Analysis of Methyl Methacrylate Copolymers by Gas Chromatography JOHN STRASSBURGER, GERHARD M. BRAUER, MAX TRYON, and ALPHONSE F. FORZlATl Denfal Research Section, National Bureau o f Standards, Washington 25, D. C.
b Detection, identification, and quantitative determination of copolymers of methyl methacrylate were accomplished by gas chromatographic analysis of the pyrolysis products. The procedure distinguishes between polymer mixtures and copolymers, and detects the presence of 0.2% of copolymer. Composition can b e determined quantitatively, with a precision of &OS%, from standard curves of the ratio of peak areas a t known composition. The analysis takes only a few minutes, can b e readily adapted to other copolymers, and is especially useful for cross-linked, insoluble materials.
G
AS
CHROMATOGRAPHIC
EXPERIMENTAL
Sample Preparation. T h e copolymers were thermally polymerized (without catalyst) after removal of t h e inhibitor from t h e monomer, either by extraction with sodium hydroxide or by distillation. Solutions of varying percentages of monomers (by volume) were placed in test ANALYTICAL CHEMISTRY
D
Figure 1.
Depolymerization. PROCEDURE A. A 2- t o 3-gram sample was placed in a small still pot (10 cm. long and
Pyrolysis apparatus
CLAMP
/
STANDARD T A P E R , G L A S S
/
AKALYSIS
of the pyrolysis products of polymers has been suggested by a number of investigators for the rapid qualitative identification of polymers. Davison, Slaney, and Wragg (2), Haslam and coworkers (4, 6 ) , Radell and Strutz (6), and the authors (1. 7) used chromatograms of pyrolysis products to identify acrylate and methacrylate polymers singly or in mixtures. A literature survey indicates, however, that the quantitative determination of components of copolymers directly from the chromatograms of their pyrolysis products has not been reported. The aim of this study was to determine the suitability of gas chromatographic techniques for the identification of the components of methyl methacrylate (MlIA) copolymers and polymer mixtures containing varying percentages of methyl acrylate (Jf4) and ethyl acrylate (EA), acrylic acid (AAc) and methacrylic acid (llh-4c) , ethyl methacrylate and ethylene dimethacrylate (EDhI.1) ; the detection of small amounts of copolymeric constituents; and the direct quantitative determination of the composition of copolymers from the pyrolysis products.
454
r,u
for 2 to 3 days to complete the polymerization. Apparatus. Analyses were made using a Consolidated Electrodynamics Corp. chromatograph, Type X-26201, with dinonyl phthalate as liquid phase and ground firebrick (30- t o 70-mesh) as the solid support. The columns were prepared according t o the procedure described by Dimbat, Porter, and Stross (3).
POLYMER ,SAMPLE
NICHROME WIRE C O I L STAhDARD TAPER PLUG, TEFLON --HEAVY
COPPER WIRE
Figure 2.
LEADS ( 2 )
Sample introduction system
tubes, a n d nitrogen was bubbled through each sample for 5 minutes. T h e test tubes were immediately stoppered with a n aluminum foilcovered cork or were sealed. They were placed in a n oven and were held at 60" C. until the contents had polymerized to a solid mass. They were again placed in an oven at 100" C.
Table 1.
