Blending Studies of Poly(siloxane acetylene) and Poly(carborane

Chapter 21

Blending Studies of Poly(siloxane acetylene) and Poly(carborane siloxane acetylene) Downloaded by NORTH CAROLINA STATE UNIV on October 1, 2012 | http://pubs.acs.org Publication Date: December 15, 1997 | doi: 10.1021/bk-1998-0681.ch021

1

Teddy M . Keller and David Y. Son

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

High temperature thermosets and ceramics have been synthesized by heat treatment of various blends of poly (siloxane-acetylene) and poly (carborane-siloxane-acetylene). The polymeric blends give high char yields on pyrolysis, and the resultant chars show excellent oxidative stability to at least 1500 °C. The thermosets and ceramic chars show similar oxidative stability to previously studied copolymers containing varying amounts of siloxane, carborane, and acetylene units within the backbone. It has been determined that only a small percentage of carborane is necessary to provide this oxidation protection. Thus, these precursor linear hybrid polymers are more cost-effective than previous polymers which contained carborane in each repeating unit.

Our current interest in inorganic-organic linear hybrid polymers as precursors to high temperature thermosets and ceramics has led us to investigate the synthesis of novel materials containing silicon, carborane, and acetylenic segments. Several poly (carboranesiloxane-acetylene^ 1 and 2 and poly (siloxane-acetylene)s 3 have been synthesized (see Scheme 1) and are being evaluated as high temperature matrix materials for composites and as precursor materials to ceramics for applications under extreme environmental conditions. 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. Siloxane-acetylenic polymers have also been made but lack the thermal and oxidative stability that the carborane units possess. The chemistry involved in synthesizing poly(siloxane) and poly(carborane-siloxane) has been modified to accommodate the inclusion of an acetylenic unit in the backbone. The novel linear 1,2

3

1

Current address: Department of Chemistry, Southern Methodist University, Dallas, T X 75275

248

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

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

Downloaded by NORTH CAROLINA STATE UNIV on October 1, 2012 | http://pubs.acs.org Publication Date: December 15, 1997 | doi: 10.1021/bk-1998-0681.ch021

21.

249

Blending of Acetylene Polymers

K E L L E R & SON

CH

C H

I I 3

C H

3

- C = C - C = C - S i - O - S i - C B

I

CH

ÇH

ÇH

3

I

CH

C H

3

1

0

H

0

H

1

10 10

C H

3

j

CH

C H

I I

3

,

3

C H

3

CH

3

C H

3

3

• C = C — C = C - S i — O - S i —

I

(

C H

3

3

C - S i - O - S i —

0

3

C H

3

j

0

C - S i - O - S i —

10 10 1

C H

3

C H

3

I

- C = C - C = C - S i - O - S i - C B

I

1

CH

3

I

C H

3

3

2a-d a, x/y = 50/50 b, x/y = 25/75 c, x/y =10/90 d, x/y = 5/95

ÇH

3

- C = C - C = C - S i I CH

f

CH

3

(-0—Si— \ 3

I

C H

3

3a, n=1 3b, n=2

ÇH

3

ÇH

C H

3

- S i - O - S i - C B I I CH C H 3

3

1

0

H

1

0

3

C H

3

C - S i - O - S i - C I

I

C H

I

3

C H

3

ÇH

,

3

I

C H

ÇH

3

i-O-Si

Cl—Si

\

3

f-CI

I

C H

,

/ 3

5a, n=1 5b, n=2

Scheme 1. Preparation of linear inorganic-organic hybrid polymers


n

250

SYNTHESIS AND C H A R A C T E R I Z A T I O N O F A D V A N C E D M A T E R I A L S


polymers have the advantage of being extremely easy to process and convert into thermosets or ceramics since they are either liquids at room temperature or low melting solids and are soluble in most organic solvents. They are designed as thermoset polymeric precursors. The cross-linked density of the thermosets 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 during processing at lower temperatures and reacts either thermally or photochemically to form conjugated polymeric cross-links without the evolution of volatiles.

ÇH

ÇH

3

ÇH3 ÇH3

3

Si—O-Si—CB H C -Si-O-Si— 10

CH3

10

CH3

CH3 CH3

1 ÇH3ÇH3

ÇH

ÇH

3

CH3CH

3

I

-Si-O-SiCB H C^i-aSi10

CH3CH3

CH

3

I

-Si-OSi—•

10

CH

3

CH3CH3

3

2a-d a, x/y = 50/50 b, x/y = 25/75 c, x/y = 10/90 d, x/y = 5/95 ÇH

3

ÇH

3

Sin CH

3

CH3

3a, n=1 3b, n=2 This paper is concerned with blending 1 and 3 in an attempt to arrive at similar thermoset and ceramic compositions as found for copolymer 2 upon thermal treatment. Thermal analysis studies were performed on thermosets 4 and ceramics 5 obtained from various blends of 1 and 3a.


