Decaborane(14)-based polymers

Prog. t’olym. Sci., Vol. 21, 707-773, 1996 Copyright 0 1996 Elsevier Science Ldd

Pergamon

Printed in Great Britain. All rights reserved. 0079-6700196 $32.00

PII:

SOO79-6700(96)00002-O

DECABORANE(14)-BASED

POLYMERS*

S. PACKIRISAMY Polymers and Special Chemicals Division, Propellant and Special Chemicals Group, PCMEntity, Sarabhai Space Center, Thiruvananthapuram 695 022, India

Vikram

CONTENTS 1. Introduction 2. Polymeric Lewis base adducts of decaborane 2.1. Synthesis of polymers 2.1.1. Polymers derived from phosphorus-containing ligands 2.1.2. Polymers obtained from dinitriles 2.1.3. Polymers derived from diamines 2.1.4. Miscellaneous polymers 2.2. Thermal properties 2.2.1. TGA and DSC studies 2.2.2. Conversion of [B1aH12.G--L.], to [B&& by heat treatment 2.3, “B-NMR spectral studies 2.3.1. Structural changes in polar solvents 2.3.2. Structural changes on heat treatment 3. Polymers derived from carboranes 3.1. Polymers having carborane units in the main chain 3.1.1. Poly(carborane-siloxane)s 3.1.2. Poly(carborane-siloxane-acetylene) 3.1.3. Polyaryl(ether-ketone-carborane)s 3.1.4. Polyarylenes containing carborane units 3.15. Other polymers 3.2. Polymers having carborane units as pendant groups 3.2.1. Poly(carborane-siloxane)s 3.2.2. Polymers derived from vinyl, isopropenyl, acryloyl and ally1 derivatives of o-carborane 3.2.3. Polyphosphazenes containing carborane units 4. Decaborane-based polymers as precursors for ceramics 4.1. Polymers derived from carboranes as precursors for ceramics 4.1.1. Poly(carborane-siloxane)s 4.1.2. Poly(carborane-siloxane-acetylene) 4.1.3. Carborane-containing polymers devoid of oxygen 4.2. Polymeric Lewis base adducts of decaborane as precursors for ceramics 4.2.1. Ceramics derived from phosphorus-containing polymers 4.2.2. Ceramics derived from [BraHrz.diamine], 4.2.3. Preceramic polyblends 4.2.4 Metal borides 5. Decaborane-based polymers as atomic oxygen resistant coatings *Dedicated to Professor Mukul Biswas, Department occasion of his 60th birthday.

of Chemistry,

707

Indian Institute of Technology,

Kharagpur,

708 709 711 711 712 713 715 716 716 717 718 718 719 721 723 723 724 725 728 729 730 730 730 732 734 735 735 738 738 739 739 742 744 747 751.

India on the

708

S. PACKIRISAh4Y

5.1. Simulation of low earth orbit (LEO) atomic oxygen environment 5.2. Effect of atomic oxygen on siloxane coatings and [B10H12.~L.]n for atomic oxygen-resistant 5.3. Tailoring of poly(carborane-siloxane)s coatings 5.4. Mass loss measurements of coatings on exposure to atomic oxygen 5.5. Surface analyses of coatings 5.5.1. Scanning electron microscopy studies 5.5.2. Optical properties 5.5.3. XPS depth profile analysis 6. Concluding remarks Acknowledgements References

752 753 753 756 758 758 761 761 765 766 767

1. INTRODUCTION

The classic synthetic studies made by Alfred Stock’ in the early 20th century gave birth to a family of two classes of boranes having the composition BnHn+4and B,Hn+6. Theoretical and structural understanding of these boranes remained obscure until 1947 when the structure of diborane (B2H6) was resolved.2’3 Pioneering research work by Lipscomb and coworkers4” in the second half of this century on the theoretical analyses and X-ray diffraction studies of boranes led to a greater understanding of these compounds. Studies on boranes paved the way for major advancements of molecular orbital theory, viz. three-centered two-electron bonds and the extension of the principle of three-center and multi-center bonding to higher boranes in order to rationalize their observed geometrical structures and chemical stabilities. 4-7 Today, borane chemistry is no longer a laboratory curiosity and is involved in many interdisciplinary areas and technological developments. Besides diborane (B2H6), decaborane(l4) is the most studied boron hydride. The term “decaborane” refers to compounds having the molecular formulae BraHr4and BraHr,jand these two compounds are given the name, decaborane(l4) and decaborane(l6), respectively, where the numbers in parenthesis indicate the number of hydrogen atoms present in the molecule. Decaborane(l4) belongs to the series B,Hn+4which have non-closed polyhedral structures, designated “nido”-decaborane(14) (&o-the Latin word for a nest). Decaborane(l6) falls in the series B,H,+h and contains two BsHa units fused together and is designated conjuncto-decaborane(16). In this review we deal with nido-decaborane(l4) and for the sake of convenience through out the text it will be referred to as “decaborane”. Decaborane is synthesized by pyrolysis of diborane, B2H6. The synthetic procedures, structure and reactions of decaborane have been reviewed extensively. 4,8-11 The initial interest on decaborane has been to use it as a high energy fuel for military applications. The finding by Schaeffer12 in 1957 that decaborane reacts with Lewis bases (L) to form a diadduct, Br0Hr2h (a difunctional precursor), paved the way for its entry into the field of polymer science. A major breakthrough was made in 1963 when it was found that Br0Hr21+on reaction with acetylene gave carborane monomers, C2Br0Hr2,which have a 12 atom (10 B and 2 C) closed skeleton, an icosahedron.13F14Following this discovery, several polymers were synthesized from carboranes of which only poly(carboranesiloxane)s have gained technological significance. l5 Almost at the same time, it was found

DECABORAh’E(14)-BASED POLYMERS

709

that the use of bidendate ligands for the preparation of adducts of the B1OH1& type resulted in the formation of polymers. 16-r8However, these polymers did not gain much significance until recently when such polymers were evaluated as processable precursors for boron carbide, boron nitride and metal boride ceramics 19-31and as atomic oxygen-resistant coatings for space applications. 32-35 The present review deals with the development of polymers obtained from decaborane. The synthesis of these polymers can be broadly classified into two categories: (1) polymeric Lewis base adducts of decaborane, which are obtained by the direct synthesis involving decaborane and bidendate ligands, and (2) polymers derived from carborane monomers, which are in turn synthesized from decaborane. Sections 2 and 3 following the Introduction deal with the synthesis of the above polymers. Section 2 presents the synthesis and properties of polymeric Lewis base adducts of decaborane synthesized from various bidendate ligands. Among the carborane-based polymers poly(carborane-siloxane)s have been the major development and they find extensive applications as high temperature polymers. This topic merits a separate review and, in fact, it has been reviewed on various occasions. 15,36-39 Hence, a brief mention is made on this important class of polymers in Section 3 and a detailed account has not been attempted in this review. Section 3 also presents some recent developments on polymers derived from carboranes including poly(carborane-siloxane-acetylene), polyaryl(ether-ketone-carborane)s, polyarylenes and other polymers. Polymeric Lewis base adducts of decaborane and polymers derived from carborane are excellent precursors for ceramics as they have high boron to carbon ratios. Particularly, poly(carborane-siloxane)s are useful for obtaining mixed non-oxide ceramics, B&/Sic, which are advantageous in certain specific applications. On the other hand, polymeric Lewis base adducts of decaborane can be synthesized almost in quantitative yield in a single step and the variety of Lewis bases that can be used provides an opportunity for designing polymers with varying boron to carbon ratios. Recent investigations pertaining to the evaluation of polymers derived from carborane and polymeric Lewis base adducts of decaborane as precursors for ceramics are reviewed in Section 4. Carborane moieties present in poly(carborane-siloxane)s and the decaborane moiety present in polymeric Lewis base adducts can take up to 15 oxygen atoms on reaction with atomic oxygen. This results in an enormous weight increase countering shrinkage-induced cracks on coatings exposed to atomic oxygen. Tailoring of polymers suited to the above requirements and their evaluation as atomic oxygen-resistant coatings are presented in Section 5. Finally, based on the present survey of developments on decaborane-based polymers, an attempt has been made to identify future research directions. These remarks are presented in the concluding section (Section 6).

2. POLYMERIC

LEWIS

BASE ADDUCTS

OF DECABORANE

Decaborane reacts with Lewis bases (ligands, L) resulting in the formation of BrOH1& (Scheme 1). This reaction was discovered by Schaffer12 when he found that Br0H12(CH3CN)2precipitated from a solution of decaborane in acetonitrile. Following this discovery, a number of such derivatives were prepared using various nitriles,40’41 amines,42-46 phosphines,47-49 sulfides50951and other ligands.41,47’50’52-54

S. PACKIRISAMY

710

Decaborane(l4) 0:

Boron

D iadduc t

810 H14

BID H12L2

0: Hydrogen

Scheme 1. Formation of B10H12L+

W'12L2

t a'-

W',2l;

+

‘-2

Scheme 2. Ligand exchange reaction of B&J.+

nB&+n

:L-L:

-r

B&2:

L-L

j

n

+ nH2

Scheme 3. Synthesis of polymeric Lewis base adducts of decaborane(l4).

In the formation of B&I& hydrogen atoms are displaced by electron pair donor atoms at the 6 and 9 positions of the Bid& frame work. There is no polyhedral rearrangement during the reaction and the only structural difference involves the relocation of the B-H-B 3-centered 2-electron bridge while going from one nido-structure to the other.8 An alternative synthetic route to bis(ligand adducts) of decaborane is to employ the displacement of one ligand by another (Scheme 2), having a higher bonding strength 41,42,47,55-59 q=j., e d’rsp1acement reactions indicate the following increasing bonding strength of ligand, L:’ (CH3)2S < CHsCN < (C2HJ2NCN < (C2H5)&s = (C?H,S)J’ = HCON(CH& = CHsCON(CH& < P(OqH& = (C6H50)aP = (&H&N

z C,H5P(OC4H,),

= C5H5N = (C6H&P

An exhaustive list of bis(ligand adducts) of decaborane, BiOH1&, obtained by a direct reaction of L with Bi0Hi4 and by ligand displacement reactions, and their properties, are given elsewhere. ’ It is obvious that if a bidendate ligand is used in place of L for the synthesis of Bi&Ir&, a linear polymer can be obtained (Scheme 3). Following the first report by ParshaUl on the synthesis of a decaborane-based linear polymer from PQ, P’Q’-tetraethylethylenediphosphine (l), various polymers were synthesized from Lewis bases such as phosphines, nitriles and amines.

711

DECABORANE(14)-BASED POLYMERS

Table 1. Lewis bases used for the synthesis of [BIOH&---L.],, Reference

Sl. no.

Bidendate

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Et2PCH2CH2PEt2 Et*P(o-C&)PEt2 Et2P(l,4-CH2C6HSCH2)PEt2 Ph2PCH2PPh2 Ph2PCH2CH2PPh2 Ph2PCH2CH2CH2PPh2 Ph2PC=CPPh2 Ph2POPPh2 PhzPN=PPh2CHZCHzPPh2=NPPh2 Ph2PN=PPh2(1,4-C6H4)PPhz=NPPh2 PhzPNHNHPPhz NC(CH&CN NC(CH&CN NC(CH&CN NC(CH&CN NC(CH&CN H2NCH2CH2NH2 (CH&NCH&HzN(CH& (CH&NCHzCHzNHz 85% CH3NHCH2CH2NHCHd 15% CH3NHCH2CH2NHCH3 mixture HN(CHzCHzNH& NH(CH2CH2)2NH N(CH2CHzhN H2NCH2CH2CH2NH2 H,N(l,4-c&)NHz H2N(CH&Si(CH&OSi(CH&(CH2)3NHz H2NCH2CH20CH2CH20CH2CH2NHz H2NCH2CH2CHZOCH2CH20CH2CHzCH2NH2 H2NCH2CH2CH,0CH,CH20CH2CHzOCH&ZH2CH2NHz

21. 22. 23. 24. 25. 26. 27. 28. 29.

ligands

16,25 16 16 19 19 19 19 l&21,27,60 18,61 18,61 19 62 62 62 30,63 62 22,28 22,25,28 28,63 22.28 22 22,25 22 22 27 32 32 32 32

2.1. Synthesis of polymers The various bidendate ligands used for the synthesis of polymeric Lewis base adducts of decaborane are listed in Table 1. In general, these polymers can be prepared in organic solvents such as benzene, hexane, toluene, xylene, diethylether and tetrahydrofuran (THF). Most of the polymers are soluble in polar solvents such as dimethylacetamide (DMAc), dimethylformamide (DMF), N-methylpyrrolidone (NMP), hexamethylphosphoramide (HMPA) and dimethylsulfoxide (DMSO). The solubility of these polymers appears to depend on the nature of the bridging ligands. In selective cases, the polymers obtained are also soluble in ether-type solvents. 2.1.1. Polymers derived from phosphorus-containing ligands Parshall16 reported the synthesis of polymers from decaborane and diphosphines and used them in fuel compositions. In a typical example, decaborane and I were reacted in 1:l mole ratio in benzene at room temperature to obtain a polymer. Seventy parts of this polymer and 500 parts of sodium nitrate were ground together in a mortar. The mixture was not detonated by a sharp hammer blow and was apparently unaffected when heated to 200°C in a sealed glass tube. The mixture did not ignite when the tube was heated over an

