Cross-Linking and Isomerization Reactions of an Acetylene

Chapter 33

Cross-Linking and Isomerization Reactions of an Acetylene-Terminated Polyisoimide Prepolymer 1

Downloaded by UNIV OF ROCHESTER on November 4, 2014 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch033

R. H. Bott, L. T. Taylor , and T. C. Ward Department of Chemistry and Polymer Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0699 Acetylene terminated polyimide prepolymers have many advantages over conventional polyimides in the areas of processing and solvent resistance. In addition, the presence of the isoimide structure further extends the the utility of these systems by modification of the solubility properties and glass transition temperature. This work discusses the thermal crosslinking and isomerization reactions occurring in the acetylene terminated isoimide prepolymer: Thermid IP600. The techniques of Fourier Transform Infrared Spectrometry and Differential Scanning Calorimetry are used to determine the contribution of these two reactions during the thermal cure including their kinetics at 183°C. The i n c r e a s i n g need f o r h i g h s e r v i c e temperature a d h e s i v e s and s t r u c t u r a l m a t r i x r e s i n s has l e d t o t h e development of many new p o l y m e r i c systems i n r e c e n t y e a r s . One o f t h e most i n t e r e s t i n g and p o t e n t i a l l y u s e f u l o f t h e s e new polymers i s p o l y i m i d e s . Polyimides a r e noted f o r t h e i r e x c e l l e n t thermal and m e c h a n i c a l p r o p e r t i e s b u t t h e i r u t i l i t y has been s e v e r e l y l i m i t e d due t o problems w i t h f a b r i c a t i o n and p r o c e s s i n g o f t h e s e polymers ( 1 - 3 ) . N e v e r t h e l e s s , t h e c a r e f u l d e s i g n o f p o l y i m i d e s can l e a d t o enhanced processability. I n t h i s r e s p e c t , s e v e r a l approaches have been i n v e s t i g a t e d and found t o be u s e f u l . One d e s i g n method which has improved t h e p r o c e s s a b i l i t y o f l i n e a r a r o m a t i c p o l y i m i d e s i s t h e i n t r o d u c t i o n of m e t a - s u b s t i t u t e d aromatic diamines f o r para s u b s t i t u t e d analogs (4,5). T h i s procedure, while improving t h e p r o c e s s a b i l i t y , a l s o has t h e p o s s i b l e d e t r i m e n t a l e f f e c t of l o w e r i n g t h e g l a s s t r a n s i t i o n temperature. A n o t h e r method which has been s u c c e s s f u l l y u t i l i z e d i n i m p r o v i n g p o l y i m i d e p r o c e s s i n g and s o l u b i l i t y c h a r a c t e r i s t i c s i s t h e i n c o r p o r a t i o n o f b u l k y s i d e g r o u p s such as p h e n y l a t e d diamine monomers ( 6 ) . A l t h o u g h t h e s e m a t e r i a l s m a i n t a i n a h i g h g l a s s t r a n s i t i o n temperature t h e i r r e s i s t a n c e t o s o l v e n t s may be s a c r i f i c e d . P r o c e s s a b i l i t y can a l s o

Correspondence should be addressed to this author. 0097-6156/88/0364-0459$06.00/0 © 1988 American Chemical Society

In Chemical Reactions on Polymers; Benham, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


