duplicate by the continuous procedure ( I ) , in which the HCN was volatilized away from interferences and collected in pure 0.03M NaOH for measurement. Procedure was exactly as described in the literature, except for one difference: the recovered HCN was measured potentiometrically instead of amperometrically. Results are summarized in Table 111, with individual determinations listed, rather than the averages of duplicates, so that some idea of the precision of the method may be conveyed. Not only is the precision of both methods good, but agreement between them is excellent, the root mean squared relative difference between them being around 2 %. Attempt to Apply the Potentiometric Procedure to Low Cyanide Grasses. Toward the end of this investigation, considerable interest arose in the analysis of low cyanide materials, particularly in some varieties of corn which are resistant to insects and root worm. Consequently, several materials, including low cyanide Sudan grasses, and corn and tomato leaves, were analyzed potentiometrically and also by
the volatilization procedure, as described above. The blank was determined for each material by sparging. The potentiometric method gave cyanide contents (2-13 ppm) that ranged up to twice as high as the cyanide contents determined by volatilization. Until this discrepancy is resolved, the potentiometric method is not recommended for hydrolyzates of low cyanide materials. ACKNOWLEDGMENT
Thanks are extended to L. E. Schrader of the Department of Agronomy for his cooperation in supplying the grass samples. RECEIVED for review January 7, 1971. Accepted March 2, 1971. This work was supported in part through a postdoctoral fellowship for D. B. E. provided by Hatch Funds from the U. S. Department of Agriculture, and in part through Atomic Energy Commission Grant No. AT(11-1)1082.
Analysis of Acrylic Polymers Using Combined Zeisel Reaction-Gas Chromatography and Infrared Spectrometry D. G. Anderson, K . E. Isakson, D. L. Snow, D. J. Tessari, and J. T. Vandeberg DeSoto, Inc., Administratiue and Research Center, 1700 S . Mt. Prospect Road, Des Plaines, Ill,
A quantitative method for the analysis of acrylic polymers was developed using the Zeisel reaction and gas chromatography. The rate of the cleavage reaction was studied and found to depend on the monomer composition and/or the stereoconfiguration of the polymer. Also, the reaction products from hydroxylcontaining monomers were identified. Infrared techniques were developed for the quantitative determination of styrene and the qualitative identification of acrylic acid, methacrylic acid, and the half ester of maleic acid.
THESUPERIOR FILM properties and durability of thermosetting acrylic polymers make them extremely popular as vehicles in coatings compositions. These resins have diverse applications in automobile, container, and home appliance finishes. With increased acceptance by the coatings industry, it has been necessary to develop an accurate and reliable method for the analysis of thermosetting acrylic resins. A survey of the literature indicates that no comprehensive and reliable technique exists for the analysis of acrylic polymers. Previous workers have proposed pyrolysis-gas chromatography for the chracterization of polymers (1-8). This (1) J. Strassburger, G. M. Brauer, M. Tyron, and A. F. Forziati, ANAL.CHEM., 32, 454 (1960). (2) E. A. Radell and H. C. Strutz, ibid., 31, 1890 (1959). (3) R. S. Lehrle and J. C. Robb, Nature, 183, 1671 (1959). (4) S . Straus and S. L. Madorsky, J . Res. Nut. Bur. Stand., A., 50, 165 (1953). (5) F. A. Lehrnann and G. M. Brauer, ANAL.CHEM.,33, 673 (1961). (6) K. Ettre and P. F. Varadi, ibid., 34, 752 (1962). (7) Ibid., 35, 69 (1963). (8) G. G. Esposito and M. H. Swann, J . Gas Chromatogr.,3,282 (1965).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
