Wetting Kinetics Study of Modified Polyimide Surfaces Containing

Jun 4, 2003 - OMNOVA Solutions, Inc., 2990 Gilchrist Road, Akron, Ohio 44305 ... The areal density of incipient carboxylic acid groups is proportional...
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Langmuir 2003, 19, 5763-5770

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Wetting Kinetics Study of Modified Polyimide Surfaces Containing Ionizable Functional Groups Richard R. Thomas OMNOVA Solutions, Inc., 2990 Gilchrist Road, Akron, Ohio 44305 Received February 5, 2003. In Final Form: April 30, 2003 Contact angle relaxation of partially wetting drops was studied on native and base hydrolyzed pyromellitic dianhydride (PMDA)-4,4′-oxydianiline (ODA) polyimide surfaces as a function of wetting liquid pH. Upon base hydrolysis, followed by acid neutralization, carboxylic acid groups are formed on the native polyimide surface. The areal density of incipient carboxylic acid groups is proportional to base hydrolysis time. Wetting kinetics were examined on polyimide surfaces as a function of hydrolysis time and wetting liquid pH using the molecular-kinetic theoretical approach. The results of parametric fitting of experimental contact angle relaxation data using the molecular-kinetic approach indicate that the kinetics and thermodynamics of wetting were nearly invariant when using a pH 2 probe liquid for native compared to hydrolyzed surfaces. In contrast, the kinetics and thermodynamics of wetting using a pH 11 probe liquid exhibited large differences based on the extent of modification and ionization of the surface. The differences in kinetics appear to be due to a lesser interaction of the pH 11 probe liquid with modified surfaces.

Introduction The adhesion of one material to another is critical in many industrial processes. In many cases, the adherend is applied from a liquid system comprising the material of interest in a solvent package. A necessary, but not sufficient, property is that this liquid system wet the substrate. Often, the coating is applied under relatively high shear conditions such as spin coating or spraying. This is termed forced wetting. While the equilibrium contact angle of the coating may be sufficient to ensure wetting at long times, the contact angle of the wetting fluid must first relax to its equilibrium value from a relatively higher contact angle after initial application. In practical situations, this contact angle relaxation to equilibrium can take seconds. If the contact angle relaxation time to equilibrium is much larger than the coating application inverse shear rate, then the contact angle could be very high, resulting in a decreased contact area at a critical time in the application. Therefore, it is crucial to understand the wetting kinetics of liquids on a substrate. Two main theories have been advanced to explain contact angle relaxation. One is based on the hydrodynamic theory of wetting.1 This theory has been applied to cases of relatively viscous liquids (small Reynolds number, Re; Re ) FrU/µ, where F is density, r is droplet radius, U is spreading velocity, and µ is viscosity) in cases where the interfacial tension between the liquid and substrate is relative low, leading to a contact angle at or near zero at long times (small capillary number, Ca; Ca ) µU/γLV, where γLV is interfacial tension). The second theory is molecular-kinetics using the Eyring activated-rate theory for liquid transport.2-4 Using molecular-kinetic theory, viscous effects are ignored and the driving force for contact angle relaxation is based on the difference between an equilibrium contact angle and one at some time during relaxation. It must be mentioned that there is no distinct boundary between the use of hydrodynamic versus mo(1) Tanner, L. H. J. Phys. D: Appl. Phys. 1979, 12, 1473. (2) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969, 30, 421. (3) Blake, T. D.; Clarke, A.; De Coninck, J.; de Ruijter, M. J. Langmuir 1997, 13, 2164. (4) de Ruijter, M. J.; De Coninck, J.; Blake, T. D.; Clarke, A.; Rankin, A. Langmuir 1997, 13, 7293.

lecular-kinetic wetting theories and, in many cases, both could be applied equally. In fact, evidence has been presented that a molecular-kinetic regime precedes a distinct hydrodynamic regime during droplet spreading with an approximate characteristic time for the mechanistic crossover.5,6 This observation has led to a combined molecular-kinetic/hydrodynamic theory approach that has been employed successfully to follow contact angle relaxation kinetics over wide time regimes and velocities.6,7 The substantial difference between the two theories lies in the dissipation mechanism. For hydrodynamic wetting, viscous dissipation is rate-determining in the drop during spreading. For molecular-kinetic wetting, the main dissipation mechanism is the result of molecular displacements between substrate and wetting fluid at the threephase boundary. Polyimides are polymers used widely in the electronics industry due to thermal and mechanical stability and relatively good dielectric properties. During the fabrication of integrated circuits, many layers of different materials are produced. This requires that the polyimide surface be coated with other materials. The method of choice is spin coating. This is a relatively high shear rate coating process. Therefore, knowledge of wetting kinetics would be useful in understanding the intimate details of the coating process. In previous studies, it was found that the surface of the native polyimide could be modified by base hydrolysis.8 Base hydrolysis creates carboxylic acid functional groups on the polyimide surface. It has been shown that the equilibrium contact angles on these surfaces are a sensitive function of probe liquid pH. Furthermore, the areal density of incipient carboxylic acid groups on the modified polymide surface can be controlled by base hydrolysis conditions (time, base concentration, and temperature). From study of contact angles as a function (5) de Ruijter, M. J.; Charlot, M.; Voue´, M.; De Coninck, J. Langmuir 2000, 16, 2363. (6) de Ruijter, M. J.; De Coninck, J.; Oshanin, G. Langmuir 1999, 15, 2209. (7) Schneemilch, M.; Hayes, R. A.; Petrov, J. G.; Ralston, J. Langmuir 1998, 14, 7047. (8) Thomas, R. R.; Buchwalter, S. L.; Buchwalter, L. P.; Chao, T. H. Macromolecules 1992, 25, 4559.

