Langmuir 1996, 12, 5247-5249
5247
Quantification of Ionizable Functional Groups on a Hydrolyzed Polyimide Surface Richard R. Thomas E. I. du Pont de Nemours and Company, Jackson Laboratory, Deepwater, New Jersey 08023 Received June 6, 1996. In Final Form: September 12, 1996X Surfaces of a hydrolyzed PMDA-ODA polyimide have been examined by contact angle titrations. From an analysis of the data and the free energy changes occurring during ionization, an estimate can be made of the number of carboxylic acid groups created by hydrolysis. Calculations estimate that (5-7) × 1017 -CO2H/m2 are formed during alkaline hydrolysis (0.25 M NaOH) from times of 5-120 min. These results are consistent with X-ray scattering data and unit cell area values obtained on aromatic polyimides.
Results and Discussion In an earlier report, we examined the surface of a hydrolyzed polyimide and performed subsequent chemical reactions on that surface.1 When subjected to alkaline hydrolysis, the surface of a polyimide prepared by thermal imidization of the reaction product of 1,2,4,5-benzenetetracarboxylic dianhydride and 4,4′-oxydianiline (PMDAODA), undergoes a ring-opening reaction to the poly(amic acid) upon neutralization. The reaction scheme is shown in Figure 1. A study of the hydrolyzed surface by the contact angle titration method2-5 proved to be an excellent probe of the carboxylic acids present. As observed in studies by others, the effect of ionization of the carboxylic acids is dramatic as the pH of the probe liquid is varied. Shown in Figure 2 are the advancing contact angle, θadv, data obtained as a function of pH for polyimide surfaces which were subjected to alkaline hydrolysis and acidification. The receding contact angles, θrec, measured on the modified surface behave similarly with pH (θadv - θrec ≈ 20°, pH 2-11). For comparison, θadv values for the unmodified polyimide ≈75° and are invariant with changes in pH.1 For pH values 6), the carboxylic acid groups are converted to the more wettable -CO2- anions. This scheme is depicted in Figure 3 showing the para-diacid isomer only. The decrease in contact angles as the surface is converted to one rich in carboxylate anions is a consequence of the interfacial tensions at the three-phase boundary of the wetting liquid and is given by the Young equation6
γLV cos θ ) γSV - γSL
(1)
where γLV, γSV, and γSL are the interfacial tensions at the liquid/vapor, solid/vapor, and solid/liquid interfaces, respectively, and θ is the angle of contact of a probe liquid at the three phase boundary (solid/liquid/vapor). The wettability of surfaces with ionizable functional groups is more complicated due to the fact that additional free X
Figure 1. Scheme for hydrolysis of PMDA-ODA surface. Only para-diacid isomer is shown.
Abstract published in Advance ACS Abstracts, October 15, 1996.
(1) Thomas, R. R.; Buchwalter, S. L.; Buchwalter, L. P.; Chao, T. H. Macromolecules 1992, 25, 4559. (2) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1, 725. (3) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921. (4) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (5) Wamser, C. C.; Gilbert, M. I. Langmuir 1992, 8, 1608. (6) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Interscience Publishers: New York, 1990; Chapter X.
S0743-7463(96)00556-2 CCC: $12.00
Figure 2. Advancing contact angles as a function of probe liquid pH for PMDA-ODA polyimide surfaces which were hydrolyzed for 5 (9), 10 (b), 15 (2), 30 (1), 60 ([), and 120 (0) min in 0.25 M NaOH followed by acidification.
energy terms must be included in γSL to account for the formation of the ionizable surface relative to the neutral surface. It is these additional free energy terms, however, which allow for the quantification of ionizable functional groups. Such a measurement is difficult, if not impossible, with neutral surfaces. Chatelier et al.7 have developed the relevant equations which will be utilized in the current study. The assumption was made that γSL was the sum of two terms (7) Chatelier, R. C.; Drummond, C. J.; Chan, D. Y. C.; Vasic, Z. R.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 4122.
© 1996 American Chemical Society
5248 Langmuir, Vol. 12, No. 22, 1996
Letters
Figure 3. Conversion of carboxylic acid groups to carboxylate anions during a contact angle titration. Only para-diacid isomer is shown.
γSL ) γSL° + γSLion
(2)
where γSL° is the intrinsic solid/liquid interfacial tension of the neutral surface and γLVion represents the free energy of formation of the ionizable surface relative to the neutral one. For ionizable acidic surfaces, the observed contact angle as a function of pH, cos θ (pH), was then given by
cos θ (pH) ) cos θ (pzc) + ∆FSLion (pH)/γLV
Figure 4. Contact angle titration showing experimental data (9) for a PMDA-ODA polyimide which was hydrolyzed with 0.25 M NaOH for 120 min followed by acidification and values calculated (b) according to eq 3. The curved lines are nonlinear least-squares curve fits of experimental data (s) and values of cos (- - -) calculated according to eq 12.