procedure is good for qualitative polymer analysis but has limited quantitative application. Determination of maleate and acrylate esters in polymers by Zeisel cleavage of the acrylate ester linkages, followed by gas chromatographic analysis of the reaction products, has also been reported (9-18). We chose the Zeisel reaction (Figure 1) for the cleavage of the acrylic esters and gas chromatography for the analysis of the alkyl iodides formed. Using this procedure, the recovery of alkyl iodides is greater than 95 for polymers containing between 10 and 90% of the methyl, ethyl, and butyl esters of acrylic and methacrylic acid. In addition, the use of isopropylbenzene as the trapping solvent allows the determination of all C1 to Cd alkyl iodides. Quantitative cleavage of the acrylate and methacrylate esters is also observed in the presence of modifying monomer units such as styrene. In order to achieve specific properties, acrylic polymers are often modified by copolymerization with monomers such as styrene, vinyl acetate, vinyl chloride, and acrylamide. The Zeisel-gas chromatographic procedure does not permit the (9) J. Haslarn, J. B. Hamilton, and A. R. Jeffs, Analyst,83, 66 (1958). (10) D. L. Miller, E. P. Sarnsel, and J. G. Cobler, ANAL.CHEM., 33, 677 (1961). (11) R. Kretz, 2. Anal. Chem., 176, 421 (1960). (12) A. Steyerrnark, J. Ass. Ofic. Agr. Chem., 38, 367 (1955). (13) S. Ehrlich-Rogozinsky and A. Patchornik, ANAL.CHEM.,36, 840 (1964). (14) W. C. Easterbrook and J. B. Hamilton, Analyst, 78, 551 (1953). (15) W. Kirsten and S. Ehrlich-Rogozinsky, Mikrochim. Acta., 4, 786(1955). (16) S. Vertalier and F. Martin, Chim. Anal. (Paris), 40, 80 (1958). (17) K. F. Sporek and M. D. Danyi, ANAL.CHEM., 34,1527 (1962). (18) G. Castello, G. D’Arnato, and E. Biagini, J. Chromatogr., 41, 313 (1959).
quantitative estimation of these monomer units. Qualitative identification of modifying monomer units is normally ascertained using infrared (19) and nuclear magnetic resonance spectrometry (20). In some cases, these techniques permit the quantitative estimation of these monomer units. If an unknown polymer contains only one monomer other than an ester of acrylic or methacrylic acid, its concentration may be determined by difference. If more than one modifying monomer is present, an “absolute” procedure for one or more components must be used. Styrene has been analyzed using a variety of chemical and instrumental techniques (21-26). However, no reference is given to the absolute determination of styrene using infrared spectrophotometry, an approach which we feel gives the most utility. The styrene band at 700 cm-l, which is the phenyl ring out-of-plane bending mode, is used because it provides specificity, freedom from interferences, and an absorption that is directly proportional to the styrene content. The presence of carboxylic acid modification of acrylic polymers is generally confirmed by titrimetric methods (27,28). These procedures determine the total acid equivalents but do not identify the acid present. Careful examination of the carbonyl region in the infrared spectra of cast films of acid containing acrylic polymers indicates a difference in the position of the acid carbonyl band when different acids are present. This difference allows the discrimination of acrylic, methacrylic, and the half ester of maleic acid when these monomer units are incorporated into a polymer above 10 weight per cent. EXPERIMENTAL
Apparatus. Gas chromatographic separations were performed using a Hewlett-Packard 5756 gas chromatograph equipped with flame ionization detector. A stainless steel column, 10 ft x 0.125-in. o.d., was packed with 20% wjw DC-401 on 60-80 mesh Gas Pak WAB. The helium flow rate was 10 ml/min with an inlet pressure of 65 psi. Injection port and detector temperatures were 210 and 250 “C, respectively, and the column temperature was 150 “C. Solution infrared spectra were obtained using a Beckman IR-12 infrared spectrophotometer, which was purged with dry air to minimize absorption caused by water vapor. The instrument was operated at a single beam to double beam ratio of l / l , constant slits of 3.0 mm and a pen period of 2. The samples were scanned between 650 and 750 cm-’ at 17 cm-l/min with a 4X abscissa scale expansion. A liquid cell, having potassium bromide windows and a path length of 0.062 mm was used. The effective cell path length was measured by the usual method (19). Cast film spectra were also obtained using a Beckman IR-12 infrared spectrophotometer. The instrument was operated at a single beam to double beam ratio of l / l , using the standard slit program, and a pen period of 2. The region from 1650 (19) L. C. Afremow, D. J. Tessari, K. E. Isakson, D. Netzel, and
