(15) Kitson. R. E.. BSAL. CHEW.25. . 1470 (1953). (16) Kraus, G., Short, J., Thornton, V., Ru.bber & Plastics Age 880 (October 1957). (I?) Luongo, J. P., J . A p p l . Polymer Sei. 3- r. 3- 0 2- (1960). ~ \ - - - -
(18) Powell, W. R.,WADC Tech. Note 57-413, ilSTIA Document 151189,
Wright-Patterson AFB, Ohio. (19) Reding, F. P., Lovell, C. M., J. Polymer Sci. 21, 158 (1956).
(20) Silas, R. S., Yates, J., Thornton, V., ANAL.CHEM.31, 529 (1959). (21) St,averly,F.W. et al., Ind. Eng. Chem. 48, 778 (1956). (22) Sutherland, G.B. B. M., Jones, A., Discussions Faraday Soc. 9, 281 (1950). (23) Washburn, W. H., Scheske, F. A,, ANAL.CHEM.29, 346 (1957). (24) Whetsel, K. E.,Roberson, Pi. E., Krell, M. W., Ibid., 30, 1598 (1958). (25) Whiffen, D. H., Torkington, P.,
Thompson, H. W., Trans. Faraday Xoc. 41, 200 (1945). (28) Willbourn, A. H.,J. Polymer Sci. 34,Nottingham Symposium, 569 (1959). (27) Wood, D.L., Luongo, 9.P., Modern Plastics 38, 132 (March 1961). RECEIVEDfor review July 17, 1961. Accepted September 18, 1961. Division of Analytical Chemistry, 139th Meeting, ACS, dt. Louis, Mo., March 1961.
yroIysis Techniques D. A. VASSALLO Polychemicals Department, E. 1. du Ponf de Nemours & Co., Inc., Wilmingfon, Del. lb Important pyrolysis techniques for characterizing organic polymers have included thermogravimetry and hotwire degradation, Modifications to a fhermobalance furnace have permitted reliable measurements of sample temperature, control of atmosphere over the sample, and adequate diffusion of gas through the assembly. A convenient hot-wire pyrolysis unit was designed for use with gas chromatography, mass spectrometry, or infrared spectrophotometry. "Pyrolytic analysis" was used for rapid, sensitive analyses by thermal conductivity, after oxidation of volatiles to carbon dioxide, water, and other oxides.
viscosity is lowered but reaction between pyrolysis products may be enhanced. Thin films give best results under most conditions. The type of information desired-eg., identification of a polymer, analysis of volatile or nonvolatile products-determines whether a pyrolysis should be performed on a hot filament or in a tube furnace. Extensive use has been made of trapping systems for collecting pyrolyzates (12). However, direct determination of volatile products by gas chromatography or mass spectrometry often reduces time for analysis, simplifies operational steps, and minimizes further chemical reaction.
studies provide information on thermal stability and degradation kinetics, and permit identification of polymers and pyrolysis products. Reliable and reproducible measurements are obtained only through careful control in sampling and pyrolysis, and high precision in detection and separation or identification. The low order of diffusion of degradation products through the viscous polymer mass must be considered in the sampling step. As pyrolysis temperature is increased,
Most methods for determining thermal and oxidative stability have not been very practical for routine use in high polymer characterizations. They are time-consuming and usually measure only gross physical effects. It is better to reserve these procedures for final evaluation of a few compositions and apply a faster scouting method for screening purposes. Thermogravimetric analysis (TGA) is well suited to such use. Weight loss, continuously measured us. time or temperature, provides a rapid means for following the degradation of polymers over a wide temperaturerange. When stepwise degradation occurs, the effects of stabilizers on these reactions can be resolved. Additives which perform well under thermogravimetric investigation then can be tested by more time-consuming methods. The thermogravimetric approach has been used in the study of flame retardants at the Forest Products Laboratory ( I ) . Thermogravimetry or other pyrolysis techniques facilitete polymer identification either through knowledge gained of degradation behavior or identification of pyrolysis products. Detailed degradation kinetics and mechanism in-
THERMOGRAVIMETRY YROLYSIS
BO 70 60
THICKNESS
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'0
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TIME, minuter
Figure 1. Effect of thickness on weight loss of Moplen M/2 polypropylene 260' C. in air
vestigations can be made conveniently by thermogravimetry, since conditions for rigorous isothermal degradation studies can be selected quickly ( I , 6,6). Critical factors in any pyrolysis are sample thickness, temperature, and atmosphere. The effcct of thickness on weight loss behavior of Moplen M/2 polypropylene a t 260" 6.in air is shown in Figure 1. These data were obtained using constant sample weight, but varying sample area. The thinner samples degraded much more rapidly than the thicker ones. To reproduce results to =t2%, the 10-mil sample thickness had to be controlled to 0.1 mil or less. This effect of sample thickness was very much diminished at 2 mils, and for still thinner samples there was little or no effect. Madorsky (12) compared data obtained by several workers (6, 8, 1.2, 13) for the degradation of poiystyrenc samples of comparable molecular weight. Wide differences in the temperatures reported for similar volatilization rates were thought due to variations in sample thickness, errors in measurement of sample temperature, and delays in vaporization of volatile products due
