[acs symposium series] synthesis and characterization of advanced materials volume 681 || synthesis...

Chapter 20

Synthesis and Characterization of Linear Tetraphenyl-Tetramethyldisiloxane

Diacetylene Copolymers

Eric J. Houser and Teddy M. Keller1

Chemistry Division, Materials Chemistry Branch, Naval Research Laboratory, Code 6120, 4555 Overlook Avenue, SW, Washington, DC 20375-5320

New phenyl substituted siloxyl diacetylene polymers have been prepared in high yields. The syntheses are one-pot, two step procedures involving the reaction of dilithiobutadiyne with various dichlorodisiloxanes. Spectroscopic and microanalytical data support the proposed structures. The polymers are thermally crosslinked to give hard, void-free thermosets which can be further heated to give ceramic products with high weight retention.

Materials possessing high thermal and oxidative stabilities are in high demand for applications in the aerospace and related industries. Polymeric fiber-reinforced carbon-carbon composites are frequently used in many of these applications due to their high thermal stability and strength to weight ratio, but these materials suffer from a relatively low oxidative stability (7, 2). Research has shown that the oxidative stability of carbon-based materials can be greatly increased by the introduction of inorganic elements such as silicon or boron (2).

Preceramic polymers have been used successfully in the synthesis of a variety of useful ceramics such as SiC, S i 3 N 4 and B N (3-5). The use of polymers in the formation of ceramic materials has potential processing advantages such as in forming complex shapes and giving homogeneous final ceramic products. However, obtaining pure materials, such as SiC, from polymer precursors can prove difficult in that small amounts of carbon and silicon oxide are frequently present in the final product (3). The pyrolysis of several silylene or siloxyl diacetylene polymers to SiC-containing ceramics has been reported (6).

In the field of composite materials, inorganic-organic hybrid polymers offer great promise as precursors to ceramic matrix materials. In these applications, high purity ceramics are often not necessary and preceramic polymers allow the introduction of inorganic elements such as silicon and boron in quantities which can be directed by polymer structure and stoichiometry.

Our research efforts have focused on the synthesis of hybrid inorganic-organic diacetylenic polymers as precursors to materials with high thermal and oxidative

1Corresponding author

236 This chapter not subject to copyright. Published 1998 American Chemical Society

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In Synthesis and Characterization of Advanced Materials; Serio, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

20. HOUSER & K E L L E R Linear Diacetylene Copolymers 237

stabilities (7,8). The diacetylene units act as crosslinking agents which is necessary for high weight retention upon pyrolysis (9). Heat treatment of these polymers to 300 °C gives hard, void-free thermosets. The thermosets can be further pyrolyzed up to 1500 °C affording ceramics which exhibit excellent thermal and oxidative stabilities. We recently reported on the properties of the diacetylene polymer poly(tetramethyldisiloxyldiacetylene) (8). We were interested in the effects of phenyl substitution on the thermal and oxidative stability in this type of polymer. To this end we have synthesized several new polymers and copolymers via the reaction of 1,3-dichloro-l,3-dimethyl-l,3-diphenyldisiloxane, 1, or 1,3-dichlorotetraphenyldisiloxane, 2, and 1,3-dichlorotetramethyldisiloxane, 3, with 1,4-dilithiobutadiyne, 4. The molar ratios of 2 and 3 were varied in order to tailor the structure of the copolymers. Related copolymers, formed from 4 and l,7-bis(chlorotetramethyldisiloxyl)-m-carborane, 6, were also prepared. We hereby present preliminary work describing the syntheses, characterization, and thermal evaluation of these new hybrid polymers.

