Fire and Polymers II - American Chemical Society

Chapter 18

High-Temperature Copolymers from Inorganic—Organic Hybrid Polymer and Multi-ethynylbenzene

Downloaded by UNIV OF LIVERPOOL on October 11, 2017 | http://pubs.acs.org Publication Date: July 21, 1995 | doi: 10.1021/bk-1995-0599.ch018

Teddy M . Keller Materials Chemistry Branch, Code 6127, Naval Research Laboratory, Washington, DC 20375-5320

Considerable interest has been shown in the uses of polyfunctional arylacetylenes as precursors to carbon. Carbon erodes rapidly in air at temperatures as low as 400°C. Much effort is thus being devoted at developing techniques for protecting carbon/carbon composites against oxidation at elevated temperatures. Phenolic resin systems and petroleum and coal tar pitches are currently used as the carbon matrix precursor material. There are numerous problems associated with these carbon precursor materials. We are engaged in the synthesis of compounds containing three or more phenylethynyl groups substituted on the benzene ring and inorganic-organic hybrid polymers. Copolymers of a multi-phenylethynylbenzene and a hybrid polymer show outstanding flame resistance or oxidative stability. Both compounds contain acetylenic units for conversion to the copolymer. The resistance to oxidation is a function of the amount of the hybrid polymer present in the copolymer.

Carbon-carbon (C-C) composites are strong, lightweight, high temperature materials that are used as ablators in short duration rocket and reentry systems and are currently being developed for structural applications in advanced missiles, aircraft, and aerospace vehicles. Many future applications for C-C composites require operation at elevated temperatures in an oxidizing environment. Depending on the application, they are expected to be used for periods ranging from minutes to a few thousands hours at temperatures above 1000°C and approaching 2000°C. Unfortunately, there is a major problem in using such materials in an oxidizing environment. Carbon erodes rapidly in air at temperatures as low as 400°C. Great effort is thus being devoted at developing techniques for protecting C-C composites against oxidation at elevated temperatures (1-4). Much of this interest has arisen from recent plans of the U.S. Government to build a National Aerospace Plane

This chapter not subject to U.S. copyright Published 1995 American Chemical Society Nelson; Fire and Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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(NASP) which would require lightweight structural materials stable up to 1500°C in air. Carbon-carbon composites are known to retain good mechanical properties up to 2000°C under inert conditions. An effective method of protecting carbon from oxidation is to establish a barrier against oxygen penetration in the form of an external coating. Most of these coatings rely on oxide films as oxygen diffusion barriers. The development of external coatings such as ceramics to protect C-C composites was initiated about 20 years ago to provide reusable thermal protection for the shuttle orbiter vehicles. Because of the large differences in thermal expansion characteristics of carbon fibers and ceramic materials, few coatings have been found to withstand thermal cycling without cracking. Thick CVD coatings of silicon carbide (SiC) are currently used to protect C-C composites at temperature up to 1300°C. Microcracks in the SiC coating can lead to catastrophic failure since penetration of oxygen through the cracks will result in rapid oxidation of the C-C composite to carbon monoxide and carbon dioxide (5). The strategy that has proven most successful dealing with cracked external coatings is to employ a boron-rich inner coating beneath the cracked outer coating that acts as the oxygen barrier. In this scheme, oxygen penetrating the crack oxidizes the boron layer to produce a compliant sealant glass (B 0 ) that fills and seals the crack. Prominent coating combinations consist of a SiC outer coating and boron-rich inner coatings that consist of elemental boron or B C. The silicon-based ceramics are used for outer coatings because of their excellent oxidative stability, refractoriness, and relatively low thermal dimensional changes. Another approach often used in combination with external barrier coating is to add elemental boron, B C, SiC, and phosphorous compounds to precursor carbon matrix material during processing (6-9). On exposure to air at elevated temperatures, these additives are expected to oxidize and provide in-depth oxidation protection. Experience has shown that it is difficult to achieve a uniform dispersion of the particulate additive throughout the composite and substantial amounts of the carbonaceous material is oxidized before the additive can become effective. Considerable interest has been shown in the uses of polyfunctional arylacetylenes in the preparation of thermally stable polymers (10-14) and recently as precursors to carbon (15-17). Phenolic resin systems and petroleum and coal tar pitches are currently used as the carbon matrix precursor material. There are numerous problems associated with these carbon precursor materials such as difficulty in processability, low char yield, and lack of consistency of pitch composition. We are interested in aromatic containing acetylenic compounds as a carbon source that have low melting points, have a broad processing window which is defined as the temperature difference between the melting point and the exothermic polymerization reaction, can be easily polymerized through the acetylene units to thermosets, and lose little weight during curing and pyrolysis to carbon under atmospheric conditions. Of further importance is the fact that the processing of C-C composites from acetylene-substituted aromatics reduces the number of required impregnation cycles relative to pitch and phenolics and drastically diminishes the pressure requirements. Our strategy for the synthesis of these materials involves the preparation of multiple-substituted benzenes bearing phenylethynyl groups. Secondary acetylenes have been shown to be less reactive or 2

