Polymer Characterization by CoupIed Thermogravimetry-Gas Chromatography Jen Chiu Plastics Department, E. I . du Pont de Nemours & Co., Du Pont Experimental Station, Wilmington, Del. A technique combining thermogravimetry (TG) and gas chromatography (GC) is described, which follows the weight changes of a sample as it is heated under controlled conditions, collects the volatiles at various stages as shown on the TG curve, and then analyzes them intermittently by GC. The technique features precise control of temperature and atmosphere and use of minimum thermal energy to perform pyrolysis, therefore reducing the production of secondary products and providing simple and reproducible GC scans. Impurities, such as solvents, monomers, additives, etc., can be analyzed separately from the polymers. Both qualitative and quantitative information on the various components in the sample can be obtained: In favorable cases, polymer blends and microstructures can be studied.
PYROLYSIS-GAS CHROMATOGRAPHY (GC) has been established as a technique for characterization of high polymers (I-.?), Pyrolysis of the sample is usually carried out either on a hot wire or ribbon inside the injection port, or in an oven-type unit from which the pyrolysis products can be produced and then injected into the gas chromatograph. The advantages and disadvantages of each method have been extensively discussed by various authors (4-10). Thermogravimetry (TG) is a well known technique for following changes in mass of a material as a function of temperature. Its applications to high polymers have included the study of thermal stability, analysis of additives, and characterization of polymer blends and copolymers (11). The possibility of coupling the two powerful techniques, thermogravimetry and gas chromatography, has been reported (12, I.?). However, no experimental results have been given in the literature. This presentation describes a combined TG-GC technique which follows the weight changes of a sample as it is heated under controlled conditions, collects the volatiles a t various stages as indicated on the TG curve, and then analyzes them intermittently by GC. In a way, this approach is similar to the oven-pyrolysis GC method, except that the extent of pyrolysis is now guided and measured quantitatively by the
TG curve. Accordingly, this technique possesses many of the features and drawbacks of the oven-pyrolysis method. Because of the addition of a sensitive thermogravimetric analyzer, the whole apparatus is inevitably more expensive than an ordinary pyrolysis-GC unit. The total analysis time can be considerably longer, if a complete step-by-step analysis is to be made. However, in cases where isothermal TG operation is adequate, the time required is comparable to ovenpyrolysis GC. Other limitations will be discussed in later sections. On the other hand, the coupling of TG and GC offers the following features from the weight loss information: Only a minimum amount of energy is applied, and applied slowly, to break down the structure, thus reducing the production of secondary decomposition products. The gas chromatograms are simple and reproducible. Impurities in the sample, such as solvents, additives, monomers, etc., can be separated out prior to the pyrolysis of the main species, and analyzed by GC. Interaction among impurities and main decomposition products can be minimized. The amount of impurities can be determined from the TG scan. Quantitative information on the various components in the sample, including polymer blends and copolymer compositions in many cases, can be obtained from the weight loss curves. Large sample sizes can be accommodated for analysis of trace amounts of materials present in a large matrix. Also, a small sample can be injected into GC by taking only a small fraction of the pyrolysis products. The temperature and the extent of each pyrolysis step can be controlled and measured precisely. Isothermal measurements also can be made. Both effluence and residue at each weight change step can be recovered for studies by other analytical methods. Any physical form of the specimen can be handled. Coupling of other analytical techniques, such as differential thermal analysis (DTA), electrothermal analysis (ETA), infrared spectroscopy (IR), and mass spectrometry (MS), to either TG or GC is possible.
