Ind. Eng. Chem. Res. 1987,26, 667-671
dodecanethiol in both deaerated benzene and tetradecane solvents at 120 "C. The variation in product mix was studied over reaction times of 15-180 min. A common variety of products was observed for all reaction time periods. The yield of individual components, however, varied significantly with reaction time. The major product derived from tBHP was tert-butyl alcohol. Other observed tBHP products included methane, acetone, and isobutylene. The major product from the hexyl sulfide oxidation was hexyl sulfoxide. Other sulfur-containing products were hexyl sulfone, hexyl disulfide, and hexyl thiosulfinate. The major product from dodecanethiol was dodecyl disulfide. Oxidized products included dodecyl sulfoxide, dodecyl sulfide, and dodecyl sulfone. Trace products included hexene, hexane, and hexanal from hexyl sulfide and dodecane and dodecanal from dodecanethiol. Solvent participation was noted by the formation of toluene from benzene and tetradecanones and tetradecanols from tetradecane. No oxygenated products of the benzene were observed. Kegistry No. tBHP, 75-91-2; (C6H13)2S, 6294-31-1; (C6H13)2S=0, 2180-20-3; C&, 71-43-2; C12H25SHI 112-55-0; (C12H2s)2Sz, 2757-37-1; tetradecane, 629-59-4.
Literature Cited Barnard-Smith, D. G.; Ford, J. F. Chem. Commun. 1965, 2, 120. Benson, S. W. J. Chem. Phys. 1964, 40, 1007. Bernard, D.; Bateman, L.; Cole, E.; Cunneen, J. Chem. Ind. 1958, 918. Calvert, J. G.; Pitts, J. N. Photochemistry; Wiley-Interscience: New York, 1966. Cohen, S. G.; Aktipis, S. J . Am. Chem. SOC.1966,88, 3587. Correa, P. E.; Riley, 0. P. J. Org. Chem. 1985, 50, 1787. Curci, R.; Giovine, A.; Modena, G. Tetrahedron 1966,22, 1235. Denison, G. Ind. Eng. Chem. 1944,36,477. Denisov, E. T. Liquid Phase Reaction Rate Constants;IFI/Plenum: New York, 1974. Gray, P.; Williams, A. Chem. Rev. 1959, 59, 239.
667
Hazlett, R. N. Frontiers of Free Radical Chemistry; Academic: New York, 1980. Henbest, H. B.; Khan, K. A. Chem. Commun. 1968,17,1036. Hiatt, R. R. Frontiers of Free Radical Chemistry; Academic: New York, 1980. Hiatt, R. R.; Irwin, K. C. J. Org. Chem. 1968, 33, 1436. Hiatt, R. R.; Mill, T.; Mayo, F. R. J. Org. Chem. 1968, 33, 1436. Howard, J. A. The Chemistry of Functional Groups, Peroxides; Wiley-Interscience: New York, 1983. Howard, J. A.; Ingold, K. U. Can. J . Chem. 1969,47, 3797. Kice, J. L. Free Radicals; Wiley-Interscience: New York, 1973. 1960,56, 1296. Kirk, A. D.; Knox, J. H. Trans. Faraday SOC. Migita, T.; Kosugi, M.; Takayama, K.; Nakagawa, Y. Tetrahedron 1973, 29, 51. Morse, B. K. J. Am. Chem. SOC.1957, 79, 3375. Mosher, H. S.; Durham, L. J. J. Am. Chem. SOC.1960, 82, 4537. Mushrush, G. W.; Hazlett, R. N.; Eaton, H. G. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 290. Ohno, A.; Ohnishi, Y. Int. J . Sulfur Chem. 1971,3, 203. Rahman, A.; Williams, A. J . Chem. SOC.B 1970, 1391. Richardson, W. H. J . Am. Chem. Soc. 1965, 87, 1096. Schrauzer, G. N.; Sibert, J. W. J. Am. Chem. SOC.1970, 92, 3509. Schwartz, F. G.; Whisman, M. L.; Ward, C. C. Bull.-U. S., Bur. Mines 1964,626, 1. Scott, G. Autoxidation; Elsevier: Amsterdam, 1965; Chapter 3. Sweely, C. C.; Bentley, R.; Makits, M.; Wells, W. W. J . Am. Chem. SOC.1963,85, 2497. Taylor, W. F. Ind. Eng. Chem. Prod. Res. Deu. 1974, 13, 133. Taylor, W. F.; Wallace, T. J. Ind. Eng. Chem. Prod. Res. Dew 1967, 6, 258. Taylor, W. F.; Wallace, T. J. Ind. Eng. Chem. Prod. Res. Deu. 1968a, 7, 198. Taylor, W. F.; Wallace, T. J. SAE Trans. 196813, 76, 2811. Thompson, R. B.; Druge, L. W.; Chenicek, J. A. Ind. Eng. Chem. 1949, 41, 2715. Van Swet, H.; Kooyman, E. C. Red. Trau. Chim. Pays-Bas 1968,87, 45. Wagner, P. J.; Zepp, R. C. J . Am. Chem. SOC.1974, 94, 285. Wallace, T. J. Adu. Pet. Chem. Refin. 1964, 9, 378. Wallace, T. J. J. Org. Chem. 1966, 31, 1217. Walling, C. Free Radicals in Solution; Wiley-Interscience: New York, 1957.
