Gas Chromatographic Analysis of Polymers after Oxidative Degra da tio n SIR: Results of preliminary work on the characterization of polymeric and high molecular weight organic compounds by using an oxidative degradation technique are reported. Although much more work will be necessary to develop and refine it, a t this stage the method does appear feasible and should provide useful information on the degradative nature of the organic materials. T o date most of the work has dealt with oxidative degradation. However, other forms of degradation such as hydrolytic fission have also been briefly explored. The method is essentially a characterization of polymers or their oxidative behavior on the basis of their oxidation products, in the same way that determining the kind and amount of pyrolysis products is used to identify a polymer or indicate its thermal stability. With the new method the oxidation products of the polymers are produced in a short precolumn just ahead of the chromatographic column, immediately swept into a chromatographic column for separation, and then detected to produce a regular chromatogram of the products. There are numerous reports on pyrolysis studies with polymers, but in 1954, Davison et aZ. ( 4 ) reported the first application of gas chromatography to the identification of pyrolysis products. Subsequent improvements included units which pyrolyzed the polymers and immediately swept the products into a gas chromatographic column for separation (6, 7 ) . Oxidation of relatively nonvolatile organic compounds, like pyrolysis, has been studied extensively primarily with nongas chromatographic methods. In addition, complete oxidation of organic compounds to COZand H20followed by gas chromatographic analysis of these products has been used for carbonhydrogen determinations (5, 9, 11). Stein et aZ. (IO) described a microcat,alytic reactor placed immediately before the gas chromatographic column for studying oxidation of hydrocarbons. Dal Nogare and Juvet (3) mention the possibility of oxidative degradation of liquid phases when oxygen is present in the carrier gas. Oxidation products from diesters and siloxanes, compounds of the types utilized as liquid phases, have been identified and reported (1,2 ) .
Kieser and Sissons (8) reported the formation of volatile compounds on gasliquid chromatographic columns but noted that the volatile compounds were present in the liquid phase before application, EXPERIMENTAL
Apparatus. A Wilkens Instrument A-90-C gas chromatograph equipped with the Wilkens flame-ionization attachment was used for the analyses. The instrument was fitted with a stainless steel column 10 feet X l / 4 inch packed with a substrate of 15% Carbowax 20M plus 5y0 stearic acid on 60- to 80-mesh Gas Pack S. Selection of the liquid phase is not critical, but a polar liquid phase is desirable because of the probable polar nature of the oxidation products. This chromatographic column was thermostated a t approximately 100' C. The hydrogen and air flow rates to the detector were approximately 50 and 250 cc./minute, respectively. The electrometer output was recorded on a Sargent Model M R recorder with a chart speed of 3 minutes/inch and a full-scale sensitivity of 1.25 mv. Precolumn. The precolumn is a 12X l/r-inch stainless steel tube packed with the material to be studied coated on a suitable support. The precolumn is attached directly to the head of the chromatographic column. A Swagelok T attached to the front of the precolumn and fitted with a silicone rubber septum serves as the injection port. Helium a t a flowrate of approximately 70 cc./minute was the carrier gas. The precolumn was kept a t a constant temperature between 100' and 600' C. by a heating tape connected to a variable transformer. The upper temperature limit for the precolumn is set by the volatility and thermal stability of the material being studied. Procedure. When possible, the substrate for the precolumn is prepared in the usual manner. When the polymer is highly insoluble, 0.1 to 1.0 gram of granular polymer mixed with the support can be packed into the precolumn. If the granular material is packed alone, it must be amenable to packing into the tube and allow the carrier gas to flow a t operating temperatures. The freshly packed column is then conditioned, overnight if necessary, a t the temperature a t which it will be operated. Although Chromosorb P was used for the studies reported, other supports such as Gas Pack, firebrick, and glass beads are equally suitable.