J. T. Vandeberg, “Infrared Spectroscopy-Its Use in the Coatings Industry,” Federation of Societies for Paint Technology,
Philadelphia, Pa., 1969. L. C. Afremow, J. Pairit Techno/., 40, 503 (1968). J. T. Vandeberg, Appl. Spectrosc., 22,304 (1968). A. S. Wexler, ANAL.CHEM., 36, 1829 (1964). K. Ito and Y. Yamashita, Polym. Lett., 3, 625 (1965). A. V. Tobolsky, A. Eisenberg, and K. F. O’Driscoll, ANAL. CHEM., 31,203 (1959). (25) M. H. Swann, ibid., 25, 1735 (1953). (26) C. D. Miller and 0. D. Shreve, ibid., 28, 200 (1956). (27) F. Denes, N. Asadei, and C. Simionescu, ibid., 40, 629 (1968). (28) D. G. Anderson and D. J. Tessari, J . Paint Techno/., 42, 119
(20) (21) (22) (23) (24)
(1970).
/
\
/
\
Figure 1. Zeisel reaction between styrenated acrylic polymer and hydriodic acid
R
H, CHI, CsHj, CdHo; R’ = H, CH,
to 1750 cm-1 was scanned at 1 cm-l/min with an abscissa expansion of 10X . Reagents. “SpectroGrade” acetone, methyl ethyl ketone, hexane, 1,1,2-trichloroethane, and isopropylbenzene (Eastman Organic Chemicals) were used as received. Tetrahydrofuran (E.I. du Pont de Nemours) was distilled over triphenyl phosphite and nitrogen to remove any water and butylated hydroxy toluene inhibitor. After distillation, the tetrahydrofuran was stored in amber bottles over thin strips of copper metal to prevent the formation of peroxides. Hydriodic acid (Fisher Scientific Company) was freshly distilled over hypophosphorous acid and nitrogen. Only the 57% hydriodic acid azeotrope boiling at 127 “C was retained. The distillate was stabilized with hypophosphorous acid. Stereospecific poly(methy1 methacrylates) (Rohm and Haas) were used without further purification. Cationically polymerized polystyrene with a narrow molecular weight distribution was purchased from Arro Laboratories. The polystyrene standard was dried at atmospheric pressure for 24 hours at 100 “C prior to use. The remaining polymers were synthesized by the Resin Research Department of DeSoto, Inc. and dried as outlined in the Procedure section of this paper. Procedure. DRYINGOF POLYMERSAMPLES.All resins were precipitated by dropwise addition to hexane agitating in a Waring Blendor. The precipitated resins were then redissolved in a 3 :1 mixture of acetone and methyl ethyl ketone to approximately 25% nonvolatile resin. Thin films of the resin solutions were uniformly cast on clean glass plates using a smooth glass rod and immediately stored at 50 “C and 0.1 mm Hg for a minimum of 24 hours to remove the solvents. The film thickness of the dried polymers was kept below 0.5 mil to ensure complete solvent removal. The films were removed from the glass plates using oil free razor blades and stored in vials. Molecular weight distributions, using gel permeation chromatography, were determined for the polymer solutions and the precipitated, dried polymers. No evidence of fractionation or curing of the polymers during the precipitation and drying steps was observed. ZEISELCLEAVAGE REACTION.Approximately 200 mg of the dried polymer were placed in the reaction flask. To that were added 10 ml of molten phenol, 5 ml of glacial acetic acid, and 5 ml of acetic anhydride to swell and dissolve the polymer and scavenge any water that might be present. The reaction flask was then placed in a heating mantle held at 125 “C until the polymer was completely dissolved or sufficiently swollen to allow attack by the hydriodic acid. While the sample mixture was heating, 5 ml of a 1 % wjv solution of 1,1,2-trichloroethane in isopropylbenzene were pipetted into the receiver, which was then placed in a dry ice-acetone bath. After the solution was obtained, the