Solution Characterization of Polyamic Acids and Polyimides - ACS

19

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 7, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch019

Solution Characterization of Polyamic Acids and Polyimides P. METZGER COTTS and W. VOLKSEN IBM Research Laboratory, San Jose, CA 95193 The low dielectric constant, ease of processing, and high temperature stability of polyimide materials have led to the widespread use of these materials as insulating layers in electronic devices. Several characteristics of these materials are not yet well understood, such as the instability of solutions with time, structural changes on curing, and polyelectrolyte effects observed in dilute and concentrated solutions. In this study we have investigated these and other characteristics of the polyamic acid and polyimide formed from the condensation of pyromellitic dianhydride and 4,4'-diaminodiphenylether. The polyamic acids were studied in solution using various solvents, and the cured polyimide was studied in concentrated sulfuric acid. Techniques used included low-angle light scattering, osmometry and viscometry. Behavior characteristic of dilute polyelectrolyte solutions was only observed in NMP which had not been purified by distillation. In all other solvents studied, normal behavior characteristic of neutral, flexible chain polymers dissolved in good solvents was observed. Comparison of data for samples before and after cure showed that structural changes during cure were limited essentially to imidization with little degradation in molecular weight and no evidence of cross-linking, for samples cured thermally to 300 degrees C. The condensation of pyromellitic dianhydride (PMDA) with 4,4-diaminodiphenylether (DAPE) to a precursor polyamic acid followed by cyclodehydration (cure), produces a polyimide (Figure 1) which is widely used in the electronics industry as a high temperature insulating material. This particular polyimide is available in several forms commercially and has been the subject of !

0097-6156/84/0242-0227506.00/0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

228

POLYMERS IN ELECTRONICS

v®5•,jria, Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 7, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch019

ο

ο PMDA

DAPE

r

ο Polyamic acid chemical and/or thermal cure

F i g u r e 1. S y n t h e s i s and c u r e o f t h e p o l y i m i d e and p r e c u r s o r p o l y a m i c a c i d from t h e c o n d e n s a t i o n o f p y r o m e l l i t i c d i a n h y d r i d e (PMDA) and 4 , 4 - d i a m i n o d i p h e n y l e t h e r (DAPE). 1

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


19.

COTTS A N D VOLKSEN

Polyamic Acids and

Polyimides

229

many investigations (1-5). However, some aspects of i t s synthesis, physical properties and processing requirements are not yet completely understood; such as control of molecular weight and polydispersity, solution s t a b i l i t y , polyelectrolyte effects and changes i n molecular structure with cure. We have investgated the properties of solutions of both the polyamic acid and the polyimide to increase our understanding of this particular polymer, and the complete class of polyimide materials. The use of dilute solutions for our studies permitted observation of molecular parameters essentially independent of interactions between polymer chains. Techniques used included low angle light scattering (LALS), viscometry (η) and membrane osmometry (OS). Synthesis of PMDA/DAPE samples in our laboratory (7) permitted conditions of the synthesis to be carefully controlled, eliminating many of the uncertainties inherent in a commercial synthesis. We were able to directly measure weight average molecular weights by LALS on the cured polyimide i n concentrated sulfuric acid, and the precursor polyamic acid i n organic solvents. Until recently reported from this laboratory (6), the molecular weights of cured PMDA/DAPE samples could only be compared by relative viscosity or final mechanical properties, where the interpretation can be ambiguous. Experimental Samples of polyamic acid were obtained commercially (DuPont) as concentrated solutions or were synthesized i n this laboratory i n N-methylpyrrolidone (NMP), with the polymerization solutions stored under argon u n t i l use (7). A l l dilute solutions were prepared by dilution from the concentrated solution with d i s t i l l e d NMP. Cured polyimide samples were either commercially available films (Kapton), or were cured in this laboratory from either commercial or laboratory synthesized polyamic acids, using a thermal or a combination chemical/thermal cure. Solvents used were a l l reagent grade, and at times were r e d i s t i l l e d before use. Low angle light scattering (LALS) was measured using a Chromatix KMX-6 light scattering photometer. Weight average molecular weights (M^ were obtained by determining the Rayleigh factor, R Q , of several different concentrations and extrapolating to i n f i n i t e dilution to obtain M , using Equation 1 below: w

