symmetric and
asymmetric alkylaminoborazines. The thermolysis of 2,4,6-[(CH3)2N]3B3N3H3,
2,4-[(CH3)2N]2-6-(CH3HN)B3N3H3 and 2-[(CH3)2N]-4,6-(CH3HN)2B3N3H3 led to
different types of polyborazines. Polyborazine filaments were prepared and
subjected to thermal treatment up to 1800°C to obtain h-BN fibers.
20 Chapter 1
1.2.2 Decaborane(14)-based polymers
The reaction of decaborane(14), B10H14, with Lewis bases (ligands, L) was
reported for the first time by Schaeffer in 1957.204 Following this discovery, a
number of such derivatives were prepared using various nitriles, amines,
phosphines, sulfides and other ligands. In the formation of B10H12L2 (Scheme 1. 6),
hydrogen atoms at 6 and 9 position of the B10H14 framework are displaced by
electron pair donor atoms.205
Scheme 1. 6. Formation of B10H12.L2
Incidentally, the ligand, L, in B10H12.L2 can be displaced by another ligand
having a higher bonding strength, thus offering an alternative synthetic route for
the synthesis of bis(ligand adduct)s of decaborane. It is obvious that if a bidendate
ligand is used in place of L for the synthesis of B10H12.L2, a linear polymer can be
obtained. (Scheme 1.7)
B10H14 + L~~~~~~~L [B10H12.L~~~~~~~L]n
Scheme 1.7. Synthesis of decaborane-based polymers
Following the report by Parshall206 on the synthesis of a decaborane-
based linear polymer from P,P,P',P'-tetraethylenediphosphine, various polymers
were synthesized from Lewis bases such as phosphines, nitriles and amines.
Polymeric Lewis base adducts of decaborane can be prepared in organic
solvents such as benzene, hexane, toluene, xylene, diethylether and THF.
Introduction 21
Most of these polymers are soluble in polar solvents such as dimethylacetamide
(DMAc), dimethylformamide (DMF), N-methylpyrolidone (NMP), hexamethyl-
phosphoramide (HMPA) and dimethylsulfoxide (DMSO). The solubility of these
polymers appears to depend on the nature of the bridging ligand.
In general, [B10H12.L~~~~~~~L]n polymers give high char residue
(70-90%) in inert atmosphere. The fact that these polymers lose very little weight
on pyrolysis suggests the presence of "thermolabile functionality".207 In the case of
[B10H12.L~~~~~~~L]n polymers, the acidic bridging B-H-B bonds of the B10H12.L2
cage unit may serve as "thermolabile functionality”. On thermal treatment,
B10H12.L2 cage may be converted into (LH+)2 B10H102-.
Seyferth and Rees207 reported that decaborane-based polymers undergo
degradation in DMAc. Packirisamy et al.208, 209 studied the effect of various polar
solvents on the stability of these polymers using 11B-NMR spectroscopy. 11B-NMR
spectra of the decaborane-based polymers were recorded in DMF and DMSO.
It was found that the polymer has undergone degradation in these solvents
resulting in the formation of more than one degradation products. One of the main
degradation products is B10H102-. Studies revealed that there is no salt formation
during the synthesis, but on standing the complex incorporates oxygen resulting in
the formation of small amount of the salt. Polar solvents favor proton transfer from
B10H12 cage of a B10H12L2 complex to the ligand and in a less polar solvent such a
proton transfer is less favored. The proton transfer mechanism which is operative
in polar solvents is shown below:
B10H12.L2 ↔ B10H12.L + L
B10H12.L + Base → B10H11- + [Base.H]+ + L
B10H11- + Base → B10H102- + [Base.H]+
Packirisamy et al.9,208,209 synthesized B and Si containing polymer which is
the first decaborane polymer reported soluble in THF (less polar solvent compared
to DMAc, DMF and DMSO) through the reaction of decaborane with
1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane and evaluated it as an atomic
oxygen (AO) resistant coating for low earth orbit space structures.
22 Chapter 1
Decaborane-based polymers serve as precursors for boron nitride and
boron carbide ceramic materials9,210-213 and as atomic oxygen resistant
coatings.9,208,209 Ambadas122 synthesized B and Si containing polymers through
the reaction of decaborane with different types of aminosiloxanes, namely,
3-aminopropyl- triethoxysilane (APTEOS), 3-aminopropylmethyldiethoxysilane
(APMDEOS) and aminopropyl terminated polydimethylsiloxane (APPDMSiO).
New decaborane-based, single-source molecular and polymeric
precursors to B4C were developed by Sneddon et al.214 that enable the formation
of boron carbide in processed forms, including nanofibers, nanocylinders and
nanoporous materials. Sneddon et al.215,216 reported the synthesis,
characterization and ceramic conversion of a polyborosilazane that can be
prepared via the reaction of commercially available polysilazane, namely
CERASET®, with decaborane. Depending on the pyrolysis conditions, these new
polymers can serve as excellent precursors to SiC/BN ceramic compositions.
Forsthoefel and Sneddon217 obtained metal boride precursors by
dispersing metal oxides in decaborane-dicyanopentane polymer. Pyrolysis of the
metaloxide/polymer dispersions gave metal borides, including TiB2, ZrB2 and HfB2
in high yield. Metal-boride/nitride composites including TiB2/TiN and HfB2/HfN
materials were also readily obtained by pyrolysis of metal dispersions in
polyborazylene, (B3N3H4)x. The high ceramic yield of metal boride/nitride systems
(~ 98%) enables their use for the controlled formation of shaped monolithic
TiB2/TiN or HfB2/HfN materials on pyrolysis. Better results were obtained when
polyhexenyldecaborane polymer was used. Boron-containing ceramics, in
particular, ceramic fibers were obtained by heat treatment of precursor fibers
drawn from decaborane-amine and decaborane-phosphine polymers in inert
atmosphere.218-221
1.2.3 Polymers derived from vinylpentaborane
2-(Vinylpentaborane) when heated at 125°C gets converted to
poly(vinylpentaborane).222,223 A crosslinked polymer was obtained when
poly(vinylpentaborane) was heated to 140°C, where B-B linkages were formed by
Introduction 23
the loss of hydrogen and the crosslinked polymer on pyrolysis in argon at 1200°C
gave B4C in 77% yield.
1.2.4 Polymers derived from boric acid
Wada et al.224 reported the conversion of borate esters synthesized from
boric acid and 1,2,3-propanetriol to ceramics in 1985, though the synthesis and
properties of this polymer was reported as early as 1964.225 A glassy, colorless
transparent product was formed which showed spinnability on heating. Pyrolysis of
this polymer at 1200°C yielded B2O3 regardless of the gas used and duration of
heating. When the polymer was heated above 1300 °C in nitrogen, BN and B4C
were formed, whereas B4C was formed when the polymer was heated in argon at
1400°C. The crystallinity of the ceramics increased with increase in heat treatment
temperature and heating time. Wada et al.226 studied the reaction between boric
acid and diethanolamine as well as triethanolamine as a means of preparing
precursors for BN ceramics. A glassy polymer obtained on pyrolysis under
nitrogen at 1400°C gave BN and small amount of B4C.
