Under the conditions of the recommended procedure, Figure 2 shows the excellent decontamination of tin113 from an old fission product mixture to which have been added iron-59 and antimony-124. The respective radionuclides were added in such amounts as to test the higher limits of decontamination. Figure 3 shows typical decontamination data for the isolation of tin from a young fission product mixture. Five milligrams of natural uranium were ir, radiated for 30 minutes (flux = 6 X 1011n./sq. cm./sec.) and allowed to cool for 1 hour, so that short-lived fission product tin isotopes would decay out. Natural uranium, rather than enriched uranium, was used in order to test decontamination from neptunium-239, a typical actinide element. X slight modification of the procedure was used to isolate tin; the final tartaric acid strip was made 2N in hydrochloric acid and the tin was re-extracted into 0.5M
TTA-hexone. This allowed higher decontamination as well as placing the tin in a phase which could be easily dried for subsequent beta and gamma countA gamma spectrum of the ing. separated tin was recorded after allowing 2 hours for the tin-128 daughter, 10.1-minute antimony-128, to grow in to equilibrium. Normalized counting rates for both original fission product mixture and final tin product are shown in Figure 3. The tin-128 spectrum closely reproduces the work of Dropesky and Orth (2). The decay of the tin128-antimony-128 was followed by beta counting. A half life of 59.8 =t 0.8 minutes was also in good agreement with the most recent value, 57 5 minutes (2).
*
ACKNOWLEDGMENT
The authors gratefully acknowledge the capable assistance of J. S. Eldridge and P. Crowther in interpreting some of the nuclear data.
LITERATURE CITED
( 1 ) Courtney, C. R., Gustafson, R. L.,
Chabarek, S., Jr., Martell, A. C., J . A m . Chem. SOC.80,2121 (1958). (2) Dropesky, B. J., Orth, C. J., J . Inorg. Sucl. Chem. 24, 1301 (1962). ( 3 ) Goto, H., Kakita, Y., Furukawa, T., S i p p o n Kagaku Zasshi 79, 1513 (1958). ( 4 ) Luke, C. L., ANAL.CHEM.28, 1276 (19.56\. (5) I b i d . , 31, 1803 (1959). ( 6 ) McBride, J. P., Stoughton, R. W., U. S. At. Energy Comm. Declassified Documents, ORNL-286, 122, ORNL4 9 9 , 9 0 (1949). ( 7 ) Moore, F. L., ANAL. CHEM.28, 997 (1956). (8) Moore, F. L., “Metals Analysis with \ - - - - ,
TTA,” Symposium on Solvent Extraction in the Analysis of Metals, ASTM Spec. Tech. Publ. No. 238 (1958). ( 9 ) Nervik, W. E., “The Radiochemistry of Tin,” Office of Technical Services, Dept. Commerce, NAS-NS-3023 (1960). (10) Pappas, A. C., Wiles, D. R., J . Inorg. ic’ucl. Chem. 2 , 69 (1956). (11) Poskanzer, A. M., Foreman, B. M., Jr., Ibid., 16, 323 (1961). RECEIVEDfor review January 22, 1964. Accepted March 5 , 1964.
A p pIicat io n of Pyr o lysis- Gas Chromatography to Polymer Characterization BARNEY GROTEN Analytical Research Division, Esso Research & Engineering Co., P. 0. Box 12 1 , linden, N. 1.
b Pyrolysis-gas chromatography has been applied to several phases of polymer analysis. Qualitative fingerprinting conditions have been defined so that a large number (>150) of polymers can be identified using essentially singular pyrolysis and chromatograph conditions. Quantitative analytical schemes have been devised for ester types in cellulosics,. and for styrene content in SBR vulcanizates. Finally, the technique has been applied to the elucidation of polymer microstructure, where it is useful for differentiating blends from true copolymers of ethylene and propylene, and for indicating major differences in stereoregularity in polypropylene.
