Title: Chemically derived ceramic composites
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Title: Chemically derived ceramic composites
Physical Description: x, 272 leaves : ill. ; 28 cm.
Language: English
Creator: Lee, Burtrand Insung
Publication Date: 1986
Copyright Date: 1986
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Subject: Ceramic materials   ( lcsh )
Ceramics   ( lcsh )
Materials Science and Engineering thesis Ph. D
Dissertations, Academic -- Materials Science and Engineering -- UF
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Thesis: Thesis (Ph. D.)--University of Florida, 1986.
Bibliography: Bibliography: leaves 264-271.
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General Note: Vita.
Statement of Responsibility: by Burtrand Insung Lee.
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oclc - 015266718

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CHEMICALLY DERIVED CERAMIC COMPOSITES


By

BURTRAND INSUNG LEE






















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA
1986






ACKNOWLEDGEMENTS

It is difficult for me to acknowledge everyone who gave helping

hands during the tenure of my research as a graduate student at the

University of Florida.

It is hardly necessary to mention my advisor Professor Larry L.

Hench for his inspiring support and stimulating thoughts. His sense of

humor maintained a relaxed research atmosphere. He helped me to mature

as an independent scientist. I am especially grateful for the freedom

and independence he has given me throughout this research. It was in-

deed a privilege and pleasure to work with him.

Dr. Robin Sinclair at 3M Co. not only provided the facility to

synthesize some of polysilanes but also taught me the techniques in

polymer chemistry. The analytical services of 3M Co. which provided

analytical data of the polysilanes are acknowledged with gratitude. Dr.

Curt Schilling at Union Carbide is also acknowledged for his support by

providing some of their experimental polysilanes.

My thanks are extended to D. Dunnagan and U. Folz for thermal anal-

yses, E. Jenkins for SEM, W. Acree for XRD, Dr. J. Newkirk and C. Turner

for TEM, Dr. A. Gupta for GC, S. Yoon and S. Kong for proton NMR, Lester

at the Engineering Machine Shop of the University of Florida, G. LaTorre

for FT-IR, S. Kang for density and microhardness measurements, and

Professor Batich and Dr. S. Kurinec for help in XPS.

The financial support of the U.S. Air Force Office of Scientific

Research through contract no. F49620-83C 0072 was certainly an essential

part of my life at the University of Florida and is especially acknowl-

edged as is the encouragement of Dr. D. R. Ulrich, contract monitor.






I am also grateful to the members of my supervisory committee,

Professors C. Batich, M. D. Sacks, L. Malvern, D. Clark, and E. D.

Whitney, for their advice and reading of the entire manuscript of this

dissertation.

On the nontechnical side, special personal thanks goes to my

family. Instead of complaining about not spending much time with them,

they rather were truly the Gatorade, "a thirst quencher." My mother-in-

law, in particular, played too great a role to describe.






TA3LE JF CO:T;TE:TS



Pa e

ACKNOW LE DGE'MENTS .................................................. ii

:ST )F ABBREVIATIONS, ACRONY'~S, PIUTIALISMS, AND SYMBOLS......... vi

ABSTRACT................................................ ......... ix

:uAPTERS

OVERViE OF C'-EMICALLY DER /ED CERAMICS ....................

SILICON CAR3IDE FROM ORGANOSILANE PRECURSORS

:"troauction ............................. ................ 10
Experimental ................................................ 16
Results ..................................................... 31
Discussion................................................. 6-
Conc i sions ................................................. 96

III. SILICON CARBIDE/SILICA COMPOSITES FROM CAR3OSILANES
AN:i A-KOXYS: LANES

Introduction .............................................. luu
Experimental ............................................... 1U2
Results ..................................................... 10
Discussion.............................................. ... 142
Conclusions........................................... .... 152

IV. SILICON CARBIDE/SILICA COMPOSITES FROM COMMERCIAL
SILICON CARBIDE AND SILICON TETRALKOXIOE

Introduction...... ..................... ................. 155
Experimental ........... ...... ..................... .... 155
Results.................................................. 167
Discussion ............................................... 213
Conclusions............................................. ... 230

V. OTHER CHEMICALLY DERIVED CERAMIC COMPOSITES
Introduction.............................................. 233
Experinental................................................ 234
Results.................................................... 235
Discussion. ...................... ......................... 238
Conclusions .............................................. 257






Table of Contents (continued)


Page

VI. CONCLUSIONS AND RECOMMENDATIONS............................ 259

REFERENCES...................................................... 264

BIOGRAPHICAL SKETCH.............. o................ ................ 272






LIST OF ABBREVIATIONS, ACRONYMS, INITIALISMS, AND SYMBOLS


Ac

AIBN

A-PSS

B.E.

BPO

-C=C or '-

-C-C=C or b

CDC13

06D6

CFRI

DCCA

DCP

DMDCS

DSC

DTGA

EDS

en

EtOH

FID

FT-IR

GC

GPC

IR

J-PSS


Acetate ion or group

Azobisisobutyronitrile

Allylic polysilastyrene

Binding energy in eV

Benzoyl peroxide

Vinyl group

Allyl group

Deuterochloroform

Deuterobenzene

Chemical free radical initiator

Drying control chemical additive

Dicumyl peroxide

Demethyldichlorosilane; Me2SiC12

Differential scanning calorimetry

Derivative thermogravimetric analysis

Energy dispersive x-ray spectroscopy

Ethylenediamine

Ethanol

Flame ionization detector

Fourier transform infrared

Gas chromatography

Gel permeation chromatography

Infrared

PSS prepared by Shinnisso Kako Co., Japan






Me Methyl group, -CH3

Rn Number average molecular weight

MeC12 Methylene chloride

MPDCS Methylphenyl dicnlorosilane, MeSiPhC12

Mrad Mega rad

M.W. Molecular weight

PC Polycarbosilane

PDMS Polydimethyl silane

PDS Polydimethyl silane

Ph Phenyl group, -C6H5

PrOH Isopropyl alcohol

PS Polysilane

PSS Polysilastyrene

PSS-0 Oligomer fraction of polysilastyrene

P-PSS Petrarch's polysilastyrene

R Reflectance, or diameter to length ratio of a fiber

RT Roon temperature

T Temperature, or transmittance

TEOS Tetraethylorthosilicate: tetraethoxysilane

TMOS Tetramethoxysilane

TMS Tetramethylsilane

SEM Scanning electron microscopy

SS Silastyrene, oligomer fraction of polysilastyrene

STEM Scanning transmission electron microscopy

TEM Transmission electron microscopy







TGA Thermogravimetric analysis

TMA Thermomechanical analysis

UV Ultraviolet

v/o Volume percent

ViSO Vinylic silane oligomer

ViSP Vinylic silane polymer

w/o Weight percent

w/v Weight percent volume

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction


Greek Symbols

a Linear coefficient of thermal expansion

y Uncharged high energy electromagnetic quanta

p Density, g/cc

X Wavelength

v Frequency

v Wavenumber

A Differential value

a Stress, strength


Subscripts

x Crystalline

g Glass

c Carbide


viii









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the degree of Doctor of Philosophy



CHEMICALLY DERIVED CERAMIC COMPOSITES



By

BURTRAND INSUNG LEE



May 198b

Chairman: Dr. Larry L. Hench
Major Department: Materials Science and Engineering

Silicon carbide was made from various organosilane precursors by

crosslinking and pyrolyzing them in an inert atmosphere. Crosslinking

of these silane precursors was studied by various means. The most suc-

cessful means of crosslinking was found to be via a chemical free radi-

cal initiator, dicumyl peroxide. The mechanism of crosslinking for the

precursors was determined.

Pyrolyses of the silane precursors were carried out and increased

ceramic yields after crosslinking were shown as compared with uncross-

linked precursors. The ceramic yields determined by TGA ranged from 10-

70% depending on the precursors and the crosslinking treatments.

Partially densified sol-gel derived silica monoliths were impreg-

nated with the silane precursors while the silica monoliths were still






highly porous. Diamond microhardness values increased 2-3 times from

unimpregnated gel derived silica monolith. A noderaze increase in frac-

ture toughness, KIC, and flexural strengths was achieved. Optical and

mechanical properties, and porosity data are presented.

Using sol-gel silica precursors and the processing techniques of

polysilanes to obtain s-SiC, molecular composites of SiC witn Si02, with

Ti02 and with A1203 were made in monoliths and powder forms. Monolithic

composites with a molecularly dispersed SiC phase in the Si02 gel matrix

showed a hardening effect by the SiC phase.

The molecular composite powders of SiC/A1203 showed no or little

crystallization of either phase after heating to 14000C.

Monolithic silicon carbide/silica composites were made using com-

mercially available fibrous silicon carbides and a tetralkoxysilane

precursor. Modest to low flexural strengths were obtained after heat

treating to 900-14000C, because of the high porosity in the composite.

Cold pressing of the SiC and silica sol slurry improved the density and

flexural strengths. Notched 3-point fracture toughness values, KIC, was

as high as 7 MPa.ml/2. Excellent thermal shock resistance and oxidation

resistance of these composites are shown.








CHAPTER I
OVERVIEW OF CHEMICALLY DERIVED CERAMICS


Ceramic materials are of critical importance in high technology

(high-tech) areas where unique combination of properties, such as high

strength, strength retention at high temperature, low thermal and elec-

trical conductivity, high hardness and wear resistance, and high chemi-

cal stability are required. However, because of their brittle nature,

ceramic materials produce problems in design reliability in high perfor-

mance structural applications resulting in catastrophic failures under

stress. This inherent problem combined with poor cost effectiveness in

fabrication of complex shapes severely limits wider applicability of

current ceramic materials.