(1) B. Groten, “Gas Effluent Analysis,” W. Lodding, Ed., Chap. 4, Marcel Dekker, New York, 1967. (2) R. L. Levy, “Chromatographic Reviews,” Vol. 8, M. Lederer, Ed., Elsevier, Amsterdam, 1966, p 48. (3) Sixth Symposium on Pyrolysis and Reaction Gas Chromatography, Ecole Polytechnique, Paris, France, Sept. 15,1966. J. Gas Cliromarogr., 5, 1, 53, 107, 437 (1967). (4) K. Ettre and P. F. Varadi, ANAL.CHEM., 35,69 (1963). (5) M. Dimbat and F. T. Eggertsen, Microchem. J., 9, 500 (1965). (6) J. Zulaica and G. Guiochon, Bull. SOC.Chim. France, 1966,1343. (7) D. Deur-Siftar, T. Bistricki, and T. Tandi, J. Cliromatogr., 24, 404 (1966). (8) S. G. Perry, J . Gas Chromatogr., 5,77 (1967). (9) R. L. Levy, ibid., p 107. (10) F. Farre-Rius and G. Guiochon, ibid., p 457. (11) J. Chiu, Appl. Polym. Symp., 2,25 (1966). (12) J. Chiu, Laboratory Management, 3, No. 7, 29 (1965). (13) A. S. Kenyon, “Techniques and Methods of Polymer Evaluation,”Vol. 1, P. E. Slade, Jr., L. T. Jenkins, Ed., Marcel Dekker, New York, 1966, p 228.
Apparatus. The instrumentation is designed in such a way that the thermogravimetric analysis unit and the gas chromatograph are connected to allow for either combined or separate analysis. A schematic diagram of the coupled TG-GC system is shown in Figure 1. A Du Pont 950 Thermogravimetric analyzer of the D u Pont 900 Thermal Analysis System (Instrument Products Division, E. I. du Pont de Nemours & Company, Wilmington, Del.) and the F & M 810 Research gas chromatograph (F & M Scientific Division, Hewlett Packard Co., Avondale, Pa.) were mainly used for this work. The Du Pont unit was chosen because of its high sensitivity, small furnace tube volume (ca. 50 ml), horizontal gas flow, and versatile instrumentation. The F & M instrument was preferred in this work, because the physical arrangements of its injection port and carrier gas flow were more conveniently adaptable to the Du Pont unit, and its oven was more versatile for further modifications.
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The coupling unit is a borosilicate glass structure which consists of four switching stopcocks, A , B, C,and D,linked with 8-mm diameter glass tubing of 2-mm bore diameter, and equipped with standard 12/5 spherical joints for outer connections. A U-shaped glass trap No. 1 is clamped firmly to stopcocks A and B through two spherical joints, and placed in a suitable refrigerant for sample collection. Glass trap No. 2 is attached to stopcock B to protect trap No. 1 from gas condensation from the atmosphere. A switching valve E is installed in the carrier gas line of the G C unit to direct the gas flow either directly to the injection port for straight G C analysis, or through a regulating valve F to the coupling unit for combined TG-GC operation. All the stopcocks and sample transportation lines are heated with Glassohm resistance tapes (110 V, 22 ohms, 5 A) controlled by powerstats. A small collar heater controlled by a separate powerstat is used to maintain a proper temperature of the TG furnace tube end. Trap No. 1 can also be heated by a separate powerstat to volatilize the condensates prior to the injection. Even though the Du Pont TGA unit has a purge gas inlet on the side of the balance housing, an additional purge gas inlet is added at the end of the cold envelope to sweep out more completely the volatile pyrolysis products. In the present work, dual stainless steel packed columns, 6 feet long x '/*-inch diameter, purchased from F & M Scientific, are used, which contain Chromosorb W (acidwashed, 80-100 mesh) coated with 10% silicone rubber UC W-98. Unless otherwise specified, the column temperature is programmed from 70 to 250 "C at a rate of 6 "C per minute. Helium is used as the carrier gas at an inlet pressure of 100 psig and a flow rate of 30 ml per minute. The injection port temperature is 200 "C, and the detector block temperature, 250 "C. A dual hydrogen flame detector is used. Retention times and peak areas are read from an electronic digital integrator Model CRS 100 (Infotronics Corporation, Houston, Texas). The sample injection system is made from a No. 17 syringe needle silver-soldered to a '/*-inch diameter copper tubing tee, as shown in the upper drawing of Figure 2. One end of the tee is cut flat to install a rubber septum for direct G C injection. Another end is connected to the coupling unit to receive the sample from T G pyrolysis. The whole injection system can readily be inserted into the injection port of the F & M 810 gas chromatograph, the schematic diagram of which is shown at the bottom of Figure 2. Evidently, the coupling is an added feature which does not introduce any major modification to the existing TG or GC instruments. Procedure. In starting a typical TG-GC operation, the stopcocks are manipulated in such a way that the carrier gas flows through the switching valve E, regulating valve F, and switching stopcocks D and C to the injection port. The T G analysis is started, and the atmosphere gas flows through the switching stopcocks A and B, and the two traps to the atmosphere. Proper refrigerants are added to the traps, when a