Mondal et al.227 used the borate ester synthesized by condensation of
boric acid and triethanolamine to obtain a hard, carbon coating with boron nitride
and carbonitride on glass fiber which was used for filtration of molten aluminum.
Tian et al.228 synthesized crystalline hexagonal B(N1-xCx) and cubic B-C-N
ceramics from a precursor produced from melamine and boric acid by application
of high temperature and high pressure. A low temperature synthetic route for
boron carbide was developed by Mondal and Banthia.229 A polymeric precursor
was synthesized by the reaction of boric acid and polyvinylalcohol which on
pyrolysis at 400°C and 800°C gave B4C. X-ray diffraction shows that B4C obtained
by this method has orthorhombic crystal structure.
1.3 Boron and silicon containing polymers
Synthesis of boron and silicon containing polymers has gained importance
due to their superior thermo-chemical properties.230,231 These polymers on
pyrolysis at higher temperatures give non-oxide ceramics such as silicon boride,
24 Chapter 1
SiC-B4C and borosilicon carbonitride. Ceramics derived from boron and silicon
containing precursors are found to remain amorphous even up to 2000°C.232
Boron and silicon containing ceramic, when exposed to oxidizing
environment at high temperature, forms a protective borosilicate glassy layer on
the surface, which prevents further oxidation of the ceramic. Boron and silicon
containing polymers are mainly prepared by the following four synthetic methods:
(i) by reacting functional monomers of boron with functional monomers of silicon,
(ii) by reacting functional monomers containing both boron and silicon with
functional monomers of silicon, (iii) by chemical modification of organosilicon
polymers with boron compounds and (iv) through the reaction of single source
precursor containing both boron and silicon with suitable comonomers.
1.3.1 From functional monomers of boron with functional monomers of
silicon
Borosiloxane oligomers/polymers synthesized from boric acid/borate
esters with functional monomers of silicon fall under this category. As the present
thesis deals with borosiloxane oligomers, the literature on this topic is dealt with in
a separate section.
Synthesis of boron and silicon containing polymers reported by Ricitiello
et al.233-235 involves the dehalogenation of borontrichloride and
dialkyldichlorosilane such as DMDCS or MVDCS in presence or absence of
methyliodide in hydrocarbon solvents. Gutenberger et al.236 synthesized
poly(borodiphenylcarbosilane) free of oxygen by reacting DPDCS,
dibromomethane and phenyldichloroborane or borontribromide with sodium.
Sundar and Keller237 synthesized acetylinic polymers by reacting
1,4-dilithiobutadiyne with phenylboron dichloride and DMDCS. The incorporation
of silicon and boron into the diacetylinic polymer was found to enhance the
oxidation stability of the polymer. They also reported238 the synthesis of
borocarbosilicate polymer containing diacetylinic functionalities with improved
thermal properties, by reacting DPDCS and 1,4-dilithiobutadiyne with
Introduction 25
phenylboronic acid. Boron and silicon containing polymers having acetylinic units
in the backbone were also synthesized by reacting bis(dimethylchlorosilyl) -
benzene with phenylboronic acid.239
1.3.2 From functional monomers of boron and silicon with functional monomers of silicon
Poly(carborane-siloxanes)s synthesized from functional monomers of
boron and silicon with functional monomers of silicon represent another class of
boron and silicon containing polymers. Poly(carborane-siloxane)s marketed under
the trade name "Dexsil" contain carborane units along with siloxane units in the
backbone (Structure 8). Dexsil polymers are prepared by the condensation
polymerization of bis(methoxydimethylsilyl)-m-carborane with alkylchlorosilane or
alkylchlorosiloxane or bis(chlorosilyl)-m-carborane derivatives in presence of a
catalyst240-245
Structure 8. Structures of different poly(carborane-siloxane)s
Dexsil 202 is a viscous liquid and consists of m-carborane cage attached
to a dialkyl or diarylsiloxane unit with a small amount of 1-vinyl-o-carbonyl side
group to enhance the crosslinking. Dexsil polymer serves as a precursor for boron
and silicon containing ceramics.246,247 The incorporation of carborane units
prevents the degradation of polysiloxanes by ring forming depolymerization and
n Dexsil 100 crystalline polymer
CH 3
Si B 10 H 10 C C Si O
CH 3
CH 3
CH 3
n Dexsil 200 elastomer
Si B 10 H 10 C C Si O Si O
CH 3
CH 3
CH 3
CH 3
CH 3
CH 3
n
Dexsil 300 elastomer
Si B 10 H 10 C C Si O Si O
CH 3
CH 3
CH 3
CH 3
CH 3
CH 3
Si
CH 3
CH 3
26 Chapter 1
enhances the resistivity towards high temperature degradation particularly in inert
atmosphere. Pyrolysis of these polymers in nitrogen atmosphere led to the
degradation of carborane cage at around 700°C leaving an amorphous
borosiliconoxycarbide (BSiOC), which got converted to β-SiC and B4C on further
heating to 1300°C by the carbothermal reduction of BSiOC phase.
A highly functionalized carborane-siloxane polymer was synthesized by
Kulig et al.248 from l,7-bis(hydroxydimethylsilyl)-m-carborane and methyltriacetoxy-
silane. The polymer formed has one acetoxy crosslinking site per repeat unit. This
polymer has a molecular weight of 6,000 and is soluble in THF and toluene. This
polymer was evaluated as an AO resistant coating for low earth orbit space
structures.248,249
1.3.3 Chemical modification of organosilicon polymers
Niebylski250,251 synthesized organoborosilazane polymers by reacting
trialkoxy, triaryloxy or tri(arylalkoxy)boroxine with polysilazane. Chemical
modification of hydridopolysilazane was performed by reacting it wi th liquid
borazine at low temperatures to obtain boron and silicon containing
polymers.252,253 Chemical modification of hydridopolysilazanes with
monofunctional boranes such as pinacolborane (Me2CO)2BH, l,3-dimethyl-l,3-
diaza-2-borocyclopentane (CH2NMe)2BH and 2,4-diethylborazine (Et2B3N3H4) has
also been reported.254
Chemical modification of silazanes with borane-dimethylsulfide was
studied by Seyferth et al.255 In this method oligomeric silazane (CH3SiHNH)n was
treated with BH3.S(CH3)2 to obtain a crosslinked polyborosilazane. The polymer
obtained as a low melting white solid has molecular weight of 800.