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Nelson; Fire and Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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exotherm at a higher temperature relative to primary acetylenes. Moreover, some primary acetylenes have been reported to react explosively (18). We are engaged in the synthesis of compounds containing three or more phenylethynyl groups substituted on the aromatic unit and inorganic-organic hybrid polymers. Various multi-secondary acetylene substituted aromatic hydrocarbons were synthesized and evaluated as to their ease of homopolymerization and their ease of carbonization after being polymerized. Out of our studies, 1,2,4,5-tetrakis(phenylethynyl)benzene 1 has been found to exhibit outstanding properties as a carbon precursor material (19). Moreover, apoly(carborane-siloxane-acetylene) 2 has been shown to exhibit exceptional oxidative properties to 1000°C (20). This paper is concerned with the synthesis of copolymers from 1 and 2 and characteristic studies pertaining to the oxidation stability at elevated temperatures. Experimental Thermal analyses were performed with a DuPont 2100 thermal analyzer equipped with a thermogravimetric analyzer (TGA, heating rate 10°C/min) and a differential scanning calorimeter (DSC, heating rate 10°C/min) at a gas flow rate of 50 cc/rnin. The reported glass transition temperature (T ) was identified as the midpoint of the endothermic displacement between the linear baselines. Thermal and oxidative studies were achieved in nitrogen and air, respectively. The TGA studies were performed on melts and films of the copolymers. A l l pyrolysis studies were performed under atmospheric conditions. All aging studies were accomplished in a TGA chamber. l,7-Bis(chlorotetramethyldisiloxyl)-/w-carborane was purchased from Dexsil Corporation and was used as received. g

Synthesis of 1,2,4,5-Tetrakis(phenylethynyl)benzene 1. Phenylacetylene (4.697 g, 45.98 mmol), 1,2,4,5-tetrabromobenzene (4.113 g, 10.45 mmol), triethylamine (29.1 ml, 209 mmol), pyridine (16.9 ml, 209 mmol) and a magnetic stirring bar were added to a 250 ml round bottom flask. The flask was fitted with a septum and then chilled in an isopropanol/dry ice bath. After the flask had cooled, the mixture was degassed several times by the alternate application of partial vacuum and argon. To the flask was added palladium catalyst, which consisted of Pd(PPh )Cl (0.147 g, 0.209 mmol), Cul (0.139 g, 0.731 mmol) and PPh (0.294 g, 1.120 mmol). The septum was refitted and the flask was again degassed. The flask was warmed up to room temperature, then placed in an oil bath at 80°C, and stirred overnight resulting in the formation of a copious amount of a white precipitate. The product mixture was poured into 200 ml of water. The product was collected by suction filtration, washed several times with water, and dried. Recrystallization from methylene chloride and ethanol afforded l,2,4,5-tetrakis(phenylethynyl)benzene I in 84% yield; mp: found 194-196°C, lit.l93-194°C (18). 3