10.1021/la034200a CCC: $25.00 © 2003 American Chemical Society Published on Web 06/04/2003

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of probe liquid pH, the areal density of carboxylic acid groups created can be estimated.9 The goal of the present study is to examine contact angle relaxation on native and base hydrolyzed pyromellitic dianhydride (PMDA)-4,4′-oxydianiline (ODA) polyimide surfaces as a function of probe liquid pH (specifically, 2 and 11) using molecular-kinetic theory. Experimental Section Materials. The precursor poly(amic acid) was a generous gift from Dr. G. Hougham of IBM T. J. Watson Research Center. It was obtained from DuPont under the tradename of RC5878 (equivalent to Pyralin 2545). Polyimide films were prepared by spin coating onto silicon wafers after treatment with an adhesion promotor ((aminopropyl)triethoxysilane). The poly(amic acid) was converted thermally to PMDA-ODA polyimide under a nitrogen atmosphere by heating to 200 °C at 5 °C/min, holding at 200 °C for 15 min, and ramping to 400 °C at 5 °C/min, followed by a 30 min dwell at the final temperature. The final film thickness ≈ 5 µm. The polyimide surfaces were verified by contact angle goniometry and X-ray photoelectron spectroscopic examination and found to be free from any extraneous oxidation. (Aminopropyl)triethoxysilane and pHydrion buffers were purchased from Aldrich Chemical Co. and used as received. Potassium nitrate and sodium hydroxide were ACS Reagent Grade from SargentWelch and used as received. Acetic acid was from J. T. Baker Co. Polyimide Surface Modification. A hydrolysis solution at 0.02 M NaOH and 0.4 M KNO3 was prepared and maintained at 30 °C in a thermostated bath controlled to (1 °C. After hydrolyzing the surface for the appropriate time, the samples were rinsed with distilled water and then immersed in 0.1 M CH3CO2H for 15 min to form the free carboxylic acid. The samples were then given a brief rinse with distilled water adjusted to pH 7 with acetic acid and dried under a stream of 99.999% nitrogen gas. Methods. Contact angle goniometry and contact angle relaxation studies were performed using The Tracker (ThetaDyne Instruments, Charlotte, NC) operating in contact angle mode. Typically, 1-5 µL of probe liquid was used for both contact angle goniometry and relaxation studies. For relaxation studies, the data collection rate was set equal to 10 s-1. Contact angle data had a precision ≈ (0.5°. X-ray photoelectron spectroscopic studies were performed with a 300 W achromatic Mg KR anode using a VG ESCALAB Mk II spectrometer.

Results and Discussion Characterization of Modifed Polyimide Surfaces. The PMDA-ODA polyimide used in the current study is prepared by thermal imidization of the poly(amic acid) precursor copolymer from 1,2,4,5-benzenetetracarboxylic dianhydride (pyromellitic dianhydride; PMDA) and 4,4′oxydianiline (ODA) and is shown in Figure 1 A. Poly(amic acid) solutions were spin coated on silicon wafers and imidized to a final temperature of 400 °C under nitrogen to a thickness ≈ 5 µm. Surface modification was performed by base hydrolysis using 0.02 M aqueous NaOH while maintaining a high, constant electrolyte strength with 0.4 M KNO3 at 30 °C using a thermostated water bath. Neutralization was performed using 0.1 M CH3CO2H for 15 min. The acetic acid solution was then diluted to pH 7 for a final rinse followed by drying with a stream of 99.999% nitrogen. Polyimide samples were base-hydrolyzed for 5, 10, 20, 30, and 60 min. The base hydrolysis modification scheme is shown in Figure 1B. The hydrolyzed polyimide surfaces were then analyzed by contact angle titration (contact angle goniometry as a function of probe liquid pH). Contact angle titration data for all the hydrolyzed and native polyimide surfaces are shown in Figure 2. Not surprisingly, contact angles are invariant with pH for the native, unmodified surface. (9) Thomas, R. R. Langmuir 1996, 12, 5247.

Figure 1. Structure of PMDA-ODA polyimide (A), poly(amic acid) (B), and poly(amate) salt (C). Only the para isomer is shown for poly(amic acid) and the poly(amate) salt.

Figure 2. Contact angle titration data for PMDA-ODA surfaces after 0 (0), 5 (b), 10 (2), 20 (1), 30 ([), and 60 (9) min of hydrolysis using 0.02 M NaOH followed by neutralization.