712

S. PACKIRISAMY

nHOP(C6H5)2B,~y2p(C,HS)20H

+

2n(C21ds13N

~~~~~~~~H,),oP(c,H,)Il,, + Z~(C~H~)JN.HCI Scheme 4. Synthesis of [BIOH1&---L.],,

from phosphorus-containing

ligands. l8

open flame until the tube softened. However, smooth rapid combustion occurred when the mixture was ignited with a hot wire. This composition could be used as a rocket fuel. In another example, a blend of the above polymer and bis(tri-n-butylphosphine)decaborane (II) was found to be useful in a fuel composition containing potassium perchlorate. l6 In this composition the polymeric phosphinodecaborane in addition to acting as fuel ingredient, serves as a binder. The role of II in the above composition was to control the rate of fuel combustion and to improve the plasticity of the composition. Rees and Seyferth19 synthesized decaborane-based polymers using diphosphine ligands such as Ph2PCH2PPh2,Ph2PCH2CH2PPh2,Ph2PCH2CH2CH2PPh2,Ph2PC = CPPhZ and then investigated their pyrolysis to obtain boron-based ceramic materials, the details of which will be discussed later. Schroeder et al.,l’ while attempting to prepare bis(diethylaminodiphenylphosphine)decaborane, B10H12[Ph2PN(&H5)2]2(III), by treating bis(chlorodiphenylphosphine)decaborane, B10H1@h2PC1)2(IV), with triethylamine, obtained the bistriethylammonium salt of bis(hydroxydiphenylphosphine)decaborane, [B10H12(Ph2POH)2](V), and a polymeric material, [Br0H12.P(Ph)20P(Ph)&. Possibly, the above polymer containing the P-O-P linkage was formed by the condensation polymerization of IV and V, a compound easily obtained by hydrolysis of IV in organic solvents. Attempts to react equimolar quantities of IV and V in refluxing benzene under strictly anhydrous conditions did not result in the formation of the desired polymer and the addition of two molar equivalents of dry triethylamine to the reaction mixture resulted in the formation of the expected polymer (Scheme 4) in good yield. Molecular weight determination of this polymer by light-scattering in NMP gave a value of 27,000. In more recent years, Seyferth et aZ.19Y21investigated the conversion of this polymer to B& ceramics, the details of which will be presented in Section 4. Adopting a similar method as used for the synthesis of [Br0Hr2.PPh20PPh2.],, a polymer of the formula [B10H12.Ph~PNHNHPPh2.],was prepared by Rees and Seyferth19 through the reaction of IV and B10H12(Ph2PNHNH2)2. Decaborane polymers containing phosphonitrile bonding system were synthesizedr’ through the reaction of bis(azidodiphenylphosphine)decaborane, B1&Ir2.[P(Ph)2.Na12, with diphosphines in refluxing benzene (Scheme 5). 2.1.2. Polymers obtained from dinitriles Green et a1.62 mvestigated . the synthesis of polymeric Lewis base adducts of decaborane and NC(CH2),CN. In this series, when n = 0 there was no reaction. The rate of reaction with

DECABORANE(14)-BASED POLYMERS

nNj P(Phl~B&+W’d~

NdmP(Pt+R

-

713

P(Phlz

where R = CH2Cl+jl&-CsH, Scheme 5. Synthesis of [BI&.L--IL.],

from phosphorus-containing

ligands. I*

malononitrile, where IZ= 1, was very low. The reaction of other dinitriles with decaborane where n = 2,3,4 and 8 proceeded at a reasonable rate. It was postulated that steric hindrance was a contributing factor to the lack of reactivity when 12= 0 and 1. A resin was obtained from the reaction of equirnolar quantities of adiponitrile and decaborane, 62 which according to wet analysis, was composed of at least 10 repeating units. When excess decaborane was used, the product composition approached a 1: 1 molar ratio indicating much higher molecular weights. These solid products were readily plasticized with nitriles to yield gums and oils depending on the concentration of the plasticizer. In contrast to the high melting resin obtained from adiponitrile, the product obtained from the reaction of sebaconitrile and decaborane softened at 80-100°C. 2.1.3. Polymers derived fram diamines Initial attempts to synthesize polymers using diamines as ligands were made by Green et a1.62 They could obtain only a monomeric product containing decaborane and diamine units in 12 mole ratio. Cragg et aL41 reported the synthesis of polymers of the type where Y = methyl or ethyl. However, these polymers were [B~oH~~.Y~NCH~CH~~~.I~ reported to be insoluble. Soluble linear polymers using diamines as ligands were reported almost simultaneously by Johnson25 and by Seyferth and Rees.27 Diethylether is found to be a suitable solvent for the synthesis of these polymers. The polymers synthesized in diethylether contained small amounts of bound solvent that could be removed by prolonged heating at 140°C under vacuum. It is observed that the etherated polymers were more soluble than the ether-free ones.22 These polymers were mainly synthesized for use as potential precursors for boronbased ceramics, 227Z,27*28 which will be dealt with in some detail in the section on preceramic polymers. Molecular weight determinations using vapor pressure osmometry in acetone and DMF indicated a molecular weight in excess of 50,000 for [B10H12.(CH3)2NCH2CH2N(CH3)2.]n (VI) and [BroHr2.H2NCH&H2NH2.1n(VII). GPC studies of the former in DMF using polystyrene as a standard, suggested a molecular weight above 100,000. Dynamic laser light scattering studies of this polymer in HMPA gave a molecular weight of 325,000. The above observations suggest that high molecular weight polymers are formed in the reaction of decaborane with diamines. 22T28 It is interesting to note that the polymers precipitate out of the reaction medium even in the very early stages of the reaction. It is believed that one of the requirements for the formation of high molecular weight polymers is that the growing polymer chain should remain in solution during the reaction. Premature precipitation of

714

S. PACKIRISAMY

CH,

CH,

BIoH~~~YN-CH~CH~CH~~I-O-~~-CH~C~CHZ-NHZ. &H:, Bx, Hjz*H?N-

8,0H,rHzN

hHj

C~C~0CH2CH20CH2C~-NH2

[ B,oH,,’ H, N - CH2 CH,CH,OCH,

t

( DBP-1

3

CH20CH2 CHIC$-dn

-CH2CH2CHzOCYCH,CXH2CHZOCH2Ct+CH2-NHf

Fig. 1. Chemical structures of [B1&12.L--L.]n

)

I n

containing

n (

(

DBP-2) D B P-3

)

1 n(DBP-4) siloxane and ether linkages. 32

the growing polymer chain from the reaction medium would result in low molecular weight polymers. Thus, the formation of high molecular weight polymers from the reaction of decaborane with diamines was not completely understood. If the molecular weight data are any indication, they suggest a very rapid build-up of molecular weight before the precipitation of polymeric chains in the reaction medium. It is believed that diadducts of decaborane are formed as reaction intermediates in the initial stages, which further react with decaborane resulting in the formation of high molecular weight polymers. Recently, Packirisamy and Litt32,33reported the synthesis of polymers from diamines containing flexible Si-0-Si or ether linkages in order to obtain polymers (DBP-1,2,3 and -4; Fig. 1) which are useful as atomic oxygen-resistant coatings. Wide angle X-ray diffraction (WAXD) studies of the polymers indicated that they are amorphous in nature. Comparison of the WAXD of [Br0H12.H2NCH2CH&H2NH&,(VIII) with those of DBP3 and -4 reveals that the incorporation of ether linkage reduces the ordered structure present to some extent in the former. The coatings obtained from the former tend to develop cracks on bending whereas flexible coatings were obtained from the polymers containing siloxane and other ether linkages. The polymer (DBP-1) obtained from 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (IX) was soluble in THF and in polar solvents. The polymers containing ether linkages (DBP-2,3 and -4) were insoluble in THF but were soluble in a 1:l mixture of THF and DMAc or DMF. The possible end-use of these polymers as atomic oxygen-resistant coatings will be discussed later. Reaction of decaborane with a mixture of diamines results in a copolymer. The ‘H-NMR spectrum of a copolymer obtained by reacting decaborane with 1:l (molar) mixture of H2NCH2CH2NH2 and (CH&NCH2CH2N(CH$ suggests a random distribution of the diamines. 22 Polymers were also prepared using a mixture of ethyl-substituted decaboranes22 that had been synthesized by Friedel-Craft’s ethylation of decaborane and found by gas chromatographic analysis to consist of 6.9% EtB 1,,H i3, 81.4% Et2Br0Hr2, 10.4% Et3Bn,Hr1, and 1.3% Et4BiOHr0.This mixture was used to prepare polymers such as [EtnBrOHiZ_n.WfWW3N.lm [Et,Br,&-,.Me2NCH2CH2NMe2.], and {[B10Hi2.Me2NCH&H2NMe2.]1,8[Et*nB10H12_..Me2NCH2CH2NMe2.]1.~}.

DECABORANE(14)-BASED POLYMERS

f

O-C-N-&&2---_F-O-B~H123;1 I I

ri

cl+J

Scheme 6. Synthesis of decaborane-based

715

CH3 H

polymer from B10H14and dimethylacetamide.

Q

2.1 A. Miscellaneous polymers

Polymers from dimethylacetamide and dimethylformamide: Green et aZ.62reported the synthesis of polymers obtained through the reaction of decaborane and ethyldecaborane with DMAc. When DMAc was allowed to react with decaborane or ethyldecaborane in equimolar quantities at 50-60°C in an inert atmosphere, a homogeneous red solution was obtained that turned darker and increasingly viscous with time. Finally, a solid product was obtained. The polymerization presumably takes place through the involvement of carbonyl group forming C-O-B bonds (Scheme 6). No evidence was provided by the investigators supporting the structure suggested. The solid resin obtained in the above reaction could be plasticized with additional ethyldecaborane to give a gum-like product. Products of varying consistency ranging from syrups to gums to resins were obtained depending upon the quantity of ethyldecaborane charged. Johnson24,26synthesized a polymer by reacting decaborane and DMF. During the reaction initially a bis(dimethylformamidato) complex of decaborane precipitated in the reaction medium which formed a polymer on heating at 100°C. The polymer did not precipitate out of the reaction medium (DMF) at concentrations below 1 M. A preceramic polyblend was obtained by dissolving polyacrylonitrile in this solution. Polymers containing Bi0H8 units: the synthesis of a series of polymeric Lewis base adducts containing BloHs units was disclosed in a patent by Drinkard.@ (X). This com{BIoHQ.]S(CH3)212) on reaction with ammonia affords [Nl$ ]2[B10H10]2pound can be converted to BlaHs(N& (XI), a diazonium compound, by treating with nitrous acid and reacting the resulting product with a reducing agent such as the alkali metal tetrahydroborates and metal acid combinations or catalytic reducing compounds. XI on reaction with N,N, N’,N-tetraethylterephthalamide (XII) in a nitrogen atmosphere at 95-115°C for 3.7 h followed by heating at 150-160°C afforded a polymer (Scheme 7). Alternately, this polymer can also be synthesized by reacting X with XII at 140°C for 1 h in a nitrogen atmosphere followed by heating at 150-160°C under reduced pressure for 2 h. During this reaction, ammonia and hydrogen were evolved. The syntheses of several such polymers having B10H8units were described in this patent using different synthetic procedures. These polymers can be used for preparing molded articles for use as light-transparent neutron barriers or space vehicle windows, which need to have resistance to outer-space radiation. In addition, they can be used as components of solid, high-energy fuels, as primers on polymeric substrates for adhesion improvement and as surface coatings.64

716

S. PACKIRISAMY

nbH6(N2

h+ dC2%

12 N

Scheme 7. Synthesis of B10H8 unit containing polymer.64

2.2. Thermal properties The polymeric Lewis base adducts of decaborane synthesized in the 1960s were poorly studied for their thermal properties. However, in recent years, considerable work has been done towards understanding their thermal behavior in order to use them as precursors for ceramics.

2.2.1. TGA and DSC studies In general, [B1aH12.~L.]n polymers give char residue in the range 60-90% (in inert atmosphere). The TGA curves of VI22 and [BIOHr2.NC(CH2)&N.], (XIII)31 are shown in Figs 2 and 3, respectively, as examples. For VI major weight loss occurs around 400°C. There is very little further weight loss above 550°C. Similar observation has been made in the case of XIII. TGA studies of this polymer shows initial weight loss occurring at 150°C followed by a gradual weight loss in the region 350-500°C. In agreement with this observation, a strong exotherm was found in the DSC trace of this polymer corresponding to the first weight loss. The ceramic conversion reaction was complete at 500°C with a resulting 86% ceramic yield. Studies on the thermal properties of siloxane and ether linkage containing polymers DBP-1,2,3 and -4 suggest that the overall thermal stability of the polymers depends on 100

cq

75

E .P

60

5 25

0 60

150

240

330

420

510

Temperature

600

690

790

870

960

(*C)

Fig. 2. TGA curvesz2 of [B&2.(CH&NCH&H,N(CH&.]~(-)

ad

[HN(CH3)2CH2CH2(CH3)zNH12+[BloHlo12-(- -).

DECABORANE(14)-BASED

Q P 5 .F e 3

POLYMERS

717

80 70 60 50 40

I

0

200

1

1

400

600

Temperature

M)o

(‘c)

Fig. 3. TGA curve of [Bl,-,H12.NC(CH&CN.]..

the ligand. 32135 DSC curves show exothermic effects in the temperature range 175-225°C for DBP-2,3 and -4. Almost in the same temperature range, weight loss is observed for these polymers in the TGA curves. The peak temperatures of the exothermic peaks in the DSC curves of DBP-2,3 and -4 decrease with an increase in the ether linkage and C content. TGA studies of these polymers indicate that the initial decomposition temperature and the overall thermal stability decrease with an increase in the ether and -CHz- linkages. Interestingly, no sudden exothermic effect is observed in the DSC curve of DBP-1. TGA curves of the polymers show that DBP-1 is stable up to 400°C whereas the ether linkage containing polymers are stable up to 180-200°C. The improved thermal stability of DBP-1 is attributed to the presence of siloxane linkages in the polymer. These polymers give a char residue of 70-83% at 800°C in a nitrogen atmosphere. DSC and TGA studies suggest the following trend for the overall thermal stability of these polymers: DBP-l>DBP-2>DBP-3>DBP-4

2.2.2. Conversion of [B&YlpL --L.],,

to [B&Tl,]2- by heat treatment

It is generally the case that linear polymers, in the absence of thermolabile functionality that would serve to crosslink them during the initial stages of pyrolysis, will not give high anaerobic char residue when they are pyrolyzed. In most of the polymers, thermal scission results in the evolution of small, usually cyclic, molecules that are swept out of the hot zone by the carrier gas, leaving behind little or no ceramic residue. Polymeric Lewis base adducts of decaborane are linear and hence, are expected to behave in a similar way. The fact that these polymers lose very little weight on pyrolysis suggests the presence of “thermolabile functionality”. In the case of [BIOHi2.~--L,ln polymers, the acidic bridging B-H-B bonds of the B1&112.L2 cage unit may serve as “thermolabile” functionality. On thermal treatment, the B10H12.k cage unit may be converted into (LH’)Z BloH&. llB-NMR spectral studies of the product obtained by heating VI for 6 min at 210°C suggest the formation of [(CH3)2NHCH$H2NH(CH3)J2+[B10H10]2-(XIV). XIV on

718

S. PACKIRISAMY

pyrolsis to 1000°C (lO°C/min, in argon) produced a ceramic material identical to that obtained in the pyrolysis of VI. In Fig. 2, TGA curves of VI and XIV are compared. It is seen that the TGA curves of the polymer and the corresponding salt are almost identical. It is likely that intermediate heating to cu. 200°C converts the initially covalent, linear polymers via proton-transfer reactions (which will be discussed later) to ionic salts containing the very thermally stable [B10H10]2anion. The salt is not volatile and hence, remains in the hot zone as the temperature is increased to 1000°C and is ultimately converted into the final boron carbonitride ceramic product. 2.3. “B-NMR spectral studies “B-NMR studies for the polymers [B1&112.~L.]n indicate that the BraHr2cage units present in these polymers are identical to the ones present in B1aHr2.c. “B-NMR of [Br0Hr2.diamine], polymers showed broad resonances with a general pattern of 6 = 0 to 5, -15 to -20 and -35 to -40 ppm in an 2:2:1 integrated intensity.22 2.3.1. Structural changes in polar solvents Seyferth and Rees22 reported that decaborane-based polymers undergo degradation in DMSO. However, a detailed study on the effect of solvents on the degradation of [Br0Hr2.L---L.], polymers has not been reported. Recently, Packirisamy and Litt32Y35 studied the effect of various polar solvents on the stability of these polymers using ‘lB-NMR. rlB-NMR spectra of VI, VIII, and DBP-1,2,3 and -4 were recorded in DMAc, DMF and DMSO at various time intervals. The llB-NMR spectra of DBP-1 in DMF and DMSO are given in Fig. 4 as an example. It is observed that this polymer undergoes degradation in these two solvents resulting in the formation of more than one degradation products. A prominent peak in the region 6 = -29 to -30 ppm is attributed to the (B1,-,H10)2salt. The nature of the other degradation products is not clearly understood. The

Fig. 4. “B-NMR spectra35 of DBP-1 in (a) dimethylsulfoxide and (b) dimethylformamide.

719

DECABORANE(14)-BASED POLYMERS

I

I

1

I

I

20

I

I

-30

I

I

I

-10

6(rwm1 Fig. 5. “B-NMR

spectra3’ of the reaction mixture of decaborane and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane in THF after (a) 90 min, (b) 24 h and (c) 17 days.

