Quantitative Pyrolysis Gas Chromatography of Some Acrylic Copolymers and Homopolymers J. K. Haken Department of Polymer Science, University of New South Wales, Kensington, N.S. W. Australia, 2033
T. R. McKay Philips Industries Ltd., Pye-Unicam Division, Sydney, N. S. W. Australia, 2000
The degradation behavior of alkyl polyacrylate and alkyl methacrylate homopolymers and copolymers has been examined using a Curie Point pyrolysis system. A technique for distinguishing between copolymers and their homopolymer mixtures is presented together with relationships described as Homopolymer Fragmentation Indices (HFI) and Copolymer Fragmentation Indices (CFI) which allow calculation from pyrolysis data of the composition of the unknown mixture or copolymer.
While qualitative degradation studies of polyacrylate and polymethacrylate esters have appeared in many reports and several bibliographies ( I , 2 ) , much less quantitative data are available. Many of the earlier works considered relatively large samples, i . e . , 2-15 mg, and are of limited value as the desirability of small sample size with minimization of both heating variations and secondary reactions is now well known as a prerequisite in achieving precise results in pyrolysis studies. The mechanism of degradation of polyacrylic esters with essentially quantitative depolymerization of polymethacrylates and of minor monomer formation with polyacrylates has been reviewed by McCormick ( 3 ) . The most detailed study relative to the quantitative assay of polyacrylic esters is that of hlcCormick ( 3 ) who has substantiated the earlier observations of Strassburger and his coworkers ( 4 ) and of Gatrell and Mao ( 5 ) that the yield of an acrylate monomer is substantially increased when pyrolysis of a copolymer containing a polymethacrylate is carried out as compared to pyrolysis of homopolymer or mixture containing polyacrylate homopolymer. The same worker showed that it is possible to distinguish between a copolymer and a mixture of homopolymers and suggested the possibility of determining the polymer composition. Pyrolysis was carried out using single shot and stepwise degradation on a helical filament with five polymer systems, i. e . , ethyl acrylate-methyl methacrylate, ethyl acrylate-butyl methacrylate, 2-ethylhexyl acrylatemethyl methacrylate, butyl acrylate-methyl methacrylate, and butyl methacrylate-methyl methacrylate, but the range of concentrations studied was very limited. Pulse mode pyrolyzers according to the survey of the Pyrolysis Gas Chromatography Sub Group of The Gas Chromatography M . P. Stevens, "Characterization and Analysis of Polymers by Gas Chromatography." Marcel Dekker, New York, N . Y . . 1969. G . M . Brauer in "Thermal Characterisation Techniques," P. E. Slade, Jr., and L . T. Jenkins, Ed.,Marcel Dekker, New Y o r k , N . Y . ,
1970. H. McCormick. d. Chromatogr.. 40, 1 (1969). J. Strassburger, G . M . Brauer. M . Tyron, and A. F. Forziati. Anal. Chem., 32,454 (1960), R . L. Gatrell ahd T. J. Mayo, Anal. Chem., 37, 1294 (1965).
Discussion presently form 80% of all units used (6-8). Certain theoretical aspects of pyrolysis behavior have been described by Farre-Rius and Guiochon (9). For reproducible operation, radio frequency devices are advantageous provided that the experimental characteristics are stabilized, although improved filament pyrolyzers have recently been reported (10 ) . The present work considers the pyrolysis with a radiofrequency pyrolyzer of combinations of homopolymers and copolymers of the methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, and 2-ethylhexyl esters of acrylic and methacrylic acids. A constant fractional recovery of methacrylate esters from homopolymers, copolymers, or mixtures is demonstrated while with acrylates separate constants dependent on the presence of the polyacrylate as homopolymer or copolymer are evident. The use of these constants described as Homopolymer Fragmentation Indices (HFI) and Copolymer Fragmentation Indices (CFI) allows the composition of an unknown copolymer or mixture of homopolymers to be determined from pyrolysis data.