Receiued for review March 31, 1986 Accepted December 8 , 1 9 8 6
Comparison of Experimental Methods Used To Follow Polymer Film Weathering by Infrared Spectroscopy Linda M. Briggs, David R. Bauer, and Roscoe 0. Carter III* Research S t a f f , Ford Motor Company, Dearborn, Michigan 48121
Changes in polymer film composition during weathering can, in principle, be determined by following changes in infrared band intensities. In this report, infrared reflection and transmission methods will be compared and evaluated as techniques t o evaluate polymer film weathering. It has been found that when spectra are recorded by using the reflection method, band intensity ratios are strongly and nonlinearly related to the film thickness. This can be a serious problem for infrared studies of polymer film weathering since the films generally erode during weathering. The effect has been explained in terms of classical interference phenomena. These effects cannot be compensated for by internal standards. The interference effect is much smaller in the transmission experiment, but it is still present. In practice, it is found that band intensity ratios vary by a t most 10% when measured in the transmission mode, while the same ratios varied 60% in the reflection mode for the same range of thickness variation. It is our contention that it is possible to follow significant chemical composition changes by using transmission techniques. If care is taken to sample the same region consistently, physical thinning can also be determined for nonuniformly thick films. When organic polymer coatings are exposed to ultraviolet light and humidity, chemical changes occur which can ultimately lead to the loss of desirable physical properties. A variety of techniques can be used to follow these 0888-5885/87/2626-0667$01.50/0
chemical changes, including infrared, nuclear magnetic resonance, and ultraviolet spectroscopies. We have used infrared techniques to follow cure (Bauer and Dickie, 1980),hydrolysis (Bauer, 1982),and ultraviolet (Bauer and 0 1987 American Chemical Society
668 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987
Briggs, 1984) induced degradation in melamine formaldehyde cross-linked, styrene-modified acrylic copolymer films. The nature of the coatings and the exposure conditions employed place certain constraints on the sampling methods used. Following chemical changes as a function of exposure time requires among other things the ability to quantitatively measure band intensities relative to an internal standard. These coatings have strong carbonyl stretching mode absorptions so that thin films (i.e., 10 pm) must be used to obtain spectra with maximum absorbance less than unity. Thin, free-standing films are not strong enough to survive the extensive exposure. Since the coatings are cross-linked, conventional solution techniques cannot be employed. Thicker films can be pulverized and presented for analysis in a KBr pellet (Bauer et al., 1984). This technique is destructive and requires a separate sample for each sample exposure. It also requires a somewhat tedious sample preparation, and small changes in coating composition or exposure conditions from sample to sample can lead to scatter in the kinetic data. For these reasons, a nondestructive assay is preferred. In the course of our studies, we have evaluated infrared sampling techniques based on both reflection and transmission methods. In this report, the results of that evaluation are reported. We contrast measurements of relative band intensities as determined by transmission and reflection techniques from different substrates. The results are discussed in terms of the effect of interferences on band intensities in the two basic sampling methods.