degradation is usually faster in DMSO. Similar degradation behavior is observed for the case of VI, VIII and DBP-2,3 and -4. In an attempt to understand if [B1,,H1$ is formed during the synthesis of the polymers, a “B-NMR spectrum of the reaction mixture of decaborane and IX in THF was recorded at different time intervals (Fig. 5).32 This study suggests that there is no salt formation initially as evidenced by the absence of a prominent peak at 6 = -29 to -30 ppm. A small amount of salt is formed on standing. However, the reaction product undergoes changes as evidenced by the llB-NMR spectra. The reason for this behavior is not clearly understood. Probably on standing, the decaborane-diamine complex incorporates oxygen. Similar studies could not be carried out with other decaborane-based polymers in THF as they were insoluble. The above study suggests that polar solvents favor proton transfer from the B1eH12cage of a BloH12~ complex to the ligand and, in a less polar solvent, such a proton transfer is less favored. The proton transfer mechanism (which will be presented later) proposed by Hawthorne and Pitochelli65 is probably operative in polar solvents. 2.3.2. Structural changes on heat treatment Seyferth and Rees22 analyzed the products obtained by heating VI in refluxing xylene using llB-NMR (Fig. 6) in an attempt to understand the intermediates formed during pyrolysis. It was observed that under the above conditions this polymer undergoes conversion to XIV in more than 80% yield. Such proton transfer from the BloH12cage of a B10Hr2.k complex to the ligand had been demonstrated by Hawthorne and Pitochelli,65 and the following mechanism, in the case of a basic amine ligand, was shown to be operative (Scheme 8).

720

S. PACKIRISAMY

1

1

0

l

20

I

0

-20

*

A_

-40

d(PPm)

Fig. 6. “B-NMR spectraz2 in dimethylformamide: (A) [BI,&.(CH&NCH2CH~N(CH&.]~, (B) sample A heated for 1 h in refluxing xylene, (C) sample A heated overnight in refluxing xylene and (D) Sample A heated for 1 day in reflwing xylene.

‘lB-NMR spectral studies thus suggest that [BIOH&---L.],, particularly in the case of a basic amine ligand, undergoes conversion to [HL--LH]2’[B1&110]2- on standing in polar solvents or on heat treatment. In the above transformation, BroHr2 units having an open cage (nido) structure change to BloH& having closed cage structure (Fig. 7).9,22The formation of such structures during thermal treatment enhances the overall thermal stability of the polymers as discussed earlier. ~HQ.~L m

BI&L

+ L

B&?L + base -

B&,-+lbo

BaHt+bax

&H$+@stst-ij+

‘%

tj++ L

Scheme 8. Proton transfer 22*65 from B10H12b to the ligand, L.

721

DECABORANE(14)-BASED POLYMERS

d-&f7% Fig. 7. Structurez2 of [BI$IIa]2-.

3. POLYMERS

DERIVED

FROM

CARBORANES

The term ‘ ‘carborane” is used to refer to the compounds composed of boron, hydrogen and carbon in which both boron and carbon atoms are incorporated into three dimensional, polyhedron skeletons. They can be classified into three major groups: close-, nido- and arachno-carboranes. The close-carboranes have the general formula CaBn_aHn_a+z. The most common members of this class have a = 2, i.e. C2Bn_2H,,and one or both of the C can be replaced by an isoelectronic B- to give CB, _ ,Hi and B,Hi-, respectively. Likewise C can be replaced by an isoelectronic BH unit to give CB,_lH,+l. Nido-carboranes and arachno-carboranes have the general formulae CaB,_,H,_,+4and CaB,_,H,_,+6,respectively. In this review we deal with close-carboranes having the formula CzBloHlz which are derived from decaborane. The diadduct of decaborane (B1OH1&) can react with acetylene

L

L

HC’=CH L

Diadduct BloHl&

+ ZL:+ H2

Ortho-carborane k2-C2

0

Boron

0:BH

l

Hydrogen

l :CH Scheme 9. Synthesis of o-carborane.

w-42

722

S. PACKIRISAMY

Ortho-urboraqe

Metr-carbordnc

42 -C2 810 H12

0: BH

.:

1.7-C2&)

Pard - carborane

H12

1,12-C2 BloH12

CH

Scheme 10. Isomerization of o-carborane to m- and p-carborane.

to form a 12 atom closed skeleton, an icosahedran, that contains 10 boron and two carbon atoms across the top of the decaborane basket as shown in the Scheme 9.13,14This compound, 1,2-dicarba-&so-dodecaborane (12) was named o-carborane. On heating at 470600°C in an inert atmosphere, o-carborane undergoes an irreversible thermal reaction resulting in the formation of the meta isomer [1,7-dicarba-&so-dodecaborane (12)] in nearly quantitative yield66-72 (Scheme 10). Further heating to about 650-700°C in inert atmosphere results in another isomerization forming p-carborane [1,12-dicarba-closododecaborane (12)] 68T69,73,74 (Scheme 10). These isomers of carborane (Br0Hr2C2) are normally represented as indicated below:

o-carborane

m-carborane

p-carborana

Derivatives of o-carborane substituted at carbon can be prepared by two general methods. The first method involves the reaction of substituted acetylenes with bis(ligand) decaborane compounds. 75-78In the second approach, metallation of o-carborane is carried followed by treatment with reagents such as carbon dioxide, halogens, organic out 14379-81 and inorganic halides, epoxides, aldehydes and others to obtain mono- and di-C-functional o-carboranes. Derivatives of m- and p-carborane are obtained mostly by following the second approach. The carborane cage is generally stable towards organic reactions and it is possible to carry out a variety of reactions on substituent groups attached to carborane A detailed account of the syntheses and properties while leaving the cage system intact. 9~15 of mono- and difunctional derivatives of carboranes is provided elsewhere. l5 Several polymeric systems containing carborane moieties have been synthesized from carborane derivatives. These polymeric systems can be broadly classified into the following two groups: (1) polymers having o, m- and p-carborane in the main chain and (2) polymers in which carborane cages are attached to the main chain as pendant groups.

DECABORANEQ4)-BASED

HC -

CH

Cqb&Li c

\w B1o HIO

LiC -CLi ;f/w

723

POLYMERS

(CH3 12 SiC’2,

Cl(CH312SiC ,--~~l(CHJ)2Ct ’

BtoMo

CI(CH3)2SiCBloHIo

CSi(CH312CI

CH30(CH3)2SiCBw

HioCSi(CH312

Dexsil 200

0CH3

elastomer

Dexsil 100 crystalline pdymer

Dexsil 300 elastomer Scheme 11. Synthesis of Dexil polymers.

3.1. Polymers having carborane units in the muin chain 3.1.1. Poly(carborane-siEne)s The development of exceptionally heat-resistant poly(m-carborane-siloxane)s by a group of chemists at the Olin Corporation was a major breakthrough in the early 1960s. This family of polymers is known under the trade name Dexil. They consist of dialkyl or diarylsiloxane units linked by m-carborane cage structures, with a small portion of lvinyl-o-carbonyl side groups to promote crosslinking.82-85 The Dexil polymers are obtained by the condensation polymerization of bis(methoxydimethylsily)-m-carborane with bis(chlorosilyl)-m-carborane derivatives, alkylchlorosilanes or alkylchlorosiloxanes in the presence of a catalyst83-87 (Scheme 11).

S. PACKIRISAMY

724

0

200

400

600

Temperature

000

1000

PC )

Fig. 8. Effect of the siloxane content on thermal stability (in argon at S”C/min.) of poly(carborane-siloxane)s39, {Si(CH3)2CB10H10CSi(CH3)2_o-[Si(CH,),-o-}~: A: x = 0, B: x = 1, C: x =i2, D: x = 3, E: x = 4, F: x = 5, G: x = 6, H: x = 80 and PDMS: polydimethylsiloxane.

The presence of the carborane moiety in the backbone of polysiloxanes blocks ring forming depolymerization reactions by which polysiloxanes normally degrade on extended heating. Hence, the poly(carborane-siloxane)s show higher thermal stabilities and improved resistances towards high temperature degradation, particularly in inert atmospheres. The thermal stabilities of carborane-siloxane polymers is governed by the relative quantity of the carborane moiety present in the polymer. It is observed in Fig. 8 that the onset of degradative weight loss and the amount of residue formed after degradation are gradually increased when the carborane content is increased or, in other words, when the siloxy content is decreased. 37,38,88 A recent review by Dvornic and Lenz39 deals with the synthesis of carborane-siloxane polymers by various synthetic routes, their thermal properties and their applications as high temperature elastomers. Hence, a detailed account of this vast research area is not presented in this review. 3.1.2. Poly(carborane-siloxane-acetylene) As discussed above, poly(carborane-siloxane) elastomers show superior thermo-oxidative properties. It would be desirable to incorporate functional groups into the polymer backbone that could be utilized for crosslinking to obtain thermosetting resins. Henderson and Keller89Yg0 reported the synthesis of poly(carborane-siloxane-acetylene) (XV) (Scheme 12). Incorporation of acetylenic groups in the poly(carborane-siloxane) backbone provides many advantages. The acetylenic group remains dormant during processing under ambient conditions. During reaction by thermal or photochemical means, a crosslink is formed by an addition polymerization reaction without the formation of volatile by-products. XV was synthesized following the method used for the preparation of silylene-diacetylene polymers.91’92 Equal amounts of dilithiobutadiene and 1,7-bis(chlorotetramethyldisiloxy)-m-carborane were reacted to generate a linear polymer in good yield. This polymer (XV) is soluble in most common organic solvents. GPC analysis indicated the presence of low molecular weight species (500) as well as high average molecular weight

DECABORANE(14)-BASED

POLYMERS

725

Cl

Cl

i=i=c Cl

Cl

Cl

-

n-BuLi

Li

Cl

X

ZLI

/

CH3

I

CH3

CH3

CH3

I I Cl-Si-0-Si-CB10

I HloC-Si-0-Si-Cl

I

I

CH3

CH3

CH3 I

I

CH3

/ CH3

--

-f-

I

Si-

CH3 o-si

1

CH3

ht Ceramic

I -

CBIO

HIO C -Si

-O-S1

1

CH3

\t -hclThcrmoscl

or light

Polym~

Scheme 12. Synthesis of poly(carborane-siloxane-acetylene).

89

polymers (MW = 4900 and MN = 2400), averaging about 10 units. Heating to 150°C under reduced pressure removed the lower molecular weight polymeric species, leaving a 9295% overall yield. The liquid linear polymer XV is readily converted into a thermoset through thermal crosslinking. A shiny void-free dark brown solid was produced by thermal curing of XV at 300, 350 and 400°C for 2 h at each temperature either under inert conditions or in air. Gelation occurred during the initial heat treatment at 300°C. An FTIR spectrum of the above crosslinked polymer shows the disappearance of the acetylenic absorption at 2175 cm-r and the appearance of a new weak peak around 1600 cm-’ (C=C). A crosslinked polymer can also be obtained by the UV irradiation of XV. In the DSC curve, a small exotherm was apparent from about 150-225°C and was attributed to the presence of a small amount of primary terminated acetylinic units. This peak was absent in XV heated at 150°C for 30 min. A large broad exotherm commencing at 250°C and peaking at 350°C was attributed to the reaction of the secondary acetylenic units to form the crosslink. This exotherm was absent after heat treatment of XV at 320°C and 375°C for 30 min. A TGA curve for XV (in an argon atmosphere) indicated a char yield of 85%. This polymer can be used as a preceramic polymer to obtain B&/Sic. This will be discussed in the section on ceramics later in this review. 3.1.3. Polyaryl(ether-ketone-carborane)s Colquhoun et a1.93 reported the synthesis of thermally stable polyaryl(ether-ketonecarborane)s. They studied the effect of incorporating icosahedral carborane units into the

S. PACKIRISAMY

726

Q 0

0

Fig. 9. Monomers used for the synthesis of polyaryl(ether-ketone-carborane)s.

93

structures of an already thermally-resistant group of linear polymers, the aromatic polyether-(ketone)s. Aromatic poly(ether-ketone)s can be synthesized by two known routes,94 viz. basepromoted nucleophilic condensation of a phenol with a halogeno- or nitrobenzophenone (the polyether synthesis) or by acid-promoted electrophilic attack of an aromatic carboxylic acid or acyl chloride on an aromatic ether (the polyketone synthesis). Since icosahedral carboranes are known to be susceptible to nucleophilic degradation by alkoxide ions,95 but are stable in the presence of strong acids, the synthesis of poly(ether-ketone-carborane)s was carried out by the acid promoted electrophilic route.

727

DECABORANE(14)-BASED POLYMERS

n 810Hto

Jn

n

1

@“~~7cgo@pJcsc~~ 0 :

XIX

BIOHIO Fig. 10. Polyaryl(ether-ketone-carborane)s g3synthesized from monomers shown in Fig. 9.

J n

The monomer combinations for the synthesis of various polyaryl(ether-ketone-carborane)s are shown in Fig. 9. The polymerizations were carried out at room temperature for 20-60 h in anhydrous trifluoro sulfonic acid. All four polymers, XVI-IX (Fig. 1O)93were amorphous in nature and had glass transition temperatures in the range of 174-214°C. Polymers XVI, XVII and XVIII were readily soluble in chloroform and dichloromethane. Polymer XIX was essentially insoluble in all organic solvents at room temperature. TGA in nitrogen of polymer XVI showed an evolution of volatiles beginning at 35O”C, but between this temperature and lOOO”C,the total weight loss from the sample was no more than 11%. Similar results were found for polymers XVIII and XIX, but for polymer XVII (containing the lowest weight fraction of carborane), the weight loss was a little higher (cu. 15%). In striking contrast to these results, TGA of the conventional, all-organic poly(ether-ether-ketone) (PEEK), [-OArOArCOAr-], where Ar = 1,4-CsH4, shows a much higher temperature for the onset of volatiles production (cu. 540°C). However, for this system, a total weight loss of nearly 50% occurs on heating to 1000°C under nitrogen. In view of the high carbon:boron ratio in these new polymers, inert atmosphere pyrolysis could eventually lead to a ceramic residue of boron carbide (B&) dispersed in a graphite matrix.

S. PACKIRISAMY

728

OEt

OEt

I CH3C

-Ar

-C

I

Cy

I

OEt

i-

A&C-

cy

Ar’

I

OEt

OEt

Scheme 13. Synthesis of polyarylenes.

lo1

3.1.4. Polyarylenes containing carborane units One of the methods used for the preparation of polyarylene-type polymers is through the polycondensation of mono- and diacetylaromatic compounds in the presence of a ketalization agent96 or their ethyl ketals.97 Low molecular weight prepolymers with active acetyl and ketal end-groups96-98 are formed in the first stage of the reaction (Scheme 13). The polycondensation shown in Scheme 13 does not give only one kind of product and in fact, along with benzene rings, dipnone (P-methylchalcone), polyvinylene-, and polyvinylenedefective fragments can be formed.98 The prepolymers on heat treatment give rise to threedimensional, highly heat-resistant and thermally-stable polymers. Teplyakov et aZ.99-‘04reported the synthesis and properties of polyarylenes obtained by the polycondensation of mono- and diacetyl/diketal aromatic compounds containing o-, pand m-carborane units (Fig. 11). Model studies carried out with 4-acetylbenzyl-o-carborane diethylketal suggest that the carborane-containing polymer from diacetyldiaryl carboranes or their ketals contains dipnone, linear and cyclic polyvinylene fragments in addition to 1,3,5_trisubstituted benzene rings. to1 The prepolymers are highly soluble in a number of organic solvents such as chloroform, toluene, cyclohexane, chlorobenzene, dioxane, acetone, THF and benzene. These 0

cHi*cy_c-CH 3

\o/ bO%l

YE1

OEt Had

@0$--c d Et

BtoHm

-C-CH2-@fCHJ

\Ol WIO

Fig. 11. Monomers

OEt

used for the synthesis

of polyarylene

containing

carborane units. 1018104

DECABORANE(14)-BASED

POLYMERS

729

prepolymers, when exposed to an inert atmosphere at 450°C for 4 h or by pressing at 400450°C under a pressure of 1000 kg/cm2, resulted in the formation of infusible and insoluble three-dimensional po1ymers.r” In the IR spectra of these polymers, the absorptions at 1060, 1120, 1150 and 1680 cm-‘, corresponding to the vibrations of ketal and acetyl end-groups fully disappear. The above observation suggests a reaction of the end-groups or their degradation at higher temperatures in the curing process to give network structures. TGA studies suggest that these polymers possess very good thermal stabilities when compared to polymers derived from acetyls and keta1s.l” Studies on the structural characteristics of model compoundsl” suggest that molecules of C, C’-diphenyl substituted o-carborane derivatives are subjected to a certain steric strain, which gives rise to significant elongation and twisting of the C-C bond in the carborane nucleus and furthermore to a distortion of the bond angles and planar geometry of the benzene rings. Other features being equal, such effects could cause a decrease in the thermal stability of polymers containing these units in the main chain.

3.1.5. Other polymers The synthesis of polyimides containing m-carborane units was reported by Zhubanov et al. lo5Friedel-Craft’s acylation of diphenyl ether by m-carboranedicarbonyl dichloride in a 2:l ratio followed by the condensation of a diketone product with trimellitic acid chloride in a 1:2 ratio under step-wise heating at 383-413°C yielded 1,7-bis-{4-[4-,4-dicarboxybenzoylo)phenoxy]benzoylo} carborane dianhydride (XX). The polymerization of XX with diaminodiphenylether affords a polyether-polyketone-polyimide which has high thermo-oxidative stability. Korshak et al. lo6 synthesized polymeric Schiff bases through the reaction of m-carboranylene diamine or 1,7-bis(4-aminophenyloxy carbonyl)-m-carborane with aromatic dialdehydes in a chloroform-methanol mixture. Oxazolidinone group containing polyisocyanurates with improved elasticity and physicomechanical properties were prepared by the copolymerization of 4,4’-diphenylmethane diisocyanate with epoxy oligomers based on 1,2-bis(4-hydroxyphenyl)carborane at a 1:l to a 4:l mole ratio in the presence of 2-phenylimidazole catalyst. lo7 The composition was cured stepwise at 120,140,170 and 200°C by holding at each temperature for < 1 h. It was observed that at 120°C the curing rate increased significantly with increasing diisocyanate:carborane epoxy oligomer ratio from 1:l to 4:l. The curing was independent of the component ratio at 140-200°C. The reaction of diphenyl-m-carborane dicarboxylate with 3,3’-diaminobenzidine yields a low molecular weight polybenzimidazole containing m-carborane in the polymer backbone. lo8 Comparison of the TGA curves for aromatic polybenzimidazole (from 3,3diaminobenzidine and diphenyl isophthalate) and carborane-containing polybenzimidazole indicates a distinct superiority in thermal stability for the latter product. The synthesis and properties of polyesters,‘09-113polyformals,1149115 polyoxadiazoles1’6 containing o-carborane units, and polymers containing p- and m-carborane units linked by Si, Ge, Sn, Pb, P, S and Hg have all been reported851117-121 and are reviewed elsewhere. r5

