X-Ray Photoelectron Spectroscopy of Fluoropolymers C. R. Ginnard and W. M. Riggs Central Research Department, E . I. du Pont de Nemours and Company, Wilmington, Del. 19898 X-RAY PHOTOELECTRON SPECTROSCOPY (XPS or ESCA) is rapidly becoming an established method in chemical research (1-3). The technique provides a sensitive probe of the electronic environment ot atoms in molecules. A number of studies by ESCA have shown that the inner level electron binding energies (Eb) of an atom are sensitive to the over-all charge on the atom. Since chemical effects such as oxidation state (4) and the electronegativities of substituents (5) determine the charge, chemical ”shifts” are observed in photoelectron spectra. Part of the usefulness of ESCA as an analytical tool lies in the application of these “shifts” to the structural analysis of organic materials. Polymer characterization appears especially promising since solid samples are easily handled by ESCA and some structural information may be obtained even on insoluble materials that cannot be studied by NMR. This study was undertaken to determine the effects of fluorine substitution in simple polymer systems as models for future work with more structurally complex molecules. Fluoropolymers are good subjects for investigation for two reasons. First, fluorine is the most electronegative of’ the elements. The carbon 1s chemical shifts are therefore as large as any seen. Second, the polymers are stable and special handling techniques are not generally required to obtain clean surfaces representative of the material. The series of polymers, -(CH2-CH2)-,, -(CH2-CHF)-,, -(CHzCF?)-,, -(CHF-CF,)-,, and -(CF2-CF2)-,, was chosen for this initial study to determine the effect of direct fluorine substitution on carbon in a simple aliphatic polymer system, the effect of substitution on the adjacent carbon atoms in these systems, and whether it will be possible to correlate some calculated value such as ligand electronegativity with chemical shift, thus achieving predictive ability. EXPERIMENTAL
A Varian IEE-15 electron spectrometer, equipped with an aluminum anode X-ray source, was used in this work. It was operated at an analyzer energy of 100 volts, so that the width of a gold 4f7,z standard peak is 1.8 eV FWHM. Under these conditions, fluoropolymer carbon peaks are approximately 2.0 eV FWHM. Carefully prepared films of the polymer samples were mounted on the cylindrical sample holder with double-sided tape. A gold wire was secured around the sample for binding energy calibration purposes. A multiple scan signal averaging technique was used to obtain a sufficient signal-to-noise ratio to locate individual peak maxima to within about =k0.07 eV. All peak positions were corrected to the measured Au 4f7,2peak position (83.3 eV), thus compensating for the effects of sample charging. The precision of a corrected binding energy measurement is thus approximately 1 0 . 1 5 eV. (1) Kai Siegbahn et al., “ESCA,” Almquist and Wiksells, Uppsala 1967. (2) D. M. Hercules, ANAL.CHEM., 42(1), 20A (1970). (3) J. M. Hollander and D. A. Shirley, Ann. Rev. Nucl. Sci., 20, 435 (1970). (4) 0. Nilsson, C. H. Norberg, J. E. Bergmark, A. Fahlman, C. Nordling, and K. Siegbahn, Helv. Phys. Acta., 41, 1064 (1968). (5) T. D. Thomas, J . Amer. Chem. Soc., 92,4184 (1970). 1310
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
Table I.
Observed and Charge-Corrected Binding Energies Observed Eba
Corrected Eb5 Polymer c 1s Au 4f7j2 Chargea C 1s -(CHz-CHz)-, 285.9 86.1 $2.8 283.1 -(CHF-CHz)-, 288.6, 286.5 86.4 $3.1 285.5, 283.4 87.1 +3.8 289.2, 283.7 -(CFzCHz)-n 293.0, 287.5 87.3 +4.0 289.6, 287.4 -(CF2CHF)-, 293.6, 291.4 +5.0 290.3 88.3 -( CFz-CFz)-n 295.3 a In electron volts.
