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Kinetics of Alkaline Hydrolysis of a Polyimide Surface Lori E. Stephans,*,† Anthony Myles,† and Richard R. Thomas*,‡ Rowan University, Department of Chemistry and Physics, Glassboro, New Jersey 08028 and E. I. du Pont de Nemours & Company, Jackson Laboratory, Deepwater, New Jersey 08023 Received August 16, 1999. In Final Form: February 2, 2000 The alkaline hydrolysis of a polyimide (PMDA-ODA) surface was studied as a function of time, temperature and hydroxide ion concentration. Quantification of the number of carboxylic acid groups formed on the modified polyimide surface was accomplished by analysis of data from contact angle titration experiments. Using a large excess of base, pseudo-first-order kinetics were found, yielding kobs ≈ 0.1-0.9 min-1 for conversion of polyimide to poly(amic acid) depending on [OH-]. From the dependence of kobs on [OH-], a rate equation is proposed. Conversion of the polyimide surface to one of poly(amic acid) was found to reach a limiting value with a formation constant, K, in the range 2-10 L‚mol-1.
Introduction Polyimides have been used as an interlayer dielectric in the electronics industry for several decades due to their excellent thermal, mechanical, and chemical properties in addition to their relatively low dielectric constant. The use of polyimides has necessitated schemes for removal or etching of the polymer or modification of the surface to facilitate adhesive bonding of subsequent polymer or metal layers. One of the more successful schemes involves etching or modification of the surface with alkaline solutions. Numerous reports have appeared in the literature that detail studies on alkaline etching or surface modification of polyimide surfaces.1-10 Common to all these schemes is ring-opening of the imide in alkaline solution to eventually reform the original precursor poly(amic acid) upon neutralization. This scheme is depicted in Figure 1. Several kinetic studies have been performed during polyimide etching.4,5,10 The alkaline solution and reaction conditions chosen were aggressive and complicated by the formation of a distinct layer of hydrolyzed material that adhered well to the underlying polymer. The layer is, presumably, partially or completely hydrolyzed polyimide and often required mechanical agitation to facilitate removal. These studies did not address the early stages of etching that would involve surface modification only by hydrolysis of the polyimide as shown in Figure 1. A kinetic study of the early stages of polyimide hydrolysis is difficult because of the small amount of material being converted to the poly(amic acid) on the polyimide * To whom correspondence should be addressed. † Rowan University. ‡ E. I. du Pont de Nemours & Company. (1) Lee, K.-W.; Kowalczyk, S. P.; Shaw, J. M. Macromolecules 1990, 23, 2097. (2) Lee, K.-W.; Kowalczyk, S. P.; Shaw, J. M.; Adamopoulos, E. J. Adhes. Sci. Technol. 1991, 49, 108. (3) Lee, K.-W.; Viehbeck, A. IBM J. Res. Dev. 1994, 38, 457. (4) Pawlowski, W. P.; Coolbaugh, D. D. Polym. Mater. Sci. Eng. 1988, 59, 68. (5) Pawlowski, W. P.; Coolbaugh, D. D.; Johnson, C. J. J. Appl. Polym. Sci. 1991, 43, 1379. (6) Plechaty, M. M.; Thomas, R. R. J. Electrochem. Soc. 1992, 139, 810. (7) Stoffel, N. C.; Hsieh, M.; Chandra, S.; Kramer, E. J. Chem. Mater. 1996, 8, 1035. (8) Thomas, R. R.; Buchwalter, S. L.; Buchwalter, L. P.; Chao, T. H. Macromolecules 1992, 25, 4559. (9) Thomas, R. R. Langmuir 1996, 12, 5247. (10) Xue, G. Angew. Makromol. Chem. 1986, 142, 61.