S. PACKIRISAMY

730

-CL

nCI-Si

+ n Cl -Si-

Cl

I (CY),

I

I

$!!on.

H20

Scheme 14. Synthesis of polysiloxanes containing o-carborane units as pendant groups.

3.2. Polymers having carborane units as pendant groups 3.2.1. Poly(carborane-siloxane)s

The condensation of o-carboranyl dichlorosilanes during hydrolysis or their copolymerization with dichlorosilanes or dialkoxysilanes afford poly(o-carboranylorganosiloxane)s (Scheme 14). 111~122~123 o-Carboranyl substituted linear polysiloxanes can also be obtained by the ring opening polymerization of cyclic o-carboranyl siloxanes. 12* TGA studies indicated that the incorporation of carborane nuclei into the pendant groups did not result in any significant improvement in thermal stabilities of polysiloxanes and, in fact, the thermo-oxidative cleavage of the o-carboranyl pendant groups occurred at lower temperatures than required for the thermo-oxidative cleavage of Si-0 and Si-C bonds. ‘2~ For this reason, this approach to the synthesis of high temperature polysiloxane elastomers was abandoned and more attention was focused on polysiloxanes that contained carborane nuclei in their backbones. 3.2.2. Polymers derived ji-om vinyl, isopropenyl, acryloyl and ally1 derivatives of o-carborane

Vinyl and isopropenyl o-carboranes do not polymerize via a radical mechanism due to the high electronegativity of the carborane nucleus and steric hindrances. 1Z~126 However, they do copolymerize with methyl methacrylate (MMA), styrene and other vinyl monomers in limited quantities and produce copolymers having low molecular weights.126-129 When the carborane nucleus is separated from these functional groups by some bridging group, then such a compound easily participates in radical homo- and copolymerization. 13o-132For instance, acryloyl or methacryloyl derivatives of o-carborane *11P126J32-134

DECABORANE(14)-BASED

%

7

-CR’

cY=c-c-o-cH2~0,

POLYMERS

731

e

WI0 R=H, Scheme 15. Polymerization

R’=H. CHJ

Cb,

of acryloyl and methacryloyl

derivatives

of o-carborane.

are active in homo- and copolymerization (Scheme 15) and 4-(1’~o-carboranyl)styrene undergoes copolymerization with MMA resulting in the formation of high molecular weight polymers. 13’ Recently, Teplyakov et al. 135reported the radical copolymerization of MMA with isopropenyl derivatives of o-carborane, which contain spacer groups between the isopropenyl group and the carborane unit (Fig. 12). Bulk copolymerization of the above monomers with MMA was conducted in the presence of 0.5 wt% benzoyl peroxide at 80°C and at concentrations of the carborane-containing monomer from 0.5 to 8.3 mol%. Copolymers prepared from the monofunctional monomers are transparent, colorless substances that are soluble in benzene, chloroform, acetone and other organic solvents. The content of XXI and XXII (Fig. 12) fragments in the copolymer is similar to that in the initial mixture. This is indicative of the high reactivity of the unsaturated carboranecontaining monomers in the copolymerization with MMA. However, the average degree of polymerization decreases with increasing amounts of XXI and XXII in the reaction mixture as indicated by a sharp drop in intrinsic viscosity. This fact can be explained by chain transfer to the monomer. Probably, this reaction is the result of a displacement of a mobile H,C =C

Cf$-$y

+

XXI

CY

y”=i

BIoH,

~CH3-_F~~

--C~~i=CH2

CH3

Ell0Hl.l XxnI

CH3

“‘=i~‘~~~r~~i=“* CY

W-40

CH3

xxv

Fig. 12. Isopropenyl

derivatives

of o-carborane. 135

732

S. PACKIRISAMY

hydrogen atom of an unsubstituted C-H group of o-carborane. The new radical is stabilized by conjugation with a phenyl ring in the monomer and is less active in the chain growth reaction, and by interacting with radicals of the growing polymer chain may lead to chain termination. It was also interesting to investigate the ability of the bifunctional carborane-containing monomers, XXIII and XXIV to copolymerize with MMA. XXIII was used in amounts of 1.0 and 3.0 mol%, and XXIV, in amounts of 0.5, 1.0 and 2.0 mol% due to its lower solubility in MMA. Copolymerization resulted in the formation of crosslinked copolymers with nearly quantitative gel fraction yields. The amount of carborane-containing fragments was found to be significantly higher than in the feed. This has been attributed to transfer to the monomer and chain termination caused by the formation of a stable radical. For copolymers based on XXI and XXII, the beginning of weight loss occurs at 200°C. However, with increasing quantities of carborane-containing comonomer, the weight of the residue also increases and at 400°C it comprises up to 55%. TGA data reveal that the thermal stability of the crosslinked copolymers based on XXIII and XXIV with a low boron content (2%) exceeds that of PMMA only slightly. On the other hand, a copolymer based on XXIII with a boron content of 6.1 wt% afforded a residue of 53% at 400°C. Copolymers of MMA with carborane-containing bifunctional monomers such as diallyl esters of carboxylic acids can also be obtained.136 An increase in the thermo-oxidative stability of this copolymer was observed and this increase was attributed to an inhibition of chain depolymerization by boroxyl radicals. 3.2.3. Polyphosphazenes

containing carborane units

Polyorganophosphazenes 137-139have interesting chemical and physical properties such as resistance to alkali and acids (except concentrated sulfuric acid), water and oil repellent characteristics, high degrees of fire resistivity and good flexibility at low temperatures (-60°C to -8O’C). The alternating phosphorus and nitrogen atoms in the backbone of phosphazene polymers is responsible for a high degree of torsional mobility and accounts for low glass transition temperature (T& values and transparency of the polymer to ultraviolet radiation.‘37 The susceptibility of the phosphorus-chlorine bond to hydrolysis can be overcome by nucleophilic substitution reactions involving the replacement of halogen atoms in the polydichlorophosphazenes by alkyl, aryl, amino, alkoxy and aryloxy group~.r~~-~~~ It was anticipated that the incorporation of carboranyl units into polyphosphazenes would improve their thermal stabilities. Allcock et al. 141-143and FewelllM reported the synthesis and properties of polyphosphazenes containing alkyl/phenyl carborane as pendant groups. The addition of an alkyl/phenyl carborane group on the hexachlorocyclotriphosphazene ring followed by thermal polymerization and the replacement of remaining chlorine atoms with trifluoroethoxy groups afford several carborane-containing polyphosphazenes. The synthesis of poly(phenylcarboranyl-di-trifluoroethoxy-phosphazene) (XXV) is shown in Scheme 16. 144 TGA studies for this polymer in nitrogen and air showed char yields of 61 and 57%, respectively at 800°C. The thermal properties of poly(bistrifluoroethoxy)phosphazene and

733

DECABORANE(14)-BASED POLYMERS

Scheme 16. Synthesis of poly@henylcarboranyl-di-trifluoroethoxy-phosph~ene).

144

its carborane-substituted analog (XXV) are summarized in Table 2.1U The high char yield in air of the carborane-substituted polymer is a unique property and is indicative of its stability and resistance to thermo-oxidative degradation. Pyrolysis mass spectroscopy studies of carborane-containing polyphosphazenes 144 suggest that initial weight loss is not due to thermal degradation but rather to a purging process that involves the removal of low-molecular-weight compounds and products related to the hydrolysis of residual P-Cl bonds from atmospheric moisture. The primary thermal reaction involves the loss of trifluoroethoxy groups from the polymer during pyrolysis near 300°C. This results in crosslinking of the phosphazene chains, producing a thermally stable polymer residue. The second thermal reaction occurs during pyrolysis at 400°C and involves the loss of phenylcarborane from the polymer resulting in additional crosslinking and a highly thermally thermo-oxidatively-resistant char residue. The carborane group has a considerable propensity for electron withdrawal or acceptability, which is responsible for its inductive characteristics, its interactions with the phenyl ring and its greater electron mobility that allows the phenylcarborane to function as an Table 2. Comparison

of thermal stability of poly(bistrifluoroethoxy)phosphazene

and its carborane analog’44

Nitrogen atmosphere % Char yield PDF (“C) TkU at 800°C

PDTa(oC)

Poly(phenylcarboranylditrifluoroethoxyphosphazene) (XXV)

110

395

61

110

380

57

Polymer XXV dried at 75°C for 16 h

120

465

59

120

455

65

Poly (2,2,2-trifluoroethoxy)phosphazene

370

410

0

180

412

0

Polymer

a PDT = Polymer decomposition temperature. bT ,,,= = temperature at which maximum decomposition

occurs.

Air atmosphere T;, % Char yield at 800°C

134

S. PACKIRISAMT

energy sink. The phenylcarborane in the inhibit helical coil formations which are compounds via depolymerization. ‘4~The has resulted in a substantial improvement retarding depolymerization.

4. DECABORANE-BASED

PN backbone may sterically hinder and thereby low energy pathways to the formation of cyclic phenylcarborane group in the polymer backbone in the overall thermal stability of the polymer by

POLYMERS CERAMICS

AS PRECURSORS

FOR

Advanced ceramic materials are widely used in aerospace environments, in the fabrication of heat engines, for high speed cutting tools and electronic sensors for extreme environments and as oxidation-resistant ceramic coatings. However, their potential applications are limited by their inherent brittleness and difficult processing conditions. The pyrolysis of organometallic polymers into ceramics overcomes some of the problems encountered with the conventional processing techniques for ceramics. Processing techniques common to organic polymers can be applied to preceramic polymers. These polymers can be drawn into fibers, molded into desired products, dissolved in suitable solvents and applied as coatings on substrates of choice and then pyrolyzed to ceramics to obtain the corresponding ceramic end-products. They can also be used as binders for ceramic powders that are difficult to sinter and as matrix resins for making ceramic matrix composites and for obtaining ultrafine ceramic powders. The conversion of polymers into ceramics requires relatively low temperatures when compared to conventional ceramic processing techniques. The polymeric route to ceramics also provides the opportunity to obtain novel ceramic alloys with wide ranging properties. The polymer that is chosen as a precursor for a ceramic should be meltable or soluble in common organic solvents and should give a high char residue on pyrolysis. The polymer precursor should be designed in such a way that the volume change associated with pyrolytic conversion is as small as possible. Although the concept of polymer pyrolysis to ceramics was first reported1451146 in the 196Os, there was not much interest until, Yajima147 and Verbeckr4’ reported the synthesis of Sic and S&N4from preceramic polymers in the 1970s. Over the last two decades, many preceramic polymers have been developed for SiC,‘49-170Si3N4,171-182Si-C-N, 183-187 B4C

212%188,189 BN,%190-197

B4C_SiC,89,90’198-203

etc.

Ceramic

fibep,‘~-“O

ceramic

coat_

ceramic components230-234and ceramic matrix composites235-242have all ings, 190~191T221-229 been made using these polymers. A number of review articles have appeared concerning these developments.B3-259 For obtaining B4C ceramics, a high boron to carbon ratio is required. Polymers derived from carboranes and polymeric Lewis base adducts of decaborane meet this requirement. As discussed earlier, the presence of carborane and decaborane moieties in the polymer backbone stabilizes the polymer against degradation thereby increasing the ceramic yield. This avoids an added stabilization step, which is often carried out with the other preceramic polymers. In this section, various non-oxide ceramics such as B&/SiC, B&,BN and metal borides obtained from polymers containing carborane and decaborane moieties are reviewed.

DECABORANE(14)-BASED POLYMERS

735

4.1.Polymers derived porn carboranes as precursors for ceramics 4.1.1. Poly(carborane-siloxane)s

The conversion of poly(carborane-siloxane)s Dexi1260202 and Urasi1261was studied by Waker et al. 199These two polymers differ mainly in their molecular weights. Dexil202 is a low molecular weight (1000-2000) polymer having the consistency of honey where as Urasil 261is a rubbery solid with a molecular weight of approximately 30,000-50,000. On pyrolysis in an inert atmosphere (nitrogen or argon), Dexil 202 and Urasil gave char residues of 60 and 70%, respectively. When Dexil 202 was subjected to long term (50100 h) low temperature (75-200°C) heat treatments or irradiated using 6oCo y-irradiation with dosages ranging from 8 to 700 Mrad at approximately 100 Mrad intervals, a rubbery material similar to Urasil was obtained, which gave a char yield approaching that of the later. Zheng et al. 202studied the structural changes that take place during the pyrolysis of Dexil 202 using 29Si-NMR, FTIR, X-ray diffraction (XRD), DTA and TGA. The major weight loss up to 600°C comes from the cleavage of silicon-alkyl bonds resulting in volatile products. An endothermic peak was noticed in the DTA curve which was attributed to the dissociation of these bonds. In the IR spectrum, characteristic peaks corresponding to B-H bonds of the carborane cage were present for the material heated up to 700°C and these peaks were absent when the heat treatment exceeded 730°C. The above observation suggests that the carborane cage undergoes dissociation around this temperature. An endothermic peak was present in the DTA curve around 730°C which may be related to the above structural change. Probably, above this temperature, a ceramic phase consisting of B-Si-C-O bonds is formed. XRD studies indicated that this phase was amorphous (Fig. 13).202

20

60

40 28

Fig. 13. XRJI of pyrolyzed

products of Dexil 202 at various stages of pyrolysis in an inert atmosphere. *02

736

S. PACKIRISAMY

I

IS0

1

loo

,

I

so

0 d

Fig. 14. 29Si-h4AS-NMR

of pyrolyzed

I

1

I

I

-50

-100

-I50

-200

(wm)

products of Dexil 202 at various atmosphere. *02

stages of pyrolysis

in an inert

Inert atmosphere pyrolysis at 1000°C probably initiates a carbothermal reduction of the amorphous boron-silicon-oxycarbide phase. The above inference is supported by 29SiMAS-NMR spectra (Fig. 14),202 which show broad peaks suggesting the presence of various structural intermediates of silicon such as OSiC3 (6 = -10 ppm), 02SiCz (6 = -19 ppm) and Sic, (6 = -0 ppm). The formation of B-C bonds and the crystallization of amorphous phase probably occurs above 1000°C. The DTA curve shows an endothermic peak around 12OO”C,which may be attributed to the above changes. XRD of the polymer after pyrolysis at 1300°C shows peaks corresponding to both P-Sic and B4C (Fig. 13). Furthermore, the 29Si-MAS-NMR spectrum of this sample includes a sharp peak at 6 = -16.20 ppm, which is characteristic of &Sic. With further pyrolysis at higher temperatures, crystallization and crystal growth continues, which is evidenced by the characteristic peaks in both the XRD (Fig. 13) and 29Si-MAS-NMR becoming sharper (Fig. 14).

DECABORANE(14)-BASED

Table 3. Properties Starting material Dexil202 graphite particles

POLYMERS

of ceramic matrix composites

Pyrolysis products (M%)

Typical densit r k/cm )

737

made from Dexil 2021w Open porosity (%)

Flexural strength (MPa)

Wt lossa at 1ooo”c (%)

+

70 SiC/EL& + 30 carbon particles

1.60

> 0.3

1.5

<1

Dexil 202 + Rayon cloth Dexil202 + Sic fibers

20 SiC/BJ + 80 carbon fibers 87 Sic/EL& + 13 Sic fibers

1.42

> 5.7

26

<1

1.4

Nb

9.6


Dexil 202 infiltrated carbon fiber composite

10 Sic&C + 90 carbon fiber composite

1.17

Nb

124

90

Dexil 202 infiltrated carbon fiber composite

12 Sic/B& + 88 carbon fiber composite

1.33

Nb

158

88

1.20

15

69

100

Carbon fiber composite (as received) a After 3-4 h in an open furnace. b Not measured.

