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Solution Properties of Partially Fluorinated Unsaturated Esters and Surface Properties of Polyester Coatings Containing Partially Fluorinated Unsaturated Esters as Cross-linkers Richard R. Thomas,*,† David F. Glaspey,† David C. DuBois,† Jack R. Kirchner,†,| Douglas R. Anton,‡ Kathryn G. Lloyd,§ and Katherine M. Stika§ E. I. du Pont de Nemours and Company, Jackson Laboratory, Deepwater, New Jersey 08023, E. I. du Pont de Nemours and Company, Marshall Laboratory, Philadelphia, Pennsylvania 19146, and E. I. du Pont de Nemours and Company, Corporate Center for Analytical Sciences, Experimental Station, Wilmington, Delaware 19898-0323 Received March 10, 2000. In Final Form: May 15, 2000 Simple, reactive amphiphilic molecules were prepared by condensation of perfluorooctylethyl alcohol with elaidic, oleic, and linoleic acids. The fluorinated, unsaturated esters were found to be surface active in polyester and urethane-modified alkyd coatings. In addition, they function much like typical drying oils used in these coatings through oxidative cross-linking. The surfaces of the coatings were examined by contact angle goniometry and X-ray photoelectron and time-of-flight secondary ion mass spectroscopy. The solution properties of these fluorinated derivatives were studied by measurement of surface tension isotherms, vapor pressure osmometry, and variable temperature 19F NMR spectroscopy in n-butyl acetate solution. Analysis of the data revealed that aggregation is occurring at low (99% C8F17 telomer) and used as received. Elaidic (98%), oleic (99+%), and linoleic (99%) acids were obtained from Aldrich Chemical Co. and used as received. (23) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Sauer, B. B.; Stika, K. M.; Swartzfager, D. G. Macromolecules 1997, 30, 2883. (24) Thomas, R. R.; Anton, D. R.; Graham, W. F.; Darmon, M. J.; Stika, K. M. Macromolecules 1998, 31, 4595. (25) Anton, D. R. U.S. Patent 5,637, 657, 1997. (26) Turner, G. P. A. Introduction to Paint Chemistry and Principles of Paint Technology, 3rd ed.; Chapman and Hall: London, 1988. (27) Gilby, A. R.; Alexander, A. E. Aust. J. Chem. 1956, 9, 347.
Langmuir, Vol. 16, No. 17, 2000 6899 Squalene was from Eastman Chemical Co. and purified by fractional crystallization. N-Butyl acetate (Spectral Grade) was from Acros Chemical Co. and phosphorus acid was from Aldrich Chemical Co. Two coating systems were chosen for evaluation. One was a medium oil polyester alkyd from McWhorter Industries and sold under the tradename Duramac 2768 and the other was a urethane-modified alkyd from Enterprise Paint Co. and sold under the tradename Designers Choice Polyurethane Clear Gloss Finish. Melting points were determined as the onset of the melting endotherm by differential scanning calorimetry (Perkin-Elmer DSC). Preparation of Perfluorooctylethyl Linoleate, Z,ZF(CF2)8(CH2)2O2C(CH2)7CHdCHCH2CHdCH(CH2)4CH3 (3). A three-necked 100-mL round-bottomed flask fitted with nitrogen inlet/outlet, thermometer, paddle stirrer, and a short-path distillation head, condenser, and receiver is charged with 24.94 g (88 mmol) of linoleic acid, 45.76 g (99 mmol) of perfluorooctylethyl alcohol, and 0.12 g of 70% aqueous H3PO3. The reaction mass was heated with stirring to 140-150 °C for ∼48 h. The sample was allowed to cool under positive nitrogen pressure. The reaction mass yielded 58.02 g (91% yield based on linoleic acid) of perfluorooctylethyl linoleate as a pale yellow liquid: mp -9.2 °C. 1H NMR δ: 0.2-2.1 (-CH2-, 23 H), 2.2-2.4 (-CH2-, t, 2 H), 2.5-2.8 (-CH2-, t of t, 2 H), 2.7-2.9 (-CH2-, t, 2 H), 4.3-4.5 (-CH2-, t, 2 H), 5.2-5.5 (dCH-, m, J ≈ 10 Hz, 4 H). Anal. Calcd for C28H35O2F17: F, 44.4. Found: 43.3. Preparation of Perfluorooctylethyl Elaidate, E-F(CF2)8(CH2)2O2C(CH2)7CHdCH(CH2)7CH3 (1) and Perfluorooctylethyl Oleate, Z-F(CF2)8(CH2)2O2C(CH2)7CHdCH(CH2)7CH3 (2). The preparation of perfluorooctylethyl elaidate 1 and perfluorooctylethyl oleate 2 was performed in a fashion similar to the synthesis of perfluorooctylethyl linoleate 3. Esters 1 and 2 were isolated in high yield and purity as a white, waxy semisolid and as a pale yellow liquid, respectively. Analytical data for 1: mp 24.9 °C. Analytical data for 2: mp 8.5 °C. 1H NMR δ: 0.8-2.1 (-CH2-, -CH3, m, 29 H), 2.2-2.4 (-CH2-, t, 2 H), 2.5-2.8 (-CH2-, t of t, 2 H), 4.3-4.5 (-CH2-, t, 2 H), 5.2-5.5 (dCH-, d of t, J ≈ 10 Hz, 2 H). 