Experimental

A l l reactions were carried out under an inert atmosphere using standard Schlenk techniques unless otherwise noted. Tetrahydrofuran (THF) was distilled from sodium/benzophenone under inert atmosphere immediately prior to use. Hexachlorobutadiene was obtained from Aldrich Chemical Co. and purified by distillation. n-Butyllithium (2.5M in hexanes) was obtained from Aldrich Chemical Company and used as received. 1,3-Dichloro-l ,3-dimethyl-l ,3-diphenyldisiloxane was purchased from Huls Chemical Company and purified by distillation under N 2 from Mg chips. 1,3-Dichlorotetramethyldisiloxane and 1,3-dichlorotetraphenyldisiloxane were obtained from Gelest, Inc. and were purified by distillation under N 2 from Mg chips. 1,7-Bis(chlorotetramethyldisiloxyl)-m-carborane was obtained from Dexsil Corp. and used as received. 1,4-Dilithiobutadiyne was prepared according to literature procedures (6a). Caution: The synthesis of 1,4-dilithiobutadiyne as described involves the use of n-BuLi which is pyrophoric and should thus be used only with the necessary precautions. Thermogravimetric analyses (TGA) were performed on a TA Instruments SDT 2960 DTA-TGA thermogravimetric analyzer. Aging studies were carried out on a DuPont 951 thermogravimetric analyzer in air (60 mL/min.). Differential scanning calorimetry analyses (DSC) were performed on a DuPont 910 instrument. A l l thermal measurements were carried out at a heating rate of 10 °C/min and a nitrogen flow rate of 60 mL/min. unless otherwise noted. Gel permeation chromatography (GPC) data were collected using a Hewlett-Packard Series 1050 pump and two Altex μ-spherogel columns (size 103

and 104 Â, respectively) connected in series. GPC values were referenced to polystyrene standards. Infrared spectra were obtained using a Nicolet Magna 750 FTIR spectrometer. ]H and 1 3 C N M R spectra were recorded on a Bruker AC-300 N M R spectrometer in CDC13. Elemental analyses were performed by Ε and R Microanalytical Labs. Corona, N.Y.

Preparation of poly(l,3-dimethyl-l,3-diphenyldisiloxyIdiacetylene), 7. A mixture of 1,4-dilithiobutadiyne (6.3 mmol) in THF(10 mL)/hexane (10 mL) was cooled in an ice bath. To this slurry, was added a solution of 1,3-chloro-1,3-dimethyl-1,3-

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238 SYNTHESIS AND CHARACTERIZATION OF ADVANCED MATERIALS

diphenyldisiloxane (2.0 g, 6.3 mmol) in THF (5 mL). The ice bath was removed and the resulting solution was stirred for 90 minutes at room temperature at which time the reaction was judged to be complete by the disappearance of a band at 2140 cm'1 in the infrared spectrum (10). The solution was then diluted with 30 mL of diethyl ether and 50 mL of ice water. The aqueous layer was separated and extracted twice with diethyl ether (20 mL). The organic extracts were combined, dried over MgS0 4 , and filtered through a Celite pad. Removal of volatiles under reduced pressure gave a viscous brown material. Further heating of the polymeric mixture to 75 °C at 0.1 Torr for six hours afforded a brown solid (1.67 g, 87 %). Elem. Anal. (C 1 8 H 1 6 Si 2 0) Calcd: C., 71.00; H, 5.30; Si, 18.45. Found: C., 71.03; H, 5.37; Si, 18.57.

Preparation of poly(tetraphenyldisiloxyldiacetylene), 5a. The procedure that was used in the preparation of 7 was adapted for the synthesis of 5a. The reaction of 1,4-dilithiobutadiyne (6.3 mmol) with 1,3-dichlorotetraphenyldisiloxane (2.88 g, 6.3 mmol) gave a brown solution of crude polymer. Work-up gave a solid, brown product (2.3 lg, 85 %). Elem. Anal. (C 2 8H 2 0Si 2O) Calcd: C., 78.46; H, 4.70; Si, 13.10. Found: C., 76.71; H, 5.21; Si, 12.40.