Dexil202-based materials retained the shapes and structural integrity. 199However, a fair amount of cracking is commonly observed. Typical pyrolysis products of this polymer had a black shiny appearance similar to glassy carbon. The pyrolyzed product had a density of 1.8 g/cc, an average flexural strength of 7.6 MPa and a Vickers hardness of lo-16 kPa. The density value suggests the presence of a substantial amount of porosity (about 45%; mostly closed). The low flexural strength observed is attributed to stresses and cracking due to shrinkage. On heating in air at 1000°C for 3-4 h, the pyrolyzed body lost only 0.5% weight indicating a high oxidative stability for this material. Dexil202 has been utilized as a matrix resin for ceramic matrix composites. 199Table 3 199 outlines the starting materials for the composite samples and the pyrolysis results. Most of the products possessed improved mechanical integrity with little or no obvious macrocracking. The densities were substantially lower than the material obtained by the pyrolysis of Dexil202 itself. It was observed that the addition of (-2 pm) graphite particles to Dexil 202 doubled the strength and gave a rougher fracture surface. Copyrolysis of the composite of this polymer and rayon resulted in bodies with over a three-fold increase in strength over the pure polymer, despite the lower densities. SEM studies of this composite showed a substantial porosity and irregularities between the fiber bundles indicating problems of hand lay-up especially for small samples. Infiltration of this polymer into the carboncarbon composite followed by pyrolysis significantly increased its strength. While there was actually a lower post-pyrolysis density in the carbon-carbon composite infiltrated with 10 wt% of this polymer than the control composite, strength was increased by 80% by

738

S. PACIURISAMY

polymer impregnation and pyrolysis. A strength increase of 130% was obtained after impregnation with 12.5% of this polymer followed by pyrolysis. On the other hand, the strength of the composite made from placing aligned Sic fibers in Dexil202 polymer then pyrolyzing was not much greater than the monolith made from Dexil202 alone. SEM examination of the above composite showed that good wetting and bonding occurred. However, extensive shrinkage cracking occurred in the pyrolyzed matrix between the Sic fibers, which might be the reason for the limited composite strength. This matrix cracking was attributed to the matrix bonding to the Sic fibers and shrinkage, while the fibers do not shrink.199 While the carbon-carbon composite infiltrated with Dexil202 produced better strength, its oxidation resistance was inferior to composites made from Dexil202 + graphite particles and Dexil 202 + rayon. In fact, for the latter two composites, temperatures in the range 14001500°C were found to be necessary to eliminate the carbon by oxidation. The graphite particles were probably protected by the ceramic coating obtained from the pyrolysis of Dexil202. 199 Improved oxidation resistance of the composite from Dexil202 + rayon cloth inspite of having open porosity (Table 3) between fiber bundles suggests that synergistic effects of enhanced wetting, bonding and interdiffusion during copyrolysis play a vital role. 19’ Despite the fact that Dexil202 can be converted into crystalline B&/Sic ceramics, the poor rheological properties of the polymer make it very difficult to produce fibers or thin films. This failure is attributed to the sharp drop in viscosity over a narrow temperature range (30-75°C). Consequently, attempts to prepare B&/Sic films and fibers from Dexil 202 polymer were not successful.202 4.1.2. Poly(carborane-siloxane-acetylene) The synthesis and thermal properties of poly(carborane-siloxane-acetylene) (XV) were presented in Section 3.1.2. Henderson and Keller89’90reported that (XV) can be used as a matrix resin for advanced composites and as a precursor for B&/Sic ceramics. Pyrolysis of XV in a stream of argon to 900°C at lO”C/min affords a black monolith in 85% ceramic yield which retains its shape except for some shrinkage. During heat treatment of XV, the polymerization of acetylinic groups takes place resulting in the formation of a thermoset. Elemental analysis data (before and after heat treatment) suggest that the major weight percentage changes were at the expense of carbon and hydrogen. The ceramic material obtained by heat treatment at 900°C is amorphous as indicated by XRD and contains a large excess of carbon. Further details regarding the heat treatment of XV to obtain a B&/Sic crystalline material and its properties are not available. As in the case of poly(carborane-siloxane)s, XV may also require temperatures above 1600°C to effect the carbothermal reduction. 4.1.3. Carborane-containing polymers devoid of oxygen

The rather high temperatures required for the production of B&/Sic ceramics from Dexil 202 are mainly due to the temperature dependence of the carbothermal reduction of the silicon oxide components formed during pyrolysis. To overcome this problem, Zheng et aL202 synthesized several oxygen-free carborane molecules that could possibly be converted to B&/Sic ceramics at lower temperatures. Unfortunately, these molecules were volatile due to their low molecular weights and the TGA results indicated complete weight loss at temperatures around 450°C.

DECABORANE(14)-BASED

739

POLYMERS

w

y3

HJC-Si-Cs

C-C

-C-CC~~C-S~-CH~ \I

c H3

0

I CH3

Bloho

Fig. 15. Carborane monomer (XXVI) containing ethynyl and trimethylsilyl gro~ps.~~

UV treatment

60

240

420 tcmperat

Fig. 16. Comparison

of thermal stability

/

600 we

760

960

(OC)

of XXVI and crosslinked treatment). 202

XXVI (by UV and autoclave

However, a high temperature and high pressure autoclave treatment of the these monomers resulted in polymers capable of giving high char residue. A TGA curve of the polymer obtained by autoclave treatment of the monomer XXVI (Fig. 15) is given in Fig. 16. 202The monomer can also be polymerized by exposure to UV irradiation and the polymer obtained gave only 20% ceramic residue. The non-transparency of the polymer formed by UV irradiation is the primary reason for the low ceramic yield and a short penetration depth would severely limit the degree of crosslinking of the polymer. It is expected that polymers derived from carborane monomers devoid of oxygen will require relatively lower temperatures for ceramic conversion. These polymers may be useful for the preparation of B& and B,C/SiC thin films and fibers. 4.2. Polymeric Lewis base adducts of decaborane as precursors for ceramics 42.1. Ceramics derived j?om phosphorus-containing

polymers

Seyferth et al. 1gY21,27 studied the pyrolysis of polymeric Lewis base adducts of decaborane derived from phosphorus-containing ligands such as Ph2POPPh2 (POP), PhzPN= PPh2CH2CH2PPh2PPh2 (PNP), Ph2PNHNHPPh2 (PNNP), Ph2PCH2PPh2 (PMP),

740

S. PACKIRISAMY

Table 4. Phosphorus-containing

polymers investigated

as ceramic precursors”

Polymer linker”

Ceramic yield at 1000°C (%)

cc NHNH

93 52 92 69 57

a All polymers have the structural formula [BIaHt2PhzP-linker-PPh21,.

Ph2PCH2CH2PPh2 (PEP), Ph2PCH2CH2PPh2 (PPP) and Ph,PC-CPPh,(PCCP). The ceramic yields obtained for the above polymers on pyrolysis to 1000°C in argon atmosphere are compared in Table 4.19 Of these polymers, the polymer derived from POP, [B10H12.Ph2POPPh2.],(XXVII) gave the most promising results for ceramic monoliths and as binders for B& when compared to the other polymers and hence, this system was investigated in greater detail by Seyferth et aL21 The structural changes that takes place during the pyrolysis of XXVII were followed by TGA, energy disperse X-ray spectroscopy (EDS; for qualitative measurement of P) and XRD. Table 521 summarizes. The XRD and EDS results. Even though XXVII gives a high char yield that includes, B2C it is evident that the pyrolysis product is carbon rich. Although, the ceramic samples are carbon rich, some of the carbon initially present in the polymer is lost during pyrolysis. It is observed that oxygen and phosphorus loss is not complete until temperature greater than 1500°C are reached. Comparison of elemental analysis data for pyrolyzed power samples and bulk samples suggest that both shorter dimensions and increased temperatures enhance phosphorus and oxygen losses. FTIR spectral studies for the powder sample of XXVII at various stages of pyrolysis suggest that up to 350°C cleavage of B-H bonds and the concominant formation of B-B bonds take place; cleavage of C-H bonds and concominant formation of B-C bonds occurs in the temperature range of 350-550°C. Removal of phosphorus and oxygen take place above 1000”C.21 XRD results of as-fired powder and the surfaces of bulk samples (before and after removal of 1 mm of exterior thickness) are summarized in Table 5. No sample showed Table 5. Summary of crystalline phases observed by XRD and EDS determined phosphorus content in materials produced from [BIaH12.POPPh2.], at various stages of processing” Temp., “C

Time, h

Sample

P(EDS)

B4C

Relative peak intensity (XRD) C BP B13P2

1,000 1,500 1,500 1,500 1,500 1,500 1,550 1,870 2,090 2,350

0.5 10.0 10.0 10.0 25.0 25.0 0.5 0.5 0.5 0.5

bulk powder bulk bulk” bulk bulk” bulk bulk bulk bulk

yes

0

0

0

62 26 0 62 0 0 42 18 9

100 70 47 80 70 43 100 100 100

90 100 65 100 71 100 0 0 0

a After - 1 mm removed by surface grinding.

yes yes yes yes no no

0 0

42 100 9 100 53 0 0 0

DECABORANE(14)-BASEDPOLYMERS

0

500

1000

1500

2ooo

741

2soo

Temperature(.c) Fig. 17. Density of the material produced from [BloHIZ.Ph2POPPH2.],as a function of pyrolysis temperature. ‘I’

evidence of crystallinity for firing temperatures below 1500°C. This is common for the extremely fine, sometimes microcrystalline, structures that result from polymer pyrolysis. 199With increasing firing temperatures, crystalline phases emerged with progressively sharper diffraction patterns. From the results presented in Table 5, it was observed that the sample fired as powder and the bulk samples differ considerably. After firing at 1500°C for 10 h, fired powder samples exhibit crystalline B&, C and B13P2.Exterior surfaces of bulk samples subjected to the same firing cycle revealed these phases in addition to BP and the concentration of BP increases with the distance from the surface. Crystalline B&was first observed only on the exterior surfaces of the bulk samples fired at 1500°C. B& and C diffraction patterns became well developed for bulk samples only for firing temperatures of 1870°C and higher. When the sample was heated to 2090°C B& was present throughout the bulk sample.21 Densities of the bulk samples (measured after cooling to ambient temperature by both calipers and Hg porosmetry) increase continuously with firing temperatures up to 2250°C (Fig. 17).21 The density of the solid phase including closed pores (skeletal density) increases at a higher rate than the bulk density in the temperature range above 1000°C. At its maximum, the skeletal density (as determined by Hg porosmetry) reaches about 90100% of the theoretical value for B& + C body having the overall composition, B 29.15% and C 65.14% (for the bulk sample fired at 2350”C).20,21 The results of BET surface area determinations for powder, bulk, ground bulk and ground powder samples suggest that materials fired as powder and bulk differ from one another over the entire range of temperatures investigated.20,21 Bulk samples exhibit progressively reduced surface areas from 250°C to the highest temperatures studied (2350°C). The ground bulk samples show that the reduced surface area does not result from conventional densification mechanism. Rather, internal pores become isolated from the outer surface. BET studies suggest that, in the bulk samples, high surface area pores evolve during the initial weight loss up to 300°C and at firing temperatures above lOOO”C,small isolated pores are formed. Weight loss measurements for pyrolysis and surface area measurements suggest that much of the weight loss from XXVII occurs after the pores become isolated in bulk

142

S. PACKIRISAMY

samples, whereas pores within the smaller diameter powder samples retain pathways to the exterior surfaces. 2oy21 Isolation of pores, before gaseous products have completely escaped, presents obvious problems with respect to internal pressures. SEM studies of a bulk sample fired at 1500°C for 25 h indicate the formation of crack originated from an isolated pore within a bulk sample. The potential for this type of damage will impose dimensional limits upon parts produced by pyrolysis of fabricated shapes from XXVII unless firing cycles are identified that do not cause premature closure of pores. 4.2.2. Ceramics derived from [Bl~lz.diamine], The above study on the conversion of XXVII reveals that this system has the following major problems: (1) the ceramic produced in the pyrolysis of this polymer contained large amounts of free carbon due to the presence of phenyl substituents on phosphorus, (2) the phosphorus and oxygen content of the ceramic could be reduced to minimal values for powder samples by processing at 15OO”C,but for the complete removal of these elements, high temperatures (ca. 2200°C) were required; and (3) although this polymer served as a low-loss binder for ceramic powders, it was not suitable for the production of ceramic fibers. 22728 In an attempt to overcome the above problems Seyferth and Rees22>23Y27p28763 investigated the conversion of decaborane-diamine adducts as precursors for B& and BN ceramics. The synthesis of this type of polymers, their characterization, thermal properties and the structural changes that take place during the heat treatment were discussed in Section 2. The pyrolysis of these polymers to B& and BN ceramics is described here. The [B1aH12.diamine], polymers were converted to ceramic powders by pyrolysis to 1000 and 1500°C (10Wmin heating rate) in a stream of argon. The results of these experiments are summarized in Table 6.22 In general, the ceramic yields obtained by TGA were slightly higher than those obtained in the furnace pyrolysis of 1 g samples of the same polymer. The ceramic products of the pyrolysis of the [B10H12.diamine],polymers to 1000°C are brown or black to silver/gray solids that are amorphous by XRD. The diffuse reflectance Fourier transform (DRIFT) spectra of these pyrolysis products show broad absorptions in the 3400-2480 cm-’ range that are attributable to the presence of residual N-H, C-H and B-H bonds at this stage of processing. Further heating of these samples to 1500°C under an argon atmosphere results in the disappearance of these bands in the DRIFT spectra of the products thus obtained and appearance of crystalline hexagonal boron nitride peaks in XRD. Broad diffraction lines attributable to the presence of B& are present in the XRD but their width (2”) and weak intensity ( < 5% relative to BN lines) suggest that only a relatively minor portion of the sample is composed of crystalline B&. As noted above, all ceramic samples pyrolyzed to 1000°C in argon are amorphous. Probably a three-dimensional covalent lattice of “boron carbonitride” is present, in which the boron atoms are more or less randomly bonded to carbon and nitrogen atoms. After heat treatment at 15OO”C,XRD patterns attributable to the presence of partially crystalline boron carbide and crystalline hexagonal boron nitride are observed, but this gives no information about what fraction of the sample has crystallized. The DRIFT spectra of the ceramic samples show absorption bands in the 1090-1105 cm-’ region that could be attributed to the presence of B-C bonds.

81 93* 73 94

61 83

1000 1000 1000 1500 1000 1500 1000 1000

16.9 28.8

15.2 22.6 19.3 19.5 25.9 26.8 28.5 22.7

49.7 46.1 53.4 60.5 46.6 47.8 51.0 54.3

19.3 20.0 20.1 19.4 18.4 22.3

22.2 20.5 16.8 15.7 19.9

14.7 10.4 19.6 17.3 17.6

61.6 68.9 60.5 67.0 62.5

N

C

analysis data.

0.64 0.73 0.50 0.55 0.65 0.54

0.27

1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.70

0.65 0.59 0.92 1.13 0.77

1.0

1.0 1.0 1.0 1.0 1.0

9.4 7.0 18.6 18.9 18.7 12.5

53.2 57.5 42.9 44.7 48.1 47.8 37.4 35.4 38.5 36.5 33.2 39.7 0.52 0.41 1.00 1.07 1.16 0.65

16.2

31.5 52.3

6.2

57.1 0.64

2.3 15.4,B 6.5 2.0 4.4

57.8 48.0 62.8 70.1 60.4

weight %e B& C

36.7

39.9 36.5 30.7 27.9 35.2

BN

0.35

0.12 0.97,B 0.44 0.15 0.26

Ceramic composition molard BN B.,C C

under an argon atmosphere**

B

Elem. anal.,’ wt %

a Maximum temperature to which sample was heated. b Defined as 100 (mass out/mass in), has no reflection on composition. “,As found by elemental analysis. ” Elemental analysis data converted to molar ratios for phases suspected of being present. e Weight percent composition of phases suspected of being present, calculated from elemental *From 1000 to 1500°C.

90 73

78

93f

1000 1500 1000 1500 1000

HZNCH2CH2NH2 H2NCH,CH2NH2 Me2NCH2CH2NMe2 Me2NCH2CH2NMe2 H2NCH2CHzNHfiezNCH,CHzNMez copolymer H2NCH2CH2NH2-Me2NCH2CH2NMe2 physical mixture 85% MeHNCI-12CH2NHMe/15% MeHNCHaCHaNHr mixture H,NCH,CHaCH,NH, H2NCH2CH2CH2NH, HN(CH,CH2)2NH HN(CH&H&NH N(CHaCH&N HN(CH&H,NH,),

Ceramic yieldb

Temp., “C”

of [B10H12.diamine], polymers

Diamine

Table 6. Data for pyrolysis

744

S. PACKIRISAMY

