Characterization of Cobalt(II) Chloride-Modified Condensation

Chapter 27

Characterization of Cobalt (II) Chloride-Modified Condensation Polyimide Films

Downloaded by NATL TAIWAN UNIV on July 1, 2015 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch027

Properties Before and After Solvent Extraction J. D. Rancourt and L. T. Taylor Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, V A 24061-0699

Cobalt(II) chloride was dissolved in poly(amide acid)/ Ν,Ν-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bulk of the film. Two properties of these films, surface resistivity and bulk thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. M e t a l i o n m o d i f i e d p o l y i m i d e f i l m s have been p r e p a r e d t o o b t a i n m a t e r i a l s having mechanical, e l e c t r i c a l , o p t i c a l , adhesive, and surface chemical p r o p e r t i e s d i f f e r e n t from nonmodified polyimide films. F o r example, t h e t e n s i l e modulus o f metal i o n m o d i f i e d p o l y i m i d e f i l m s was i n c r e a s e d ( b o t h a t r o o m t e m p e r a t u r e a n d 200°C) w h e r e a s e l o n g a t i o n was r e d u c e d c o m p a r e d w i t h t h e n o n m o d i f i e d p o l y i m i d e (1,). A l t h o u g h c e r t a i n p o l y i m i d e s a r e k n o w n t o b e e x c e l l e n t a d h e s i v e s {2) l a p s h e a r s t r e n g t h ( b e t w e e n t i t a n i u m a d h e r e n d s ) a t e l e v a t e d t e m p e r a t u r e (275°C) w a s i n c r e a s e d b y i n c o r p o r a t i o n o f t r i s ( a c e t y l a c e t o n a t o ) a l u m i n u m ( I I I ) (3). Highly conductive, r e f l e c t i v e polyimide films containing a palladium metal s u r f a c e were p r e p a r e d a n d c h a r a c t e r i z e d (4). The t h e r m a l s t a b i l i t y o f t h e s e f i l m s w a s r e d u c e d a b o u t 200°C, b u t t h e y w e r e u s e f u l a s novel m e t a l - f i l l e d electrodes( 5 ) . Much o f t h e r e c e n t w o r k p e r t a i n i n g t o m e t a l i o n m o d i f i e d p o l y i m i d e s h a s been aimed a t i n c r e a s i n g t h e s u r f a c e e l e c t r i c a l conductivity of condensation polyimides (6,7,8). T h i s has been accomplished t o a s i g n i f i c a n t extent (surface r e s i s t i v i t y reduced up t o 1 1 o r d e r s o f m a g n i t u d e ( 8 , 9 ) ) b u t i n m o s t s y s t e m s t h e b u l k 0097-6156/88/0367-0395S06.00/0 © 1988 A m e r i c a n C h e m i c a l Society

In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

CROSS-LINKED POLYMERS

396

thermal s t a b i l i t y of the polymer i s a l s o reduced. The p u r p o s e o f t h i s w o r k was t o d e t e r m i n e t h e e f f e c t s o l v e n t e x t r a c t i o n h a s o n t h e properties of cobalt c h l o r i d e modified polyimide f i l m s . The g o a l was t o o b t a i n e n h a n c e d b u l k t h e r m a l s t a b i l i t y a n d r e t a i n t h e l o w surface e l e c t r i c a l r e s i s t i v i t y c h a r a c t e r i s t i c of these systems. The d a t a i n d i c a t e t h a t t h e p r o p e r t i e s o f t h e l o w e r g l a s s t r a n s i t i o n temperature metal i o nmodified polyimides a r e a l t e r e d more t h a n t h e p r o p e r t i e s o f t h e h i g h e r g l a s s t r a n s i t i o n t e m p e r a t u r e metal i o nmodified polyimides. E x t r a c t i o n removes b o t h c o b a l t and c h l o r i n e from t h e f i l m s and s l i g h t l y i n c r e a s e s bulk thermal s t a b i l i t y and both s u r f a c e r e s i s t i v i t y and b u l k electrical resistivity. D e t a i l s p e r t a i n i n g t o t h e s t r u c t u r e , a n a l y s i s and p r o p e r t i e s of these novel g r a d i e n t composites a r e discussed.