When the precolumn has been conditioned and thermostated a t the desired temperature and the chromatographic column has been thermostated a t approximately 100' C., 1 cc. of oxygen is injected with a gastight syringe. As the oxygen passes through the heated precolumn, a portion of the organic polymer is oxidized. The oxidation products enter the chromatographic column as a plug. They are separated and detected as they are eluted to produce the chromatogram. DISCUSSION
Figure 1 shows the chromatograms of Carbowax 20M and UCON Polar. The components are the oxidation products produced in the precolumn by the oxygen. The precolumns were thermostated a t 203' and 202.5' C., respectively. The carrier gas flow rate is not extremely critical. However, too low a flow rate caused unsymmetrical peaks, and too high a flow rate reduced the amount of oxidation products formed because of the short residence time of oxygen in the column. This effect is presumed to be general and not specific for the compounds studied. The temperature selected for the chromatographic column depends on the liquid phase being studied. However, the temperature selected should not be so high that the liquid phase is oxidized by the oxygen. Programmed temperature might be helpful after the oxygen has been eluted from the chromatographic column. Note that the chromatograms are different. At its simplest, this technique can serve as a qualitative tool as does pyrolysis because these chromatograms are characteristic and always produced for Carbowax 20M and UCON Polar under similar operating conditions. A different chromatogram for polystyrene is illustrated in Figure 2. The same support must be used to obtain reproducible results because different supports give chromatograms that differ qualitatively. Whether this is due to differences in surface area or catalytic effects is not known. If the identity of the oxidation products shown in Figures 1 and 2 (or for any material being studied) were known, the "weak link" in a polymer chain could be studied. The chemical bonds VOL. 38, NO. 2, FEBRUARY 1966
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Gas chromatogram of oxidation products for polystyrene
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that underwent oxidation could be surmised, and in the case of polymers, if such oxidation were undesirable, perhaps a polymer could be designed with the "weak link" eliminated. Figure 3 illustrates another applicat'ion of the method-that is, ranking the relative severity of degradation. All three materials are polyphenyl ethers. The compounds in Figure 3 are: A , a &ring polyphenyl ether; B, Convalex 10; and C, 05124. The last two are five-ring materials which differ in their relative isomer distribution. All precolumns were filled with a substrate of 15% of the polyphenyl ether on 35 to 80-mesh Chromosorb P. The six-ring polyphenyl ether produced the least number of oxidation products, and OS124 (five-ring polyphenyl ether) produced the most, even a t a lower temperature (250' C.). Thus, the chromatograms suggest that oxidative degradation was least severe for the six-ring ether and most severe for the 053124. Some or all of these peaks may not be due to the polyphenyl ethers but to additives that are known to be present and that are probably different for each material. However, this does not detract from the value of the data and it suggests another use of the method: observation of the effect of various additives, such as catalysts or antioxidants, on the oxidative or other degradative character of the organic material being studied.
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Column length, 1 foot.
Precolumn temperature, 240' C.
At present, the technique does not lend itself well to quantitative determinations due to variations in relative peak intensities for a given sample. Additional applications include ozone and hydrolytic reactions and/or stability obtained by injecting ozone or water. Also, by varying the temperature of the precolumn relative stabilities of the chemical bonds in the material being studied can be ascertained. Thermal and ultraviolet stabilities can
be determined by irradiating quartz or borosilicate glass precolumns coated with the compounds with intense xenon or ultraviolet light, respectively. Oxidation stabilities and mechanisms for animal and vegetable oils can also be studied. LITERATURE CITED
(1) Atkins, D. C., Jr., Baker, H. R., Murphy, C. M., Zisman, W. A., Znd. Eng. Chem. 39, 491 (1947).
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Gas chromatograms of oxidation products for polyphenyl ethers A B. C.
Six-ring polyphenyl ether Convalex 10 OS124 VOL. 38, NO. 2, FEBRUARY 1966
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(2) Atkins, D. C., Jr., Murphy, C. M., Saunders, C. E., Ibzd., p. 1395. (3) Dal Nogare, S., Juvet, R. S., Jr., “GasLiquid Chromatography,” p. 123, Interscience, New York, 1962. (4) Davison, W. H. T., Slaney, S., Wragg, A. L. Chem. I n d . (London) 44, 1356 (1954j. (5) Duswalt, A. A., Brandt, W. W., ANAL. CHEM.32, 272 (1960). (6) Guillet, J. E., Wooten, W. C Combs, R. L., J. A p p l . Polymer S k 3, 61 (1960).
(7) Janak, J., 3rd Conference on Analytical Chemistry, Prague, September 1959. (8) Kieser, M. E., Sissons, D. J., Nature 185, 529 (1960). (9) Luskina, B. M., Syavtsillo, S. V., Terent’ev, A. T., Turkel’taub, N. M., Doklady Akad. N a u k , S.S.S.R.141, 869 (1961). (10) Stein, K. C., Feenan, J. J., Thompson, G. P., Shultz, J. F., Hofer, L. J. E., Anderson, R. B., I n d . Eng. Chem. 52, 671 (1960).
(11) Sundberg, 0. E., Maresh, C., ANAL. CHEM. 32, 274 (1960).
ROBERT G. SCHOLZ JAMES BEDNARCZYH TERRY YAMAUCHI IIT Research Institute Technology Center Chicago, Ill. RECEIVEDfor review August 26, 1965. Accepted December 6, 1965.