21.

K E L L E R & SON

Blending of Acetylene Polymers

251

Experimental


13

The synthesis of 1,2, and 3 have been reported previously. A l l reactions were carried out in an inert atmosphere unless otherwise noted. Solvents were purified by established procedures. 1,3-Dichlorotetramethyldisiloxane and 1,5-dichlorohexamethyltrisiloxane were obtained from Silar Laboratories and used as received. w-Butyllithium (2.5 M in hexane) was obtained from Aldrich and used as received. 1,7-Bis(chlorotetramethyldisiloxyl)-m-carborane 1 was purchased from Dexsil Corporation. Hexachlorobutadiene was obtained from Aldrich and distilled before use. Cure and thermal analysis studies were performed on various mixtures of 1 and 3a in milligram quantities. Thermogravimetric analyses (TGA) were performed on a DuPont SDT 2960 Simultaneous DTA-TGA analyzer. Differential scanning calorimetry analyses (DSC) were performed on a DuPont 910 instrument. Unless otherwise noted, all thermal experiments were carried out at a heating rate of 10 °C/min and a nitrogen flow rate of 50 cc/min. Synthesis of poly (carborane-siloxane-acetylene) 1. 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 (5.58 g, 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 hours. The 1,4-dilithiobutadiyne in THF was then cooled to -78 °C. At this time, an equimolar amount of l,7-bis(chlorotetramethyldisiloxyl)-m-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 1 was extracted into ether. The ethereal layer was washed several times with water until the washing was neutral, separated, and dried over anhydrous 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 analysis indicated the presence of low molecular weight species (-500) as well as higher average molecular weight polymers (Mw«4900, Mn«2400). Heating of 1 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

Synthesis of poly (siloxane-acetylene) 3a. A mixture of 1,4-dilithiobutadiyne (6.3 mmol) in THF/hexane was cooled in a dry ice/acetone bath. To this mixture, 1,3dichlorotetramethyldisiloxane (1.24 mL, 6.3 mmol) was added dropwise over 15 min. After addition, the cold bath was removed and the mixture was stirred at room temperature for two hours. The tan mixture was poured into 20 mL of ice-cooled saturated aqueous ammonium chloride solution with stirring. The mixture was filtered through a Celite pad and the layers were separated. The aqueous layer was extracted twice with Et 0 and the combined organic layers were washed twice with distilled water and once with saturated aqueous NaCl solution. The dark brown organic layer was dried 2


252


over anhydrous magnesium sulfate and filtered. Most of the volatiles were removed at reduced pressure and the residue was heated at 75 °C for three hours at 0.1 torr to give 3a as a thick, dark brown material (1.04 g, 92%). Polymer 3a slowly solidifies on standing at room temperature and liquefies at approximately 70 °C. *H N M R (ppm) 0.30 (s, 12H, -Si(CH )); C N M R (ppm) 1.7, 1.9 (-Si(CH )), 84.9 (-Si-CC-), 86.9 (-Si-CC-). Anal. Calcd. for (C H OSi ) :C.,53.31; H, 6.66; Si, 31.16. Found:C.,55.81; H , 7.61; Si, 27.19. 13

3

3


8

12

2

n

Preparation of Homogeneous Mixtures from 1 and 3a. Molar mixtures (50/50,25/75, and 10/90 weight amounts) of 1 and 3a were weighed into a vial, mixed by dissolution in THF, and concentrated at reduced pressure. These compositions were used for thermal analysis studies. Preparation of Thermoset 4. Various mixtures of 1 and 3a were weighed into a T G A pan and cured by heating at 200, 250, 350, and 450 °C for 4 hours at each temperature under an inert atmosphere. Conversion to Ceramic 5. Various mixtures of 1 and 3a or 4 were weighed into a T G A pan and heated to either 1000 °C or 1500°C under inert conditions. Upon cooling the ceramic chars were reheated to 1500 °C under a flow of air to determine the oxidative stability. Oxidative aging studies. Various mixtures of 1 and 3a were weighed into a T G A pan and either cured to a thermoset 4 or converted into a ceramic 5. Thermoset 4 was then heated in sequence in a flow of air (50 cc/min) at 200, 250, 300, 350, and 400 °C for 5 hours at each temperature. Addtional heating at 450 °C was performed up to 15 hours. The ceramic compositions 5 were heated in sequence at 400, 500, and 600 °C for 5 hours. Further heat exposure at 700 °C was carried-out up to 15 hours. Results and Discussion Several molar mixtures (50/50,25/75, and 10/90) of 1 and 3a were prepared for cure and thermal analysis studies. Homogenous mixtures were obtained by dissolving the linear polymers 1 and 3a in THF. After thorough mixing, the solvent was removed by distillation at reduced pressure. The resulting mixtures as prepared were viscous compositions. However, gummy, semicrystalline compositions formed after several days. Upon heating to 100°C., the mixtures existed as viscous liquids.