Experimental Chemicals. 3 , 4 , 3 ' , 4 ' - B e n z o p h e n o n e t e t r a c a r b o x y l i c d i a n h y d r i d e , BTDA ( A l d r i c h C h e m i c a l C o . ) w a s v a c u u m d r i e d a t 100°C f o r t w o h o u r s . 4 , 4 - O x y d i a n i l i n e , ODA ( A l d r i c h C h e m i c a l C o . ) was s u b l i m e d a t 1 8 5 ° C and l e s s t h a n 1 - t o r r p r e s s u r e . 4,4 -Bis(3,4-dicarboxyphenoxy)d i p h e n y l s u l f i d e d i a n h y d r i d e , BDSDA, was u s e d a s r e c e i v e d f r o m NASA, L a n g l e y R e s e a r c h C e n t e r , H a m p t o n , V A . R e a g e n t g r a d e Ν,Ν-dimethyl a c e t a m i d e , DMAc, d i s t i l l e d i n g l a s s , was s t o r e d o v e r m o l e c u l a r s i e v e s under a n i t r o g e n atmosphere and sparged w i t h d r y n i t r o g e n p r i o r t o u s e . A n h y d r o u s c o b a l t ( I I ) c h l o r i d e was p r e p a r e d b y h e a t i n g t h e h e x a h y d r a t e f o r t h r e e h o u r s a t 1 2 0 ° C u n d e r vacuum. 1

1

P o l y m e r S y n t h e s i s a n d M o d i f i c a t i o n . The c o n d e n s a t i o n r e a c t i o n b e t w e e n e i t h e r BTDA o r BDSDA a n d ODA was p e r f o r m e d i n DMAc a t room temperature under a n i t r o g e n atmosphere. ODA ( 0 . 0 0 4 m o l e ) w a s a d d e d t o a n i t r o g e n - p u r g e d g l a s s s e p t u m b o t t l e w i t h 7 m l DMAc. One o f t h e d i a n h y d r i d e s ( 0 . 0 0 4 m o l e ) was t h e n a d d e d t o t h e d i a m i n e s o l u t i o n w i t h a n a d d i t i o n a l m i l l i l i t e r o f DMAc r e s u l t i n g i n 1 5 - 2 5 w t % s o l i d s d e p e n d i n g u p o n t h e monomer c o m b i n a t i o n . The r e s u l t i n g s o l u t i o n was s t i r r e d f o r 2 0 - 2 4 h o u r s t o f o r m t h e p o l y ( a m i d e a c i d ) , a polyimide precursor. F o r the modified p o l y i m i d e s , anhydrous c o b a l t ( I I ) c h l o r i d e ( 0 . 0 0 1 m o l e ) was a d d e d a s a s o l i d w i t h i n o n e h a l f hour a f t e r t h e d i a n h y d r i d e . Film Preparation. P o l y ( a m i d e a c i d ) s o l u t i o n s were c e n t r i f u g e d a t c a . 1700 rpm, p o u r e d o n t o c l e a n , d u s t - f r e e s o d a - l i m e g l a s s p l a t e s , a n d s p r e a d w i t h a d o c t o r b l a d e w i t h a 16 m i l o r 8 m i l b l a d e g a p t o o b t a i n f i l m s having a f i n a l cured t h i c k n e s s o f approximately 3.4 or 1 . 5 m i l , r e s p e c t i v e l y . T h e f i l m s w e r e c u r e d a t 80°C f o r 2 0 - 3 0 minutes w i t h subsequent d r y i n g and thermal i m i d i z a t i o n i n a f o r c e d a i r , s t a t i c a i r , o r an i n e r t gas oven w i t h c o n t r o l l e d h u m i d i t y a t 100°, 2 0 0 ° a n d 300°C e a c h f o r o n e h o u r . A f t e r c o o l i n g t o room t e m p e r a t u r e , some o f t h e p o l y i m i d e s w e r e r e m o v e d f r o m t h e g l a s s p l a t e by soaking i n d i s t i l l e d water. The s u r f a c e o f t h e f i l m i n contact with the soda-lime glass plate during i m i d i z a t i o n i s r e f e r r e d t o as t h e g l a s s - s i d e ; w h i l e , t h a t i n contact w i t h t h e cure atmosphere o f t h e c u r i n g oven i s r e f e r r e d t o as t h e a i r - s i d e .


27.

RANCOURT ANDTAYLOR

Cobalt(II)