Studies on the llB-NMR spectra of [B10H12.(CH3)2NCH2CH2N(CHs)Z.]~ (VI) heated in refluxing xylene suggested that, during heat treatment, the polymer gets converted to [(CH&,NHCH$HJW(CH&]2t[B1c,Ht0]2- (XIV).22 During pyrolysis, this salt possibly decomposes to another covalent polymer, [BtaHs.diamine], which on further heat treatment gives B4C and BN ceramics. Pyrolysis of some [B10H12.diamine],samples in a stream of ammonia, rather than argon, usually resulted in the formation of a white (rather than a dark brown to black) ceramic residue. Analysis of the resulting ceramic residues in most cases showed a carbon content of less than 0.6% (Table 7).22 The yields of ceramic residue obtained in these furnace pyrolysis carried out in a stream of ammonia to 1000°C are quite high, e.g. 70% for [BloHr2.N(CH2CH&N.]. (XXVIII) and 84% for VI. A “carbon kick-out” reaction occurs between 350 and 650°C when [B10Ht2.diamine], polymers are pyrolyzed in a stream of ammonia.22 Thus, when such pyrolysis of XXVIII were interrupted at 250,500 and 650°C the carbon contents of the solid residues formed at these temperatures were 10.6,4.26 and 0.77%, respectively. In the 350°C sample, the B/N ratio was 0.48; in the 650°C sample it was 0.93. In an argon stream the [B1aHr2.diamine], polymers undergo conversion to [H.diamine.H]2t[B1aH10]2-salts at temperatures above 2OO”C,so it is likely that this process also occurs in pyrolysis carried out in a stream of ammonia prior to the “carbon kick-out” reaction. The “carbon kick-out” mechanism starts at temperatures where the polymer already is decomposing. Also, gaseous ammonia begins to decompose at 450-500°C and traces of organic matter lower its decomposition temperature. Thus, the “carbon kick-out” reaction may not be a process involving the intact ammonia molecule. It is likely that .NH2 and :NH radicals, present in small quantities in decomposing ammonia at higher temperatures, may be responsible for “carbon kickout” in a complicated free radical process.22 Preceramic polymers can be used as binders for ceramic powders. Various [B1aH12.diamine], polymers were used at a loading of 16.7 wt% as binders for B& or BN powders. In most cases, shape retention of the fired composite bar (compared to the “green” composite bar) was excellent. A bar of the XXVIII polymer alone was also pyrolyzed. The resulting ceramic bar retained its shape, but with a 48% volume shrinkage and had an apparent density of 1.68 g/cm3. The ceramic bar could not be broken manually whereas the unfired ceramic composites were broken easily. 22 Short (-5 cm) fibers were hand-drawn from a syrup of these polymers in DMF. Such fibers were pyrolyzed either in an argon or ammonia stream to yield boron carbonitride or boron nitride fibers, respectively. 22 SEM photomicrographs show these fibers to be solid, spherical, l-3 pm in diameter and smooth-surfaced. They contained flaws of less than 0.01 pm. Melt-spinning of the [B1aHr2.diamine], polymers was not possible because of their high melting (decomposition) points. 4.2.3. Preceramic polyblends Metal carbide fibers can be produced by incorporating metallic additives into a carbon fiber product, the precarbonaceous polymer forming solution, the polymer spinning solution or the polymer fiber subsequent to spinning and converting the metallic compounds in situ to metal carbides upon thermal conversion. In the above methods, the precarbonaceous

43.6

from elemental

56.4

51.1

45.4

BN ceramic

Cal. for BN a Maximum temperature to which sample was heated. b Defined as 100 (mass out/mass in), has no reflection on composition. ’ As found by elemental analysis. d Elemental analysis data converted to molar ratios, normalized to Bme. e Weight percent composition of phases suspected of being present, calculated f Purchased BN commercially available. g Yield to 1000°C is given first, yield from 1000 to 1500°C is given second.

45.9 55.7

53.9

N

48.4 46.8 57.8 51.4

60,989 7O,g98 61,g95 88

1,500 1,500 1,000 1,500

39.0 42.6

42.4

B

1.000

0.869

0.892 0.905 0.943 1.081

0.909 0.982

1.008

0.011

0.008 0.004 0.0002 0.012

0.010 0.078

0.016

Ceramic composition weight %d BN C

analysis data, assuming (BN),,(C,).

0.54

0.37 0.18 0.53 < 0.01

0.42 0.78

3.69

C

Elem anal., wt %

in an ammonia atmospherez2

41.9 39.9 42.1 41.2

56,695 70

62

Ceramic yieldb

of [BIoHrz.diamine],

1,500 1,000

1,000

IBldr,,.H,NCH,CH,NH,.l,

[B&r2.Me2NCH2CH2NMe2.BIoHIoEt2. [BloH12.N(CH2CH2),N.l, Me2NCH&H2NMe2.], Et2NH[BroHr2.Me2NCH&H2NMez.]&NEt2 [B,d-&oBtz.N(CHzCH&N], [BIoHIo.Et2Me~NCH2CHzNMe2.J, IB~~H~~.[HN(CHZCH~NH~)~.IU~]~

Temp,a “C

Polymer

Table 7. Data for the pyrolysis

molar”

100.0

97.8

99.6 99.8 99.99 99.4

99.5 96.4

99.2

BN

2.1

0.4 0.2 0.01 0.6

0.5 3.6

0.8

C

g

d

$

g g

g 2 y

6

g

746

S.PACKlRISAh4Y

polymer acts as the source of carbon. Important ceramic fibers formed by such a method include boron carbide-containing fibers. The addition of boron carbide to carbon fiber is known to increase the fiber strength and more particularly, to increase the oxidative stability of the carbon fibers.24Y26 Methods of incorporating boron into carbon fibers to form boron carbide fibers involve the treatment of carbon fibers with gaseous boron halides or impregnation with boric oxides, boric acid, organic borates, boranes, metal borides, boron nitride or boron silicide. 262-269On heat treatment of these fibers, boron reacts with carbon fibers to yield boron carbide. For producing an ideal boron carbide fiber, boron levels in the fibers must reach about 78 wt%. Unfortunately, the above methods yield only a small amount of boron loading (2-5 wt%). Such small boron loadings do not result in any appreciable improvement in the oxidative stability of the loaded carbon fibers at elevated temperatures. Substantial boron loadings can be achieved by blending polymeric Lewis base adducts of decaborane with precarbonaceous polymers.B’26 The fibers can be spun by wet or dry spinning methods. The concentrations of the polymeric materials in the spinning solution can vary widely and depend on the particular spinning process, e.g. dry or wet, used to form the fibers. Typically, for wet spinning, concentrations of the polymeric materials between about 5 and 20% by weight can be used whereas for dry spinning, concentrations of up to about 80% are useful. It is extremely difficult to obtain boron-containing polymer concentrations near 80% and, thus, for dry spinning, a much higher level of the precarbonaceous polymer relative to the boron-containing polymer must be utilized. In such instances, the boron content of the formed fibers will be relatively low and, thus, dry spinning is not a preferred method of forming boron carbide fibers wherein the amount of boron relative to carbon should approach 3:l. On the other hand, the dry spinning process may be useful in forming boron nitride, boron phosphide or boron metalloid ceramic fibers as minimum amounts of boron-containing polymer is needed to form an intact fiber. High levels of the precarbonaceous polymer do not adversely affect the non-carbide ceramic products since the polymer is burned away and is not present as a carbon source. Preferably, wet spinning is used to form the fibers since the greater amounts of solvent allow the use of a greater amount of boron-containing polymer relative to the precarbonaceous polymer. 24,26 After a newly-formed fiber is spun, it can be stretched or drawn to about lOO-300% of its original length by conventional techniques and stabilized by thermal treatment or subjected to a chemical stabilization treatment using a radical forming agent such as diazidoformamide, which effects the desired crosslinking structure in the fiber substrate at ambient temperatures. 24,26 In the subsequent pyrolysis step of the process, the preceramic fiber (either charred or uncharred) is subjected to temperatures between about llOO-1800°C. Different gases can be used to pyrolyze the fibers. Use of inert gases will lead to the formation of metal carbides while reactive gases including ammonia, phosphine and metalloid-containing gases such as metal hydrides, including germane, arsine, stibione, silane, etc., will lead to boron nitride, boron phosphides and boron-metallic ceramics, respectively. Following the procedures described above, JohnsonBp26reported the synthesis of boron carbide and boron nitride fibers from a preceramic polyblend obtained by blending a decaborane-dimethylformamide polymer and polyacrylonitrile. Ceramic fibers containing

DECABORANE(14)-BASED

b402+*0,c

+

POLYMERS

747

~c-Ma,+zco

Scheme 17. Conversion of metal oxides to metal borides.

boron carbide ranging from 1030% and boron nitride fibers having 75% boron nitride content were made from the preceramic blends. 4.2.4. Metal borides Boron forms binary compounds with most metals, and these materials are in general high-melting, extremely hard solids with high degrees of thermal stability and chemical inertness. In addition, many metal borides also have metal-like conductivities and/or unusual magnetic properties. As a result, these materials have numerous possible structural and electronic applications. 170-272 A number of different high-temperature powder techniques have been used for the preparation of metal borides and the most common method is the reduction of the metal oxide with boron carbide and carbon270-272(Scheme 17). This method has been successfully applied to the large-scale preparation of hexaborides, tetraborides and diborides of most transition metals. This reaction, although efficient, is not useful for the syntheses of coatings or shaped materials since neither the metal oxides nor B4C are processable. In addition, the final products are contaminated with carbon and oxygen. 31Processable boron-containing polymers can be employed successfully to obtain these borides in the form of films, coatings, fiber or shaped material. The high boron content of polymeric Lewis base adducts of decaborane coupled with their processability qualify them as a good boron source for the preparation of metal borides. Processable precursors to metal borides can be obtained by dispersing a metal source such as metal oxides in the boron carbide polymeric precursor followed by heating the mixture to obtain metal borides. In this process, the in situ generation of boron carbide followed by reduction to produce the boride or direct reaction of the polymer with the metal oxide to give the final boride product probably take place. Su and Sneddon30’31reported the preparation of metal borides such as TiB2, ZrBz, HfB2, TaB2 and NbBz through heat treatment of the corresponding metal oxides with decaboranedicyanopentane polymer (XIII) in an inert atmosphere (Scheme 18).31 The metal oxide/polymer dispersions were prepared by adding a metal oxide to a THF solution of the polymer, followed by vacuum evaporation of the solvent. The metal/boron/ carbon ratios in the dispersions were adjusted by varying the metal oxide/polymer ratios in the mixtures. In all the samples, about 25% excess of the polymer over that required for the

+ MO2 or M205 IA MB where Scheme 18.

M=Ti. Zr, Hf, Nb,Ta

Conversion of metal oxide and [BI&2.NC(CH2)5CN], (XIII) mixture to metal boride.31

748

S. PACKIRISAhJY

100 -s

90

Y

E 2

00

g

70

0

400

000

1200

1600

Temperature (.C 1 Fig. 18. TGA trace3’ for Ti02 + XIII mixture.

stoichiometric formation of the diboride was used. Submicron-sized metal oxides were found to give the most homogeneous mixtures.30731 The conversion process for the metal oxide/polymer dispersions was followed by the TGA-DTA-MS method. As shown in Fig. 1830three weight-loss events were observed for all metal oxide dispersions. An initial two-step weight-loss event (8-15%), consistent with the evaporation of residual THF solvent and polymer decomposition, occurred in the 50580°C range for each sample and was found to be independent of the oxide identity. Mass spectral analyses of the volatile decomposition products showed predominantly THF during the first weight loss (50-180°C) and methane in the second (300-580°C). A strong exotherm was observed in the DTA at the end of the first weight loss, suggesting that the decomposition of the polymer also occurred at this temperature. The XRD analyses of the Ti02, Nbz05 and Taz05 polymer dispersions pyrolyzed at 580°C showed only peaks corresponding to metal oxides, suggesting an initial polymer decomposition to amorphous BCN materials. No significant change was found in the 580-1250°C region. The major weight losses for all samples took place at temperatures higher than 1250°C corresponding to subsequent reduction of the metal oxides to metal borides. The metal oxide/polymer dispersions employing TiOz, ZrOz, Hf02, Nbz05 and Ta205 gave the metal diborides as crystalline products as indicated by the XRD patterns. Elemental analyses of the Ti, Zr, Nb and Ta boride samples prepared at >145O”C showed 1:2 metal:boron ratios. However, the HfO:! dispersions were only about 60% reacted (as estimated by their XRD patterns) at this temperature and required higher temperatures to give complete conversion. The 0, C and N impurities were found to depend upon the reaction temperature and time. 31 The subsequent reduction of the metal oxides to metal diborides was found to proceed differently depending upon the metal. The experimental data for the Ti02/polymer system suggest that, following initial polymer decomposition, titanium borate, titanium carbide and boron oxide are formed in the reaction of Ti02 with the BCN material by one or more of the reactions, and at higher temperatures, further reaction occurs to produce the final titanium diboride product (Scheme 19).31 In the Nbz05/ and TazOs/polymer systems, the XRD patterns of the ceramics prepared at

DECABORANE(14)-BASED

Ti Q/polymer 3Ti02

-Ti

3TiQ+lOB

BO3 + 2TiC 4-B?%

-3TiB2i-2B20j

TiO2 + 3C -

TiC+B&

+

749

-Ti@I”BCN’+H2

-t38+2C

2TiBg

POLYMERS

X

TiC+ZCO

-2TiC

+ B,4+3CO

+ 2C -Ti&

+3CO

Scheme 19. Conversion of TiOz and XIII mixture31 to TiB*.

MzOJpolymer

MI 05 +7C

-2MCi5CO

-3MB

3M205t19B

+3MB,

-t 5 B,O,

-2MB2+2MB+9C0

4MC +3&0,$~ MB+B M

I”BCN I’

-M,O,

=

-MB Nb

2 or

Ta

Scheme 20. Conversion of Nb20gTa205 and XIII mixture31 to NbBZ/TaB*.

MO?+ 2B +2C--MB?+

2C0

or

3hK++3C+hB-3MC+2B2

0,

MC + B,O, +2C xMB,+XO M=

Zr or t-if

Scheme 21. Conversion of ZrO#IfOz and XIII mixture31 to ZrBz/HfBZ

different temperatures suggest the reaction sequence, shown in Scheme 20 for the reduction of the metal oxides. These reactions involve the initial formation of metal carbide, Bz03 and/or metal monoborides followed by the conversion of these species to the diborides at higher temperatures ( > 1450°C) (Scheme 20).31 In contrast to the above observations, no intermediate species were observed in the ZrO;?/ and HfOJpolymer systems (Scheme 21).3* The XRD patterns of the materials obtained before completion of the reactions showed only the unreacted metal oxides and the diborides. No reaction was observed between the Zr02 and the BCN material at lOOO”C,but the 1450°C material contained 90% crystalline ZrBz, together with unreacted ZrOz. The reaction was complete when the sample was pyrolyzed at 1450°C for 21 h. Similar results were

750

S. PACKIRISAMY

observed in the HfOz/polymer system, except that a higher reaction temperature (1900°C) was necessary to drive the reaction to completion. These observations suggest that Zr02 and HfOz react with the BCN material in one step or alternately, that the initial formation of carbides and boron oxide is followed by a fast reaction to produce the borides. The latter explanation would imply that the ZrC and HfC are much more reactive than TiC and that they react with boron oxide to give diborides immediately after their formation. SEM and TEM studies30,31suggest that the grain sizes and microstructure of the metalboride powders depend upon the metal and the processing conditions. SEM of the TiB2 and ZrBz powder samples showed particle sizes of 2-5 pm at 2OOO”C,but submicron crystallites at lower temperatures. Both the TiB2 and ZrB2 samples prepared at 1300°C (2 h) contained nanometer-sized TiBz or ZrBz particles. As suggested by the higher than theoretical ceramic yields (theoretical for TiB2: 48.9%; obs. 58.2%; theoretical for ZrB2 61.1%; obs. 67.6%) and the XRD results, both samples at this temperature contained either small amounts of intermediate species or unreacted oxide. When the samples were heated at 1450°C for 1 h, the amorphous phase between the TiB2 and ZrBz grains decreased and further crystallization of the borides was observed. The samples annealed at 1450°C for 21 h showed significant grain growth. In agreement with these results, the XRD spectra for TiB2 and ZrBz prepared at higher temperatures or longer times exhibited sharper diffraction peaks than those from samples prepared at lower temperatures. In the NbBz and TaB2 systems, the TEM micrographs of the 1300°C materials (2.5 h) showed larger grains (0.5-1.0 pm) than the TiBz and ZrBz samples (50-100 nm) annealed at the same temperature. The NbB2 and TaBz samples prepared at 1480°C had an average grain size of 1.0 pm, as estimated by TEM. Only slight grain growth was found for the 1900°C samples. Metal-boride coatings have been used in a number of important practical applications. For example, TiBz has been used as a coating material for the protection of electrodes from chemical corrosion and for a hard coating on tools.270-272Chemical vapor deposition (Cm)273-276 or physical vapor deposition (PVD)277-279methods are widely used for the generation of titanium diboride and zirconium diboride films. A solution method for achieving such coatings would clearly have a number of advantages. Su and Sneddon3t evaluated the use of decaborane polymer/metal oxide systems described above for the formation of metal boride coatings on graphite surfaces. In a typical process, a suspension of TiOz in THF was prepared by the complete mixing of a submicron Ti02 powder, decaborane-dicyanopentane polymer and a dispersant by means of ultrasonic agitation. The precursor coatings were obtained by either dipping the substrate into the emulsion or by dispersing the emulsion on the carbon plates. The thickness of TiBz dispersion was adjusted by either multiple coatings or by increasing the viscosity of the precursor solution. Subsequent pyrolysis of the coated carbon plates under argon or in vacuum above 1450°C produced TiB2 coatings.31 An Auger analysis of the elemental composition of the coatings annealed at 2000°C showed a nearly 1:2 titanium:boron ratio. The XRD pattern shows crystalline TiB2, in addition to the diffraction peaks arising from the graphite substrate. Consistent with bulk pyrolysis results, no other crystalline species were observed. The SEM of a coated graphite plate pyrolyzed at 1450°C for 2 h shows that a homogeneous, porous TiB2 coating is formed on the surface. When the coatings were heated at 2OOO”C,larger crystals

DECABORANE(14)-BASED POLYMERS

7.51

