Determination of Functional Groups in Copolymers by Infrared Spectrometry F. D. BRAKO and A. S. WEXLER Dewey and Almy Chemical Division, W.
b A useful technique is described for differentiating the presence or absence of functional groups such as hydroxyl, carboxylic acid or ester in a difficult situation such as in the analysis of polymers containing small percentage components of such groups. Films of latices or polymers are subjected to chemical treatment which results in marked changes in the infrared spectrum which can be associated with the disappearance of a functional group or its replacement by another functional group. The changes in the spectrum resulting from specific chemical reaction have application to analysis of organic compounds in general as well as for polymers.
T
HE DETECTION and determination of functional groups in organic compounds and polymers are major analytical problems usually solved by wet chemical methods involving specific reagents which react with specific groups in the molecule. Quantitative results are obtained by application of stoichiometric reactions such as titration with alkali, bromination, and acetylation. Qualitative results are obtained by class reactions and formation of derivatives. Polymers and large organic molecules which do not constitute well defined chemical entities, such as plasticizers and rosinates, do not readily yield to conventional wet chemical methods. The combination of infrared spectroscopy with wet chemical analysis is very powerful in analysis of polymers, surfactants, plasticizers, and other high molecular weight materials for determination of functional groups. Infrared data readily may be interpreted negatively so that one may definitely preclude the presence of hydroxyl, carbonyl, amine, amide, nitrile, ester, carboxylic, aromatic, methylene, tertiary butyl, and terminal vinyl groups if the corresponding group vibrations are absent in the infrared spectrogram. More difficult is the assignment of functional groups where multiple or several alternate possibilities exist as in the mixture of a carboxylic and a keto group or in the assignment of a band to an olefinic group. Of course, one can carry out sequences of reactions in test tubes and submit
1944
ANALYTICAL CHEMISTRY
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each step to infrared analysis. Much time can be saved and fewer false steps taken if organic materials can be examined by the spectroscopist at each step or stage in analysis, since infrared will give a detailed picture of many of the functional groups and the results or chemical reactions or transformations. Hence, the infrared spectroscopist must work in close collaboration with the organic or polymer chemist for maximum results. The analysis of high molecular weight material of limited solubility in solvents may pose a problem in wet chemical analysis for determination of chemical groups. A way to bypass this problem is to obtain infrared reflectance spectra of these as films, layers, or deposits of samples. These thin films or layers will readily react under appropriate conditions with selected reagents. An infrared spectrum of the film may be recorded directly superimposed over the original spectrum after carrying out an appropriate reaction. In this way the smallest details of change may frequently be detected by this sort of "in situ" functional group determination by infrared spectrometry. The technique is particularly appropriate in the analysis of film formers. EXPERIMENTAL
Surfactants were cast as films from solutions in solvents on sheets of aluminum with a highly reflecting surface. Polymer latices were spread as thin films on aluminum sheets. Films were allowed to dry in a stream of warm air. Infrared spectra were obtained a t moderately high resolution conditions over the range of 4000 to 400 cm.-' on a Beckman I R 9 spectrophotometer. Films were mounted in a Beckman specular reflectance attachment. Detection of the Carboxylate Group (Figure 1). The salt or carboxylate form of an organic acid is characterized by an intense, broad band a t about 1550 em.-' On exposure to HCl vapor, the 1550 cm.-' peak disappears. A new, somewhat less intense band appears at 1715 cm.-' This band is characteristic of the carbonyl group in carboxylic acids. Many changes are also observed in the skeletal vibration region. Noteworthy is the loss of sharp bands on exposure to HCl. Hydrogen bonding and loss of crystallinity may account for many of
