Cyclopolymerization and Polymers with Chain-Ring Structures

22 Triazine Ring Cross-linked Polyimides and Refractory Materials Derived from Them LI-CHEN HSU and WARREN H. PHILIPP

Downloaded by MONASH UNIV on November 29, 2015 | http://pubs.acs.org Publication Date: August 12, 1982 | doi: 10.1021/bk-1982-0195.ch022

NASA, Lewis Research Center, M.S. 106-1, Cleveland, OH 44135

Catalytic cyclopolymerization or polycyclotrimerization of imide oligomers containing nitrile groups with or without graphite fibers is described. The graphite fiber reinforced triaryl-s-triazine ring (TSTR) cross-linked polyimides with ring-chain structures have good mechanical properties at elevated temperatures. On pyrolysis, the TSTR cross -linked polyimides were converted to refractory type materials which are believed to be graphitic type ladder polymers containing some nitrogen in their cyclic structures. The polyimide c l a s s o f polymers a r e known t o possess a h i g h degree o f thermal s t a b i l i t y . They decompose i n an i n e r t atmosphere around 500°C and i n a i r about 400°C as i n d i c a t e d by thermog r a v i m e t r i c a n a l y s i s (1). Because o f the great thermal s t a b i l i t y of 2 , 4 , 6 - t r i p h e n y l t r i a z i n e {2) (decomposes above 486°C), copolym e r i z a t i o n o f t h i s compound with imides should l e a d t o enhanced heat r e s i s t a n t m a t e r i a l s i n the form o f t r i a r y l - s - t r i a z i n e r i n g (TSTR) c r o s s - l i n k e d (XL) polyimides. T h i s paper d e s c r i b e s the chemistry, r e a c t i o n mechanism, and method o f p r e p a r a t i o n o f these h i g h temperature m a t e r i a l s . T h e i r p o s s i b l e s t r u c t u r e s and r e f r a c t o r y p r o p e r t i e s a r e a l s o d i s c u s s e d . Processable Aromatic N i t r i l e - M o d i f i e d Imide Precursors The f i r s t step i n making TSTR c r o s s - l i n k e d polyimides i s t o synthesize the processable aromatic n i t r i l e - m o d i f i e d imide p r e c u r s o r s o f the type ( I ) , (II) and (III)

This chapter not subject to U.S. copyright. Published, 1982, American Chemical Society

In Cyclopolymerization and Polymers with Chain-Ring Structures; Butler, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

286

POLYMERS

Ο

Ο

I c

I c

c I

NC

\

/

ο

ο

ο


C

ο

Rj—Ν

/

NC

C

I

I

CN

/ X.

Ο

ο I

C

\

R*

/

CN

Γ C

I

I η

ο

ο

Ο

I c\

C

/ SN - R j - N / R|

I

\

I c.

N-R«-N

Ο

I c • \

ο

I c

/ \ C

Ο

I c

0

ο

I

NC

Ο

\

c I

WITH CHAIN-RING STRUCTURES

° (110

ο

I c / \

I

Λ

R

c I

c I

C

ο

ο

ο

I

X

from a t e t r a c a r b o x y l i c a c i d dianhydride o r i t s d e r i v a t i v e s , and a diamine having the s t r u c t u r a l formula. HN 2

R

2

—

NH~

with a n i t r i l e c r o s s - l i n k i n g agent having one of the f o l l o w i n g s t r u c t u r a l formulas: Ο CN H N-R 2

3

>x'

.

^ c ^

^CN

V

x

η i s an i n t e g e r u s u a l l y from 0 to 3. R]_, R 2 , a r y l r a d i c a l s , X i s a monoradical i n c l u d i n g Η Cyclopolymerization

and R (3)·

4

are

or P o l y c y c l o t r i m e r i z a t i o n

C r o s s - l i n k i n g i n v o l v i n g c y c l o p o l y m e r i z a t i o n or p o l y c y c l o t r i m e r i z a t i o n of the aromatic n i t r i l e i n the p r e c u r s o r i s accomplished by thermal curing as i l l u s t r a t e d by the f o l l o w i n g reaction:

+ BTDA

MDA

H r; ?

PABN


22.