Previous work has shown that the magnitudes of contact angles on base hydrolyzed polyimide surfaces are a sensitive function of probe liquid pH, as shown in Figure 2.8 The carboxylic acid groups have a pKa (≈7) that is accessible through the pH range examined. Below the pKa, the surface is predominately in the free acid form (less wettable), and above the pKa, the acid groups are in the carboxylate form (more wettable) with a sigmoidally shaped transition between the two pH extremes. The difference in magnitude between the two contact angle extremes may be attributed to the difference in intrinsic wettability between the two species: one ionized and the other neutral.10 Previous studies have shown that contact angle titration data can be analyzed using a free energy approach to wetting for ionizable surfaces to yield an estimate of the number of ionizable functional groups (in this case, carboxylic acid) per unit area, Ns.9 Using the contact angle titration data shown in Figure 2 and the methodology detailed elsewhere, Ns values were estimated for polyimide samples that were hydrolyzed for 5, 10, 20, 30, and 60 min. Values are shown in Table 1 with a kinetic plot exhibited in Figure 3. The mechanism for alkaline (10) Thomas, R. R.; Stephans, L. E. J. Colloid Interface Sci. 2002, 251, 339.

Polyimide Surfaces with Ionizable Functional Groups

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Table 1. Molecular-Dynamic Wetting and Surface Modification Parameters for PMDA-ODA Surfaces sample

pH

λ (Å)

K° (×10-3 s-1)

n (×10-18 m-2)

θi (deg)

θf (deg)

χa (mN/m)

Ns (×10-17 RCO2H/m2)

control #1 control #2 control #1 control #2 5 min hydrolysis #1 5 min hydrolysis #2 5 min hydrolysis #1 5 min hydrolysis #2 10 min hydrolysis #1 10 min hydrolysis #2 10 min hydrolysis #1 10 min hydrolysis #2 20 min hydrolysis #1 20 min hydrolysis #2 20 min hydrolysis #1 20 min hydrolysis #2 30 min hydrolysis #1 30 min hydrolysis #2 30 min hydrolysis #1 30 min hydrolysis #2 60 min hydrolysis #1 60 min hydrolysis #2 60 min hydrolysis #1 60 min hydrolysis #2

2 2 11 11 2 2 11 11 2 2 11 11 2 2 11 11 2 2 11 11 2 2 11 11

8.89 ( 0.002 9.26 ( 0.02 9.27 ( 0.02 9.20 ( 0.1 9.21 ( 0.01 9.27 ( 0.02 9.04 ( 0.02 8.44 ( 0.2 9.18 ( 0.03 8.82 ( 0.04 8.75 ( 0.3 9.20 ( 0.01 8.93 ( 0.01 8.92 ( 0.01 8.40 ( 0.1 8.74 ( 0.05 8.89 ( 0.01 8.83 ( 0.03 8.94 ( 0.1 8.85 ( 1 8.88 ( 0.02 8.92 ( 0.03 10.26 ( 0.1 10.86 ( 0.2

5.86 ( 0.3 7.29 ( 0.8 5.94 ( 0.3 8.29 ( 1 9.32 ( 0.4 6.97 ( 0.3 10.1 ( 0.03 19.0 ( 1 9.08 ( 0.6 5.45 ( 0.3 17.3 ( 1 9.39 ( 0.3 6.32 ( 0.3 6.14 ( 0.3 23.0 ( 0.5 25.0 ( 1 5.81 ( 0.3 5.45 ( 0.2 27.33 ( 0.8 15.6 ( 2 5.75 ( 0.4 6.21 ( 0.5 27.2 ( 0.8 30.4 ( 2

1.26 ( 0.0004 1.17 ( 0.004 1.16 ( 0.004 1.18 ( 0.003 1.18 ( 0.003 1.16 ( 0.005 1.22 ( 0.006 1.40 ( 0.08 1.19 ( 0.007 1.28 ( 0.01 1.31 ( 0.08 1.18 ( 0.003 1.25 ( 0.002 1.26 ( 0.002 1.42 ( 0.03 1.31 ( 0.01 1.27 ( 0.002 1.28 ( 0.008 1.25 ( 0.03 1.28 ( 0.3 1.27 ( 0.006 1.26 ( 0.008 0.950 ( 0.02 0.848 ( 0.02

69.2 66.0 76.7 76.3 50.5 60.8 57.0 50.8 66.6 62.4 52.0 53.3 66.6 62.4 42.5 43.3 64.2 68.0 40.4 53.0 60.7 61.9 37.9 37.5

66.0 62.9 71.3 69.8 44.3 56.0 48.6 42.4 61.8 59.3 43.9 44.4 61.8 59.3 36.6 37.8 60.7 64.1 35.8 43.8 58.2 59.8 31.7 31.8

0.18 0.20 0.35 0.26 0.12 0.18 0.25 0.24 0.18 0.13 0.29 0.26 0.14 0.47 0.24 0.16 0.52 0.16 0.22 0.58 0.08 0.09 0.34 0.13

