Anal. Chem. 1996, 68, 2477-2481
Qualitative Identification of Fumaric Acid and Itaconic Acid in Emulsion Polymers Frank Cheng-Yu Wang,* John G. Green, and Bruce B. Gerhart
Analytical Sciences Laboratory, The Dow Chemical Company, Midland, Michigan 48667
A pyrolysis gas chromatography (Py-GC) technique has been used for the qualitative analysis of fumaric acid and itaconic acid as low-level monomers polymerized with other major monomers in emulsion polymers. In order for fumaric acid and itaconic acid to be detected through pyrolysis, the acids are derivatized with primary amines such as methylamine and ethylamine to form a cyclic imide. The detection of derivatized fumaric acid and itaconic acid is accomplished by atomic emission detection (AED). The structures of the derivatization-pyrolysis products have been elucidated by mass spectrometry. Fumaric acid and itaconic acid are widely used as comonomers in various types of polymer systems to supply functionality. For example, fumaric acid has been used in polymer blends as a compatibilizer to enhance the grafting between two different types of polymer systems, such as poly(phenyl oxide) and polystyrene. Fumaric acid and itaconic acid have also been used in emulsion polymer (latex) systems to increase copolymer strength, provide stability, enhance adhesion, expand swellability in water, and define the surface chemistry. Because these acids are normally added at low concentrations (1-2% by weight), their qualitative and quantitative analysis is a difficult task. Traditionally, the unreacted fumaric acid and itaconic acid monomers can be separated from the polymer by selective solvent extraction or directly collecting serum from the emulsion polymer system. The unreacted-acid-containing solutions can be extracted and filtered to permit identification and quantitative analysis by a suitable liquid chromatography (LC) technique.1,2 The analysis of fumaric acid and itaconic acid in the polymer chain, however, is typically determined by acid-base titration. Even with multiple endpoint titration curve analysis, which may provide information on the cumulative strengths of the polymerized fumaric acid and itaconic acid, there is no effective direct technique for the qualitative and quantitative analysis of fumaric acid and itaconic acid in the polymer chain. Pyrolysis gas chromatography (Py-GC) employing various detection systems is the technique usually used to qualitatively and quantitatively analyze major components and low-level additives in polymers.3-5 The technique utilizes thermal energy to break down polymers to monomers and small oligomers. The mixture of pyrolysis products is directly passed into a gas chromatograph for separation. However, there are numerous low(1) Blake, J. D.; Clarke, M. L.; Richard, G. N. J. Chromatogr. 1987, 398, 256. (2) Badoud, R.; Pratz, G. J. Chromatogr. 1986, 360 (1), 119. (3) Irwin, W. I. Analytical Pyrolysis: A Comprehensive Guide; Marcel Dekker: New York, 1982. (4) Liebman, S. A.; Levy, E. J. Polymer Analysis; Marcel Dekker: New York, 1985. (5) Wampler, T. P. J. Anal. Appl. Pyrol. 1989, 16, 291. S0003-2700(96)00025-X CCC: $12.00
© 1996 American Chemical Society
level comonomers and additives that may not be appropriately separated at the same time as the major monomers. These lowlevel comonomers and additives frequently appear with poor peak shape under the chromatographic conditions established for analysis of the major monomers (such as a polar additive in a nonpolar capillary column). Additionally, these peaks may have been overlooked because they exist as converted products in the chromatogram after the pyrolysis-induced reaction (such as vinyl acetate converted to acetic acid). In general, the acid functional group is not suitable for analysis by pyrolysis. The major pyrolysis reaction mechanism for the acid functional group in polymer chains is decarboxylation, as shown below:
The pyrolysis of all vinylcarboxylic acids results in decarboxylation to the vinyl portion of the molecule. This vinyl moiety product is the same as the fragment products from other common monomers. The analysis of vinylmonocarboxylic acids (acrylic acid and methacrylic acid) in the polymer chain has been achieved by direct pyrolysis with a solvent trapping method6 and by a derivatization method, whereby methylation is performed on the acid groups.7,8 Simultaneous pyrolysis methylation or other derivatization reactions are reported for polyesters, phenolic resins, and other materials that yield polar, poorly separated pyrolysis products.9-11 The methylated functional group or other derivatives survive through the pyrolysis reaction to give the ester products, which are very stable for GC and GC/MS analysis. Most derivatizing agents require particular reaction environments to achieve complete reaction. For instance, as a result of competitive reactions, water is prohibited in carboxylic acid functional group methylation. This fact causes the limitation that methylation cannot be used in aqueous emulsion polymer systems (latex). Although the methylation can be performed after the latex is dried, the extent of derivatization is not reproducible because the form of the latex was changed. To have complete open access between derivatizing agents and the target functional groups (6) Wang, F. C.; Gerhart, B. B.; Smith, C. G. Anal. Chem. 1995, 67, 3681. (7) Cutie`, S. S.; Buzanowski, W. C.; Berdasco, J. A. J. Chromatogr. 1990, 513, 93. (8) Buzanowski, W. C.; Cutie`, S. S.; Howell, R.; Papenfuss, R. J. Chromatogr. A 1994, 677, 355. (9) van der Peyl, G. L. Q.; Linnartz, T. C. T.; van Rossum, C. A. J. J.; Zeelenberg, M. J. Anal. Appl. Pyrol. 1991, 19, 279. (10) Challinor, J. M. J. Anal. Appl. Pyrol. 1991, 20, 15. (11) Challinor, J. M. J. Anal. Appl. Pyrol. 1991, 16, 323.