Ceramic materials derived from chemical reagents have the potential

to overcome these problems by

1) Low temperature processing compared with traditional ceramic

processing,

2) Starting chemical compounds that can easily be purified to in-

crease the purity of the ceramic materials,

3) Having versatility in forming complex shapes and precise control

of each step in the processing,

4) Rendering homogeneous mixing; uniformity and, thus, reliability

of the formed bodies can be improved, and

5) A unique combination of microstructure and phase assemblages not

obtainable by traditional ceramic processes may be obtained.











Ceramic materials obtained by chemical processing are a rather

recent development, despite the fact that the science behind the pro-

cessing existed long before the ceramic applications were realized.

Traditional ceramic science has been based more on physics and has been

developed by optimizing the physical behavior with the microstructure of

the materials.

The term "ultrastructure processing" of ceramics has been intro-

duced1 to represent the chemical manipulation and control of surfaces

and interfaces during the earliest stages of formation in atomic or

molecular scales. The so called "high-tech" ceramics are largely based

on "ultrastructure processing" as this is one way in which enyineeriny

ceramics can potentially yield properties approaching the theoretical

limit.

Organic chemistry, once an anathema to ceramists, is recognized as

a major source of new ceramic materials. By using ordinary chemicals as

precursors to ceramic materials, one can study and control the chemical

process in every step during the evolution of ceramics, from the start-

ing chemical to the final product. Greater versatility in fabrication

with more precise control of the process leading to an extremely homo-

geneous composite with superior properties is the goal and advantage of

this approach.

Ceramic fibers, optical glasses, ultrapure and ultrafine powders,

and ceramic monolithic parts are some of the demonstrated materials

derived chemically.1,2 Figure I-1 summarizes some of the chemical pro-

cesses.























































Fig. I-1. Flow Diagram of Some of Chemically Derived Ceramics
and Composites








The sol-gel method, as shown in Fig. I-1, is a notable example of

obtaining oxide ceramics from metal-organic precursors. Pure monolithic

parts, thin coatings, matrices for reinforced composites, etc. have been

produced with controlled properties. Uranium oxide fuels were fabri-

cated by the sol-gel method at Oak Ridge National Laboratory in the

1970's.3 Active research is underway to understand the fundamental

chemistry in the reaction steps, as well as in the applications.2,4 All

facets of chemistry are involved. For example, a nuclear magnetic

resonance technique has been found helpful in understanding the reaction

mechanism of the sol-gel process.2

It has been shown2 that certain chemical additives change the phys-

ical-chemical state during sol-gel transformation. The mechanism of how

these additives function chemically is not fully understood. Various

dopants may be added to the sol as a chemical reagent by forming a mo-

lecularly homogeneous solution.

Tne most understood sol-gel process is in the production of silica

glass. Silica sol-gel reactions involve hydrolysis and polycondensation

steps of a metal-organic precursor, as shown in eqs. I-i and 1-2.

(%41OR)4 + n H20 + Si-OH + 4 ROH

2 = Si-OH + E Si-O-Si = + H20 (1-2)

The hydrolysis and polycondensation reactions initiate at numerous

sites within the Si(OR)4 precursor + H20 solution as mixing occurs.

They eventually form a three dimensional linkage of Si-O-Si in a sub-

micron scale and are called sol particles. The sol particles come in

contact to form a gel network. As the gel network is aged at an ele-

vated temperature, the monolithic body is strengthened and becomes more

like a ceramic body.






5



Some other oxide ceramic materials derived chemically may be given

below, 1) alumina from A1(OR)3 by Yoldas,5'6 2) lead titanate by Gurko-

vich and Blum,7 3) inaium tin oxide films by Arfsten et al.,8 4) mono-

sized SiO2 and Ti02 powders by Barringer et al.,9 and 5) single and mix-

ed phase oxide powders by Mazdiyasni.10

Similar to the sol-gel process of obtaining metal oxides, pyrolysis

of organometallic precursors results in nonoxide ceramic materials of

the constituent elements. Thus far, successful examples are silicon

carbide (SiC) and silicon nitride (Si3N4) from polymers containing sili-

con-carbon and silicon-nitrogen bonds in the backbone.4 Boron nitride

and boron carbide can also be made from organometallic precursors.4

Titanium carbide, titanium nitride, titanium boride, silicon boride, and

aluminum nitride may be possible from organometallic precursors.4 Table

I-1 lists ceramic materials that can be made from chemical processing of

organometallic compounds.4

It is difficult to make complex shapes of dense refractory ceramics

such as SiC or Si3N4 using conventional high temperature sintering, hot

pressing, or hot isostatic pressing methods without a sintering aid.

Grain boundary phases are often introduced in materials, degrading high

temperature performance and oxidation resistance. Refractory carDide

and nitride fibers are nearly impossible to make using traditional pro-

cessing methods.

In making fibers from pyrolysis of an organometallic precursor,

densification accompanies pyrolysis, which eliminates a separate sinter-

ing process. By analogy to a carbon fiber made from a carbon polymer,







Table I-1.
Ceramic Products That Can Be Made From Organometallic Precursors.*


Products Available


Theoretical
Density, g/cc


Products Under
Development


Theoretical
Density, g/cc


Silicon Carbide (SiC)

Silicon Nitride (Si3N4)

Boron Nitride (RN)

Boron Carbide (B4C)


3.2

3.2

2.2

2.5


Aluminum Boride (A1B12)

Calcium Boride (CaB6)

Silicon Boride (SiB6)

Titanium Boride (TiB2)

Titanium Carbide (FiC)

Aluminum Nitride (A1N)

Titanium Nitride (TiN)

Boron Phosphide (BP,BbP)

Titanium Silicide (TiSi, TiSi2)


* Based on Ref. 11.


2.6

2.5

2.4

4.4

4.9

3.3

5.2

3.0, 2.6

4.2, 4.0










organometallic precursors offer a means to make refractory carbides and

nitrides at potentially lower temperatures with the easy forming opera-

tions of traditional polymers.

One of the primary thrusts in applications of these materials is in

gas turbine engines because these materials are strong and stable at

temperatures no metal can withstand, and they also have high thermal

shock resistance, which is necessary for heat engine components. Cer-

amic materials derived from polymer precursors in the form of foam may

also be used for thermal insulation, filtration, and packing. Thin

films may be applied in electronic devices and metal-ceramic joints.

Extremely homogeneously doped high temperature semiconductors may be

made this way as well. Boron nitride fiber made from organometallic

precursors can be used as a dielectric material where alumina and silica

fibers are less desirable. These organometallic precursor materials can

also be used as binders in powder forming processes. There is also a

high probability of obtaining infrared transmitting films of sulfides

and selenides, superconductive fibers of NbN, NbC, silicides, sulfides,

and borides via polymer precursor pyrolysis.

Intrinsic flaw sensitivity and brittleness continue to impede the

broader applications of monolithic ceramic components. These instrinsic

weaknesses can be overcome by incorporating a high modulus, small diam-

eter ceramic reinforcing phase in a ceramic matrix to change the failure

mechanisms to tough, noncatastrophic modes.

Composite materials on the ultrastructural level can be achieved by

mixing polymer precursors containing constituent elements, e.g., a










polysilane mixed with polyphenylborazole yields a composite of SiC/BN.3

Polymers containing Si, C, and N can be used to obtain a SiC/Si3N4 com-

posite.4

A reaction of polycarbosilane and Ti(OR)4 can yield a composite of

SiC/TiC.12 In this composite process Ti(OR)4 not only provides the TiC

phase, but also crosslinks the polycarbosilane, hence maintaining the

shape of the green body during the subsequent heat treatments and in-

creasing the ceramic yield. The structural scale of these precursor

based composites is in the 1-10 nm range, as compared with the 1 to 100

pm or larger range of composites made by traditional processes.13

Composites are leading a new era in structural engineering. The

development of high performance materials and advances in fabrication

technology are laying the groundwork for revolutionary changes in

structural design. In order to go forward with high speed in ceramic

composite technology, it is necessary for engineers to break away from

engineering thought processes that have been developed over decades of

working with conventional materials.

Active research on ceramic matrix composites began no earlier than

1982, according to Persh.14 Even then, the ceramic matrix composites

were based more on applied physics using the traditional processing

methods, such as hot pressing matrix phase powder with a reinforcing

phase.

The objectives of this dissertation are thus based on an explora-

tory study and development of new methods to obtain ceramic materials

derived by chemical means. Processing and properties of SiC/Si02























composites utilizing techniques of sol-gel derived SiO2 and SiC via

organosilanes are the main topics of this work. Other ceramic compos-

ites from chemical origins are also part of this dissertation. The pri-

mary motivation and objectives for the work presented in this disserta-

tion are an interest in the development of new processing methods based

on chemical processes and an understanding of these processes. For cer-

amic and composite materials in this work, the emphasis is more on con-

cepts rather than the final products with exciting quantitative data in

part because concepts are felt to be of greatest use to those developing

ceramic composites.

The more elaborate and topical introductions are given in the

beginning of each chapter.