(2-4 pm) were formed. Due to the grain coarsening, the coatings prepared at high temperatures were more porous than those made at low temperatures. These results suggest that polymer/submicron metal oxide dispersions along with a suitable surfactant should be useful for the generation of a variety of metal-boride coatings of potential technological importance. 31 Seyferth et al. 28reported that metal borides can be synthesized by pyrolyzing a mixture of metal powder and [BrOHlz.diamine], polymer. Thus, a composite prepared from the polymer and Ti powder (Ti:B ratio = 1:2) on pyrolysis to 1500°C in a stream of argon gave a ceramic residue in which TiB2 was the. only crystalline phase present as evidenced by the XRD pattern. Elemental analysis (61.74% Ti, 24.60% B, 3.24% N and 1.56% C) indicated that about 7% excess of Ti (over that required to form TiB2 with the B in the sample) was present; therefore, the remainder of the ceramic residue may have been an amorphous titanium carbonitride. Ceramic products containing other metal borides (TiB2, HfB2, CrB and CrB2, MOB and MoB2, WB and W2B and LaB6) could be prepared by a similar procedure in high yield. In the case of Ti and Zr powder composites, pyrolysis in a stream of ammonia to 800°C followed by heating to 1500°C in argon resulted in the formation of a ceramic containing a mixture of the respective crystalline metal boride and nitride. 5. DECABORANE-BASED POLYMERS AS ATOMIC OXYGEN-RESISTANT COATINGS

The photodissociation of molecular oxygen in the upper atmosphere (200-700 km) results in the formation of atomic oxygen (AO).28o It is the most predominant neutral species in the low earth orbit (LEO) environment281-283(Fig. 19). Although the A0 density is quite low at altitudes where LEO spacecraft typically operate, the high orbital speeds (about 8 km/s) required to maintain the spacecraft in orbit can result in high incident fluxes. The collision energy of impact between A0 and the ram surfaces of the spacecraft is about 5 eV.282,284 As this energy is above the threshold energy for gas/surface interactions, the atmosphere interacts with the spacecraft materials in a number of ways adversely affecting their properties. 700 600 300

200

._

2

100

lo5 lo6

10'

10'

Number Fig. 19. Atmospheric

composition

lo9

1o’O 10"

dcnsitytcz

lo'* )

as a function of altitude. ***

752

S. PACKIRISAMY

Flight experiments conducted during space shuttle flights285-291STS-5, STS-8, STS-32, STS-41 and STS-44, and materials exposure experiments conducted on long duration exposure facility (LDEF)292-295have shown that the LEO environment causes oxidative degradation of many candidate spacecraft materials. Polymer matrix composites, tribomaterials, and thermal control, optical and space power components interact with AO, which results in a deterioration of their performance.296-301Thus, spacecraft materials in LEO environment require protective coatings in order to meet the long life expectancy of the spacecraft. Several approaches to protective coatings for A0 have been reported. Metal oxide coatings such as aluminum oxide,302,303silicon dioxide302-3Wand indium tin oxide305-307 have been recommended since they have negligible erosion rates. However, a lack of flexibility, the requirement of elaborate sputtering techniques for application on complex shapes and their susceptibility to pin hole defects due to shadowing of dust particles limit their application. These coatings also easily crack on thermal cycling since the substrates, usually polymers, have much higher thermal expansion coefficients than the inorganic coatings. In recent years, there has been considerable interest on the use of polymeric materials as protective coatings for AO. Fluorinated polymers,308-311 silicones,312-314 fluorinated polyphosphazenes,315 poly(carborane-siloxane)s316-320 and polymeric Lewis base adducts of decaborane32-35 have all been considered for this purpose. The ease and flexibility of application of polymeric materials by spin-coating, dip-coating or spraying allow the coating of flexible structures or complex shapes. In this review, the developments relating to AO-resistant coatings obtained from poly(carborane-siloxane)s and polymeric Lewis base adducts of decaborane are reviewed. 5.1. Simulation of low earth orbit (LEO) atomic oxygen environment To evaluate the performance of coatings and to qualify them for use in LEO space environments, it is necessary to simulate these conditions in ground-based testing laboratories. However, there appears to be no facility that exactly simulates the LEO atomic oxygen (AO) environment. Several techniques have been used to produce oxygen atoms or ions with or without acceleration for impingement on test samples. Plasma asher, a plasma chamber where oxygen plasma is produced using radio frequencies, serves as an inexpensive source for exposing different coatings to A0 in ground-based experiments 317-319,321-324 In Table 8,325plasma asher and LEO environment are compared. It is seen that Plasma asher and LEO environment differ considerably in their compositions and hence, it is difficult to extrapolate the results observed using a plasma asher to predict the lifetime of polymeric coatings in LEO environment. Nevertheless, ground-based experiments serve the purpose of identifying and obtaining preliminary evaluation data on candidate materials for LEO environment. Evaluation of protective coatings in groundbased experiments involves the following: (a) mass-loss measurements at regular intervals of the substrate protected by the coating in order to understand the effectiveness of protection, (b) surface analyses of the coatings by SEM, XPS (X-ray photoelectron spectroscopy) and optical microscopy before and after exposure to AO, (c) measurement of optical properties of the coatings before and after exposure to A0 and (d) subjecting the coated

DECABORANE(14)-BASED

POLYMERS

753

Table 8. Comparison of plasma asher and LEO environments325 Environment

LEO Plasma asher

A0 Fluxa

lOI5

10’9-20

Energy (eV

02 molecule Fhlxa Energy (ev)

5 0.04-0.06

Electron Density Energy (cmm3) (ev)

Fluxa

Wave length (nm)

105-lo6 109-10’2

4 x 10” 10’2-1014

121.6 130.0

0.1 l-10

uv

a Flux in particles/cm*.s

substrate to several thousand cycles of temperature variations ranging from -80 to 120°C (close to the temperature variations encountered in LEO environment). 5.2. Effect of atomic oxygen on siloxane coatings In order to understand the need for polymers having polyhedral boron hydride moieties in their backbone as AO-resistant coatings, it is necessary to consider the performances of polysiloxanes which have been extensively studied. Early flight data for A0 exposure with silicones326 mdicated * A0 erosion yields between 1 and 2 orders of magnitude below those of Kapton327 H polyimides. Ground-based laboratory exposure of silicones to A0 has demonstrated protection of underlying organic materials.328 Since silicones develop a glassy SiO2 surface on A0 attack, they have been widely used to protect underlying oxidizable organic materials. On reaction with AO, the silicone coatings (d = 1 g/cc) lose their organic components and undergo a density increase to that of amorphous Si02 (d = 2.4 g/cc). Due to this density increase, the coatings shrink. Extensive exposure to A0 causes macroscopic shrinkage which results in crack formation within the coating (Fig. 20).316A0 can then reach the substrate through these cracks. This problem can be overcome by incorporating carborane units into the siloxane backbone. A carborane moiety contains 10 boron atoms which takes-up 15 oxygen atoms on oxidation. Hence, carboranesiloxane polymers can undergo considerable weight increase (50%) on reaction with AO, which would offset the volume shrinkage due the increase in density [from 1 to 1.92 g/cc for Br00r5.(Si0&]. 5.3. Tailoring of poly(carborane-siloxane)s

and [B&12.L--L.], oxygen-resistant coatings

for atomic

Litt and coworkers317-320developed poly(carborane-siloxane)s coatings, Pl, P2 and P5, and evaluated their effectiveness as AO-resistant coatings. The chemical structures of these polymers are shown in Fig. 21.320 Pl was developed by end-capping a commercially available Dexil 300260GC polymer with triacetoxysilane. The acetoxy end-capped polymer (Pl) reacts on exposure to moisture to form acetic acid and silanol end groups which then self-condense. Curing can be catalyzed by dibutyl tin dioctanoate. Films were cured at room temperature and humidity for 2 days and then at 50°C overnight. The coatings were not transparent. Optical microscopy and SEM observations showed that the films tended to crystallize during curing at room temperature. 316 To overcome this problem, a highly functionalized polymer (P2) was synthesized by a

S. PACKIRISAMY

754

Fig. 20. SEM of a crack in a siloxane (P4) coating.33S320

0

a-&-O H+Z-_Si

I

CBloH,&-Si-O-5

(Pl)

I

w+o 0

HO

-I W

H

I

O-Si-CB,,,H,oC

CH,

(P4)

CH3

CH,

-ii-O-Ai &H3

OH k

j n

Fig. 21. Chemical structures of Pl,P2,P4 and P5 polymers.3”

(P5)

DECABORANE(14)-BASED

POLYMERS

75.5

reaction of methyltriacetoxysilane and 1,7-bis(hydroxydimethylsilyl)-m-carborane. 317This produced a prepolymer with one acetoxy crosslinking site per repeat unit that had a polystyrene equivalent molecular weight of 6000. The solubility of P2 in THF and toluene indicated that steric hindrance prevented the third acetoxy group from reacting. This material cures in a manner similar to Pl. The P2 coatings took over 1 week to cure because of their low molecular weights. The long cure times for the moisture-cured P2 coatings prompted a search for faster cure systems. The reported curing of poly(methylhydrosiloxane) (P4) with heat and peroxides led to the development of a cure based on the UV light generation of radicals. Poly(carborane-siloxane)s (Pi!) having molecular weights of 3300 and 13,000 were synthesized from bis(hydroxydimethylsilyl)-m-carborane by two different synthetic routes. 320In the first procedure, P5 was prepared by reacting 1,7-bis(hydroxydimethyl)-m-carborane with methyldiacetoxysilane in toluene under reflux conditions. During the reaction, acetic acid was distilled off. The reaction was stopped when the peak molecular weight of the polymer reached 3300. In the second procedure, following Hedaya’s work,329 high molecular weight (13,000) poly(hydrocarborane-siloxane) (P5) was obtained by reacting equimolecular amounts of 1,7-bis(hydroxydimethylsilyl)-mcarborane and bis(aminodimethyl)methylsilane in toluene under reflux. As will be seen from the forthcoming discussion on the mass loss of poly(carboranesiloxane) coatings, the performance of these coatings is superior to those of polysiloxanes and many other systems. However, the high cost and the non-availability of the carboranebased monomers have limited their use as protective coatings for AO. As discussed earlier, poly(carborane-siloxane)s are synthesized from decaborane involving several synthetic steps. Thus, it would be advantageous if polymers synthesized directly from decaborane in a one-step synthesis could meet the requirements for AO-resistant coatings. As in the case of carborane units present in poly(carborane-siloxane)s, the Br0Hr2repeat unit present in polymeric Lewis base adducts of decaborane, [BroHr&---L],, can take-up 15 oxygen atoms resulting in considerable weight increase, thus meeting one major requirement. Hence, attempts were made by Packirisamy and Litt32-35to design polymeric Lewis base adducts of decaborane suitable for atomic oxygen-resistant coatings. Initially, the suitability of the polymers, [BlaH12.H2NCH2CH2CH2NH~.],, (VIII) and [B10H12.N(CH3)2CH2CH2N(CH3)2.]~ (VI) which were reported to be precursors for ceramics, were evaluated as AO-resistant coatings. 32 Unlike poly(carborane-siloxane)s, the above polymers are soluble only in polar solvents such as DMAc, DMF and DMSO. The coatings obtained from the above two polymeric Lewis base adducts of decaborane were translucent. AO-resistant coatings designed for application on Kapton and on silver mirrors must be transparent. Moreover, use of polar solvents resulted in the formation of defect sites due to dewetting. On exposure of the coatings on Kapton coupons to AO, it was observed that undercutting occurred at the defect sites due to reaction of the substrates with AO. In order to overcome the solubility problem and also to increase the flexibility of the polymeric chain, polymeric Lewis base adducts of decaborane, DBP-1,2,3 and -4 (Fig. l), were synthesized from diamines containing Si-0-Si or ether linkages, the details of which were presented earlier (Section 2.1.3). DBP-1 containing the Si-0-Si linkage is soluble in THF and hence, the coatings were prepared from solutions of this polymer in THF.

7.56

S. PACKIRISAMY

DBP-2,3 and -4 were insoluble in THF and the coatings were prepared from polymer solutions in a 1:l mixture of THF and DMAc. 5.4. Mass loss measurements of coatings on exposure to atomic oxygen Kapton polyimide film, which is widely used for space applications, was used as a substrate for the evaluation of the performance of the coatings. A convenient procedure involves measuring the mass loss at different time intervals of the Kapton film protected with the coating and of unprotected Kapton film on exposure to atomic oxygen. The mass loss of protected and unprotected Kapton can be plotted against the exposure time. Alternately, the mass loss data can be plotted against A0 fluence, expressed in atoms/cm2. A0 fluence can be calculated from the mass loss data of unprotected Kapton film using the following equation

where AM is the mass loss in grams, A is the surface area of the sample and E is the erosion yield expressed in cm3/atom. In the case of Kapton polyimide film the value of E is 3.0 x lo-% cm3/atom. Five and 25 wt% solutions of P2 in toluene were spin-coated onto Kapton or silver substrates. Subsequent exposure to moisture in the air for 1 week at room temperature cured the prepolymer to a final clear coating. Since only one side was coated, two samples were sandwiched together with double stick Kapton tape. Coatings of P4 and P5 were prepared by spin-coating a dilute solution (10%) of the polymer in toluene onto Kapton substrates (125 pm thick). Oxidative crosslinking of the polymers was carried out using a photocatalyst. The coatings were cured for 10 min by exposure to ultraviolet light in a Loctite Corp. Zeta 7000 photocure unit. The other side was then coated and cured the same way. For testing the A0 resistance of DBP-1,2,3 and -4 based coatings, an improved experimental technique was used.330 Circular Kapton coupons of 1.5 cm diameter were spincoated with a 20% solution of polymer in THF or 1:l mixture of THF and DMAc on one side and gold-coated on the other side. They were dried in an air oven at 38°C for 10 h and then under vacuum at 60°C for 24 h. This procedure permits the exposure of only one surface to AO. The other side and the edges are protected from A0 by the gold coating. The coatings prepared from P2, P4, P5, DBP-1 and DBP-4 were exposed to A0 in a plasma asher for periods ranging from 1 week to 5 weeks. A highly crosslinked 1.1 pm thick P2 coating on Kapton was exposed to A0 equivalent to 5 years of space exposure. The coated sample showed no macroscopic change in appearance and lost very little mass, whereas the uncoated 125 pm thick control became opaque and lost three quarters of its initial weight (Fig. 22).317The performance of an 8% PTFE/92%Si02 ion beam-deposited coatings is shown for comparison. Of the carborane-siloxane polymer-based coatings studied, the UV-cured P5 coatings on Kapton substrates provided the best protection against AO. A Kapton coupon coated with 0.2 pm thick P5 showed negligible mass loss after 5 weeks in the asher (Fig. 23).320It was only after extensive ashing that the sample began to lose weight.

DECABORANE(14)-BASED POLYMERS

Exposure Fig. 22. Mass loss of unprotected

757

time (hr)

and P2 protected Kapton on exposure to atomic oxygen. 317

In the case of the DBP-1 coating on Kapton obtained from the solution of the polymer in THF, practically no weight loss was observed on exposure to A0 (Fig. 24).33 Polymers DBP-2,3 and -4 were insoluble in THF and when DMF or DMAc was used as the solvent for preparing the coatings, dewetting was observed in some areas of the coatings, which then became vulnerable to A0 attack. This problem was overcome by using a 1:l mixture of THF and DMAc for preparing the polymer solutions. The coatings obtained from DBP-2 and DBP3 were translucent and coatings from DBP-4 were transparent.

Knpton x 0.01

Atomic

Fig. 23. Mass loss of unprotected

oxygen fluence (atomdcmz)

and P5 protected Kapton on exposure to atomic oxygen.320

758

S. PACKIRISAMY

Fig. 24. Mass loss of unprotected and DBP-1 protected Kapton on exposure to atomic oxygen.33

The contamination of space components by the degradation products of silicone coatings is of great concern.328 In view of this, the coating from DBP4 would be of particular interest as it offers a transparent coating from a polymer which is devoid of silicon. The mass-loss data suggest that the performance of DBP4 coating is inferior to that of DBP-1 coating. The mass loss of Kapton protected by DBP4 was four times higher than that of DBP-1 over a period of 1 week exposure in the plasma asher. The reason for this observation can be understood from SEM data, which will be discussed later. 5.5. Surface analyses of coatings The changes that have taken place on exposure to A0 can be well understood by