contents of the reaction flask were cooled, 25 ml of hydriodic acid were added, and the apparatus was assembled as shown in Figure 2. The nitrogen flow rate was set at 10 ml/min, and the reaction flask placed jn an oil bath at 132 + 1 “C. The position of the reaction flask in the oil bath was extremely important. The reaction mixture in the flask was level with the oil in the ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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STYRENEDETERMINATION. Solutions of the polymers studied were prepared in volumetric flasks by dissolving known weights of the dried polymers in tetrahydrofuran or acetone. The infrared cell was rinsed twice with the sample solution before use. The optimum region for performing quantitative infrared analysis was between 0.3 and 0.6 absorbance unit. Typical polymer concentrations were 3-8 wt within the optimum region. Before scanning the spectrum, the filled infrared cell was placed in the instrument for 15 minutes to allow the sample to reach temperature equilibrium. The region of the spectrum between 650 and 750 cm-1 was scanned, and the absorbance of the 700 cm-1 band was determined using the “base-line technique” (19). IDENTIFICATION OF ACID. Approximately 5 polymer solutions, in methyl ethyl ketone, were cast onto well polished, optically flat, cesium iodide crystals. Care was taken to ensure that the entire front surface of the crystal was uniformly covered with the resin solution. Most of the solvent was allowed to evaporate from the polymer film at room temperature. The film thickness was adjusted to give approximately 1.0 absorbance unit at the 1733 cm-1 band. After proper film thickness was achieved, the film was dried for 1 hour in a 100 “C vacuum oven under 0.1 mm Hg. The infrared spectrum of the cast film was obtained from 1650 to 1750 cm-l. While this region was being scanned, the chart paper was calibrated against the optical readout of the infrared spectrophotometer. The position of maximum absorption of the acid carbonyl was then accurately determined to the nearest wavenumber.
40
14
RESULTS AND DISCUSSION
Acrylic Monomer Analysis. Early in the development of the Zeisel-gas chromatographic procedure, it was evident that hydriodic acid concentration is very important in obtaining quantitative cleavage of acrylic esters. The use of commercially available 57 hydriodic acid, designated suitable for methoxyl determination, does not consistently lead to quantitative results. Several authors have reported the importance of the hydriodic acid concentration (14,29), and a method for purification has been given (30). We find that optimum results are obtained when freshly distilled hydriodic acid is used. Extreme care must be exercised during the distillation since explosions can occur (31). To ascertain the quantitative applicability of this method, several polymers were analyzed. Table I indicates the known
Figure 2. Apparatus for Zeisel cleavage of acrylic esters bath. Care was taken to prevent the refluxing mixture from reaching the desiccant. ANALYSIS OF ALKYL IODIDES.Periodically, 2 4 samples of the reaction products were removed from the receiver and analyzed by gas chromatography. Prior to taking a sample, the receiver was loosened, and the trapping solution shaken around the glass spiral to form a homogenous solution of the alkyl iodides and the internal standard. Care was taken not to raise the delivery tube above the liquid level in the receiver. The reaction was allowed to proceed until the ratio of the longest chain alkyl iodide to the internal standard became constant. Details of the determination of relative response factors and calculation of copolymer composition are given in the Appendix.
(29) R. Belcher, M. K. Bhatty, and T. S. West, J . Ckem. Soc., 4, 4480 (1957). (30) A. Steyermark, “Quantitative Organic Microanalysis,” The Blakiston Company, New York, N. Y., 1951, p 231. (31) “Reagent Chemicals, American Chemical Society Specifica-
tions,’’ American Chemical Society Publications, Washington, D. C., 1968, p 277.