19F NMR (vs trichlorofluoromethane) δ: -80.9 (-CF3, t, J ≈ 12 Hz, 3 F), -113.3 (-CF2-, m, 2 F), -121.5 to 121.7 (-CF2-, m, 6 F), -122.5 (-CF2-, m, 2 F), -123.4 (-CF2-, m, 2 F), -126.0 (-CF2-, m, 2 F). Anal. Calcd for C28H37O2F17: F, 44.3. Found: F, 43.3. Surface Tension Measurements. Surface tension measurements were performed using the pendant drop method on a VCA 2500 contact angle goniometer from AST Products, Inc. The device utilizes a CCD camera to capture the drop image and software to analyze the drop profile by nonlinear least-squares curve fitting. The drop was allowed to equilibrate for ∼15 s before measurement. It is necessary to input solution density values to solve for surface tension by the pendant drop method. These were measured using a Mettler/Paar DMA46 densiometer. Solutions were prepared by dilution of the pure esters in n-butyl acetate. Vapor Pressure Osmometry. Measurements were performed using a Wescan Model 233 vapor pressure osmometer (Jupiter Instrument Co.). The instrument was calibrated with squalene in n-butyl acetate. Solutions were prepared by dilution of the pure esters in n-butyl acetate. Measurements were taken with a cell temperature of 81.0 ( 0.1 °C. NMR Spectroscopy. 1H (300 MHz) and 19F NMR (282.2 MHz) measurements were performed using a Varian VXR-300 spectrometer. Samples were dissolved in acetone-d6 for 1H and n-butyl acetate for 19F experiments. The residual proton resonance of acetone-d6 was used as a reference for 1H and the fluorine signal of trichlorofluoromethane was used for 19F, both as internal standards. XPS Spectroscopy. XPS data were collected in both survey and high-resolution mode on a Physical Electronics (PHI) LS5000 and 5400 system equipped with a Mg anode operating at 300 W. Typical analysis areas were 2 × 8 mm. Data were recorded at both 30° and 90° takeoff angles, resulting in analysis depths of ∼50 and 100 Å, respectively. The scan time was chosen to maximize signal-to-noise and minimize beam damage. Typically, this was in the range of 10-20 min. Since fluorine is relatively more sensitive to beam damage than the other elements present, its signal was collected first. Quantification was done by
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Figure 1. Reaction scheme for preparation of the perfluorooctylethyl esters of elaidic 1, oleic 2, and linoleic 3 acids.
Figure 2. Surface tension isotherm data for the perfluorooctylethyl ester of elaidic (2), oleic (b), and linoleic (9) acid in n-butyl acetate. integration of the atomic signals and corrected with Scofield sensitivity factors adjusted for the instrument lens and detector design. The element specific detection limit was on the order of 0.5 atom %. ToF-SIMS Spectroscopy. Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) experiments were performed using a Physical Electronics (PHI) Model 7200 reflector-design time-offlight mass spectrometer. An 8-keV bunched cesium primary ion source was used to generate the spectroscopic data. Charge buildup at the insulating sample surface was compensated by use of a pulsed electron flood gun.
Results and Discussion Synthesis of Perfluorooctylethyl Alcohol Esters of C18 Unsaturated Fatty Acids. The amphiphiles used in the current study are based on the phosphorus acidcatalyzed condensation of perfluorooctylethyl alcohol with a C18 unsaturated fatty acid. The reaction scheme for the preparation of perfluorooctylethyl elaidate, 1, is shown in Figure 1. Similar methods were used to produce the perfluorooctylethyl alcohol esters of oleic 2 and linoleic 3 acids. The reaction proceeded in high yield and resulted in high-purity (>90%) products. All the derivatives analyzed satisfactorily by 1H and 19F NMR spectroscopy and GC/MS. Surface Tension Measurements. Surface tension measurements were conducted to evaluate the surface activities of the perfluorooctylethyl esters of elaidic 1, oleic 2, and linoleic 3 acid. Surface tension isotherms for 1, 2, and 3 in n-butyl acetate solution are shown in Figure 2. The fluorinated, unsaturated, fatty acid esters prepared are all surface active as judged by their ability to lower the surface tension of n-butyl acetate. The surface tension of the pure perflourooctylethyl ester of oleic 2 and linoleic
Figure 3. Free energy of adsorption determination for (9) linoleic, (b) oleic, and (2) elaidic acid esters of perfluorooctylethyl alcohol in n-butyl acetate solution using eq 1. The solid, dashed, and dashed-dot lines are from a linear least-squares fit to the data for linoleic, oleic, and elaidic acid esters, respectively.