-
Experimental Section The acrylic copolymer used was prepared by conventional free-radical polymerization. The monomer composition is as follows: butyl methacrylate, 23 % ; hydroxyethyl acrylate, 30%; acrylic acid, 2%; styrene, 25%; ethylhexyl acrylate, 20%, with a molecular weight of 2200. The cured film was comprised of the acrylic copolymer and a partially alkylated melamine cross-linker (Cyme1 325 from American Cyanamid). The polymer to cross-linker ratio was 70:30. Films were baked in a forced air oven at 130 "C for 20 min. Accelerated weathering was achieved by exposing the acrylic melamine films to constant ultraviolet light a t constant temperature and humidity. These conditions were controlled by using a modified Atlas weatherometer (Gerlock et al., 1984). Standard FS-20 sunlamps were used. The ultraviolet light intensity was around 1mW/cm2 with the peak wavelength at 300 nm. The humidity was controlled by holding the temperature of the water in the bottom of the weatherometer constant while rapidly circulating the air in the weatherometer. Samples were cast on a variety of substrates by using a draw-down technique. Samples for the transmission experiments were cast on KRS-6 windows. Substrates for the reflection experiments included polished anodized aluminum, chromium-plated aluminum, mirror-polished steel, and glass flats sputter coated with gold. Film thicknesses were approximately 10 pm. Reflection spectra were obtained by using two spectrometers. One was a Perkin-Elmer Model 283 spectrometer, equipped with an external reflection accessory (incidence angle 30 deg from normal). Spectra were referenced against reflection from the bare substrate. The other spectrometer was a Nicolet 7001 bench interfaced to a PDP 11/60 computer. Interferograms were detected by using a liquid-nitrogen-cooled HgCdTe detector. Resolution of 2 cm-' was used. Samples investigated by external reflection were placed in a Harrick Horizontal Reflection Stage Accessory which afforded a reflection angle of 1 2 deg.
LX'?
3630
3290 2 K 3 2430 2220 >( 1 i
E '* U I! E E F S
,EX t
21C E 3 3
-30
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Figure 1. (a) Infrared transmission spectrum from 4000 to 500 cm-' of a fully cured acrylic melamine film on a KRS-5 plate. (b) Infrared spectrum of the f i in (a) after 600 h of exposure to ultraviolet light and humidity. Peaks of significance, marked with arrows, corresponding to carbonyl, hydrocarbon, melamine-methoxy, melamine triazine ring, and styrene are at 1735, 2960, 913, 815, and 700 cm-', respectively.
Results Figure l a is an example of a transmission infrared spectrum of a fully cured acrylic melamine film before any "weathering" has occurred. Figure l b is an infrared spectrum of the same film after 600 h of exposure to ultraviolet light and humidity. The comparison of spectra obtained from various substrates by external reflection spectroscopy with those obtained in the transmission mode was initiated when it was discovered that relative intensities of certain bands in the spectra obtained by using the relfection accessory varied in an unexpected and irregular manner as a function of exposure time. These samples were acrylic melamine films cast on sputtered gold. The band shapes and frequencies were consistent; only the relative intensities seemed to be affected. Further investigation of cured, unweathered films revealed that peak intensity ratios varied widely with location on the same sample. The variation was as great as 60% for some peaks (see Figure 2). Peak intensity ratio refers to the ratio absorbance of xlabsorbance of styrene at 700 cm-l, where x is either the carbonyl band a t 1735 cm-' or the hydrocarbon band at 2960 cm-l or the melamine band at 815 cm-'. Four locations on a single 6.5-cm2film were used to obtain the spectra. The same coating cast on chromium and polished steel showed similar, but less dramatic, spectral intensity variations. It is unlikely that the observed effect is due to interfacial interaction, since it was observed on all substrates, but is largest on gold, an inert substrate. One possible explanation to account for the observed variation in peak intensity is that the polymer/cross-linker resin is inhomogeneous and yields films that are also inhomogeneous. However, if there are small inhomogeneous regions, sampling larger areas should produce more uniform results. Varying the sample area examined from 1.0 to 4.2 mm2 and to 32 mm2 did not reduce
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 669
5t E 4 -
E >
c
-0
(0 7.