Preparation of tetraphenyl/tetramethyldisiloxyldiacetylene copolymers, 5b-d. A similar procedure to that used in the preparation of 7 was used to prepare copolymers 5b-d by reacting 1,4-dilithiobutadiyne with appropriate amounts of both 1,3-dichlorotetraphenyldisiloxane and 1,3-dichlorotetramethyldisiloxane.

75/25 tetraphenyldisiloxyl/tetramethyldisiloxyl units, 5b. The reaction of 1,4-dilithiobutadiyne (6.38 mmol) with 1,3-dichlorotetraphenyldisiloxane (2.16 g, 4.8 mmol) and 1,3-dichlorotetramethyldisiloxane (0.33 g, 1.6 mmol) gave a brown solution. Workup gave a solid, brown product (1.83 g, 78 %). Elem. Anal. (C 2 3 H, 8 Si 2 0) Calcd: C., 75.36; H, 4.95; Si, 15.32. Found: C., 75.13; H, 5.14; Si, 15.08.

50/50 tetraphenyl/tetramethyldisiloxyl, 5c. The reaction of 1,4-dilithiobutadiyne (6.38 mmol) with 1,3-dichlorotetraphenyldisiloxane (1.44 g, 3.2 mmol) and 1,3-dichlorotetramethyldisiloxane (0.65 g, 3.2 mmol) gave a brown solution. Workup gave a solid, brown product (1.58 g, 81 %). Elem. Anal. (C 1 8 H 1 6 Si 2 0) Calcd: C., 71.00; H, 5.30; Si, 18.45. Found: C., 71.15; H, 5.46; Si, 18.74.

25/75 tetraphenyl/tetramethyldisiloxyl, 5d. The reaction of 1,4-dilithiobutadiyne (6.38 mmol) with 1,3-dichlorotetraphenyldisiloxane (0.72 g, 1.6 mmol) and 1,3-dichlorotetramethyldisiloxane (0.97 g, 4.8 mmol) gave a brown solution. Workup gave a solid, brown product (1.31 g, 84 %). Elem. Anal. (C 1 3 H, 4 Si 2 0) Calcd: C., 64.41; H, 5.82; Si, 23.17. Found: C., 64.15; H, 6.23; Si, 23.27.

Preparation of tetraphenyIdisiloxy 1/1,7-bis(tetramethyldisiloxyl)-m-carborane copolymers, 5e-f.

90/10 tetraphenyldisiloxyl/l,7-bis(tetramethyldisiloxyl)-m-carborane, 5e. A similar procedure to that used in the preparation of 7 was used in the reaction of 1,4-

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dilithiobutadiyne (6.38 mmol) with 1,3-dichlorotetraphenyldisiloxane (2.59 g, 5.74 mmol) and l,7-bis(tetramethyldisiloxyl)-m-carborane (0.30 g, 0.63 mmol). Work-up gave a solid, brown product (2.34 g, 85 %). Elem. Anal. ( C 2 6 6 H 2 1 4 8 1 3 2 0 , ,B) Calcd: C., 74.08; H, 5.00; Si, 14.33; B, 2.51. Found: C., 73.71; H, 5.26; Si, 14.68; B, 2.33.

60/40 tetraphenyldisiloxyl/l,7-bis(tetramethyldisiloxyl)-m-carborane, 5f. A similar procedure to that used in the preparation of 7 was used in the reaction of 1,4-dilithiobutadiyne (6.38 mmol) with 1,3-dichlorotetraphenyldisiloxane (1.73 g, 3.83 mmol) and l,7-bis(tetramethyldisiloxyl)-m-carborane (1.22 g, 2.55 mmol). Work-up gave a solid, brown product (2.35 g, 84 %). Elem. Anal. (C 2 2 4 H 2 5 6 S i 2 8 0 , 4 B 4 ) Calcd: C., 61.27; H, 5.88; Si, 17.91; B, 9.85. Found: C., 60.98; H, 6.13; Si, 17.94; B, 9.87.