Polyborosilazane was also derived from perhydropolysilazane and
trimethylborate.256 Polyborosilazane with low oxygen content was synthesized by
reacting perhydrosilazane with tris(dimethylamino)borane.257
The synthesis and thermal behavior of boron-modified polysilazanes
{B[C2H4Si(R)NH]3}n and polysilylcarbodiimides {B[C2H4Si(R)NCN]3}n were reported
by Aldinger and Weinmann (Scheme 1. 8).258
Introduction 27
Si
BCHR
R'CH3
ClX
CH2H
NH3
R=H,CH3,ClR'=C2H4Si(R)Cl2
X=NH,N=C=NR=H,CH3,X0.6 R=H,CH3
R'=C2H4Si(R)H2
Me3SiN=C=NSiMe3
X=NH,N=C=NR'=C2H4SI(R)X
Si-B-C-N
Cl
R
Si
Si
Si
C
R
R
R
BR'
R' CHCH3
H H
or BH3-SMe2 NH3,[nBuLi]
CH
CH3BR'
R'
R=H,CH3,X0.6
Ceramization
ceramic
n
12
3
nX
Scheme 1. 8. Synthesis of boron-modified silazanes
These Si-B-C-N ceramic precursors were obtained by treating vinyl
substituted polysilylcarbodiimides [(H2C=CH)(R)-SiNCN]n with borane-dimethyl-
sulfide BH3.S(CH3)2. The polysilylcarbodiimides themselves were synthesized via
treatment of vinyl-substituted chlorosilanes (H2C=CH)(R)SiCl2 with cyanamide,
H2N-C≡N, in the presence of pyridine, or via the reaction of the vinyl-substituted
chlorosilanes with bis(trimethylsilyl)carbodiimide, (H3C)3SiN=C=N-Si(CH3)3.
Alternatively, these compounds were synthesized via hydroboration of vinyl-
substituted chlorosilanes with borane-dimethylsulphide, borane-trimethyl-amide or
borane-triethylanhydride yielding tris[(chlorosilyl)ethyl]boranes initially.
The synthesis by a novel reaction pathway and the polymer-to-ceramic
conversion of boron-containing polysilylcarbodiimides were reported by
Weinmann et al.155,259 Polysilylcarbodiimides were prepared by reaction of
tris(hydridosilylethyl)boranes with different amounts of cyanamide in THF.
28 Chapter 1
Thermolysis of such compounds in an argon atmosphere delivers Si-B-C-N
ceramics in 65-85% yield.
Polysilazanes with polymerizable pendant groups were synthesized by
Weinmann et al.260 These pendant groups enable crosslinking thereby minimizing
depolymerization during ceramization.
A new kind of polyborosilazane was synthesized by Zeng et al.261 from
hexamethylcyclotrisilazane by exposure to borontrichloride. It was observed that
the boron content of the precursor increased as the exposure time was increased
whereas the C/Si ratio remained constant. The pyrolysis of the precursor was
carried out at 1250°C, 1600°C and 1800°C in nitrogen. Heat treatment of the
polymer at 1800°C resulted in BN crystals.
Schiavon et al.262,263 synthesized poly(borosilazanes) with Si/B molar
ratios of 3 and 9, via hydroboration, of vinyl-substituted cyclotrisilazane,
[CH2=CH(CH3)SiNH]3, with borane-trimethylamine adduct, BH3.N(CH3)3.
The polyborosilazanes were pyrolyzed up to 1000°C under argon atmosphere,
which resulted in amorphous silicon boron carbonitride (SiBCN) glasses. The
SiBCN glass with B/Si= 3 was amorphous up to 1700°C and at this temperature,
β-SiC and β-Si3N4 were formed. At 2000°C, β-SiC and BN were formed. These
ceramics have a number of advantages when compared to the corresponding
B-free Si-C-N system, which include higher thermal stability, oxidation resistance,
crystallization temperature and ceramic yield.
1.3.4 From single source precursors
Baldus et al.264,265 synthesized polyborosilazane by reacting a single
source precursor, viz., chlorosilylaminodichloroborane [Cl3Si-NH-BCl2] with
methylamine to obtain a processable polymer in 80% yield. The polyborosilazane
consists of [Si3NCH3]3 six membered ring connected via. HN-B- and -N(CH3)-B-
units respectively. This polymer has an average molecular weight of 2,500 and
melts at 140°C. Prolonged heating at temperatures above 180°C caused further
crosslinking and the polymer became infusible. Pyrolysis of this polymer in argon
or nitrogen at 1650°C led to the formation of amorphous borosiliconcarbonitride
Introduction 29
[Si-B-N-C] ceramic in 70% yield and the ceramic obtained did not crystallize on
heating in nitrogen up to 1900°C. This polymer was also used for coating and for
infiltration of woven carbon fibers, carbon fiber composites and porous ceramic
materials.265
Riedel266 explored the hydroboration of MVDCS with borane-
dimethylsulfide [(CH3)2S.BH3]. Addition of (CH3)2S.BH3 to MVDCS in toluene
resulted in the formation of a precursor tris(dichloromethylsilylethyl)borane. This
organosilylboron was subsequently polycondensed with ammonia at 0°C in
tetrahydrodfuran to obtain a polyorganoborosilazane in 80% yield. Pyrolysis of this
polymer in argon at 1000°C resulted in the formation of SiBCN ceramic in 62%
yield. The crystallization of this ceramic commenced at 1700°C and the
crystallization was completed above 2000°C.
The requirement of high pyrolysis temperature for carbothermal reduction
of silicon oxide components has led to the development of oxygen free carborane-
silicon compounds. Zheng et al.247 synthesized several oxygen free boron and
silicon containing monomers. These molecules were volatile compounds due to
their low molecular weight.
The high temperature properties of poly(carborane-siloxane)s were
improved by incorporating acetylinic units in the polymer backbone. Henderson
and Keller267,268 synthesized poly(carborane-siloxane-acetylene) polymers by
reacting equal amount of 1,4-dilithiobutadiyne and 1,7-bis(chlorotetramethyl-
disiloxane)-m-carborane. A linear polymer soluble in common organic solvents
and having molecular weight wM of 4,900 was obtained in 97% yield.
The incorporation of acetylinic units in the polymer backbone has the
added advantage that they remain dormant during ambient temperatures, but can
be used for crosslinking at higher temperatures. The linear poly(carborane-
siloxane-acetylene) can be readily converted to a thermoset by thermal
crosslinking, where additional polymerization reaction occurs with out the
formation of volatile by-products. Pyrolysis of this polymer yielded a black solid
ceramic in 85% yield.
30 Chapter 1
Bucca and Keller269 investigated the possibility of incorporating boron
and silicon in arylacetylenic compounds. These monomers can be converted
to thermally stable thermosets, which on pyrolysis give ceramic like
materials. When these materials were exposed to oxidizing environment at
elevated temperatures protective inorganic oxides were formed on the surface.