2

3

Polymerization and Carbonization of 1,2,4,5-Tetrakis(phenylethynyl)benzene 1 Under Inert Conditions. The monomer 1 (15.1 mg) was weighed into a TGA pan and cured by heating under a nitrogen atmosphere at 225°C for 2 hours, at 300°C for 2 hours, and at 400°C for 2 hours resulting in the formation of a solid thermosetting


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polymeric material. During the heat treatment, the sample lost 1.1% weight. Upon cooling, a TGA thermogram was taken between 30 and 1000°C resulting in a char yield of 85%. An alternate procedure involves performing the polymerization and carbonization in one step. A sample of 1 was heated between 30 and 1000°C under inert conditions. At 1000°C, the carbon residue exhibited a char yield of 85%.


Oxidation of Carbon Formed from 1,2,4,5-Tetrakis(phenylethynyl)benzene 1. A TGA thermogram was taken of the carbon residue between 30 and 1000°C in a flow of air at 50 cc/min. The sample started to slowly lose weight at approximately 500°C with catastrophic decomposition occurring between 600 and 800°C. Synthesis of Poly(butadiyne-l,7-Bis(tetramethyldisiloxyl)-/«-Carborane) 2. In a typical synthesis, a 2.5M hexane solution of «-BuLi (34.2 ml, 85.5 mmol) in 12.0 ml of THF was cooled to -78°C under an argon atmosphere. Hexachlorobutadiene (5.58g, 21.4 mmol) in 2.0 ml THF was added dropwise by cannula. The reaction was allowed to warm to room temperature and stirred for 2 hr. The dilithiobutadiyne in THF was then cooled to -78°C. At this time, an equimolar amount of 1,7bis(chlorotetramethyldisiloxyl)-w-carborane (10.22 g, 21.4 mmol) in 4.0 ml THF was added dropwise by cannula while stirring. The temperature of the reaction mixture was allowed to slowly rise to room temperature. While stirring the mixture for 1 hour, a copious amount of white solid (LiCl) was formed. The reaction mixture was poured into 100 ml of dilute hydrochloric acid resulting in dissolution of the salt and the separation of a viscous oil. The polymer 2 was extracted into ether. The ethereal layer was washed several times with water until the washing was neutral, separated, and dried over sodium sulfate. The ether was evaporated at reduced pressure leaving a dark-brown viscous polymer 2. A 97% yield (9.50 g) was obtained after drying in vacuo. GPC analysis indicated the presence of low molecular weight species («500) as well as higher average molecular weight polymers (Mw«4900, Mn«2400). Heating of 2 under vacuum at 150°C removed lower molecular weight volatiles giving a 92% overall yield. Major FTIR peaks (cm ): 2963 (C-H); 2600 (B-H); 2175 (C^C); 1260 (Si-C); and 1080 (Si-O). 1

Pyrolysis of Poly(butadiyne-l,7-Bis(tetramethyIdisiloxyl)-/n-Carborane) 2 in Nitrogen. A sample (24 mg) of 2 was weighed into a platinum TGA pan and heated at 10°C/min to 1000°C under a nitrogen atmosphere at a gas flow rate of 50 cc/min resulting in a ceramic yield of 87%. Upon cooling back to room temperature, the ceramic material was heated at 10°C/min to 1000°C under a flow rate of air at 50 cc/min. During the oxidative heat treatment, the ceramic material gained weight (1-2 weight percent) attributed to oxidation on the surface. Pyrolysis of Poly(butadiyne-l,7-Bis(tetramethyldisiloxyl)-m-Carborane) 2 in Air. A sample (13.7 mg) of 2 was weighed into an platinum TGA pan and heated at 10°C/min to 1000°C under a flow of air at 50 cc/min resulting in a ceramic yield of 92%. The ceramic was aged at 1000°C for 4 hours resulting in a slight weight gain attributed to the formation of a protective layer enriched in silicon oxide. Moreover, the sample retained its structural integrity.