30
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I
O,"
t
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Figure 2. Pyrolysis of polytetrafluoroeth ylene
480' C. various atmospheres VOL. 33, NO. 13, DECEMBER 1961
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THERMOCOUPLE
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to slow diffusion through the viscous polymer. D3erences of as much as 60' C. have been observed between the temperature measured by a thermocouple in the sample and the temperature a t the wall of a thermobalance furnace. Meaningful comparisons of results obtained on different equipment can be made only if the temperatures reported are those of the sample. The rate of volatilization during pyrolysis is sometimes controlled by mass action, and therefore the sample atmosphere must be controlled. As shown in Figure 2, polytetrafluoroethylene degradation in the presence of monomer is slow, compared to degradation under vacuum (14). An intermediate rate of degradation is found under autogenous vapor pressure. Volatilization of polytetrafluoroethylene under 35-mm. pressure of monomer becomes equivalent t o degradation under pyrolysis products of less than or equal pressure after 400 minutes a t 480" C. At this point the volatilization rate has greatly decreased. When the pyrolysis products are continuously removed, the degradation is much more rapid. Degradation in a flowing nitrogen stream is not shown here, but again the degradation is rapid and almost equivalent t o vacuum. Most commercial thermobalances do not provide adequate control of temperature and atmosphere for high precision work. In Figure 3, modifications to a Stanton thermobalance are shown which greatly improve its reliability. Instead of crucibles supplied with the balance, flat platinum or glass cups are used to contain the samples. These provide constant area for the pyrolysis of films. To reduce convection currents within the 96y0 silica (Vycor) envelope, a portion of the volume is filled with glass wool. Gaseous atmospheres intrcduced through the side arm are preheated before diffusing to the sample. through the tube over a wider area, further reduces convection. Temperature is measured with an andlary thermocouple piaced about
one-half inch over the sample. Even though the temperature is measured this Close to the sample, a -5' correction must be applied during programmed temperature operation. The atmosphere of flowing nitrogen is Controlled by a calibrated flowmeter. Early work showed that oxygen was able to diffuse against a flow of 1 liter per minute of nitrogen through the large orifice. By covering the bottom of the envelope to reduce the diameter of the hole from 2 inches to less than inch, a flow of 250 cc. of nitrogen per minute was sufficient to prevent oxygen diffusion. The degradation temperature of a polymer is easily fouad by TGA, but a correlation with techniques requiring different conditions often must be made to optimize utility. Pyrolysis a t a mass spectrometer inlet system, for example, requires a high vacuum. A comparison of weight loss ve. temperature for poly(methyl methacrylate) under different conditions is shown in Figure 4. (In general, similar results are obtained for other polymers.) The difference a t any level of degradation between isothermal vacuum pyrolysis (19) and pyrolysis in flowing nitrogen is about 10" C., while pyrolysis in nitrogen with a 5" 6. per minute programmed temperature rise differs by as much as 50" to 60" C. After a single TGA run, the mass spectroscopist can estimate the temperature needed to arrive at a certain level of volatilization. When dealing with research polymers available in limited amounts, this type of extrapolation is particularly useful. Similarly, infrared spectrophotometric polymer identification, when infrared discrimination is blocked by fillers, is facilitated by examination of pyrolyzates (4). TGA can be used to select pyrolysis conditions. OTHER PYROLYSIS TECHNIQUES
A pyrolysis technique that has been receiving increasing attention in recent years employs a hot filament or hot wire (3,4, 7, 10, 11). The method can be applied in conjunction with mass or infrared spectroscopy or gas chromatography. The basic sample holder used in our laboratory is shown in Figure 5. This attachment can be placed a t the inlet of a gas chromatographic column or adapted for direct injection into the heated inlet system of a mass spectrometer through a hypodermic needle as shown in the figure. From 1 to 5 mg.
