Polymers for High Technology - American Chemical Society

Chapter 23 Stress-Dependent Solvent Removal in Poly(amic acid) Coatings

Downloaded by FUDAN UNIV on December 27, 2016 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch023

C. L. Bauer and R. J. Farris Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003 The development of residual stresses in poly(amic acid) coatings (precursors to polyimide films) was investigated using force-temperature experiments. Results indicate that the residual solvent content is dependent on the stress level. Experiments were performed using poly(N,N'-bis(phenoxyphenyl)pyromellitamic acid) at temperatures between 60° and 80° C to insure no cycloimidization. By altering the balance between internal stress and the driving force for solvent evaporation further solvent removal may occur. Because of their extremely good thermal and electrical properties, thin polyimide films are widely used in the electronics industry. One commonly used polyimide is poly(Ν,Ν'-bis(phenoxyphenyl)pyromellitimide). This may be prepared from the reaction of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) in a two step process (Figure 1). The first step involves a solution reaction forming the polyUmic acid) (PAA). After solvent removal this material can be thermally cyclized to the polyimide (PI). To improve properties, it is often annealed at temperatures up to 400° C. In its final state, this polyimide cannot be processed; thus processing occurs before the imidization step. PAA solutions may be spin-coated onto the appropriate substrates and then thermally treated. This sequence establishes the molecular order in the material {1±2) and in-plane orientation (3). Further development of residual stresses is also associated with~this coating process. Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4^5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history.

0097-6156/87/0346-0270$06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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I n i t i a l l y a f t e r c o a t i n g a f l a t , p l a n a r s u b s t r a t e , as s o l v e n t i s removed, s t r e s s e s do not d e v e l o p i n the f i l m s i n c e the l i q u i d f r e e l y shrinks. When s u f f i c i e n t s o l v e n t has e v a p o r a t e d , the m a t e r i a l g e l s and e v e n t u a l l y s o l i d i f i e s . Assuming complete a d h e s i o n t o the s u b s t r a t e , the m a t e r i a l i s then c o n s t r a i n e d i n the p l a n e of the f i l m but can f r e e l y c o n t r a c t normal t o t h i s p l a n e . As f u r t h e r s o l v e n t e v a p o r a t e s , the m a t e r i a l s h r i n k s and d e v e l o p s s t r e s s e s i n the p l a n e due t o the c o n s t r a i n t s . There a r e no shear s t r e s s e s at the c o a t i n g / s u b s t r a t e i n t e r f a c e , e x c e p t at the e d g e s . These shear s t r e s s e s at the edges r e s u l t from an e q u i l i b r i u m f o r c e b a l a n c e and decay t o z e r o away from the edge. Because t h e r e e x i s t s a s t r o n g i n t e r p l a y between s t r e s s and s w e l l i n g ( s o l v e n t c o n t e n t ) i t may be i m p o s s i b l e t o remove a l l the s o l v e n t d u r i n g the e v a p o r a t i o n s t e p . A s t e a d y s t a t e s t r e s s l e v e l and r e s i d u a l s o l v e n t c o n t e n t may be a t t a i n e d (5^6) and the s t r e s s i n some c o a t i n g s has been shown t o be r e l a t e d t o t h e volume f r a c t i o n o f s o l v e n t ( 7 ) . Additional solvent may be removed by e x p o s i n g the m a t e r i a l t o h i g h e r t e m p e r a t u r e s which c o n t r i b u t e s to f u r t h e r shrinkage. The s o l v e n t removal p r o c e s s i n c o a t i n g s has been a d d r e s s e d as a t r a n s p o r t problem (8,9) and from a mechanics v i e w p o i n t . Several methods have been d e v e l o p e d t o measure r e s i d u a l s t r e s s e s i n c o a t i n g s . For o r g a n i c c o a t i n g s many of t h e s e methods u t i l i z e p l a t e or beam d e f l e c t i o n (6, 1 0 - 1 3 ) . Only r e c e n t l y have t h e y been a p p l i e d t o PI coatings. In the f o r m a t i o n of PI f i l m s , the m a t e r i a l undergoes a s o l v e n t removal s t e p p l u s s e v e r a l t h e r m a l t r e a t m e n t s t o c y c l i z e the polymer and a n n e a l i t . Each of t h e s e p r o c e s s e s c o n t r i b u t e s t o the development of r e s i d u a l s t r e s s e s . G o l d s m i t h , e t . a l . (14) have shown t h a t the r e s u l t i n g s t r e s s e s from c u r i n g PI f i l m s a r e independent of f i l m t h i c k n e s s and the maximum room temperature s t r e s s d e v e l o p e d f o r a f u l l y c u r e d f i l m i s 70 MPa. With s e v e r a l s t a g e s of p r o c e s s i n g , i t may be p o s s i b l e to o p t i m i z e the f i n a l s t r e s s s t a t e . Therefore i t i s important to u n d e r s t a n d the m a t e r i a l b e h a v i o r t h r o u g h a l l the p r o c e s s i n g s t a g e s . In t h i s p a p e r , the i m p o r t a n c e of the c o a t i n g p r o c e s s i n PI f i l m s and the i n t e r r e l a t i o n s h i p of s t r e s s l e v e l and s w e l l i n g i n PAA f i l m s was i n v e s t i g a t e d by a d i r e c t measurement of the s t r e s s . Experimental Materials. The p o l y i m i d e p r e c u r s o r used was DuPont PI5878 p o l y ( a m i c a c i d ) and i s based on PMDA-ODA. N-methylpyrrolidinone (Aldrich #M7960-3) (NMP) was used as the s o l v e n t . Force/Displacement-Temperature Experiments. F o r c e as a f u n c t i o n of temperature was o b t a i n e d by p l a c i n g a sample i n a v e r t i c a l g l a s s oven and g r i p p i n g b o t h ends, w i t h one end a t t a c h e d t o a l o a d c e l l and the o t h e r t o an a d j u s t a b l e mount. A slow n i t r o g e n flow was i n t r o d u c e d through the bottom and an RTD probe was p l a c e d near the m i d d l e of the sample. Output from the probe and l o a d c e l l was r e c o r d e d on a Bascom-Turner Instrument model 4000. With a sample i n p l a c e s e v e r a l h e a t i n g and c o o l i n g c y c l e s were performed at v a r i o u s l o a d s . T y p i c a l l y , the temperature was r a p i d l y i n c r e a s e d t o 80° C , m a i n t a i n e d at t h i s l e v e l f o r a g i v e n time frame and then c o o l e d .