CHAPTER II
SILICON CARBIDE FROM ORGANOSILANE PRECURSORS


Introduction

Many ceramic materials have specific properties that make them

ideal for energy related systems. Silicon carbide (SiC) is one of the

leading candidates for high temperature structural applications because

of its low density, high-temperature strength, chemical stability, re-

fractoriness, high thermal shock resistance, and creep resistance. To

achieve these desirable properties of silicon carbide, it is necessary

to develop a reproducible and reliable method for producing the material

in complex shapes and with a controlled ultrastructure.

In the conventional process for producing SiC material, silica in

the form of sand and carbon in the form of a coke are reacted together

at 24000C in an electric furnace. The SiC produced is in relatively

large grains which are subsequently ground to the desired size.15

The Cutler process16 was developed to produce SiC material with

superior properties and cost effectiveness by using rice hulls. From

this process, the commercially known a-SiC whisker Silar" by ARCO is

obtained. The major advantage of it is that it has a much lower pro-

cessing temperature, ~16000C, than the more traditional process.

The increasing search for new types of high-strength materials, and

for performance improvement in the existing ceramics, has pushed several

nonconventional approaches to ceramic synthesis.










As presented in Chapter I, obtaining nonoxide ceramics via pyroly-

sis of organometallic precursors has potential advantages over the con-

ventional methods of producing materials in low temperature process,

higher purity, fabrication of complex shapes, greater homogeneity, new

fabrication procedures leading to continuous fiber, coatings, and

impregnated porous structures. At a more fundamental level, polymer

routes can allow control over the microstructure of the intended ceramic

product with a unique combination of microstructure and phase assem-

blages and important consequences for both physical and chemical proper-

ties.4 Some of the more important applications of nonoxide ceramic

materials obtained via polymer routes are listed in Table II-1.

The first use of organic polymers to produce an inorganic refrac-

tory material was probably the development of graphite fiber from poly-

acrylonitrile in late 1950.17 Other ceramic materials from organometal-

lic polymers were first noted by Chantrell and Popper.19 A partial

history of the development of nonoxide ceramics from organometallic pre-

cursors is given in Table II-2.4

However, early workerss0-52 in organosilanes (OS) genuinely be-

lieved that polysilanes were worthless and regarded them as undesirable

by-products of a faulty synthesis. Tnis all changed in 197b when Yajima

and his coworkers22"30,53-55 demonstrated that the polydimethylsilane

that was regarded as an undesirable by-product by previous investi-

gatorsso-52 is indeed a precursor to $-SiC. The reaction scheme, as

shown in Fig. II-1, is the polymerization of dimethyldichlorosilane

[(CH3)2SiCI2] by dechlorination to yield polydimethylsilane (PDMS). The




















Table II-l.
Some of the Demonstrated Applications of
Nonoxide Ceramics Derived from Organonetallic Precursors


Form


Fiber



Monolith


Foam


Powder



Thin Film


Thermosetting polymer


Applications


Reinforcement for composites
weaves, wovens, mattes


Monolithic bodies for high temperature parts


Filters, packing, insulation, heat exchanger


Press to bulk body,
Filler material


High temperature electronic devices


Metal-ceramic, ceramic-ceramic joints, bind-
er in powder forming
















Table 11-2
A Partial History of Nonoxide Ceramics Via Polymer Routes


Precursor
Year Polymers

1960 phosphonitric
chlorides

1965 unknown


1974-75 polysilanes


1976-81

1976


1978

1979-80


polycarbosilanes

polyphenylborazole


carboranesiloxane

polycarbosilanes


1980-81 polysilastyrene
polycarbosilanes

1981 polytitanocarbosilane

1982 polysilazanes

1982 polysilazanes

1984 vinylic polysilane


Ceramic
Products

P-N


BN, A1N,
Si3N4, SiC

Si-C-N


SiC

BN


SiC-B4C

SiC


SiC
SiC, Si-C-N

Si-Ti-C

Si-C-N

Si 3N4
SiC


Investigators

Ainger, Herbert


Chantrell,
Popper

Verbeek and
Winter et al.

Yajima et al.

Taniguchi, Harada
Maeda

Rice et al.

Schilling,
Williams, Wesson

West et al.
Baney and Gaul

Yajima et al.

Penn et al.

Seyferth, Wiseman

Schilling and
Williams


Ref.

18


19


20, 21


22-3U


31

32, 33


34-39

40
41-46

12

47

48, 49

39
















C8H CI CH C Crosslinking by
C S/ N Polymerizoaton 'I thermal rearongement mel spinng oxygen in air
by dechlorlnatlon L'S in ot 500 C t- ot 200"C
CH3 CI CH3H
Dimethyldichlorosllaon Polydimethylsilone Polycorbosilone


Heat treatment In 6i C fr /
Crosslinked, infusible fiber an at e -SiC fiber + CH4 # H2
on inert atmosphere
at 12000C



Fig. 11-1. Reaction Steps for the Yajima Process to Produce B-SiC via Polycarbosilane and
Oxygen Crosslinking, % Yield Shown above the Intermediates and the Product.









polymer chain then is rearranged to make it more reactive for a thermo-

setting condition. The rearranged polymer called polycarbosilane has

alternating silicon and carbon atoms. The thermosetting or crosslinking

carried out in air is a necessary step in the Yajima SiC fiber synthesis

to maintain the fiber shape during the subsequent heat treatments. Dur-

ing the heat treatment, hydrocarbon products are eliminated to yield a

ceramic char of SiC.

Since Yajima and coworkers22-30,53-55 developed a B-SiC fiber with

excellent mechanical properties from the polycarbosilane, there have

been several other precursors potentially superior to the Yajima pro-

cess.34-40 These are vinylic silanes developed by Wesson and

Williams34-36 and Schilling et al.3,839 and polydimethyl phenylmethyl-

silane, better known as polysilastyrene (PSS) developed by West et al.40

The vinylic silanes are reactive under a thermal crosslinking con-

dition but, because of their low viscosity (liquid at room temperature),

control of viscosity to draw fibers may require extra steps. On the

other hand, PSS is a solid with good solubility in common solvents and

possesses excellent tractability with good melt viscosity. However, it

possesses no reactive functional groups for crosslinking. It was noted

by West et al.40 that the polymer strongly absorbs UV light at ~330 nm.

Irradiation of UV with XZ330 nm on PSS was shown to crosslink the poly-

mer on the surface.40 However, the practicality of UV crosslinking of

PSS for larger structural ceramics is in question. An alternate way to

achieve bulk crosslinking is needed.

It is important to note that vinylic silanes and PSS are poten-

tially superior to polycarbosilane because they can be processed without









the separate thermal rearrangement and oxygen crosslinking (Fig. II-i),

as required in the Yajima process. Oxygen crosslinking undoubtedly

introduces Si-O-Si in the network and ends up as silica in the final

product S-SiC. In order for the potential advantages of vinylic silanes

and PSS to be realized, it is essential that crosslinking methods be

developed which will avoid oxygen in the SiC lattice following pyrol-

ysis.

The term polymer used here is defined as any organic or organo-

metallic compound that is not a monomer. However, in some specific

cases, oligomers are distinguished from polymers.

It is the objective of this work to investigate crosslinking

methods and some applications of vinylic silanes and PSS precursors to

SiC. In this chapter the following topics are investigated and dis-

cussed: 1) synthesis and characterization of the polymers, 2) modifica-

tion of the polymers for crosslinking, and 3) crosslinking and pyrolysis

to obtain SiC.



Experimental

Preparation of equimolar dimethyl phenylmethyl copolymer

Reagent grade toluene for a solvent was dried with sodium metal in

~1 g sodium per 1 1 toluene by refluxing for 24 hours followed by dis-

tillation through a one way air sealed glass apparatus using mineral oil

bubbler.

A starting monomer dimethyldichlorosilane (Me2SiC12) from Aldrich

Chemical Co. was purified by distillation using a trap-vacuum technique

with liquid nitrogen. Methylphenyl dichlorosilane (PhMeSiC12) monomer








also from Aldrich Chemical Co. was vacuum distilled in a Yamato model

rotary evaporator at 800C.

A reagent grade sodium metal bar was cut to 47.5 g and placed in a

dry 2 liter 3-necked round bottom flask. These operations were carried

out in a glove box with N2 atmosphere. The dried and distilled toluene

(850 ml) was added to the flask and the polymerization reaction appar-

atus was set up, as shown in Fig. 11-2.

Sodium and toluene were mixed by stirring and heating to obtain a

molten mixture of sodium dispersed in the solvent. Heating was discon-

tinued to add dichlorosilane monomers. A premixed solution of 61 ml of

Me2SiC12, 81 ml of PhMeSiCl2, and 50 ml of dry toluene was added slowly

through the air sealed side arm, while stirring was continued and N2 gas

was continuously flowing through the apparatus.

The rate of addition of the premixed dichlorosilane monomers was

adjusted to maintain the gentle refluxing temperature of ~980C, since

the initial dechlorination reaction is highly exothermic. A typical

duration of the monomer addition was ~30 min. Upon completion of the

monomer addition, external heating was restored to achieve a gentle

reflux. The reaction flask was kept dark by wrapping it with aluminum

foil. The reflux continued for 1.5 hours before cooling the reaction

mixture to room temperature and then poured into an isopropyl alcohol

bath (PrOH) with stirring.

Fractionation of the reaction products was carried out by first

separating them in PrOH. The polymer fraction was precipitated out

while the oligomer fraction remained in the solution. The oligomer















--- N2 out


N2 in i .