EXPERIMENTAL G a s Chromatography. Gas chromatography was conducted on a modified F & M 810/29 dual column research chromatograph with flame ionization detection and fitted with a n improved flow control system. Two 12-ft fi,-in. 0.d. aluminum columns packed with 10% OV-1 on 60-80 mesh Chromosorb W were programmed between 100 and 220 "C a t 10 "C/min with the top temperature held for 5 min before automatic recycling occurred. Helium was used as carrier gas at 40 ml/min. T h e amplifier sensitivity used was 16 X lo2 A f.s.d. for methacrylate polymers and 2 X 102 A f.s.d. for minor components of acrylate polymers. F i l a m e n t Pyrolysis. Commercial platinum pyrolysis probes with ribbon filaments [Hewlett-Packard (Aust) Pty. Ltd.] were used. Pyrolysis conditions were controlled using a variable voltage power source and a n electronic timer with accurate interval selection from 0.1 to 99 sec. T h e calibration curves provided with the probes are prepared for one gas flow rate and are of little value as the temperatures shown are far in excess of those experienced such that a major extrapolation of the calibration curve is necessary to determine the current to be applied. In service, resistance pyrolysis elements decrease in resistivity with continued service, and to maintain a uniform temperature a n increase in the applied voltage is necessary. To allow calibration a chamber consisting of a simulated gas chromatographic injection system was constructed ( I I ) from transparent material. In use, the filament was charged with a series of small crystals of known melting (6) S. G . Perry, J. Chrornatogr. Sci., 7, 193 (1969). (7) W. B. Coupe, C. E. R. Jones, and S. G . Perry, J. Chromatogr., 47, 291 (1970). ( 8 ) A. G. Douglas, J. Chromafogr. Sci., 9, 321 (1972). (9) F. Farre-Rius and G . Guiochon, Anal. Chem., 40, 998 (1968). (10) R. L. Levy, D. L. Fanter. and C. J. Wolf, Anal. Chem., 4 4 , 38 119721. (11) J. K. 'Haken, T. R. McKay, Unisearch Ltd., Aust. Prov. Patent 8111/72 (29.2.72).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
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Table I. Monomer Yields with Curie Point and Filament Pyrolysis Polymethyl methacrylate Filament Teomp,
Polymethyl acrylate Filament
Stepwise
Curie Pt
Te?p,
Yield.
C
%
OC
%
"C
%
, . .
...
450 550 650 750
84.0 98.0 98.0 92.0
350 480 510 610 770
68.0 98.0 98.0 98.0 94.0
350 450
7.0 28.0 57.0 8.0
550 650
points, and a calibration curve of melting point and applied voltage produced. Radiofrequency Pyrolysis. A Philips Curie Point pyrolyzer was used. The ferromagnetic sample probes were prepared by forming a flat surface with light hammering for lengths of 1 cm a t the end of the wires and then folding back these prepared tips to form a closed loop. Solutions of the polymers were prepared and 0.50 f 0.2 pl of solution was deposited on wires with Curie Points of 358, 480, 510, 610, and 770 " C . The amount of polymethyl methacrylate deposited was 10 f 0.4 pg and the approximate film thickness was calculated to be 0.5 p . The coated wires were stored for 24 hr to allow evaporation of solvent and the residual material was removed by allowing several minutes to elapse after mounting in the pyrolysis head prior to firing. Stepwise or Sequence T e m p e r a t u r e Filament Pyrolysis. The procedure used was as described by Barlow. Lehrle, and Robb (12, 1 3 ) and by McCormick ( 3 ) .The polymer sample was deposited on the filament by evaporation of a measured volume of solution. The sample was mounted in the injection port a t 150 "C and, after allowing sufficient time for removal of residual solvent, pyrolysis was carried out a t 250 "C for 10 sec and then a t higher temperatures, each increasing by 100 " C . The effective sample was the residue remaining after the preceding lower temperature degradation.
Curie Pt
Stepwise
Yield.