Figure 1. Alkaline hydrolysis of polyimide to poly(amic acid). Note that only one (para) of the two possible isomers is shown.
surface. In an earlier study from this laboratory, it was demonstrated that the contact angle titration technique (contact angle versus pH)11 could be used to quantify the conversion of polyimide to poly(amic acid).9 The surface sensitivity of this technique makes it ideal for a study of kinetics of surface reactions involving ionizable functional groups such as those present on a poly(amic acid) surface. In addition, the technique can provide insight into kinetics and mechanisms of reactions occurring in two dimensions. Experimental Section The precursor poly(amic acid) was obtained from DuPont and sold under the tradename Pyralin 2545. The PMDA-ODA polyimide films were prepared by spin coating the poly(amic acid) solution on silicon wafers after application of an adhesion promoter. Curing of poly(amic acid) to the polyimide film was accomplished by heating to 200 °C and holding for 30 min, followed by ramping the temperature to 350 °C and holding for 60 min in a nitrogen atmosphere. Film thicknesses were measured to be ≈2.7 µm. NaOH and NaNO3 (Aldrich; reagent grade) were used as received. Kinetic studies of alkaline hydrolysis were performed by immersing the polyimide samples in an aqueous solution of NaOH of the appropriate concentration for a specified period of time. Constant ionic strength was maintained by using NaNO3 (0.4 (11) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725.
10.1021/la991105m CCC: $19.00 © 2000 American Chemical Society Published on Web 03/31/2000
Alkaline Hydrolysis of a Polyimide Surface
Figure 2. Contact angle titration for polyimide using 0.05 M NaOH as a function of hydrolysis time at 30 °C. Shown are data after 0 (0), 2 (b), 5 (9), 15 (2), 30 (1), 60 ([) and 120 (O) min of hydrolysis.
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Figure 3. Conversion of poly(amic acid) to poly(amate) anion in alkaline solution.
M). A thermostatically controlled, constant-temperature bath was employed for hydrolysis experiments and was precise to (1 °C. After alkaline hydrolysis, the samples were soaked in CH3CO2H (0.1 M) for 30 min, and then rinsed extensively with distilled water and dried with a stream of nitrogen. Contact angles were collected using a Rame´-Hart model 100 contact angle goniometer. The pH of the probe liquid was adjusted and maintained with pHydrion (Aldrich) buffers and verified independently with a pH meter. Data were collected on four sessile drops for each pH with measurements taken on each side of the drop. Standard deviations were typically (3°. Quantification of the number of carboxylic acid groups created on the polyimide surface during hydrolysis was accomplished by a method derived in earlier work. 9
Results and Discussion Kinetics of Polyimide Hydrolysis. The PMDA-ODA polyimide used in this study is prepared by thermal imidization of 1,2,4,5-benzene-tetracarboxylic dianhydride (pyromellitic dianhydride) and 4,4′-oxydianiline and is shown in Figure 1. Samples were spin coated on silicon wafers and imidized to a temperature of 350 °C and a thickness ≈2.7 µm. Specimens were then hydrolyzed with various concentrations of aqueous NaOH (0.02-1.7 M) and for various times (0-120 min) using NaNO3 (0.4 M) to maintain constant ionic strength. Neutralization was achieved with CH3CO2H (0.1 M) and was followed by extensive rinsing with distilled water and drying with a stream of nitrogen. After conversion of the polyimide to the poly(amic acid), quantification of the number of carboxylic acid groups created, Ns, was done by evaluation of the contact angle titration curves. Typical contact angle data are shown in Figure 2 for 0.05 M NaOH hydrolysis at 30 °C. The decrease in contact angle as pH increases clearly reflects the conversion of poly(amic acid) to the poly(amate) anion at higher pH (see the scheme shown in Figure 3). In addition to the large inflection (pH 5-7) observed in the contact angle data as the carboxylic acid moiety is ionized, another smaller inflection is seen at lower pH values (pH 3-5). Upon alkaline hydrolysis, an amide group is formed in addition to the carboxylic acid moiety. The amide can also ionize during changes in pH of the contact angle probe liquid. The difference in magnitude between the two contact angle inflections may be attributed to the difference in intrinsic wettability between the two ionizable species.8 Values of pKa for amides in solution typically
Figure 4. Kinetic data for conversion of polyimide to poly(amic acid) after treatment with 0.02 M NaOH (using 0.4 M NaNO3) at 30 °C. The line is a nonlinear least-squares fit to eq 1.