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(carboxylic acid), it is always preferable to perform the derivatization reaction in the liquid phase. However, limitations exist in finding a solvent that suitably swells the polymer. The miscibility between polymer solutions and the derivatizing agent solution remains the major challenge. In this study, a Py-GC technique has been used for the qualitative analysis of fumaric acid and itaconic acid in the emulsion polymer as low-level comonomers polymerized with other major monomers. In order for fumaric acid and itaconic acid to be detected through pyrolysis, the acids are derivatized with primary amines such as methylamine and ethylamine to form a cyclic imide. The detection of derivatized of fumaric acid and itaconic acid is accomplished using an atomic emission detection (AED). The peaks used for acid identification have been identified by mass spectrometry. Other techniques have also been used to analyze fumaric acid and itaconic acid in polymers, such as infrared spectroscopy (IR)12 and nuclear magnetic resonance spectroscopy (NMR).13 Both techniques are able to identify the existence of the acid functional groups at the concentration levels examined here, but they are inadequate for use in further qualitative identification because of a lack of significantly different functional groups between the fumaric acid and itaconic acid and/or interferences from other monomers in the polymer, as well as a lack of sensitivity at lower acid concentrations. EXPERIMENTAL SECTION Sample Preparation. (1) Derivatizing Agents. The methylamines (40 wt % in water, Catalog No. 42,646-6; 2 M in THF, Catalog No. 39,505-6) and the ethylamines (70 wt % in water, Catalog No. 15,639-6; 2 M in THF, Catalog No. 39,507-2), purchased from Aldrich Chemical Co., were used without further purification. (2) Vinyldicarboxylic Acid Copolymer Standard. The styrene/maleic anhydride copolymer (50/50, Catalog No. 049), purchased from Scientific Polymer Products, Inc., was used without further purification. A 5% by weight standard solution was prepared by dissolving appropriate copolymer powder in water (pH > 7) to obtain the desired concentration. (3) Emulsion Polymer Preparation. (a) Emulsion Polymers (Latex). Two latexes, A and B, were made in our laboratories. Latex A had 49.5% solid polymer which contained (by weight) 50% styrene, 48% butadiene, and 2% fumaric acid, while latex B had 56.3% polymer solids which contained (by weight) 46% styrene, 52% butadiene, and 2% itaconic acid. (b) Swelling Emulsion Polymer with Tetrahydrofuran (THF). Each latex (1.0 mL) was mixed with 1.0 mL of distilled water and then mixed with 10 mL of methanol, followed by mixing with 0.2 mL of 1 N HCl solution (Fisher Scientific Co. Catalog No. SA48-4), and centrifuged to separate polymer solid and serum. The solid portion was collected and mixed with 10 mL of THF. The polymer solid/THF solution was shaken until all the solid polymer had swollen. (4) Derivatization. Primary amine (2.0 mL, 2 M in THF) solution was added to each swollen latex solution. Two drops (∼0.2 mL) of concentrated primary amine (40 wt % in water) solution was added into the standard styrene/maleic acid (5 mL) (12) Chattopadyay, B.; Balasubramanian, K.; Ramaswamy, D.; Srinivasan, K. S. V. Polym. Commun. 1990, 31 (1), 15. (13) Barboiu, V.; Streba, E.; Luca, C.; Simionescu, C. I. J. Polym. Sci., Part A: Polym. Chem. 1995, 33 (3), 389.