n 3 n
0
\ X
2 2 -
*
I
b ) HYDROCARBON
1
2 3 LOCATlON
4
Figure 2. Absorbance peak height ratios from a reflection spectrum of an acrylic melamine film cured on a gold substrate: (a) z = carbonyl, (b) z = hydrocarbon, and (c) z = melamine.
.3 I-
r
2 W X
Y
a
.2
W
a
z 0 m a a
g
5r KRS-5
.I
K
n
> r
100 200 300 400 500 600 700 800 900
TIME, HOURS
Figure 3. Film erosion due to “weathering” indicated by the intensity of the hydrocarbon absorption as a function of exposure time.
the variation observed. The sample area was changed by changing the aperture of the FTIR spectrometer, thus changing the sample area examined. The variation was somewhat less using the 30-deg geometry in the P-E 282 instrument. However, the sample area in this case is about 250 mm2. There is no phase separation in the liquid state of the coating system, and none was apparent as the coating cured. The coating system is homogeneous and does not produce localized concentration differences. The distance between the carbonyl, hydrocarbon, and styrene functionalities within the polymer itself is on the order of 100 A so that chemical inhomogeneity would be impossible to the degree to which variation was observed. It is proposed that the intensity variation is due to optical interference effects inherent in thin film spectroscopy. This is a problem of greater magnitude in reflection experiments than in transmission experiments. Thickness variations in the original sample cause the spectra to vary from spot to spot. The coating erosion during exposure accounts for the variations during weatherivg. An illustration of the film erosion process as indicated by the general decrease in the hydrocarbon intensity at 2960 cm-’ as measured in transmission is presented in Figure 3. The optical interference phenomenon as the basis of the variation of intensity with film thickness will be discussed in more detail in the next section. Also discussed below will
‘t
c ) MELAMINE
1
2 3 LOCAT I O N
4
Figure 4. Absorbance peak height ratios from a transmission spectrum of an acrylic melamine film on a KRS-5 substrate as a function of location: (a) z = carbonyl, (b) x = hydrocarbon, and (c) z = melamine.
be reduction of such problems in the transmission experiments. Films cast on KRS-5 plates and measured in transmission showed a real decrease in intensity ratio variation, even though the films were not of uniform film thickness. Four locations were examined (see Figure 41, and the greatest deviation from the mean was 10%. This provides further support for the belief that the coating system is indeed homogeneous and that the large variations described above are due to the reflection sampling technique. The transmission method has two additional advantages. First, thicker films can be used since the optical path length is less than half that in the reflectance mode. Thicker films can be weathered longer. A thin film becomes too fragile to survive the weathering protocol. Second, through the use of a simple sample holder made to fit the KRS-5 plates snugly, it is possible to sample the same spot on the coating, allowing film erosion rates to be estimated (see Figure 3) along with the rate of chemical composition change.
Discussion In the reflection experiment, the infrared radiation passes through an absorbing film, reflects from the substrate, and passes back through the film; the intensity of the radiation is diminished by the reflection efficiency on the substrate and the absorption by the film. This process is accurately described by classical electrodynamics. The resulting reflectivity is found from the Fresnel equations derived from Maxwell’s equations with the proper boundary conditions. Although this formalism has been derived in several ways (Heavens, 1965; Born and Wolf, 1970),there has been limited application of this theoretical basis to describe realistic experiments in infrared molecular spectroscopy. The parameters which must be considered in this treatment are the real and imaginary parts of the optical constants of the film and the substrate, which are functions of the frequency of the radiation. In addition, the experimental parameters such as the angle of incidence and the polarization of the light must also be accounted for. In the case of the typical transmission experiment, changes in the real part of the film optical constants play a minimal role in the infrared spectrum. When reflection techniques are used, however, band maxima may shift and the band shape may change due to large changes in the real part of the optical constant for the sample in the
670 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987
n0
Film
fd
nl
I
Substrate
n2
Figure 5. Diagram of film system to be modeled in calculations.