1

+

3 a

Cure

Thermoset 4

PyrotysJS

Ceramic 5

DSC Studies. DSC analyses of blends of 1 and 3a show a homogeneous reaction initially to a thermoset. The DSC scans to 400 °C of the blends exhibit only one cure exotherm for each of the compositions studied (see Figures 1 and 2). For example, mole percent mixtures (10/90, 25/75 and 50/50) of 1 and 3a display exotherms (polymerization



21.

K E L L E R & SON

Blending ofAcetylene Polymers

253

T E M P E R A T U R E (°C) Figure 1. DSC thermograms of 1 and 3a

0.2

5 ο

8

o.o

100

200

300

400

Temperature (°C) Figure 2. DSC thermograms of various mixtures of l/3a: (A) 10/90, (B) 25/75, and (C)50/50


254


reaction) peaking at 296,298 and 328 °C., respectively. It is apparent from the observed cure temperature for the blends that 3a being more reactive initially forms radicals that are not selective in the chain propagation reaction with the acetylenic units of both land 3a. Samples that have been heat treated to 400 C do not exhibit characteristic exothermic transitions. Copolymer 2c shows a similar DSC thermogram with a strong exotherm at approximately 300 °C. 4

e


5

Thermal and Oxidative Stability. The thermal and oxidative stability of various mixtures of 1 and 3a was determined to 1500 °C by simultaneous TGA/DTA analysis. The scans were run at 10 °C/min at a gas flow of 50 cc/min in either nitrogen or air. When heated to 1000°C and 1500 °C under inert conditions, the various mixtures containing 1 and 3a afforded char yields of 79-80 and 77-78%, respectively. During the heat treatment, similar exothermic transitions (DTA) as found during the DSC scans were observed. Moreover, above 1000 °C., an exothermic transition is observed which is attributed to the formation of crystalline ceramic components such as SiC and B C . Upon cooling, the carbon/ceramic masses were reheated to 1500 °C in air. The oxidative stability of the charred mass was found to be a function of the amount of 1 present and the initial heat treatment. Charred samples obtained from heat treatment to 1000 °C and 1500°C of 10/90,25/75, and 50/50 molar weight percent of 1 to 3a showed chars of 98, 98, 99% and 90, 97, 98% respectively, when reheated in air (see Figures 3 and 4). The major difference observed was in the chars formed from the 10/90 mixtures. The crystallme-containing compositions (see Figure 4) lost most of their weight between 700 and 800 °C. For the amorphous compositions (see Figure 3), weight losses occurred between 600 and 700 °C and above 1000 °C. As the temperature was further increased, an acceleration in the weight loss was observed. These results indicate that the oxidative stability of the carbon/ceramic mass depends on the morphology. 4

Oxidative Aging Studies. Aging studies were performed on the thermosets derived from various blending compositions of 1 and 3a. The compositions were cured by heating at 200,250, 350, and 450 °C for 4 hours at each temperature under a nitrogen atmosphere. Aging of the thermoset was studied by heating the sample in air for 5 hours in sequence at 250, 300, 350, and 400 °C followed by 15 hours at 450 °C. Copolymer 2c and the 10/90 composition showed similar thermo-oxidative stability upon conversion into a thermoset (see Figure 5). The stabilizing effect of the carborane unit was apparent. A l l of the samples gained weight during the oxidative exposure up to 350 °C. Moreover, less oxidation occurred on the surface as the amount of 1 increased. More extreme heat treatment at 400 and 450 °C showed an enticement in oxidative stability with greater amounts of carborane (see Figure 6). The 50/50 mixture exhibited outstanding oxidative performance during the entire heat exposure. When 3a was cured and aged under identical conditions, the sample gained almost 7% during the heat exposures from 200 to 300 °C. While at 350 and 400 °C., the sample had lost about 14% weight. Upon exposure at 450

Blending Studies of Poly(siloxane acetylene) and Poly(carborane

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