Ore concentrates were decomposed by fusion with sodium peroxide and dissolution in acid (3) followed by controlled potential reduction and oxidation of the tin. Results obtained using two NBS samples are given in Table 111. I n order to obtain good precision the samples had to be ground, put through a 200-mesh sieve, and dried a t 110’ C. Then the difference between the high and low values in both sets of results was 0.13%, but the averages do not agree with the provisional values issued by the National Bureau of Standards. I t is possible that sample inhomogeniety may be responsible for these discrepancies; for the method used by the Sational Bureau of Standards and the one described in this paper employ essentially the same procedure for decomposition and dissolution of the
samples. Also, spectrographic examination of the samples did not disclose appreciable amounts of constituents that interfere with the coulometric method. ACKNOWLEDGMENT
The authors are grateful to K. A. Cook who assisted in solving electronic problems and P. R. Segatto who contributed valuable suggestions. LITERATURE CITED
(1) Bard, A. J., Anal. Chim.Acta 22, 577
(1960).
( 2 ) Bard, A. J., Lingane, J. J., Zbid., 20, 463 (1959).
(3) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 285, Wiley, New York, 1953.
(4) Lingane, J. J., “Electroanal tical Chemistry,” 2nd ed., p. 364, {iterscience, New York, 1958. (5) Illeites, L., ANAL. CHEM. 27, 1116 (1955). (6) Meites, L., Anal. Chim.Acfa 18, 364 (1958). ( 7 ) lleites, L.. Zbid., 20, 456 (1959). (8) Willard, H. H., Diehl, H., “Advanced Quantitative Analysis,” p. 328, Van Nostrand, Ken, York, 1943. (9) Willard, H. H., Furman, N. H., Bricker, C. E.:, “Elements of Quantitative Analysis, 4th ed., p. 422, Van Nostrand, Princeton, 1956. W. hl. WISE J. P. WILLIAMS
Glass Research and Development Corning Glass Works Corning, X. Y. PRESEKTED a t the Sixteenth Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1, 1965.
Analysis of Copolymers of Methyl Methacrylate and an Alkyl Acrylate by Pyrolysis-Gas Chromatography SIR: The use of pyrolysis-gas chromatography for the examination of polymers has become well known (1-6, 8-12, 14, 15), and this technique was chosen when it became necessary to determine the composition of a limited number of acrylic copolymer samples. I n the course of that work a series of known copolymers was also analyzed as a means of evaluating the method. Analyses of copolymers consisting of two types of methacrylate units-e.g., methyl methacrylate-ethyl methacrylate-gave quantitative recovery of the monomers, as anticipated. Investigation of the copolymers containing methyl methacrylate and an alkyl acrylate-e.g., methyl acrylate-led to rather unexpected results. Although poly(methy1 acrylate), unlike poly(methyl methacrylate) , does not undergo an “unzipping” reaction, the recovery of acrylate comonomers amounted to about 40% of the theoretical value; this was unexpected, considering the reported behavior of polymers of acrylate type ( 7 ) . The consistency of the results obtained with several different copolymers of methyl methacrylate and alkyl acrylate suggested that a satisfactory anaIyticaI procedure could be developed for this type of copolymer.