Operational Conditions
Procedure Procedure A B 6 6 Column length, feet Bore (inside diameter), inch '/16 '/I0 Column material 25y0 (wt.) dinonyl phthalate on ground- firebrick, 30- to (0-mesh Carrier gas Helium Helium Flow rate, ml./min. 50 50 Pressure head, p s i . 20 7.5 Column temperature, C. 140 140 20 PI. 10-15 mg. Sample quantity
18-mm. outer diameter) which n a s enclosed within a temperature-controlled air jacket (Figure I). T h e still pot, a glass tube closed at one end, had a ground-glass ball joint that could be connected to the delivery tube by a ground-glass socket joint and clamp. The temperature was raised rapidly to 350" C. and the pyrolyzate was led into a test tube immersed in an ice bath. No effort vias made to collect gaseous pyrolysis products and hence they did not appear in the chromatograms. The pyrolysis condensates were stored in a refrigerator prior to analysis. Tventy microliters were introduced into the injection valve of the chromatograph by a Gilmont micropipet syringe. Operating details are given in Table I. PROCEDURE B. An alternative pfocedure -xas used for the quantitative determinations of copolymer compositions. To avoid loss of volatile constituents of the pyrolysis, and to keep the experimental conditions as constant as possible, the polymer was degraded directly in the carrier gas stream. The inlet
system of the chromatograph was modified as shown in Figure 2. The sample was pyrolyzed on a Nichrome No. 30 wire coil having a total resistance of 7 ohms. The ends of the coil were silver soldered to two copper wire leads connected to a variable transformer by which the temperature of the coil could be adjusted. The copper wire leads were sealed into a standard taper (14/35) Teflon plug which was inserted into a (14/35) female glass joint. The glass joint was connected to the metal inlet system by two glass-to-Kovar seals. Thus, the helium gas passed over the sample and carried away vaporized pyrolysis products into the chromatographic column. A 10- to 15-mg. polymer sample was placed in the pyrolysis coil. The plug containing the coil was clamped into the glass joint, forming an air-tight seal. The voltage of the transformer was adjusted to produce a coil temperature of about 450' C. At this temperature poly(methy1 methacrylate) degraded solely into its monomer which appeared as a single, sharp peak on the chromatogram. The coil was heated for 2 minutes but this time could be shortened as the sample pyrolyzed in less than 60 seconds. Operating details are given in Table I. RESULTS AND DISCUSSION
Qualitative Identification. Various copolymers containing 50, 90, and 99% of methyl methacrylate were identified by chroma.tographic analysis of their pyrolysis products using Procedure A. Components of the pyrolysis products were detected by characteristic peaks a t definite retention times which were reproducible within 5 5 seconds. Variations in the amounts of monomeric constituents in the copolymer could be readily distinguished by changes in the peak heights on the chromatograms. Figures
3 to 5 show typical chromatograms of copolymers. Similar monomeric components of the copolymers, such aa methyl and ethyl acrylate (Figure 3) or acrylic and methacrylic acid (Figure 4), give peaks a t approximately the same retention times. However, the pyrolysis products of these copolymers are in different proportions, as evidenced by the variations in the areas under the peaks as well as the peak heights. Small amounts of copolymeric components could be readily detected. The presence of 1% of copolymer in poly(methy1 methacrylate) produced a distinct additional peak besides the single major peak due to methyl methacrylate. Even 0.2% of copolymeric constituents could be detected, but their qualitative identification from the chromatograms was not readily possible. With modification of the sensing device, it should be possible to increase considerably the sensitivity of detection of copolymeric constituents. Poly(methy1 methacrylate) is degraded with the formation of its monomer as the only depolymerization product. Even when other copolymeric constituents are present, poly(methy1 methacrylate) reverts to its monomer on heating. Hence, the chromatograms of all methyl methacrylate copolymers will have one peak due to this component and other peaks which are caused by the other constituent. Methyl methacrylate always gave the most prominent peak in the chromatograms and, in the accompanying figures, its peak height has been scaled down by a factor of 10. Although it was not the main object of this study, some of the peaks are identified by comparison with known compounds suspected to be present. For methyl acrylate polymer, the peaks
were partially identified by comparison with mass spectrographic data reported by Straus and Madorsky (8). The chromatogram of the liquid pyrolysis products of ethylene dimethacrylate polymer showed one major peak and 11 other peaks. Mass spectrographic analysis of the same pyrolysis mixture indicated the presence of about 40 mole % of CZHAO (ethylene oxide), 9 mole % of C7H14 and C310, 6 mole % of CsHls and C8H14, 5 mole % of C&r, 4 mole % of methanol, 3 mole % of benzene, methylacetylene, and C,H12, as well as 1 to 3 mole % ' of four other compounds. Homogeneous copolymers can be distinguished from mechanical polymer mixtures of the same composition, aa the chromatograms of their pyrolysis products are not identical (Figure 5 ) . Although similar degradation products are formed, their quantities-as determined by peak heights-are different. For instance, the amount of methanol in the pyrolysis products of the mechanical polymer mixture of methyl methacrylate and methyl acrylate polymers is much larger than in the pyrolysis condensate of the copolymer of the same composition. Similar chromatograms were obtained when the pure polymers were mixed before polymerization or when the pyrolyzates were mixed after polymerization in the same ratio as in the original polymer mixture. Quantitative Determination. MECHANICAL M I X T U R E S . A series of mixtures of pyrolysis products, mixed after depolymerization using Procedure A, was employed as standards to evaluate the precision of the method and to study the reproducibility of the instrument. The per cent composition of the pyrolysis products varied from 50 t o 9C% of
90% M M A - 1 0 % M A A c COPOLYMER MMA
MMA
r\
-
5 0 % MMA 5 0 % EA COPOLYMER
0 TIME
Figure 3.