GAS
Figure 2. Sample injection system for TG-GC desired stage is reached as shown on the T G curve. After the trapping, both stopcocks A and B are closed to the outside, and connected to stopcocks C and D. Trap No. 1 is now heated sufficiently but not excessively to volatilize the condensed products. Stopcock D is then switched toward stopcock A to pressurize the system before stopcock C is turned to inject the vaporized sample to the GC unit. After the injection, the stopcocks are returned to their original positions and are ready for the next trapping. RESULTS AND DISCUSSION
One of the essential factors in achieving high performance in gas chromatography is to use a small sample injection port and a fast plug injection of the sample vapor to reduce the A-term or the eddy diffusion term contribution to plate height according to the familiar van Deemter equation (14). Our present design involves the intermediate step of vaporizing the sample in the trap and transporting the vapors to the injection port. This inevitably reduces the column efficiency in spite of efforts made to minimize such an effect with a small volume trap, approximately 0.6 ml, and the shortest possible transportation line of a total volume of 1.8 ml. For instance, G C analysis of a sample of methyl caproate (bp 151 "C) at a constant column temperature of 80 "C showed a column efficiency of 1630 theoretical plates and a plate height of 0.11 cm in the TG-GC system compared to 1900 and 0.096 by direct injection. The delay in retention time in the TG-GC injection was less than 0.1 min, thus allowing direct injection of model compounds into the modified injection port for identification purposes without going through the T G system. It is generally recognized by pyrolysis-gas chromatographers that pyrograms are highly dependent on the pyrolysis temperature, particularly in the case of polymers (4, 15, Id). Lehmann and Brauer (15) have shown beautifully the change of the pattern of products with various pyrolysis temperatures for both polystyrene and poly(methy1 methacrylate). Simple pyrograms showing mainly the monomers were not obtained until pyrolysis temperatures were reduced to 425 "C. In fact, it has become common practice for gas chromatographers to try several temperatures to pyrolyze a sample in order to locate the proper one. However, this temperature may not be easy to duplicate from laboratory to laboratory because (14) R. Kieselbach, ANAL.CHEM., 33, 806 (1961). (15) F. A. Lehmann and G. M. Brauer, ibid.,p 673.
(16) H. J. O'Neill, R. E. Putscher, A. Dynako, and C . Boquist, J . Gas Chromatogr., 1, No. 2,28 (1963). VOL. 40, NO. 10, AUGUST 1968
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Figure 4. TG-GC of a polystyrene(PS)-poly(methy1 methacrylate) (PMMA) blend and a styrene-methyl methacrylate copolymer (S-MMA)
TG conditions: sample size, 5 mg; He flow, 160 ml/min; heating rate, 5 "C/min
of the variation in pyrolysis units. When a typical commercial polystyrene and a poly(methy1 methacrylate) homopolymer were analyzed in the coupled TG-GC system under standard conditions, the pyrograms showed mainly the expected monomers for both polystyrene and poly(methy1 methacrylate) and also the dimer for polystyrene. No painstaking experiments were necessary to find the correct pyrolysis conditions. Such simple pyrograms are very convenient for characterization work and structure studies. Three consecutive runs were made on polystyrene under identical conditions with 2-mg samples. The results read from the electronic digital integrator are shown in Table I. A reproducibility of 2z in retention time measurements and
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Table I. TG-GC of Polystyrene Monomer Dimer Area, Area, Retention, loo0 Retention, loo0 min counts min counts 4.6 4.6 4.5
238.1 235.6 229.6
ANALYTICAL CHEMISTRY
22.6 22.1 22.7
17.1 18.1 17.9
4z in peak area measurements is apparently obtainable; We are currently investigating the use of this technique for quantitative determination of complex systems. In the following sections, several examples will be given of the use of this technique for effective qualitative characterization of polymer systems. Polystyrene-Poly(a-methylstyrene)Blend. Coupled TGG C is a powerful tool for studying polymer blends. This can be illustrated by a blend of polystyrene and poly(amethylstyrene). As shown in the TG scans in Figure 3, poly(a-methylstyrene) is much less stable than polystyrene. A physical mixture of approximately one to one ratio prepared by solution casting techniques shows a first weight loss of 2 % caused by the volatilization of residual solvent benzene, which can be easily collected and identified by GC. The second weight loss step is a measure of the amount of poly(a-methylstyrene), and the third step, a measure of polystyrene. The products from the various weight loss steps are analyzed by GC, and their pyrograms are shown as bar graphs in Figure 3 to compare with those of model substances, Evidently, pyrolysis of the total mixture shows all the components in a more complex chromatogram. On the other hand, a stepwise analysis provides both qualitative and quantitative determination of each component in the sample.