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General Polymerization Procedure for Blending of 1 and 2. Various concentrations of 1 and 2 were weighed into aluminum planchets and heated at 200°C to melt L The resulting melt was mixed thoroughly by stirring and a sample was removed for evaluation by DSC and TGA thermal analyses. All TGA studies were performed to 1000°C.


Pyrolysis of Various Mixtures of 1 and 2 Under Inert Conditions. Mixtures of 1 and 2 were weighed into a TGA pan, cured to a thermoset, and converted into a char by heating at 10°C/min to 1000°C under a flow of nitrogen. All samples showed char yield of 80-85%. Oxidative Stability of Chars from Blending of 1 and 2. The chars were cooled back to 50°C and rescanned to 1000°C under an air atmosphere at 10°C/min at a flow rate of 50cc/min. Steady improvements in the stability were observed as the concentration of 2 was increased. Results and Discussion Synthesis of Copolymer. While investigating the thermal and oxidative properties of poly(butadiyne-1,7-bis(tetramethyldisiloxyl)-/w-carborane) 2, we became interested in using this material to protect carbon derived from acetylenic aromatic hydrocarbons against oxidation at elevated temperatures. Our scheme involves the blending of various concentration of l,2,4,5-tetrakis(phenylethynyl)benzene I and 2, heating to 1000°C under inert conditions, and determining the oxidative stability of the char. Uniform dispersion of 1 and 2 in the melt could be readily achieved with polymerization through the acetylenic units affording a homogeneous polymer. The synthesis of 1,2,4,5-tetrakis(phenylethynyl)benzene I and poly(butadiynel,7-bis(tetramethyldisiloxyl)-/w-carborane) 2 have been reported previously (19,20). Compound 1 was prepared from the reaction of 1,2,4,5-tetrabromobenzene and phenylacetylene in the presence of a catalytic amount of a palladium salt. The poly(carborane-siloxane-acetylene) 2 was synthesized from the reaction of 1,7bis(chlorotetramethyldisiloxyl)-/w-carborane and dilithiobutadiyne. Ph-C=C,

,C=C-Ph

1 H

Ç 3

H

H

Ç 3

Ç 3

ψ

=—=—Si-O-Si-CB H C-Si-O-SiCH CH CH Ch 10

3

3

10

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The acetylenic functionality in both 1 and 2 provides many attractive advantages relative to other cross-linking centers. An acetylene moiety remains inactive during processing at lower temperatures and reacts thermally to form conjugated polymeric cross-links without the evolution of volatiles.