Chloride-Modified

Films

397


E x t r a c t i o n of F i l m s . C o b a l t m o d i f i e d p o l y i m i d e f i l m s were e x t r a c t e d by one of s e v e r a l t e c h n i q u e s : (1) f i l m s on t h e c a s t i n g p l a t e w e r e s o a k e d i n a t r a y o f d i s t i l l e d w a t e r a t room t e m p e r a t u r e , (2) f i l m s were s o x h l e t t e x t r a c t e d w i t h d i s t i l l e d w a t e r , (3) f i l m s w e r e s o a k e d i n DMAc a t r o o m t e m p e r a t u r e , a n d ( 4 ) f i l m s w e r e s o x h l e t t e x t r a c t e d w i t h DMAc. Transmission Electron Microscopy; Transmission electron microscopy d a t a w e r e o b t a i n e d by p e r s o n n e l i n t h e U l t r a s t r u c t u r e L a b o r a t o r y a t the V i r g i n i a - M a r y l a n d C o l l e g e of V e t e r i n a r y M e d i c i n e u s i n g a JEOL 100CX-II transmission e l e c t r o n microscope. Samples were imbedded i n P o l y - b e d 812 epoxy r e s i n and c u r e d a t 5 0 - 6 0 ° C f o r 2 - 3 d a y s . S a m p l e s w e r e t h e n s e c t i o n e d t o b e t w e e n 8 0 0 a n d 1 0 0 0 Â on e i t h e r a S o r v a l MT2B o r a n L K B I V U l t r a m i c r o t o m e u s i n g g l a s s k n i v e s a n d w e r e p l a c e d o n 2 0 0 mesh c o p p e r g r i d s . Spectroscopic Methods. X - r a y p h o t o e l e c t r o n s p e c t r a were r e c o r d e d w i t h a P e r k i n - E l m e r P h i M o d e l 5 3 0 0 ESCA s y s t e m . Auger E l e c t r o n s p e c t r o s c o p y and d e p t h p r o f i l i n g v i a argon i o n e t c h i n g were r e c o r d e d w i t h a P e r k i n - E l m e r P h i Model 610 S c a n n i n g Auger Microprobe System. E n e r g y d i s p e r s i v e X - r a y a n a l y s i s was o b t a i n e d by p e r s o n n e l i n t h e U l t r a s t r u c t u r e L a b o r a t o r y a t t h e V i r g i n i a Maryland C o l l e g e of V e t e r i n a r y Medicine u s i n g a Tracor Northern 5500 X - r a y m i c r o a n a l y s i s s y s t e m . S e c t i o n s were prepared i n a m a n n e r s i m i l a r t o t h a t u s e d f o r TEM e v a l u a t i o n e x c e p t t h a t t h e s a m p l e s were s e c t i o n e d t o b e t w e e n 900 and 1200 Â and w e r e p l a c e d on 190 mesh c a r b o n c o a t e d n y l o n g r i d s . Thermal Methods. T h e r m o g r a v i m e t r i c a n a l y s i s of a s i n g l e d i s k (1/4" d i a m e t e r ) o f e a c h f i l m was o b t a i n e d w i t h a P e r k i n - E l m e r M o d e l T G S - 2 t h e r m o g r a v i m e t r i c system a t 20°C/min h e a t i n g r a t e i n dynamic a i r purge. 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 i c a n a l y s i s of two o r more d i s k s o f e a c h f i l m s e a l e d a l u m i n u m s a m p l e p a n s was o b t a i n e d w i t h a P e r k i n - E l m e r Model DSC-4 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 e r a t 10°C/min h e a t i n g r a t e i n i n a dynamic n i t r o g e n purge. E l e c t r i c a l Methods. Room t e m p e r a t u r e s u r f a c e a n d v o l u m e d i r e c t c u r r e n t e l e c t r i c a l r e s i s t i v i t i e s o f 8 5 mm d i a m e t e r p o l y m e r f i l m s were d e t e r m i n e d u s i n g a K e i t h l e y h i g h v o l t a g e s o u r c e (Model 2 4 0 A ) , a K e i t h l e y e l e c t r o m e t e r (Model 610C), and a K e i t h l e y t h r e e e l e c t r o d e assembly (Model 6105 R e s i s t i v i t y A d a p t e r ) . Variable t e m p e r a t u r e e l e c t r i c a l r e s i s t i v i t y d e t e r m i n a t i o n s were o b t a i n e d w i t h a computer c o n t r o l l e d i n s t r u m e n t developed i n our l a b o r a t o r y (10). The s y s t e m c o n t r o l s s a m p l e t e m p e r a t u r e , a t m o s p h e r e a n d m e a s u r e m e n t mode i n a d d i t i o n t o a u t o m a t i c a l l y s t o r i n g , p r i n t i n g , and p l o t t i n g the data w i t h the a s s o c i a t e d p e r i p h e r a l s . The e l e c t r o d e geometry f o r the v a r i a b l e temperature t e s t c e l l i s the same a s t h e K e i t h l e y 6 1 0 5 R e s i s t i v i t y A d a p t e r . R e s u l t s and

Discussion

General Properties. I t h a s b e e n shown ( 6 ) t h a t t h e i n c o r p o r a t i o n o f c o b a l t c h l o r i d e i n t o a p o l y ( a m i d e a c i d ) s o l u t i o n and p r o c e s s i n g t h e s o l v e n t c a s t f i l m t o t e m p e r a t u r e s up t o 300°C i n an a p p r o p r i a t e