These inorganic oxide layers act as a barrier to protect the interior from
degradation. Of late, they reported270 that boron and silicon containing acetylinic
monomer viz., l,7-bis(4-phenylethylphenyltetramethyl-disiloxy)-m-carborane can
be copolymerized with l,2,4,5-tetrakis(phenylethynyl)-benzene to obtain thermo-
sets and chars having outstanding thermooxidative properties.
Jansen and Kroschel271 synthesized an amorphous ceramic, SiBNC,
obtained from 1,1-dichloro-N-(trichlorosilyl)boraneamine and N-methyl polyboro-
silazane. The pyrolysis proceeds in three clearly divided steps, which can be
assigned to completion of the polycondensation (200-350°C), fragmentation (580-
620°C) and elimination of residual hydrogen (1000-1300°C).
1.4 Polyborosiloxanes
Polysiloxanes undergo degradation by forming cyclics by back-biting
mechanism. This problem has been overcome through the incorporation of
borosiloxane and metallosiloxane linkages. Polyborosiloxanes have been an
exploration target because of their thermodynamically stable backbone linkage,
despite their high susceptibility to hydrolysis. They cannot be used themselves as
high temperature polymers but show interesting properties and can be used as
precursors for ceramic binders, additives and coatings. They are also investigated
in sol-gel chemistry as precursor sols and/or as precursors for borosilicate
glasses. Several methods have been described for the synthesis of polymers
containing -Si-O-B-O-linkages using di- and trifunctional silicon compounds272,273
as the source of silicon, and boric acid, borontrioxide, borontrichloride, boron
tribromide and borate esters as the source of boron. The literature on
borosiloxanes is presented in this thesis under the following four categories: (i)
borosiloxanes from boric acid, (ii) borosiloxanes from borate esters, (iii) sol-gel
process and (iv) organically modified SiO2-B2O3 gels.
Introduction 31
1.4.1 Borosiloxanes from boric acid
Boric acid reacts with chlorosilane, alkoxysilane or silanol as shown below
resulting in the formation of borosiloxane linkage.274, 275
3R3SiCl + B(OH)3 (R3SiO)3B + 3HCl
3R3SiOR’ + B(OH)3 (R3SiO)3B + 3R’OH
3R3SiOH + B(OH)3 (R3SiO)3B + 3H2O
Dimethyldiethoxysilane reacts with boric acid to give viscous liquid soluble
in organic solvents. When methyltriethoxysilane is used, infusible and insoluble
solids are formed. Polyborosiloxanes obtained by the reaction of boric acid with
diethyldiethoxysilane are more stable to hydrolysis. In these cases, the side
reaction to form triethoxyborane is controlled.
Ambadas et al.276 prepared borosiloxane oligomers through the
condensation of boric acid with PTMOS and VTEOS in diglyme at 150-160°C. These oligomers give ceramic residue in the range of 68-85%. Borosiloxane
oligomer containing vinyl group gives higher ceramic residue compared to the one
containing phenyl group and this is attributed to the crosslinking of vinyl groups.
Yajima et al.273 synthesized poly(borodiphenylsiloxane) by reacting boric acid with
DPDCS in n-butylether under nitrogen atmosphere for 18 h at 300°C
(Scheme 1.9).
Scheme 1. 9. Synthesis of poly(borodiphenylsiloxane)
The above polymer was obtained in 92% yield and found to be far more
moisture resistant than its analog prepared from boric acid and DMDCS.
Poly(borodiphenylsiloxane) softens at 100°C and is stable up to 500°C when
heated in air. The use of this polymer as a catalyst for the thermal conversion of
PDMS to polycarbosilane has been discussed in the section on polysilanes.
Ph
+Cl ClSi
Ph
B(OH)3Cl Si
Ph
Ph
Ph
O B Si OO
n
n-butylether
300°C
32 Chapter 1
Kobayashi277 reported the preparation of borodiphenylsiloxanol by the
condensation reaction of boric acid and diphenylsilanediol and then
polycondensing the borodiphenylsiloxanol to obtain borosiloxanes with wM
ranging from 800-5000.
Hoshii et al.278-280 synthesized borodiphenylsiloxane oligomers by reacting
diphenylsilanediol with boric acid (in different mole ratios) at 300°C in nitrogen
atmosphere in presence of dibutylether and N-methyl-2-pyrrolidone. The
oligomers have molecular weight ( nM ) in the range of 510 to 610 and gave a
ceramic residue of 59-70% when pyrolyzed at 1000°C.
1.4.2 Polymers from borate esters
Among the borate esters tris(trialkylsiloxy)boranes, (R3SiO)3B (R= Me, Et)
and bis(tri-t-butoxysiloxy)-n-butoxyborane have been reported as model
compounds for preparation of polyborosiloxanes.274 The reaction of chlorosilanes
or silanols with trialkoxyboranes277 results in the formation of polyborosiloxanes.
Partially hydrolyzed tetraethoxysilane reacts with trialkoxyborane, followed by further hydrolytic polycondensation to give borosilicate via sols and borosiloxane gels.281
Since polyborosiloxanes formed in a sol-gel process are unstable and
could not be isolated, heterofunctional polycondensation of tetraacetoxysilane and
tri-n-butoxyborane has been investigated to prepare and characterize the
polymers.281 The reaction provides only a low molecular weight polymer
( nM = 300-400). However, polyborosiloxanes of nM more than 5000 (HPBS),
which show good spinnability, were obtained by controlled alkoxylation, followed
by partial hydrolysis and final condensation in a sealed flask (Scheme 1.10). Dry
spinning of polymers (HPBS) provides SiO2-B2O3 precursor fibers.282
Scheme 1. 10. Preparation of polyborosiloxanes and/or SiO2-B2O3 gels
Si(OAc) 4 B(OBu n ) 3
THF Reflux BS
TAS TBB
Alkoxylation with EtOH Partial hydrolysis PBS
PBS HPBS Condensation/60 C
+
Introduction 33
Polyborosiloxanes and gel fibers were more easily prepared by reacting silicic acid and tributylborate in various molar ratios as shown in Scheme 1.11. The solution was concentrated in vacuum and then allowed to stand in a sealed flask at room temperature or at 60°C for a certain time to form a viscous solution from which gel fibers could be drawn just before gelation.
Scheme 1.11. Preparation of polyborosiloxanes from silicic acid Beckett et al.283 reported the synthesis of alkali-free borosilicate gel from
Si(OEt)4 and (MeO)3B3O3 in non-aqueous solvents. The formation of the gel proceeds via rapid transesterification/oxygen transfer with elimination of B(OR)3 followed by removal of volatiles from the gel and then drying at 60°C and subsequent heating in air at 600°C resulted in alkali-free borosilicate glass.