ι

1 + 2

> High Temperature Copolymer


Carbon/Ceramic Mass DSC Studies. Cure studies of 1 and 2 were performed by DSC analysis to 400°C (see Figure 1). A thermogram of I shows an endothermic transition (m.p.) at 195°C and an exothermic transition at 290°C. Upon cooling another thermogram was obtained showing a T at 164°C and a strong exotherm commencing at approximately 300°C. A sample of 1 that had been cured by heating under inert conditions at 225°C for 2 hours, at 300°C for 2 hours, and at 400°C for 4 hours did not exhibit a T . A DSC thermogram of 2 shows a small broad exotherm from about 150 to 225°C which was attributed to the presence of a small amount of primary terminated acetylenic units. This peak was absent when 2 was heated at 150°C for 30 minutes under reduced pressure. These low molecular weight components must be removed to ensure the formation of a void-free thermoset. A larger broad exotherm commencing at 250°C and peaking at 350°C was attributed to the reaction of the acetylene functions to form the cross-links. This exotherm was absent after heat treatment of 2 at 320°C and 375°C, respectively, for 30 minutes. The polymer 2 could be degassed at temperatures below 150°C without any apparent reaction of the acetylenic units. Compound 2 displays only an exothermic transition at 346°C. The exothermic transitions are attributed to polymerization through the acetylenic units. Fully cured samples of 1 and 2 did not exhibit a T , which enhances their importance for structural applications. DSC analyses of blends of 1 and 2 show a homogeneous reaction initially to a thermoset. The DSC scans of the blends show only one cure exotherm for each of the compositions studied. For example, weight percent mixtures (90/10 and 50/50) of 1 and 2 display endotherms (m.p. 1) and exotherms (polymerization reaction) peaking at 195°C, 293°C and 193°C, 300°C, respectively (see Figure 2). It is apparent from the observed cure temperature for the blends that I being more reactive initially forms radicals that are not selective in the chain propagation reaction with the acetylenic units of both I and 2 (27). Charred samples that have been heat treated to 1000°C do not exhibit characteristic endothermic and exothermic transitions. g

g

g

Pyrolysis Studies. The thermal stability of 1 was determined under inert conditions. During the heat treatment to 1000°C, the acetylenic compound 1 is initially converted into a dark brown thermoset polymer, which behaves as a precursor polymer for further conversion into carbon. Pyrolysis of 1 to 1000°C under inert conditions afforded a char yield of 85% and a density of 1.45 g/cc. Very little



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100

200

300

400

T E M P E R A T U R E (°C)

Figure 1. DSC thermogram of: 1 (solid line) and 2 (dash line).

Figure 2. DSC thermogram from weight percent mixtures of 1 and 2: 90/10 (solid line) and 50/50 (dash line).



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weight loss (1-2%) occurred below 500°C. A TGA thermogram of the pyrolyzed product (carbon) in air resulted in catastrophic degradation occurring between 600 and 800°C (see Figure 3). Polymer 1 possesses exceptional thermal and oxidative stability to 1000°C (see Figure 4). It is a viscous liquid that is soluble in most common organic solvents and is easy to process into structural components. Pyrolysis of 2 to 1000°C in nitrogen, resulted in a ceramic yield of 85%. Further heat treatment of the ceramic at 1000°C for 12 hours resulted in no additional weight loss. When the ceramic material was cooled back to 50°C and rescanned to 1000°C in air, the sample gained weight («2%) attributed to surface oxidation. A TGA thermogram of 2, which was heated to 1000°C in air, exhibited a ceramic yield of 92%. Further TGA aging studies of the ceramic in air revealed that additional weight loss did not occur and that the sample actually increased in weight as observed previously. When the aged sample was cooled and heated to 1000°C under nitrogen, no weight changes were observed. These observations show the stability of the ceramic material under both inert and oxidative conditions. The thermal and oxidative stability of various mixtures of I and 2 was determined to 1000°C by TGA analysis. Studies have been performed on samples containing 0-50% by weight of 2. The scans were run at 10°C/min at a gas flow of 50 cc/min in either nitrogen or air. Samples containing various amounts of I and 2 afforded char yields of 85% when heated to 1000°C under inert conditions. Upon cooling, the carbon/ceramic mass was reheated to 1000°C in air. The oxidative stability of the charred mass was found to be a function of the amount of 2 present. Charred samples obtained from 5, 10, 20, 35, and 50% by weight of 2 showed chars of 12, 27, 58, 92, and 99.5%, respectively, when heated to 1000°C in air (see Figure 5). TGA scans of the oxidized chars were completely stable in air to 1000°C. These results indicate that the oxidative stability of the copolymer and carbon/ceramic mass is enhanced as the concentration of 2 is increased. Aging Studies In Air. The carbonaceous mass produced from the pyrolysis of \ to 1000°C under inert conditions was aged in a flow of air at 400°C and 500°C (see Figure 6). Heat treatment at 400°C resulted in an initial weight gain attributed to the absorption and interaction of oxygen with the carbon prior to oxidative breakdown. After «45 minutes, a cessation of the weight gain was observed. Shortly thereafter, the sample started to gradually lose weight. After 6 hours the sample had lost «2.5% weight. Upon exposure to air at 500°C, a carbon char commenced to lose weight immediately. Moreover, the rate of breakdown increased as a function of time. After 1 hour, the sample had lost about 9% weight. Isothermal aging studies were performed on the ceramic formedfrom2 under oxidative conditions. A sample of 2 was heated under a nitrogen atmosphere to 1000°C to afford a ceramic yield of 85%. Upon cooling, the ceramic sample was aged in air at 500, 600, and 700°C. After each aging study, the sample was cooled to room temperature. When heat treated at 500°C for 20 hours, the sample gained 0.11% weight. The sample was then aged at 600°C for 6 hours. While heating up to 50 minutes, the sample lost weight (0.25%) and then gained 0.05% weight upon heating for an additional 5 hours. After 3 hours at 600°C, no further weight loss occurred. For heat treatment at 700°C, the sample initially lost weight (0.16%)