398


atmosphere r e s u l t s i n a h i g h l y a n i s o t r o p i c polymeric s t r u c t u r e . X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y was u s e d t o d e t e r m i n e t h e f i n a l chemical s t a t e of the dopant. T y p i c a l l y , the c o b a l t c h l o r i d e forms c o b a l t o x i d e n e a r t h e a i r - s i d e o f t h e p o l y m e r f i l m as shown by Auger e l e c t r o n spectroscopy w i t h depth p r o f i l i n g v i a argon i o n sputtering. The e l e c t r i c a l p r o p e r t i e s o f t h e p o l y m e r s a r e a l s o anisotropic. The a i r - s i d e b e h a v e s a s a c o n d u c t o r , w h e r e a s , t h e b u l k b e h a v e s as a d i e l e c t r i c w i t h b o t h d i e l e c t r i c l o s s and d.c. conduction. V a r i a b l e temperature e l e c t r i c a l r e s i s t i v i t y determina t i o n s suggest t h a t the near-surface r e g i o n of the polymer contains d i s c r e t e metal o x i d e domains because the s u r f a c e r e s i s t i v i t y t h e r m o r e v e r s i b l y i n c r e a s e s as t h e sample i s h e a t e d t h r o u g h t h e g l a s s t r a n s i t i o n temperature of the polymer (independently d e t e r m i n e d by 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 ) . An a d d i t i o n a l f e a t u r e o f t h e s e g r a d i e n t c o m p o s i t e s i s t h a t , a l t h o u g h some o f t h e dopant i s converted to c o b a l t oxide, the bulk r e s i s t i v i t y i s s t i l l many o r d e r s o f m a g n i t u d e g r e a t e r t h a n i t s h o u l d be f o r t h e a m o u n t of i n c o r p o r a t e d c o b a l t c h l o r i d e . This leads to the s p e c u l a t i o n t h a t the d i e l e c t r i c c o n s t a n t of the polymer i s not h i g h enough t o s u p p o r t d i s s o c i a t i o n o f an a p p r e c i a b l e amount of t h e c o b a l t s a l t . The s t r u c t u r a l f e a t u r e s e n v i s i o n e d b a s e d o n t h e a n a l y t i c a l t e c h n i q u e s e m p l o y e d f o r t h e c o b a l t c h l o r i d e m o d i f i e d BTDA-ODA p o l y i m i d e f i l m s c u r e d i n a m o i s t a i r atmosphere were v e r i f i e d u s i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p i c e v a l u a t i o n s of ultramicrotomed of c r o s s s e c t i o n s ( F i g u r e 1, p a r t s A, Β a n d E ) . EDAX v e r i f i e d t h a t t h e p a r t i c l e s d i s p e r s e d t h r o u g h o u t t h e f i l m were composed p r i m a r i l y o f c o b a l t a n d c h l o r i n e . I t was a t t e m p t e d , o n t h e b a s i s o f t h e s e d a t a , t o use e x t r a c t i o n t e c h n i q u e s t o m o d i f y t h e p o l y m e r s t r u c t u r e and p r o p e r t i e s . T h e r m a l P r o p e r t i e s . E a c h o f t h e p o l y i m i d e f i l m s a m p l e s was e v a l u a t e d by 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 t o d e t e r m i n e t h e g l a s s t r a n s i t i o n t e m p e r a t u r e ( T a b l e I ) . The g e n e r a l o b s e r v a t i o n i s t h a t t h e BTDA-ODA p o l y i m i d e f i l m s h a v e a h i g h e r g l a s s t r a n s i t i o n t e m p e r a t u r e t h a n t h e BDSDA-ODA p o l y i m i d e f i l m s w h e t h e r t h e y a r e nonmodified or are modified with c o b a l t c h l o r i d e . This i s i n agreement w i t h t h e work o f F r y e (11) i n w h i c h t h e d i a n h y d r i d e m o i e t y , n o t t h e d i a m i n e , was f o u n d t o c o n t r o l t h e p o l y i m i d e g l a s s t r a n s i t i o n temperature· More s p e c i f i c a l l y , t h e g l a s s t r a n s i t i o n t e m p e r a t u r e of t h e BDSDA-ODA p o l y i m i d e f i l m i s s l i g h t l y h i g h e r f o r t h e m o d i f i e d t h a n for the nonmodified sample. During the f i r s t heating c y c l e there i s an e n d o t h e r m w h i c h i s a b s e n t on s u b s e q u e n t r e h e a t i n g t h a t h a s been a t t r i b u t e d t o a s t r e s s r e l a x a t i o n phenomenon i n t h e g l a s s t r a n s i t i o n r e g i o n f o r e a c h BDSDA-ODA s a m p l e . The g l a s s t r a n s i t i o n i s a l s o i n c r e a s e d s l i g h t l y a f t e r t h e f i r s t h e a t - c o o l c y c l e due m o s t l i k e l y to annealing (Figure 2). DSC a n a l y s i s t o o n l y 250°C (50° l o w e r t h a n t h e p r o c e s s t e m p e r a t u r e , 300°C) o f a s a m p l e a l s o r e s u l t s i n l o s s o f t h e s t r e s s r e l a x a t i o n e n d o t h e r m and an i n c r e a s e i n t h e glass t r a n s i t i o n temperature a f t e r t h i s f i r s t heat-cool c y c l e . We t a k e t h i s as f u r t h e r s u p p o r t t h a t t h e h i g h e r g l a s s t r a n s i t i o n t e m p e r a t u r e o b s e r v e d d u r i n g t h e s e c o n d h e a t i n g c y c l e i s due t o a n n e a l i n g and n o t t o f u r t h e r r e a c t i o n ( i m i d i z a t i o n ) o r t o f u r t h e r s o l v e n t (DMAc) l o s s .


