35 Pyrolysis Gas Chromatography, Mass Spectrometry, and Fourier Transform IR Spectroscopy Introductory Material S. A. LIEBMAN and E . J. LEVY Chemical Data Systems, Inc., Oxford, PA 19363
DESPITE
SUCCESSFUL PYROLYSIS GAS CHROMATOGRAPHIC (PGC) appli-
cations in laboratories throughout the world (1), diverse experimentation has made standardization of the method difficult. However, modern pyrolyzers coupled with high-performance chromatographic and spectroscopic detection systems now provide the means to study polymers, in varied sample forms and amounts, in a reproducible and informative manner. Currently, A S T M committees are assessing a standardization method for pyrolysis using the molecular thermometer concept (2). Also, statistical evaluation of the reproducibility of the method, as well as an aid in data interpretation, has been applied to key systems. This statistical or chemometrics approach permits handling of complex chromatographic and spectral results in an efficient, informative way by using pattern recognition, multivariate, or factor analysis techniques (3). Features of unique pyrolysis experimentation and interpretive aids to define microstructure and degradation of macromolecular systems have been emphasized by many researchers. Much of the information obtained by pyrolysis gas chromatography (GC)/mass spectroscopy (MS) or Fourier transform IR spectroscopy (FTIR) would be difficult to obtain by traditional spectroscopic or chromatographic techniques alone. If a single word were needed to describe the major attribute of analytical pyrolysis, it would be versatility—the ability to analyze diverse types of synthetic, biopolymers, and geopolymers in a realistic, informative manner. Pyrolysis Basics Experimentalists have used a wide range of homemade and commercial pyrolysis units for the analysis of complex materials (4). However, the most successful systems in modern laboratories are those that 0065-2393/83/0203-0617$06.00/0 © 1983 American Chemical Society
618
POLYMER CHARACTERIZATION
provide the n e e d e d control of pyrolysis temperature, heating rate, a n d other parameters. T h e factors affecting filament pyrolysis are as follows: 1. C u r r e n t , voltage, or final temperature 2. H e a t i n g rate (dependent on current, filament mass and geometry, a n d rate of heat dissipation) 3. V e l o c i t y of carrier gas 4. D u r a t i o n of pyrolysis process 5. S a m p l i n g method, a n d amount and form of sample 6. Variations i n c o i l / r i b b o n geometry 7. Pressure variations i n pyrolysis u n i t T h e pyroprobe ( C h e m i c a l D a t a Systems, Inc.), w h i c h uses resistive heating a n d temperature sensing w i t h a p l a t i n u m filament or r i b b o n , specialty microfurnaces, lasers, and C u r i e point pyrolyzer arrangements, are used w i t h varying degrees of sophistication. A large majority of analytical p r o b l e m solving may be performed w i t h rather moderate instrumentation, such as interfacing the pyrolysis u n i t to a G C capable of m o d e r n high-performance operation (i.e., capillary c o l u m n separation, m u l t i p l e detectors, a n d close temperature and flow control). A typical p o l y m e r fingerprint (or pyrogram) of the pyrolysis fragments separated i n such a G C analysis provides micro structural information w h e n studies are conducted w i t h proper experimentation. F o r example, the pyrograms resulting from the pulse pyrolysis w i t h a P y r o probe at 750 °C of atactic and isotactic polypropylenes (PP) are easily distinguishable patterns (Figure 1). T h e f u l l interpretive value of analytical pyrolysis data for microstructural studies of P P systems has b e e n reported (5), a n d is discussed regarding other polymers i n this volume. Extended
Applications
R e s i d u a l V o l a t i l e s . A n o t h e r aspect of p o l y m e r characterization involves the analysis of trace residuals—solvents, monomers, a n d additives. W i t h m i n o r experimental modifications, static headspace analysis may be conducted by isolating the Pyroprobe interface from the G C flow stream and heating the sample for the desired time/ temperature interval (e.g., 10 m i n at 150 °C) b e l o w the degradation temperature. T h e volatiles are t h e n swept from the interface b y the H e carrier stream into the appropriate G C c o l u m n h e l d at ambient or subambient temperature to trap the e v o l v e d gases. Temperature programming of the c o l u m n i n the normal manner provides a direct analysis of the sample. F i g u r e 2 shows this comparative analysis of volatiles of two P P samples obtained from different manufacturers. T h e residual volatile contents are significantly different. S i m i l a r l y ,
35.