IN
MINUTES
Typical chromatograms of pyrolysis products
(Change In scale is indicated by numbers on figure)
TIME I N
Figure 4.
MINUTES
Typical chromatograms of pyrolysis products
(Change In scale is Indicated by numbers on figure)
VOL. 32, NO. 4, APRIL 1960
455
methyl methacrylate. Each sample wm analyzed a t least three times. Because of the lack of sufficient thermal conductivity data for the components of the pyrolysis mixtures, it is difficult to determine accurately the absolute quantities of the compounds present from the areas under the peak. However, in all of our chromatograms a few sharp, characteristic, symmetrical peaks of reproducible height are present, Two or more representative peaks that are attributable to two components of the mixture are chosen for the calculation of the peak height ratios. From the ratios of a series of standard mixtureseach of known polymeric composition-
a calibration curve, which serves for the determination of the composition of an unknonm mixture, is obtained. The relative areas under the peaks have also been used for calculating the polymeric composition. Because the curves selected from the chromatogranis are similar in base width and shape, use of the peak height ratios is simpler and gives results of the same order of precision as those arrived at from measuring the areas under the peak by a planimeter. Figure 6 shows a plot of the average peak height ratio us. per cent composition for methyl methacrylate-methyl acrylate polymer mixtures. ;is the two iliain pyrolysis products of poly(methyl acrylate) are methanol and methyl acrylate and that of poly(methyl methacrylate) is its monomer. the ratio of the peak heights of methanol mcthyl acrylate methyl methacrylatp
COPOLYMERS. Procedure A. lho copolymeric systems, that of methyl methacrylate-methyl acrylate and methyl methacrylate-ethylene dimethacrylate, were studied. T n o specimens were made for the first copolymer system and four specimens 13-ere made for the second system, for each composition. From the standard curves of peak height ratio us. composition, the copolymer composition can be determined with a precision on the order of 1 2 or 3% in the range of 50 to 80% poly(methyl methacrylate), and on the order of =kl.Oyewhen the percentage of polj(methyl methacrylate) is more than 80%. A polymer prepared froni a commercial monomer solution containing 81% of methyl methacrylate and 19% of ethylene diniethacrylate was anal? zed using this standard analysis curve. The results nere ne11 nithin 1% of its true composition. The precision of the analysis n a s ala ays better n ith materials containiiig high percentages of methj.1 niethacrj late .In analysis of the data indicates that this change in precision did not reqult from instriiniental errors, or even from preparation of the polymer. Therefore, the greatest inaccuracy n as due to the method of depolymerization. despite the fact that precautions n ere taken to repeat thiq procedure under identical condition.. Copolymers containing about equal aniounts of methyl niethacrylate and a second coniponent are more likely to be influenced by slight changes in the operating procedure of the depolymerization than are polymers high in methyl methacrylate, as the degradation mechanism of the former is more complex and results in the formation of a larger number of very volatile products. Some volatile constituents (such as methanol or methyl or e t h j l acrylate) may be lost, el-en though the receiver in n hicli the pyrol-
+
n
t
80%MMA-ZO%MA
"i;,"
TIME
8ObMMA-20XMA POLYMER M I X T U R E
is used. Triplicate chromatographic analyses of the pyrolysis mixture shon. little variation in the peak height ratios The standard deviations of these ratios lie within the diameters of the circles surrounding the average values. From this curve, the per cent compo4ion of an unknown polymer mixture can be determined to better than 5 0 . 5 % . The slope of the peak height ratiocomposition curve will vary slightly if the ratios of the neak heights, methanol mechyl acrylate methyl methacrylate Or methyl methacrylate methanol methvl acrylate are substituted for methyl methacrylate' However, precision of the analysis is not changed appreciably by using t1ie.e alternative ratios.