HSU AND PHILIPP

Triazine Ring Cross-linked Polyimides

287

He*


[it

JM

The key t o the success o f t h i s process i s a combination o f s p e c i f i c c a t a l y s t s and optimum r e a c t i o n c o n d i t i o n s . E f f e c t i v e c a t a l y s t s f o r t r i m e r i z a t i o n o f aromatic n i t r i l e s a r e l i s t e d i n Table I (4_). Optimum r e a c t i o n c o n d i t i o n s f o r p r o c e s s i n g the aromatic n i t r i l e - m o d i f i e d imide p r e c u r s o r s depend on the chemical s t r u c t u r e and c h a r a c t e r i s t i c p r o p e r t y o f the i n d i v i d u a l p r e c u r s o r o f concern. In general, y i e l d o f the polymeric p r o ducts i n c r e a s e s with the i n c r e a s e o f r e a c t i o n temperature, pressure, time, and c o n c e n t r a t i o n o f c a t a l y s t w i t h i n the range of p r a c t i c a l experimental l i m i t s ( 5 ) . Proposed Reaction Mechanism F i g . 1 i l l u s t r a t e s the "Push-Pull" mechanism suggested by H. Smith Broadbent (6) f o r c a t a l y t i c t r i m e r i z a t i o n o f aromatic n i t r i l e s i n g e n e r a l . T h i s mechanism should apply t o the aromatic n i t r i l e - m o d i f i e d imide p r e c u r s o r s as w e l l : (a) The tautomeric c a t a l y s t , f o r example 2-hydroxypyridine, approaches a n i t r i l e molecule and donates a proton t o i t through a six-member r i n g environment. (b) T h i s a c t i v a t e d molecule (cation) a t t r a c t s a second n i t r i l e molecule and t r a n s f e r s the p o s i t i v e charge t o i t . (c) Because o f the i n f l u e n c e o f the r e s o n a n c e - s t a b i l i z e d c a t a l y s t anion, a t h i r d n i t r i l e molecule w i l l j o i n t o complete a t r i a z i n e r i n g with the r e l e a s e o f the proton r a t h e r than forming a l i n e a r configuration. TSTR Polyimides with Ring-Chain S t r u c t u r e s Although the key f a c t o r s f o r the p r e p a r a t i o n o f TSTR cont a i n i n g polyimides by c y c l o p o l y m e r i z a t i o n o r p o l y c y c l o t r i m e r i z a t i o n o f aromatic n i t r i l e - m o d i f i e d imide p r e c u r s o r s a r e the s p e c i f i c c a t a l y s t s and optimum p r o c e s s i n g c o n d i t i o n s used, t a i l o r e d d e s i g n o f polymer s t r u c t u r e s t i l l p l a y s an important r o l e so f a r as d e s i r a b l e p r o p e r t i e s and r e p r o d u c i b i l i t y are concerned. We b e l i e v e that c y c l o p o l y m e r i z a t i o n o f aromatic n i t r i l e modif i e d imide p r e c u r s o r s having s t r u c t u r e s ( I ) , ( I I ) , o r (III) would r e s u l t i n TSTR c r o s s - l i n k e d polyimides with the Ring-Chain Ring type s t r u c t u r e shown i n Fig.2. I f an excess o f aromatic n i t r i l e c r o s s - l i n k i n g agent i s used, the TSTR polyimides would have the Ring-Chain Chain type s t r u c t u r e shown i n F i g . 3 . Since both types or TSTR c o n t a i n i n g polyimides are composed o f a Ring-Chain


288

POLYMERS

Table I .

WITH

CHAIN-RING

STRUCTURES

EFFECTIVE CATALYSTS FOR TRIMERIZING AROMATIC NITRILES CATALYST

FORMULA


ORGANIC SULFONIC AND SULFINIC ACIDS p-TOLUENE SULFONIC ACID BENZENE SULFONIC ACID

CH C H S0 OH · H 0 C H S0 OH · H 0 3

4

6

5

6

2

2

2

2

S0 OH

1 -NAPHTHALENE

SULFONIC ACID

JQJ^J

2 -NAPHTHALENE

SULFONIC ACID

JJ^J^J-S0 OH

·

2

2

S0

H

^ 2

BENZENESULFINIC ACID

ORGANIC PHOSPHONIC AND PHOSPHINIC ACIDS TRICHLOROMETHYL PHOSPHONIC ACID PHENYL PHOSPHONIC ACID PHENYL PHOSPHINIC ACID

CCI PO(OH) 3

2

C H P0(OH) C H P0(OH)H 6

5

6

5

2

METALLIC ACETYLACETONATES FERRIC ACETYLACETONATE ZINC ACETYLACETONATE

(0 Η 0 ) Fe ?