Kai )
[-CO2-][H+] [-CO2H]
(7)
Since ψ0 ∝ sinh-1 (δ0), the extent of ionization, R, is related to the number of ionizable functional groups, NS, through the following relationship.
(3)
where cos θ (pzc) is the contact angle of the uncharged surface (point of zero charge) and ∆FSLion (pH) is the free energy of formation of the ionizable surface. Additionally, ∆FSLion (pH) is assumed to be the sum of two terms
∆FSLion (pH) ) ∆FSLelec (pH) + ∆FSLchem (pH) (4) where ∆FSLelec (pH) is the electrostatic free energy of a charged surface relative to the neutral one and ∆FSLchem (pH) is the chemical free energy change, all as a function of the probe liquid pH. The detailed derivations of the free energy terms involved in surface ionization are given in the work of Chatelier et al.7 but will be highlighted here. It was shown that ∆FSLelec (pH) is given by the following expression
R ) δ0/eNS
Due to the nature of this relationship, eqs 5 and 6 have to be solved simultaneously. This necessitates making assumptions as to the value of NS and, therefore, introduces uncertainties in the quantitative determination of the number of ionizable functional groups (-CO2H) on the surface. Fortunately, in the present case, values of R can be determined experimentally from contact angle titration (cos θ vs pH) plots. From the expression for R
[-CO2-] R) [-CO2-] + [-CO2H]
0κ(2kT/e)2[cosh{eψ0/2kT} - 1] (5)
∆FSLchem (pH) ) kTNS{(1 - R) ln(1 - R) + R ln R + R ln([H+]/Kai)} (6)
R (pH) )
cos θ (pH) - cos θ (pH 2) cos θ (pH 11) - cos θ (pH 2)
(10)
Shown in Figure 4 is one set of data taken from Figure 2 and replotted as cos θ vs pH for analysis. Values of cos θ (pH 2), cos θ (pH 11), and pKaeff for the other hydrolysis times are listed in Table 1. There is excellent agreement statistically between the values of cos θ obtained experimentally and the values calculated according to eq 3 by varying NS and minimizing “goodness of fit”, χ2,8 using [H+] from the buffer solutions. Similarly good statistical fits are seen with data for other hydrolysis times. The calculated values of cos θ (pH) were determined using the value of Kai of 1.6 × 10-4 for phthalamic acid.9 (8)
where NS is the number of ionizable functional groups (-CO2H, in the present case), R is the extent of ionization, and Kai is the dissociation constant of the surface and is defined by the expression below.
(9)
and eq 1, R can be determined experimentally from contact angle data as a function of pH by the following equation.2
∆FSLelec (pH) ) δ0ψ0 -
where δ0 is the surface charge density, ψ0 is the electrical potential at the liquid/solid interface, 0 is permittivity of free space, is the dielectric constant of the medium (aqueous buffers, in this case), κ is the Debye reciprocal length, k is the Boltzmann constant, T is temperature, and e is the elementary electrostatic charge. The free energy due to the change in chemical free energy with pH was given by
(8)
11
χ2 )
∑ [cos θ (pH)
obs
- cos θ (pH)calcd]2/degree of freedom
pH ) 2
(9) Bender, M. L.; Chow, Y.-L.; Chloupek, F. J. Am. Chem. Soc. 1958, 80, 5380.
Letters
Langmuir, Vol. 12, No. 22, 1996 5249 Table 1. Data Obtained on Surfaces of Hydrolyzed PMDA-ODA Polyimidea
hydrolysis time (min)b
cos θ (pH 2)c
cos θ (pH 11)c
pKaeff c
NS (×10-17 RCO2H/m2)d
χ2 (×104)e
5 10 15 30 60 120
0.543 ( 0.008 0.511 ( 0.009 0.492 ( 0.005 0.523 ( 0.02 0.525 ( 0.01 0.521 ( 0.02
0.865 ( 0.01 0.881 ( 0.02 0.872 ( 0.01 0.932 ( 0.01 0.942 ( 0.008 0.946 ( 0.01
6.40 6.56 6.42 6.18 6.23 6.27
5.47 6.40 6.45 6.81 7.04 7.14
8.99 6.24 11.2 5.02 5.02 6.45
a PMDA-ODA samples were prepared from DuPont Pyralin 2545 and cured to a temperature of 400 °C in an inert environment. Samples hydrolyzed in 0.25 M NaOH and then converted to free acid by reaction with 0.25 M acetic acid for an equal period of time.c From eq 12. d From eq 3 using kT ) 4.1162 × 10-21 J/molecule, e ) 1.6021 × 10-19 C/charge, ) 80, 0 ) 8.85 × 10-12 C/(V‚m), γLV ) 7.28 × 10-2 N/m, and Kai ) 1.6 × 10-4. Values of κ were estimated for the buffer solutions.13 e From fit of eq 3 by varying NS and minimizing χ2.
b
Figure 5. Unit cell of PMDA-ODA polyimide.