Results and Discussion

Synthesis. The synthesis of the tetraphenyl/tetramethyldisiloxyldiacetylene polymers and copolymers is shown in Scheme 1. The polymeric materials 5a-f were isolated as brown solids with yields in the 78-85 % range. These materials were prepared having molar ratios of 4 to 3 incorporated into the main chain as follows: 100:0, 5a; 75:25, 5b; 50:50, 5c; and 25:75, 5d. Copolymers 5e and 5f were prepared having 2 and 6 incorporated into the main chain in molar ratios of 90:10 and 60:40, respectively. The synthetic strategy used is a one-pot, two step reaction sequence adapted from previously published procedures (7,8). The reaction of 4 with appropriate ratios of 2 and either 3 or 6 resulted in the formation of 5a-f with concomitant formation of L iCl . Poly(l,3-dimethyl-l,3-diphenyldisiloxyl diacetylene), 7, was prepared similarly from the reaction of 4 with l,3-dichloro-l,3-dimethyl-l,3-diphenyldisiloxane.

The polymers and copolymers 5a-f were characterized by FTIR spectroscopy. A strong absorption around 2068 cm"1 confirms the presence of the internal butadiyne groups. The structure of polymer 5a was supported by absorptions at 3048 cm"1 (Ar-H), 1123 cm"1 (Si-Ph), and 1053 cm"1 (Si-O-Si). Copolymers 5b-d showed additional stretches at 1261 and 850-790 cm"1 (Si-CH 3) consistent with the proposed copolymer composition. Copolymers 5e-f showed an additional absorption at 2595 cm"1 (B-H) verifying the presence of the carborane moiety.

Ή N M R spectroscopy supports the proposed structures of 5a-f with resonances in the 7.7-7.2 ppm region for the phenyl groups and 0.2-0.6 ppm for the silicon methyl protons with intensities consistent with the expected copolymer composition. 1 3 0{'Η} N M R showed two resonances in the 83-90 ppm region for the diacetylenic carbons. The resonances for the phenyl and methyl carbons occurred in the 132-138 and 0.3-2.0 ppm regions, respectively. For 5e-f the carborane cage carbons appeared at 68 ppm.

Poly(l,3-dimethyl-l,3-diphenyldisiloxyldiacetylene), 7, was also characterized using gel permeation chromatography (GPC). GPC analysis showed a broad molecular weight distribution with a peak maximum at approximately 10,000 (relative to polystyrene) molecular weight. This polymer was chosen as a representative material and the GPC results are consistent with previous observations (77).

Thermal curing of 7 was studied by DSC. When heated to 450 °C., polymer 7 showed a prominent exotherm with a peak maximum at 314 °C. Upon cooling and rerunning the sample, an essentially featureless DSC trace was observed (Figure 1). This

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R R

- C ^ C - C ^ C — S i — O — S i -

Ph Ph

R=Ph (5a) R=Me (7)

Ph Ph -C = C - C = C - S i - 0 - S i -

Ph Ph

ÇH 3 C H 3

. c = C - C = C - S i - o - S i - -

C H 3 C H 3

x/y = 75/25 (5b), 50/50 (5c), 25/75 (5d)

C H 3 C H 3 Ç H3 ÇH 3

- C = C - C = C - S i - O - S i - C B 1 0 H 1 ( ) C S i - O - S i —

C H 3 C H 3 C H 3 C H 3

Ph Ph C = c - C ^ C - S i - 0 - S i -

Ph Ph

x/y =10/90 (5e), 40/60 (5f)

R R Ç H 3 ÇH 3 ÇH 3 C H 3

C I — S i — Ο — S i — C l C l - S i - O - S i - C B , 0 H 1 0 C - S i - O - S i - C l

R R C H 3 C H 3 C H 3 C H 3

R=l,3-Me,-1,3-Ph,(l) 6 R=Ph4(2)~ R=Me 4 (3)

Scheme 1. Preparation of Linear Inorganic-Organic Hybrid Diacetylene Polymers.