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Samples c o n s i s t e d of a s t r i p of rubber (6 cm χ 1.5 cm χ 0.25 mm) w i t h the ends g l u e d between aluminum f o i l . T h i s was then d i p p e d i n t o a 15% PAA/85% NMP s o l u t i o n and p l a c e d i n t o the f o r c e - t e m p e r a t u r e (F-T) a p p a r a t u s , g r i p p i n g o n l y the aluminum t a b s . To o b t a i n d i s p l a c e m e n t ( s t r a i n ) as a f u n c t i o n o f t e m p e r a t u r e , the l o a d c e l l i n the above a p p a r a t u s was r e p l a c e d w i t h a l i n e a r variable d i f f e r e n t i a l transducer. To d e t e r m i n e i f any c y c l o i m i d i z a t i o n o c c u r r e d w i t h t h e s e h e a t i n g / c o o l i n g c y c l e s , i n f r a - r e d s p e c t r a were o b t a i n e d and i n s p e c t e d f o r i m i d e bands and the absence of N-H s t r e t c h . L i n e a r Mass E x p e r i m e n t s . U s i n g l o n g s t r i p s of n a t u r a l rubber (60 cm χ 2 mm χ 0.5 mm) c o a t e d w i t h a 15% PAA s o l u t i o n i n NMP, the mass per u n i t l e n g t h of the sample was o b t a i n e d as a f u n c t i o n of t e m p e r a t u r e . The t e c h n i q u e i n v o l v e s measuring the t e n s i o n on the sample and the time of f l i g h t of a t r a v e l i n g wave on the sample (M. C h i p a l k a t t i , J . H u t c h i n s o n and R . J . F a r r i s , Rev. Sci. Instr., in press.). These q u a n t i t i e s are then r e l a t e d t o the m a s s / l e n g t h u s i n g the wave e q u a t i o n s f o r a f l e x i b l e s t r i n g which y i e l d s : mass/length = t e n s i o n / ( v e l o c i t y )