tions were applied, in turn, to a nichrome wire filament; about 7 drops of 2.5% w./w. solution were used for an analysis. The filament had a resistance of 5 ohms and was heated by an a x . electrical current; the voltage was controlled within k 0 . 2 volt and set wherever desired within the range, 3 to 10 volts. A constant flow of helium over the filament was maintained during the pyrolysis t o carry the products into the gas chromatograph. It was absolutely essential to determine the influence of the filament voltage-i.e., the pyrolysis temperature-on the analytical values that
?4?4
M26
-
r‘
MMA-MA (1:ll
I
MMA-MA 0.11
EXPERIMENTAL
Samples. Using ti conventional solution polymerization technique ( I S ) , some 20 random copolymers were synthesized specifically for evaluation of the pyrolysis-gas chromatography procedure. I n each case, methyl methacrylate was chosen to be the base monomer. The various copolymer samples were dissolved in chloroform and the solu1294
ANALYTICAL CHEMISTRY
M A - M A I9:l)
4
6 8 FILAMENT VOLTAGE
1
0
Figure 1 . Analysis of methyl methacrylate-methyl acrylate copolymers Effect of filament voltage
were obtained. Figure 1 is an extreme example of the type of relationship found when a series of analyses was performed on a single copolymer at different voltage settings. I n all the cases reported here, the proper setting was in the range of 6 to 8 volts (estimated filament temperature 400-600’ C.). Some of the results were checked by analyzing the residual liquor from the copolymer synthesis. The analytical conditions were the same as for analysis of the corresponding solid copolymer, except that 5 to 10 11. of the residual liquor were injected into the pyrolysis chamber by syringe. Experiments also were performed on poly(methy1 methacrylate) and on poly(methyl acrylate). I n these cases, weighed pieces of the samples were placed directly in the filament for pyrolysis since there would be only one monomer present. I n addition, a 50 : 50 mixture of these two homopolymers mas analyzed in the same manner as the copolymers. Apparatus. T h e instrument used was a Perkin-Elmer 154D vapor fractometer with a thermistor detector. It was equipped with ?he microreactor accessory which contained two 6-port gas sampling valves. The pyrolysis chamber was connected to the lower of these valves by l/s-inch Teflon tubing. The separations were effected by means of a &foot, l/r-inch PerkinElmer column “A” followed by a 5 ’ / 2 foot, 1/4-~nch9% silicone grease column (Dow Corning high vacuum stopcock grease on 20/40 Fluoropak 80). The helium flow rate was 70 ml./minute a t ambient temperature. The column temperature was set to 75’ C., 100’ C., or 125” C. as desired, to maintain thc separation time within 1 hour. Treatment of Data. T h e peak areas were measured by means of a
Table 1.
Comparison
of
Copolymer Preparation Data with Results of Analyses
A Reaction mixture, initial mole ratio MMA/comonomer
Copolymer studied
B Reaction mixture, initial yo by vol. of comonomer (calcd.)
D
C
E
F
Reaction Residual Solid mixture, liquor, copolymer, analyzed analyzed analyzed Fractional yo by vol. ro by vol. yo by vol. recovery of coof coof coof comonomer monomer monomer monomer 0.45 0.41 0.37 0.43
MMA-EA MMA-BA MMA-EMA IVlMA-PAT A ,1lMA-BAlA
Note: MMA EMA PMA BMA
= = = =
1:l 3: 1 9: 1 1:l 3: 1 9:l
50.3 25.2 10.1 57.1 30.8 12.9
... ...
...
59.8 28.4 9.6
22.8 11.3 4.1
0.38 0.40 0.43
54.0 31.6 13.1
61.7 32.7 15.5
24.5 12.9 5.3
0.40 0.40 0.34
1:l 3: 1 9: 1 1:l 3:l 9:l
54.0 28.2 11.6 57.0 30.7 12.9
... ... ... ... ... ...
... ...
...
53.0 26.3 10.7
... ... ...
56.9 32.4 13.2
56.1 29.1 12.1
...
1:1 3: 1 9: 1
59.8 33.1 14.2
*..
57.7 30.5 12.7
Methyl methacrylate Ethyl methacrylate n-Propyl methacrylate n-Butyl methacrylate
29.7 13.4
...
... ...
... ...