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solution. All samples were shaken for 30 min. The standard copolymer solution was used directly for the Py-GC/AED and PyGC/MS studies. The latex solvent was removed by placing the mixture on a hot plate (temperature 80 °C) until all the solvent had vaporized. Instrumentation. (1) Pyrolysis Gas Chromatography/ Atomic Emission Detection (Py-GC/AED). Samples of derivatized latex solid were weighed (∼300 mg) into quartz tubes; the quartz tube was then placed into a platinum (Pt) coil-type pyroprobe. A sample (∼3 µL) of derivatized standard styrene/ maleic anhydride copolymer solution was deposited onto a Pt ribbon-type pyroprobe. All pyroprobes were equilibrated for 10 min in a 250 °C interface connected to the injection port of a Hewlett-Packard Model 5890 gas chromatograph equipped with a 5921A atomic emission detector. Samples were pyrolyzed (CDS 120 Pyroprobe) at a set temperature of 700 °C, with a maximum heating ramp (∼20 °C/ms) for a 20 s interval. The pyrolysis products were carried by the helium carrier gas through the injection port. The separation was performed on a fused-silica capillary column (J & W DB-5, 30 m × 0.25 mm i.d., 0.5 µm film) using a linear temperature program (40 °C for 8.5 min, then 10 °C/min ramp to 250 °C and a 15 min hold), with 10 psi head pressure, 1.0 mL/min carrier gas flow rate, and a 20:1 split ratio. The signal was recorded with 4.5 min delay. The transfer line between GC and the AED was a deactivated fused-silica capillary column. The maximum temperature of the transfer line was set at 250 °C. The GC/AED used O2 and H2 as reagent gases (flow rate, 0.07 mL/min for both gases), with detection at 174 and 193.1 nm emission lines for nitrogen and carbon, respectively. (2) Pyrolysis Gas Chromatography/Mass Spectrometry (Py-GC/MS). For the Py-GC/MS study, the Py-GC conditions above were the same, except the separation was performed on a fused-silica capillary column (J & W DB-5MS, 30 m × 0.25 mm i.d., 1.0 µm film) using a linear temperature program (40 °C for 4 min, then 10 °C/min ramp to 280 °C and a 12 min hold). The mass spectrometer is a Fisons TRIO-1 quadrupole system. An electron ionization mass spectrum was obtained every second over the range 29-350 Daltons, with the detector multiplier at 425 V, a scanning electron current of 1500 mA, a source temperature of 180 °C, and the transfer line between GC and MS set at 280 °C. Safety Considerations. The derivatization agent is a strong base and is corrosive. Personal protective devices should be used when performing the derivatization reaction. The latex (solution) and styrene/maleic anhydride copolymer (powder) are considered as low-hazard materials, but any skin contact or inhalation of vapor/powder should be avoided. RESULTS AND DISCUSSION In this study, the copolymer of styrene/maleic anhydride (50/ 50) has been chosen as a reference standard. Maleic acid is a cis-form vinyldicarboxylic acid, compared with trans-form fumaric acid. After polymerization, the distinction between cis and trans acid is lost, because the former double bond is now a single bond. There is no structural difference between maleic acid and fumaric acid in the polymer chain. The derivatization reaction and pyrolysis degradation mechanisms14 are similar in those emulsion polymers containing fumaric acid and itaconic acid. The major reason for the choice of this copolymer as a reference standard is its high concentration of vinyldicarboxylic acid. There should (14) Sharp, J. L.; Paterson, G. Analyst (London) 1980, 105, 517.
Figure 1. AED carbon trace pyrogram of derivatized 50/50 styrene/ maleic anhydride copolymer standard. The highest peak in the pyrogram is that of styrene monomer. The inset shows an expression of the retention time between 14 and 24 min. The assignment of peaks labeled from 1 to 5 is given in Table 1.
Figure 3. Py-GC/MS total ion current (TIC) pyrogram of 50/50 styrene/maleic acid copolymer standard. Table 1. Py-GC/MS Peak Assignment of Derivatized 50/50 Styrene/Maleic Anhydride Copolymer
Figure 2. AED nitrogen trace pyrogram of derivatized 50/50 styrene/ maleic anhydride copolymer standard. The inset shows an expression of the retention time between 14 and 24 min. The assignment of peaks labeled from 1 to 5 is given in Table 1.
be less interference from other monomers or their fragments and the derivatized acid peak and its fragments. Consequently, the peaks of interest should be easier to distinguish and their retention times more clearly defined. For the same reasons, the Py-GC/ MS identification of the peaks of interest is facilitated with this reference polymer. Figures 1 and 2 show the carbon and nitrogen traces of the reference standard subjected to Py-GC/AED. The retention time index and relative peak intensity were reproducible to within 5% for all the pyrolysis products, as was demonstrated by six duplicate experiments performed using a standard polymer. Figure 3 shows the Py-GC/MS total ion current (TIC) of the same polymer. The peaks of interest have been structurally identified as listed in Table 1. The derivatization reaction mechanism of this study is based on the following reaction:
The pyrolysis products reflect the chain scission to saturated or
a
Order of intensity.