Table I. Recovery of Alkyl Iodides from the Zeisel Cleavage of Acrylic Polymersn Butyl Methyl Methyl Ethyl Ethyl acrylate, acrylate, methacrylate, acrylate, methacrylate, Polymer 1 2 3 4
z
a
z
50.1 (50.0) 59.9 (60.0) 29.7 (30.0) 59.8 (60.0) 19.9 (20.0)
z
33.4 (33.3)
31.9(33.3) 39.8 (40.0) 29.8 (30.0) 9.8 (10.0) 19.8 (20.0)
19.8 (20.0)
30.1 (30.0) 29.9 (30.0) 60.1 (60.0)
z
32.7 (33.3)
19.5 (20.0) 30.5 (30.0)
30.0 (30.0) 29.6 (30.0) 30.1 (30.0)
All values are the average of at least three determinations and are reported as:
896
z
33.0(33.3) 33.6(33.3)
5
6 7 8 9 10 11
z
33.7 (33.3)
Butyl methacrylate,
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
29.8 (30.0) 39.7 (40.0) 9.5 (10.0)
zmonomer found (zmonomer in polymer).
I
Table 11. Reproducibility of Alkyl Iodide Recovery for Acrylate Terpolymer Ethyl Butyl Methyl acrylate, acrylate, acrylate, % % Run 33.1 33.2 1 33.1 33.4 33.6 2 33.5 33.0 33.5 3 33.7 33.1 32.7 4 34.0 33.3 33.1 5 33.2
z
0
1
2
3
4
5
TIME “ours
polymer compositions and the experimentally determined values calculated from the recovery of alkyl iodides. In all cases studied, the calculated recovery is 95% or greater for polymers containing between 10 and 100% acrylic monomer. Because of these results, we feel this technique may be extended to acrylic monomer levels as low as 1%. A statistical evaluation of the procedure was performed by analyzing the same polymer five times. These results are shown in Table I1 and indicate a 99 % confidence interval of 0.8. The presence of monomers such as styrene, acrylonitrile, vinyl acetate, unmodified acrylamide, or acrylic acid does not change the recovery of the acrylate or methacrylate esters. When hydriodic acid reacts with hydroxyethyl-acrylate or methacrylate, ethyl iodide is formed. This is analogous to the reaction between hydriodic acid and ethylene oxide adducts (3.2, 33). When hydroxypropyl acrylate or hydroxypropyl me:hscrylate is present, isopropyl iodide and propionaldehyde are produced. Similar reaction products are isolated from the reaction of propylene oxide adducts with hydriodic acid (34). However, when polymers containing these monomers are analyzed, the recovery of ethyl iodide and isopropyl iodide is not quantitative. We assume this is due to the formation of volatile intermediates which are lost prior to conversion to the iodide. The rate at which the alkyl iodides are produced from the cleavage of acrylate and methacrylate esters is determined by periodic sampling and analysis of the products from polymers 1 and 2 from Table I. Figures 3 and 4 indicate the recovery of the monomers at selected time intervals. These data suggest that as the length of alkyl chain on the acrylate and methacrylate ester increases, the reaction time necessary for complete alkyl iodide recovery increases. Also, an acrylate ester undergoes cleavage at a faster rate than the corresponding methacrylate ester. With the type of kinetic data shown in Figures 3 and 4, one may be able to qualitatively determine if the component of an unknown polymer that undergoes cleavage is an acrylate or methacrylate. Additional analytical data, however, are necessary to confirm such results. The presence of monomer sequences such as styrene in the copolymer also influence the rate of alkyl iodide production as shown in Figure 5 . Another factor which affects the rates of alkyl iodide production is the stereoconfiguration of the monomers in the polymer backbone. Figure 6 shows the recovery of poly(methy1 methacrylate) polymers which were polymerized in various stereo forms (34). Based on these data, the stereospecific poly(methy1 methacrylates) are listed in decreasing order of rate of cleavage: isotactic, syndiotactic, stereoblock, and conventional. (32) S. Siggia, J. Amer. Oil Chem. SOC.,35, 643 (1958). (33) D. Hummel, “Identification and Analysis of Surface Active Agents,” Interscience, New York, N. Y.,1962, p 198. (34) W. Merz, Fresenius’ 2.Anal. Chem., 232, 82 (1967).