Values of the effective dissociation constant of carboxylic acid groups on the surface, Kaeff
Kaeff ) [H+]
1-R R
(11)
can be calculated by combining eqs 10 and 11 to give
cos θ (pH) )
cos θ (pH 11) - cos θ (pH 2) ([H+]/Kaeff) + 1
+ cos θ (pH 2) (12)
and applying eq 12 to the experimental data using nonlinear least-squares curve fitting to solve for Kaeff. The calculated values of pKaeff for all the hydrolysis times are several units higher than the pKai for phthalamic acid. Such a trend has been discussed by us and others and seems to reflect the difficulty in generating charged groups at the polymer/water interface.1-5 It is likely that the values observed for pKaeff reflect a change in the local dielectric environment at the air/contact angle fluid phase. A positive shift in pKaeff vs pKai would be expected as the presence of the air phase would increase the free energy of the charged layer of -CO2-. A recent theoretical study suggests that small displacements (∼1 Å) of ionizable functional groups from the interface will have a substantial impact on pKaeff values.10 From our previous work,11 it was shown that the modified polyimide/air interface is likely not very sharp. A diffuse interface is also expected with the systems examined by Whitesides et al.2-4 The resulting spatial variation in ionizable functional groups relative to the interface and its effect on dielectric constant will lead to changes in dissociation behavior compared to that observed in solution. A unique feature arising from a fit of experimental cos θ (pH) data using eq 3 is an estimate of the number of ionizable functional groups (-CO2H in this case) by
varying NS in eq 6 and adjusting δ0 in eq 5 using eq 8. Shown in Table 1 are values of NS calculated after various hydrolysis times. As anticipated, the value of NS increases with increasing hydrolysis times. The calculated values of NS are meaningful only if they are a reasonable reflection of the actual number of carboxylic acid groups present on the surface of the hydrolyzed PMDA-ODA polyimide. Fortunately, data exist that allow an estimation of the number of carboxylic acid groups that might be expected. Using grazing incidence X-ray scattering, Russell et al. determined that the air surface of PMDA-ODA polyimide was more ordered than in the bulk and, for thermal treatments above 300 °C, crystalline-like ordering is observed.12 This fact, coupled with a unit cell area estimation of 202 Å2 (Figure 5),12 allows an appraisal of ∼1.98 × 1018 -CO2H/ m2 to be made for a completely hydrolyzed polyimide surface. Earlier work1 has shown that contact angles could not be obtained on poly(amic acid) surfaces (a model for the completely hydrolyzed polyimide surface) when the probe liquid pH was greater than 7 due to dissolution (θadv ≈ 52 ( 1.8°, pH 2-6). It was also demonstrated that under the hydrolysis conditions employed (0.25 M NaOH; 5-120 min), little or no dissolution of the hydrolyzed polymer occurred, indicating incomplete hydrolysis of the surface. These two facts suggest that the total density of carboxylic acid groups that could be detectable by contact angle measurements should be 0 < NS < 1.98 × 1018. The contact angle data shown in Figure 2 appear to reach a limiting value for pH values >9 as the hydrolysis time approaches 120 min. It is concluded that the surface is reaching completion levels in conversion of polyimide to poly(amic acid). These facts taken together with the calculated NS values of (5.47-7.14) × 1017 -CO2H/m2 obtained on hydrolyzed surfaces are quite consistent. The quantification of functional groups on surfaces using contact angle titrations should have great utility for the study of surfaces and reactions thereon. For example, a kinetic study of surface reactions is now possible. Currently, we are examining the kinetics of base hydrolysis of polyimide surfaces using this type of measurement. Acknowledgment. The author wishes to thank Drs. R. H. Dettre and B. B. Sauer for helpful discussions. LA960556T (10) Smart, J. L.; McCammon, A. J. Am. Chem. Soc. 1996, 118, 2283. (11) Plechaty, M. M.; Thomas, R. R. J. Electrochem. Soc. 1992, 139, 810. (12) Factor, B. J.; Russell, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847. (13) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986; Chapter 12.