fall in the range 0-2, whereas those for carboxylic acids are 3-5. Interestingly, both values of pKa determined from the hydrolyzed polyimide surface (vide infra) are shifted substantially compared with solution analogues. A shift of pKa on ionizable surfaces has been observed previously and is due to unfavorable free energy changes associated with forming charged groups at an air interface.11 The effect of the surface pKa shift on reaction kinetics and mechanism will be discussed later. Alternatively the inflection in contact angle data at pH 4 could be due to an interaction of the buffer with the surface. The definitive reason for this inflection requires further study. Details of the Ns calculation are given in the work of Thomas9 and Chatelier et al.12 This method has been shown to provide a reasonable estimate of the number of carboxylic acid groups on a hydrolyzed polyimide surface. Once Ns has been determined as a function of time, a kinetic analysis can be attempted. A plot of Ns as a function of hydrolysis time using 0.02 M NaOH (with 0.4 M NaNO3) is shown in Figure 4. The kinetics of hydrolysis could not be fit by a simple first-order expression because the conversion to poly(amic acid) reached a limiting value at long reaction times. Earlier work showed that alkaline (12) Chatelier, R. C.; Drummond, C. J.; Chan, D. Y. C.; Vasic, Z. R.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 4122.
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Table 1. Kinetic Parameters for Polyimide Hydrolysis Performed at 30 °C [NaOH], mol‚L-1
kobs (min-1)
C∞ (×10-17), -CO2H m-2)
χ2a
0.17 0.10 0.050 0.020
0.859 ( 0.09 0.597 ( 0.2 0.288 ( 0.04 0.148 ( 0.02
2.96 ( 0.05 3.81 ( 0.2 3.49 ( 0.1 3.63 ( 0.1
7.36 × 1031 2.40 × 1033 4.35 × 1032 3.15 × 1032
a
χ2 ) ∑t[Ct(exptl) - Ct(calcd)]2/degrees of freedom.
hydrolysis, similar to conditions used here, resulted in little mass loss.8 Attempts were made to image the surface by atomic force microscopy but were not successful because of the transfer of a thin layer of film from the hydrolyzed surface to the tip that obscured the image. The formation of a distinct layer of hydrolyzed material during alkaline etching of polyimides has been observed previously.4,5,10 The layer appears to be partially or completely hydrolyzed polyimide that would not be expected to be soluble in either the reaction medium or the contact angle probe liquid. The hydrolyzed layer appears as a solvent-swollen, gellike material when observed on a macroscopic scale during gross etching of PMDA-ODA polyimide. The removal of the hydrolyzed layer required more aggressive reaction conditions than those employed in the current study because of substantial adhesion of the hydrolyzed film to the underlying polyimide. Etching must proceed through a reaction with a diffusion mechanism. It appears, under mild reaction conditions, that diffusion of reactant [OH-] through the hydrolyzed polyimide is rate determining. With diffusion being the rate-determining step, a balance is established between hydrolyzed material and underlying unmodified polyimide. In essence, the hydrolyzed polyimide is a self-limiting layer that retards further hydrolysis. Because a large excess of NaOH was used to modify the polyimide surface and its concentration remained constant during hydrolysis, the kinetic data were analyzed with a psuedo-first-order kinetic expression reaching a limiting value
Ct ) C∞[1-exp(-kt)]
(1)
where Ct is the concentration of carboxylic acid groups measured on the modified polyimide surface at time t, C∞ is the equilibrium concentration of carboxylic acid groups at long reaction times, and k is the pseudo-first-order rate constant for conversion of imide to amic acid. The solid line shown in Figure 4 represents a nonlinear least-squares fit of the experimental data to eq 1. Reasonable fits were obtained using the psuedo-first-order expression, and kinetic data for polyimide hydrolysis are shown in Table 1. Although there have been no similar studies on the reaction kinetics occurring on surfaces of polyimides, the values of kobs presented here (0.1-0.9 min-1) compare favorably with those from studies involving phthalimide hydrolysis in solution under alkaline conditions.13,14 Values of kobs obtained from these studies have been in the range 10-1-104 min-1. Mechanism of Polyimide Hydrolysis. The kinetics of hydrolysis of phthalimide-type small molecules in homogeneous solution have been examined.13,15 Unlike the current study, these studies involved precursor imides (13) Khan, M. N.; Khan, A. A. J. Chem. Soc., Perkin Tran. 2 1979, 796. (14) McClellend, R. A.; Seaman, N. E.; Duff, J. M.; Branston, R. E. Can. J. Chem. 1985, 63, 121. (15) Khan, M. N.; Khan, A. A. J. Org. Chem. 1975, 40, 1793.