In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. A Β C D Ε

5.0 0.1 0.1 5.0 0.2

μηη μηη μηη μηη μηη

1

F i g u r e 1. T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h of u l t r a m i c r o t o m e d c r o r - r - n e c t i o n of a c o b a l t c h l o r i d e m o d i f i e d BTDA-ODA p o l y i m i d i f i l n b e f o r e (A, B, and E) and a f t e r (C and D) e x t r a c t i o n .

\


In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. No Yes

BDSDA/ODA

BDSDA/ODA

δ

9 Yes

#9,

24 h r w a t e r

Nonextracted soak

532

1.21

i n dynamic a i r

512

1.24

385 580

15.00 0.41

215

215

217

217

300 212

300 306 300

273,357 269 276,336 505 504 505

2.30 2.60 2.00

212

302 302 272,345 279,360

503 505

284

2.07 1.83

275

Nonextracted #2, 24 h r w a t e r s o a k #3, 24 h r w a t e r soxhlett Nonextracted #5, 24 h r DMAc s o a k #6, 24 h r DMAc soxhlett Nonextracted

553

b

c AGT (°C) 2nd 1st Heat Heat

1.36

a

PDT (°O

Nonextracted

Treatment

Mass Lost (%)

P r o p e r t i e s o f Dry A i rCured C o b a l t C h l o r i d e Modified Condensation Polyimide Films

200 C Temperature c o r r e s p o n d i n g t o 10% w e i g h t l o s t AGT = A p p a r e n t g l a s s t r a n s i t i o n t e m p e r a t u r e

BDSDA/ODA

Yes Yes Yes

BTDA/ODA BTDA/ODA BTDA/ODA

5 6 7

10

Yes Yes Yes


2 3 4

No

BTDA/ODA

1

Doped

Polyimide

Thermal

Film No.

Table I .


(Λ

w

Ο r

w α

%

s


RANCOURT AND TAYLOR

Cobalt(II)

Chloride-Modified

Films

F i g u r e 2. 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 i c a n a l y s i s o f a c o b a l t c h l o r i d e m o d i f i e d BDSDA-ODA p o l y i m i d e f i l m .


401


402


DSC analysis of the cobalt chloride modified BTDA-ODA polyimide films i s more complex than the BDSDA-ODA case. During the f i r s t heating cycle two changes i n heat capacity occur with the BTDA-ODA polyimide films which d i r e c t l y suggest the presence of two glass t r a n s i t i o n temperatures and thereby two types of polyimide phases. This i s p a r t i c u l a r l y interesting i n view of the TEM crosssection (Figure 1 , part A) that shows a depletion zone, approximately 5% of the sample thickness, and a zone which shows discrete p a r t i c l e s dispersed throughout the f i l m . The higher glass t r a n s i t i o n temperature, which correlates well with thermomechanical analysis data (Figure 3 , part C), may be due to a f i l l e r - e f f e c t . The lower glass t r a n s i t i o n temperature may r e f l e c t the polyimide i n the depleted zone. The magnitude of the higher temperature glasst r a n s i t i o n of 350°C i s greater than the magnitude of the glasst r a n s i t i o n of 2 8 0 ° C which suggests more of the sample i s responsible for the higher glass t r a n s i t i o n . This i n combination with the TEM data supports the assignment of the 3 5 0 C Tg to the p a r t i c l e - c o n t a i n i n g portion of the sample. Heating to 400°C during DSC analysis may allow p a r t i a l polymer relaxation around the cobalt chloride p a r t i c l e s . This may be the reason for the intermediate glass t r a n s i t i o n temperature observed during the second heating c y c l e : 302°C instead of ~280 and ~350°C. DSC analysis of the films after extraction revealed that the extraction solvents (DMAc or water) and the extraction conditions (soak at room temperature or soxhlett extraction) do not seem to have an appreciable influence on the glass t r a n s i t i o n temperature of either the cobalt chloride modified BTDA-ODA or BDSDA-ODA polyimide f i l m s . Each sample was evaluated by thermogravimetry to determine i f the thermal s t a b i l i t y could be enhanced by removing some residual cobalt chloride. The BTDA-ODA polyimide f i l m thermal s t a b i l i t y i s reduced about 50°C due to the cobalt chloride dopant. Soaking or extraction with water has no positive e f f e c t on the thermal s t a b i l i t y whereas soxhlett extraction with DMAc severely degrades the polymer s t a b i l i t y . For the BDSDA-ODA polyimide films the incorporation of cobalt chloride also reduces the bulk polymer thermal s t a b i l i t y . Soaking t h i s f i l m i n water for 24 hours, however, increased the bulk thermal s t a b i l i t y s l i g h t l y from 5 1 2 ° to 532°C. The v o l a t i l e content of the nonmodified and cobalt chloride modified polyimide films was estimated by determining the mass loss that occurred up to 2 0 0 ° C during the thermogravimetric evaluation. In general, compared with the nonmodified controls, i o n incorporation does not s i g n i f i c a n t l y a l t e r the v o l a t i l e content of the cobalt chloride modified polyimides. Also, the v o l a t i l e content i s comparable to that observed f o r DuPont Kapton polyimide f i l m ( 1 2 ) ( i . e . 1 . 3 wt% at 50% R.H., 2 . 9 wt% at 100% R.H.). In no case did soaking or extracting the polyimide films s i g n i f i c a n t l y a l t e r the v o l a t i l e content of the f i l m except f o r the DMAc extracted BTDA-ODA samples which contained about 15% v o l a t i l e s . e

