Pyrolysis Gas Chromatography of Polymers Pyrolysis makes possible the GC characterization of complex intractable materials
Pyrolysis gas chromatography (PGC), although a relatively recent analytical technique, actually is a combination of something old and something new. Pyrolysis, or controlled thermal fragmentation, has been used for many years to elucidate organic structures. More than 100 years ago isoprene was isolated from the pyrolysis of natural rubber, thereby identifying it as the monomeric unit in rubber. Gas chromatography (GC), on the other hand, is a relatively recent technique t h a t has gained wide acceptance as an extremely powerful and sensitive analytical method, b u t with a major limitation—the material to be analyzed must be admitted as a vapor. With present chromatographic colu m n technology, the compound must have a vapor pressure of at least several hundred pascals (a few Torr) at 350 °C in order to pass through the column. T h u s , if a nonvolatile material is t o be analyzed, it must be transformed into a compound(s) with sufficient vapor pressure. Pyrolysis gas chromatography originally evolved to extend the tremendous separation capability and resolution of GC to the characterization of nonvolatile materials, such as polymers. In the past few years, however, P G C has been applied to the analysis of many other materials such as paints, drugs, textiles, and even bacteria. Pyrolytic fragmentation is not a random phenomenon, but a process which can, in principle, be predicted from thermodynamic and kinetic data. However, it is difficult to predict a pyrolysis pattern before the experiment. Usually a trial and error procedure is used to determine which peaks in the chromatogram of the pyrolysis products, usually called the pyrogram, are related or are unique to the compon e n t in question. In many cases the relationships are obvious and are readily predictable on the basis of good chemical intuition. For a given set of ther-
mal energy input parameters, the molecular bonds in the material rupture in a statistically controlled manner, and the resultant radicals react via known reactions yielding a highly reproducible product distribution. Gas chromatography is also known to be a highly reproducible science, provided t h a t the operating parameters are carefully controlled. Thus, the marriage of pyrolysis with gas chromatography should yield a highly reproducible analytical system. T h e expectations were high and the method so simple t h a t many workers designed their own systems. T h e ease with which a system can be constructed, however, was a mixed blessing. T h e number of units described during the first few years following the developm e n t of P G C almost equalled the number of publications dealing with the technique. As a result, interlaboratory reproducibility was poor, prompting some workers to state, erroneously, t h a t P G C is intrinsically nonreproducible. Fortunately, second-generation pyrolysis units have emerged t h a t eliminate many of the undesirable features of the original units. Before we discuss the applications of P G C to polymer analysis, the design of various pyrolyzers and the major factors affecting the reproducibility of the PGC technique will be discussed. Pyrolyzers
Pyrolyzers can be conveniently classified into two major groups according to the method by which the fragmentation energy is delivered to the sample: • pulse-mode pyrolyzers • continuous-mode pyrolyzers Pulse-Mode Pyrolyzers. T h e two most commonly used pulsed-mode pyrolyzers are (1) resistively heated electrical filaments or ribbons and (2) radio-frequency induction-heated wires (Curie-point method). Other methods for applying a fragmentation
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GC Column
Figure 1. Typical filament pyrolyzer system The carrier gas purges the interior of the quartz tube and sweeps the volatile pyrolysis products into the GC. The power supply for the filament can range from a simple variable AC supply to sophisticated controllers with capacitive discharge and temperature feedback control systems. The walls of the quartz tube and the connections to the GC must be heated to prevent condensation of the pyrolysis products
0003-2700/80/0351-348A$01.00/0 © 1980 American Chemical Society
Report
Clarence J. Wolf Michael A. Grayson Dale L. Fanter McDonnell Douglas Research Laboratories St. Louis, Mo. 63166
^Ferromagnetic Wire Septum
Carrier Gas Preheater
rf Coils
Heated GC Inlet
Figure 2. Diagram of typical Curie-point pyrolyzer The ferromagnetic wire is heated in the rf field to its Curie point, where the alloy becomes paramagnetic and ceases to absorb rf energy. The Curie point of the wire thus provides a predetermined pyrolysis temperature