Silane 1
mixture



SThermometer










Toluene+Sodium .
+ Silan ,.. .









Heating mantle


Fig. 11-2. Apparatus for Polymerization Reactions for
Synthesizing Polysilastyrene











fraction in PrOH was collected by distilling off the solvent by a rotary

evaporator. The excess sodium residue was decomposed in PrUH. The oli-

gomer fraction collected was redissolved in toluene and washed with dis-

tilled water three times in a 200 ml separatory funnel to extract any

residual salt product. Then the toluene solution of the oligomer was

rotoevaporated to obtain a viscous oligomer fraction of PSS (PSS-0)

which was kept in a brown bottle after vacuum drying and weighing.

The PrOH insoluble fraction was washed with 200 ml PrOH and with

200 ml EtOH twice after draining the PrOH by filtering. After the solid

polymer fraction was dried in a vacuum oven for five hours at 550C, it

was redissolved in warm toluene and the toluene insoluble fraction was

separated out. The toluene insoluble fraction was thought to be a

highly crosslinked polydimethylsilane. This fraction was washed with

water five times in a separatory funnel and dried in a vacuum oven at

80C for ten hours.

The toluene soluble fraction (PSS-P) in toluene solution was washed

with water five times to insure that all unreacted Si-C1 is hydrolyzed

out. This was done by titrating the effluent with AgNO3 solution. Then

PSS-P in toluene was reprecipitated in 7 liters of PrOH. The bright

white precipitate was collected by filtration and dried in a vacuum oven

at ~500C for ten hours. A pure PSS-P should appear as a white powder or

a clear, colorless solid.

The variations in reaction conditions for the subsequent runs are

given in Table 11-3.




















Table 11-3. Summary of Reaction Conditions for
Preparation of Polysilastyrene and Product Designation.


Mole Dichlorosilanes
Mole Na
Volume Toluene


Addition
Time, min.


0.5 m~ole each
2.05 mole
11


0.5 mole each
2.09 mole
11


0.5 mole MePhSiC12
0.4 mole Me2SiC12
0.1 mole Allyl MeSiC12
2.04 mole Na
1 1 toluene


Reaction
Time, hr.


Reaction
Temp., C


Product
I.D.



PSS-10
PSS-1P



PSS-20
PSS-2P



A-PSS-P
A-PSS-0










Infrared spectra for the polymerization products were taken by

using a Perkin-Elmer IR Spectrophotometer Model 283 with KBr Pellet in

transmission mode and also by a Nicolet MX-1 FT-IR Spectrophotometer in

diffuse reflectance mode. Proton NMR spectra were obtained by using a

Varian XL-100 with CDC13 or C6U6 as solvents without the TMS reference.

Molecular weight distributions of the products were determined by gel

permeation chromatography (GPC) using polystyrene as a reference in THF

solvent and using a refractive index detector.

Other PSS samples were provided by Shinnisso Kako Co. of Japan

through the 3M Co. (J-PSS1 and J-PSS2), courtesy of Dr. R. Sinclair and

also by Petrarch Systems, Inc. (P-PSS), courtesy of Dr. B. Arkles.

Vinylic silanes were provided by Union Carbide, courtesy of Dr. C.

Schilling. They are oligomer and polymer fractions of


Me Me

Me3Si {i 1 i }-SiMe3

H C=C


Two kinds of siloxane substituted PDMS containing a hydride functional

group or phenyl group were provided by Petrarch System, Inc., courtesy

of Dr. B. Arkles.

The structure of each polymer unit and physical state of all the

polysilanes used in this study are given in Table 11-4.

Crosslinking and pyrolysis

For crosslinking via y-ray irradiation, PSS was melt-coated on thin

stainless steel plates in glass test tubes with vacuum, Ar, N2, He,"air,










Table 11-4. Structure Formulas of Polysilane Unit and
Physical State of the Organosilanes at Room Temperature


Organosilane


Structure
Formula


-Me Me

- Si-Si

-Ph Me-


Me Me



-Ph Me-
(where m < n)


Physical State
At Room Temperature


White to dull yellow
solid





Yellow to brownish
viscous liquid


Allylic PSS

A-PSS-P

A-PSS-0




Vinylic Silanes
ViSP
ViSO


Siloxane PDMS
Hydride


--Si-Si--

TMe P



Me Me-

Me3Si tSiHSi- SiMe3
JLA Jx-ly


H

-Si--0- Si-- (SiMe2)8


Light yellow solid

Brownish viscous
liquid




Colorless clear
viscous liquid

Colorless clear low
viscosity liquid


Yellowish viscous
clear liquid


Ph

Phenyl -Si- O--1i--(SiMe2)8


Yellowish viscous
cloudy liquid


PSS-P






PSS-O










or N20 atmospheres. Some portions of the silane precursors were dis-

solved in benzene in glass test tubes and sealed for irradiation. The

vinylic silanes were placed in evacuated oorosilicate test tubes. The

glass tubes containing silane samples were irradiated with y-radiation

from a 60Co source at 1" distance for various lengths of time up to 29

days at room temperature.

Chemical free radical initiators (CFRI), benzoyl peroxide (BPO),

aszobisisobutyronitrile (AIBN), and dicumyl peroxide (DCP) obtained from

Polyscience Co. were recrystallized from methanol before use. A few

grams of silane were dissolved in 5-10 ml of benzene in a test tube or

in a 3-neck round bottom flask and then the silane solution was degassed

with an inert gas. After 30-60 min., a CFRI in the range of 3-10 wt%

was added under an inert atmosphere and the crosslinking reaction was

carried out with heating on a hot plate or by a heating mantle, as shown

in Fig. 11-3. The crosslinking reaction in the 3-neck flask was allowed

to reflux for twelve hours with continuous stirring before cooling to

room temperature. The crosslinked product was extracted and washed with

methanol.

Crosslinking via DCP was carried out also in a sealed Teflon con-

tainer for ViSP, ViSO, and PSS. About 1-2 g of vinylic silanes were

well mixed with 0.05-0.07 g DCP by a spatula under N2. Polysilastyrene

was also mixed with DCP after the polymer was made into a thick solution

in toluene. The silane + DCP mixtures were cured in an oven at 110-

1500C after the containers were tightly sealed. Other portions of poly-

silanes were cured without DCP under the same condition.

























Water
out


Y Thermometer









Reacting
mixture


Heating mantle

Magnetic stirrer


Fig. 11-3. Apparatus for Crosslinking of Polysilastyrene


N2 out
.0- -


N2 in










For crosslinking via Pt4+, a 1.2 x 10-4 M solution of Pt4+ was

prepared by dissolving 3.1 mg of H2PtC16.6H20 in 50 ml of a mixed sol-

vent of acetone (20 ml), ethyl ether (16 ml), toluene (10 ml), and EtOH

(4 ml). Crosslinking via Pt4+ was tried for A-PSS, ViSP, and ViSO pre-

cursors by adding 10-6-10-12 moles of Pt4+ to 0.05-0.4 g of the silanes.

Crosslinkings of Petrarch's siloxane PDMS were carried out by

adding drops of concentrated THF/H20 solution of Zn(Ac)2, SnCl4,

[Co(en)312(S04)3, SnC12, triethanolamine, ZrCl4, DCP, BPO, AIBN, Pt4+

and ethanolic NaOH followed by heating and curing in an air sealed glass

vial up to 1500C or heating in N2 gas up to 3000C.

Detection and confirmation of crosslinking of the polymers were

tested by using FT-IR, solubility in a solvent (benzene or THF), and

fusion at ~2000C. Pyrolysis of the polymers was carried out in a high

temperature furnace with an inert gas flowing at a rate of -100 ml/min.

and also in a DuPont TGA 951 Thermogravimetric Analyzer with N2 or Ar

continuous flowing with a heating rate of 10OC/min. Differential scan-

ning calorimetry (DSC) using the same DuPont Model 951 was carried out

in continuous Ar flowing with a heating rate of 5C/min.

Crosslinking reaction products of PSS via DCP were identified by

GC's in order to elucidate the reaction mechanism. The product gasses

were introduced into a Tracor GC 550 and an HP 5880A GC. The detailed

experimental conditions and sample preparation are as follows:

Approximately 0.1 g of PSS-1P was dissolved in 1.5 ml of degassed

benzene, and then ~0.01 g DCP added and mixed in a 20 ml glass vial. The

thick solution was vacuum dried to evaporate benzene at room temperature












for two hours. The dried sample (PSS/DCP) was placed in a glass vial

with a rubber septum or in a Pyrex glass loop directly attached to the

Tracor GC injection port. The samples in the glass containers were

heated to ~3000C by a Bunsen burner for 0.5-1 min. The product gasses

were either directly introduced to the silica gel column of a Tracor 550

through a valve or drawn by a syringe through the septum. The gas drawn

by a syringe was dissolved in a methylene chloride (MeC12) solvent; a

few microliters of this solution was injected into the capillary column

of an HP 5880A GC.

The GC parameters used are given below.

Instrument: Tracor 550 and HP 5880A interfaced to an HP 85
computer

Column: Silica gel 2.7 m 60-200 mesh and glass capillary

Detector: Flame ionization

Column T: 450C and programmed from 800C to 2500C at 200/
min.