5

2

3

(C H 0 ) Zn 5

7

2

2

·

2 H

2

0

HETEROCYCLIC AMIDES 2-HYDROXYPYRIDINE 2-PYRIMIDINOL

^->O~H

0 H

2-QUINOLINOL


2 H

2

0

Hsu

AND

PHILIPP


289


22.

Figure 1. Proposed PUSH-PULL mechanism for catalytic trimerization of aromatic nitriles.


POLYMERS WITH CHAIN-RING


290

Figure 2.

Figure 3.

STRUCTURES

Triaryl-s-triazine ring polymer with ring-chain ring structure. Key: 0, triazine ring; R, imide oligomer.

Triaryl-s-triazine ring polymer with ring-chain chain structure. Key: 0, triazinering;R', NH» etc.


22.

HSU AND

PHILIPP


291

backbone, they are expected to be extremely s t i f f . Those having a predominant Ring-Chain Ring type s t r u c t u r e would be r i g i d or b r i t t l e , whereas those having more Ring-Chain Chain type s t r u c t u r e s are expected to be more f l e x i b l e , E: :per ime n ta 1 1

M a t e r l a I s . 3,3',4,4 -Benzophenonetetracarboxylic dianhyd r i d e (BTDA), 4 , 4 - m e t h y l e n e d i a n i l i n e (MDA), p - a m i n o b e n z o n i t r i l e (PABN), and p - t o l u e n e s u l f o n i c a c i d monohydrate (PTSA) were of commercial sources and used as r e c e i v e d . Graphite f i b e r used as the r e i n f o r c i n g f i l l e r was Hercules HMS grade. Downloaded by MONASH UNIV on November 29, 2015 | http://pubs.acs.org Publication Date: August 12, 1982 | doi: 10.1021/bk-1982-0195.ch022

,

Synthesis of TSTR C r o s s - l i n k e d Polyimides (3). In a 100m i l l i l i t e r g l a s s f l a s k , 9.27 grams (0.00302 mole) o f BTDA was heated with 29 grams of anhydrous methanol u n t i l a c l e a r s o l u t i o n was obtained. A f t e r c o o l i n g , 4.02 grams (0.00202 mole) of MDA, 2.36 grams (0.00200 mole) of PABN, and about 0.1 gram (0.25 mole percent on the b a s i s of the n i t r i l e content) of PTSA were i n t r o duced with s t i r r i n g u n t i l a l l was d i s s o l v e d . T h i s c l e a r s o l u t i o n of monomeric r e a c t a n t s was t r a n s f e r r e d to a 4 5 - m i l l i l i t e r s t a i n l e s s s t e e l pressure v e s s e l equipped w i t h a 1000 p s i pressure gauge, a 1000 p s i b u r s t d i s k , and a needle v a l v e . The v e s s e l without the l i d was heated g r a d u a l l y to about 100°C so t h a t most of the methanol and some o f the by-product (methanol and water) would evaporate o f f . The r e a c t i o n v e s s e l was then i n s t a l l e d with the l i d and heated under reduced pressure a t a temperature up to about 200°C u n t i l no more methanol and/or water came o f f . The v e s s e l was then f i l l e d with dry n i t r o g e n gas and a d j u s t e d t o a pressure of 750 p s i and heated a t 350-400°C f o r 8 hours. The polymeric product presumed to be TSTR c r o s s - l i n k e d polyimide was found to be very hard and dark brown i n c o l o r . I t d i d not melt when heated up to 340°C. I t s KBr i n f r a r e d spectrum showed the absence o f the n i t r i l e band a t 2240 cm"" , but the broad bands at 1520 and 1380 cm" , which are c h a r a c t e r i s t i c s - t r i a z i n e bands Ç7), i n a d d i t i o n to imide bands a t 1795, 1755, 1735 cm"" appeared. 1

1

1

Another run was made with n e a r l y the same procedure and composition except t h a t 2.5 grams (0.00210 mole) of PABN was used. The r e s u l t i n g polymeric product was a l s o dark brown i n c o l o r , and d i d not melt up to 340°C; i t appeared t o be s l i g h t l y s o f t e r than the product of the previous run. Comparison of the KBr i n f r a r e d s p e c t r a of the two m a t e r i a l s showed no s i g n i f i c a n t difference. Graphite F i b e r R e i n f o r c e d TSTR Polyimide Composites (3). The c l e a r s o l u t i o n s of monomeric r e a c t a n t s prepared a c c o r d i n g to the method d e s c r i b e d i n the preceding s e c t i o n were used to make prepregs by drum winding and impregnating Hercules HMS g r a p h i t e f i b e r with a r e s i n / f i b e r content of about 60/40 by weight. The prepregs were heated from 50 to 120°C f o r a couple of hours t o