0 0 0 0 1.46 ( 0.5 1.46 ( 0.5 1.46 ( 0.5 1.46 ( 0.5 3.48 ( 1 3.48 ( 1 3.48 ( 1 3.48 ( 1 3.96 ( 0.8 3.96 ( 0.8 3.96 ( 0.8 3.96 ( 0.8 3.83 ( 0.5 3.83 ( 0.5 3.83 (0.5 3.83 ( 0.5 4.40 ( 0.3 4.40 ( 0.3 4.40 ( 0.3 4.40 ( 0.3

a χ ) {[1/(m - 2)]∑m (θexptl - θtheory)2}1/2, where m is the number of data points and the superscripts exptl and theory indicate experimental i)1 i i and theoretical, respectively, values of the contact angle θ.

Figure 3. Kinetic plot for creation of carboxylic acid groups during surface modification of PMDA-ODA polyimide with 0.02 M NaOH. The solid line is a nonlinear least-squares fit to the expression Ct ) C∞[1 - exp(-kt)].

hydrolysis of a PMDA-ODA polyimide surface has been studied previously,11 and it was shown that, analytically, the kinetic data can be described by the pseudo-first-order expression Ct ) C∞[1 - exp(-kt)], where Ct is the areal density of carboxylic acid groups created at time t, C∞ is the equilibrium carboxylic acid group areal density, and k is the pseudo-first-order rate constant for conversion of imide to amic acid. For the present case, C∞ ) (4.26 ( 0.3) × 1017 RCO2H/m2 and k ) 0.121 ( 0.02 min-1, and the fitted line is shown in Figure 3. Contact Angle Relaxation Theory. The base hydrolyzed, modified polyimide surface presents an excellent substrate for contact angle relaxation studies. First, the surfaces are characterized well; second, the extent and nature of modification are known; and finally, it is known that the wettability of the surfaces is pH dependent. Furthermore, PMDA-ODA has a Tg > 450 °C,12 mitigating bulk or surface mobility at room temperature. Previous studies have shown that the surface modified by base hydrolysis is not subject to reconstruction on time scales (11) Stephans, L. E.; Myles, A.; Thomas, R. R. Langmuir 2000, 16, 4706. (12) Mittal, K. L. In Polyimides: Synthesis, Characterization and Applications; Mittal, K. L., Ed.; Plenum: New York, 1982; Vol. 1.

far beyond the measurements performed in the current work.8 Hydrolyzed polyimide surfaces should, therefore, be a good substrate on which to test wetting kinetic theories, since there are reasonable differences between equilibrium contact angles when examined at pH extremes. Much theoretical work has been described in the literature in regard to contact angle relaxation. Predominately, two theories have been forwarded.4 The hydrodynamic wetting theory is applicable particularly to fluids with a moderate capillary number, where viscous effects are predominant and an equilibrium contact angle near or at zero is obtained (low interfacial tension). The molecular-kinetic theory has been applied successfully to systems that have lower capillary numbers, where interfacial tension effects are dominant and a finite equilibrium contact angle is reached.2-4 Since the probe liquid used in the present study is water, viscous effects should be negligible (low capillary number), and a finite contact angle is seen at relaxation, the molecular-kinetic theory seems most appropriate for analysis of the contact angle relaxation data obtained in the current study. The driving force for contact angle relaxation according to the molecularkinetic theory is a product of the out-of-balance difference between the equilibrium contact angle, θf, and the angle at some time before relaxation, θ. It must be noted that there is no distinct boundary in the use of hydrodynamic versus molecular-kinetic wetting theories. The molecularkinetic theory was applied here, as it tends to provide greater information regarding the chemical nature of the surface. The basis for the molecular-kinetic theory is the Eyring activated-rate theory for liquid transport forwarded by Blake and Haynes2

ν ) 2K°λ sinh

[

γLV (cos θf - cos θ) 2nkBT

]

) (K + - K-)λ ) Knetλ

(1)

where υ is the wetting line velocity, K° is the quasiequilibrium frequency of molecular displacements, λ is

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the distance between two neighboring adsorption sites, γLV is the interfacial tension, n is the number of adsorption sites/unit area, kB is the Boltzmann constant, T is absolute temperature, θ is the contact angle at time t, θf is the contact angle at equilibrium, K+ is the molecular displacement frequency in the wetting direction, K- is the molecular displacement frequency in the dewetting direction, and Knet is the net molecular displacement frequency. At equilibrium, the drop is at rest and Knet ) 0 and K+ ) K- ) K°. Using small drops (1-5 µL), the effect of gravity can be neglected and the spherical cap approach adopted to yield the radius, r, of the droplet as a function of contact angle, θ, and volume, V.

r)

(

3V sin3 θ π(2 - 3 cos θ + cos3 θ)

)

1/3

(2)

Figure 4. Contact angle relaxation data for native PMDAODA polyimide surface at pH 2 (0) and pH 11 (b) along with solid lines fitted using eq 5.