Synthesis of borosiloxanes through the reaction of borontrialkoxide with dialkyldiacetoxysilane or tetraacetoxysilane was reported by Kasgoz et al.284,285
These polymers were investigated as precursors for preparation of homogeneous borosiloxane gels.
Pillot et al.286 prepared polyborosiloxanes by the condensation of polychlorosilane and an alkylborate, B(OR)3, in presence of a catalyst. This polymer on nitridation followed by pyrolysis gave crystalline boron and silicon oxynitride ceramics.
1.4.3 Sol-gel process
It involves the preparative techniques heavily based on chemical reactions, for example, the generation of ceramic type materials by the hydrolysis of an organometallic compound. The sol-gel name applied to this approach comes
HO Si O H
OH
B(OBun)3
OH n
Mn=1000
Mn=6000
nOH
HO
OH
OSi B O
OBun
Bun
x
nOH
HO
OBun
OSi B O
OBun
Bun
x
nBuOH/Conc.
34 Chapter 1
from the fact that, in at least some of these synthesis, a dispersion of particles called a sol is produced and these particles can aggregate into gels as intermediates to final ceramic like materials. The most important reaction of this type involves the catalytic hydrolysis of tetraethoxysilane (TEOS).287 Hydrolysis and condensation reactions in an anhydrous sol-gel system comprising TEOS, boric acid and ethanol were studied by Zha et al.288 Boric acid is able to hydrolyse TEOS directly, and subsequent condensation reactions form borosiloxane and siloxane linkages whereas in an aqueous system, borosiloxanes are unstable to hydrolysis and are formed only upon heat treatment of the gel. The anhydrous mixture is stable indefinitely against gelation, but can be readily gelled by addition of NaOH in ethanol and this system is useful for preparing borosilicate glasses at lower temperatures with good homogeneity.
Borophosphosilicate glass–ceramics has been synthesized by Li et al.289
by sol–gel process using boric acid, phosphoric acid and TEOS as starting
precursors. The borophosphosilicate glass–ceramics can be sintered below 950°C
in air. It was found that the SiO2 glass phase wraps up BPO4 crystallites and
Si3(PO4)4 crystallites exist when the SiO2 content is decreased. The glass
ceramics show good potential as dielectric materials in super high frequency multi-
layer chip inductors (MLCI).
Different precursors were synthesized through the sol-gel process using
tetraethoxysilane, triethylborate and polydimethylsiloxane by polycondensation
between the Si(OH) groups from the hydrolyzed tetraethoxysilane and the
polydimethylsiloxane.290 Mixed vitreous oxycarbide glasses containing silicon and
boron were prepared by pyrolysis of the hybrid precursors in nitrogen at 1100°C.
The spectral studies indicate that boron is not incorporated into the network until
pyrolysis of the precursors that form the oxycarbides. The oxycarbides were
porous made of spherical interconnected particles and the porosity and mean pore
size of the glasses increased with increase in triethylborate content.
Sodium silicate solutions were mixed with boric acid in order to form
sodium borosilicate gels and the gelation process was studied by Boschel and
Roggendorf.291 The effect of composition and temperature on the gel point was
studied in detail.
Introduction 35
A patent literature292 discloses a process for the borosilicate glass
formation. Silicontetraalkoxide and borate ester were combined with water in
excess of that required for hydrolysis of alkoxide groups and the alkanol and water
formed were removed by fractional distillation from the reaction product to obtain
borosilicate glass.
1.4.4 Organically modified SiO2-B2O3 gels
Preparation of binary SiO2-B2O3 gels as precursor for borosilicate glasses
has been reported by several authors293-298 using sol-gel process. Sol-gel process
was chosen with the aim of preparing, via cocondensation reactions between
TEOS and triethylborate or boric acid, a more homogenous network compared to
the traditional melting route.
SiO2-B2O3 gels, have been extensively studied as precursors for
borosilicates or oxynitride glasses. It is found that the boron atoms are present in
the silica matrix as a separate B2O3 and/or B(OH)3 phase.296,297 This is probably
due to the well known reactivity of the B-O-Si bonds towards hydrolysis. The
borosiloxane linkages that might form in the early stages of the sol-gel process are
consumed at the end when increasing amount of water is produced via
condensation.298 Soraru et al.299 have recently shown that if a modified silicon
alkoxide, such as RSi(OEt)3, where R = Me, Et or Vi is cohydrolyzed with
tetraethylborate, then a large amount of trigonal BO3 units are incorporated in the
hybrid siloxane-network via B-O-Si bridges, leading to borosiloxane gels. The
resulting gels appear homogeneous, clear and no evidence of the formation of a
crystalline B(OH)3 can be detected. The retention of B-O-Si bonds in the hybrid
borosilicate gels is partly due to: (i) the low hydrolysis ratio used in the sol-gel
synthesis (H2O)/Si = 1.5), (ii) the in situ protection that the hydrophobic organic
groups bonded to silicon provide against water attack and (iii) the higher electron
density at the silicon atoms in the organically substituted gels which strengthen
the Si-O bonds.
In a modification of the above approach, the production of homogenous
borosilicate gels were tried by a new process in which the modified silicon
36 Chapter 1
alkoxides are directly reacted with boric acid without any external addition of
water.300 Here the hybrid borosilicate gels were synthesized according to a new
route, by adding boric acid to the liquid silicon alkoxide and by stirring until its
complete dissolution. The clear solutions were then cast in plastic test tubes that
were left open for gelation. Wet gels were dried for at least 10 days at 60°C. 11B-MAS NMR spectroscopic studies showed the formation of homogeneous
borosilicate gels and the incorporation of boron atoms into the siloxane network
via B-O-Si bridges. On the other hand, when a conventional tetrafunctional silicon
alkoxide such as TMOS or TEOS is reacted with boric acid the expected
borosilicate gels are not formed. The homogeneous borosilicate, obtained from
boric acid and alkoxysilanes, on pyrolysis results in the formation of homogeneous
amorphous borosilicon oxycarbide (BSiOC) glasses.301
The structural conversion of silicate gel to glass was investigated by Yang
and Woo302 in the temperature range of 150-850°C by using solid-state 1H- and 29Si-NMR spectroscopy. The conversion process is continuous and dependent on
the stabilization temperature. The silicate gel to glass conversion process is
completed at 850°C. The incorporation of boron in silicate backbone provides the
formation of ≡Si-O-B= bonds which results in the formation of borosilicate gels.
1.4.5 Applications of borosiloxanes
The end uses of preceramic polymers are dealt in detail in next section.
As the present thesis deals mainly with borosiloxane oligomers, specific end uses
of polyborosiloxanes are dealt in some detail in this sub section.