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HOUSER & K E L L E R Linear Diacetylene Copolymers

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exotherm is attributed to the thermally induced crosslinking of the internal diacetylene groups. This assignment is supported by infrared studies which showed the disappearance of the butadiyne stretch (2160 cm"1) upon heating a sample of 7 to 450 °C (Figure 2). DSC studies on copolymers 5b-f showed similar behavior with exotherm peak maxima occurring at 311 °C (5b), 300 °C (5c), 298 °C (5d), 315 °C (5e), and 311 °C (5f).

Thermogravimetric analysis of 5a-f showed these materials have high thermal stabilities. When heated to 1500 °C (10 °C/min) under a nitrogen atmosphere these polymers gave char yields of 68 % (5a), 71 % (5b), 77 % (5c), 76 % (5d), 72 % (5e), and 74 % (5f). Oxidative stabilities of the chars obtained from heat treatment of the polymers to 1500 °C under N 2 were determined by cooling to the sample to room temperature and heating to 1500 °C under an air atmosphere. The chars obtained from 5a-d showed initial weight loss beginning around 650 °C with weight retentions at 1500 °C of 38 %, 5a; 43 %, 5b; 63 %, 5c; and 84 %, 5d (Figure 3). The trend of lower oxidative stability in air with an increasing mole percent of phenyl groups in 5a-d is consistent with the formation of a char with a higher carbon content (12). It should be noted that precursor materials with high carbon content do not always give high carbon ceramic chars (75). Oxidative stabilities of the chars obtained from heat treatment of 5e-f to 1500 °C showed increased oxidative stability over the chars of 5a-d with similar phenyl group content. At 1500 °C in air the weight retentions for the chars of 5e and 5f were 52 % and 99 %, respectively (Figure 4). This latter result clearly demonstrates the increased oxidative stability obtained from incorporating both boron and silicon into the polymeric chain (7). Examination of 7 by TGA showed 78 % weight retention at 1500 °C under a nitrogen atmosphere. Heating a sample of the resulting char of 7 to 1500 °C in air gave a weight retention of 57 %. Thus, the thermooxidative properties of 7 are similar to those of copolymer 5c.

Aging studies were performed on the thermoset and ceramic derived from poly( 1,3-dimethyl-1,3-diphenyldisiloxyldiacetylene), 7. The thermoset sample was prepared by heat treatment of the polymer to 450 °C (10 °C/min) under N 2 (98 % weight retention). Aging of the thermoset was studied by heating the sample in air for 5 hours sequentially at 250, 300, 350, and 400 °C followed by 15 hours at 450 °C (Figure 5). After 5 hours at 250 °C the thermoset showed a slight weight gain with 101 % weight retention. After 5 hours at 300 °C the thermoset showed a slight weight loss returning to 100 % weight retention. More substantial weight loss was observed during aging at higher temperatures with weight retentions of 97 and 91 % after 5 hours at 350 and 400 °C., respectively. After 15 hours at 450 °C., the sample showed 44 % weight retention which had nearly reached a plateau. This behavior is similar to that of other crosslinked siloxane systems (14).

Aging studies were also performed on the char obtained from heat treatment of 7. The char sample was prepared by heating the polymer to 1000 °C (10 °C/min) under N 2 . Aging of the ceramic was studied by heating the sample in air sequentially at 400 °C., 500 °C., 600 °C., and 700 °C for five hours at each temperature (Figure 6). The aging at 400 °C resulted in a slight weight gain with 101 % weight retention after 5 hours. Higher temperatures resulted in significant weight loss with weight retentions of 83 and 50 % after 5 hours at 500 and 600 °C., respectively, with the weight retention stabilizing at 49 % at 700 °C.