E l e c t r i c a l C h a r a c t e r i s t i c s . The d.c. e l e c t r i c a l r e s i s t i v i t y of the polyimide films was determined as a function of temperature i n vacuum. Insofar as the nonmodified polyimides are concerned the BTDA-ODA polyimide f i l m has a s l i g h t l y higher volume and surface


Cobalt(II)

RANCOURT AND TAYLOR

Chloride-Modified

Films

403


0.75

H

50

1

1

100

1

1—I

150

1—I

1

1

1—I—I—I

200 250 300 350 TEMPERATURE (°C)

1

1

1

400

F i g u r e 3. Comparison o f 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 i c d a t a ( p a r t s A and B) w i t h thermomechanical a n a l y s i s d a t a ( p a r t C) f o r a c o b a l t c h l o r i d e m o d i f i e d BTDA-ODA p o l y i m i d e f i l m .



404


r e s i s t i v i t y t h a n t h e BDSDA-ODA p o l y i m i d e a s e x p e c t e d b a s e d o n b o t h the higher g l a s s t r a n s i t i o n temperature (lower i o n i c m o b i l i t y ) and lower d i e l e c t r i c constant (lower free i o ncontent) of t h e former. For both systems i n c o r p o r a t i o n o f c o b a l t c h l o r i d e reduces t h e bulk a n d s u r f a c e r e s i s t i v i t y a n d , a s e x p e c t e d , m o r e s o f o r t h e BDSDA-ODA p o l y i m i d e f i l m s ( T a b l e I I ) . The u s e o f e x t r a c t i o n t e c h n i q u e s t o i n c r e a s e t h e b u l k r e s i s t i v i t y o f t h e f i l m s was e f f e c t i v e i n a l l c a s e s e x c e p t f o r t h e 24 h o u r w a t e r s o a k o f t h e c o b a l t c h l o r i d e m o d i f i e d BTDA-ODA p o l y i m i d e f i l m . A l t h o u g h t h e volume r e s i s t i v i t y , i n g e n e r a l , was i n c r e a s e d , s o t o o was t h e a i r - s i d e r e s i s t i v i t y . The a i r - s i d e r e s i s t i v i t y o f t h e c o b a l t c h l o r i d e m o d i f i e d p o l y i m i d e f i l m was i n c r e a s e d t o t h e v a l u e o b s e r v e d w i t h a n o n m o d i f i e d BTDAODA p o l y i m i d e f i l m ; w h i l e , t h e c o b a l t c h l o r i d e m o d i f i e d BDSDA-ODA p o l y i m i d e f i l m had an i n c r e a s e i n s u r f a c e r e s i s t i v i t y of o n l y about three orders of magnitude a f t e r soaking t h i s f i l m i n water. The v a r i a b l e temperature a i r - s i d e surface r e s i s t i v i t y p r o f i l e s f o r the c o b a l t c h l o r i d e m o d i f i e d BDSDA-ODA p o l y i m i d e f i l m b e f o r e a n d a f t e r a w a t e r s o a k a r e s h o w n i n F i g u r e 4. Metal I o n Content. The m e t a l i o nc o n t e n t o f c o b a l t c h l o r i d e m o d i f i e d c o n d e n s a t i o n p o l y i m i d e f i l m s was a s s e s s e d u s i n g e l e m e n t a l analysis. The d a t a ( T a b l e I I I ) i n d i c a t e t h a t t h e w e i g h t percentage of both c o b a l t and c h l o r i n e a r e u s u a l l y reduced f o l l o w i n g soaking or e x t r a c t i o n . However, e x c e p t f o r t h e p o l y i m i d e f i l m t h a t was S o x h l e t t e x t r a c t e d w i t h DMAc t h e m e t a l i o n c o n t e n t i s n o t r e d u c e d v e r y much. I t i s b e l i e v e d t h a t t h e p o l y i m i d e f i l m , e v e n w i t h i o n i n c o r p o r a t i o n , i s an e f f i c i e n t b a r r i e r a g a i n s t e x t r a c t i o n . Proof t h a t i f t h e p o l y i m i d e were more p e r m e a b l e t h e n g r e a t e r e x t r a c t i o n e f f i c i e n c y w o u l d b e a