M A = Methyl acrylate
EA BA
= =
Ethyl acrylate n-Butyl acrylate
With respect to the homopolymer poly(methy1 acrylate), yields of less than 2% of monomer have been reported (7) when i t was pyrolyzed under static conditions. Under the dynamic conditions employed in this study, however, the maximum recovery of methyl acrylate was 11%. This finding is significant because i t may help to explain the high yields of methyl acrylate that were obtained from the copolymers. Among the factors which possibly contributed t o these results are the high filament temperature and catalytic properties of the nichrome wire. T o compare a true copolymer and a physical mixture of the corresponding homopolymers, analyses were performed on a 50:50 mixture of poly(methy1 acrylate) and poly(methy1 methacrylate). The analytical value obtained for methyl acrylate was 9.570-i.e., 19% of the theoretical value. B y contrast, the value obtained for methyl acrylate in the 1 : l copolymer was 2070-i.e.J 41Y0 of the theoretical value. Thus i t is clear that the correction factor derived for the copolymer does not apply to the homopolymer mixture. ACKNOWLEDGMENT
planimeter. Relative detector response correction factors were obtained with 1-pl. portions of each monomer. The analytical values were obtained by dividing the corrected peak area for the monomer by the corrected total peak area. Peaks for residual solvent showed u p on the chromatograms; they did not interfere with monomer peaks and were ignored for the purpose of these calculations. The size of the air peak varied with the filament voltage and it was assumed that the products of fragmentation were not separated by these columns; the chromatograms were very simple in consequence. The similarity between corresponding values in columns B and D (or C) of Table I indicated that the monomers had reacted in the proportions in which they were mixed. On this assumption, the values in column F were obtained by dividing the numbers in column .E by the corresponding numbers :n column D. The numbers in columns C, D, and E are average values from several determinations. DISCUSSION
Those copolymers consisting of two types of methacrylates yielded quantitative results (Table I) which demonstrate the adequacy of the method. I n addition, a sample of poly(methy1 methacrylate), when pyrolyzed a t the optimum filament voltage, produced at least monomer, in excellent agreement with values reported b y Lehmann and Brauer (IO).
None of the copolymers of methyl methacrylate and a n alkyl acrylate yielded quantitative results. However, these copolymers did show greater recovery of acrylate monomer than was expected. Furthermore, each of the three series that were studied had a definite tendency to give a fractional recovery (about 40%) of acrylate monomer which was consistent within that series. (The copolymers listed in Table I were synthesized several times and corresponding information was obtained for each group of samples. These data illustrated well the variation in composition from one batch of copolymer to another, but the results were not significantly different in terms of fractional yield.) Any given copolymer was analyzed from two to four times to prepare the data in Table I; the extreme variation between duplicate values ranged from 0 to about 1% absolute. Since there occurs a consistent fractional recovery of acrylate comonomer, a n empirical correction factor may be calculated. It is evident, of course, that a method dependent upon such factors would be limited in scope; the accuracy and applicable range of concentration would be reduced because not all of the comonomer can be recovered for measurement. Within these limitations, however, the data presented here suggest that it is possible to analyze such copolymers with accuracy sufficient for many applications.
The authors thank R. W. Lietz for designing and constructing the pyrolyzer apparatus used in this study LITERATURE CITED
(1) Barlow, A., Lehrle, R. S., Robb, J. C., Polymer 2 , 2 7 (1961). ( 2 ) Barrall, E. hl., 11, Porter, R. S., Johnson, J. F., ANAL.CHEM.35, 73 (1963). (3) Bombaugh, K. J., Cook, C. E., Clampitt, B. H., Ibid., 35, 1834 (1963). (4) Ettre, Kitty, Varadi, P. F., Ibid., 35, 69 (1963). (5) Guillet, J. E., Wooten, W. C., Combs, R. L., J . Appl. Polymer Sci. 3, 61 (1960). (6) Haslam, J., Jeffs, A. R., J . Appl. Chem. 7 , 2 4 (1957). (7) Jellinek, H. H.,,G., “Degradation of Vinyl Polymers, p. 163, Academic Press, New York, 1955. (8) Jones, C. E. R., Moyles, A. F., Nature 189,222 (1961). (9) Leghissa, S., Carazzolo, G. A., Ann. Chim. (Rome) 49, 1621 (1959). (10) Lehmann, F. A., Brauer, G. AI., ANAL.CHEM.33,673 (1961). (11) Parriss, W. H., Holland, P. D., Brit. Plastics 33, 372 (August 1960). (12) Radell, E. A., Strutz, H. C., ANAL. CHEM.31,1890 (1959). (13) Riddle, E. H., “Monomeric Acrylic Esters,” p. 59, Reinhold, New York, 1954. (14) Strassburger, J., Brauer, G. hl., Tryon, M., Forziati, A. F., ANAL. CHEM.32,454 (1960). (15) Vassallo, D. A., Ibid., 33, 1823 (1961).
ROGERL. GATRELL T. J. Mao
Research Laboratories General Motors Corp. Warren, Mich. 48090 VOL 37, NO. 10, SEPTEMBER 1965
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