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Table 11. TG-GC of Polystyrene-Poly(methy1 Methacrylate) Area. lo00 counts Beiizene
MMA
Styrene
Styrene dimer
151.0 17.2
3.2 5.3 84.4 15.0
1.2 249.1
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Styrene-Methyl Methacrylate Homopolymer Blend and Copolymer. In a polymer research laboratory, it is frequently desirable, but always difficult to distinguish between a blend of two homopolymers and their copolymers. Common techniques including solvent extraction, infrared spectroscopy, magnetic resonance, and differential thermal analysis, are useful in many cases, but futile in others. A technique based on the coupled TG-GC system has been established and can supplement greatly the existing techniques. This method is illustrated by a styrene-methyl methacrylate system. Figure 4 shows the T G scans of a 40:60 styrenemethyl methacrylate copolymer and a corresponding blend prepared from benzene solution. Except the first weight loss caused by solvent vaporization, the two T G scans do not seem to provide any convincing evidence to distinguish these two samples. As shown in the bar graphs in Figure 4, the gas chromatograms of the total pyrolysis products showed two points of interest. First, the blend shows the presence of solvent benzene. Second, a significant amount of styrene dimer is observed in the case of the blend, but not the copolymer. However, the production of dimer is not general for all polymer systems, and the amount of dimer produced is also relative. Our new technique is based on the comparison of the pyrolysis products distribution during the first 10-20 decomposition and the last 10-20%, as indicated by steps #3 and $4 in Figure 4. The results are summarized in Table 11. It is obvious that a blend of polystyrene and poly(methy1 methacrylate) produces predominantly MMA in the initial stage of decomposition and predominantly styrene in the final stage. On the other hand, a styrene-methyl methacrylate copolymer produces both MMA and styrene monomers as the main products in both the initial and the final stages of decomposition. Furthermore, the production of a relatively large amount of styrene dimer in the case of a homopolymer or a blend, and only a small amount in a random copolymer strongly suggests the possibility of using such a technique for studying block and graft copolymers. Polyethylene-Poly(viny1 Acetate) System. The use of thermogravimetry for quantitative analysis of ethylene-vinyl acetate copolymers has been previously reported (11). Acetic acid gas is evolved rapidly and quantitatively during the initial period of thermal decomposition in an inert atmosphere. The combination of G C and T G adds another feature of separating and identifying the substances evolved during the various weight loss stages, thus further establishing the decomposition mechanism proposed. Figure 5 shows the T G and GC scans for pyrolysis of a low-density polyethylene, a
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Figure 5. TG-GC of polyethylene (PE), poly(viny1 acetate) (PVAc), and an ethylene-vinyl acetate copolymer (E-VAc)
TG conditions: sample size, 2 mg; He flow, 160 ml/min; heating rate, 5 O C/min
poly(viny1 acetate) homopolymer, and an ethylene-vinyl acetate copolymer containing 30 by weight of vinyl acetate. These G C scans were obtained by programming the column from 40 to 240 "C. The weight loss step #1 liberates mainly acetic acid with a retention time of 3.2 minutes for either poly(vinyl acetate) or ethylene-vinyl acetate copolymer. The VOL. 40, NO. 10, AUGUST 1968
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an expanded weight scale, we can measure an amount of polymer coating as small as 0.01 of the glass weight from its weight loss at elevated temperatures. The nature of the coating can then be identified by the G C scans of the volatilized products. For instance, Figure 6 shows the T G curve of a commercial coated glass fiber. A sample size of 100 mg was used, and the weight scale suppressed to read 0.4 mg per inch. The total weight loss of 0 . 8 x obtained is a measure of the amount of the organic coating. The two-step weight loss pattern and the decomposition temperature range are reminiscent of those of poly(viny1 acetate). Indeed, as shown in Figure 6, the pyrogram of the weight loss step #1 shows acetic acid, and that of step #2 very similar to that observed earlier for poly(viny1 acetate). The presence of other peaks suggests other components in the sample.