200
250
300 350 400 450
500
TEMPERATURE, 'C
Figure 4. TGA of poly(rnethy1 methwrysate)
of sample is placed on the resistive heated tantalum filament and pyrolyzed by applying a current t o the filament. Filament temperature is regulated by use of tantalum ribbon of known resistance-temperature characteristics. A wire basket also can be used when a rapid and complete pyrolysis is anticipated. The attachment can be heated externally to prevent condensation of pyrolyzate. This technique used in conjunction with gas chromatography shows promise for rapid analyses of many homo- and copolymers. Currently, it is useful for analyses of polymers which depolymerize to monomer. Reliable methyl methscrylate-styrene copolymer analyses were achieved by Jones and Moyles (9) using a similar technique. Recently, a high sensitivity pyrolysis technique was developed a t Los Alamos for the study of explosive mixtures (16). The method, called "pyrolytic analysid' by the authors, can be applied to polymers also. The sample is heated in u carrier stream, preferably helium; and volatiles are oxidized by copper oxide to carbon dioxide, water, and other oxides. The thermal conductivity of this stream is compared to the original carrier gas using any of the thermal conductivity detectors. The results are recorded as relative thermal conductivity us. sample temperature or time. A curve which resembles the derivative of a thermogram is obtained under programmed temperature conditions, and rate of volatilization is determined under isothermal conditions. A block diagram of components used in our laboratory is shown in Figure 6. The oxidation step serves two purposes: (1) Condensation in the detector is prevented, and the detector
Figure 5. Hot-wire pyrolysis unli
can be operated a t a lower temperature, where it is inherently more sensitive. (2) The molar concentration of volatiles is greatly increased; thus, detection of low volatilization levels is facilitated. The system has three heated zones. The temperature of the pyrolysis zone is programmed and contains the sample in a platinum boat. The combustion unit is a t 800" C. for oxidation of the volatiles with copper oxide. The detector (matched 100,000-ohm Veco thermistors operated at 10 ma.) is maintained a t 125" C. to eliminate condensation of water and to prevent interaction of carbon dioxide and water. The method cannot replace thermogravimetry; but for comparing polymers of similar structure, this method has more discrimination and is more rapid. The method is still under study to optimize flow, heating rate, and sample size. A weight loss of only 2 pg. already can be detected a t much faster heating rates than those used in thermogravimetry .
COMBUSTION cuo BOO%
w -
I t
I I
THERMAL CONDUCTIVITY DETECTOR RECORDER OUT
Figure 6. ment
IN
Pyrolytic analysis equip-
The pyrolysis techniques described should be sufficient for most problems. Thermogravimetry is most useful for gross degradation studies and correlation with other analytical techniques. Hot-filament pyrolysis facilitates identification of pyrolysis products. Pyrolytic analysis is most useful for detecting minor differences in similar polymers. These techniques, modified for specific applications, will find wide application. LITERATURE CITED
(1) Aminco Lab News 16, No. 3, 5 (1960).
(2) Anderson, D. A,, Freeman, E. S., J . Appl. Polymer Sci. 1 , 192 (1959). (3) Barlow, A,, Lehrle, R. S., Robb, J. C., Polymer 2, 27 (1961). (4) Cleverly, B., Hermann, R., J. -4ppZ. Chem. 10, 192 (1960). (5) Freeman, E. S., CarroI, B., J. Phys. C h m . 62,394 (1958). (6) Grassie, N., Kerr, Wa W., Trans. Faraday SOC.53, 234 (1957)(7) Janak, J., .Vatwe 185, 684 (1960). (8) Jellinek, H. H. G., J. Polymer Sci.