[Kc/R ] 0

1 / 2

= (M )" w

1 / 2

+ A c(M )" 2

1 / 2

w

(1)

which i s s t r i c t l y valid only at 0 = 0 ° and i s used here without correction for the low angles measured with this instrument (~4°). This square root equation is often found to be linear over a larger concentration range than the usual equation especially for polymers in very good solvents ( Α 2 » 0 ) as is found here. The intrinsic viscosity ([η]) for each sample was obtained by measuring the viscosities of several dilute solutions of the polymer and extrapolating to i n f i n i t e dilution using the usual relations :


230


l / c =

[η] + Κ ' [ η ] ο + . . .

(2)

2

2

l n ( T i ) / c = [η] + (k'-1/2)[η] c + ...

(3)

rel

where η /c is the reduced viscosity and 1η(η | )/c is the inherent viscosity. Linear plots were obtained except where polyelectrolyte effects were observed, which w i l l be discussed below. Concentrated solution viscometry was measured using a Brookfield viscometer equipped with a pressure transducer and temperature controller to allow simultaneous recording of temperature and viscosity. Membrane osmometry was measured on a few samples to determine the number average molecular weight (M ). Regenerated cellulose membranes were used, and were found to be highly swollen in NMP, leading to both very long equilibration times and excellent retention of low molecular weights. Table I below shows the results of these and the above measurements for the samples reported here. Studies on solutions over a period of months, to be discussed below, were made on solutions stored under argon at room temperature u n t i l just before dilution and/or measurement. A l l dilutions were made with freshly d i s t i l l e d NMP, unless specific effects of the non-distilled NMP were being investigated.


§ ρ

Γ6

n

Results and Discussion Equilibrium Molecular Parameters. Polyelectrolyte effects, evidenced by increased solution viscosity and reduced scattering intensity at very low concentrations, were observed for polyamic acid solutions in NMP which had not been r e d i s t i l l e d over ?2®5' ^ attribute this effect to the abstraction of protons from the amic acid by amine impurities in the NMP. This is discussed i n more detail i n an earlier study (6). Measurements reported by other workers i n other amide solvents such as dimethylacetamide, have been made with LiBr added to suppress the polyelectrolyte affect (3) . Measurements described here were made on solutions in NMP which had been r e d i s t i l l e d over ?2®5 * P°ly l ° l y effects were observed. Polymerizations such as the condensation of PMDA and DAPE are expected to follow step-growth kinetics and lead to a most probable distribution. Control of molecular weight should be possible via a stoichiometric imbalance, i f the purity of the monomers i s well controlled. The control of molecular weight by stoichiometric imbalance has been shown i n this laboratory for the samples used here (7). These samples span a range of weight average molecular weights from 2000 to 250,000, where the highest molecular weight sample was obtained with equal stoichiometry. These molecular weights were a l l measured using LALS on solutions in d i s t i l l e d NMP. More details on the use of this technique to determine molecular weights for PMDA/DAPE polymers can be found in an earlier paper (6). Membrane osmometry results on two of the samples confirmed that M /M was in fact two as expected for a most probable distribution. e

y

w

anc

n

o

e

e c t r

t e

n


19.

COTTS AND VOLKSEN

Polyamic Acids and

Polyimides

231

In some cases, notably when polymerizations were carried out at higher concentrations, i n i t i a l molecular weights were higher than expected and decreased slowly over a period of days to the expected M . This was accompanied by a corresponding decrease in the concentrated solution viscosity. Results obtained for two samples are shown in Figure 2. Once the equilibrium molecular weight had been obtained, the viscosity and M measured by LALS remained constant for several days. Results reported in Table I were measured on these stable equilibrated samples. In a l l cases, intrinsic viscosities were measured within 24 hours of the LALS measurement so that relations between these parameters would be independent of any changes in the sample with time. We attribute this observed decrease in M with time to a very slow approach to the equilibrium distribution in samples polymerized at high concentration. This is caused by the combined effects of the very high reaction rate of the anhydride-amine condensation and the relative insolubility of the PMDA monomer in NMP. The i n i t i a l local stoichiometry differences equilibrate later to the expected distribution through the reversibility of the condensation reaction. Details of the synthesis have been discussed in an earlier report (7). Reports from other laboratories indicating degradation in molecular weight with time (2), or synthesis of molecular weights increasing with concentration of the polymerization solution (5) may also be evidence of this observed slow equilibration. w