+
IN M I N U T E S
Figure 5. Typical chromatograms of pyrolysis products (Change in scole i s indicated by numbers on flgure)
4.01
t
RAT,O. M A ( P E A K I + P E A K 2 ) M M A PEAK
3.0k
01
I
10
I
I
20
30
40
I
50
PER C E N T M A I N M I X T U R E
Figure 6. Curve of a series of mixtures of MMA-MA pyrolysis products Peak 1. Peak 2.
456
Methanol M A monomer
ANALYTKAL CHEMISTRY
4.d
I
I
10 20 PER C E N T ( V 0 L . )
I
30 EDMA I N
I
40 COPOLYMER
I
50
Figure 7. Standard curve for analysis of MMAEDMA copolymers
ysis products are collected is cooled in an ice bath. Although chromatographic analysis of the pyrolysis products was carried out nithin 24 to 48 hours after degradation, some change in the multicomponent reaction mixture may have taken place. Marked changes in the composit ion, as evidenced by variations of the chromatograms, were observed after storage of the pyrolysis liquids for a few months. Procedure R. To alleviate the shortcomings of this pyrolysis technique, Procedure 13 was adopted in which the decomposition products of the polymer sample are carried directly into the column. -1nalyais of methyl methacrylateethyl niethacrylate copolymers gave a linear peak area ratio-composition curve with a slope approximating unity over the 100 to 50% methyl methacrylate composil ion range examined. Statistical analysis of the results of six determinations a t each composition indicates that there is no evidence of differences betiveen duplicate specimens of the same composition. The standard deviation nieasuriiig the variability of single measureinents is approximately 0.48y0 methyl methacrylate. This value is the same over the composition range investigated. If three replicate measurement:; are averaged, the true value can be predicted with 95% confidence t o be an interval extending from 0.55% of methyl methacrylate below the average to 0.55% of methyl methacrylate above the average. Similar results were obtained for the analvsis of methyl methacrylate-methyl acrylate copolymers (Table 11). The peak area ratio-composition curve is again a straight line with a slope of 0.78. The deviation of the slope from unity may be due to the slight overlap of the methyl methacrylate peak with another peak, the possible formation of small amounts of methyl methacrylate on pyrolysis of poly(methy1 acrylate), and the difference in the thermal conductivity of the pyrolysis products. Pyrolj sis of methyl methacrylateethylene dimethacrylate copolymers on the hot iilament gives a chromatogram with only one major peak, methyl methacrylate. Because a number of peaks are present on the chromatograms of the liquid obtained on pyrolysis in air (Figure 5), the decomposition of this copolymer differs when conducted in air (Procedure A) and in the helium carrier gas (Procedure B). ilpparently, in the helium atmosphere, a reaction takes place between the depolymerized methyl methacrylate monomer and some other degradation products with the formation of a relatively nonvolatile compound. The
coniposition of methyl niethacrylateethylene dimethacrylate copolymers can, however. be determined by pyrolyzing a weighed sample on the hot filament and using the ratio of sample weight to area of the methyl methacrylate peak for obtaining the standard analysis curve (Figure 7). This calibration curve is not linear. -4 quadratic function provides a reasonably good fit, showing a slope that increases n ith the concentration of ethylene dimethacrylate. The equation of this quadratic is: y = 5 007
i0 0 2 6 2 4 ~+ 0 0 0 1 5 0 6 ~ ~(1)
where y = n-eight of sample/methyl methacrylate peak area and 2 = per cent ethylene dimethacrylate. The standard deviation characterizing the variability between triplicate determinations of the same sample is 0.06 ratio unit. There is also no evidence of sample-to-sample variability.