2

R e s u l t s and D i s c u s s i o n R e s u l t s of f o r c e - t e m p e r a t u r e e x p e r i m e n t s f o r a PAA s o l u t i o n on rubber are shown i n F i g u r e 2 . F o r the f i r s t c y c l e , the i n i t i a l l o a d i s t h a t of the rubber s i n c e the c o a t i n g i s s t i l l i n the l i q u i d s t a t e and cannot s u p p o r t a l o a d . A t t h i s l o a d the rubber i s j u s t below i t s t h e r m o e l a s t i c i n v e r s i o n p o i n t and i t s c o n t r i b u t i o n t o the f o r c e change i s n e g l i g i b l e . As the sample i s h e a t e d , i t e x p e r i e n c e s a p o s i t i v e t h e r m a l e x p a n s i o n . S h r i n k a g e does not o c c u r u n t i l a c r i t i c a l amount of s o l v e n t i s removed and the c o a t i n g s o l i d i f i e s , at which p o i n t the f o r c e i n c r e a s e s as s o l v e n t i s removed. When c o o l e d t o room t e m p e r a t u r e , the f o r c e (at 2 5 ° C) has i n c r e a s e d by 250g due t o s o l v e n t l o s s i n the PAA c o a t i n g . C o n s i d e r i n g o n l y the c o a t i n g on the two p l a n a r f a c e s o f the r u b b e r , the change i n l o a d f o r one c y c l e i s 125g/coat. By a d j u s t i n g one of the mounts i n the F - T a p p a r a t u s , the l o a d was reduced t o 100g, a p p r o x i m a t e l y the same s t a r t i n g f o r c e as i n the f i r s t c y c l e . On h e a t i n g and c o o l i n g the sample f o r the second c y c l e , the p a t h t r a v e r s e d i s s i m i l a r t o t h a t of the f i r s t c y c l e . If instead the l o a d was l e f t unchanged a f t e r the f i r s t c y c l e ( t h a t i s at 375g) the h e a t i n g / c o o l i n g c u r v e r e v e r s i b l y f o l l o w s the upper p a t h ( c o o l i n g ) of c y c l e - 1 (between 375g at 2 5 ° C and 165g at 8 5 ° C ) . For comparison w i t h o t h e r samples the f i n a l c r o s s s e c t i o n a l area of the c o a t i n g (-1.4 cm χ ~ 1 0 μιη) was used t o compute s t r e s s e s i n the films. F o r the above e x p e r i m e n t , the s t r e s s change due t o s h r i n k a g e w i t h a one d i m e n s i o n a l c o n s t r a i n t was found t o be 8 M P a / c o a t . Assuming p l a n a r symmetry, f o r a two d i m e n s i o n a l l y c o n s t r a i n e d f i l m the s h r i n k a g e s t r e s s would approach 16 MPa i n e q u a l b i a x i a l t e n s i o n . R e s u l t s i n F i g u r e 2 suggest t h a t the e v a p o r a t i o n p r o c e s s i s dependent on the s t r e s s l e v e l . Figure 3 i l l u s t r a t e s this further. F o r c e - t e m p e r a t u r e c y c l e s were performed as b e f o r e , except a f t e r the