Since V is constant over the measurement lifetime,

∂r ∂r ∂θ ) ∂t ∂θ ∂t

(3)

from which can be derived the differential equation for the change in r with t.

∂θ 3V ∂r )υ)∂t ∂t π

1/3

( )

(1 - cos θ)2

(2 - 3 cos θ + cos3 θ)4/3

(4)

Combining eqs 1 and 4 leads finally to the differential equation for contact angle relaxation.

∂θ ) ∂t 2K°λ sinh -

[

]

γLV(cos θf - cos θ) (2 - 3 cos θ + cos3 θ)4/3 2nkBT (3V/π)1/3(1 - cos θ)2

(5)

To simply data reduction, two adjustable parameters were chosen

a ) 2K°λ b)

γLV 2nkBT

λ ≈ n-1/2 The numerical approach taken in the current study is that given by de Ruijter et al.4 The raw contact angle versus time data were subjected to linear interpolation between experimental time limits to discretize the data in the time domain necessary for solution of eq 5 by a fourth-order, time-adaptive Runge-Kutta algorithm.13 The step size in the Runge-Kutta algorithm was chosen to be 0.1 s to match the experimental data that was collected at ∼10 s-1. The tolerance was set to 10 ppb. The values of θi and θf were chosen to match best the experimental data. The values of θf are reasonable compared to equilibrium values shown in Figure 2. The difference between theoretical and experimental data was performed by minimizing χ2 using the downhill simplex (13) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes (The Art of Scientific Computing); Cambridge University Press: New York, 1986; Chapter 16.

Figure 5. Contact angle relaxation data for a PMDA-ODA polyimide surface hydrolyzed for 10 min with 0.02 M NaOH at pH 2 (0) and pH 11 (b) along with solid lines fitted using eq 5.

method.14,15 Typically, χ values obtained were in the range of 0.1°, that is, far less than the estimated precision of the contact angle measurement of (0.5°. Since no direct estimation of errors is possible with a Runge-Kutta analysis of experimental data, errors were estimated on the basis of a Monte Carlo simulation (100 cycles). Simulated contact angle relaxation values were obtained by replacing 1/e of experimental data by normally distributed random numbers generated using the experimental data as a mean and a standard deviation of 1°. The experimental data to be replaced were chosen at random and varied between simulations. Acceptable theoretical fits were obtained with χ in the range 0.10.6°, since the experimental precision ≈ 0.5°. According to eq 6, a and b are related linearly, and a plot for one data set along with a normal fit to a and b values generated by Monte Carlo simulation is shown in the Supporting Information. Contact Angle Relaxation: Macroscopic Observation. Contact angle relaxation and fits from molecularkinetic theory (eq 5) are shown in Figures 4-7 for native polyimide and the polyimide surface hydrolyzed for 10, 30, and 60 min, respectively, using 0.02 M NaOH followed by neutralization. Results from parametric fitting using molecular-kinetic theory for the modified PMDA-ODA polyimide surfaces are given in Table 1. One thing is obvious from inspection of the contact angle relaxation curves for all samples. The change in contact angle from (14) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes (The Art of Scientific Computing); Cambridge University Press: New York, 1986; Chapter 10. (15) Sprott, J. C. Numerical Recipes: Routines and Examples in Basic; Oxford University Press: Oxford, 1998; Chapter 10.

Polyimide Surfaces with Ionizable Functional Groups

Figure 6. Contact angle relaxation data for a PMDA-ODA polyimide surface hydrolyzed for 30 min with 0.02 M NaOH at pH 2 (0) and pH 11 (b) along with solid lines fitted using eq 5.

Figure 7. Contact angle relaxation data for a PMDA-ODA polyimide surface hydrolyzed for 60 min with 0.02 M NaOH at pH 2 (0) and pH 11 (b) along with solid lines fitted using eq 5.

its initial (θi) to final (θf) value is relatively small and θf reaches a finite value at equilibrium. The experimental precision of the contact angle data is estimated at (0.5°. Several relaxation experiments were performed on each sample. In some cases, the fitted parameters and relaxation curves are nearly identical. In other cases, the curves are approximate in shape but there is more variance in the fitted paramters a and b. Considering the contact angle hysteresis that can be observed with this system (≈20°), 8 the relaxation curves and fitted parameters are quite robust and the error between samples hydrolyzed differently is beyond the error in the fitted parameters. Using the fitted parameters a and b, λ, K°, and n were calculated from eq 6 and are shown in Table 1. Shown in Figure 4 are the contact angle relaxation data for the native PMDA-ODA surface at pH 2 and 11. The shapes of the curves for data collected at pH 2 and 11 are nearly identical. This is manifested in similar values of λ, K°, and n obtained by fitting of the experimental data to eq 5 and shown in Table 1. Since the native polyimide surface does not possess any ionizable functional groups or, hence, pKa, there is little change in relaxation behavior at the pH extremes. It can be seen by examination of the polyimide surface hydrolyzed for 10, 30, and 60 min with 0.02 M NaOH shown in Figures 5-7, respectively, that there is a gradual change in the contact angle relaxation curve when using a pH 11 probe liquid while the curve using a pH 2 liquid remains relatively unchanged from the native surface. The values obtained from fitting experimental contact angle relaxation data to eq 5 reflect this gradual change. Data for other hydrolysis times are given as Supporting Information.