Figure 3. Recovery of alkyl iodides from Zeisel cleavage of acrylate terpolymer Methyl acrylate A Ethyl acrylate 0 Butyl acrylate
0
1
2
3
4
5
TIME (Hours)
Figure 4. Recovery of alkyl iodides from Zeisel cleavage of methacrylate terpolymer Methyl methacrylate A Ethyl methacrylate 0 Butyl methacrylate
0
1
2
3 TIME IHourrl
4
5
Figure 5. Recovery of ethyl iodide from the Zeisel cleavage of styreneethyl acrylate copolymers 100% Ethyl acrylate A 60% Ethyl acrylate, 40% styrene 0 30% Ethyl acrylate, 70% styrene The products formed during the Zeisel cleavage of acrylic esters do not allow the qualitative determination of the monomers in the polymer. This is because the same alklyl iodide is formed from both the acrylate and methacrylate ester. For example, ethyl iodide is formed from the cleavage of both ethyl acrylate and ethyl methacrylate. If the concenANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
897
O
I
0
1
2
3
TIME
(Hours]
4
Table VI. Position of Acid Carbonyl in Acid-Containing Acrylate Polymers Position of Type of acid COIH (cm-1) Methacrylic 1700 Methacrylic 1701 Methacrylic 1702 Methacrylic 1699 Acrylic 1707 Acrylic 1707 Acrylic 1707 Maleic half ester 1710 Maleic half ester 1708 Maleic half ester 1708 Maleic half ester 1709
5
Figure 6. Recovery of methyl iodide from Zeisel cleavage of stereo specific poly(methy1 methacrylates) Isotactic Stereoblocked A Syndiotactic e Conventional D
Table 111. Absolute Absorptivity Values Obtained for Styrene Absorptivity value, Component licm g Solvent Styrene 1.903 Acetone Styrene 1.911 Tetrahydrofuran ~~~
~~
~~~
~
Table IV. Infrared Analysis for Bound Styrene in Styrenated Acrylic Polymers Styrene Calculated, Observed Sample I 5.0 5.0, 4.9 2 7.5 7.1, 6.9 3 20.0 19.6, 19.5 4 24.3 24.4, 24.3 5 24.3 25.6, 24.6 6 25.0 25.1, 25.8, 24.24 7 35.0 36.3, 35.6 8 35.0 34.7, 35.2 9 40.0 38.9, 40.1 10 60.7 59.1, 60.3, 59.7a 11 70.0 69.3, 70.3, 69.5” Per cent styrene values determined in tetrahydrofuran. All remaining values were determined using acetone as the solvent.
z
z
Q
Table V. Reproducibility of Styrene Analysis Styrene, Sample Deviation A 70.5 $0.5 B 70.3 $0.3 C 69.6 -0.4 D 69.3 -0.7 E 69.5 -0.5 F 70.9 $0.9
z
trations of the monomers are substantial, qualitative monomer identification can sometimes be obtained using infrared spectrometry or a residual monomer analysis of the bulk polymer. However, any component in the polymer which reacts with hydriodic acid to yield an alkyl iodide interferes with this analysis. Some of these components include etherified melamines, etherified methylolated acrylamides, ester type 898
*
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, J U N E 1971
plasticizers, phenol-formaldehyderesins, etc. . . . These materials either have to be removed prior to analysis or the level of etherification has to be determined and corrected for. A forthcoming publication will include data on such a procedure. Styrene Analysis. An absorptivity value was determined for the 700 cm-l out-of-plane ring bending vibration 01 polystyrene (Table 111). Details of this calculation are given in the Appendix. The accuracy of this method on typical acid modified styrenated acrylic polymers is shown in Table IV. In the 5 to 70% styrene range, determined values are better than 95% relative. The reproducibility of the method was also determined by analyzing six solutions of the same polymer. The results shown in Table V indicate a standard deviation of 0.6 and a 99% confidence interval of 1.05. Any additional monosubstituted aromatic compounds or other material having absorption bands in this region interfere with this analysis. Acid Characterization. Infrared spectrometry can be used to distinguish acrylic acid, methacrylic acid, and the half ester of maleic acid if those acids are individually incorporated into acrylic polymers at a concentration greater than 10%. Infrared spectra showing the carbonyl region of polymers containing these acids are shown in Figure 7. Based on the data in Table VI, the average frequency of the acid carbonyl absorption is 1707 cm-’ (acrylic acid), 1700 cm-l (methacrylic acid), and 1709 cm-l (half ester of maleic acid). The inductive effect of the a-methyl group in methacrylic acid causes the acid carbonyl to absorb at a lower frequency. It is not known why the acid carbonyl in the half ester of maleic acid absorbs at a higher frequency than the other acids