3 acids was measured to be 18.3 ( 0.2 and 19.2 ( 0.1 mN/m, respectively. The perfluorooctylethyl elaidic acid ester 1 was a solid at room temperature, which prevented a surface tension measurement. From surface tension isotherm data, standard free energies of adsorption, ∆G°, can be calculated for the various esters according to28
∆G° ) -RT ln(π/χ)χf0
(1)
where R is the gas constant, T is the absolute temperature, π is the surface pressure ()γ0 - γe), γ0 is the surface tension of the pure solvent, γe is the surface tension of solution, and χ is the mole fraction of solute, by extrapolation to infinite solute dilution. The data used to determine ∆G° are shown in Figure 3. As anticipated from the observed surface activity, ∆G values are 2 × 10-2 mol/L based on slopes of ∆[M]osmotic/∆[M]stoichiometric < 1. Shown also in Figures 4-6 are number-average aggregation values calculated from (42) Chen, J.; Jiang, M.; Zhang, Y.; Zhou, H. Macromolecules 1999, 32, 2, 4861. (43) Matsumoto, K.; Kubota, M.; Matsuoka, H.; Yamaoka, H. Langmuir 1999, 15, 7122. (44) Preuschen, J.; Menchen, S.; Winnik, M. A.; Heuer, A.; Spiess, H. W. Macromolecules 1999, 32, 2690. (45) Seery, T. A. P.; Yassina, M.; Hogen-Esch, T. E.; Amis, E. J. Macromolecules 1992, 25, 4784. (46) Zhang, Y.; Wu, C.; Fang, Q.; Zhang, Y.-X. Macromolecules 1996, 29, 2494. (47) Fung, B. M.; Mamrosh, D. L.; O’Rear, E. A.; Frech, C. B.; Afzal, J. J. Phys. Chem. 1988, 92, 4405. (48) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991, 95, 1829. (49) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935. (50) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942. (51) Muller, N. Langmuir 1994, 10, 2202. (52) Petit, F.; Iliopoulos, I.; Audebert, R.; Szo¨nyi, S. Langmuir 1997, 13, 4229. (53) Tadros, T. F. J. Colloid Interface Sci. 1980, 74, 196. (54) Zhang, Y.-X.; Yang, J.; Da, A.-H.; Fu, Y.-Q. Polym. New. Technol. 1996, 8, 169. (55) Duns, G. J.; Reeves, L. W.; Yang, D. W.; Williams, D. S. J. Colloid Interface Sci. 1991, 145, 270. (56) Fendler, E. J.; Fendler, J. H.; Medary, R. T.; El Seoud, O. A. J. Phys. Chem. 1973, 77, 1432. (57) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. J. Am. Chem. Soc. 1994, 116, 5579. (58) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 1995, 11, 977. (59) Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361. (60) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. Chem. Lett. 1989, 1737. (61) Kuwahara, H.; Ishikawa, Y.; Kunitake, T. Chem. Lett. 1993, 1161. (62) Kunitake, T.; Okahata, Y.; Shimomura, M.; Tasunami, S.-i.; Takarabe, K. J. Am. Chem. Soc. 1981, 103, 5401.
Langmuir, Vol. 16, No. 17, 2000 6903
Figure 4. Vapor pressure osmometry data for the perfluorooctylethyl ester of elaidic acid. Osmotic concentration (9) and aggregation number (b) are shown.
Figure 5. Vapor pressure osmometry data for the perfluorooctylethyl ester of oleic acid. Osmotic concentration (9) and aggregation number (b) are shown.