t t a i n e d was o b t a i n e d b y t h e f o l l o w i n g experiment. T h e same TEM g r i d , u s e d t o o b t a i n t h e m i c r o g r a p h s h o w n i n F i g u r e 1 , p a r t A was s o a k e d i n d i s t i l l e d w a t e r . Prior to s o a k i n g , EDAX was u s e d t o v e r i f y t h a t t h e p a r t i c l e s w e r e p r i m a r i l y composed o f c o b a l t a n d c h l o r i n e and s h o u l d t h e r e f o r e be s o l u b l e i n water. A f t e r s o a k i n g t h e g r i d i n w a t e r f o r 24 h o u r s t h e t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h shown i n F i g u r e 1 , p a r t s C a n d D was o b t a i n e d . A l l t h e s p h e r i c a l p a r t i c l e s a r e absent b u t most o f the near-surface deposit i s present. The s p h e r i c a l p a r t i c l e s a r e removed because t h e u l t r a m i c r o t o m e d c r o s s - s e c t i o n t h i c k n e s s , c o m p a r a b l e t o t h e d i a m e t e r o f t h e s e p a r t i c l e s , makes t h e p a r t i c l e s a c c e s s i b l e t o t h e w a t e r w h i c h c a n d i s s o l v e them. Thus, had t h e polyimide allowed s u f f i c i e n t solvent into the f i l m the desired i n c r e a s e i n volume r e s i s t i v i t y and b u l k t h e r m a l s t a b i l i t y would h a v e b e e n r e a l i z e d . T h e s u r f a c e r e s i s t i v i t y was i n c r e a s e d a f t e r s o a k i n g b e c a u s e t h e c o n t i n u i t y o f t h e n e a r - s u r f a c e m e t a l o x i d e was lessened. A d d i t i o n a l d e t a i l s r e g a r d i n g t h e samples were o b t a i n e d b y i n f e r r i n g f r o m t h e e l e m e n t a l a n a l y s i s t h e amount o f c o b a l t p r e s e n t as o x i d e a n d c h l o r i d e . The o n l y source o f c h l o r i n e i n t h e s e samples i s t h e c o b a l t c h l o r i d e dopant. Thus, based on t h e c h l o r i n e c o n c e n t r a t i o n we c a n e s t i m a t e t h e a m o u n t o f c o b a l t t h a t m u s t b e present as c o b a l t c h l o r i d e . F o r e x a m p l e , f i l m #5 i s c a l c u l a t e d t o c o n t a i n 78.7% o f t h e c o b a l t a s c h l o r i d e and by d i f f e r e n c e a b o u t 2 1 % as c o b a l t o x i d e . F i l m #7 ( w h i c h i s f i l m #5 a f t e r b o t h a 24 h o u r s o a k a n d a 24 h o u r e x t r a c t i o n w i t h DMAc) c o n t a i n s 2 0 % o f t h e c o b a l t


In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. No Yes Yes Yes

Yes Yes Yes

No Yes Yes

BTDA/ODA



BDSDA/ODA

BDSDA/ODA

BDSDA/ODA

1

2 3 4

5 6 7

8

9

10

#9,

24 h r w a t e r

Nonextracted

Nonextracted

Nonextracted #5, 24 h r DMAc #6, 24 h r DMAc soxhlett

r

Nonextracted #2, 24 h r w a t e r #3 24 h r w a t e r soxhlett

Nonextracted

Treatment

soak

soak

soak

Log^Q o f a v e r a g e o f f i r s t h e a t i n g a n d f i r s t i n v a c u u m w i t h +100 VDC Room t e m p e r a t u r e a t 1 0 0 0 Hz i n a m b i e n t F i l m was d e s t r o y e d

Doped

Polyimide

e

a t 200 C

12.80

16.22

cooling cycle

9.65

15.48 13.48

16.22

c

c

c

14.88 15.53

5.69 4.52

14.76 16.07

14.14 14.53 15.33

5.23 4.03

15.56 14.97

15.77

2.76

3.5

16.82

Air-Side Resist, (ohm)

16.04

Volume Dielectrig Constant

Volume Resist. (ohm cm) a

E l e c t r i c a l P r o p e r t i e s of Dry A i r Cured Cobalt C h l o r i d e Modified Condensation Polyimide Films

Film No.

Table I I .


406


Ιβ.Οτ ° οο ο


15.0

° ο.