LiEBMAN
I·
Pyrolysis GC, Mass Spectrometry, and FTIR 619
A N D LEVY
W
H
··
s e
(β
Ό
>0
M
IN
TIME—MINUTES
Figure 1. Pyrolysis capillary GC of polypropylenes at a rate of 750 °CI 10 s. Key: top, isotactic; and bottom, atactic. the analytical pyrolysis experiment may be easily configured to s i m u late some process or in-use exposure condition (e.g., heating i n air at 200 °C for 2 min). T h e same sample may then be subjected to flash pyrolysis conditions to p r o v i d e f u l l characterization data. I n this m a n ner, m i n o r variations or residuals from polymerization or purification schemes do not complicate the microstructural analysis. M o r e impor-
620
POLYMER CHARACTERIZATION
TIME
iuU Figure 2. Volatiles in polypropylenes. tantly, performance data may be correlated to the presence or absence of some components that may be critical for p o l y m e r i z a t i o n or processing conditions. P r o g r a m m e d P y r o l y s i s and F T I R . I n addition to the flash pyrolysis mode, analysts may use a programmed pyrolysis or timeresolved P G C configuration. T h i s approach was needed to relate degradation fragments p r o d u c e d from conventional thermogravimetric analyses ( T G A ) to products produced b y pyrolysis. T h i s result was achieved by programming the Pyroprobe c o i l at a heating rate of 2, 5, 10, etc., °C/min to correspond to the rate used i n the T G A experiments
35.
L i E B M A N AND LEVY
Pyrolysis GC, Mass Spectrometry, and FTIR 621
(6). Interfacing the Pyroprobe to an automatic G C sampling device, or F T I R (7) p r o v i d e d the necessary information. P G C data were obtained for v i n y l c h l o r i d e - p r o p y l e n e copolymers a n d complex composites i n microstructural a n d degradation studies. L e p h a r d t reported previously (8, 9) that e v o l v e d gas analysis w i t h an F T I R to serve as a sensitive spectral monitor for chosen volatiles. Figures 3—5 present the information frorn this type of experimentation. L i g n i n and cellulose samples were heated linearly and the released volatiles were r e m o v e d continuously b y the N carrier gas, and t h e n passed through a heated gas c e l l m o u n t e d i n the spectrometer. D u r i n g the sample heating, I R spectra of the e v o l v i n g species were collected repetitively. Spectral absorbances at frequencies k n o w n to be due to the chosen c h e m i c a l species were subsequently plotted vs. temperature to obtain e v o l u t i o n profiles. These e v o l u t i o n profiles are shown for the above systems. T h e Kraft p i n e l i g n i n samples prov i d e d an e v o l u t i o n profile of p h e n o l i c species (Figure 3). T h e spectral subtraction technique p e r m i t t e d separation of two phenols (A and B) and patterns for each were obtained (Figure 4). A d d i t i o n a l l y , S 0 detection gave information on the cross-linking system. A cellulose sam2
2
fl flfl 1 flfl flfl Oflfl flfl Oflfl flfl ÂOOiflflCflfl flfl fiflfl flfl *7flfl flfl Qflfl flfl 0*00
100.00
£00.00
300· 00
400» 00
300· RJ
TEMPERATURE.
C
000. DD
/00.DO 000· 00
Figure 3. FTIR-evolved gas analysis. Phenol A and Phenol Β evolution profiles from lignin. (Reproduced from Ref. 8. Copyright 1977, Ameri can Chemical Society.)
622
POLYMER CHARACTERIZATION
1650 1550 1450 1350 1250 1150 1050 950 CM"'
850
750
Figure 4. FTIR-evolved gas analysis. IR spectra (1650 — 750 cm' ) Phenol A (top) and Phenol Β (bottom). 1
of
pie was examined i n a thermal d e p o l y m e r i z a t i o n mode b y f o l l o w i n g an apparent end-group specific product (formic acid) vs. a c h a i n p r o d uct ( H 0 ) (Figure 5). T h e e v o l v e d gas F T I R experiments provide i n v a l u a b l e insight regarding kinetics a n d mechanisms i n thermal degradation studies. T h e ability to optimize product formulations w i t h respect to specialty 2
35.