Figure 6. Vapor pressure osmometry data for the perfluorooctylethyl ester of linoleic acid. Osmotic concentration (9) and aggregation number (b) are shown.
the ratio [M]stoichiometric/[M]osmotic. Aggregation values range from 3 to 7 at the highest concentrations (∼8 × 10-2) studied. While few studies have been conducted on fluorinated amphiphiles in nonaqueous solution, those that have also find aggregation occurring at relatively low concentrations.55,58,63 In addition, the aggregate numbers are low (2-5) when compared to those in similar systems in aqueous solution. NMR evidence suggests that the aggregates formed have the perfluoroalkyl portion of the molecule directed toward the core of the aggregate.55 (63) Binks, B. P.; Fletcher, P. D. I.; Kotsev, S. N.; Thompson, R. L. Langmuir 1997, 13, 6669.
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Table 4. Vapor Pressure Osmometric and 19F NMR Aggregation and Formation Constant Values for Perfluorooctylethyl Esters of Elaidic 1, Oleic 2, and Linoleic 3 Acids in n-Butyl Acetate samplea
[M] (mol/L)
T (K)
1 2 3 3 3 3 3
8.3 × 10-2 8.4 × 10-2 8.3 × 10-2 8.3 × 10-3 to 8.3 × 10-2 8.3 × 10-3 to 8.3 × 10-2 8.3 × 10-3 to 8.3 × 10-2 8.3 × 10-3 to 8.3 × 10-2
354.15 354.15 354.15 248.15 273.15 298.15 323.15
osmometric aggregation valueb 5.5 ( 0.5 6.1 ( 0.8 3.6 ( 0.3
19F NMR aggregation value, nc
K × 10-3 (mol-(n-1))
3.4 ( 0.7 3.5 ( 0.7 3.5 ( 0.8 3.3 ( 0.8
1.7 ( 1 2.7 ( 1 2.9 ( 1 1.4 ( 1
a 1 ) perfluorooctylethyl elaidate; 2 ) perfluorooctylethyl oleate; 3 ) perfluorooctylethyl linoleate. b At indicated concentration. c Evaluated over indicated concentration range.
Figure 7. Proton-decoupled 19F NMR spectrum of the perfluorooctylethyl ester of linoleic acid in n-butyl acetate at room temperature.
The presence of aggregates was confirmed also by variable temperature 19F NMR measurements of perfluorooctylethyl ester of linoleic acid 3 in n-butyl acetate solution. Shown in Figure 7 is the proton-decoupled 19F NMR spectrum of perfluorooctylethyl ester of linoleic acid 3 in n-butyl acetate solution at room temperature. Unlike VPO measurements that have a limited temperature range available for a given solvent, NMR experiments can be conducted over a wide temperature range limited only by phase transitions of the solvent. The 19F resonance of the terminal -CF3 group occurs at approximately -81 ppm vs trichlorofluoromethane with a frequency that was found to be dependent on concentration of fluorinated ester. The terminal -CF3 resonance was chosen for study because of its unique chemical shift relative to the internal -CF2- units and its environment should be particularly sensitive to aggregation. The absence of distinct monomer and aggregate resonances indicates that exchange between the two species is occurring rapidly on the NMR time scale and the following analysis is valid. On the basis of chemical shift of the -CF3 resonance vs concentration, number average aggregation values, n, and aggregate formation constants, K, can be estimated according to55
log ([m]t - [m]m) ) n log[m]m + log nK
(3)
where [m]t is the total molal concentration of amphiphile, [m]m is the molal concentration of monomeric units, n is the aggregation value, and K is the aggregate formation constant. Concentrations are expressed in molality, [m], to avoid solvent contraction/expansion corrections necessary with temperature changes. The data are shown in Figure 8 C. The molal monomer concentration, [m]m, is evaluated as χ[m]t where χ is the number fraction of monomers as a function of solute concentration and given by the expression
δobs ) δmχ + δa(1 - χ)
(4)
Figure 8. Analysis of 19F NMR data according to eqs 3 and 4 for the perfluorooctylethyl ester of linoleic acid in n-butyl acetate at 298.15 K. The chemical shift of the monomer (δm) is determined from the y intercept in (A) and the chemical shift of the aggregate (δa) is determined from the y intercept in (B). Aggregation values and formation constants are evaluated from the slope and y intercept, respectively, in (C). Solids lines are from linear least-squares fits of data.
where δobs, δm, and δa are the observed chemical shifts of the desired resonance as a function of concentration, resonance due to monomer, and resonance due to aggregate, respectively. The values of δm and δa are evaluated as the y intercept of plots of δobs vs [m]t (A) and 1/[m]t (B), respectively, and plots are shown in Figure 8. The NMR measurements for 3 support the VPO data well. Over the temperature range 248-323 K, aggregation values were nearly invariant at 3.3-3.5. Vapor pressure osmometric evaluation (Figure 6) gave aggregation values ≈3.5 at ∼8 × 10-2 mol/L and are in remarkably good agreement. Aggregation values and formation constants for all the fluorinated ester derivatives are given in Table 4. Since monomer and aggregate concentrations can be determined as well as aggregation values as a function of temperature, the thermodynamic parameters for aggregation can be estimated according to64
∆G°a ) -RT ln βa ∆H°a ) RT (d ln βa/dT) ∆S°a ) R ln βa + ∆H°a/T
(5)
where ∆G°a, ∆H°a, and ∆S°a are the standard free energy, enthalpy, and entropy of aggregation, respectively. βa is the stepwise aggregate formation constant and arises from a mass-action analysis; therefore, βa is a product of all the (64) Kertes, A. S. in Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1; p 445.