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200

400

600

800

1000


Figure 3. Thermal stability to 1000°C of: 1 (solid line) under nitrogen atmosphere and carbon char from 1 (dash line) in flow of air.

100

H ι

50

ID

0

4 0

.

200

•

400

,

600

•

800

1

1000


Figure 4. Thermal stability to 1000°C of: 2 (solid line) under nitrogen and ceramic char from 2 (dash line) in flow of air.



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during the first 25 minutes. Between 25 and 300 minutes, the sample gradually increased in weight (0.29%). Another sample of 2 heated under a flow of air at 50 cc/min to 1000°C and held at this temperature for 10 hours afforded a ceramic yield of 87%. When the ceramic was further heated in air at 500°C for 12 hours, no apparent weight changes occurred. Regardless of the heat treatment, the samples retained their structural integrity except for some shrinkage during pyrolysis. Extreme aging conditions show the importance of silicon and boron in the protection of carbon-based systems against oxidation. Two ceramic compositions prepared from 50/50 and 65/35 weight percent blends of I and 2 were initially processed to 1000°C under a nitrogen atmosphere to form a char yield of 83% in each case. Upon cooling, the chars were heat treated to 1000°C under a flow of air and aged for 2 hours resulting in weight losses of 4.5 and 12.5% for the 50/50 and 65/35 weight percent compositions, respectively (see Figure 7). Since carbon commences to degrade oxidatively at approximately 400°C, similar aging studies were performed on the ceramic materials formed from various blends of I and 2. Two ceramic compositions prepared by heat treatment under a nitrogen atmosphere to 1000°Cfrom80/20 and 50/50 weight percent blends of 1 and 2 were aged in air at 400°C and 500°C. On exposure at 400°C, the char from the 80/20 mixture immediately commenced to gain weight to a maximum of 1.48%. The sample then gradually lost weight and was at 100% weight retention after 5.2 hours of heat treatment. Heat treatment of another charred sample at 500°C resulted in an immediate weight loss with a weight retention of 97.6% after 5 hours. The char from the 50/50 mixture showed outstanding oxidative stability. The copolymer quickly gained about 0.42% weight with very little further weight change during the 20 hour heat treatment. Upon increasing the temperature to 500°C, the polymer still displayed excellent stability with a weight retention of about 98.8% after isothermal aging for 20 hours. The oxidized film that developed during the heat treatment at 400°C formed a protective barrier against oxidation. On exposure of the copolymers formed from I and 2 to an oxidizing environment, a protective film initially develops that deters or alleviates further oxidation at a given temperature. The formation of the oxidized film and any weight loss associated with the exposure was accelerated by heat treatment of the carbonaceous/ceramic mass to 1000°C in air. Such samples were prepared from 50/50 and 65/35 weight percent blends of 1 and 2 heated to 1000°C, consecutively, in nitrogen and air and then isothermally aged in air in sequence for 10 hours each at 600 and 700°C (see Figure 8). The chars from the 50/50 and 65/35 weight percent blends gained and lost about 0.1% and 18% weight, respectively, at 600°C. During the heat treatment at 700°C, the samples lost about 0.4% and 4% weight. The superior performance of the 50/50 blend shows that critical amounts of boron and silicon are necessary to protect a carbon-based material against oxidation. Conclusion Copolymers of 1 and 2 show outstanding oxidative stability. Both compounds contain acetylenic units for conversion to the copolymer. The resistance to oxidation was a function of the amount of 2 present in the copolymer. The studies show that carbon can be protected from oxidation at various temperatures by proper