region of an absorption. This effect has been described previously (Allara, 1978) and can cause serious problems when interpreting reflection-absorption spectra. The optical constants of the supporting, reflecting substrate also have a significant effect on the resulting spectrum, since the electric field strength of the radiation in the vicinity of the interface and the intensity of the returning light depend on the nature of the substrate. Any changes in the interfacial region, oxide growth, or reactions between the substrate and the sample during the study can produce effects throughout the spectrum. However, by initially assuming that these possibilities do not present dire problems and assuming that the reflection geometry and polarization of the light is well controlled, film thickness is the variable which changes during the course of the weathering experiment. Film thickness, along with the angle of incidence and the optical constants of the film, determines the path length of the radiation in the sample. As discussed above, the film thickness used is, generally, of the same magnitude as the radiation wavelength. As the experiment proceeds, the thickness is diminished from the air interface, while other chemistry may be proceeding in the "bulk" of the film. The result is not only film thinning but, undoubtedly, surface roughening. The film thickness is bound into the physics of the reflection process and, thus, is related to the reflection intensity through the wavelength of the radiation. The amplitude of the reflected ( R )and transmitted (2') radiation [or that polarized parallel (p) or perpendicular (s) to the plane of reflection (see Figure 5) as given by Heavens (1965) is (Heavens, 1964)
d
= 2 ~ ~ n cos ,d 4
where the r, and t, quantities are the appropriate Fresnel coefficients, d is the film thickness, I$ is the angle of incidence, n, is the refractive index of the film, and z, is the frequency of light in wavenumbers (cm-'). One example of the interaction of the light with a film is the fringe pattern that occurs in the transmission spectrum of the polystyrene film commonly used as a wavelength standard in the mid-infrared. The films are thinner in a reflection experiment, and the effect of the interference pattern may be even greater, causing a great deal of distortion in the spectrum. The fringe patterns are not usually apparent in our typical reflection spectrum because the beam is converging and the sample area is large enough to include a varied path length. The result is an averaged spectrum in regions where there is no absorption. Due to the way in which the optical resonances are produced in regions of absorption, the averaging process does not result in this
500
IWO I500 WAVENUMBERS
2600
Figure 6. Contours computed for the extinction contribution to the 2000-500-cm-' region for films of thickness in the range 3-9 pm. The experimental conditions are reflection a t 12 deg to the normal with both polarizations for a film with a refractive index of n = 1.45 and k = 0.008 on gold, n = 12 and k = 85.
500
2600
Io00
WAVENUMBERS
Figure 7. Intensity contours for the extinction contribution to the 200C-500-cm-' region for films of thickness in the range 3-9 pm. The experimental conditions are transmission a t normal incidence with both polarizations for a film with a refractive index of n = 1.45 and k = 0.008 on KRS-5, n = 2.4 and k = 0.