850 (1948); 4, 13 (1949). (9) Jones, C. E., Moyles, A. F., Aralurs 189, 222 (1961). (10) kehmann, F. A,, Brauer, G. M., ANAL.CHEX.33, 673 (1961). (11) Lehrle, B. S., Robb, 6. e., Nature 183, 1671 (1959). (12) Madorsky, S. L., J . Research .?'atl. Bur. Standards 62,219 (1959). (13) Madorsky, S. b.,Strauss, S., Ibid., 53, 361 (1954). (14) Siegle, J. C . , Muus, L. T., private communication. (15) Rogers, B. N., Yasuda, S. K., Zinn, J., ANAL.CHEW 32, 672 (1960).
RECEIVEDfor review July 10, 1961. Accepted Se tember 22, 1961. Division of Analyticaf Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
.IS
ole of Carbon-14-Tagged Standards in t f Acrylonitrile-Vinyl Acetate Copolymers MYRON E. GIBSON, Jr., and ROBERT H. HEIDNER Chemstrand Research Center, Inc., Durham, N. C.
b The preparation of copolymers of vinyl acetate and acrylonitrile using vinyl acetate-1 -C14 has allowed primary standards to b e established for evaluating analytical procedures. Copolymers containing about 4 to 25% vinyl acetate were prepared and vinyl acetate contents determined by liquid scintillation counting techniques. Using these polymers as standards, wet chemical and instrumental methods of analysis have been developed and eva luated
.
R
studies of new copolymer systems generally require reasonably accurate, precise methods for determining the individual components of the copolymer. Such data are used to calculate reactivity ratios, check material balances, and establish uniformity of polymer composition. Many copolymer systems lend themselves to an analysis by infrared or ultraviolet spectrophotometric techniques but to a lesser degree by chemical methods of analysis. -4major problem with a spectrophotometric approach, however, is the need for standard copolymers of known composition for calibration purposes. -bsorptivities based on the ESEARCH
absorbance of individual homopolymers are not always identical with those obtained on copolymers. Consequently, each new system must a t least be checked in this regard. In the case of acrylonitrile-vinyl acetate (AN-VA) copolymers, the authors have selected a system which appeared amenable to analysis by chemical as well as spectrophotometric techniques. Further, the composition is such that, in theory, standards could be established on the basis of elemental analysis for carbon, hydrogen, oxygen, and nitrogen. Very little is disclosed in the literature pertaining to the quantitative determination of composition in SNVA copolymers. Takayama and Kadota (6) reported a lengthy method for determining the vinyl acetate content of AN-VA copolymers by colorimetry. Vinyl chloride-vinyl acetate copolymers mere analyzed for poly-VA by Lardrra, Cernia, and Mori (4) by hydrolyzing with sodium ethoside in a neutral solvent such as dioxane. After hydrolysis was completed, water was addcd and the mixture was acidified with sulfuric acid. Free acetic acid was then distilled off and determined by titrimetry. This same type of approach was used
by Inglis (2) to hydrolyze fructose tetraacetate quantitatively. Samples nere refluxed in aqueous KOH, the resulting hydrolyzate was made acid with sulfuric acid, and acetic acid, liberated by this procedure, was steam-distilled and titrated. Kennett (3) describes a saponification method for VA in poly(vinyl acetals) and various terpoiymers. The polymers were dissolved in organic solvents (pyridine and methanol) and then hydrolyzed with 0.LY KOH. Hydrolysis procedures have also been developed and used in the authors' laboratory as detailed below. Certain discrepancies, however, in the results by chemical and elemental analyses, coupled with recognized weaknesses of each, made necessary the use of an arbitral technique. Radioassay of polymers prepared with carbon-14-tagged vinyl acetate was selected for this role, since it would serve as a direct measure of the VA content and would not be subject to the questions associated with the other methods. Two possibilities were considered for carbon-14-tagged YA. Vinyl-l,2C14 acetate was listed as commercially available. I t was ultimately ruled out for use in initial work because the influence of the radioactive carbon atoms VOL. 33, NO. 13, DECEMBER 1961
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