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Figure 8. Variation of λ with hydrolysis time using 0.02 M NaOH on a PMDA-ODA polyimide surface with pH 2 (9) and pH 11 (b) probe liquids.

Figure 9. Variation of K° with hydrolysis time using 0.02 M NaOH on a PMDA-ODA polyimide surface with pH 2 (9) and pH 11 (b) probe liquids.

The behavior of the fitted parameters λ and K° can be observed best when plotted against hydrolysis time, as shown in Figures 8 and 9, respectively. Values of λ and K° are nearly invariant with hydrolysis time using the pH 2 probe liquid. Contrast this to the behavior seen with λ and K° when a pH 11 probe liquid is used. For both λ and K°, a steady increase is seen with increasing hydrolysis time. Contact Angle Relaxtion: Molecular Scale Observations. It is evident from this study and those previous8 that base hydrolysis of PMDA-ODA polymide surfaces results in the creation of carboxylic acid groups (in essence, a reversion to poly(amic acid) at the surface), as illustrated in Figure 2. It is also evident that the magnitude of modification (creation of carboxylic acid groups) can be controlled by varying hydrolysis conditions (specifically NaOH concentration).11 Having observed differences based on extent of modification and probe liquid pH, the parameters λ and K° fitted to eq 5, in light of eq 6, on a molecular scale are examined. The findings reported here are quite different from those reported during a molecular-kinetic wetting study of polymer surfaces with a variety of liquids. 4 Specifically, the values obtained for λ as a function of pH and hydrolysis time are unusual. In the previous study, the average length of a molecular displacement, λ, was found to be very close to the radius of gyration of the solvent employed. In the current study, values of λ are much larger than those expected based on a radius of gyration of water ≈1.5 Å by a factor of 5-7× using both pH 2 and pH 11 probe liquids. This indicates that the probe liquid (buffered) water does not seem to form a close-packed layer on the polyimide surface. A large area fraction of the polyimide surface is dominated by phenyl groups. It is known well that phenyl

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Figure 10. PM3 calculated unit cells for native PMDA-ODA polyimide (A) and the para (B) and meta (C) isomers of poly(amic acid).

groups are not wetted particularly well by water and are comparable to other groups containing C-H bonds only. For example, contact angles of water on polystyrene are ≈91° for advancing and ≈84° for receding.16 The large values obtained for λ in the current study may be an indication of a lessened interaction of water on the hydrophobic portion (phenyl group) of the polyimide surface regardless of its state of modification. Furthermore, λ increases with increasing hydrolysis time using a pH 11 probe liquid only from ∼8 to ∼11 Å. At first glance, this seems odd. From an analysis of contact angle titration data collected after various hydrolysis times, carboxylic acid groups are created on the polyimide surface in a quantity related directly to hydrolysis time, as shown in Table 1 and Figure 3. To a first approximation it would seem that λ should decrease or at least remain invariant. Note that λ does remain invariant for all hydrolysis times using a pH 2 probe liquid. Insight as to this unusual result can be obtained by an examination of PMDA-ODA polyimide and precursor poly(amic acid) structures. It is known well that the poly(amic acid) precursor to polyimide is found in the meta and para isomeric states in regard to backbone linkages.17 Naturally, these configurational differences are removed upon polyimide formation. Upon base hydrolysis or conversion of the polyimide surface to poly(amic acid), the formation of meta and para isomeric states would be expected statistically. Shown in Figure 10 are calculated structures (PM3) of PMDA-ODA polyimide along the para and meta isomers of poly(amic acid). The projected length of the monomer unit along the axis of the fully extended chain is estimated to be ≈16, 15, and 17 Å for the native polyimide and the p- and m-pyromellitamic acid linkages, respectively. The values reported here are similar to those given in the literature based on X-ray diffraction studies.17,18 Overall, there appear to be no gross structural differences between the native polyimide and the poly(amic acids). An attempt was made to correlate the observed decrease in adsorption site density using a pH 11 probe liquid, n, with distances between relatively hydrophilic groups on the different (16) Wu, S. Polymer Interface and Adhesion; Marcell Dekker: New York, 1982; Chapter 4. (17) Takahashi, N.; Yoon, D. Y.; Parrish, W. Macromolecules 1984, 17, 2583. (18) Factor, B. J.; Russell, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847.