Polyborosiloxanes find end-uses as catalysts for the conversion of
polydimethylsilane to polycarbosilane, binders for making SiC components and
precursors for SiC-B4C ceramics and thermal protection coatings.272,276,303-315
Polyborosiloxanes synthesized by sol-gel process, are used for preparing
borosilicate bulk bodies, powders, thin films and fibers with high purity and
molecular homogeneity under mild conditions.284,316,317 Borosilicates are known to
be especially useful as cladding material for wave guides. Since they have a lower
refractive index than pure fused silica, low attenuation in glass fibers is more
Introduction 37
easily achieved.318-320 They have also been used as passivation layers for
microelectronic devices and as heat-resistant inorganic fibers.321-323
Polyborosiloxanes serve as binders, coatings and heat-resistant paints for
far infrared ray irradiators80, exhaust systems of cars and electric wires.324,325 A
metal plate can be coated to a thickness of 5 µm with a polyborosiloxane layer
containing metal oxides to form a solar collector.326
A patent literature explains a process for producing a heat-resistant
ceramic sintered body, which comprises preparing polycarbosilane containing
siloxane bonds by addition of polyborosiloxane containing phenyl groups with a
part of the side chain of Si and skeletal structure B, Si and O attached to
polysilane and heating the reaction mixture in an inert atmosphere. The
polycarbosilane obtained was mixed with a ceramic powder from the group
consisting of oxides, carbides, borides and silicides and sintering the mixture at a
temperature from 800 to 2000°C.327
Polyborosiloxane find wide variety of end-uses such as adhesive for
joining two substrates,328 curing agents for epoxy resins,329-331 and as heat curable
polymers.332
1.5 End uses of preceramic polymers
Preceramic polymers find wide range of applications as high temperature
polymers, high temperature rubbers, thermal protection systems, and oxidation
resistant coatings, matrix resins for ceramic matrix composites, precursors for
ceramic fibers, binders for making ceramic components and precursors for
obtaining ultra-fine ceramic powders. End-uses of some of the preceramic
polymers whose synthesis were dealt with in the preceding sections are discussed
in this section.
1.5.1 Protective coatings
Organometallic polymers find application as protective coatings for a
variety of substrates including carbon/carbon composites, carbon/graphite fibers,
ceramic fibers, ceramic matrix composites, metals and metal oxides.333-339
Protective coatings derived from organometallic polymers offer potential
38 Chapter 1
advantages over the conventional methods. The advantages include short
processing time, processing flexibility, ability to form coating on complex shaped
objects, processing at low temperature enabling ceramic formation under milder
conditions, high purity of the final ceramic, precise control of ceramic composition,
ability to apply multiple coating with relative ease and tailoring the coating
chemistry to obtain desired properties.336 A detailed discussion on various
preceramic polymer systems used as coatings is made in chapter 3.5.
Besides their end-uses as high temperature resistant materials,
precursors for ceramics and oxidation resistant coatings for carbon-based
substrates, inorganic and organometallic polymers find utility as atomic oxygen
(AO) resistant coatings for low earth orbit (LEO) space structures.208 A
comprehensive account of different polymeric systems used as AO resistant
coatings is given in Chapter 3.7.
1.5.2 Ceramic fibers
Preparation of ceramic fibers from organosilicon polymers was first
reported by Yajima et al.2,339,340 SiC fiber was obtained by a process which starts
from DMDCS and involves the synthesis of polysilane and its conversion to
polycarbosilane by heat treating it in an inert atmosphere under pressure around
450°C or in presence of borodiphenylsiloxane catalyst at 350°C. Green fibers (10-
20 µ m diameter) were obtained by melt spinning the precursor. They were made
infusible by surface oxidation and then subjected to pyrolysis at 1400°C to get SiC
fibers. The presence of oxygen in the final ceramic fibers is a draw back
associated with this process.
Silicon carbide fibers with low oxygen content (<2%) were prepared by
suitably modifying the spinning or curing process of green fibers followed by
pyrolysis.341 SiC fiber with low oxygen content was also obtained by pyrolyzing the
green fibers drawn from a blend of polycarbosilane and hydroxy-terminated
polybutadiene.342 Hasegawa and Okamura343 observed that the incorporation of
hetero atoms such as titanium, zirconium or vanadium into the precursor polymer
Introduction 39
enhanced the thermal stability of the ceramic fibers formed. Meyer et al.344
explored the use of silylene-acetylene polymer as a precursor for preparing SiC
fibers. Preparation of silicon oxynitride fibers by pyrolysis of polycarbosilane fibers
in ammonia345 and silicon carbonitride fibers by pyrolysis of polysilazane fibers in
inert atmosphere were reported.139,140 Silicon nitride ceramic fibers were also
prepared by heating the polymeric fibers derived from perhydropolysilazane.346
Various other preceramic polymers have been prepared and evaluated as
precursors for SiC fibers.347-353 Fibers made of SiBNC were derived from a single
source precursor, Cl3Si(NH)BCl2 and it retained the mechanical strength, as
measured by creep resistance and tensile strength, up to 1400°C.354 A variety of
borylborazine-based polymers were successfully converted into BN fibers via the
preceramic polymer route.355 Recently, Sneddon et al.214 developed new
decaborane-based polymeric precursors to B4C nanofibers.
1.5.3 Binders for ceramics
Current techniques of preparing ceramic components are based on
pressureless sintering of green body. The major drawback associated with this
process is that the voids often remain in the final structure and these voids have a
deleterious effect on the structural properties of ceramic. When preceramic
polymer is used for making green body, the void that would have formed during
organic binder burn-out would be replaced by a polymer-derived ceramic.356 The
first reported literature in this field was by Yajima et al.357 where a three
dimensional polycarbosilane was used as a binder for SiC sintered bodies.
Following this invention, Yajima et al.304 studied borodiphenylsiloxane as binders
for SiC and Si3N4 sintered bodies. Polysilastyrene358 was used as a binder for
making SiC components from β-SiC powder, and polycarbosilane and
polysilazane were used as binders for making injection moulded silicon nitride
parts.359,360 Sawai et al.152 reported that the compaction of SiC ceramic
was improved when polycarbosilane containing organofluoric groups was used
as a binder.
40 Chapter 1
1.5.4 Matrix resins for CMC
CMCs are tough materials and show a high failure stress when the fiber-
matrix bonding is either too strong or too weak. The bonding strength is usually
obtained through the use of the fiber coating referred to as the interphase. CMCs
are used as thermostructural materials under severe service conditions like high
temperatures under load and in corrosive atmospheres. The most common CMCs
used are non-oxide CMCs mainly C-SiC and SiC-SiC, the fibers being specified
first. There are three constituents in a CMC, which are the reinforcement, a matrix
and generally an interphase. The nature, volume fraction and arrangement of the
constituents depend on a number of factors including the function of the part to be
fabricated, the service conditions and the cost.