P
TECHNIQUES are among the oldest approaches to the study of the structure of polymeric systems (16). The early literature contains examples of the destructive distillation of large batches of a polymer, followed by isolation and identification of the individual low molecular weight compounds resulting therefrom (8). The advent of modern spectroscopic and gas chromatographic instrumentation has now given pyrolysis renewed imYROLYSIS
1206
ANALYTICAL CHEMISTRY
portance as a useful technique for the analysis of macromolecular materials. The aims of the present work were threefold: to devise a system whereby routine identification procedures-Le., fingerprinting-could be performed for the widest variety of polymers with the minimal assortment of columns and chromatograph conditions; to apply the pyrolysis technique in a quantitative manner for a few selected systems where intractable samples or a very lengthy procedure made traditional approaches undesirable; and to determine whether pyrolysis-gas chromatography (P-GC) could be useful in the elucidation of the microstructure of polymers. Pyrolysis-gas chromatographic procedures have generally followed one of two basic patterns. I n the flash technique, a sample is deposited on an electrically heatable filament, or placed in a boat in a furnace, and the G C carrier stream used to transport the pyrolyzate directly to the column. The alternative procedure involves heating of the polymer in a separate enclosure, trapping the off-gases and admitting the pyrolyzate to the chromatograph after a given collection interval. Barlow, Lehrle, and Robb ( I ) , Ettre and Varadi ( 6 ) , and others
have discussed the relative merits of the sampling techniques. Each approach has certain advantages and the type of information desired from P-GC should, therefore, dictate the sampling method to be employed. The trapping technique is superior for the analysis of trace solvents in polymers (11). The apparatus described in this paper, however, is of the flash pyrolysis type, employing a platinum heating coil. EXPERIMENTAL
The pyrolysis apparatus is constructed from an isothermal PerkinElmer 154 chromatograph, a n Aminco pyrolysis accessory and a six-way gas sampling valve. The arrangement is shown in Figure 1. The valve and pyrolyzer assembly are mounted in ports cut through the oven insulation in the chromatograph. This placement exposes the block to oven temperatures and, thus heated, prevents condensation of the less volatile portions of the pyrolyzate. The functions of the valve are to reduce pressure fluctuations, and to prevent large quantities of air from entering the hot system when the pyrolyzer is opened for a sample change. I n addition, this arrangement also allows the instrument to be employed
P~ET
ANALYTICAL HEATED OUTLET
K T ~ N
FLOWMETER)
MODIFIED P.E. GAS SAMPLING
HERMISTOR ETECTOR
-MODIFICATIONS -ORIGINAL
RE.-154
PYROLYZER BLOCK
TUBING
Figure 1 . Perkin-Elmer 154 gas chromatograph modified for pyrolysis
as either a pyrolysis apparatus or as a conventional GC by simply turning the gas valve. Thus, when peak identification is required, known materials can be injwted into the GC in the usual manner, and retention times matched with those found in the pyrolyzate pattern (vide infra). The pyrolyzer chamber consists of a two-piece stainless steel block containing carrier gas connections and the platinum heater coil. Samples, of approximately 1 milligram, are weighed into 2-nim. 0.d. quartz capillary tubes and placed in the ceni,er of the coil. After a run is completed, the tube is removed and any residue may then be recovered. Pyrolysis temperatures are controlled by a variable transformer in the heater circuit, and the duration of pyrolysis is regulated b q an adjustable cam operating a microswitch. The
Figure 2. Temperature profiles achieved within quartz sample capillary Elapsed time (x-axis) reads from right to left
transformer was calibrated by measuring the maximum temperature actually achieved within the quartz sample capillary rather than Pt-coil temperature. Since the total heating-peakingcooling cycle is important in pyrolysis studies, a complete profile was obtained for each transformer setting, and at various total heating times. Typical cycles are shown in Figure 2. These curves show that the attainment of maximum temperature is quite rapid under all conditions. The cycle was fairly insensitive to minor changes in carrier flow rate. The column used in most of this work was a 1/4-inch by 12-foot 20% Carbowax on Diatoport P. Water insoluble Ucon (5%) OII an 8-foot Haloport F column was used for analysis
of the cellulose esters. Chromatograph conditions were: bridge, 8 volts; attenuation, 2 X unless otherwise specified; helium flow rate, 45 cc. per minute; pressure, 10 p.s.i.; column temperature, 150' C. (100' for Ucon column). RESULTS A N D DISCUSSION
Qualitative Analysis. Some of the earliest workers in P-GC demonstrated the utility of the tool as a means of rapidly identifying a broad variety of polymeric materials (1,2,4,6,9,10, 12). Most of these studies were limited, however, to one, or at most a few selected systems. Recently, Cox and
POLYSTYRENE
(X=
- 4)
Figure 3. Pyrograms of various
+CH~CHZ+~ polymers
VOL. 36, NO. 7 , JUNE 1964
1207
r
POLYPROPYLENE
POLYETHYLENE
4 F i g u r e 4. Pyrograms of various poly-a-olefins
m
POLY-3-METHYL
A
1
7 ,r
POLY-4-METHYLPENTENE- I
BUTENE-I
I I A
1
A
i-
Ellis (3)presented a qualitative study of a somewhat wider variety of plastics, resins, and elastomers. We have now examined over 150 different materials, including plastics, elastomers, resins, and natural products, homopolymers, blends and copolymers, filled and unfilled, pure laboratory preparations and molded articles of commerce.