460

CHEMICAL REACTIONS ON POLYMERS

be improved by d i l u t i n g t h e r i g i d imide f u n c t i o n a l i t y i n the polymer c h a i n through the use of b l o c k c o p o l y m e r i z a t i o n w i t h a f l e x i b l e segment such as a s i l o x a n e ( 7 - 9 ) . These approaches a l l r e l y on enhancing the t h e r m o p l a s t i c i t y o f t h e p o l y i m i d e s through i n c o r p o r a t i o n of f l e x i b i l i z i n g l i n k a g e s . F i n a l l y , p r o c e s s a b i l i t y and f a b r i c a t i o n a s p e c t s of p o l y i m i d e s have been improved through the use of low m o l e c u l a r w e i g h t imide o l i g o m e r s t e r m i n a t e d w i t h a c e t y l e n i c groups (10-11). The m a t e r i a l (MC-600) shown i n F i g u r e 1 i s an example of a c o m m e r c i a l l y a v a i l a b l e p r o d u c t ( N a t i o n a l S t a r c h and C h e m i c a l C o . ) . These m a t e r i a l s have improved s o l u b i l i t y and processing c h a r a c t e r i s t i c s while maintaining both a high g l a s s t r a n s i t i o n temperature and good s o l v e n t r e s i s t a n c e due t o t h e i r h i g h l y c r o s s l i n k e d n a t u r e f o l l o w i n g t h e r m a l c u r e of the a c e t y l e n e groups. T h i s approach a l s o has problems i n terms of p r o c e s s i n g parameters. P r e l i m i n a r y r e a c t i o n s of the t e r m i n a l a c e t y l e n e groups d u r i n g t h e r m a l c u r e l e a d t o a r e s t r i c t i o n o f f l o w and w e t t i n g p r o p e r t i e s b e f o r e good c o n t a c t i s a c h i e v e d ( 1 2 ) . In a d d i t i o n , s i n c e t h e g l a s s t r a n s i t i o n temperature of t h e s e imide o l i g o m e r s i s q u i t e h i g h (~200°C) t h e c r o s s l i n k i n g r e a c t i o n proceeds v e r y r a p i d l y r e s u l t i n g i n an i n f u s i b l e , r i g i d network. Once t h e g l a s s t r a n s i t i o n temperature has been exceeded, enough m o b i l i t y i s a v a i l a b l e i n t h e system f o r r a p i d c r o s s l i n k i n g of the t e r m i n a l a c e t y l e n e groups. In t h i s c a s e , above 200°C ( t h e Tg of the uncured polymer) t h i s c r o s s l i n k i n g r e a c t i o n p r o c e e d s v e r y r a p i d l y . The g e l time f o r MC-600 has been e s t i m a t e d a t l e s s than t h r e e minutes a t a temperature of 250°C ( 1 3 ) . T h i s s h o r t g e l time s e v e r e l y r e s t r i c t s the uses of t h e m a t e r i a l i n a p p l i c a t i o n s such as m a t r i x r e s i n s and a d h e s i v e s where good f l o w i s n e c e s s a r y p r i o r t o t h e o n s e t of gellation. In o r d e r t o c i r c u m v e n t t h i s problem of r a p i d g e l l a t i o n an i s o m e r i c imide s t r u c t u r e , termed i s o i m i d e , has been i n t r o d u c e d i n t o t h e s e systems (13,14). M a t e r i a l s with t h i s f u n c t i o n a l i t y ( i . e . , IP-600) ( F i g u r e 2) e x h i b i t improved s o l u b i l i t y as w e l l as l o n g e r g e l times and lower g l a s s t r a n s i t i o n temperatures (~160°C vs ~200°C f o r t h e c o r r e s p o n d i n g i m i d e o l i g o m e r ) . I n i t i a l l y , i t was thought t h a t the p r e s e n c e of the i s o i m i d e s t r u c t u r e , as an u n f a v o r a b l e s i d e r e a c t i o n p r o d u c t i n p o l y i m i d e s , l e d t o premature t h e r m a l d e c o m p o s i t i o n of p o l y i m i d e s t h r o u g h l o s s of CO^ from the i m i n o l a c t o n e h e t e r o c y c l i c r i n g ( 1 5 ) . However, l a t e r work (16) showed t h a t the i s o i m i d e f u n c t i o n a l i t y t h e r m a l l y i s o m e r i z e d t o the i m i d e f u n c t i o n a l i t y ( F i g u r e 3) p r i o r t o any s i g n i f i c a n t d e g r a d a t i o n o f the polymer backbone. S i n c e the u t i l i t y of t h e s e m a t e r i a l s i s improved by t h e i n c o r p o r a t i o n of t h e s e r e a c t i v e f u n c t i o n a l i t i e s w i t h o u t s e v e r e l y d e c r e a s i n g o t h e r f a v o r a b l e p r o p e r t i e s such as t h e r m o o x i d a t i v e s t a b i l i t y and s o l v e n t r e s i s t a n c e the c h e m i s t r y of the i s o i m i d e i s o m e r i z a t i o n and a c e t y l e n e c r o s s l i n k i n g r e a c t i o n s i s of considerable interest. P r e v i o u s work i n our l a b o r a t o r y has shown t h a t t h e s e m a t e r i a l s , when l o a d e d w i t h m e t a l powders, p r o v i d e a c o n v e n i e n t and e f f e c t i v e method of o p t i m i z i n g t h e e l e c t r i c a l conductance and t h e r m a l s t a b i l i t y of aluminum c o n d u c t o r j o i n t s . The g o a l o f t h i s work i s t o e l u c i d a t e t h e r e l a t i o n s h i p between t h e t h e r m a l i s o m e r i z a t i o n and c r o s s l i n k i n g r e a c t i o n s o c c u r r i n g i n t h i s acetylene terminated p o l y i s o i m i d e oligomer: Thermid IP600. The