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102


X

86 0

40

80

120

TIME (MIN)

Figure 7. Oxidative aging studies at 1000°C on char from blends of I and 2: 50/50 weight percent (solid line) and 65/35 weight percent (dash line).

400

800

1200

TIME (MIN)

Figure 8. Oxidative studies on chars from blends of 1 and 2 preheated to 1000°C in flow of air before aging at 600 and 700°C: 50/50 weight percent (solid line) and 65/35 weight percent (dash line).


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incorporation of silicon and boron units into a carbon precursor material. Further studies are underway to evaluate and exploit the copolymers as matrix materials for high temperature composites and carbon/ceramic composites. Acknowledgments


Acknowledgment is made to the Office of Naval Research for financial support of this work. The author wishes to thank Dr. Ken M . Jones and Dr. David Y. Son for the synthesis of l,2,4,5-tetrakis(phenylethynyl)benzene and poly(butadiyne-l,7bis(tetramethyldisiloxyl)-/w-carborane), respectively. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Strife, J. R.; Sheehan, J. E. Bull. Am. Ceram. Soc. 1988, 67, 369. Sheehan, J. E. Carbon 1989, 27, 709. McKee, D. W. Carbon 1987, 25, 551. McKee, D. W.; Spiro, C. L.; Lamby, E. J. Carbon 1984, 22, 507. Kim, D. P.; Economy, J. Chem. Mater. 1993, 5, 1216. Luthra, K. L. Carbon 1988, 26, 217. McKee, D. W. Carbon 1986, 24, 736. Jawed, I.; Nagle, D. C. Mat. Res. Bull. 1986, 21, 1391. Rakszawski, J. F.; Parker, W. E. Carbon 1964, 2, 53. Stille, J. K.; Harris, F. W.; Rukutis, R. O.; Mukamal, H. J. Polym. Sci., Part Β 1966, 4, 791. Samyn, C.; Marvel, C. J. Polym. Sci., Polym. Chem. 1975, 13, 1095. Frank, H.; Marvel, C. J. Polym. Sci., Polym. Chem. 1976, 14, 2785. Banihashemi, Α.; Marvel, C. J. Polym. Sci., Polym. Chem. 1977, 15, 2667. Hergenrother, P. M. J. Polym. Sci., Polym. Chem. 1982, 20, 2131. Economy, J.; Jung, H.; Gogeva, T. Carbon 1992, 30, 81. Zaldivar, R. J.; Rellick, G. S. SAMPE Journal 1991, 27, 29. Stephens, Ε. B.; Tour, J. M . Macromolecules 1993, 26, 2420. Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489. Jones, Κ. M.; Keller, T. M . Polym. Mat. Sci. & Eng. 1993, 68, 97. Henderson, L. J.; Keller, T. M . Polym. Prep. 1993, 34(1), 345. Sastri, S. B.; Keller, T. M.; Jones, Κ. M.; Armistead, J. P. Macromolecules 1993, 26(23), 6171.

RECEIVED November 18,

1994


Fire and Polymers II - American Chemical Society

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