unsaturated derivatized products that contain the nitrogen-containing five-membered ring compounds (N-alkyl-2,5-pyrrolidinedione and 1-alkyl-1H-pyrrole-2,5-dione). The emulsion polymers used in this study do not show any pyrolysis products of the derivatized acids without use of the swelling procedure. This indicates that the emulsion polymers need to swell to ensure that the polymer structure is sufficiently open to provide enough physical contact with the derivatizing agent. The swelling procedure described in the Experimental Section is adequate for styrene/butadiene types of emulsion polymers. Other types of emulsions may require other procedures and solvents in order to provide the best swelling results. Figures 4 and 5 show the carbon and nitrogen traces of polymer A (2% fumaric acid) Py-GC/AED results, and Figures 6 and 7 show similar traces for polymer B (2% itaconic acid). The nitrogen traces from reference standard (Figure 2) and both samples (Figures 5 and 7) show the same retention time of peaks, as labeled peaks 2, 3, 4, and 5. This is evidence that low levels of fumaric acid and itaconic acid in the polymer have been successAnalytical Chemistry, Vol. 68, No. 15, August 1, 1996
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Figure 4. AED carbon trace pyrogram of emulsion polymer sample A (styrene/butadiene/fumaric acid 50/48/2). The inset shows an expression of the retention time between 14 and 24 min.
Figure 6. AED carbon trace pyrogram of emulsion polymer sample B (styrene/butadiene/fumaric acid 46/52/2). The inset shows an expression of the retention time between 14 and 24 min.
Figure 5. AED nitrogen trace pyrogram of emulsion polymer sample A (styrene/butadiene/fumaric acid 50/48/2). The inset shows an expression of the retention time between 14 and 24 min. The assignment of peaks labeled from 2 to 5 is given in Table 1.
Figure 7. AED nitrogen trace pyrogram of emulsion polymer sample B (styrene/butadiene/fumaric acid 46/52/2). The inset shows an expression of the retention time between 14 and 24 min. The assignment of peaks labeled from 2 to 5 is given in Table 1.
fully derivatized and that the derivatized products have been detected. The vinyldicarboxylic acid is different in polymer A (2% fumaric acid) and polymer B (2% itaconic acid), but the major pyrolysis products of the derivatized acid are the same. The only way to distinguish them is by the relative peak intensity of the derivatized acid peaks. The derivatized fumaric acid forms a high intensity peak 3 (1-methyl-2,5-pyrrolidinedione) compared with the itaconic acid, which has a high-intensity peak 5 (1-methyl-3-methylene2,5-pyrrolidinedione), which directly reflects their difference in the monomer structure. There is no evidence of the detection of peak 1 in both polymers as compared to the reference standard. This may be because the fragment is an unfavorable degradation product in the emulsion polymer. This result also can be used as an indication that there is no free vinyldicarboxylic acid in both polymers A and B (i.e., not bound into the polymer chain). The reference standard as well as the emulsion polymers was also derivatized with ethylamine. The pyrolysis peak patterns were similar to those for the samples derivatized with methylamine, except they all elute at a later time because of the high molecular weight of the fragments. This is additional evidence that the derivatization reaction worked and the pyrolysis products
have been effectively detected. This type of derivatization reaction should work with all primary amines. Which primary amine to use for derivatization reaction should depend on (1) the interference in the separation between monomers, fragments, and derivatized products [For example, if there is a nitrogen-containing monomer (such as acrylonitrile) in the latex, the pyrolysis peak patterns of acids that were derivatized with a specific primary amine may elute in a time region causing overlap with the nitrogen-containing fragments. This will increase the difficulty of the peak pattern identification.] and (2) the percent yield of the derivatization reaction and the pyrolysis/detection efficiencies. In this study, the pyrolysis efficiency and the ability to detect acids derivatized with ethylamine are lower than those for acids derivatized with methylamine. The methylamine derivatization is a better choice for this case. From the GC detector point of view, the AED played an important role in detecting the derivatized acids. The carbon traces of the emulsion polymer samples (Figures 4 and 6) show numerous overlapping components in the retention time region of interest. Without elemental-selective detection, it is difficult to clearly reveal the peak pattern of interest for comparison with the reference standard. The mass spectrometer detector always
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has powerful direct identification capability, but in this case, the mass spectrum in any retention region was too complicated to draw any positive identification for the acid-derivatized pyrolysis products. CONCLUSION The determination of vinyldicarboxylic acids in emulsion polymers is extremely difficult because the low-level acids do not survive through pyrolysis and traditional derivatization techniques cannot be applied due to the presence of the aqueous environment. The combination of derivatization with primary amine solutions and detection with the atomic emission detector makes vinyldicarboxylic acid qualitative analysis possible. The sample prepara-
tion in this study is very critical, especially in the derivatization reaction, to assure that dicarboxylic acids have sufficiently bonded with the amines. This study demonstrates that specific element detection can provide the selectivity necessary for reliable peak pattern recognition. The use of multiple peaks for detection of the acids helps avoid interference.
Received for review January 11, 1996. Accepted May 14, 1996.X AC960025+ X
Abstract published in Advance ACS Abstracts, June 15, 1996.
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