14.0+ _ 13.0-Ε

χ: Ο

Βο

- Ι2.0-11.0(Ο

ω

10.0 Ο ο ,

9.0-8.0 0

Η

20

h

40

Η

60

80

1

100

1

120

1

140

1

160

τ-

180

200

TEMPERATURE ( Ο β

F i g u r e 4. Comparison o f a i r - s i d e e l e c t r i c a l r e s i s t i v i t y o f a c o b a l t c h l o r i d e m o d i f i e d BDSDA-ODA p o l y i m i d e f i l m b e f o r e and a f t e r soaking i n d i s t i l l e d water.


In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988. Yes Yes Yes Yes No Yes Yes

BTDA-ODA BTDA-ODA BTDA-ODA BTDA-ODA

BDSDA-ODA

BDSDA-ODA BDSDA-ODA

8

9 10

5 5 6 7

1.66 1.47

2.03 2.03 2.02 1.99

Nonextracted #9,24 hr water soak

2.44 2.44

0.01

Nonextracted

2.74 2.74 2.74 2.74 2.42 2.47 2.32 0.66

0.00

3.30 3.30

2.62 0.16

2.74 2.74 2.74 2.74 2.37 2.63 2.30 2.00

0.00

Cobalt, wt% Found Expect,

0.00

3.30

2.29

3.30 3.30 3.30 3.30

Nonextracted Nonextracted #5,24 hr DMAc soak #6,24 hr DMAc extr.

2.52 2.17

3.20

BTDA-ODA BTDA-ODA BTDA-ODA BTDA-ODA

2 2 3 4

No Nonextracted Nonextracted #2,24 hr water soak #3,24 hr water extr.

BTDA-ODA

1

0.00

Chlorine , wt% Expect. Found

Yes Yes Yes Yes

Treatment 0.04

Doped Nonextracted

Film No. Polyimide

Table I I I . Elemental Composition of Dry A i r Cured Cobalt Chloride Modified Condensation Polyimide Films


408


as c o b a l t c h l o r i d e and 80% o f t h e c o b a l t as o x i d e . As a c r o s s check, t h e d a t a f o r f i l m #5 i m p l i e s t h a t complete e x t r a c t i o n o f a l l the c o b a l t c h l o r i d e b u t no e x t r a c t i o n o f c o b a l t o x i d e s h o u l d r e s u l t i n 0.53 wt% c o b a l t ( a s c o b a l t o x i d e ) f o r f i l m #7. T h i s i s i n good agreement w i t h t h e e x p e r i m e n t a l d a t a (0.66 wt%) f o r f i l m #7 suggesting mostly c o b a l t c h l o r i d e , not c o b a l t oxide, i s extracted from t h e f i l m by t h i s h a r s h t e c h n i q u e . Acknowledgment


The f i n a n c i a l a s s i s t a n c e o f t h e N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n , Langley Research Center i s g r a t e f u l l y appreciated. The s e r v i c e s of L . H o r n i n g i n p e r f o r m i n g many o f t h e e x t r a c t i o n experiments i s appreciated.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Taylor, L. T. and St. Clair, A. K. J. Appl. Polym. Sci., 1983, 28, 2393. Progar, D. J. and St. Clair, T. L. 7th National Sampe Technical Conference, Albuquerque, NM, Oct. 1975. Taylor, L. T.; St. Clair, A. K. NASA Langley Research Center, "Aluminum Ion Containing Polyimide Adhesives", U. S. Patent 284,461 (1981). Wohlford, T. L.; Schaff, J.; Taylor, L. T.; St. Clair, A. K.; Furtsch, Τ. Α.; Khor, E. Conductive Polymers; Seymour, R. B., Ed.; Plenum Publishing Corporation: New York, 1981, p 7. Furtsch, Τ. Α.; Finklea, H. O.; Taylor, L. T. Polyimides: Synthesis, Characterization and Applications, Vol. 2; Mittall, K. L., Ed.; Plenum Press: New York, 1984, p 1157. Rancourt, J. D.; Boggess, R. K.; Horning, L. S.; Taylor, L. T. J. Electrochem. Soc. 1987, 134, 85. Ezzell, S. Α.; Furtsch, Τ. Α.; Taylor, L. T. J. Polym. Sci. Polym. Chem. Ed., 1983, 21, 865. Ezzell, S. A. and Taylor, L. T. Macromolecules, 1984, 17, 1627. Rancourt, J. D.; Porta, G. M.; Taylor, L. T. Thin Films, submitted for publication, 1987. Rancourt, J. D.; Swartzentruber, J. L.; Taylor, L. T. Am.Lab. March 1986, 68. Frye, M. Polyimides: Synthesis, Characterization and Applications; Mittal, K. L., Ed.; Plenum Press: New York, 1984, p 377. DuPont Kapton Polyimide Film, DuPont DeNemours Literature.

RECEIVED October

7, 1987


Characterization of Cobalt(II) Chloride-Modified Condensation

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