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Figure 6. TG-GC of a commercial polymer coated glass fiber TG conditions: sample size, 100 mg; He flow, 160 ml/min; heating rate, 5 O C/min
second step involves the breakdown of the ethylene segments. Polyethylene produces a beautiful pyramidal pyrogram with a series of evenly spaced doublet peaks believed to be homolog unsaturated and saturated normal hydrocarbons (17, 18). By introducing comonomer units such as vinyl acetate, the ethylene segments are interrupted and the ratio of the homolog peaks is altered. The higher the vinyl acetate content, the larger is the alteration of the regular pattern. Poly(vinyl 'acetate) homopolymer represents the extreme of such an alteration. By careful analysis of the pyrogram and proper identification of the individual peaks, valuable information could be obtained of the sequence distribution of the copolymer chain. Polymer Coated Glass Fibers. Analysis of small amounts of polymer coatings firmly bound to the glass surface is extremely difficult by ordinary analytical means. Coupled TG-GC is ideally suited for such an analysis. By using a highly sensitive thermobalance and a large sample size with (17) B. Kolb, G. Kemmer, K. H. Kaiser, E. W. Cieplinski, and L. S. Ettre, 2. Anal. Cliem., 209, 302 (1965). (18) D. Noffz and W. Pfab, ibid., 228, 188 (1967).
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A technique combining thermogravimetry and gas chromatography has been described, which provides information on both the weight-temperature profile and the nature of decomposition products. The technique features precise control of temperature and atmosphere and use of minimum thermal energy to perform pyrolysis, thus reducing the production of secondary decomposition products and resulting in simpler and more reproducible GC scans. Impurities such as solvents, monomers, additives, etc., are analyzed separately from the polymers. Examples have been shown to illustrate the use of such a technique to characterize polymer blends and copolymers. It is possible that microstructures of polymers can be studied from their fragmentation pattern. With the use of a flame ionization detector, this technique is ideal for studying thermal stability of polymers and for determining trace amounts of materials present in an inert matrix. As coupled gas chromatography and mass spectrometry or infrared spectroscopy has been in use for years, the addition of thermogravimetry provides a very valuable analytical system for difficult problems. On the other hand, caution should always be exercised whenever techniques are coupled. Optimum conditions for individual techniques cannot always be achieved in a combined technique as is evident from the reduction in column efficiency in our coupled TG-GC system. We should also be aware of possible interaction among freshly formed fragment molecules in the trap. The temperature of the transportation lines and the trap should be varied to avoid condensation of high boiling components. If necessary, the high boilers can be collected in a separate trap before the coupling unit. Although the present glass coupling unit has the feature of easy cleaning, a metal unit is more feasible for automation, It is hoped that a completely automated TG-GC system can be developed in the near future. RECEIVED for review March 11, 1968. Accepted May 17, 1968. Third Middle Atlantic Regional Meeting, ACS, Philadelphia, Pa., February 1968.