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t e c h n i q u e s of F o u r i e r T r a n s f o r m I n f r a r e d s p e c t r o m e t r y and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y have been shown t o be u s e f u l i n d e t e r m i n i n g the c u r e s t a t e s of a c e t y l e n e - t e r m i n a t e d r e s i n s such as i m i d e s and s u l f o n e s (17-20), These t e c h n i q u e s w i l l be a p p l i e d t o the c u r e r e a c t i o n s o c c u r r i n g i n the IP600 a c e t y l e n e t e r m i n a t e d i s o i m i d e system.


Experimental F o u r i e r T r a n s f o r m I n f r a r e d S p e c t r o m e t r y (FTIR) was performed on a N i c o l e t 6000 I n f r a r e d S p e c t r o m e t e r . S p e c t r a were o b t a i n e d a t 2 cm r e s o l u t i o n and were an average o f 10 s c a n s . The samples were p r e p a r e d by c a s t i n g from a 5% (w/w) s o l u t i o n i n THF a t h i n f i l m onto e i t h e r a KBr o r KC1 p l a t e . The s o l v e n t was removed by vacuum d r y i n g a t 60-80°C. The FTIR c u r e e x p e r i m e n t s were performed u s i n g a Barnes Model C019-020 h e a t e d c e l l . I n o r d e r t o more a c c u r a t e l y m o n i t o r t h e t e m p e r a t u r e of the c u r i n g r e a c t i o n s a t t h e KBr p l a t e , h o l e s were d r i l l e d i n t o the s i d e o f the sample p l a t e s and a thermocouple was inserted. I n t h i s way t h e temperature o f the c r y s t a l c o u l d be i n d e p e n d e n t l y m o n i t o r e d . D i f f e r e n t i a l S c a n n i n g C a l o r i m e t r y (DSC) was performed on a P e r k i n Elmer DSC-4 a t h e a t i n g r a t e s of 10, 5 and 2.5°C/min w i t h a n i t r o g e n purge. Thermogravimetry (TG) was p e r f o r m e d i n a P e r k i n Elmer TGS-2 t h e r m o g r a v i m e t r i c a n a l y z e r a t 10°C/min i n a dynamic a i r atmosphere a t a purge r a t e o f 50 c c / m i n . T h e r m i d IP-600 was o b t a i n e d from N a t i o n a l S t a r c h and Chemical C o r p o r a t i o n , P.O. Box 6500, B r i d g e w a t e r , NJ, 08807. R e s u l t s and

Discussion

The i n c o r p o r a t i o n of the i s o i m i d e f u n c t i o n a l i t y i n t o a c e t y l e n e t e r m i n a t e d o l i g o m e r s has been shown t o r e s u l t i n enhanced s o l u b i l i t y , a lower g l a s s t r a n s i t i o n temperature and improved p r o c e s s a b i l i t y compared t o t h e analogous i m i d e o l i g o m e r s ( 1 9 ) . Chemical changes o c c u r r i n g i n c u r i n g systems can be m o n i t o r e d i n a v a r i e t y o f ways, two o f t h e most common o f which a r e DSC and FTIR. DSC p r o v i d e s an o v e r a l l e n e r g e t i c p r o f i l e o f the r e a c t i o n s o c c u r r i n g i n a g i v e n temperature range; w h i l e FTIR p r o v i d e s d e t a i l e d s t r u c t u r a l i n f o r m a t i o n c o n c e r n i n g the r e l a t i v e c o n c e n t r a t i o n s of f u n c t i o n a l i t i e s p r e s e n t i n t h e system. By e s t a b l i s h i n g a r e a c t i o n energy p r o f i l e u s i n g DSC one can e f f e c t i v e l y choose the p r o p e r t e m p e r a t u r e s a t which t o c o n d u c t i s o t h e r m a l FTIR e x p e r i m e n t s . These i s o t h e r m a l FTIR e x p e r i m e n t s can then be used t o e s t a b l i s h the n a t u r e and k i n e t i c s of t h e c h e m i c a l changes o c c u r r i n g i n t h e system a s a f u n c t i o n of time a t the g i v e n i s o t h e r m a l t e m p e r a t u r e . The DSC thermograms of MC-600 and IP-600 a r e shown i n F i g u r e 4. I t i s a p p a r e n t from t h e s e scans t h a t the i s o i m i d e o l i g o m e r (IP-600) has not o n l y a lower g l a s s t r a n s i t i o n temperature b u t a l s o a l a r g e r exotherm of r e a c t i o n than the c o r r e s p o n d i n g i m i d e o l i g o m e r (~ -80 c a l / g f o r IP-600 vs ~ -40 c a l / g f o r MC-600). T h i s o b s e r v a t i o n a l o n g w i t h the f a c t t h a t t h e s e o l i g o m e r s a r e of the same nominal m o l e c u l a r w e i g h t (~1100 amu) l e a d s t o t h e c o n c l u s i o n t h a t t h e i s o m e r i z a t i o n of the i s o i m i d e t o t h e i m i d e f u n c t i o n a l i t y i s s u f f i c i e n t l y e x o t h e r m i c t o c o n t r i b u t e s i g n i f i c a n t l y t o t h e exotherm of the n e t r e a c t i o n measured by DSC. A c t i v a t i o n e n e r g i e s f o r t h e s e two samples were