POLYMERS WITH CHAIN-RING STRUCTURES


292

reduce the s o l v e n t content to l e s s than 10 percent by weight. P l i e s o f p a r t i a l l y i m i d i z e d prepregs were then stacked between aluminum f o i l and heated i n an oven a t 200 to 250°C f o r s e v e r a l hours f o r complete i m i d i z a t i o n . The i m i d i z e d prepregs were then placed i n a mold and cured a t 350°C under pressures i n the range of 200 t o about 1000 p s i f o r s e v e r a l hours. The prepregs were f u r t h e r post cured i n an oven a t 316°C f o r an a d d i t i o n a l s i x t e e n hours. The f l e x u r a l strengths ( t h r e e - p o i n t method) o f these two u n i d i r e c t i o n a l HMS graphite f i b e r r e i n f o r c e d TSTR polyimide comp o s i t e s with or without post c u r i n g were measured with a u n i v e r s a l I n s t r o n machine a t room temperature and a t 316°C. These r e s u l t s are shown i n Table I I . Table I I .

FLEXURAL STRENGTH OF GRAPHITE FIBER/POLYIMIDE COMPOSITES 0

Specimen TSTR-XL-PI/HMS 1 2

a

F l e x u r a l Strength, p s i 316° C (600°F) Room Post cure No Temperature Post Cure (16 h r s . 316°C) 155,000 130,200

148,000 145,700

167,500 157,500

R e s i n / f i b e r = "40/60 by weight.

Bunsen Burner Burning T e s t . A sample o f g r a p h i t e f i b e r r e i n f o r c e d TSTR polyimide composite was exposed to a Bunsen burner flame a t about 1100 t o 1250°C ( F i g . 4 ) . A f t e r 20 minutes i n the flame, the composite s t i l l r e t a i n e d i t s i n t e g r i t y . No r e l e a s e of g r a p h i t e f i b e r was observed ( F i g . 5 ) . A sample o f g r a p h i t e f i b e r r e i n f o r c e d composite made from PMR-15 polyimide (8) under the same c o n d i t i o n s d i s i n t e g r a t e d w i t h i n a couple o f minutes with only g r a p h i t e f i b e r s remaining as a l s o shown i n F i g . 5. Results and D i s c u s s i o n Property-Strueture R e l a t i o n s h i p . Table I I shows the f l e x u r a l strengths of the HMS. graphite f i b e r r e i n f o r c e d TSTR polyimide composites at room temperature and a t 316°C with or without p o s t c u r i n g . The data c l e a r l y i n d i c a t e t h a t the g r a p h i t e f i b e r r e i n f o r c e d composites prepared from aromatic n i t r i l e - m o d i f i e d imide oligomers e x h i b i t e d very good r e t e n t i o n o f f l e x u r a l s t r e n g t h d u r i n g s h o r t time exposure i n a i r a t 316°C. More important, the data a l s o i n d i c a t e t h a t the f l e x u r a l s t r e n g t h o f the composites improved a f t e r a 16-hour post cure a t 316°C. Apparently t h i s r e s u l t e d from an increase i n TSTR formation d u r i n g post cure.


HSU AND PHILIPP


293


22.

Figure 5. Bunsen burner burning test of graphitefiberreinforced polyimide composites. Key: a, TSTR-XL-PI/GrF specimen before test; b, TSTR-XL-PI/GrF specimen after test; and c, PMR-15-PI/GrF specimen after test.



