Poly(carboraneâsiloxaneâacetylene) - American Chemical Society

Chapter 31

Poly(carborane—siloxane—acetylene) as Precursor to High-Temperature Thermoset and Ceramic

Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch031

Leslie J. Henderson, Jr. and Teddy M . Keller Materials Chemistry Branch, Code 6127, Naval Research Laboratory, Washington, DC 20375-5320

A linear carborane-siloxane-acetylenic hybrid polymer has been synthesized and characterized. The poly (carborane-siloxane-acetylene) is a viscous liquid at room temperature and can be easily processed to a high temperature polymeric thermoset under either thermal or photochemical conditions. Thermal curing of the linear polymer to the thermoset is accomplished above 150°C. This novel thermosetting polymer is readily converted into a ceramic material by heating to 1000°C under inert or oxidative conditions. The thermoset and ceramic material are stable in air at elevated temperatures.

The search for polymeric materials that will maintain their properties for extended periods above 300°C in an oxidative environment is being conducted by many investigators over a broad spectrum of disciplines. These polymers are usually composed of aromatic and/or heterocyclic units connected by flexible groups to impart processability and to enhance mechanical properties. Relatively few of the many polymers that have been synthesized have been exploited commercially. Moreover, there is little evidence that any marked improvements in thermal and oxidative stabilities of organic or carbon-based polymers will be forthcoming over currently available resin systems. An emerging technology that holds promise for extending the temperature stability of polymers is inorganic-organic hybrid polymers. The polymeric hybrid combines the desirable features of inorganics and organics within the same polymeric system such as thermal and oxidative stability and processability. The development of carborane-siloxane polymers in the early 1960s was a major breakthrough in the search for high temperature elastomers (1). Poly(carboranesiloxane) elastomers show superior thermal and oxidative properties and retarded depolymerization (2,3) at elevated temperatures relative to poly(siloxanes). The carborane unit is incorporated into a polymeric chain to impart either high temperature and/or specialized chemical resistance. Patents have been issued for This chapter not subject to U.S. copyright Published 1994 American Chemical Society In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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thermally stable elastomers consisting of meta- or para-carborane and either silyl or siloxyl units. Some have been produced commercially. However, they lack acetylenic groups or any other functional group for cross-linking purposes to produce thermosetting polymers. Researchers have successfully incorporated carboranes into the backbone of most of the common types of addition and condensation polymers and as pendant groups or side chains (4). It was recognized that the attachment of a carborane side group onto the backbone of a polymer did not enhance the thermal properties. However, if the carborane becomes a part of the polymeric backbone, appreciable improvements in thermal stability were achieved. In our continuing investigations of high temperature polymers, carboranesiloxane-acetylenic polymers have been synthesized and are being evaluated as high temperature matrix materials for composites and as precursor materials for ceramics. The major advantage of our approach is that the desirable features of inorganics and organics such as high thermal and oxidative stability and processability are incorporated into the same polymeric chain. The siloxane units provide thermal and chain flexibility to polymeric materials. Silylene-acetylenic polymers have also been made but lack the thermally and oxidatively stable carborane units. The chemistry involved in synthesizing poly(carborane-siloxane) has been modified to accommodate the inclusion of an acetylenic unit in the backbone. The presence of the acetylenic linkages within the backbone provides the opportunity to convert the initially formed liquid linear polymer into a thermoset. The viscosity of the linear polymer depends on the molecular weight. The cross-linked density of the poly(carborane-siloxane-acetylene) is easily controlled as a function of the quantity of reactants used in the synthesis. The acetylenic functionality provides many attractive advantages relative to other cross-linking centers. The acetylene group remains inactive or dormant during processing at lower temperatures and will react either thermally or photochemically to form conjugated polymeric cross-links without the evolution of volatiles. This paper is concerned with the synthesis of a carborane-siloxane-acetylenic polymer, its conversion into a high temperature polymer/ceramic, aging under high temperature oxidative conditions, and characterization by FTIR, thermal analyses, optical and scanning electron microscopies, and X-ray photoelectron spectroscopy (XPS). 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 of 10°C/min) at a gas flow rate of 50 cc/min. Thermal and oxidative studies were carried out in nitrogen and air, respectively. Infrared spectral studies were performed with a Perkin-Elmer 1800 FTIR spectrophotometer purged with nitrogen gas. FTIR spectra of soluble samples were obtained from thin films solvent-cast on NaCl plates. FTIR spectra of insoluble samples (cured or pyrolyzed) were acquired via KBr pellets. Molecular weights were determined by gel-permeation chromatography (GPC) using a Hewlett-Packard Series 1050 pump equipped with two Altex μ-spherogel columns (size 10 and 10 À, respectively) connected in series. «-Butyllithium (2.5M in hexane) and 3