LiEBMAN
AND LEVY
Pyrolysis GC, Mass Spectrometry, and FTIR 623
additives (e.g., smoke suppressants, flame retardants, promoters) has b e e n a c h i e v e d w i t h such s i m u l a t i o n studies for the material sciences (JO). T h e particular advantage o f F T I R is evident over the experimentally l i m i t e d , but perhaps more quantitative approach, of automatic G C s a m p l i n g a n d analysis. Pyrolysis MS and GCIMS Synthetic P o l y m e r s . T h e g r o w i n g f i e l d of pyrolysis M S a n d G C / M S of synthetic polymers has benefited from the contributions o f many researchers u s i n g C u r i e point, Pyroprobe, laser, or other direct insertion pyrolyzers. Some o f the outstanding developments documented deal w i t h polyamides and n y l o n degradation mechanisms (I J , 12). Other work has i n v o l v e d catalytic effects on polystyrene thermal degradation at the initiation stage. T h e catalytic effect of small amounts of L e w i s acids, salts, or bases was studied w i t h condensation polymers to assess the sensitivity o f the i n i t i a l degradation temperature of the system and production of oligomers and monomers. A l s o , n e w extensions of laser microprobe mass analysis ( L A M M A ) are reported b y H e r c u l e s et a l . i n applications to synthetic polymers.
150.00
200.00
250.00
300.00
350.00
TEMPERATURE. C.
Figure 5. FTIR-evolution
profiles of formic acid and water from cellulose.
624
POLYMER CHARACTERIZATION
Natural P o l y m e r s . U n i q u e information p r o v i d e d from tempera ture programming the thermal device a n d l e a d i n g degradation p r o d ucts of natural polymers directly into the M S has b e e n r e v i e w e d (13). T h e experimental arrangements n e e d e d for such studies have b e e n specialized to provide l i m i t e d b u t significant data o n a variety of biopolymers. A n overview discussed i n this v o l u m e highlights the advances made i n the b i o c h e m i c a l a n d b i o m e d i c a l f i e l d , not only w i t h pyrolysis M S , b u t w i t h the f u l l analytical pyrolysis input. F r o m Reiner's initial report i n 1965 (14) o n fingerprinting microorganisms using flash pyrolysis G C , to the current h i g h sensitivity, sophisticated thermal-programming M S analysis, one can appreciate the ac complishments i n this f i e l d . A d d i t i o n a l l y , to be aware that advanced pyrolytic techniques have b e e n a p p l i e d to both synthetic and natural polymers w i t h directly analogous methodologies is a focal point of this volume. Interpretive
Aids
F o r the most d e m a n d i n g of analytical pyrolysis applications— complex composites, microstructural details of synthetic biopolymers and geopolymers—the i n p u t of chemometrics is p r o v i n g to be i n v a l u able. Pattern recognition a n d other statistical or computer-assisted techniques are b e i n g a p p l i e d to a i d interpretation of pyrograms at an increasing rate w i t h many successful applications (1,3). Effective data management is mandatory for the massive amount of information gen erated i n typical pyrolysis G C / M S or F T I R experiments. Illustration of such experimental designs u s i n g the chemometrics approach is discussed elsewhere i n this v o l u m e . Literature Cited 1. "Selected Bibliography of Analytical Pyrolysis (1973-1980)," Applications Laboratory, Chemical Data Systems, Inc. 2. Levy, E. J.; Walker, J. G. J. Chromatogr. Sci., in press. 3. "Chemometrics: Theory and Applications"; Kowalski, B. R., Ed.; ACS SYMPOSIUM SERIES No. 52, ACS: Washington, D.C., 1977. 4. "Selected Bibliography of Solids Pyrolysis (1960-1973)," Applications Lab oratory, Chemical Data Systems, Inc. 5. Sugimura, Y.; Nagaya, T.; Tsuge, S.; Murate, T.; Takeda, T. Mac romolecules 1980, 13 (4), 928. 6. Liebman, S. Α.; Ahlstrom, D. H.; Foltz, C. R. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 3139. 7. Liebman, S.A.;Ahlstrom, D. H.; Griffiths, P. R. J. Appl. Spectrosc. 1976, 30, 335. 8. Fenner, R. Α.; Lephardt, J. O. J. Agric. Food Chem. 1981, 29, 846. 9. Lephardt, J. O. et al.J.Appl. Spectrosc. 1980, 34 (2), 174. 10. Liebman, S. A. et al., Presented at the ACS 172nd Natl. Meet., Org. Chem. & Plastics Div., San Francisco, CA, 1976. 11. Luderwald, I. Angew. Makromol. Chem. 1978, 74, 165. 12. Ibid. 1978, 67, 193. 13. Risby, T. H.; Yergey, A. L. Anal. Chem. 1978, 50, 327A. 14. Reiner, E. Nature (London) 1965, 206, 1272. ACCEPTED October 27, 1981