294

POLYMERS WITH CHAIN-RING STRUCTURES

The composites would reach maximum r i g i d i t y and thermal s t a b i l i t y when a l l the TSTRs are completed i n the c r o s s - l i n k i n g mode. Comp l e t e c r o s s - l i n k i n g v i a t r i a z i n e r i n g formation r e s u l t s i n a Ring-Chain Ring s t r u c t u r e shown i n F i g . 2. Excess of n i t r i l e c r o s s - l i n k i n g agent above t h a t r e q u i r e d to endcap the imide oligomer takes p a r t i n p o l y c y c l o t r i m e r i z a t i o n l e a d i n g to chain t e r m i n a t i o n . I f the excess i s s u f f i c i e n t j u s t to block c r o s s l i n k i n g , then only chain extension occurs r e s u l t i n g i n the RingChain Chain s t r u c t u r e shown i n F i g . 3. Thus the c r o s s - l i n k i n g d e n s i t y of the TSTR should decrease as the amount of excess n i t r i l e i s i n c r e a s e d . A decrease i n c r o s s - l i n k i n g d e n s i t y of a polymer should increase i t s f l e x i b i l i t y . T h i s i s i l l u s t r a t e d by a comparison of the f l e x u r a l strengths of specimens 1 and 2 (Table I I ) . Both specimens were d e r i v e d from the same p r e c u r s o r except t h a t i n specimen 1, a 5% excess of PABN was used. As a n t i c i p a t e d , the f l e x u r a l s t r e n g t h of t h i s specimen i s about 10,000 p s i higher than the more completely c r o s s - l i n k e d specimen 2. Thus f l e x i b i l i t y of an otherwise r i g i d m a t e r i a l can be adj u s t e d by o p t i m i z i n g c r o s s - l i n k i n g d e n s i t y which i n t u r n can be c o n t r o l l e d by the amount of excess n i t r i l e added p r i o r t o c u r i n g . R e f r a c t o r y Type Polymeric M a t e r i a l s . P y r o l y s i s of our TSTR c r o s s - l i n k e d polyimide a t about 1000°C leads to the formation of a r e f r a c t o r y type m a t e r i a l p o s s i b l y of a g r a p h i t i c ladder p o l y meric s t r u c t u r e . These ladder polymers have p r e v i o u s l y been s y n t h e s i z e d from a r y l ketones with n i t r o g e n c o n t a i n i n g groups on the r i n g (9_) . T h i s phenomenon was not observed with convent i o n a l c r o s s - l i n k e d polyimides such as polybisnorbornenylimide which d i s i n t e g r a t e d on p y r o l y s i s ; thus the conversion of TSTR polyimides to r e f r a c t o r y type m a t e r i a l s appears to be a s s o c i a t e d with the n i t r i l e or t r i a z i n e r i n g component. In a d d i t i o n to being a high temperature polymeric matrix m a t e r i a l by i t s e l f , TSTR c r o s s - l i n k e d polyimide i s a promising p r e c u r s o r f o r the p r e p a r a t i o n of a r e f r a c t o r y matrix f o r carbon f i b e r s . Conclusion We note the f o l l o w i n g c o n c l u s i o n s : (1) C a t a l y t i c c y c l o p o l y m e r i z a t i o n of imide oligomers c o n t a i n i n g n i t r i l e groups i s a convenient method f o r s y n t h e s i z i n g TSTR c r o s s - l i n k e d polyimides. The TSTR c r o s s - l i n k i n g d e n s i t y can be c o n t r o l l e d by a d d i t i o n of excess n i t r i l e . (2) Graphite f i b e r r e i n f o r c e d TSTR c r o s s - l i n k e d polyimides have good f l e x u r a l s t r e n g t h a t e l e v a t e d temperatures (316°C). (3) P y r o l y s i s of TSTR c r o s s - l i n k e d polyimides converts them to r e f r a c t o r y type m a t e r i a l s which show promise as matrix m a t e r i a l s f o r carbon f i b e r s .



22. HSU AND PHILIPP


295

Literature Cited 1. Markle, R.A. in Carver, J.K.; Tess, R.W. Eds; " Applied Polymer Science "; American Chemical Society, Washington,D.C. 1975; p.560. 2. Blake, E.S.; Hammann, W.C.; Edwards, J.W.; Richard, T.E.; Ort, M.R. J. Chem. Eng. Data 1961, 16, 87. 3. Hsu, Li-Chen. U.S.patent 4,159,262, June 26, 1979. 4. Hsu, Li-Chen. U.S.patent 4,061,856, December 6, 1977. 5. Hsu, Li-Chen. NASA TMX-71525; 1974. 6. Broadbent, H. Smith. Brigham Young University. Invited lecture on "Catalysis" at NASA-Lewis Research Center, Sep.5, 1969. Private communication. 7. The Aldrich Library of Infrared Spectra., "T4605-1 s-Triazine." Aldrich Company, p.1202. 8. Serafini, T.T. NASA TM-79039; 1979. 9. Cassidy, P.E.; "Thermally Stable Polymers--Syntheses and Properties," Marcel Dekker Inc., New York, N.Y. 1981, p.288. RECEIVED December 27, 1981.


Cyclopolymerization and Polymers with Chain-Ring Structures

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