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Ρ ο d W

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S* § ? S"

ί

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SYNTHESIS AND CHARACTERIZATION OF ADVANCED MATERIALS

0 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 " I 0 200 400 600 800 1000 1200 1400 1600

Temperature (°C)

Figure 3. Oxidative Stability of Chars from Copolymers 5a-d.

Figure 4. TGA of 5f (N2) and Char of 5f (in air).

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HOUSER & K E L L E R Linear Diacetylene Copolymers

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Conclusion

Diacetylene disiloxyl polymers and copolymers containing phenyl and methyl substituents were synthesized and their structures characterized by FTIR, ] H and 1 3C{ ]H} N M R spectroscopies. Copolymers containing tetraphenyldisiloxyl and 1,7-bis(tetramethyldisiloxyl)-m-carborane units in the polymer backbone were also studied. DSC and T G A analyses were used to evaluate the thermal and oxidative stabilities of these new materials and their conversion into thermosets and ceramics. The copolymer-derived ceramic products exhibited good thermal stabilities up to 1500 °C under a nitrogen atmosphere. The oxidative stability of the chars of phenyl-substituted copolymers was shown to decrease with increasing percentage of phenyl groups. The oxidative stability of the char of the 60/40 tetraphenyl/l,7-bis(tetramethyldisiloxyl)-m-carborane copolymer is excellent to 1500 °C., demonstrating the protecting effect of boron and silicon in these systems. Further work is currently underway to determine the thermal, oxidative, and mechanical properties of the thermosets and ceramics obtained from these copolymers.

Acknowledgements.

Support for this work was provided by The Office of Naval Research (ONR) and is gratefully acknowledged. Eric J. Houser is also grateful to the American Society for Engineering Education (ASEE) for his postdoctoral fellowship. Thanks to Dr. Tai Ho for his GPC work.

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2500. e) Corriu, R. J. P.; Gerbier, P.; Guérin, C.; Henner, B. J. L.; Jean, Α.; Mutin, P. H. Organometallics 1992, 11, 2507.

7. Henderson, L. J.; Keller, T. M . Macromolecules 1994, 27, 1660. 8. Son, D. Y. ; Keller, T. M. Macromolecules 1995, 28, 399. 9. a) Boury, B.; Corriu, R. J.; Douglas, W. E. Chem. Mater. 1991, 3, 487. b) Allcock,

H. R.; McDonnell, G. S.; Riding, G. H.; Manners, I. Chem. Mater. 1990, 2, 425. 10. The 2140 cm-1 band in the FTIR spectrum of the crude reaction mixtures is assigned

to terminal butadiyne groups which diminish in concentration as the polymer forms. 11. Son, D. Y. ; Keller, T. M . In Fire and Polymers II: Materials and Tests for Hazard

Prevention A.C.S. Symposium Ser. No. 599; Ed. by Nelson, G. L.; American Chemical Society: Washington, D.C. 1995.

12. The apparent higher carbon content in the chars with increasing percentages of phenyl substituents in the polymer was, in general, verified by elemental analysis. A low carbon content was observed for the char obtained from 5a which is attributed to incomplete combustion of the sample. Elem. Anal. of chars from 5a: %C, 67.40; % H , 0.0; %Si, 16.76. 5b: %C, 70.84; % H , 0.0; %Si, 18.43. 5c: %C, 65.72; % H , 0.0; %Si , 22.19. 5d: %C, 62.06; % H , 0.0; %Si , 23.82.

13. See Bouillon, E.; Pailler, R.; Naslain, R.; Bacqué, Ε.; Pillot, J.-P.; Birot, M . ; Dunoguès, J.; Huong, P. V . Chem. Mater. 1991, 3, 356.

14. Kovalenko, Y. V. ; Pustyl'nik, M . L.; Bebchuk, T. S.; Muzafarova, M . N . ; Zherdev, Y. V. ; Nederosol, V . D.; Andrianov, K. A. Polym. Sci. U.S.S.R. 1978, 20, 1824.

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