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materials. For native polyimide, the distances between meta and para carbonyls is estimated to be ≈6.0 and 7.6 Å across the pyromellitimide unit and ≈10-11 Å between pyromellitimide units spanning an oxydianiline group. For the para isomer of poly(amic acid), carbonyl-carbonyl distances were estimated to be ≈7 Å across the pyromellitamic acid unit and ≈14 Å between pyromellitamic acid units. Similarly, the meta isomer yielded estimates ≈7 Å across the pyromellitamic acid ring and ≈14 Å between pyromellitamic acid units. On the basis of these estimated distances between relatively hydrophilic groups on the native and hydrolyzed polyimide surface, there is no explanation structurally that can be given for the apparent increase in n when using a pH 11 probe liquid. This is a subject for further study. It is important to note that values of λ are coincidental for the native polyimide surface regardless of probe liquid pH. Apparently, the probe liquid pH is not able to discriminate between acidic and basic functionality on the native PMDA-ODA polyimide surface. This should not be surprising, since the native polyimide surface has no carboxylic acid groups and, therefore, has no accessible pKa in the pH range of the probe liquids used. The distance between neighboring adsorption sites, λ, was calculated ()1/n2) assuming that adsorption sites are distributed uniformly. Given the slight structural differences between the meta and para forms of the poly(amic acid), along with the fact that hydrolysis of the surface does not proceed to completion (∼7-22% conversion), leaving the possibility of heterogeneity in the spatial distribution of carboxylic acid sites, the proceeding assumption may not be valid entirely. Another possible explanation is the change in solvation in the sequence polyimide f poly(amic acid) f poly(amate) that could result in even greater heterogeneity in n.10 Yet another possibility is the structural effect on the modified polyimide due to charging of carboxylic acids when the probe liquid pH exceeds the surface pKa. One point regarding the hydrolysis time dependence of λ requires further explanation. The pH 11 dependence of λ seems to lie over two distinct hydrolysis time regimes: λ is nearly invariant with hydrolysis time up to ∼30 min followed by a substantial increase for times > 30 min. Considering the fact that Ns does not change appreciably after ∼30 min of hydrolysis (Figure 3) argues that the increase in λ observed for times > 30 min is an artifact of the degree of ionization rather than the extent of hydrolysis. In addition, it is unlikely that the pH 11 probe liquid used would induce further hydrolysis considering the time scale of contact angle relaxation measurements. On the basis of prior work,11 kobs for a 10-4 (pH 11) solution would be ≈5 × 10-4 L‚mol-1‚min-1, resulting in the creation of an additional ∼2 × 1014 RCO2H/m2 after 60 s, that is, at most, a 0.1-0.05% change of Ns present on surfaces hydrolyzed for 5-60 min, respectively, using 0.02 M NaOH. The quasi-equilibrium wetting frequencies, K°, as a function of probe liquid pH and alkaline hydrolysis times for the PMDA-ODA surface are shown in Figure 10. Again, K° values are coincidental for the native polyimide surface regardless of probe liquid pH. What is surprising is that K° increases with increasing hydrolysis times using the pH 11 probe liquid only. The value of K° is a measure of the frequency of molecular displacements at equilibrium in the region of the three-phase contact line. A stronger molecular attraction of the wetting liquid with the substrate should lead to smaller frequencies. In the present case, K° increases, indicating a weaker interaction of the probe liquid with increasing modification of the polyimide surface. This is evidenced more clearly by

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Langmuir, Vol. 19, No. 14, 2003 5769

Figure 11. Variation of ∆G with hydrolysis time using 0.02 M NaOH on a PMDA-ODA polyimide surface with pH 2 (9) and pH 11 (b) probe liquids.

Figure 12. Variation of ∆g with hydrolysis time using 0.02 M NaOH on a PMDA-ODA polyimide surface with pH 2 (9) and pH 11 (b) probe liquids.

examination of molar activation free energies of wetting, ∆G, and per unit surface area, ∆g, basis,4 as shown in Figures 11 and 12.

can more than compensate for the free energy loss due to double layer charging. The molecular-kinetic approach employed here appears to provide reasonable results; however, advances in the basic theory would be necessary to describe fully the effects of double layer charging. This may be evidenced by the remarkable observation that the pH 2 probe liquid seems incapable of discriminating between a native polyimide surface and a modified one in which there is the presence of carboxylic acids. In contrast, the pH 11 probe liquid is able to readily distinguish between the two surfaces.

( )

∆G ) -NAkBT ln ∆g )

n∆G NA

hK° kBT

(6) (7)