Injector T: 1800C

Detector T: 1800C

Carrier Gas: N2

The overall experimental conditions for crosslinking of various OS

precursors are summarized in Table 11-5.

Infrared spectra, SEM micrographs, and EDS spectra were obtained by

using a Nicolet MX-1 FT-IR Spectrophotometer and a JEOL model JSM-35C

electron microscope, respectively. The assignments of IR bands are

based on reference number 56 and are given in Table 11-6.










Table II-5. Summary of Experimental Conditions
for Crosslinking of Various Polysilanes.


Means


y-ray
1-2.5 cm
distance


DCP
BPO
AIBN


DCP/y-ray
thermal

oxygen


y-ray


Solvent or
Atmosphere


Crosslinking
T, C


N20
vac.
benzene


N2
vac.
benzene
toluene
benzene
benzene/N2
vac.
air


100-250


RT
85-300

80


Length of
Crosslinking
Treatment

1-29 days






10 min-20 hrs


1 hr-5 days
10 min-4 days

3 weeks


air
Ar
vac.
benzene


1-20 days


Silane

PSS-P


PSS-0







Table II-5 (continued).


DCP
BPO
AIBN

thermal
oxygen


benzene
THF
N2
air
N2
air



toluene
N2
air sealed
air sealed
air sealed



air sealed



vac.
vac.

air sealed

vac.


20-250



150-300
800



100-300


70-170
70


70-300
120
65-150


70-300
RT

110
75
70-1500C


10 min-20 hrs



10 min-2 days
3 weeks



20 min-24 hrs


1-24 hrs
0.15-12 hrs


1-24 hrs
1-10 hrs
1-48 hrs


1-24 hrs
18 hrs-10 days

0.5-3 hrs
1-12 hrs
1-48 hrs


A-PSS


thermal


DCP
Pt4+


ViSP


ViSO


thermal
DCP
Pt4


thermal
y-ray
DCP
BPO
AIBN
Pt4+















Table 11-5 (continued).


Siloxane PDMS


ZnC14
SnC14.5H20
SnCIl
Zn(Ac)
[Co(en 312(S04)3
ethanolic NaOH
DCP
BPO
AIBN
Pt


air sealed


60-3000C


0-5 days










Table 11-6. Infrared Absorption Bands of Polysilanes


-- -1
v, cm



300

420

400-480

700-800

800-850

850-1000

1020

1100



1250

1400

1480-1580

1620

1600-1680

1710

1800

2100

2900

3050

3450

3630


Mode


Si-Si bending

H-Ph rocking

Si-Si stretching

Si-C stretching

Si-CH3 rocking

Phenyl C-H bending

Si-CH2-Si wagging

Si-O-Si stretching

Si-Ph

Si-CH3 bending

-CH3 deformation

Aromatic C=C

OH bending of adsorbed water

C=C aliphatic

C=0

-Ph

Si-H

C-H stretching in Si-CH3

C-H stretching in automatic

OH stretching

Si-OH stretching


Shape, Intensity



weak

weak



broad





shoulder

sharp

medium

strong

broad, strong

weak, sharp





medium

broad

sharp, medium

strong

narrow

broad

broad










Results

Characterizations of the precursor silanes

Some of typical IR, EDS, and NMR spectra are given in Figs. 11-4

through 11-19. The M. W. distributions, % yield of each fraction, and

MePhSi/Me2Si ratio for PSS are given in Table 11-7. The MePhSi/Me2Si

ratios were estimated by integrating the area under the peaks and nor-

malized by the number of protons in each group of the peak.

The oligomer fraction of PSS-1 in Fig. 11-4 shows some C-OH and

Si-OH (-3300 cm-1 and 3600 cm-1), Si-H (-2100 cm-1), possibly some un-

saturated carbon, i.e. C=C (-1600-1900 cm-1), strong and sharp Si-Me

stretch (~1250 cm-1), the strong and broad band for Si-O-Si, and Si-Ph

overlapped with Si-O-Si (~1100 cm-1), -Ph (-700 cm-1), and an Si-Si

stretching band at -450 cm-1. In PSS-1P, there is not as much Si-OH and

little C-OH, less Si-H, and a small but sharp peak for Si-Ph at -1100

cm-1 is shown (Fig. 11-5).

Figure 11-6 suggests that the oliyomer fraction of PSS-1 has a more

complex structure, indicated by multplets of the CH3 region (-1-2 ppm 5

scale) and the Ph-H region (8-9 ppm), than the corresponding structure

of the polymer fraction. It also shows a possible C=C band at -4.9 ppm

which is absent in the polymer fraction (Fig. 11-7). The peak at -2.6

ppm may be ascribed to to -CH2- or to Si-H.

Figure 11-8 for PSS-20 shows that a larger proportion of Si-H is

present in the oligomer, but less Si-OH and C-OH is present than in

PSS-10 (Fig. 11-6). Although PSS-10 and PSS-20 are both oligomer

fractions of PSS, the IR spectra show that they are not exactly the same











100


w -V
o -OH H 20
z60- 2

Si-Si
240-


S20 c- C=C in
o ph ]0-Si
Si-CI S-ph Si-CH3
0 I l I I I l ii
4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 400 200
WAVENUMBERS (cm")

Fig. 11-4. Infrared Spectrum of a Oligomer Fraction of Polysilastyrene ( PSS-10 )

























4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 400 200
WAVENUMBERS (cm')

Fig. 11-5. Infrared Spectrum of Polymer Fraction of Polysilastyrene ( PSS-1P )



























10 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
PPM ( )
Fig. 11-6. Proton Nuclear Magnetic Resonance Spectrum of PSS-10



















LJ.










10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
PPM (8)
Fig. 11-7. Proton Nuclear Magnetic Resonance Spetrum of PSS-1P




























10 1800 1600 1400
WAVENUMBERS(cm-<)


Fig. 11-8. Infrared Spectrum of PSS-20


100

180
z

H-60

z
< 40
0r

0~


200

























2000 1800 1600 1400 1200 1000 800 600 400 200
WAVENUMBERS (cm-)


Fig. 11-9. Infrared Spectrum of Allylic PSS Oligomer


100


























0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 I
PPM (8)

Fig. II-10. Proton Nuclear Magnetic Resonance Spectrum of Allylic PSS Oligomer











100


0
z
9
60 -

z




0 I I I I 1 I
<40





4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 600 200
WAVENUMBERS (cm-')

Fig. 11-11. Infrared Spectrum of Allylic PSS Polymer



























10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
PPM(8)

Fig. 11-12. Proton NMR Spectrum of Allylic PSS Polymer






















methyl-C


I I I I I I I I I I
10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
PPM(S)
Fig. 11-13. Carbon-13 NMR Spectrum of Allylic PSS Polymer


















t)

0







0 2.56 5.12 7.67
Energy
Fig. II-14. Energy Dispersive Spectrum of PSS






















Si-OH Si-Si


Si-H -ph

si-C 3
CH3 Si-CH3
C-H stretch

4000 3200 2400 1600 800
WAVENUMBER, cm-'
Fig. 11-15. FT-IR Spectrum of Solvent Insoluble Fraction of PSS-1

































WAVENUMBERS (cm-')


Fig. 11-16. FT-IR Spectrum of J-PSS1



















W
0
Z (D
Li
W I'
-j
LL
w





5-
I-

5600


Fig. 11-17. FT-IR Spectrum of Oligomer Fraction of Vinylic Silane (ViSO)


4400 3200 2000 1400 800
WAVENUMBERS (cm')


200










Methyl-H


Vinyl-H


10 9 8


7 6 5
ppm(S)


4 3 2 I


Fig. 11-18. Proton NMR Spectrum of Vinylic Silane Oligomer


- 1,
































10 9


7 6 5 4
ppm (8)


Fig. 11-19. Proton NMR Spectrum of Vinylic Silane Polymer


2 I






















Table 11-7. M. W. Distribution, % Yield,
and the Ratio of MePhSi/Me2Si for PSS


M. W. (in)
01 igomer
Polymer


390

10 x 103


344


% Yield
01 igomer
Polymer
Insol. Polymer


Ratio
MePhSi/Me2Si
01 igomer
Polymer


1.05
1.1


1.20
1.35


11 x 103


11 x 103


6.8 x 104


Silane


PSS-10

PSS-1P


PSS-20

PSS-2P


A-PSS-P

A-PSS-0


J-PSS1









compound; see qualitative differences at ~1720 cm-1, ~1580 cm-1, ~1250

cm-1, and ~450 cm1.

Figures II-9 and II-10 show that the allylic PSS oligomer has a

greater unsaturated carbon group, as indicated by the broader band at

~1600 crn-1; this is supported by the NMR data which show small humps

around 3-7 ppm. However, this cannot be conclusive because the bending

mode of water is also at ~1600 cm-1. Tne band shape at ~1450 cm-1 is

different from the nonallylic PSS. A significant amount of Si-O-Si may

be present in A-PSS-0. Groups including possibly Si-H and an allyl

group represent ~4% of the total protons based on the NMR data. Allylic

PSS polymer represented by Figs. II-11 and II-12 contains a smaller pro-

portion of Si-H relative to C=C. Carbon-13 NMR (Fig. II-13) does not

reveal any additional information.

Figure II-14 shows that only Si as a metallic element is present in

PSS-1P under EDS analysis.