energy pulse, such as an electrical discharge or a laser, have been used, b u t t h e y represent only a small fraction of t h e pyrolyzers presently in use. F i l a m e n t or ribbon-type pyrolyzers are probably the most widely used for polymer analysis. T h e y are simple to construct a n d typically consist of an inert wire or ribbon connected t o a high-current power supply. T h e heating element is contained within a c h a m b e r through which the carrier gas flows (usually He) a n d carries t h e pyrolysis products to the chromatographic column. An example of a typical filament pyrolyzer is shown in Figure 1. T h é organic material is placed on t h e filament and pyrolyzed by resistive heating. T h e pyrolysis tempera t u r e is controlled by the current passing through the wire. P u r e platin u m or p l a t i n u m - r h o d i u m wires or ribbons are ideal for filaments since catalytic effects are minimized. Curie-point pyrolyzers are also widely used pulse units. In these syst e m s a ferromagnetic wire, held in a flowing carrier gas stream, is heated inductively in a radio-frequency field t o its Curie point, t h e t e m p e r a t u r e at which the alloy becomes paramagnetic and ceases to absorb rf energy. T h i s pyrolysis technique provides a highly repeatable pyrolysis t e m p e r a t u r e since the Curie-point t e m p e r a t u r e of a ferromagnetic material depends only upon the composition of the alloy. A schematic diagram of a typical Curiepoint pyrolyzer is shown in Figure 2. A list of t h e Curie points for various compositions of ferromagnetic wires commonly used in P G C is given in Table I. T h e t e m p e r a t u r e s range from 358 °C for p u r e nickel t o 980 °C for a 50Fe:50Co alloy. Polymers soluble in conventional solvents can be easily applied to either the filament or ferromagnetic wire in the form of a thin film. Insoluble or intractable materials are difficult to handle by any procedure; however, in many instances the pyrolyzer wires or ribbons can be bent to accommodate
these samples. Specially shaped filam e n t s (Figure 3) and Curie-point wires (Figure 4) have been designed to pyrolyze various intractable polymers. For example, t h e filament can be wound into a basket-like shape (Figure 3c) to hold a crucible containing the polymer, or in a helical shape (Figure 3f) to hold a quartz sample t u b e containing t h e polymer. Hollow ferromagnetic tubes have been used with powdered samples. In some instances, t h e ferromagnetic wire is flattened a t the end, and a small flake of the m a t e rial is pressed between the flattened regions. A V-shaped rectangular ferromagnetic strip can be used; the polymer is placed in the groove and t h e alloy squeezed shut. E x a m p l e s of each of these types are shown in Figure 4. Continuous-Mode Pyrolyzers. Continuous-mode pyrolyzers include microreactors, tubular furnaces, a n d even resistively heated ribbons. Various types of ovens have been used, b u t basically all designs include t h r e e features: (1) a uniformly h e a t e d zone, (2) an inert sample holder for inserting and removing the sample from t h e hot zone, and (3) a low dead-volume oven. A typical continuous-mode pyrolyzer which incorporates all of the above
Table 1. Curie-Point Temperature as a Function of Composition Wire composition
Curie-Point litmperature ( ° C )
100NI 48Fe:51Ni:1Cr 49Fe:51Ni 40Fe:60Ni 30Fe:70Ni 67Ni:33Co 100Fe 40Ni:60Co 50Fe:50Co
358 440 510 590 610 660 770 900 980
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(à)
i= 7 -
(a)
(b)
(c)
(d)
(e)
(f)
(β)
Figure 3. Various geometries used in filament and ribbon pyrolyzers
Figure 4. Typical ferromagnetic wires used in Curie-point pyrolyzers with and without samples
(a) double helix, (b) single helix, (c) crucible in basket, (d) ribbon, (e) Vshaped ribbon, (f) silica tube in helical filament
(a) sample coated on wire, (b) sample coated on flat, spatula-shaped wire, (c) hemi-cylindrical wire folded to enclose sample, (d) hollow tube for pow dered sample, (e) helical wire to hold solid "chip," (f) V-shaped ribbon folded to enclose sample
features is shown in Figure 5. Microreactors and tubular furnaces can be designed to provide excellent temper ature stability and control and are particularly useful for thermal degra dation and kinetic studies. A few of the pyrolyzers, such as a resistively heated ribbon, can be oper ated as either a pulsed or continuous mode system. T h e CDS Pyroprobe 100 (Chemical Data Systems, Oxford, Pa.) is such a system; it contains both a ribbon and a coil filament pyrolyzer. T h e ribbon can be rapidly heated to a predetermined temperature and held at that temperature. Alternatively, a small quartz tube can be inserted into a coil which is resistively heated with the same circuit used to heat the ribbon. Reproducibility
T h e early history of PGC was al most a continuing search for the per fect pyrolyzer capable of elevating P G C to a highly reproducible science. T h e goal was to develop interlaborato ry reproducibility in P G C , similar to t h a t demonstrated by infrared spec troscopy (IR), mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (NMR), so t h a t it could be used as a general analytical meth od. Excellent qualitative and quanti tative intralaboratory reproducibility was achieved by many workers, but in terlaboratory reproducibility was far more difficult. Although the problem has not been completely resolved, con siderable advances have been made. Recently, the American Society for Testing and Materials (ASTM), Com mittee E-19, (7) reported on the intra-