In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

4


418

INORGANIC AND ORGANOMETALLIC POLYMERS II

hexachlorobutadiene were obtained from Aldrich. The latter was freshly distilled at reduced pressure prior to use. Tetrahydrofuran (THF), purchased from Aldrich, was dried under argon with potassium metal and freshly distilled from potassium/benzophenone prior to use. l,7-Bis(chlorotetramethyldisiloxyl)-/wcarborane 1 was purchased from Dexsil Corporation and was used as received. All syntheses were carried out under a dry argon atmosphere. Synthesis of poly(butadiyne-l,7-bis(tetramethyldisUoxyl)-/«-carborane) 1. In a typical synthesis, a 2.5M hexane solution of w-BuLi (34.2 ml, 85.5 mmol) in 12.0 ml of THF was cooled to -78°C under an argon atmosphere. Hexachlorobutadiene 2 (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 3 in THF was then cooled to -78°C. At this time, an equalmolar amount of l,7-bis(chlorotetramethyldisiloxyl)-m-carborane 4 (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 1 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 1. A 97% yield (9.50 g) was obtained after drying in vacuo. GPC indicated the presence of low molecular weight species («500) as well as higher average molecular weight polymers (Mw«4900, Mn«2400). Heating of I 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). Thermosetting Polymer 5 A . A sample of 1 (1.5490 g) was quickly heated to 300°C in argon. The polymer 1 was then cured by heating at 300, 350, and 400°C, consecutively, for 2 hours at each temperature. Upon cooling at l°C/min to ambient conditions, a void-free dark brown solid 5A, which was 96 wt% (1.4840 g) of the starting material 1, was isolated. FTIR (cm ): 2963 (C-H); 2599 (B-H); 1600 (C=C); 1410 and 1262 (Si-C); and 1080 (Si-O). An alternate procedure involves heating a sample of I (1.51 g) at 300, 350, and 400°C, consecutively, in air for 2 hours at each temperature. During the cure, the sample developed an amber film on the outer surface. The polymer was stable upon aging in air at 500°C for 100 hours. Photocrosslinking of 1 to Produce 5 B . The polymer I, dissolved in methylene chloride, was coated on a 1 inch square platinum screen, and the solvent evaporated. This process was repeated until a reasonable FTIR spectrum of i could be obtained. A mercury UV lamp with a Jarrell-Ash power supply was used without monochromator or filters to irradiate 1. The screen was mounted on an IR cell holder so that 1 could be irradiated and the assembly could be moved without disturbing the relative positions for monitoring by FTIR. Pyrolysis of Thermoset 5 A to Ceramic 6. Pyrolysis of thermoset 5A (1.4251 g) in argon to the ceramic 6 was accomplished by heating directly or at various temperatures in sequence to 1000°C. Alternatively, 5A could be heated in 1


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sequence at various temperatures to 1000°C. The sample was then cooled back to ambient conditions at 0.5°C/min producing the black solid ceramic, 6, (1.2045 g, 85% yield). FTIR (cm ): 3210; 1474; 1402; 1195; 795. 1