BAUER A N D FARRIS

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f i r s t c y c l e the a p p l i e d l o a d was m a n u a l l y i n c r e a s e d above any p r e v i o u s l e v e l . On h e a t i n g t o 80° C, l i t t l e s h r i n k a g e o c c u r r e d a f t e r 20 min. For t h e next c y c l e (C) t h e l o a d was a d j u s t e d back t o t h e o r i g i n a l l e v e l (50g). On r e h e a t i n g the sample, s o l v e n t e v a p o r a t i o n o c c u r s c a u s i n g s h r i n k a g e f o r c e s which approach t h a t of the f i r s t c y c l e . These r e s u l t s i n d i c a t e t h a t a c o u p l i n g e x i s t s between the i n t e r n a l s t r e s s i n the f i l m and the amount of s o l v e n t the m a t e r i a l can r e t a i n . I f the i n t e r n a l s t r e s s b a l a n c e s the d r i v i n g f o r c e f o r s o l v e n t removal a t a g i v e n t e m p e r a t u r e , f u r t h e r s o l v e n t can be removed by r e d u c i n g the s t r e s s . Temperature s h o u l d a l s o p l a y a s i m i l a r r o l e , where a h i g h e r temperature s h o u l d be needed t o remove a d d i t i o n a l s o l v e n t . T h i s i s c o n f i r m e d i n an experiment l i k e t h a t d e s c r i b e d above, except the temperature i s i n c r e a s e d a f t e r the second c y c l e i n s t e a d of r e d u c i n g the l o a d . I n t h i s case when t h e temperature was i n c r e a s e d t o 110° C a d d i t i o n a l s h r i n k a g e was o b s e r v e d . A f t e r t h i s p o i n t , i f the temperature i s i n c r e a s e d t o 150° C, the l o a d does not i n c r e a s e s i n c e the t h e r m a l e x p a n s i o n dominates the b e h a v i o r . However, a t temperatures above 150° C the l o a d i n c r e a s e s due t o s h r i n k a g e caused by the c y c l i z a t i o n . To determine i f t h i s b a l a n c e between s t r e s s l e v e l and s w e l l i n g i s a n o n - e q u i l i b r i u m e f f e c t , l o n g - t e r m experiments were performed. F i g u r e 4 shows the d e t e r m i n a t i o n of s t e a d y s t a t e s h r i n k a g e s t r e s s . A sample was heated under n i t r o g e n t o 62° C and kept a t t h i s temperature (10 hr.) u n t i l the s t r e s s reached a c o n s t a n t v a l u e (8.3 MPa). (On c o o l i n g t o room t e m p e r a t u r e , the s t r e s s reached 10 MPa) P r o b a b l y over an i n f i n i t e p e r i o d of t i m e , s l i g h t l y more s o l v e n t l o s s would o c c u r ; however, i n a r e a s o n a b l e time frame, a " q u a s i " - s t e a d y s t a t e i s o b t a i n e d as i l l u s t r a t e d . The temperature was then i n c r e a s e d t o 84° C. The i n i t i a l decrease i n s t r e s s w i t h t h i s temperature change i s due t o t h e r m a l e x p a n s i o n of the m a t e r i a l . I f the sample i s kept a t t h i s temperature, the s t r e s s a g a i n g r a d u a l l y i n c r e a s e s . T h i s i n c r e a s e i s due t o f u r t h e r s o l v e n t e v a p o r a t i o n and subsequent m a t e r i a l s h r i n k a g e . T h i s a d d i t i o n a l d r y i n g i s i n f l u e n c e d by two f a c t o r s . F i r s t , s i n c e the change i n s t r e s s due t o temperature i s e q u i v a l e n t t o a change due to m e c h a n i c a l d e f o r m a t i o n , then as noted above, a r e d u c t i o n i n s t r e s s a l l o w s f u r t h e r s o l v e n t t o be removed. S e c o n d l y , the h i g h e r temperature i n c r e a s e s the d r i v i n g f o r c e f o r s o l v e n t e v a p o r a t i o n . In another s e t of F-T e x p e r i m e n t s the s t r e s s was v a r i e d and the temperature h e l d c o n s t a n t ( F i g u r e 5 ) . The sample was heated t o 80° C and reached a c o n s t a n t s t r e s s of 9.5 MPa; a f t e r which the sample was c o o l e d and the s t r e s s m a n u a l l y reduced from 10 MPa t o 1 MPa. After h e a t i n g f o r 4 h r s a t 80° C the s t r e s s i n c r e a s e d t o 5.4 MPa but had not a t t a i n e d a l e v e l v a l u e . I n p a r t , t h i s can be due t o v i s c o e l a s t i c r e c o v e r y , but s i n c e t h e r e i s a measurable change i n sample mass between the two runs the s t r e s s change i s p r o b a b l y due t o s h r i n k a g e . In comparing F i g u r e s 4 and 5 t h e r e i s a n o t i c e a b l e change i n the r a t e of s h r i n k a g e f o r the d i f f e r e n t t e m p e r a t u r e s . However, the s t e a d y s t a t e s t r e s s l e v e l i s not a p p r e c i a b l y d i f f e r e n t f o r the two temperatures. To f u r t h e r u n d e r s t a n d t h e s o l v e n t removal p r o c e s s , s t r a i n as a f u n c t i o n of time (and temperature as a f u n c t i o n of time) was o b t a i n e d . R e s u l t s a r e i l l u s t r a t e d i n F i g u r e 6. A c o n s t a n t l o a d of