and interlaboratory reproducibility of a series of polymer samples sent to 24 laboratories throughout the world. They concluded that, "the correla tions have demonstrated t h a t it is pos sible to achieve reproducible pyrolysis data among laboratories, provided the carefully prescribed parameters for gas chromatography and pyrolysis are followed." T h e major factors controlling repro ducibility in PGC are: • type of pyrolyzer • pyrolysis temperature and tem perature rise-time • sample size and homogeneity • GC column and separation condi tions • pyrolyzer-GC interface Pyrolyzer. T h e type of pyrolyzer used for P G C is the overriding factor controlling the fragmentation product distribution since the temperature profiles in pulse-mode and continu ous-mode pyrolyzers are quite differ ent. In mosf pulse-mode pyrolyzers, the sample is in direct contact with the heat source, and the primary py rolysis products are quenched as they rapidly expand away from the heat source into a much cooler region. T h e temperature profile of a typical pulsemode pyrolyzer is illustrated in Figure 6. As soon as the primary products ex pand into the quench region, illus trated by the shaded area in Figure 6, the probability of further reaction is greatly reduced. Secondary reactions are minimized in pulse-mode pyrolyz ers since they occur only in the hightemperature region close to the wire. Continuous-mode pyrolyzers exhib it an entirely different set of condi
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tions. Pyrolysis is normally performed by inserting the sample into a contin uously heated zone. T h e momentary temperature profile of the sample and its surroundings under such condi tions is schematically shown in Figure 7. T h e temperature of the externally heated wall of the microfurnace is much higher than the temperature of the sample, and heat transfer is rela tively slow. T h e primary pyrolysis products released from the surface of the sample expand into the hot zone of the furnace, thus increasing the probability of secondary reactions. T h e surface of the sample is much hotter than the interior, and a typical "roast effect" may occur in which the outside is charred while the inside is medium rare. Temperature. It is well known t h a t reaction rates and mechanisms change with temperature. In PGC the equilib rium pyrolysis temperature (Tp) of the heat source is extremely important in determining the final product dis tribution, and most workers a t t e m p t to control this particular parameter carefully. Temperature influences the mechanism by either (1) affecting the initiation step or (2) controlling the rate of the subsequent reactions. T h e initiation reactions involved in the py rolysis of most organic materials in volve the generation of free radicals by the cleavage of a single bond or by the unimolecular elimination of a simple molecule such as H 2 0 or CO. Both of these processes are highly tempera ture dependent. T h e subsequent reac tions of these species via abstraction or combination reactions or by diffu sion are also markedly sensitive to
Controlled Pyrolysis Zone / /
Sample Quartz Tube
He Sampling Gas Supply
Gas Chromatographic / Column
He Carrier Gas Supply
Figure 5. Diagram of a continuous-mode pyrolyzer The sample in the quartz tube is passed through the isolation valve into the continuously heated zone
temperature. For example, Lehman and Brauer (2) showed that the yield of monomeric methyl methacrylate from the pyrolysis of polymethylmethacrylate is highly temperature dependent, decreasing from nearly 100% for pyrolysis temperatures less than 600 °C to approximately 20% a t pyrolysis temperatures greater than 850 °C (see Figure 8). From 700 to 800 °C, the monomer yield decreased from 80 to 35%, corresponding to 0.5% per degree. Temperature Rise-Time. It is well recognized t h a t pyrolysis temperature greatly affects the product distribution. What is not so well known and what may be equally important in PGC, is the effect of the heating rate of the pyrolyzer. T e m p e r a t u r e risetime, t r , is defined as the time required for the pyrolyzer to reach a temperature of (1 — l / e ) T p . Although rise times of 10 ms have been reported for Tp's as high as 600 °C, it is import a n t to note t h a t the rise time is markedly affected by the geometry of the filament, amount of sample, and design of the power supply. For most experiments, it is desirable to uniformly heat the sample to its pyrolysis temperature as rapidly as possible. Special power supplies utilizing capacitive boosted discharge have been used to increase the heating rate of filament pyrolyzers. T h e temperature risè-time of Curie-point pyrolyzers is markedly affected by the output of the rf power supply. T h u s , the advantage of reproducible pyrolysis temperatures inherent in Curie-point pyrolyzers can be lost if low-powered units are used. These systems may require hundreds of milliseconds to reach the desired pyrolysis temperature. Farre-Rius and Guiochon (3) estimated that in most of the pyrolyzers used in PGC studies, the actual pyrolysis temperature of the sample re-