w

Measurements on Cured Polyimide. Table II below shows results obtained for the cured polyimide samples dissolved in concentrated sulfuric acid. For the samples cured in this laboratory, where the precursor polyamic acid M is known, the observed molecular weight of the cured polyimide is comparable. This suggests that the final physical properties of the cured polyimide should be determined by the molecular weight of the precursor polyamic acid formed. The samples which were cured chemically with acetic anhydride and pyridine, and then heated to remove solvent, appeared to be incompletely imidized, and yielded lower molecular weights than thermally cured samples from the same polyamic acid. In addition, these samples produced a bright red solution in sulfuric acid, in contrast to the orange-gold color observed in solutions from thermally cured samples. The red color disappeared within 24 hours, when the molecular weights were determined. This color was also observed by Wallach (4), who carried out viscosity measurements on sulfuric acid solutions of chemically cured PMDA/DAPE polyimides. Wallach observed a slow decrease in the dilute solution viscosity with time over a period of hours from the i n i t i a l preparation of the solution. We have not observed any decrease in viscosity for 24 hours for solutions prepared from thermally cured samples. Polyimide samples which have been cured chemically have been shown to contain a small percentage of isoimide, which i s then converted to the more stable imide at higher temperatures (8-9). The observed red color in sulfuric acid solutions may be because protonation of



232

Table I. Molecular Parameters for Polyamic Acids 3

L S

10' M daltons W


Sample

PAA-1 PAA-2 PAA-3 PAA-4 PAA-5

3

ΙΟ" M dalton

n

(

4

L S

10 A ml/g dalton 25° C 2

4.5 9.0 37.0 77.0 250.0

43 40 29 21 16

PAA-6 PAA-7 PAA-8 PAA-9 PAA-10 PAA-11 PAA-12 PAA-13 PAA-14 PAA-15

2.0 4.0 6.0 9.1 9.8 10.4 16.0 16.0 22.0 29.0

45 42 35 38 37 48 35 30 31 25

DuPont A DuPont Β

28.0 18.0

15.0

4

0 S

10 A ml/g dalton 40° C 2

In)

'n NMP ml/g

[η] in NMP/dioxone ml/g

10

20

(cp) at -10 wt% 20 70 700 4000 200(2%)

28 50 132 245 585 14 23

12 7500(32%)

32

48 49 50 72 74 87 100

44 61

10000(18%)

120 70

0

η

30

40

50

500

60

Time (days)

F i g u r e 2. E q u i l i b r a t i o n o f t h e w e i g h t a v e r a g e m o l e c u l a r w e i g h t t o t h a t e x p e c t e d f o r a most p r o b a b l e d i s t r i b u t i o n .



19.