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RESULTS AND DISCUSSION
Electrostatic charging of the samples, frequently observed in the ESCA spectra of nonconducting materials, contributes to the binding energy shift in the C and Au spectra. We have assumed that the gold wire wrapped around the sample attains the potential of the polymer in the vicinity of the surface. Hnatowich, Hudis, Perlman, and Ragaini (6) have demonstrated the validity of this assumption in a similar case where small amounts of gold are vacuum evaporated onto insulating sample surfaces. The difference between the observed Au binding energy and the binding energy of Au metal in electrical contact with the spectrometer (83.3 eV) is then a measure of the static charge on the sample. Carbon 1s binding energies, before and after charge corrections, are shown in Table I along with the observed Au 4f,/, binding energies. It is interesting that the static charge increases as the amount of F in the sample increases. There are more than two C Is peaks in the spectra of polyvinyl fluoride, polyvinylidene fluoride, and polytrifluoroethylene because of the presence of some head-to-head and tail-to-tail units in the polymer, Polyvinyl fluoride, for example, is typically about 10% nonstereoregular (7). How(6) D. J. Hnatowich, J. Hudis, M. L. Perlman, and R. C. Ragaini, J . Appl. Phys., 42, 4883 (1971). (7) R. E. Naylor, Jr., and S. W. Lasoski, Jr., J. Polyrn. Sci., 44, l(1960).
I
'
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I .o0.8
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A
-
0.6-
% 0.4
-
0.2
-
I
282.0
284.0
286.0
288.0
290.0
1
292.0
CIS BINDING ENERGY, OV
II .o
8.0
-
20-
6.0-
5 .a
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ever the two C Is peaks which are associated with the stereoregular configuration dominate the spectra, and it is these observed binding energies which are reported in Table I. Peaks due to hydrocarbon contamination occasionally appear but these can be readily identified by their characteristic low binding energy. It is helpful for interpretation of the chemical shifts to choose the two-carbon monomer as the basic unit for consideration. The series can then be regarded as a progressively fluoro-substituted polyethylene. The data are conveniently presented by a plot of the number of F's in the monomer unit us. the C 1s Et, as in Figure 1. It is evident that the -CH2-, -CHF-, and -CFr carbons all fall within discrete binding energy regions with no overlap. Thus a -CHF- carbon is distinguishable from a -CH,- or -CFZ- carbon no matter what the substitution on the adjacent carbons. The data points connected by solid lines in Figure 1 represent shifts due to fluorine substitution directly on the carbon measured. The fact that these lines are nearly parallel indicates a constant
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change of Eb with increasing number of F's, regardless of the adjacent atom substituents. The slope of these lines is approximately 3.0 eV/F atom. Likewise the dashed lines connect data points which reveal AEb in a carbon atom as a result of substitution on the adjacent carbon. The fact that these lines are not vertical demonstrates the existence of readily measurable adjacent atom substitution effects in fluoropolymer spectra. The difference in slopes suggests a difference in the susceptibility of -CFr and -CH2 toward these effects. The slope of the -CF2- dashed line is approximately 0.3 eV/F atom. Thus, adjacent atom substitution produces binding energy changes about 10% as large as direct substitution. Obviously shifts of this magnitude cannot be neglected in the interpretation of fluoropolymer spectra. It is noteworthy, however, that with many systems involving less electronegative substituents, the effect of substitution on the adjacent atom may well be negligible. Pauling electronegativities have been used successfully to correlate observed chemical shifts with substituents on the ANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
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atom of interest ( I , 5, 8). We have attempted a correlation here by calculating the charge on the atom by the Pauling method (9), and plotting this us. observed binding energy (Figure 2A). In order to calculate the charge, we have assumed that the effect of atoms farther down the carbon chain than the adjacent carbon and atoms bound to it is negligible. Inspection of Figure 2A shows that this approach underestimates the effect of substitution on the adjacent carbon. This has been observed previously with ligand effects on binding energies in platinum complexes (8). In that case a simple method was developed for estimating effective group electronegativities so that the sum of substituent electronegativities could be correlated with observed binding energies. We have adapted the method described in Reference 8 to polymers and achieved the correlation shown in Figure 2B. According to this method, substituent group electronegativities are taken to be simply the arithmetic mean of the electronegativities of the constituent atoms. The summing is limited, however, to the central atom of the group and the atoms attached directly to it. Substituent groups which are actually part of the chain are given only ’/* weight in the summation since they are in turn part of the next unit down the chain. Thus for example when the sum of substituent electro(8) W. M. Riggs, ANAL.CHEM.,44,390 (1972). (9) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed., Cornel1 University Press, Ithaca, N.Y., 1960.