C
%
"C
%
"C
450 550 650 750
...
480 51 0 61 0 770
2.0 6.0 14.0 16.0
650 750
6.0 17.0 17.0
Oh
550
1.5 8.5 4.0
Table II. Percentage Yields of Monomer by Curie Point Pyrolyses of Homopolymers Pyrolysis Pyrolysis temp ("C) temp ("C) Acrylate Methacrylate 480a 610b 770" ester ester 480a 610b 770a Methyl 9.2 Ethyl 6.4 n-Propyl 6.8 n-Butyl 10.1 n-Pentyl 8.8 n-Hexyl 9.6 2-Ethylhexyl 4.9 a
14.0 11.3 10.8 10.7 10.7 12.0 6.8
17.1 14.0 12.5 14.0 12.6
Methyl Ethyl n-Propyl n-Butyl n-Pentyl 13.0 n-Hexyl 8.4 2-Ethylhexyl
92.0 89.0 90.0 90.6 90.1 96.0 86.0
98.3 98.0 98.5 98.0 97.0 97.3 96.4
91.8
90.0 86.2 84.0 87.0 90.2 83.2
Single determinations. Multiple determinations.
RESULTS Comparative Pyrolysis Procedures. The pyrolysis behavior and optimum degradation temperatures of polymethyl methacrylate and polymethyl acrylate were determined using a range of pyrolysis temperatures with resistance filament heating, radio frequency heating, and stepwise temperature series resistance heating. Table I shows monomer yields for the two polymers a t the temperatures used with the three pyrolysis procedures. Figu-e 1 similarly shows the pyrolysis behavior where as expected the yield of monomer from the polymethyl methacrylate is far greater than that from polymethyl acrylate; however. it is apparent that the maximum monomer yield from polymethyl methacrylate was obtained in the range 500-600 "C with each procedure, these temperatures being substantially lower than experienced with polymethyl acrylate, i e . , 650-700 "C. The temperature of maximum methyl methacrylate recovery is higher than that reported by McCormick ( 3 ) , i . e . , 410 "C, although the higher temperature of the acrylate ester is in agreement with the finding of the same worker who examined polyethyl acrylate. McCormick preferred the stepwise filament procedure for several reasons: the time required for preparing a series of samples for individual firing was too great; and also it was demonstrated that differentiation of copolymers and mixtures of homopolymers was possible. Pyrolysis of Homopolymers. It has been found convenient in the examination of homopolymers as reported ( 1 2 ) A . Barlow, R. S. Lehrle, and J. C. Robb, Polymer, 2,27 (1961) (13) R. S.Lehrleand J. C. Robb, J. Gas Chrornatogr., 5, 89 (1967).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
r
I
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, 300
400
500
600
700
800
PYROLYSIS
300
400
500
/,
I
600
700
800
TEMP E R A T U R E
Figure 1. Effect of temperature on recovery of monomer from polymethyl methacrylate and polymethyl acrylate using ( a ) Curie Point; ( b ) Filament; and (c) Stepwise Filament Pyrolysis
here and with copolymer systems to consider three reference temperatures, i e . , 480, 610. and 770 "C. The homologous polyacrylate and polymethacrylate esters were pyrolyzed at these temperatures and the yields of the appropriate monomers are shown in Table 11, and again acrylate ester recovery is small in comparison to that of methacrylate esters. Representative chromatograms are shown in Figure 2 and the peaks produced may be identified from Figure 3. The first group of peaks in Figures 2a and 2b of gaseous products and low molecular weight hydrocarbons are not effectively resolved a t the lower temperature limit of 100 "C. The yields of depolymerization products with similar molecular weights to the acrylate monomers were also small when compared to the weight of polymer examined. Peak areas were used to determine the amounts of saturated isomeric propionates, alcohols, and some hydrocarbons produced, and mass balances then accounted for only 20-3070 of the original sample, a fact due both to in-
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HOMOPOLYMERS
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