COTTS AND VOLKSEN

Polyamic Acids and

Polyimides

233

the isoimide moiety by the acid, as has been observed for other heterocyclic polymers dissolved in strong acids (10). Slow degradation of the less stable isoimide in the sufuric acid would explain the observed loss of the red color, the lower molecular weight, and the slow decrease in solution viscosity observed by Wallach. The molecular weight of ~7,000 would suggest 1-2 scissions per chain, or 1-2% residual isoimide after the 200° thermal treatment. Table II shows that a sample which was post-baked at 400°C for one hour after being thermally cured to 300°C was no longer soluble in concentrated sulfuric acid. We attribute this insolubility to a morphological change in the arrangement of the polyimide chains, leading to insolubility in the kinetic sense. Similar behavior has been observed for another heterocyclic polymer and was attributed to formation of aggregates which acted as crosslinks (jj.). In the latter study, significant partial solubility and swelling of the annealed polymer was observed. Partial cross-linking of the polyimide chains would be expected to produce a highly swollen sample in sulfuric acid, and some degree of solubility, which was not observed here. Thus, i f crosslinks arise from an aggregation process, then i t is apparently a relatively rapid process, since these samples are only annealed for 1 hour, in comparison to the 15 hour period used in reference 11. The molecular weight of the cured polyimide is essentially determined by the molecular weight of the precursor polyamic acid (see Table II and Figure 3, below). This was observed for both commercial and laboratory synthesized samples, and over a molecular weight range of 10,000 to 30,000. Thus physical properties of the f i n a l cured polyimide can be expected to be a direct function of the equilibrium molecular weight obtained for the precursor poyamic acid. Additional enhancement of these properties may arise from specific cure conditions, such as the chemical cure or post-bake above 300°C mentioned above, but these processes probably do not affect the actual molecular weight. Comparison of Chain Dimensions. Knowledge of the intrinsic viscosities and molecular weights of a series of PMDA/DAPE polyamic acids allows estimation of the unperturbed chain dimensions. Comparison may then be made with the cured polyimide, with results obtained by other workers, and with calculated values. The expressions in the literature using values for [η] obtained in good solvents generally involve extrapolation of [η]/Μ^ΐο M^= 0, where excluded volume effects are presumed to be minimal (12-14). Although this method is not s t r i c t l y valid, i t is useful for comparison purposes when direct measurement of the dimensions is impractical. The data in Table I for the polyamic acid in d i s t i l l e d NMP and in the poorer mixed solvent NMP/dioxane yield (see Figure 4): 2

( /M )* = 0.95 A Q

w

2

(4)

where is the mean square end to end distance. These dimensions


234


Table II. Measurements of Cured PI in 97% H S 0 2


Sample

Kapton® Kapton® Kapton® Kapton® Kapton®

3

L S

10" M PAA (daltons) W

Cure Conditions

_ —

—

—

—

—

—

—

—

4

3

L S

Film (/ym) Thickness

Age of Solution

ΙΟ" M PI (daltons)

7 7 12 25 50

5 hours 4 days 3 days 3 days 3 days

18 16 20 18 23

W

[η]

in H ml/g

2

100 — — — —

—

—

20 22 25

—

DuPont A DuPont A DuPont A DuPont A DuPont A DuPont A DuPont A

28 28 28 28 28 28 28

thermal. 150°C thermal, 200° C thermal, 300° C thermal, 300° C thermal, 300° C. 400° C chemical, 150°C chemical, 200°C

8-10 8-10 8-10 8-10 8-10 8-10 8-10

degraded 1 day 1 day 14 days insoluble 4 days 4 days

PAA-11 PAA-11

10.4 10.4

thermal, 300° C thermal, 300° C

8-10 25

7 hours 4 days

9 11

50

PAA-3

37.0

thermal, 300°C

8-10

1 day

30

—

—

7 9

— — — — —

—

F i g u r e 3 . L i g h t s c a t t e r i n g measurement o f w e i g h t a v e r a g e m o l e c u l a r w e i g h t s o f a p o l y i m i d e ( t , • ) and t h e p r e c u r s o r p o l y a m i c a c i d (A), s h o w i n g n e a r l y e q u i v a l e n t m o l e c u l a r w e i g h t b e f o r e and a f t e r c u r e .



19.

COTTS AND VOLKSEN

0 2 I

235

Polyamic Acids and Polyimides

I 100

I 200 M

I 300 1 / 2 w

I 400

I 500

I

1/2

(dalton )

Figure 4 . Estimation of the unperturbed dimensions of the polyamic acid from intrinsic viscosity data ( i ) : d i s t i l l e d NMP, (A): NMP/dioxane.