where NA is the Avogadro number, kB is the Boltzmann constant, h is the Planck constant, and T is temperature. The value of ∆G is invariant with hydrolysis time using a pH 2 probe liquid and decreases with increasing hydrolysis time. Apparently, there is an activation free energy of wetting decrease of 4-5 kJ/mol at the longest hydrolysis time when comparing pH 2 to pH 11 probe liquids, indicating the relative ease at which the modified polyimide surface is wetted by the higher pH probe liquid. Expectedly, and on a per unit area basis, the same phenomenon is observed for ∆g as a function of hydrolysis time and pH with the activation free energy loss amounting to 30-40 mJ/m2. The molecular-kinetic theory invoked in the current study does not take into consideration electrostatic terms that must be reconciled at an ionizable surface. Contact angle relaxation has been studied in some detail for electrowetting experiments in which the substrate to be probed is placed under a relatively high electrical potential.19-27 However, in electrowetting experiments under a large electrical potential, electrostatics aid in allowing the surface to be wetted by the probe liquid.19 The opposite is true for the case of double layer charging, as is observed with ionizable surfaces that result in a loss of free energy,28 such as in the present case. Thermodynamically, wetting can occur only if the wetting free energy gain due to the nature of the ionizable functional group (19) Welters, W. J. J.; Lambertus, G. J.; Fokkink, L. G. J. Langmuir 1998, 14, 1535. (20) Verheijen, H. J. J.; Prins, M. W. J. Langmuir 1999, 15, 6616. (21) Ivosevic, N.; Zutic, V.; Tomaic, J. Langmuir 1999, 15, 7063. (22) Schneemilch, M.; Welters, W. J. J.; Hayes, R. A.; Ralston, J. Langmuir 2000, 16, 2924. (23) Peykov, V.; Quinn, A.; Ralston, J. Colloid. Polym. Sci. 2000, 278, 789. (24) Janocha, B.; Bauser, H.; Oehr, C.; Brunner, H.; Go¨pel, W. Langmuir 2000, 16, 3349. (25) Digilov, R. Langmuir 2000, 16, 6719. (26) Blake, T. D.; Clarke, A.; Stattersfield, E. H. Langmuir 2000, 16, 2928. (27) Decamps, C.; De Coninck, J. Langmuir 2000, 16, 10150. (28) Chatelier, R. C.; Drummond, C. J.; Chan, D. Y. C.; Vasic, Z. R.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 4122.

Summary PMDA-ODA polyimide films were prepared and hydrolyzed to yield surfaces with a measured areal density of carboxylic acids, Ns. The surfaces were examined by contact angle relaxation studies as a function of probe liquid pH using molecular-kinetic theory. Relaxation kinetics were found to be invariant of probe liquid pH for the native polyimide surface. Considerable differences in relaxation kinetics were found when comparing hydrolyzed surfaces using pH 2 and 11 probe liquids. For all surfaces examined, the average length between molecular displacements, λ, was found to be larger than that anticipated (≈9 Å) based on the radius of gyration of the probe liquid and the assumption of close-packed surface adsorption. This is due to the presence, in a large area fraction, of relatively hydrophobic phenyl groups present on the polymer backbone. The value of λ was found to increase (≈11 Å) with increasing Ns for pH 11 probe liquid only. A possible explanation forwarded is the assumption that λ ) 1/n2, where n is the adsorption site density that implies a certain homogeneity in adsorption site distribution, which may not be valid entirely in the case of partial hydrolysis of the surface. Another possible explanation is the increase in solvation in the sequence polyimide f poly(amic acid) f poly(amate) that could result in even greater heterogeneity in n. It may be possible to probe this effect using miscible liquid mixtures of differing acidbase properties. The quasi-equilibrium wetting frequency, K°, was found to be invariant with probe liquid pH 2 and in the range (5-7) × 103 s-1 for all surfaces examined regardless of the state of hydrolysis. In contrast, K° was discovered to be equal to that using a pH 2 probe for the native surface but increased steadily to ∼3 × 104 s-1 as the surface became more enriched in carboxylic acid groups. This increase in K° is due to a lessened interaction of a pH 11 versus pH 2 probe liquid. The increase in equilibrium frequency of molecular displacements is manifested in molar activation free energies of wetting on an absolute, ∆G, and per unit surface area basis, ∆g.

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Using a pH 2 liquid to probe the native and hydrolyzed surfaces, both ∆G and ∆g remained constant at ≈51.5 kJ/mol and 105 mJ/m2, respectively. Using a pH 11 probe liquid, there was a steady decrease in both ∆G and ∆g to the values ≈47.5 kJ/mol and 70 mJ/m2, respectively, at the longest hydrolysis times examined. Not only is there a change in identity of functional groups at the surface of the hydrolyzed polyimide, there is also a gradual increase in surface potential as the pKa is crossed using two extreme pH probe liquids. Both factors can affect contact angle relaxation kinetics. In summary, contact angle relaxation kinetics are a sensitive function of the nature of the surface and the probe liquid employed. There are practically no changes in relaxation kinetics for native and hydrolyzed polyimide surfaces when a pH 2 probe liquid is used. In other words, it does not appear that a pH 2 probe liquid can distinguish between an interaction with an imide, carboxylic acid, or carboxylate anion. In

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contrast, the response of a pH 11 probe liquid is quite sensitive to the nature of the functional group at the surface. Acknowledgment. The author wishes to thank the reviewers for providing valuable suggestions. The author also would like to acknowledge a generous gift of the precursor poly(amic acid) from Dr. Gareth Hougham of IBM Thomas J. Watson Research Center. Supporting Information Available: Plots of the relationship between fitted parameters a and b for one data set, histogram statistics for Monte Carlo simulation of parameters a and b for one data set, and plots of contact relaxation data obtained on polyimide surfaces hydrolyzed for 5 and 20 min using 0.02 M NaOH. This material is available free of charge via the Internet at http://pubs.acs.org. LA034200A