BOTTETAL.

Acetylene-Terminated Polyisoimide Prepolymer


33.


463

464

CHEMICAL REACTIONS ON

POLYMERS

c a l c u l a t e d from the v a r i a t i o n of peak r e a c t i o n temperature w i t h heating rate. The a c t i v a t i o n e n e r g i e s so c a l c u l a t e d a l o n g w i t h the e q u a t i o n used i n t h i s c a l c u l a t i o n a r e shown i n T a b l e I . The nonagreement of a c t i v a t i o n e n e r g i e s s u g g e s t a s u b s t a n t i a l d i f f e r e n c e

Table I.

C a l c u l a t e d A c t i v a t i o n E n e r g i e s From DSC


H e a t i n g Rate:

10-5°/min

5-2.5°/min

DATA*

10-2.5°/min

IP-600

14,1

13.1

13.6

MC-600

20.4

19.1

19.5

Method of C a l c u l a t i o n : E

a

-R 1.052

A In φ Δ (1/T ) Ρ

*Kcal/mole φ = H e a t i n g Rate (°K/min) Τ = Peak R e a c t i o n Temperature

(°K)

i n r e a c t i o n pathways f o r t h e s e two samples. The method chosen f o r e x t r a c t i n g a c t i v a t i o n e n e r g i e s i s e x p e c t e d t o be more a c c u r a t e than a n a l y s i s of a s i n g l e scan f o r s i t u a t i o n s where two o v e r l a p p i n g peaks o c c u r i n the DSC s c a n ( 2 1 ) . The a c t i v a t i o n energy c a l c u l a t e d f o r the imide o l i g o m e r , MC-600, i s 19.7 ± 0.7 Kcal/mole which a g r e e s w i t h v a l u e s o b t a i n e d f o r s i m i l a r model systems such as s u l f o n e s (24.2 ± 0.7 Kcal/mole) (22) and phenoxy p h e n y l a c e t y l e n e (23.2 ± 1 Kcal/mole) ( 2 3 ) . The a c t i v a t i o n energy f o r t h e i s o i m i d e o l i g o m e r i s s u b s t a n t i a l l y lower (13.6 ± 0.5 K c a l / m o l e ) . T h i s r e s u l t i s c o n s i s t e n t w i t h t h e lower i n i t i a l r e a c t i o n temperature of the i s o i m i d e o l i g o m e r and a l s o s u g g e s t s t h a t the i s o m e r i z a t i o n r e a c t i o n may precede t h e c r o s s l i n k i n g r e a c t i o n t o some e x t e n t . In o r d e r t o b e t t e r c h a r a c t e r i z e the k i n e t i c s of t h e s e two p r o c e s s e s ( i . e . , i s o m e r i z a t i o n and c r o s s l i n k i n g ) i n the i s o i m i d e o l i g o m e r , h e a t e d c e l l FTIR experiments were c o n d u c t e d . The i n f r a r e d spectrum o f uncured IP-600 i s o i m i d e o l i g o m e r i s shown i n F i g u r e 5. T h i s spectrum shows c h a r a c t e r i s t i c a b j o r b a n c e s f o r t h e a c e t y l e n e f u n c t i o n a l i t y a t 3295 cm^ and 940 cm^ (20) and f o r the i s o i m i d e f u n c t i o n a l i t y a t 1805 cm and 930 cm (24-25). A s m a l l amount o| imide i s a l s o e v i d e n t by the p r e s e n c e of a s h o u l d e r a t 1725 cm . To m i n i m i z e the^amount o f i n t e r f e r e n c e due t o a d j a c e n t bands we chose t h e 3295 cm band f o r m o n i t o r i n g t h e c o n c e n t r a t i o n of a c e t y l e n e f u n c t i o n a l i t y and the 1805 cm band f o r m o n i t o r i n g t h e c o n c e n t r a t i o n of i s o i m i d e f u n c t i o n a l i t y . Some i n t e r f e r e n c e was s t i l l o b s e r v e d i n the case of the i s o i m i d e band due t o a 1775 cm band a s s i g n a b l e t o t h e i m i d e s t r u c t u r e . A sample was p l a c e d i n t h e c e l l and the c e l l was h e a t e d t o a temperature of 1 8 3 ° C Infrared s p e c t r a of t h e sample were then r e c o r d e d as a f u n c t i o n of time a t t h i s temperature. F i g u r e 6 shows the i s o i m i d e - i m i d e r e g i o n of t h e spectrum as a f u n c t i o n o f t i m e . I t can be seen t h a t t h e i s o i m i d e peak d e c r e a s e s r a p i d l y d u r i n g t h e f i r s t s i x minutes of t h e