that are ionizable under reaction conditions. Ionization of the precursor imide, resulting in the equilibrium formation of an imide anion, complicated the reaction mechanism and kinetics. The etching of polyimide in alkaline solution has been examined previously.4,5,10 These studies were performed under aggressive conditions (high [NaOH] and elevated temperatures), and the possibility exists that the kinetics are complicated by removal of hydrolyzed polyimide and/or complete scission of the polymer backbone.13-15 In the current study, the reaction conditions are such that hydrolyzed polymer is not removed.8 The kinetics of alkaline hydrolysis of nonionizable phthalimides14 and anilides16 have been studied and lend some insight into the mechanism of hydrolysis under the conditions employed in the current study. The mechanism proposed is shown in Figure 5 (A). The mechanism invoked to explain the overall third-order dependence on [OH-] was the equilibrium creation of an intermediate anion II formed by nucleophilic attack of OH- on a carbonyl carbon of the imide, followed by another equilibrium involving a tetrahedral dianion IV. Formation of the final hydrolysis product (amic acid) III occurred concurrently from two parallel paths involving the intermediate anion and dianion. Formally, the conversion II f III occurs through cleavage of a C-N bond with proton transfer from oxygen to nitrogen. This conversion could be envisioned as two separate steps; however, proton transfer will be much more rapid than any other mechanistic step depicted. Because proton transfer is not rate determining, combining the two steps will not affect the overall rate equation proposed. Mechanistically, these studies demonstrated that hydrolysis kinetics could be explained by an overall thirdorder process when base was used in large excess.
kobs ) k1[OH-] + k2[OH-]2
(2)
Unsuccessful attempts were made to fit the observed dependence of kobs on [OH-] using eq 2. The observed dependence of kobs on [OH-] can be explained by a modification of the mechanism proposed earlier. A reactive intermediate is formed by nucleophilic addition of OH- to the carbonyl carbon of the imide to form a monoanion adduct II in a reversible step with unreacted polyimide I, followed by the rate-determining decomposition of intermediate II to the poly(amic acid) III. This mechanistic scheme is illustrated in Figure 5 (B). From a steady-state approximation, the following is a general case expression proposed to explain the dependence of the observed pseudo-first-order rate constant on hydroxide ion concentration.
kobs )
[
]
k1k2 [OH-] k-1 + k2
(3)
A plot of kobs versus [OH-] is shown in Figure 6. From the fitted parameters, k1 k2/(k-1 + k2) ) 4.84 ( 0.4 L‚mol-1‚min-1. An alternative, but similar mechanism, can be proposed that will also show a linear dependence of kobs on [OH-] if k-1 , k2. For this case, kobs ) k1[OH-]. At present, there is no way to differentiate between the two possible alternative mechanisms proposed for alkaline hydrolysis of a polyimide surface based on the [OH-] dependence of kobs without knowing the magnitude of k-1 or k2. There does, however, appear to be a substantial difference between hydrolysis mechanisms depending on whether the reactant imide is in solution or confined to a surface. (16) Biechler, S. S.; Taft, R. W., Jr. J. Am. Chem. Soc. 1957, 79, 4927.
Alkaline Hydrolysis of a Polyimide Surface
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Figure 5. Mechanisms for alkaline hydrolyisis of polyimide to poly(amic acid). (A) Mechanism observed in solution for phthalamides and anilides. (B) Mechanism proposed to occur at surface.
Figure 6. Dependence of kobs on [OH-].