Ceramics by themselves have low fracture toughness and they fail
catastrophically. Reinforcing them with fibers increase their fracture toughness
thereby preventing catastrophic failure of the composite. Incorporation of fibers,
whiskers or particles into a ceramic matrix can result in a tough material. The
introduction of the reinforcement into the matrix results in energy-dissipating
phenomenon such as debonding at the fiber-matrix interface, crack deflection,
fiber bridging and fiber pullout.361 In this regard, proper control of the interface
region characteristics assumes great importance. The ratio of the moduli of the
reinforcement to that of the matrix is generally between 10-100 for a polymer or
metal matrix composite, while for CMC it can frequently be equal to or lower than
unity. The high ratio in polymer or metal matrix composites allows an efficient load
transfer from the matrix to the fibers through the strong interface. In a CMC, the
low modulus ratio infers that the fibers and the matrix are not much different in
their load bearing capacity. Thus, the role of the fiber reinforcement is to enhance
the toughness and strength of the composite by bridging the matrix cracks. This
requires a relatively weak interphase between the matrix and the fibers, so that
the matrix cracks induce interfacial debonding rather than failure of the fiber as the
cracks traverse the specimen’s cross-section. Accordingly, the interphase has a
key role in CMC eventhough its volume fraction is extremely low. This thin coating
which is deposited on fibers or formed in situ during composite processing serves
the functions of load transfer, crack deflection and diffusion barrier in reactive fiber
Introduction 41
matrix systems. The best interphase materials are pyrocarbon (PyC), hexagonal
BN or a layered microstructure like (SiC-PyC)m. The interphase coatings are
usually obtained by CVD process using suitable gases.361 Of late, preceramic
polymers are also used for getting interphase coatings.362 CMCs can be
processed either by conventional powder processing techniques used for
processing polycrystalline ceramics such as cold pressing and sintering, hot
pressing and reaction bonding or by newer techniques such as infiltration,
chemical vapor infiltration (CVI), sol-gel process and polymer impregnation and
pyrolysis.
Among these methods the most widely used one is the polymer infiltration
and pyrolysis. Because of the generally high cost of processing CMCs, polymer
infiltration is an attractive processing route due to its relatively low cost.
This approach allows near-net-shape molding and fabrication technology that is
able to produce nearly fully dense composites.363-367 Here, the fibers are infiltrated
with an organic polymer, which is heated to fairly high temperatures and pyrolyzed
to form a ceramic matrix. Due to the relatively low yield of polymer to ceramic,
multiple infiltrations are used to densify the composite. Polymeric precursors for
ceramic matrices allow one to use conventional polymer composite fabrication
technology that is readily available and to take advantage of processes used to
make polymer-matrix composites.364,365 By processing and pyrolyzing at lower
temperatures (compared to sintering and hot pressing) one can avoid fiber
degradation and the formation of unwanted reaction products at the fiber/matrix
interface. A detailed discussion on this topic is made in chapter 3.5.
1.5.5 Ceramic foam
There is considerable interest shown on ceramic foams which possess a
number of favorable properties such as low density, low thermal conductivity,
thermal stability, high specific strength and high resistance to chemical attack and
are suitable for industrial applications such as high temperature thermal insulation,
hot gas particulate filters, hot metal filters, catalyst supports and cores in high
temperature structural panels.368-371 The following processing routes have been
reported for the preparation of ceramic foams: (i) direct foaming of ceramic
slurries, sol-gel solutions372 or preceramic polymers (ii) chemical vapour deposition
42 Chapter 1
of various refractory materials on foamed carbon skeletons373 (iii) sintering of
hollow spheres374 and (iv) replication of polyurethane foam with ceramic
slurries.375 Of these methods, the first and the fourth one are mostly used for
making ceramic foams.
1.6 Objective and scope of the present investigation
The survey of pertinent literature on preceramic polymers presented
above reveals that the interest on these speciality polymers is ever growing. This
trend is attributed to their potential application as precursors for ceramic fibers,
ceramic coatings and ceramic foams, binders for ceramics and matrix resins for
ceramic matrix composites. Though a large amount of work has already been
done in the area of preceramic polymers and their applications, there still exists
enormous scope for developing cost effective synthetic approaches for the
preparation of novel precursors for ceramics, capable of giving high ceramic
residue. Thus, the present investigation envisages to synthesize certain speciality
oligomers/polymers containing silicon and both silicon and boron and evaluate
them as precursors for ceramics.
Among the preceramic polymers boron and silicon containing
polymers/oligomers have gained importance as they offer certain advantages
when compared to polymers which contain only boron or silicon. The ceramics
derived from such polymers remain amorphous even at temperatures exceeding
1500°C and hence, have better mechanical properties at these temperatures
compared to simple non-oxide ceramics which undergo extensive crystallization.
While several synthetic approaches are reported for the preparation of boron and
silicon containing preceramic polymers, cost effective and simple method to
prepare these polymers is by alkoxysilanes with boric acid, a cheap source of
boron. Though considerable literature is available on the synthesis of
borosiloxanes by sol-gel process there appears to be very limited information on
the synthesis of these polymers by non-aqueous sol-gel process. Thus, the main
objective of the present investigation is to synthesize novel borosiloxane
oligomers by non-aqueous sol-gel process, evaluate them as precursors for
ceramics and study their end-uses for space applications.
Introduction 43
While designing a preceramic oligomer/polymer, one should bear in mind
that it should be processable, i.e., soluble in organic solvents and/or meltable and
should give ceramic residue in high yield. To achieve this objective, it is required
to carryout a systematic investigation on the synthesis of borosiloxanes by using
different alkoxysilanes, varying the monomer feed ratios and experimental
conditions. To begin with attention has been focused on synthesizing borosiloxane
oligomers by reacting boric acid with phenyltrimethoxysilane (PTMOS) and
phenyltriethoxysilane (PTEOS). As PTEOS is much cheaper than PTMOS
emphasis has been laid on synthesizing borosiloxane oligomers from the former.
Synthesis of borosiloxane oligomer from boric acid and
vinyltriethoxysilane (VTEOS) has been reported by Ambadas.122 The choice of
VTEOS is justified from the fact that vinyl group present in the oligomer would
undergo crosslinking and hence would give high ceramic residue. The oligomer
synthesized from VTEOS is reported to give a ceramic residue of ~85% at 900°C
in inert atmosphere. However, a detailed study on the effect of monomer feed ratio
on the thermal stability and ceramic residue has not been reported. Thus, in the
present investigation attention has been focused on this aspect. As the presence
of vinylsiloxy units is expected to improve the ceramic residue, it would be of
interest to study the feasibility of synthesizing borosiloxane oligomers using a
mixture of VTEOS with other alkoxysilanes. The advantage of this approach is that
processable borosiloxane oligomers with improved ceramic residue can be
obtained. A detailed study has therefore been undertaken to synthesize
borosiloxane oligomers using mixtures of VTEOS with PTEOS and PTMOS.