BOTT ET AL.


465


33.

F i g u r e 6.

I s o i m i d e - I m i d e Region of I n f r a r e d Spectrum F u n c t i o n of Time a t 183°C.

of IP-600 as a



466


experiment (the amount of time n e c e s s a r y f o r the 183°C i s o t h e r m a l t e m p e r a t u r e t o be r e a c h e d ) and then c o n t i n u e s t o d e c r e a s e a t a steady r a t e . F i g u r e 7 shows the a c e t y l e n e r e g i o n of the spectrum a t t h e same t i m e s . Here i t appears t h a t the d e c r e a s e i n absorbance i s not as r a p i d as i n the case o f the i s o i m i d e band. F i g u r e s 8 and 9 show t h e f i r s t o r d e r k i n e t i c p l o t s f o r the i s o m e r i z a t i o n and c r o s s l i n k i n g r e a c t i o n s , r e s p e c t i v e l y . In the d a t a a n a l y s i s t h e a r e a of t h e i s o i m i d e peak was measured between c o n s i s t e n t l i m i t s chosen t o e x c l u d e any c o n t r i b u t i o n from the 1775 cm i m i d e band. These d a t a were g e n e r a t e d by measuring t h e a r e a of t h e a p p r o p r i a t e peak i n a b a s e l i n e c o r r e c t e d spectrum and r a t i o i n g t h i s a r e a t o t h a t of a r e f e r e n c e peak (which was i n v a r i e n t d u r i n g the experiment) i n the same spectrum. T h i s c o n c e n t r a t i o n i n d i c a t i v e number was t h e n r a t i o e d t o the c o n c e n t r a t i o n r a t i o o b s e r v e d on the i n i t i a l s c a n . P l o t s of the l o g o f the r a t i o of t h e c o n c e n t r a t i o n of the f u n c t i o n a l i t y at time t " t o the c o n c e n t r a t i o n of the f u n c t i o n a l i t y a t t = 0 were then c o n s t r u c t e d . In o r d e r t o i n s u r e t h a t the t r e n d s i n the d a t a were not a r t i f a c t s of t h i s p r o c e d u r e or of the b a s e l i n e c o r r e c t i o n r o u t i n e , we a l s o p l o t t e d t h e d a t a i n terms o f peak i n t e n s i t y i n absorbance u n i t s and o b s e r v e d the same t r e n d s b u t w i t h more s c a t t e r i n the d a t a . The f i r s t o r d e r p l o t f o r the i s o m e r i z a t i o n r e a c t i o n shows a good l i n e a r f i t ( c o r r e l a t i o n c o e f f i c i e n t = 0.998); w h i l e t h e r e i s n

F i g u r e 7.

A c e t y l e n e Region of I n f r a r e d Spectrum of IP-600 as F u n c t i o n of Time at 183°C.


a

BOTTETAL.



33.

J O O

H O Q H X M Q I U

O:

Cross-Linking and Isomerization Reactions of an Acetylene

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