studied. CONCLUSIONS A combination of chemical and instrumental techniques for the analysis of acrylic polymers has been presented. A quantitative method for the analysis of the methyl, ethyl, and butyl esters of acrylic and methacrylic acid was developed using the Zeisel reaction and gas chromatography. The recovery was noted to be 9 5 x or greater for polymers containing between 10 and 90% acrylic monomer. The rate of the cleavage reaction was studied and found to depend on the polymer composition and/or the stereoconfiguration of the polymer, Also, the reaction products from hydroxyl containing acrylic monomers were identified. An “absolute” infrared method was developed for the quantitative analysis of styrene in acrylic polymers. In the range of 5 to 7 0 z styrene, this procedure was found to be accurate to 95 % of the true styrene content. A qualitative in-
BUTYL
HALF-ESTER M A L E I C ACID BUTYLACRYLATE
-
STYRENE
-
BUTYLACRYLATE
Y
u
Figure 7. Infrared spectra of the carbonyl region of acid modified acrylic polymers
1734
m 0:
0
M
=
METHACRYLIC ACID ETHYLACRYLATE
WAVENUMBER
- STYRENE-
CM-'
frared technique for the identification of acrylic acid, methacrylic acid, and the half ester of maleic acid has been developed. APPENDIX Determination of Response Factors. An accurately known blend of the alkyl iodides and the internal standard (1,1,2trichloroethane) was prepared and analyzed using gas chromatography. The areas of the peaks in the chromatograms were determined and the response factors of the alkyl iodides calculated using Equation 1 :
where
response factor of the respective alkyl iodide response factor of 1,1,2-trichloroethane (arbitrarily set equal to 1 .OO) W I = weight of the respective-alkyl iodide in the calibration blend W X = weight of 1,1,24richloroethane in the calibration blend A I = area of the respective alkyl iodide peak A X = area of 1,1,2-trichloroethane peak KI KX
B
Figure 8. Gas chromatogram of alkyl iodides formed and internal standard
5
A . Iodomethane
B. C. D. E.
Iodoethane l,l,Z-Trichloroethane 1-Iodobutane Isopropylbenzene
=
=
Response factors determined at two-week intervals were found to deviate. Best results were obtained when response factors were determined just prior to analysis. Determination of Copolymer Composition. Samples from the receiver were periodically removed through the rubber septum and analyzed using gas chromatography. A typical chromatogram is given in Figure 8. The areas of the alkyl iodide and internal standard peaks were determined and the
0
2
4
6
8
10
T I M E (Minuteri
copolymer composition was calculated using Equation 2:
where: KI AI AX
WX
response factor of the respective alkyl iodide = area of the respective alkyl iodide peak in the chromatogram = area of 1,1,2-trichloroethane peak in the chromatogram = weight of 1,1,2-trichloroethane added to the receiver =
AMALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
899
W T s = weight of sample used M W A = molecular weight of acrylate or methacrylate being analyzed for M W I = molecular weight of the respective alkyl iodide
x
Duplicate determinations should agree within i3 relative. Determination of Absorptivity Value. Polystyrene solutions were prepared between 10 and 60 g/l., and their infrared spectra determined from 650 to 750 cm-’. The true absorbance of the 700 cm-’ band was then calculated using Equation 3 (27). A = AT
where: A
- Ao
(3)
= true absorbance
A T = the total recorded absorbance at the absorpA. =
tion maxima the general background absorption at the absorption maxima
The true absorbance of the 700 cm-l styrene band was then plotted against sample concentration. Using Beer’s law ( A = a h ) , an absorptivity value was readily calculated. The
absorptivity values determined in acetone and tetrahydrofuran are given in Table 111. Determination of Styrene Content. By knowing the absorbance, cell path length and absorptivity value, the concentration (g/l.) of styrene in a solution was calculated using Equation 4 : (4)
c = Ala6
where: c A 6
= = =
a =
styrene concentration (g/l.) in the solution true absorbance of 700 cm-’ band cell path length (cm) absorptivity (L/g cm)
The concentration of styrene in the polymer was calculated using Equation 5 :
% Styrene
=
C -
W
(100)
(5)
where: C = styrene concentration (g/l.) in solution W = sample concentration in solution (g/l.) RECEIVED for review November 17, 1970. Accepted March 1, 1971.