Figure II-15 for the solvent insoluble fraction of PSS-1 presumably

due to the crosslinked network indicates that the polymer may be mainly

composed of

Me Me



Me Me

An infrared spectrum of J-PSS1 (Fig. II-16) obtained from the diffuse

reflectance mode of FT-IR indicates that Si-H is present, as well as

-OH, but not as much Si-O-Si is shown. An IR spectrum for ViSO (Fig.

II-17) obtained by the same way as for J-PSS1 shows a larger proportion

of Si-H. A small but sharp band at ~1600 cm-1 may be that of C=C. The









NMR spectrum of ViSO in Fig. 11-18 shows the presence of a vinyl group

at 6.2 ppm. It is difficult to see the presence of Si-H, although snall

humps between 1-4 ppm are shown. The NMR spectrum for ViSP (Fig. 11-19)

indicates that the polymer has essentially the same structure as the

oligomer ViSO.

Crosslinking and pyrolysis

Silanes exposed to a y-ray dose greater than 200 Mrad (~30 days at

1" distance from the source at 0.3 Nrad/hr) were infusible at tempera-

tures above 2U00C and were insoluble in THF or in benzene, indicative of

crosslinking. Additions of CFRI prior to y-ray irradiation made no

difference in the crosslinking reaction rate.

Fourier Transform IR spectra of PSS samples before and after 29

days of y-ray irradiation are shown in Fig. 11-20. The polymer film on

a stainless steel plate after 29 days of irradiation in vacuum showed

insolubility in THF and infusibility upon heating up to 2500C.

Gamma-ray irradiation of the vinyl silane oligomer in vacuum showed

an increase in viscosity within twelve days from a watery fluid to a

semisolid form. The vinyl silane upon heating at ~2000C for 10 min in

N2 was transformed into a light yellow translucent solid which was in-

soluble in toluene.

Gamma-ray irradiation of PSS in a N20 atmosphere for >11 days

changed the color of PSS from translucent yellow-green to bright red-

brown. The viscosity of PSS decreased sharply, indicating the degrada-

tion of the polymer.

Among the CFRI's investigated (BPO, AIBN, and DCP), only DCP in the

range of 2-15 wt% in PSS/benzene solution showed a positive crosslinking

































2C\J I f

J 0 ( PSS -6-29 days




.C Pi


2000 1733 1466 1199 932 665 39f
WAVENUMBERS(cm"')

Fig.II-20. Reflectance FT-IR Spectra of PSS Before and After
29 Days of V-ray Irradiation in Vacuum
liEt MES c'
Fig.II20. elcac TI pcr fPSBfr n fe









reaction. The CFRI DCP has the highest decomposition temperature of

~1500C of all other CFRIs. The FT-IR spectrum of DCP crosslinked PSS at

2500C compared with that of the as-synthesized PSS is shown in Fig. II-

21. The DCP reacted polymers were insoluble in benzene and infusible at

temperatures above 2000C. The density of PSS crosslinked by DCP measur-

ed by mercury volume displacement was 0.77 g/ml. Fourier Transform IR

spectra of A-PSS samples after they were treated at different crosslink-

ing conditions are compared in Figs. II-22 and 11-23. Oxygen cross-

linking by heating in the air at 25-800C came out negative for all PSS

precursors.

In the crosslinking of ViSP without DCP under the same conditions

as with DCP, no solidification was observed within 20 hrs. However,

curing at 300C higher temperature 140C resulted in solidification of

the liquid ViSP, signifying crosslinking.

The difference in the chemical structure of the ViSP samples cross-

linked thermally compared with ViSP samples crosslinked with DCP is

shown by FT-IR spectra in Fig. 11-24.

Chromatograms of the gaseous products from a crosslinking reaction

of PSS with DCP after being separated by a GC is given in Figs. II-2b

and 11-26. Approximately 100 times more methane is produced as compared

with ethane, as shown in Fig. 11-25.

Differential scanning calorimetry thermograms are given in Figs.

II-27 and II-28 to compare the crosslinking mechanisms. In Fig. II-29,

DSCs of oligomer and polymer PSS are compared. Scanning electron micro-

graphs of the surface of DCP crosslinked and pyrolyzed PSS and ViSP are

shown in Figs. II-30 and 11-31.

















































Fig. 11-21.


1733 1466 1199 932 665 398
WAVENUMBERS (cm-1)



FT-IR Spectra of PSS-1 and PSS-1 Crosslinked
With 8% DCP



































L/, cm-'


Fig. 11-22. FT-IR Spectra of Allylic PSS Before and After Crosslinking













Allylic PSS as Synthesizud


I I -v 4 4 +
q u 0.26g A-PSS/1.2x10 mo 1e t I L /! "




A 28g A-PSS/2./xl0 moli e'




2000 1750 1500 1250 1000 750 500

WAVENUMBER, cm-1

Fig. 11-23. FT-IR Spectra Showing the Crosslinking Effect of Pt44 Catalyst on Allylic PSS





















































WAVENUMBERS (CM4)


Fig. 11-24.


FT-IR Spectra of Crosslinked Vinylic Silane
Showing the Effect of DCP, Temp., and Time.
























0
r-


C3e

x



0 I 0 I 2 3
tR, MIN.

Fig. 11-25. Gas Chromatograms of the Gaseous Products From PSS/DCP
Crosslinking Reaction Showing Methane as a Major Product


















































A tR, MIN B


Fig. 11-26.


Gas Chromatograms of the Reference and the Reaction
Product From PSS/DCP Crosslinking Reaction Dissolved
in Methylene Chloride (MeC12). The Numbers on the
Peaks Are the Retention Times in Minute.

























0
C

in situ DCP
l> \ '--- . ; \ t -_A
i Crosslinking






SDC


Precrossli nked
ex situ
0
x






0 100 200 300 400 500 600


Temp., C
Fig. 11-27. DSC Thermograms of PSS1 and PSS1/DCP Before and
After the Crosslinking Reaction









60

















0

w


J PSS








,) 1-JPSS/DCP 5%
Sp recr.osslinked
\ex situ
0 Crosslinking





i 1 I I I I
0 100 200 300 400 500 600


Temp., C
Fig. II-28. DSC Thermograms of J-PSS2 and J-PSS2/DCP After the
Croslinking Reaction









61










/
/
I








PSS-1O/DCP
In




Crosslinking
+ /

















PSS-1P/DCP
I1 in situ
SCosslinking
'I I

L I

I I I


















0 100 200 300 400 500 600
oTemp., C







Fig. I-29. DSC Thermograms of PSS-1 Oligomer and PSS-1 Polymer
Reacting in situ With DCP





















































Fig.II-30. SEM Micrographs of PSS/DCP Showinx Blisters and Pores
Generated by Gas Evolution. Top: After Crosslinking and
Pyrolysis at 4000C in Vacuum, Bottom: After Crosslinking
and Pyrolysis at 9000C in Nitrogen




















































Fig.II-31. SEM Micrographs of ViSP/DCP Showing Pores and Surface
Texture. Top: After ViSP Crosslinked With DCP at 110C,
Bottom: After Crosslinking and Pyrolysis at 9000C in N2









The more effective crosslinking conditions among the techniques

tested on the various silanes are summarized in Table 11-8.

TGA thermograms of the precursor silanes are given in Figs. 11-32

through 11-37 to show the char yield of SiC. Fourier Transform IR spec-

tra of the pyrolyzed products are given in Figs. 11-38 through 11-42.

In Fig. 11-39, the SiC product from PSS/DCP is compared with the commer-

cial B-SiC Nicalon. The spectra show the characteristic absorption

band of Si-C stretching at 793 cm-1 along with a small Si02 band at

~1040 cm-1. An XRD powder pattern of PSS showed that the pyrolyzed

product is amorphous which is identical with Nicalon.

The char yield of PSS without crosslinking was less than 20 wt%,

which is close to the char yield of ViSO, while the char yield of the

PSS/DCP systems show 52-61 wt% SiC. The char yields of pyrolyzed prod-

ucts of SiC from various silane precursors are listed in Table 11-9. An

XPS spectrum for Si2P of SiC derived from ViSP in Fig. 11-43 shows -20

atom% oxide silicon on the surface of SiC indicated by an overlapped

peak at -107 eV B.E.



Discussion

Based on IR and NMR data, PSS-1P contains a low level of Si-OH

(-3400 cm-1) and Si-H (-2100 cm-1) bonds. Unsaturated carbon components

(~1600 cm-1) may also be present at a low level. In PSS-10, significant

amounts of Si-O-Si overlapped with Si-Ph, as shown by the broad absorp-

tion band at -1100 cm-1. The formation of Si-O-Si may be caused by

water used to hydrolyze the residual Si-C1 in the polymer and the alco-

hol solvent used for fractionation.











Table 11-8. Summary of Effective Crosslinking Conditions
Found for Different Silane Systems


Silane Means Effective Conditions

PSS-P y-ray vac. 29 days at 2.56 cm
RT

CFRI DCP 110-2000C
in 10 min-10 hrs
absence of oxygen


PSS-0 CFRI DCP, 140-2500C
in 20 min-12 hrs
absence of oxygen


A-PSS Thermal 3000C in 20 min
or 1700C in 14 hrs
absence of oxygen

CFRI 1500C, 20 hrs
absence of oxygen

Pt4+ 800C, 12 hrs


ViSP Thermal 150C, 24 hrs
absence of oxygen

CFRI DCP, 1200C, 4 hrs
absence of oxygen

ViSO Thermal > 200C, 72 hrs

y-ray 10 days, polymerization
not a crosslinking

CFRI DCP/1100C, ~3 hrs
or 750C, ~12 hrs
no oxygen


Ph CFRI DCP/N2, 300C
SiOMe 20 min
SiOSiMe2





























100



80
ui
w
0

w 60
o


=40 -
140



20
20 -
Fig. 11-32.0


Fig. 11-32.