Results and Discussion The synthesis of 1 is a one pot, two stage reaction (see Scheme 1). Dilithiobutadiyne 3 was prepared by the method of Ijadi-Magshoodi and Barton (5,6). Dilithio butadiyne 3 was not isolated and was reacted with an equal-molar amount of the 1,7bisicWorotetramethyldisiloxy^-rrjhcarborane 4 to afford a dark-brown viscous polymer 1 (see Scheme 1) in quantitative yield (97-100%). GPC indicated the presence of low molecular weight species («500) as well as higher average molecular weight polymers (Mw«4900, Mn«2400). Heating under vacuum at 150°C removed the lower molecular weight components leaving a 92-95 % overall yield. The novel carborane-siloxane-acetylenic polymer 1 has the advantage of being extremely easy to process since it is a liquid at ambient temperature and is soluble in most organic solvents. It is designed as a thermoset polymeric precursor. Cross-linking of 1 can occur by thermal or photochemical means through the triplebonds of the acetylenic units to afford thermosetting polymers 5A and 5B, respectively (7-9). A shiny void-free dark brown solid 5A was produced by thermally curing 1 at 300, 350, and 400°C, consecutively, for 2 hours at each temperature either under inert condition or in air. Gelation occurred during the initial heat treatment at 300°C. An FTIR spectrum (see Figure 1) of 5A shows the disappearance of the acetylenic absorption at 2175 cm" and the appearance of a new, weak peak centered at 1600 (C=C) cm" . A spectrum of 5A cured in air also exhibited an absorption at 1714 cm" , attributed to a carbonyl group. The other characteristic peaks were still present. Pyrolysis of 5A to 1000°C yielded a black solid ceramic material 6 in 85% yield that retained its shape except for some shrinkage. The characteristic FTIR absorptions of I and 5A were now absent. The thermal polymerization of I was studied by differential scanning calorimetry (DSC) from 30 to 400°C under inert conditions (see Figure 2). A small broad exotherm is apparent from about 150 to 225°C and was attributed to the presence of a small amount of primary terminated acetylenic units. This peak was absent when 1 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 I at 320°C and 375°C, respectively, for 30 minutes. The polymer I could be degassed at temperatures up to 150°C without any apparent reaction of the acetylenic units. A fully cured sample of 5A did not exhibit a T , which enhances its importance for structural applications. Cross-linking of I by photochemical means to produce 5B was achieved in both air and argon atmospheres. Ultraviolet (UV) irradiation of the polymer 1 resulted in a decrease of the intensity of the triple bond absorption (2170 cm" ) as determined from FTIR spectroscopy. Even after irradiating for 4 days, the brown film 5B still showed some absorption due to the triple bond. Comparison of the 1

1

1

g

1



420


Scheme 1 Cl

CI

n-BuLi Cl

-Li

Cl fH

fHg

3

?

H

3

fHi

Cl-Si-O—Si-CB H C—Si-O—Si-Cl I

I

CH, 120 Sputtering Time Sample

CiAtomic") % OiAtomic1% SifAtomicW BfAtomicY^

5" 5 5 6

45 17 5 3

b

C d

8 24 20 30

6 48 66 58

a

31 11 10 9 b

Cured at maximum temperature of 400°C for 2 hours. Cured at 300°C for 4 hours in argon and at 400°C for 4 hours in air. Cured in air at 320°C, 350°C, and 400°C in sequence for 2 hours and aged for 100 hours in air at 400°C. Cured and heated at 900°C for 4 hours and aged in air at 500°C for 100 hours. c


d

material for advanced composites and further conversion into a shaped ceramic component is due to the ease of processability, high ceramic yield, and oxidative stability at elevated temperatures. Surface analysis studies of samples exposed to air indicate that the surface of both 5A and 6 is enriched with oxide forms of boron and silicon, which apparently protect the interior part against further oxidation. Further studies are underway to fully evaluate the thermal properties of 5A and 6 and to develop new chemistries pertaining to the design of high temperature polymeric materials and ceramics. Acknowledgement The authors wish to thank the Office of Naval Research for financial support of this work. The authors are grateful to Dr. Pehr Pehrsson of the Surface Branch of the Chemistry Division for the XPS studies and Dr. Tai Ho of George Mason University for his assistance with the GPC measurements. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

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426 9. 10. 11. 12.


Callstrom, M . R.; Neenan, T. X.; Whitesides, G. M . Macromolecules 1988, 21, 3528 Gee, S. M.; Little, J. A. J. Mater. Sci., 1991, 26, 1093 White, D. Α.; Oleff, S. M.; Fox, J. R. Adv. Ceram. Mater. 1983, 2, 53 Burns, G. T.; Taylor, R. B.; Xu, Y.; Zangvil, Α.; Zank, G. A. Chem. Mater. 1992, 4(6), 1313 April 25, 1994


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Poly(carboraneâsiloxaneâacetylene) - American Chemical Society

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