Elastomer Identification by Ultraviolet Spectrometry SIR:There is considerable literature on the problems of qualitative and quantitative analyses of elastomers (4). A test method for elastomer identification which has replaced most of the lengthy or less exact chemical tests, infrared (IR) spectrometry, also has some shortcomings. The test samples require a certain amount of preparation before IR spectra can be made. Organic substances other than elastomers often interfere and have to be removed by extraction. The sample then has to be either dissolved in some solvent to separate carbon black and fillers, or has to be pyrolyzed to effect the separation. The pyrolysis step is simple but the I R spectra obtained often differ from those obtained by the dissolution method and are sometimes hard to interpret. The method described here uses the selective absorption of ultraviolet radiation by gaseous pyrolyzates from different elastomers. It was developed to permit rapid qualitative tests with a minimum amount of material and no sample preparation, and can be considered as an improvement of some previous attempts. Burchfield (I) has developed a rapid chemical test for elastomers utilizing their gaseous pyrolyzates and Hummel (2) describes I R methods using gaseous pyrolyzates of elastomers. This method alone and with I R and other methods has been used and tested by the writer for a considerable period of time and has proven to be quite accurate. Because of its simplicity, savings in analyses time have been considerable. Experience from tests performed has shown that ultraviolet spectrometry can be a valuable tool for elastomer identification. The method is not meant to replace I R and other reliable elastomer identification tests, but to supplement them. THEORETICAL CONSIDERATIONS
On pyrolysis elastomers produce large amounts of monomers from which they were made, some dimers, and higher molecular weight polymer break-down 334
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products. The gaseous pyrolyzates show UV absorptions which are characteristic of individual elastomers. For example, natural rubber pyrolyzate contains isoprene monomer fraction with its typical UV absorption spectrum. If an unknown elastomer pyrolyzate has a similar spectrum, then the elastomer is probably natural rubber, or polyisoprene. I n a similar manner other elastomers can be identified by comparing the UV spectra of their pyrolyzates with known elastomer spectra, made for reference. Because electronic absorptions (9) are selective materials which do not absorb in the test region do not interfere, but this selectivity also limits the method to substances which absorb UV radiation. Contrary to what is generally believed, the UV absorption spectra made by this method are quite specific to each elastomer, or groups of elastomers. The gaseous spectra of the pyrolyzates (monomers, etc.) show vibrational fine structure of diagnostic value which would tend to disappear when using solvents. By collecting the mainly monomeric gaseous fraction, the interference of the more complex liquid pyrolyzate is eliminated. Any COz produced and most hydrocarbons with no characteristic absorption groups such as methane or its derivatives have to be present in large amounts before they will show appreciable effect on the UV absorptions. EXPERIMENTAL
Apparatus. The Bausch & Lomb Spectronic 505 recording type UV spectrophotometer was used. The absorbance measuring logarithmic gears of the instrument produced absorption curves with better definition than the linear gears. For collecting the pyrolyzate a quartz (1 cm. path) cuvette was used. Procedure. The pyrolysis of an elastomer was accomplished by pressing the red hot tip of a triangular iron file or electric resistance wire against the elastomer sample. The cuvette was held about an inch above the sample and some of the pyrolyzate
was caught in the cuvette and retained there by sealing i t immediately with a finger of the hand holding it. The cuvette was turned upright and its cover placed on quickly. The UV absorption of the pyrolyzate was recorded by medium-fast scanning from 180 mp to about 260 mp. The absorption intensity of the spectrum recorded was proportional to the amount of the gaseous pyrolyzate collected in the cuvette. The optimum pyrolyzate concentration for each elastomer was determined by a few experiments. It was sometimes best to admit more than the required amount of pyrolyzate and then let some escape by removing the cover until the desired absorbance was obtained. The spectrum obtained from the unknown was compared with known elastomer spectra. After use the cuvette was cleaned with acetone or benzene to remove pyrolysis products which had condensed on its surface. RESULTS AND DISCUSSION
The spectra, obtained from several different elastomers and other organic materials, are described here and four characteristic spectra are shown. Natural Rubber. The pyrolyzates of natural rubber and polyisoprene have similar UV spectra (Figure 1) and therefore can not be distinguished. The most characteristic absorption band is in the 220-225 mp region. From this absorption the smallest amount of isoprene that can be identified in elastomer mixtures is less than 5%. In organic materials other than elastomers, the presence of less than 3% of isoprene can be detected. The absorptions a t 210 mp and 216 mp are common with two of the butadiene peaks, but show different intensities. Because these absorption peaks become modified, they can not beusedfor natural rubber identification in certain elastomer mixtures. Styrene-Butadiene Rubber. The SBR rubber pyrolyzates (Figure 2) show the typical styrene and butadiene absorption spectra combination. The styrene absorbs a t 195-200 mp and 235-240 mp, and the butadiene has absorptions in the 200-220 mp region,