400 60
TEMP. C


TGA Char Yields of PSS-10, PSS-10 After DCP
Crosslinking, and ViSO

























N
N













I
I



I I


200


400
TEMP.


ex situ
PSS/DCP 7%
----PSS
---ALLYL PSS


-7


600
C


800


1000


Fig.II-33.


TGA Char Yields of SiC From
After DCP Crosslinking, and


PSS-1P, PSS-lP
Allylic PSS


100


80 -


60 -


40 h-


20 1





















W100


c 80



S60


A-PSS


PSS-2


0 100 2-)0 300 400 500 600 OO 800 900 om00 1160


Fig. 11-34. Solvent Insoluble Fraction of PSS Showing Low Char Yields


I I I










69


















10C---




80
S ViSP/6% DCP


60




40














Fig. 11-35. TGA Char Yields of SiC for ViSP and ViSO After
Crosslinked With DCP
















100


80


w 60



40
P-PSS/5% DPCI


P-PSS as rec'd
0




0 100 200 300 460 50 600 700 80 00 1
TIMIr'P. C
Fig. II-36. TGA Char Yields of SiC for P-PSS Before and After Crosslinking
With DCP














----- PSS-lP/IO% DCP 250 ", 2hrs
-.-. 10% DCP in situ
J-PSS2/0% DCP, 150 C, 2hrs


100 200 300 400 500 600 1I
TEMP. '(


0) mI()O


Yt() I1 U I) I U


Fig. II-37.


TGA Char Yields of SiC for PSS' Showing the Effect of Time and
Temperature of the Crosslinking Reaction


60
'1-

: 40


1 20


1 I I I I I I I I I I


"" ~^ '^~


10(


80



















O

w

a-
0

oD

S II I I I I I II
1400 1200 1000 800 600 4(
WAVENUMBERS (cm-')
Fig.II-38. FT-IR Spectrum of DCP Crosslinked PSS-1P After Pyrolysis at
960 OC in Nitrogen Atmosphere


























>00 800 600
WAVENUMBER, cm-1


Fig.II-39. FT-IR Spectra of Nicalon SiC and SiC from DCP Crosslinked PSS






























WAVENUMBERS (cm-)
Fig.II-40. FT-IR Spectrum of J-PSS2/DCP After Pyrolysis at 1000C















w
z

^~ 10
c-I n

oen \- /Si-C
0




I I I I I I I I
5600 4400 3200 2000 1400 800 200
WAVENUMBERS (cm-')

Fig.II-41. FT-IR Spectrum of ViSP/DCP after Pyrolysis at 9000C

















ViSP SiC


0-


N iccca Ion@
SiC si-o
5600 4400 3200 2000 1400 800 200
WAVENUMBER, cm-
Fig.II-42. FT-IR Spectrum of SiC Obtained from ViSP/DCP Compared With the
Spectrum of Nicalon. Note the Absence of Si-O band in ViSP/SiC











Table 11-9. Char Yield of Pyrolyzed Products
from Various Silane Precursors


Crosslinking method
and Conditions

10% DCP in situ
10% DCP, 2500C
20 min, vacuum
7% DCP, 250C
10 min, N2
5% DCP, 1100C
10 hrs, air sealed


8% DCP, 2500C
20 min, N2


Pyrolysis
Method


Char Yield
%


TGA


8000C, 1 hr, N2


5% DCP, 150C, 12 hrs
air sealed


5% DCP, 1300C, 12 hrs
air sealed


TGA


Pt4+, 80C, 12 hrs
5% DCP, 170C


ViSO


TGA
900C, 1 hr.


TGA


4% DCP, 110C, 6 hrs,
air sealed


8% DCP, 150C, 12 hrs,
air sealed


PSS-10


Silane

PSS-1P


PSS-2P




J-PSS1


J-PSS2



P-PSS


A-PSS-P


TGA




















Table 11-9 (continued).


Crosslinking method
and Conditions

7% DCP, 300C
20 min, N2


Pyrolysis
Method

TGA


Char Yield


67


Insol. PSS


Insol. A-PSS


thermal, 150C, 24 hrs
3% DCP, 120C, 3 hrs
6% DCP, 130C, 4 hrs


H

Si-0S -Me2


Ph

Si-OSi-Me2


920C, 1 hr, N2
9200C, 1 hr, N2
TGA




TGA


8% DCP, 2500C TGA


Silane

PSS-20


ViSP





















= o /
0 I
(1)
O


100 100 90


120 o l10oo go
BE,eV
Fig.II-43. XPS Spectrum of ViSP/SiC Showing -20 atomic % Oxide Silicon as
a Contaminant









All of NMR spectra show slight excess of the PhSiMe unit in the PSS

copolymer chain over the MeSiMle unit, despite the intention to form an

equimolar copolymer. This must mean that the PhSiMe monomer unit is

more reactive than the MeSiMe unit during the polymerization reaction.

This means that in order to achieve an exactly equimolar copolymer of

PSS, one would have to use a slight excess of le2SiC12 monomer.

Hydrogen directly bonded to silicon in PSS comes from the fact that

the polysilane chain ends are probably anionic in the sodium/toluene

milieu and abstract hydrogen from the alcohol that was added to quench

excess sodium. Since the Si-H is hydrolyzable to give Si-OH, when the

Si-H ends find Si-OH ends, they form a Si-O-Si linkage by a condensation

reaction.
j1e
Although early workers of polysilane50-52 considered that Si from
Me
MeSiC12 was useless because of its insolubility, introduction of a

phenyl group improves its solubility and tractability.

As given in Table 11-6, all polymerization reactions carried out

produced ~3 times larger oligomer fractions than polymer fractions.

Although it is generally thought that polymer fractions are the desired

product in these reactions, the oligomer was successfully repolymerized

and crosslinked by using a DCP as a CFRI. This is an especially import-

ant finding since the oligomer fraction in a liquid state at room temp-

erature is more convenient to impregnate porous ceramic bodies in order

to strengthen them. David,56 who pioneered PSS, has unsuccessfully

tried to repolymerize the oligomer fraction by restarting the polymeriz-

ation reaction with sodium in toluene, sodium chloride, lithium metal,

lithium t-butoxide, potassium t-butoxide, or sodium with biphenyl as an









electron transfer agent. David56 clearly concluded that "all one can do

is to fractionate them out of a polysilastyrene product and discard

them, . ." Petrarch System, Inc., the only company that makes PSS

commercially in the U.S., follows this practice.57

Oligomers of PSS are believed to be in cyclic form so that the

crosslinking mechanism is expected to be different from the polymer

fraction by opening up the six membered ring. This is partly evidenced

by a higher TGA yield (67%) than that of a polymer (Figs. 11-32 and

11-33). The DSC data (Fig. 11-29) show that oligomers require a higher

temperature for crosslinking than polymers and the decomposition begins

at ~500C lower temperature than polymers. The high char yield of the

oligomers, however, was partly caused by the loss of volatiles through

the vacuum line during the ex situ crosslinking reaction with DCP.

The longer reaction time of PSS polymerization, e.g. 10 hrs, was

believed to degrade the formed polymer by excess hot sodium. However,

it appears that a longer reaction time than ~2 hours could have been

used to increase the polymer fraction of PSS. It is not yet completely

clear to what extent the longer reaction time improves the yield of the

polymer fraction. A systematic study of the effect of reaction time on

the M. W. of the silane products must be continued.

In the allylic PSS polymer, small amounts of Si-O-Si and Si-H are

shown (Fig. II-11). The unsaturated carbon component, probably from the

allyl group, is shown at ~1600 cm-1. The mole ratio of MeSiPh to MeSiMe

is 1.2:1. The 13C NMR (Fig. 11-13) did not reveal any more evidence of

the presence of an allyl group in A-PSS-P. Some indications of allyl

groups in A-PSS-O are also shown in Fig. 11-9. The group representing









Si-H and the allyl is shown as ~4% of the total protons in A-PSS-P (Fig.

11-12). Apparently, not all the allyl methyldichlorosilane ended up in

the products. A further study to account for this is needed.

The low solubility of J-PSS1, J-PSS2, and P-PSS in the solvents

must be a result of incomplete fractionation of the insoluble high M. W.

fraction. This variation in the M. W. distribution in a polymer is most

likely affected by the fractionation procedures. The consequence of

this was observed in the different behavior of PSS under the same cross-

linking conditions used in this study.

The average number M. W. (Mn) for different PSS precursors are in

close agreement, except that of J-PSS1. The GPC chromatograms show that

the PSS polymer is bimodal with a ~4 times larger lower M. W.

portion (1Mn 9 x 103) than the higher M. W. (Tn 3 x105).

The very high M. W. fraction that is insoluble in common solvents

and infusible upon heating, appears to be polydimethylsilane with some

Si-H, Si-Ph, and Si-O-Si, as shown by the IR spectrum in Fig. 11-15.