NYLON 6
L
Figure 5.
1208
0
ANALYTICAL CHEMISTRY
Within this collection, there are remarkably few cases of exact identity, although there are definite family resemb1ances-e.g. , many nylons give roughly similar patterns, and straight chain hydrocarbons behave in a unique manner. I n each case, however, the individual members of a group almost invariably present pyrolysis patterns
i
sufficiently different to allow their identification to be made with confidence. I n collecting this basic file of chromatograms, the single set of instrument and pyrolysis conditions was selected which gave the best patterns for the widest variety of materials. I n the few situations where the standard conditions resulted in ambiguous chromatograms, only slight variations in pyrolysis times and/or temperatures were usually necessary to establish a n identity. Generally the higher the pyrolysis temperature, the greater t h e fragmentation of a polymer molecule (I+$), and, obviously, the longer the heating period, the more complete the decomposition. However, the duration of heating represents a compromise. The shortest possible period would be desirable from the standpoint of instantaneous sample deposition onto the column; a long period would allow greater decomposition, and result in a greater total amount
NYLON 610
Pyrograms of commercial nylon molding pellets
i’
of material being available for analysis. After some trials, it was found that pyrolysis a t 950" C. for 26 seconds gave the most acceptable patterns for the widest range of different polymers. These conditions result in the evolution of large quantities of Hz, CH4, CzH6, C2H4, C2Hz, and similar light gases. Compounds of intermediate volatility are also liberated, and occasionally a nonvolatile residue is obtained. Identifications are based primarily upon the moderately volatile products. The degree of individuality in the pyrolysis fingerprints is illustrated in Figures 3 to 7 . In Figure 3, the patterns
for chemically very different polymers of the general type +CH2-CH, jn are shown. In cases where the polymers are more similar in composition, such as the poly-a-olefins in Figure 4, P-GC is still quite capable of making a positive identification. The series of nylons, Figure 5 , are typical condensation polymers amenable to this pyrolysis technique. Figure 6 shows some commercial elastomers, and Figure 7 illustrates the differences among various natural fibers. With a minimum sample size of ca. 100 to 300 pg., and a total analysis time of (usually) less than 1 hour, P-GC has
-7
A , ETHYLENE-PROWLENE RUBBER
I
proved an invaluable support for infrared and other traditional polymer identification methods when the latter are beset by sampling difficulties. Dyed and filled plastics, a few flakes of unknown deposit, and carbon filled vulcanizates are typical of the difficult identifications that have been made Quantitative Analysis. An example of a procedure which is considerably accelerated through the use of P-GC is the analysis of cellulose esters. Ester pyrolysis, which results in the formation of acid and olefinic products, is a well known organic reaction. Ester types in cellulosic I
I
I
I
A . SILK
I
A
I
A
8. COTTON
b
A
C. POLYURETHANE
I
c.
I
I
I
I
I
I
I
I
Y
I I
WOOL
I
b
I
A
7 Figure 0 . Pyrograms of typical corm~,ercialelastomers, unvulcanized
Figure 7. Pyrograms of natural fibers obtained from finished cloth VOL 36,
NO. 7,
JUNE 1964
1209
r
a
I
I
I
I
I
CELLULOSE BUTYRATE
MIXED ACIDS INJECTED THROUGH STANDARDSAMPLE BLOCK
1 ACETIC
1 A
I I 15
I
20
Figure 8.