In a study of the alkaline hydrolysis of substituted anilides, Biechler and Taft observed substantial steric and electronic factors that affected the relative importance of the two competing parallel decomposition paths (anion f product r dianion) to product.16 The pathway anion f product increases in importance with increasing electrondonating power and increasing steric requirements of the substituents. The aromatic nature of the polyimide used in this study should, therefore, favor the formation of poly(amic acid) through the monoanion intermediate II and perhaps retard formation of the dianion [IV] intermediate completely. By this logic, the steric constraints imposed by using a polymeric system should also favor the formation of poly(amic acid) through intermediate II. Another factor is presented by examining kinetics at a surface compared with those of similar reactions in
homogeneous solution. Previous studies have found a profound shift of pKa of ionizable functional groups at a surface.8,9,11,17-19 The pKa shift is due to unfavorable free energy changes of generating charged groups at an interface. Such an effect was observed also in the current study, as pKa values were measured to be ≈ 6.5 versus 3.80 for N-phenylphthalamic acid in aqueous solution.20 For the entire mechanism proposed in Figure 5(A) to be operative, the dianion [IV] becomes a necessary intermediate. The generation of a monoanion intermediate [II] at an interface should be difficult on a free energy basis, whereas formation of the dianion intermediate [IV] might be precluded altogether using the same reasoning. The steric and electronic factors taken together with the complications imposed by an interface might account for the differences in mechanism proposed in the current study compared with that discussed for reactions in homogeneous solution. The kinetic studies described earlier demonstrate that a limiting concentration of hydrolyzed polyimide is reached at long reaction times. This phenomenon is shown by limiting values of the number of poly(amic acid) carboxylic acid groups created at long reaction times, C∞. The existence of a limiting concentration of hydrolyzed polyimide is substantiated by previous work that details the formation of a distinct phase of material [presumably poly(amic acid)] during etching that requires aggressive reaction conditions or mechanical agitation to remove.4,5,10 It appears as though this hydrolyzed layer is a self-limiting reaction product (vide supra) that is formed under certain (17) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921. (18) Wamser, C. C.; Gilbert, M. I. Langmuir 1992, 8, 1608. (19) Smart, J. L.; McCammon, J. A. J. Am. Chem. Soc. 1996, 118, 2283. (20) Bender, M. L.; Chow, Y.-L.; Chloupek, F. J. Am. Chem. Soc. 1958, 80, 5380.
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Table 2. Formation Constants for Polyimide Hydrolysis [NaOH] (mol‚L-1)
T (( 1 °C)
K (L‚mol-1)
0.02 0.05 0.10 0.17 0.05 0.05 0.05
30.0 30.0 30.0 30.0 4.0 34.8 40.2
9.17 ( 0.2 3.52 ( 0.1 1.92 ( 0.1 0.879 ( 0.02 3.93 ( 0.2 3.83 ( 0.2 3.11 ( 0.1
experimental conditions, such as those employed in the present study. Based on previous X-ray studies determining the unit cell of PMDA-ODA polyimide,21 an estimate can be made of the area density of RCO2H possible on a monolayer after hydrolysis (C0 ≈ 1.98 × 1018 m-2). From the value of C0, C∞, and [NaOH], formation constants, K, can be calculated.
K)
[III] -
[I][OH ]
)
C∞ C∞[OH-]
(4)
Formation constants are shown in Table 2. For all the hydrolysis experiments conducted, K is small and 2 < K < 10 L‚mol-1. The small values observed for K are not surprising considering the formation of a self-limiting poly(amic acid) phase that impedes further hydrolysis of underlying polyimide. It is also interesting to note that K is not sensitive to temperature; K remains relatively invariant with temperature over a range from 4 to 40.2 (21) Factor, B. J.; Russell, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847.
°C. These results perhaps, indicate hydrolysis to be a thermally neutral process. Conclusions The contact angle titration technique can provide quantitative information regarding the creation or change in ionizable functional groups on a polymer surface. In the present study, this method was used to follow the conversion of polyimide to poly(amic acid) in alkaline solution. The conversion of polyimide to poly(amic acid) was followed as a function of time, temperature, and [NaOH]. Using a large excess of NaOH, pseudo-first-order kinetic data were obtained with kobs in the range 0.1-0.9 min-1. Upon extended hydrolysis, the system reaches a limiting concentration of modified polyimide [poly(amic acid)]. Formation constants were in the range 2-10 L‚mol-1. These small formation constants explain the existence of a self-limiting poly(amic acid) phase observed during alkaline hydrolysis under more aggressive conditions, such as those used for etching. From the behavior of kobs as a function of [OH-], a mechanism for hydrolysis has been proposed that differs from that observed in solution. The mechanism involves nucleophilic attack of OH- on a carbonyl carbon of the imide ring to form a reactive intermediate, followed by decomposition to the observed poly(amic acid). Acknowledgment. The authors thank Dr. M. Zussman (DuPont) for providing the polyimide samples. Thanks are also given to Professors L. Dinsmore (Rowan University), D. H. Carey (Marywood University), and the reviewers for helpful comments and suggestions. LA991105M