analyzing the coatings before and after exposure by SEM, optical microscopy and by measuring their optical properties. The chemical composition of the coatings with respect to the depth can be analyzed using XI%. 5.5.1. Scanning electron microscopy studies On exposure to plasma, coatings of P2 (on Kapton) thinner than 1 pm stayed flat, whereas thicker ones became waffled (Fig. 25).317The stress-relief patterns indicate that compressive stresses are present in the film, and above a certain thickness, these stresses can cause waffling. The waffled morphology is similar to the swelling behavior of gel films when they incorporate solvent. Thus, waffling confirms that the material is increasing in volume probably by incorporating oxygen. Waffling in carborane polymers is attributed to the expansion of the film during initial oxidation as atomic oxygen inserts itself between

DECABORANE(14)-BASED POLYMERS

759

Fig. 25. SEM of P2 coating on Kapton showing waffling dependence on thickness. 319

silicon-carbon bonds, forming methoxy groups.316,317The effect of the coating thickness on waffling was observed in homogeneous regions of the films when viewed with interference microscopy. The transition from a smooth surface to a waffled region occurs between 750-1000 nm thickness. At higher magnifications in the SEM, a smooth, crackfree, glassy layer was observed on the film surface, with an estimated thickness of 0.15 pm. The rubbery coating can accommodate severe bendings of the specimen, whereas the brittle glassy layer cracks extensively (Fig. 26).319 The UV-cured poly(carborane-siloxane) from P5 was less susceptible to waffling than the moisture-cured coatings from P2.320Waffling was observed only in very thick regions, 2 pm, while the P2 coatings waffled at a thickness greater than 0.7 pm. After ashing, P5 coatings were nearly smooth and featureless. In contrast, the glassy layer on oxidized P4 coatings was extensively cracked (Fig. 20). This demonstrates the advantages of boron hydride-based polymer coatings, which showed no cracks on exposure to atomic oxygen, in contrast to P4 and other silicone-based coatings. SEM analysis, of the PS coating showed that the edges of the sample had been damaged. 320It is known that the edges are more heavily attacked in the asher. Only a few areas of the coating had failed. In these regions, undercutting occurred. The bulk of the damage was at the edges, so these ashing rates represent the upper limit of Kapton’s massloss rate using a P5 protective coatings. SEM photographs of DBP-1 coatings exposed to A0 show that the surface had no defects and that Kapton was completely protected. They further indicate that the coatings (5 pm thick) do not undergo shrinkage-induced cracks or waffling. To understand the

760

S. PACKIRISAMY

Fig. 26. SEM of a broken glassy layer in a severely bent area of the P2 coating.319

coating better after exposure to AO, the coating was scraped carefully and analyzed by SEM. SEM photographs of the broken glassy layer and the underlying unoxidized polymeric layer are shown in Figs 27 and 28.33 SEM studies of a DBP4 coating indicated that it offered very good protection for the substrate and the weight loss that was observed was mainly due to undercutting at a defect site.32 Attempts to improve the performance of DBP-4 have not met with success as the defect site can not be avoided. The formation of a defect site is probably due to the pick-up of dust particles by the coating. The slow evaporation of DMAc, a high boiling solvent, used for the preparation of the coating probably adds to this problem. SEM studies of cracks developed by severe bending of DBP-1 and DBP4 coatings exposed to A0 indicate that a crack generated in the glassy layer propagates deep into the underlying polymer, suggesting that the coatings have poor mechanical properties when compared to poly(carborane-siloxane) systems.33 As discussed earlier, severe bending of the P2 coating exposed to A0 resulted in the development of extensive cracks in the glassy layer but no cracks were observed in the polymeric coating beneath the glassy layer. Coatings based on poly(carborane-siloxane) are crosslinked systems and hence have better mechanical properties. The decaborane-based polymers investigated were of low molecular weight and underwent degradation in polar solvents. This could be the reason for the poor mechanical properties observed.

DECABORANE(14)-BASED POLYMERS

761

Fig. 27. SEM of broken glassy layer of DBP-1 coating33 on Kapton ashed for 624 h.

5.5.2. Optical properties The optical properties of the substrate should not be altered due to the protective coatings before and after exposure to AO. If the protection is not effective, it will lead to surface erosion of the substrate resulting in an alteration of optical properties. There appears to be no systematic study on the optical properties of polymeric coatings. Information is available only for silver mirrors coated with P2 polymer. It was observed that uncoated silver rapidly degrades on exposures to A0 whereas I%coated silver mirrors show only a slight loss of solar specular reflectance on exposure to A0 for 1 week (Fig. 29).317The fractional loss of the solar reflectance extrapolated to 1000 h of ashing time (about 40 years in LEO) was only 0.025. This shows that P2 offers very good protection for silver mirrors. 5.5.3. XPS depth profile analysis The nature of the glassy layers and the interfaces of the AO-resistant coatings was investigated by Xl%. The survey and high resolution spectra were collected by using a Mg (KCX= 1253.6 eV) anode at 400 W. The depth profiling of the coatings was performed using a 4-keV calibrated high-purity argon ion beam for sputtering. Under this condition, a reproducible sputtering rate of approximately 33 A/min was obtained.319 A detailed discussion on the survey spectra at different depth levels for P2 coatings is presented elsewhere. 319Here, the depth profile analyses of different coatings are compared

762

Fig. 28. SEM of polymer layer underneath

S. PACIURISAMY

the glassy layer in DBP-1 coating on Kapton ashed for 624 h.33

P2

Exposure

time ( hr 1

Fig. 29. Relative solar specular reflectance versus exposure time to atomic oxygen for unprotected protected silver mirrors. 317

and P2

DECABORANE(14)-BASED

POLYMERS

763

6

0

0

1000

2000 3ooo 4000 Sputtering

depth(A)

Fig. 30. Atomic ratio depth profiles of a P2 coating319 (normalized

to silicon) ashed for 120 h.

in order to understand their modes of interaction with AO. For this purpose, conventional depth profiling was done by averaging a smaller number of high resolution spectra for each depth level, which usually gives noisier curves. It has, however, the advantage of including a large number of experimentally-acquired data counts from many depth levels. By applying nonlinear curve fitting methods to the data, it is possible to obtain much smoother count depth profile curves. From these curves, the atomic ratio with respect to silicon was determined for each element. Such continuous depth profile curves of the atomic ratios provide excellent insight into the possible interactions between the coatings and AO. In Fig. 30319atomic ratio with respect to silicon is plotted against sputtering depth for a P2 coating exposed to AO. The approximate 2:l oxygen to silicon ratio at the surface and the very low values of boron indicate that the glassy layer’s top surface is close to a silica composition. The mechanism of boron depletion is presumably related to exposure of the sample to humidity since boric oxide can easily hydrolyze to boric and metaboric acids. These compounds can volatilize under the high vacuum of the XPS chamber. However, our recent studies on decaborane-based polymers show that the boron-poor surface might also be due to the contamination of the surface with SiOt formed by sputtering of the glass walls of the plasma asher or silicone grease contamination in the plasma asher. The oxygen curve rises to a maximum at about 140 nm and then gradually drops, while still remaining above the original polymer composition. This maximum can be attributed to the combined contribution of the silicon and boron oxides at this depth. However, the oxygen concentration stays higher than its initial concentration in the polymer even to a depth of 300 nm. This explains the increase in volume, which manifests itself as waffling in the thicker regions of the coating.319 Carbon content is almost nil in the glassy layer and sharply increases across the interface. This in fact is the best indication of the interface location, since it is expected that practically no carbon would be present in the glassy layer. Trends in the carbon profiles do not coincide with the boron curve. It is believed that the depletion of boron from the surface, perhaps by the mechanism suggested above, is a separate process independent of the glassy layer formation.319

S. PACKIRISAMY

764

6

0

500

1000 1500 2000 2500

Sputtering

depth ( i )

Fig. 31. Atomic ratio depth profiles of a P5 coating320 (normalized

to silicon) ashed for 217 h.

XPS analysis of oxidized P5 coating shows a different profile (Fig. 31)320when compared to the P2 coating. Less boron has been depleted from the surface. This was probably due to the more rigorous handling conditions developed after observing the results from P2. The surface composition corresponds to that of a borosilicate glass. The glassy layer is much thinner than in the P2 coating as shown by the presence of carbon even at 15 nm. The coating composition quickly approaches that of the PS polymer, in contrast to the observations made for the P2 coating. The boron and carbon reach the initial polymer composition by 80 nm. By 230 nm the oxygen has dropped almost to the theoretical value of 0.8 O/Si for a 50% cured film. This shows that oxygen penetration into the P5 coating is less than for the P2 coatings. The P5 coatings are more crosslinked than the P2 coatings. The tighter network of the P5 coatings could inhibit the penetration of oxygen. It was observed that the PS coating ashed for 5 weeks was oxidized to a much greater depth (Fig. 32)320than was found for the coating oxidized for 1 week. In the case of P5 2-5

3000

1000 Sputtering

5000

depth Ci 1

Fig. 32. Atomic ratio depth profiles of a P5 coating3”

(normalized

to silicon) ashed for 946 h.

DECABORANE(14)-BASED

POLYMERS

765

coating ashed for 217 h, the composition of the coating reaches that of the polymer at a depth of 80 nm, whereas in the PS coating exposed in the plasma asher for 946 h, the composition of the coating does not match with that of the polymer even at a depth of 400 nm. This implies that, with increased exposure time, A0 is able to penetrate deeper into the coating. XPS depth profile analysis of the DBP4 coating showed the presence of a 20 nm thick Si02 layer. 32,330Since this polymer does not contain Si, the deposition of Si02 over the oxidized coating might be due to the sputtering of the glass walls of the plasma asher by plasma.32 Analysis of the depth profile data for DBP-4 and DBP-1 coatings are being carried out to understand more about the interaction of A0 with these coatings.330 6. CONCLUDING

REMARKS

In this review, polymers synthesized from decaborane(l4) and their application as precursors for ceramics and atomic oxygen-resistant coatings were reviewed. Based on the survey of the pertinent literature presented in this review, the following conclusions can be drawn and the scope for future research directions can be suggested. Polymeric Lewis base adducts ofdecaborane: (1) In recent years this class of polymers has gained significance in view of its potential application as precursors for ceramics and atomic oxygen-resistant coatings. (2) The major advantage of these systems is that they can be obtained in almost quantitative yield, and on heat treatment, they give high char residues without the necessity for crosslinking. (3) Though these polymers can be synthesized by two synthetic routes, viz. directly from decaborane through the reaction with bidendate ligands or through a displacement reaction involving B1&12k and a bidendate ligand, mostly the first approach is used. It may be worth attempting the second method to investigate if the molecular weight and other characteristics of the polymer can be improved. (4) Except for a few polymers, most of the polymers reported in the literature are insoluble in non-polar/less polar solvents. This poses problems when the application aspects of these polymers are considered. Thus, it would be worth focusing on the synthesis of polymers that are soluble in relatively less polar solvents. This may require the synthesis of newer ligands. (5) Most of the polymers reported have been synthesized from conventional ligands. It would be interesting to investigate the possibility of synthesizing polymers from organometallic ligands, which would provide an opportunity for obtaining a variety of mixed ceramic systems. (6) Polymeric Lewis base adducts of decaborane undergo degradation in polar solvents. Though some work has been done to characterize the degradation products, the role of ligands and solvents in such degradation processes merits further study. Polymers derived from carborunes: (1) A variety of polymers have been synthesized from carborane monomers. However, of these polymers, only poly(carborane-siloxane)s have gained commercial significance. (2) Of the polymers reported recently, poly(carborane-siloxane-acetylene) appears to be quite promising for application as matrix resins in high temperature composites and as precursors for B&-Sic. (3) Incorporation of carborane units into high temperature polymers such as polyimides and poly(ether-ether-ketone) has further improved their thermal stabilities. The end-use of these polymers for aerospace applications and other specialized functions needs to be explored. (4) The high cost of

766

S. PACKIRISAMY

carborane monomers limits the end-uses of carborane-based materials and work needs to be directed towards more cost effective synthetic procedures. Decaborane-based polymers as precursors for ceramics: (1) The high boron content of carborane/decaborane provides an opportunity for obtaining boron carbide and boron nitride ceramics. As the carborane/decaborane moieties present in the backbone of the polymers stabilize them against degradation, no additional crosslinking is required. This is a major advantage when compared to other preceramic polymers. (2) Poly(carboranesiloxane)s provide an opportunity for obtaining mixed non-oxide ceramics B&-Sic, which are preferred especially as protective coatings for oxidizable substrates. As discussed earlier, the high cost of these polymers is the limiting factor. (3) Polymeric Lewis base adducts of decaborane provide a viable alternative to poly(carborane-siloxane)s as precursors for ceramics in view of the relatively low cost of the starting material (decarborane) when compared to carborane. A considerable amount of work has been done on the conversion of these polymers to ceramics; however, research efforts need to be directed towards producing and evaluating end-products based on these polymers. (4) Though some patent literature is available on ceramic fibers derived from polymeric Lewis base adducts of decaborane, there is inadequate information on the properties of these materials. The high boron content of these polymers is an advantage for such applications. The solubility of these polymers and their tendency for degradation in polar solvents may limit their utility for the preparation of continuous fibers. Atomic oxygen-resistant coatings:(l) The most recent application of decaborane-based polymers is their potential end-use as atomic oxygen-resistant coatings. The decarborane/ carborane moieties present in the polymer backbone can take-up 15 oxygen atoms causing an enormous weight increase, which counters shrinkage-induced cracking. (2) Poly(carborane-siloxane)s surpass the performance of polysiloxanes as atomic oxygen-resistant coatings. Due to the high cost of these polymers, polymeric Lewis base adducts of decaborane have been considered as alternatives. However, a considerable amount of research needs to be done both in ground-based laboratories and in in-flight experiments to better understand their behavior. (3) As discussed earlier, the poor solubility of polymeric Lewis base adducts of decaborane in non-polar/less polar solvents and their tendency for degradation in polar solvents are the major impediments for utilizing these systems as atomic oxygen-resistant coatings. Thus, it may be concluded that there is a considerable opportunity for further research on decaborane-based polymers. The limiting factor appears to be the high cost of decaborane and carborane monomers. ACKNOWLEDGEMENTS The author is indebted to Professor Morton Litt, Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio whose association inspired him to write this review. The author expresses his gratitude to G. Ambadas, Dr M. Rama Rao, Dr Suresh Mathew and MS Ginu Abraham for their valuable suggestions and help during the preparation of this manuscript. He is thankful to G. Rajendranathan and A. S. Radhakrishnan Nair of the Drawing Section, VSSC and to S. Surendra Babu of Central Photographic Facility, for the preparation of figures and to Mrs P. Thankamani of PSC

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Division for her assistance in typing part of this manuscript. The author is thankful to the Director, VSSC, for granting permission to publish this review and to Dr V. N. Krishnamurthy, Dr K. N. Ninan, Dr R. Ramaswamy and Dr S. S. Grover for their encouragements. Finally, the author expresses his deep sense of gratitude to his wife, Mrs P. Veda, who has encouraged him to spend many extra hours in the laboratory to complete this review. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

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