benign effect in regions of strong absorption. To demonstrate this problem, a Fortran program was written to compute and plot the effect on the reflection intensities of changes in the film thickness with wavelength at constant extinction. The extinction coefficient ( K ) is related to the absorption coefficient (a) as the product of the frequency (3) by a = 4ac~k
where c is the speed of light. The assumptions made in these calculations were that all films are homogeneous and uniform in thickness. Band shapes and maximum frequencies are ignored. Extinction is assumed to exist at all frequencies; i.e., absorption is treated as a uniform process over the entire spectral region. If the spectrum was obtained in the absorbance mode by using a nonabsorbing film of identical refractive index and corresponding thickness as a reference sample, the spectra would appear similar to trace lines in Figures 6 and 7 . The intensity, then, indicates only that part of the total intensity due to the extinction. In Figure 6, the surface represents a film on a highly reflective substrate, such as gold, subjected to
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 671 thinning from 9 to 3 pm. The extinction assumed is that of a moderate absorber under these conditions. The geometry assumed is 30 deg from the normal. Again, a constant extinction (not absorption) coefficient is used in the computation so that an increasing intensity trend is anticipated from low to high wavenumbers. The nonlinear variation with thickness at 2000 cm-l is a clear example of the type of interference present in an experiment where film thickness is changing. In terms of intensity ratios, consider the ratio of a 750-cm-' absorption to one at 1750 cm-'. At 9 pm a 750-cm-l band is at a relative minimum while a 1750 cm-l band is at a maximum. At 3 pm, both frequencies are at approximate maxima. Thus, the ratio data would imply chemical change if the two frequencies are representative of two different functional groups. In fact, no chemical changes have occurred but only thinning of the film. Similar results are produced for all metal substrates. Some improvement in the magnitude of undulation is achieved when the geometry is more nearly normal. However, the best results are predicted when the experiment is performed in the transmission mode. The plot in Figure 7 represents an experiment where the film is supported on a KRS-5 window. In this case, the deviation from linearity with thickness at 2000 cm-l is considerably improved over the case for reflection. Even under these circumstances, oscillations in the intensity due only to the extinction deviate from the ideal flat behavior, so one must always be careful not to overinterpret changes in intensities for spectra of samples where sample thickness is not controlled or accounted for in a rigorous fashion.
Conclusion This empirical and theoretical study has demonstrated fundamental limitations in infrared techniques applied to quantification for film samples with different thicknesses. As the film thickness changes, the corresponding relative intensity ratios, commonly used to access chemical composition, also change. Spectral changes, however, can occur even when no chemical differences exist. This is demonstrated by large spot-to-spot variations found in the spectral intensity ratios for films prepared on gold substrates and measured by the reflection technique. The theoretical description of the reflection method indicates that large absorption variations are expected if the film samples are not of the same thickness. Thus, when
chemical changes are combined with changes in the sample film thickness, the determination of the extent of chemical change is difficult at best. Transmission spectroscopy has been shown to reduce the range in variation produced by the small changes anticipated in the weathering experiments. Theoretically, this is the extent to which one can hope to improve the situation since similar but less dramatic variations are predicted for the transmission experiment. It is still essential that great care be used to locate the sample in the same position for each spectrum to ensure that the same region be sampled in every case. This is essential since typical samples prepared for weathering are not uniform in thickness and if the sample thickness variation is mixed in with the regular thinning due to weathering, the results will be further degraded. Therefore, we have decided to use windows fixed into metal holders which mount into the standard spectrometer bracket. During the course of an experiment, this bracket is not moved or adjusted. By taking these precautions, the unexpected variations in the intensity ratios are minimized. Registry No. (Acrylic acid)(butyl methacrylate)(styrene)(Cyme1 325)(hydroxyethyl acrylate)(ethylhexyl acrylate) (copolymer), 106568-06-3.
Literature Cited Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978,11, 1215. Bauer, D. R. J . Appl. Polym. Sci. 1982, 27, 3651. Bauer, D. R.; Briggs, L. M. In Characterization of Highly Crosslinked Polymers; Labana, s. s.,Dickie, R. A., Eds.; ACS Symposium Series 243; American Chemical Society: Washington, DC, 1984; p 271. Bauer, D. R.; Dickie, R. A. J. Polym. Sci., Polym. Phys. 1980, 18, 1997. Bauer, D. R.; Dickie, R. A.; Koenig, J. L. J . Polym. Sci., Poly. Phys. 1984, 22, 2009. Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1970. Gerlock, J. R.; Bauer, D. R.; Briggs, L. M. In Characterization of Highly Crosslinked Polymers; Labana, S. S., Dickie, R. A., Eds.; ACS Symposium Series 243; American Chemical Society: Washington, DC, 1984; p 285. Heavens, 0. S. In Physics of Thin Films; Hass, G., Thun, R. E., Eds.; Academic: New York, 1964; Vol. 2. Heavens, 0. S. Optical Properties of Thin Solid Films; Dover: New York, 1965.
Received for review March 31, 1986 Accepted December 5 , 1986