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F i g u r e 5. S t r e s s and Temperature v e r s u s Time f o r s o l u t i o n on r u b b e r , sample 4. A - f i r s t run, Β s t r e s s r e d u c e d m a n u a l l y t o 1 MPa.

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115g was applied to the sample. On heating at 65° C, the strain decreased with time as expected. After 10 hrs the strain leveled at 6.5%. When the temperature was increased to 80° C the strain increased by 0.04% due to thermal expansion. This corresponds to a thermal expansion coefficient of 3 E(-5) cm/(cm-°C); whereas Kapton (a polyimide based on PMDA-ODA from DuPont) has an expansion coefficient of 2 E(-5) cm/(cm-°C) (15). Further heating at 80° C results in a decrease in strain which must be due to shrinkage and not creep. After the equilibrium strain has been reached, if the applied load is increased there is no further change of strain with time; but if it is decreased, the strain again decreases with time. As with force-temperature experiments there is a detectable change in mass between runs to indicate solvent loss. To correlate stress changes with solvent concentration changes, linear mass experiments were performed. Preliminary results support the conclusions above and further experimentation is in progress to quantify these changes. Using this technique plus direct sample weighing, for each successive heating/cooling/load reduction cycle less solvent is removed to attain the steady state stress level. Because the solvent acts as a plasticizer, the material modulus is reduced. Then as solvent evaporates, the modulus of the film increases (with a final modulus of 1.5 ± 0.4 GPa). Therefore, less solvent needs to be evaporated to generate the same force as in earlier cycles. Conclusions These results indicate that the solvent removal process in polyUmic acid) film formation is dependent on the residual stress level. Solvent evaporation creates volume changes which in turn generate shrinkage stresses (—8 MPa at room temperature). This will continue until there is a balance between the internal stress and the driving forces for solvent removal. Further solvent may be removed if this balance is altered by a change in stress or temperature. Literature Cited 1. N. Takahashi, D.Y. Yoon and W. Parrish, Macromolecules, 17, 2583 (1984). 2. T.P. Russell, J. Polym. Sci., Polym. Phys. Ed., 22, 1105 (1984). 3. T.P. Russell, H. Gugger and J.D. Swalen, J. Polym. Sci., Polym. Phys. Ed., 21, 1745 (1983). 4. A.G. Evans, G.B. Crumley and R.E. Demaray, Oxid. Metals, 20 (56), 193 (1983). 5. K. Sato, Prog. Org. Coat., 8, 143 (1980). 6. D.Y. Perera and D.V. Eynde, J. Coat. Technol., 56 (718), 69 (1984). 7. S.G. Croll, J. Appl. Polym. Sci., 23, 847 (1979). 8. R.F. Eaton and F.G. Willeboordse, J. Coat. Technol., 52 (660), 63 (1980). 9. J. Holten-Anderson and C.M. Hansen, Prog. Org. Coat., 11, 219 (1983).


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10. E.M. Corcoran, J. Paint Technol., 41 (538), 635 (1969). 11. S.G. Croll, J. Coat. Technol., 50 (638), 33 (1978). 12. T.S. Chow, C.A. Liu and R.C. Penwell, J. Polym. Sci., Polym. Phys. Ed., 14, 1311 (1976). 13. R.N. O'Brien and W. Michalik, J. Coat. Technol., 58 (735), 25 (1986). 14. C. Goldsmith, P. Geldermans, F. Bedetti and G.A. Walker, J. Vac. Sci. Technol.,A1(2), 407 (1983). 15. "Kapton Polyimide Film" Summary of Properties, DuPont Technical Bulletin, E-50533 (1982). 1987


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