mains unknown and depends on the sample heating rate. From known thermodynamic data they estimated the half-time for decomposition of polystyrene, polytetrafluoroethylene and polyethyleneglycoladipate at several different temperatures. Their conclusions are summarized in Table II. At 400 °C, 97% of a polystyrene sample decomposes in less than 100 ms, and even a relatively stable material such as polytetrafluoroethylene is 97% decomposed in 130 ms at 600 °C. Many experimenters who report pyrolysis temperatures as high as 800 or 900 °C are heating only the surface and the bulk of the sample is' considerably cooler (the "roast effect"). Thus, it is difficult to accurately reproduce the true pyrolysis temperature in different laboratories unless the temperature conditions are carefully controlled. Sample Size and Homogeneity. T h e temperature at which the pyrolysis actually occurs is also affected by the thickness of the sample. The pyrolysis of thin films of polyethylene, weighing between 15 and 120 μg, were studied on a resistively heated fila ment boosted by a capacitive dis charge circuit. T h e temperature-time profile of the filament as a function of sample thickness was monitored by a microthermocouple welded to the fila ment. T h e empty filament reached 650 °C in less than 20 μβ and then in creased to 750 °C in another 150 μβ. When the filament was coated with a film of polyethylene, the pyrolysis temperature decreased in proportion to the weight of the film. For example, with 120 μg of polyethylene, pyrolysis began at 450 °C and the temperature slowly increased during the course of the experiment. Therefore, the true pyrolysis temperature varied between 450 and 750 °C for samples from 120 to 15 μg. It is immediately obvious
Figure 6. Temperature profile of a pulse-mode pyrolyzer
Figure 7. Temperature profile of a con tinuous-mode pyrolyzer
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t h a t small samples are required to ensure rapid heating to a well-defined temperature. T h e method used to apply the sample to the heat source may affect the overall product distribution. Materials can be conveniently divided into two classes: those which are soluble in a volatile solvent such as water, benzene, and acetone, and those which are not. T h e soluble samples can be readily coated onto a wire (either a filament/ribbon or Curie-point wire), dried and pyrolyzed in a pulse-mode instrument. Insoluble materials can be pyrolyzed with one of the filaments/ ribbons (see Figures 3 and 4) specifically designed for solids or in a continuous-mode system. In some instances an insoluble sample can be ground into a fine powder and sprinkled onto a ribbon, packed in a hollow ferromagnetic tube, or squeezed between flat portions of a ferromagnetic material. Only a few micrograms of material spread evenly as a thin film are required. Unfortunately, many materials such as rubbers cannot be ground into a fine powder even when cooled to liquid-nitrogen temperatures. When thick or bulky samples are used, three deleterious effects occur which are minimized with thin samples: (1) products must diffuse through the sample prior to volatilization at the surface; (2) a large temperature gradient exists between the interior and exterior of the sample so t h a t uniform reaction temperatures cannot be obtained; and (3) an unknown time interval is required to raise the entire sample to the pyrolysis temperature. Therefore, as has been previously pointed out, it is imperative to use as thin a sample as is practical. GC Column and Separation Conditions. A thorough discussion of GC is beyond the scope of this article, but a few points specific to P G C are worth noting. T h e chromatographic condi-
tions used for the separation of the pyrolysis products are extremely important. While this point is obvious, its real significance is often overlooked by many workers. Pyrolysis products are generally complex mixtures often containing several different functional groups. Effective analytical PGC requires an efficient separation of these widely divergent materials; therefore columns designed for separation of specific moieties are of limited value in PGC. High-resolution GC columns containing a nonspecific phase such as a high molecular weight silicone are desirable. T h e most informative results are usually obtained when the column is temperature programmed, often in combination with cryogenic cooling. T h e most common detector used in PGC is the flame ionization detector (FID) although other detectors specific to various classes of compounds can yield valuable information about the pyrolyzate. T h e degree of information required and the specificity of the products dictate the best detector to use. Mass spectrometers have been used as P G C detectors and provide a wealth of information. However, in many instances when only intralaboratory comparisons are required, a simple detector is adequate. For the detailed analysis of a complex pyrolyzate, a mass spectrometer is highly desirable. Recently, pyrolysis mass spectrometry, (PyMS) in which the pyrolyzate is not separated chromatographically prior to mass spectrometric analysis, has been developed. However, the P y M S is considerably more complex and expensive than conventional PGC. Pyrolyzer-GC Interface. T h e amount of information which can be extracted from PGC is related directly to the quality of the gas chromatographic analysis. Since the pyrolyzer is actually a GC inlet system, all rules
Table II. Half-Time for the Decomposition of Polymers as a Function of Temperature* Temperature