The linear

Me


I
Me

linkage apparently has been crosslinked via bridging oxygens of Si-O-Si

type. The TGA char yield of this fraction was <10% (Fig. 11-34), which

agrees with the result of Yajima et a1.54

In vinylic silanes, a large and sharp Si-H band at ~2080 cm-1 is

shown for ViSO in Fig. 11-16. The vinyl double bond appeared at ~1650

cm-1. However, the hydride proton is not apparent in NMR (Fig. 11-18).










This should be due to the relatively low concentration of the hydride,

but this obviously disagrees with the IR data. This point will be

discussed further in the latter part of this section.

An energy equivalent to 200 Mrad or greater y-ray irradiation

required to crosslink PSS is not unusual because of the phenyl group58

on the chain and the absence of an Si-H functional group.

In Fig. II-20 the effect of y-ray irradiation on tne PSS structure

after 29 days is shown and the sharp bands at ~700 cm-1 representing the

methyl group are lost. The DCP reacted PSS lost most of its IR bands

for Si-CH3, and Si-H (Fig. 11-21).

The enhanced crosslinking of carbon polymers in N20 under y-irradi-

ation, as shown by Okada,59 did not occur with silanes but rather the

opposite was observed.

Among the several CFRI studied, only DCP yielded an insoluble and

infusible solid of polysilanes. This is probably due to the active

methyl radical, which was not present in any other CFRI used. In the

crosslinking reaction, DCP has to be decomposed to give the methyl

radicals. This occurs at ~150-2000C.

Allylic PSS is observed to be more reactive under crosslinking con-

ditions than PSS. As shown in Fig. 11-22, A-PSS can be crosslinked

thermally or via use of the CFRI DCP. For thermal crosslinking, a temp-

erature >1700C is required for complete reaction. Allylic PSS is also

shown to be crosslinkable by Pt4+ catalyst. At least 2.4 x 10-7 mole

Pt4+ per ~0.3 g A-PSS was required for an effective crosslinking of

A-PSS, as shown by IR spectra (Fig. 11-23).









The crosslinking reaction of A-PSS must be between Si-H and C = C,

as shown in equation II-i.

catalyst
SSiH + C=C + Si-C-C-H (II-1)

The coupling reaction is catalyzed by Pt4+.60 The TGA yield of A-PSS

without a precrosslinking treatment (21%) is still greater than that of

PSS-P (12%). After being crosslinked with Pt4+, the yield increased to

35%. Differential scanning calorimetry (Fig. 11-44) shows that the

crosslinking between Si-H and --= occurs at ~240C. The CFRI DCP re-

quires temperatures >170C for complete crosslinking, but Pt4+ catalyzed

the reaction at a temperature of ~800C. This crosslinking reaction by

Pt4+ is unique to A-PSS and demonstrates the advantage of incorporating

an allyl group into polysilane synthesis.

Although a monomer with an Si-H functional group was not added in

the A-PSS synthesis, the small amount of Si-H at the chain ends was

still shown to be effective in the coupling reaction. However, inten-

tional small amounts of a monomer with a Si-H functional group, e.g.
H
C1 Si--Me should improve the crosslinkability even further.
2
Vinylic silanes can be crosslinked both thermally and via the CFRI

DCP. With DCP, the crosslinking is achieved faster and required a lower

temperature: 110C for ~4 hours as compared to 150C for 12 hours with-

out DCP. Without DCP, 150C for 12 hours treatment still did not pro-

duce complete crosslinking, as shown by the large Si-H IR peak in Fig.

11-24. This is also shown in Fig. II-45 with an expanded scale.

The vinylic silanes (ViSP and ViSO) received from Union Carbide

were reported61 to have both functional groups Si-H and -\\






85











t
o










0










0 100 200 300 400 500 600

Temp., 'C

Fig.II-44. DSC Thermogram of A-PSS-P Showing that the Thermal
Crosslinking Occurs at -240C
s







->/
^ y



i.I-4 \S /hrormo -S- hoigta h hr
Crslikn vcusat-4








0
0




0

SViSP,150'C, 12hrs


W


0w
-O


W 0




0
2400 2000 1800 1600 1400
WAVENUMBERS (cm')
Fig.II-45. FT-IR Spectrum of ViSP Showing the Effect of DCP
on Si-H Band Intensity and on Crosslinking










Although NMR confirmed the presence of -7 the presence of Si-H is

not certain. If ViSP and ViSO contain Si-H and -7 functional

groups, they should be crosslinkable via Pt4+ even more readily than

allylic PSS because -\\ is more reactive than However, this

was not observed. The vinylic silanes with a Pt4+ concentration greater

than 9 x 10-7 mole per 1 g silane and T > 8UC for several hours did not

solidify the liquid silanes. This is a question which cannot be answered

unless further work is performed.

The size of the Si-H peak in the IR spectra cannot directly be used

to estimate the degree of crosslinking because of the high bond energy

(-314 KJ/mole).54 This is shown by the Si-H IR band at -2080 cm-1 in

SiC after pyrolysis at both 10000C (Fig. 11-40), and 9000C (Fig. 11-41).

Morterra and Low62 also observed the growth of the Si-H absorption

peak when methoxylated aerosil was heated in a vacuum up to 750C, while

the absorption peak for the -CH3 stretching band at -3000 cm-1 decreased

as the length of heat treatment at 7500C in vacuum increased.

Nevertheless, it is shown in Figs. 11-22, 11-24, 11-45, and 11-46

that the degree of crosslinking appears to be a function of the Si-H

peak size. This is another area that needs to be further investigated.

A difference between vinylic and allylic silanes under crosslinking

conditions is in the reactivity of the functional groups. Vinyl groups

are more reactive than allyl groups by vinyls forming more stable radi-

cal intermediates. This was shown by the lower temperatures needed to

crosslink vinylic silanes. This advantage is somewhat curtailed by a

greater tendency of vinylic silane to be oxidized. Thus, one should





















































2000 1700 14
WAVENUMBERS(cm')


Fig.II-46. FT-IR Spectrum of a Region Showing the Effect of
Crosslinking on Si-H Band Intensities at -2080 cm 1









expect that in synthesizing more reactive silane precursors for cross-

linking there is the danger of introducing more oxygen contaminant in

the polymer and, thus, in the pyrolyzed product.

In Fig. 11-46, the as-received PSS shows a sharp and strong absor-

ption band for Si-H at ~2100 cm-1, a medium sized band for y-ray irradi-

ated PSS, and a small band for 10 wt% DCP treated PSS. This means that

PSS crosslinking can occur between Si-H's (Fig. 11-27), as well as oy

methyl free radicals from DCP at nigher temperatures. The bond energy

of = C-CH2-H is 418.4 KJ/mole63 and the bond energy of r Si-CH2-H should

be a little less than that of = C-CH2-H because of a greater electropos-

itivity of a Si atom than that of C atom. Still, the bond energy of

Si-H (314 KJ/mole)54 is much smaller than that of = Si-CH2-H, hence the

crosslinking of PSS by DCP proceeds with Si-H bonds breaking at ~150C

followed by formation of Si-C-C-Si linkages via methyl radicals at

~2500C. In Fig. 11-27, as-synthesized PSS shows a small exothermic peak

at ~160C, which probably corresponds to the crosslinking reaction via

Si-H. The DSC for J-PSS2 in Fig. 11-28 before the DCP treatment shows

negligible crosslinking via Si-H coupling during the heating schedule of

the DSC. Rearrangement of the polymer chain is thought to occur at

~400C. The small spikes at 1000C correspond to water evaporation. The

reason for sharp endothermic peaks at ~5200C is not known, but it is

thought to be the evaporation of a fraction of low volatility. After

the endothermic rearrangement, decomposition to eliminate H2, CH4, C6H6,

etc. actually begins to occur at ~4200C.

The minimum at ~2000C for PSS with in situ DCP crosslinking (Fig.

11-27) must be due to the decomposition of DCP. Under the heating rate










of DSC (50C/min), the DCP crosslinking reaction may not be able to keep

up with tne heating rate. The exothermic reaction was incomplete until

the temperature was ~2200C. This supports the previous observation of

incomplete crosslinking with DCP at temperatures below ~200C and the

low TGA char yield of the in situ DCP crosslinked PSS (Fig. 11-37).

In the DCP precrosslinked J-PSS (Fig. 11-28) no chain rearrangement

is evident. Instead of rearrangement, the decomposition begins at a

slightly lower temperature, ~400C. Tnis may mean that the molecular

rearrangements have occurred during the preceding DCP crosslinkiny

reaction. The primary chain rearrangement is probably a Kumada type,64

as shown in equation 11-2.

Me H
/
= Si-Si -+ + Si-CH2-Si = (11-2)

A pyrolysis GC study of PSS (Figs. 11-25 and 11-26) with DCP showed

a large amount of methane and acetophenone, which are some of the pro-

ducts from the proposed crosslinking reaction given in Fig. 11-47. The

amount of methane is too much to come from the Si-H coupling alone.

Thus, the methane must be formed by the methyl radicals of DCP after ab-

stracting methyl hydrogen from Si-CH3. The possibility of crosslinkage

via Si-Ph-Ph-Si is doubtful because of the greater bond energy for-_-H

(112 Kcal/mol) than for -CH2-H (104 Kcal/mol).65

During crosslinking and pyrolysis, a precursor polymer is decom-

posed and fragmented, preferably with free radical modes to achieve a

high ceramic yield. However, pyrolysis conditions strongly affect the

density of the pyrolyzed product. Density is increased during




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