Identification of Alkanes by Pyrolysis Gas Chromatography R. A. Brown Analytical & Information Dioision, ESSOResearch & Engineering Co., Linden, N . J. 07036
In a recent study, Cramers published compositional data for thermal degradation of alkanes; individual monoolefins were measured. In a simplified representation of the Rice theory ( I ) , the monoolefins originate primarily from simple cleavage of the molecule. From this concept, correlations and working rules were found to predict alkane structures accurately. Most of the C6-Cs alkanes were identified exactly and, in all cases, the principal skeletal structure was indicated. Normal alkanes are characterized by 1-olefins. These results indicate that thermal degradation can be valuable in the determination o f ’ molecular structure. Studies of other classes of organic compounds would further clarify this picture.
There is reason to believe that pyrograms should be as useful as mass spectra in providing insight to molecular structure. Indeed, one may well complement the other, as is usually the case with apparently competitive techniques. Pyrolysis causes a molecule to dissociate into a limited number of fragments which stabilize as other compounds. These can be identified and measured. Thus, all pieces of the molecule become an integral part of the molecular data which constitute the pyrogram. In the mass spectrometer, electron bombardment occurs and the molecule behaves as below :
KEULEMANS AND PERRY (2) first suggested that the basic principles of mass spectrometry apply to pyrolysis gas chromatography. Fragmentation occurs in both techniques and then the fragments are separated by a magnetic field or gas chromatograph. It is well known that mass spectra are extremely valuable in the elucidation of structure. However, only limited progress has been made in using pyrolysis fragments, called pyrograms, for structural interpretation. This is pointed out in an excellent review of pyrolysis chromatography by Levy (3). He states that identification is restricted mainly to the comparison of the pyrogram of the sample under test with those of known materials.
Only the ion, A+, is measured and the existence of B goes undetected. Therefore, some evidence of molecular structure is simply lost. Multiple cleavage can also occur in the mass spectrometer. This means many more fragments for the mass spectrum than for the pyrogram. The simplicity of the latter should be advantageous. A survey of literature shows that some progress has occurred in relating thermal degradation products to molecular structure, The objective of this study is to extend the understanding of thermnl degradation. In 1964 Dhont showed that each compound in a group of CrCs alcohols thermally decomposed to one or two principal olefins (4). A study by Levy and Paul (5) of n-alkanes, a-olefins, alcohols, mercaptans, and esters showed that pyrograms were a function of molecular structure. Thermal degra-
(1) F. 0. Rice, “Free Radical (Collected Papers of F. 0. Rice),”
The Catholic University Press, Washington, D. C., 1958. (2) A. I. M. Keulemans and S . G. Perry, “Gas Chromatography,” N. van Swaay, Ed., Butterworth, Inc., Washington, D. C . , 1962, p 356. (3) R. L. Levy, Chromatogr. Rec., 8 , 48 (1966). 900
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
A f B+A+ + B + e
(4) J. H. Dhont, Analyst, 89, 71 (1964). ( 5 ) E. J. Levy and D. G. Paul, J. Gas Chromatogr., 5 , 136 (1967).