I
I
IO
5
0
Pyrograms of cellulose esters
lower right-hand chromatogram obtained by direct injection of the free acids into the normol chromatograph sampling block
I
I
I
I
I
I
I
I
I
I
LOW STYRENE COMEKT
4 Figure 9. Pyrograms of typical SBR-polybutadiene vulcaniza tes Arrows indicate the peaks used for quantitative styrene analysis
&a
w
z
I
I
I
I
-
W K
20
In
15 -
> IHIGH STYRENE CONTENT
i-
z W u CL W
n I-
BLEND
s
VULCANIZATE
I
5
0 I
22
I
I
18
I
I
14
i
1
,
10
RETENTION TIME, MIN.
1210
rn
ANALYTICAL CHEMISTRY
I
6
I
Figure 10.
0.5 1.0 1.5 PEAK HEIGHT RATIO, -20' :
-
2.0
i 5
6'
Styrene content in SBR-PBD blends and vulcanizates Ratio of peak at ca. 20' to that at ca. 6 ' used
T
loo
I
A
I
T PEAK HEIGHT RATIO, 6.1':7.5' R.T.
.
r
Figure 1 1 Analysis of polyethylene-polypropylene blends
materials are simply pyrolyzed t o free acids which are determined from their gas chromatograms. Commercial cellulose acetate, propionate, and butyrate were analyzed by this technique. Sarnples of each material were pyrolyzed at 700" C.; the resulting pyrograms are shown in Figure 8. Also shown is the pattern for a mixture of pure metic, propionic, and butyric acids injected in the normal manner through the sample port of the chromatograph. The retention times match those of the acid products of pyrolysis quite satisfactorily. Since polymer decomposition is usually quite sensitbe to relatively minor changes in pyrclysis conditiow, quantitative analysis imposes more stringent control requiremmts than are necessary in the purely qualitative approach. Such factors as total sample size (becauqe of Ion thermal conductivities) , state of subdivision (powder zs. pellet), and mode of packing in the sample tube (ease of pyrolyzate egress to reproduce any secondary reactions) can often afflxt the yield of a particular decomposition product. For quantitative work, therefore, a complete response curve was obtained for the three esters under concideration. Acid yields were satisfactorily linear with the amount of starting ester. The composition of the resins, as determined by P-GC i; given in Table I. The three 1.00-mg. samples of cellulose propionate were aiialyzed a t different times over a 6-week period. Cellulose acetate gave no evidence of acid products other th:m acetic. The time required for a complete analysis is approximately 30 minutes, which represents a con .iderable savings over the conventional saponificationdistillation-partition terhnique ( 7 ) . ;1 difficult analytical area which is ideally suited for P-CX treatment is
Figure 12. Pyrograms of polypropylenes of varying stereoregularity
determination of the composition of carbon black-filled vulcanized rubbers. A series of SBR-polybutadiene blends and vulcanizates of known composition mere examined. Their pyrograms, shown in Figure 9, were very similar, as expected, but a significant variation in the ratio of styrene (tn = 20 minutes) to the (unidentified) aliphatic com-
ponent a t t R = 6 minutes allowed the styrene content to be assayed with a precision of = t 2 7 , (Figure 10). The P-GC technique was also applied to the quantitative analysiq of blendi: of AM plastics such as polyethylene and polypropylene. Precision, as indicated by the degree of scatter in Figure 1 1 is about =k3YG. VOL. 36, NO. 7 , JUNE 1964
121 1
Microstructure. T h e third fruitful area for application of t h e P-GC technique is in obtaining information about polymer microstructure. Differences in molecular weight and distribution, stereoregularity, branching, cross-linking, or block structure can affect the kinetics and/or mechanism of the pyrolysis of homo- and copolymers ( I S , 16). For example, true copolymers of ethylene and propylene can be distinguished from physical blends of the two materials. As shown above, a smooth curve was obtained for blends of various composition of the two commercial homopolymers. However, when a block copolymer of known (IR) composition was subjected to P-GC, the ethylene content was found to be about 30y0 too high, a figure well outside the experimental error. This behavior has also been found in other systems. I n general, copolymers do not necessarily give the same proportions of decomposition products as do blends of the same composition (1). The effects of major differences in stereoregularity and/or molecular weight on the pyrolysis products of polypropylene are illustrated in Figure 12. This system is currently undergoing further investigation. Differences between some straight chain hydrocarbons and a polymethylene and polyethylene of high molecular weight are shown in Figure 13. The regularity obvious in the pyrograms of the first three compounds is definitely perturbed in the commercial ethylene polymer. The perturbation may be due to chain branching, although data are insufficient a t the present time to treat the system rigorously. Pyrolysis-gas chromatography has already established itself as a n excellent qualitative tool, and a useful quantitative instrument in selected cases. The indication that P-GC may prove useful for rapid microstructure elucidation should open new vistas to this technique as the procedures and equipment available gein in sophistication.