negativities was calculated for -CH2- carbon in -(CF2 -CH&, the atoms shown below were considered. F H F
c-c-I cl -c-c l I
l
l
F H F
The carbon in question is considered to have four substituents’: 2 hydrogen atoms and 2 -CFz-C groups. However, the calculated -CF2-C group electronegativity is given only half the weight of a complete group since it in turn is part of the next unit down the polymer chain. An equivalent way of regarding this is to consider that the basic unit is [-(CCFz-) -CH2-] and there are thus only three substituent electronegativities to include in the sum. It is not immediately apparent on theoretical grounds that this approach should yield such good correlations. However, Thomas (5) has offered plausible rationalizations for this, at least for simple systems. This approach yields results equivalent to or better than the Pauling method through simpler arithmetic manipulations, and having succeeded for two quite different kinds of systems, may have some degree of general applicability for making quick estimates of direction and magnitude of binding energy shifts in chemical systems of interest. RECEIVED for review January 3, 1972. Accepted March 1, 1972.
Quantitative Liquid Chromatography of Sulfonylureas in Pharmaceutical Products William F. Beyer Control Analytical Research and Development; The Upjohn Company, Kalamazoo, Mich. 49001
THE SULFONYLUREA ANTIDIABETIC AGENTS (see Table I for structures) are usually determined by ultraviolet spectrometry or potentiometric titration ( I , 2). To increase specificity, thin layer chromatography has also been applied to the assay of these compounds (3-8). Attempts to develop gas liquid chromatographic (GLC) procedures for intact or derivatized intact sulfonylureas have been largely unsuccessful, primarily because of the thermal liability of these compounds at the temperatures required for chromatography. Utilizing thermal fragmentation to p toluenesulfonamide, a GLC assay for tolazamide has been
developed recently by Wickramasinghe and Shaw (9). Sabih and Sabih reported a GLC method for the determination of tolbutamide and chlorpropamide after derivitization with dimethyl sulfate (10). However, in our laboratories, we have not been able to chromatograph tolbutamide as a single peak using this procedure. Although the N-methyl derivative of tolbutamide was the principal product, approximately 10% of the methylenol ether was also formed (11). High-speed liquid chromatography has provided a highly specific and practical technique for the analysis of these antidiabetic agents, and is the subject of this report. EXPERIMENTAL
“United States Pharmacopeia.” 18th. rev., Mack Publishing Co., Easton, Pa., 1970. (2) “The National Formulary,” 13th ed., Mack Publishing Co., Easton, Pa. 1970. (3) K. C . Guven, S. Geegil, and 0. Pekin, Eczacilik Bul., 8, 158 (1966). (4) M. G. Hutzul and C. F. Wright, Can. J. Pharm. Sci., 3, 4 (1968). ( 5 ) D. L. Smith, T. J. Vecchio, and A. A. Forist, Metabolism, 15 (3), Part I (March). 1965. (6) J. Baumler and S. Rippstein, Deut. Apoth.-Zg., 107, 1647 (1967). (7) 0. Schettino and M. I. LaRotonda, Boll. SOC.Ital. Giol. S per. 1970, 46 (8), 432-6. (8) Ibid., pp 436-8. (1)
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
Apparatus. A DuPont model 820 liquid chromatograph operated at ambient temperature with a UV detector at 254 nm was used. The column was stainless steel, 100 cm long and 2.1 mm internal diameter. It was dry-packed with a hydro-carbon polymer support (1 ethylene propylene copolymer on DuPont Zipax, catalog. No. HCP-820960008, The DuPont Company, Wilmington, Del.). Peak areas were ~
(9) J. A. F. Wickramasinghe and S. R. Shaw, J . Pharm. Sci., 60, 1669 (1971). (10) K. Sabih and K. Sabih, ibid., 59 (6) 1970. (11) P. Bowman, The Upjohn Co., Kalamazoo, Mich., private communication 1971.