Epoxy resins have gained significance due to their versatility and they find
extensive application as adhesives, matrix resins, potting compounds and coating.
Considerable amount of work has been done on epoxy-functionalized siloxane
resins. However, there is very little information on borosiloxanes containing epoxy
functional group. The advantage of such a resin is that unlike vinyl-functionalized
borosiloxanes which require high temperatures (above 150°C) for curing, epoxy
functionalized borosiloxanes can be cured at room temperature. This advantage
can be exploited for infiltrating porous ceramic matrix composites (CMCs) and
then curing the infiltrated resin thereby preventing the resin ooze-out from the
44 Chapter 1
composite. Keeping in view of this, in the present work efforts have been directed
towards preparing epoxy-functionalized borosiloxane oligomers by reacting boric
acid with glycidyloxypropyltrimethoxysilane (GPTMOS). As GPTMOS contains
aliphatic linkages, it is expected that the overall thermal stability and ceramic
residue of such oligomers would be lower than the ones synthesized from other
alkoxysilanes. This disadvantage could probably be overcome by incorporating
vinylsiloxy units in addition to the epoxy functional group. In view of this,
borosiloxane oligomers have been synthesized using different mole ratios of boric
acid, VTEOS and GPTMOS and characterized.
Besides other end-uses borosiloxanes are also useful as precursors for
glass coating. Such glass coatings are expected to find application as overlay
coating to seal the cracks in SiC coating applied to C-C composite. For this
purpose it is desirable to synthesize borosiloxane oligomers devoid of carbon. If
this is difficult to achieve, then the oligomers are to be designed in such a way that
the organic groups should be knocked off completely during pyrolysis much before
the formation of glass. To achieve this objective borosiloxane oligomer has to be
synthesized by reacting boric acid with alkoxysilane having no organic
substituents. One such monomer is tetraethoxysilane (TEOS). When boric acid is
reacted with TEOS, ethanol would be formed as a by-product and hence the
product obtained should contain only B, Si and O. If some unreacted ethoxy group
is present it is likely to be knocked off well before 1000°C. In the present
investigation, attempts have been made to synthesize borosiloxane oligomers by
reacting boric acid with TEOS.
For the synthesis of borosiloxane oligomers, diglyme or 1,4-dioxane has
been used as solvent. As they are high boiling solvents, it becomes difficult to
remove them and quite often the oligomers undergo gelation while attempting to
remove the solvents. Thus, it would be advantageous to synthesize borosiloxane
oligomers, particularly the functionalized ones, by solventless process. In this
approach, boric acid and alkoxysilane would be reacted directly without using any
solvent. The by-product, alkyl alcohol, formed by the reaction of B-OH and -OR
would serve as solvent for the reaction. The advantage of the solventless process
is that the viscosity and the molecular weight of the product obtained can be
Introduction 45
controlled by controlling the amount of alkyl alcohol distilled out from the reaction
mixture. Yet another advantage is that the resin synthesized can be directly used
for infiltration without any further processing. As alkyl alcohol is low boiling, it can
be removed easily. In view of these advantages, attempts have been made to
synthesize borosiloxane oligomers, particularly vinyl- and epoxy-functionalized
ones, by reacting boric acid with alkoxysilanes without using any solvent.
As discussed earlier, borosiloxane oligomers are expected to have better
thermooxidative stability compared to other preceramic polymers. Prior to finding
out suitable end-uses of borosiloxane oligomers, it is imperative to study their
ceramic conversion. In the present study, selective systems have been subjected
to pyrolysis at 900°C followed by sintering at 1400 and 1600°C. The ceramic
conversion studies gain significance due to the fact that the presence of boron is
expected to suppress the crystallization or in other words increase the
crystallization temperature. Borosiloxane oligomers synthesized from boric acid
are expected to have 2-6% boron and so it would be interesting to find out the
effect of increasing the boron content in borosiloxane oligomers. This can be
achieved only by using caged boron compounds. For this purpose, an oligomer
containing higher boron content has been synthesized by reacting decaborane(14)
with 3-aminopropylmethyldiethoxysilane followed by reaction with boric acid. The
ceramic conversion studies of this oligomer have been carried out.
In the present investigation, the possibility of using borosiloxane oligomers
as oxidation resistant coatings for carbon-carbon (C/C) composites, precursors for
high temperature glass coating and matrix resin for carbon fiber reinforced CMC
has also been studied.
Polysilahydrocarbons belong to a new class of silicon containing polymers
in which the polymer backbone consists of Si-(C)x-Si units, where x is more than
one. In recent years, a great deal of work has been carried out in the area of
polysilahydrocarbons focusing mainly on the synthesis, microstructure and
mechanism of formation and thermal stability.111-118,120-123 However, the thermal
degradation kinetics of such polymers and their end-uses have not been studied.
46 Chapter 1
Thus, in addition to the investigation on borosiloxane oligomers, the present thesis
focuses on the thermal degradation behavior of polysilahydrocarbons and their
evaluation as atomic oxygen (AO) resistant coating for low earth orbit space
structures.
The thermal degradation kinetics of polysilahydrocarbons obtained
through the dechlorination of diorganodichlorosilanes with styrene has been
carried out using four different equations. Attempts have been made to correlate
the activation energy and pre-exponential factor with the structure of the
copolymers. Similar investigation has been carried out on the polycarbosilanes
obtained by heat treatment of polysilahydrocarbons at different temperatures.
Attempts have been made to correlate the thermal behavior of polycarbosilanes
with the structural changes that take place in polysilahydrocarbons during heat
treatment.
The atomic oxygen formed by the photodissociation of molecular oxygen
in the upper atmosphere. AO can react with most of the spacecraft materials
causing surface erosion which eventually brings down the performance of these
materials drastically. Thus, it becomes essential to develop spacecraft materials
which are resistant to AO attack or apply AO resistant coatings on the spacecraft
materials which are susceptible to AO attack. When inorganic polymers or
organometallic polymers are used as protective coatings, they form a metal oxide
layer on the surface due to the interaction of the oxidation of the polymer by AO.
This oxide layer prevents further interaction of the polymer with AO, thus offering
protection to the substrate which is otherwise susceptible to AO attack. As
polysila-hydrocarbons contain disilyl linkages, these linkages can trap oxygen
forming siloxane linkages which on further reaction with AO would result in the
formation of SiO2 layer. Keeping in view of the above advantage, in the present
work polysilahydrocarbons have been evaluated as AO resistant coatings. The
performance of these polymers has been compared with other AO resistant
materials developed in the Preceramic Polymers Laboratory of VSSC and also
with phosphazene-containing polymers.
Introduction 47
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