Table I.
I RETENTION TIME, MIN.
Figure 1 3. Hydrocarbon pyrolysis patterns, plotted on log-log scales The regularity apparent in cetane, hexatriacontane, and polymethylene is absent in polyethylene
LITERATURE CITED
(1) Barlow, A., Lehrle, R. S.,Robb, J. C., Polymer 2, 27 (1961). ( 2 ) Cobler, J. G., Samsel, E. P., SPE Trans. 2, 145 (1962). (3) Cox, B. C., Ellis, B., ANAL.CHEM.36, 90 (1964). (4) Davison, W. H. T., Slaney, S., Wragg, A. L., Chem. Ind. (London) 1954, 1356. (5) Ettre, K., Varadi, P. F., ANAL. CHEM.35, 69 (1963). (6) Karr, C., et al., Zbid., 1441 (1963). (7) Martin, A. F., in “Cellulose and Cellulose Derivatives,” Chap. XII., Vol. V of High Polymers, Interscience, N. Y . , 1955. (8) Midgley, T., Henne, A. L., J . Am. Chem. SOC.51, 1215 (1929). (9) Nelson, D. F., Yee, J. L., Kirk, P. L., Microchem. J . 6 , 225 (1962). (10) Seumann, E. W., Sadeau, H. G., ANAL.CHEM.35, 1454 (1963) (11) Quiram, E. R., unpublished work, this laboratory. (12) Radell, E. A,, Strutz, H. C., ANAL. CHEM.31, 1890 (1959).
Composition of Cellulose Esters“
Cellulose Propionate 1 00 0 50 0 75 1 00 1 00 Wt. sample, mg. Wt. yo Propionate 89 0 88 8 89 8 88 6 90 3 9 7 10 2 11 4 11 2 11 0 Wt. yo acetate Cellulose Butyrate 1 25 2 00 Av 1 00 0 75 Wt. sample, mg. Wt %acetate 29 9 29 1 29 4 29 1 3 4 3 0 2 ! 3 6 2 7 Wt %propionate 67 4 67 4 67 3 67 2 67 9 Wt. 70butyrate a Given as percentage of total ester content.
1 25 9;
i) * 0 5%
1212
ANALYTICAL CHEMISTRY
IO
Av.
7;
* 0 9%
(13) Simha, R., Advances in Chemistry 34, 157 (1962). (14) Straws, S., Modorsky, S. L., J . Res. Sat. Bur. Std. 66A, 401 (1962). (15) Wall, L. A,, in “Analytical Chemistry of Polymers,” Vol. XII, Part 11, Chap. V, G. M. Kline, ed., Interscience, S. Y., 1962. (16) Wall, L. A4., Florin, R. E., J . Res. .Vat. Bur. Std. 60, 451 (1958). RECEIVED for review Xovember 22, 1963. Accepted March 9, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1964.
Correction Theory of Stationary Electrode Polarography Single Scan and Cyclic Methods Applied to Reversible, Irreversible, and Kinetic Systems I n this article by R. S. Sicholson and Irving Shain [ ; ~ N A L . CHEM. 36, 706 (1964)l on page 722, Equation 87 should read as follows: 1
+ exp __ f f n a F ( E - EC + RT dz b+ In an.F k. E In
(Y%F
dz)]
(87)