Properties of Fluorinated Unsaturated Esters
stepwise aggregation constants up to n and is given by βa ) K ) [m]a[m]m-n, where [m]a is the molal aggregate concentration. An analysis of the 19F NMR data for the perfluorooctylethyl ester of linoleic acid 3 using eq 5 yielded ∆G°a ≈ -4.30 ( 0.2 kcal/mol, ∆H°a ≈ 6.11 ( 2 cal/mol, and ∆S°a ≈ 15.7 ( 0.8 cal/(mol‚K). The standard free energy of aggregation is negative as expected since aggregates are formed under these conditions. The aggregation process appears to be driven entropically as evidenced by the small positive value of ∆H°a and a positive value for ∆S°a. Solvent reorganization during the aggregation process would account for the relatively large positive value of ∆S°a. The driving force for aggregation in the present case appears to be similar to that for other amphiphilic molecules.53,65 In a somewhat related series of studies by Kunitake et al, the solution and adsorption properties of a variety of double-chain perfluoroalkyl amphiphiles were investigated.57,60-62 The derivatives included an oleic amide in the hydrocarbon tail. The authors claim that the main driving force for aggregation was not entropic in origin but, rather, due to cohesive energy differences between solute and solvent. However, these double-chain perfluoroalkyl amphiphiles formed bilayers that exhibit a bilayer to monomer conversion at elevated temperatures (70 °C) by VPO and differential scanning calorimetry measurements.57 From thermodynamic measurements, ∆H and ∆S of the monomer to the bilayer aggregation process were evaluated as -70 kJ/mol and -230 J/(mol‚K), respectively. In the present system, there was no evidence seen for bilayer formation and the aggregates were stable with a relatively constant aggregation number over a wide temperature range. This would account for the values of ∆H and ∆S observed in the current study. It is also interesting to note that the adsorption isotherm for the oleic amide derivative used in the previous study57 exhibited a concave upward curve, indicating stepwise adsorption, and prevented a molecular area calculation. In the present study, all the fluorinated derivatives show (65) Nusselder, J. J. H.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1992, 148, 353.
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downwardly concave surface tension isotherm curves that are typical of a simple phase diagram for adsorption predicted by the Gibbs isotherm (eq 2). Conclusions Perfluorooctylethyl ester derivatives of elaidic, oleic, and linoleic acid were prepared easily and in high yield by condensation of perfluorooctylethyl alcohol and the corresponding unsaturated fatty acid. The fluorinated derivatives are surface active and capable of lowering the surface tension in organic solvents as evidenced by surface tension isotherms and negative free energies of adsorption. The unsaturated derivatives resemble drying oils used in alkyd coatings systems and were evaluated in that capacity. When mixed with alkyd coatings, there is a substantial surface segregation of the fluorinated components, resulting in contact angles elevated above the native coating. The magnitude of surface excess appears to be low based on estimated molecular area demand. Evidence is found that the lessened excess could be due to favorable interactions between the fluorinated esters and components in the coating. It was also found that the fluorinated esters aggregate in n-butyl acetate solution as evidenced by vapor pressure osmometric and 19F NMR spectroscopic studies. Aggregates of 3-7 monomers at 8 × 10-2 mol/L were found by both techniques. A variable temperature study yielded the thermodynamic parameters for aggregation for perfluorooctylethyl ester of linoleic acid 3 as ∆G°a ≈ -4.30 ( kcal/mol, ∆H°a ≈ 6.11 ( 2 cal/mol, and ∆S°a ≈ 15.7 ( 0.8 cal/(mol‚K). The surface segregation properties of the various fluorinated esters do not seem to be influenced substantially by topology of the unsaturated hydrocarbon portion of the ester. The presence of various degrees of unsaturation and whether the olefinic portion is in the cis or trans configuration has little consequence on observed properties. Acknowledgment. The authors thank Dr. Peter Jernakoff for the 1H NMR data, Diane Davidson for the XPS data, and Wendy Justison for the ToF-SIMS data. LA0003600