236


are larger than those reported by Wallach (3) for the same polymer in dimethylacetamide (DMAc) with 0.1 Ν LiBr added to suppress the polyelectrolyte effect. As mentioned above, the data reported here are in the absence of any polyelectrolyte effect or added salt, and thus the chain dimensions are larger. We have observed precipitation of the polymer when the concentration of LiBr is 0.2-0.3 N, indicating that the solvent quality is substantially reduced by addition of salt, as in Wallach s study. If the extrapolation to M^= 0 is not sufficient to eliminate all excluded volume contribution, then the estimated unperturbed dimensions could be affected. The samples used in reference 3 were also of varying polydispersity, and the intrinsic viscosity molecular weight relation showed more uncertainty than we have observed in this study, which is also sufficient to explain the difference in estimated unperturbed chain dimensions. The limited viscosity data obtained for the cured polyimide in concentrated sulfuric acid (Table II), show that the chain dimensions of the cured material, at least in the protonated form in solution, are essentially the same as in the amic acid form. The amide linkage in the uncured polyamic acid, rather than contributing substantially to the flexibility of the chain, appears rather rigid. This is probably because of the strong resonance stabilization achieved when the amide linkage is in the planar trans configuration, and is co-planar with the aromatic rings. For example, this is observed to be the case for solutions of polyphenyleneterephthalamide, which is highly extended in solution despite the amide linkages in the chain backbone (15,16). Thus the cyclodehydration to the imide ring would not be expected to increase the chain unperturbed dimensions. It should be noted here that this measurement of chain dimensions is an equilibrium average of single molecule size in solution, and is not sensitive to hindrances to segment movement in the bulk state that contribute to the very large glass transition temperature observed for the cured polyimide in comparison to the polyamic acid. We have been able to measure both the weight average molecular weight and the intrinsic viscosity for equilibrated polyamic acid precursors in NMP, and for stable solutions of the cured polyimide in concentrated sulfuric acid. These measurements were not complicated by either polyelectrolyte effects in the amide solvent or degradation in the acid, as have been reported by other workers for these systems. The results show that molecular weight achieved in the condensation to the polyamic acid is retained in the final polyimide, at least for moderate molecular weights. The condensation reaction appears to follow the expected kinetics and leads to a most probable distribution at equilibrium. Literature Cited


1

1. Bower, G. M.; Frost, L. W. J. Pol. Sci.:A 1963, 1, 3135. 2. Frost, L. W.; Kesse, I. J. Appl. Pol. Sci. 1964, 8, 1039. 3. Wallach, M. L. J. Pol. Sci.:A-2 1967, 5, 653.


19. 4. 5. 6. 7.


8. 9. 10. 11. 12. 13. 14. 15. 16.

Polyamic Acids and Polyimides 237 Wallach, M. L. J. Pol. Sci.:A-2 1969, 7, 1995. Birshtein, T. M.; et. al. European Pol. J. 1977, 13, 375. Cotts, P. M. First Technical Conference on Polyimides November 10-12, 1982, Ellenville, Ν. Y. Volksen, W. First Technical Conference on Polyimides November 10-12, 1982, Ellenville, Ν. Y. Gay, F. P.; Berr, C. E. J. Pol. Sci.:A-1 1968, 6, 1935. Baise, A. I.; Buchwalter P. L. First Technical Conference on Polyimides November 10-12, 1982, Ellenville, Ν. Y. Berry, G. C.; Fox, T. G J. Macrom. Sci. :Chem. 1969, A3, 1125. Berry, G. C. J. Pol. Sci., Pol. Phys. Ed. 1976, 14, 451. Stockmayer, W. H.; Fixman, M. J. Pol. Sci.:C 1963, 1, 137. Kurata, M.; Stockmayer, W. H. Adv. in Pol. Sci. 1963, 3, 196. Berry, G. C. J. Pol. Sci.:B 1966, 4, 161. Wong, C.-P.; Ohnuma, H.; Berry, G. C. J. Pol Sci.: Pol. Symp. Ed., 1978, 65, 173. Erman, B.; Flory, P. J.; Hummel, J. P. Macromolecules 1980, 13, 484.

COTTS AND VOLKSEN

RECEIVED September 21,

1983


Solution Characterization of Polyamic Acids and Polyimides - ACS

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