the differences in the spectra in the salt and the acid forms in this region. Detection of the Carboxylate Group (Figure 2). The carboxylate band in rosinates is centered at about 1545 cm.-l or a somewhat lower frequency than is observed in sodium oleate. Exposure of the rosinate film to HC1 vapor results in disappearance of the 1545 cm.-' carboxylate band and its replacement by a somewhat less intense but strong band at about 1705 cm.-l which is again a t a somewhat lower frequency than the corresponding carbonyl band of the oleate. The band a t 1705 cm.-1 is associated with the carbonyl stretching frequency of the carboxylic acid group in rosin. Many other prominent changes are observed in the spectra of the film. Noteworthy is the appearance of fairly strong bands a t 1130, 1175, and 1235 cm.-' in the acid. Detection of the Carboxylic Acid Groups (Figure 3). The broad intense carbonyl band of the free acid a t about 1700 cm.-1 in rosin acid disappears, on exposure of the film to ammonia vapor. i i n intense broad band associated with Carboxylate a t 1550 cm.-l is overlapped with the methylene band a t 1460 cm.-' and with the ammonium ion band a t about 1395 cm.-l Broad bands possibly associated with the ammonium ion are also observed a t 1920 and 2200 cm.-l Comparison of the rosin acid with the acid form of the rosinate in the previous figure indicates a relatively lower aromatic content in the rosin acid judging by the intensity of the sharp band a t 1496 cm.-1 The rosin acid also differs from the acid form of the rosinate in showing a fairly strong overall absorption a t about 1025 cm.-l Detection of Ammonium Salt of a n Oxganic Acid (Figure 4). The instability of the ammonium salt is demonstrated by the complete restoration of the spectrum of the free acid after heating since the spectrum is almost identical with the spectrum of the original film of the rosin acid. There is B difference, however-losa of a band a t 1025 cm.-l which may be due to a volatile component. Detection of the Carboxylate Group in a Polymer (Figure 5 ) . Exposure to acid results in disappearance of the broad, intense band associated with carbovylxte group in polyacrylate ion a t around 1600 cm.-l Appearance of a broad intense absorption a t about 1720 cm.-1 is associated with the carbonyl of the carboxylic acid group in the
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VOL. 35, NO. 12, NOVEMBER 1963
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ANALYTICAL CHEMISTRY
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with ----- in Figure 5). The chemical transformations accomplished are ester --t carboxylate salt + carboxylic acid. Detection of Hydrates of Salts (Figure 8). The strong band in the saponified residue a t 1670 ern.-' was suspected to be associated with water of crystallization or hydration. Brief heating of the film did result in a pronounced decrease in absorption a t this wavelength. Detection of Unsaturation in Polymers (Figure 9). Brief exposure of a thin film of polybutadiene to bromine vapor results in marked changes in the infrared spectrum. Noteworthy is the almost complete disappearance of bands a t 730, 910, 965, and 1640 cm.-' associated with unsaturation. A pronounced band possibly associated with a C-Br vibration appears a t 550 em.-' Bands also appear a t 785, 1145, and 1250 cm.-' due to exposure to bromine vapor. Detection of Unsaturation in Polymers (Figure 10). Exposure of butadiene-styrene copolymer to bromine vapor results in the disappearance of bands a t 910 and 965 em.-' associated with unsaturation in the butene component of the copolymer. Some alteration of the phenyl bands a t 700 and 765 em.-' is evident. The loss of a band at 1550 cm.-l and the appearance of a band a t 1700 ern.-' are probably due to the action of acidic vapors on the carboxylate surfactant of the latex.
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____________ Copo'ymer of butadiene and styrene Copolymer of butadiene and styrene after exposure to bromine vapor
polymer. Significant changes are also observed in the 1100 to 1200 em.-' region. Heating of .,he acidified film rebulted in minor changes in the spectrum. Detection of the Carboxylic Acid Group in a Copolymer (Figure 6). Bands associated with the carboxylic acid carbonyl stretching frequencies a t 1715 to 1740 cm.-ldisappear on exposure to amnionia vapor. A ne11 defined carboxylate band appears a t 1570 crn.-l This change is sufficient to confirm that the copolymer contains carboxylic acid groups.
Detection of the Ester Group in a Polymer (Figure 7). Bands associated with the carbonyl stretching frequency of the ester st 1735 cm.J and with the C-0 bending vibrations in 1150 to 1250 cm.-l region disappear after treatment of the polymer with potassium hydroxide in methanol solvent. An intense carboxylate band is observed a t 1590 cm.-1 in the saponified residue film. Expozure of the film of saponified residue to HC1 vapor results in a further remarkable change in the spectrum which is equivalent to the spectrum of polyacrylic acid (compare
The detection of carboxylate, carboxylic, ester, olefinic, and other functional groups in polymers is demonstrated by obtaining infrared spectrograms of films before and after an appropriate treatment with a diagnostic reagent. The specular reflectance technique is particularly appropriate for analysis of films of polymers cast from latex, solvents, or from solid state by pressing. Thin films readily react n-ith appropriate reagents in vapor or liquid state. Analyses of other groups such as hydroxyl, amino, etc. is also possible. RECEIVEDfor review April 18, 1963. Accepted August 16, 1963. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1963.
Correct ion Spectrophotometric Estimation of Zirconium a s Reduced Molybdosulfatozirconic Acid In this article by G. C'. Dehne aud M. G. Alellon [ A N ~ CHEM. L 35, 1382 (1963)l on page 1383, in Table I, under column At, the final figure should read 0.275.
VOL. 35, NO. 12, NOVEMBER 1963
1947