Pyrolysis of copolymers on a hot coil surrounded by carrier gas is most suitable for their quantitative analysis. Components can be determined with a precision of i:0.5%, The reliability of the calibration curve can be further improved by increasing the number of analyses of standard polymers in the preparation of the calibration curve. Ease, speed, and accuracy of analysis make this method advantageous for the quantitative determination of components of acrylic copolymers. It is believed that similar techniques may be readily adapted to other copolymer systems. ACKNOWLEDGMENT
The authors thank John hlandel for advice in the statistical analysis of the results and F. L. RIohler and members of the Mass Spectrometry Section of the National Bureau of Standards for the mass spectrometric analysis of the pyrolysis products of poly(ethy1ene dimethacrylate) .
Table II. Analysis of Methyl Methacrylate-Methyl Acrylate Copolymers
Composition, hIM4 Peak Area & Std. Vc UMA Total Area Dev. 90
0. 91*5 f 0 002 . ~ ~. .
80 70
0.855 i:0.001 0.774 iz 0.004
50
0.680 f.0.006 0.615 i: 0.005
60
LITERATURE CITED
(1) Burns, C., Brauer, G. 31.. Foreiati, -4. F., Abstracts, p. 118, 35th Meeting, International Association for Dental
Research, Atlantic City, March 1957.
( 2 ) Davison, W. H. T., Slaney, S., Wragg, A. L., Chem. & Ind. (London) 1954,
1356.
(3) Dimbat, M., Porter, R. E., Stross, F. H., ASAL.CHEnL 28, 290 (1956).
Using the quadratic function as a calibration line, the precision of a value read from the curve varies with the ethylene dimethacrylate concentration; the precision increases m ith increasing ethylene dimethacrylate content. For values greater than 3Oy0 of ethylene dimethacrylate, the true value can be predicted m-ith 95% confidence within less than =t2% of ethylene dimethacrylate. For values less than 30% of ethylene dimethacrylate, the precision is poorer. Honever, by using a larger number of standard samples to obtain additional points for the establishment of the calibration curve, a precision of =tO.5% may be obtained. CONCLUSIONS
Chromatographic analysis of the liquid pyrolysis products obtained upon depolymerization in air can be used readily for the detection of small quantities of copolymeric constituents or impurities, and aids in the identification and characterization of the components of copolymers and polymer mixtures.
14) Haslam. J.. Hamilton, J. B., Jeffs, ' .4. R , A i a l y s t 83, 66 (1958).
(5) Haslam, J., Jeffs, A. R., J . AppE. Chem. 7 , 2 4 (1957). (6) Radell, E. A., Strutz, H. C., ANAL. CHEW31,1890 (1959). ( 7 ) Strassburger, John, Brauer, G. Ill., Forziati, A. F., Abstracts, 36th Meeting, International Association for Dental Research; J . Dental Research 37, 86
(1958).
(8) Straus, S., Madorsky, S. L., J. Research iVat1. Bur. Standards 50, 165
(1953).
RECEIVED for review August 18, 1959. Accepted January 2 2 , 1960.
Correction Determination of Cadmium in Zinc Concentrates and Other Zinc-Rich Materials. A Cation Procedure In this article by Silve Kallmann, Hans Oberthin, and Robert Liu [ANAL. CHEM.32, 58 (1960) 1, on page 60, line 5, cupric iodide should be changed to cuprous iodide.
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