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Polymer Characterization by Evolved Gas-Infrared Spectrometry Using a Dispersive Spectrometer Richard G. Davidson* and Gary I. Mathys Materials Research Laboratories, Department of Defense, P.O. Box 50, Ascot Vale 3032, Australia
A modern ratlo recording Infrared spectrophotometer has been used to characterlze polymeric materialsby analysis of gases evolved from heated samples. Both static and dynamic systems have been examlned and evolution profiles of selected gaseous components used to further characterlze the materials. Appropriate selectlons of scan speed and Infrared cell allow effective use of the relatlvely slow scan rate of the dispersive spectrophotometer. The evolved gas technique (EGA-IR) Is useful for any process In whlch gases are evolved and Is not llmlted to pyrolysls or combustion.
The use of IR spectrometry for continuous analysis of the gases evolved from heated materials was pioneered by Liebman, Ahlstrom, and Griffiths (1) and further developed by Lephardt and Fenner (2-4). Both inert and oxidative atmospheres were used to study pyrolysis and combustion behavior. The subject has been reviewed in some depth by Lephardt (5)who pointed out that Fourier transform infrared evolved gas analysis (FT-IR-EGA) can be used for both qualitative and quantitative analysis and in thermal stability and degradation studies. Components can be identified by spectral subtraction of recognizable species such as water, and evolution profiles can be plotted in terms of temperature or time. Coincident evolution profiles for two (or more) components are strongly suggestive of a common reaction origin so that more information on the underlying chemistry becomes available. Despite these perceived advantages, very little use has been made of the EGA method. Solomon and co-workers (6) have studied coal pyrolysis by FT-IR-EGA, and Roush, Luce, and Totten (7) have interfaced a TGA apparatus with FT-IR and examined inorganic materials. Although the speed and sensitivity of the FT-IR spectrometer form the basis for its use in FT-IR-EGA, the potential value of the technique for materials characterization led us to examine the possibility of using a modern ratio recording (fast response) dispersive spectrometer, with digital computing facilities, for evolved gas work. We had earlier obtained useful results from "stopped flow" working in the analysis of some epoxy resins (8), but considered that EGA-IR would be more useful in our work of characterization of polymeric materials if we could perform continuous analysis and reduce the sample size to that used in most thermal analysis work. Use of such small samples would give useful correlation with TGA work and would also reduce the problem of self-heating due to exothermic reactions. We also considered that other processes might be studied by EGA-IR, such as desorption of adsorbed or retained solvents, absorption of, or reaction with, the gas stream, evolution of gases from reacting systems, or even rates of evaporation. In this paper we discuss both the stopped flow and continuous analysis systems for evolved gas-IR analysis, with a ratio recording dispersive spectrometer, and present some results obtained with those systems to demonstrate the broad applicability of the EGA method for the characterization of polymers.
EXPERIMENTAL SECTION Apparatus. Spectra were recorded on a Perkin-Elmer 580B spectrometer and processed with a Perkin-Elmer 3600 data system. Standard PE software routines, linked by OBEY programs, were used to acquire and process spectra. Digital resolution was 5 cm-l, this being dictated by the instrument chopper frequency. (Chopper frequency limits the digitization rate ( = digital resolution) to 1reading/cycle). At the scan speed used for this work (SURVEY)PE software sets the data interval at 5 cm-l, compared with a potential rate of 3 cm-l. A smaller interval is desirable so that digital smoothing can be used without loss of significant data. A Specac 5974 multipass cell set at 1-m path length was used for "on-the-fly" work. More detailed examination of some evolution profiles was done with a 10-cm, 7.5-mL-volume cell designed and constructed at MRL. A 20-cm, 100-mL-volume UV-vis cell was adapted for stopped flow work. Furnaces were simple resistance wire wound, insulated with asbestos tape and enclosed in a jacket. Temperature control was by a programmable, software-driven device with feedback control, designed and built at MRL. The sensing element was a standard chromel-alumel thermocouple with a 1-mm-diameterstainlesssteel sheath. High purity nitrogen (CIG P/L) was passed through a column of anhydrous magnesium perchlorate ahead of the sample. Flow rate was generally 50 mL/min. Samples were placed in porcelain boats (0.2-g samples, stopped flow) or aluminum or platinum pans (5-10-mg samples). Pyrex tubes of 2 cm or 0.5 cm i.d. were used as pyrolysis tubes, as appropriate, for temperatures up to 600 "C. Gas transfer lines were either stainless steel or plastic tube, 1/16 in. 0.d. (continuousflow), or Pyrex glass, 5 mm i.d. (stopped flow). Operating Conditions. Examination of the evolution profiles presented in Lephardt and Fenner's papers (2-4) indicated that only occasionally did an evolution event occur over less than 50 OC at 5 OC/min, so for most purposes, 5- or 10-OC intervals between spectra should be adequate. At 5 "C min, scan times of 1-2 min are sufficient to define the system, and a dispersive instrument with good fast scan capability may be used for EGA work. Most of the work reported here was done at 5 "C min to allow adequate definition of evolution profiles. Because the dispersive spectrometer records slowly with respect to concentration changes in the cell, spectral subtractions can only be performed on short sections of the spectra, and then only with caution. However, since most effluents do not contain many species, there are usually isolated or easily corrected bands available for quantitative analysis and the construction of evolution profiles. The change in concentration over a single IR absorption band occupying 1-3 s is likely to be insignificant. For accurate and reproducible construction of evolution profiles, the temperature corresponding to the measured absorption based must be determined for each spectrum. We record the times at which the first, last, and an intermediate scan start, combine these with the number of scans, heating rate, and starting time of the experiment, and enter the data into a program that calculates absorbance and temperature for each band of interest. The acquisition program cycle time is not constant because the time to write files to disk increases slightly with each successive record; the data reduction program corrects for this and for the time taken for the spectrometer to reach the wavenumber of the band of interest. Dispersive spectrometers operating at high scan rates may have relatively poor signal-to-noise ratios and digitization rates that do not allow for extensive data smoothing. It is thus necessary to have a longpath, small-volume, high-transmission cell to
0003-2700/86/0358-0837$01.50/00 1986 American Chemical Society
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Flgure 1. Effect of thermocouple positlon on monomer evolutlon from
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Flgure 2. RepeatabllHyof monomer evolution profile from PMMA (1310
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maximize SIN for continuous analysis. A balance must be sought between these factors; the Specac multipass cell selected for our work has a path length of -1 m and a volume of 45 mL. Its transmission is about 40% in our instrument. The low volume gave a high effective gas concentration and also minimized the residence time and mixing. The concentration of products in the gas stream, and hence S I N , can be increased either by raising the temperature programming rate or lowering the purge gas flow rate. The temperature program must be selected to allow adequate evolution profile definition, and the gas flow to minimize secondary reactions and excessive holdup in the cell. At 50 mL/min purge gas rate, the cell reached 20% of its initial absorbance after -2 min, which is of the same order as the scan time. There was some mixing, but the profiles recorded "on-the-fly" with this cell were similar to those recorded at a fixed wavelength in a 10-cm,7.5-mL cell (purge time 20 5). This latter cell has a volume/path length ratio similar to the Specac cell (13:l cf. 20:1), but this degree of sensitivity was not achieved under dynamic conditions because of the short residence time. Its much shorter purge time allowed more detailed examination of evolution profiles when operated at a fixed wavelength. Lephardt recommended that the temperature sensing/ control thermocouple should be immersed in the sample so as to allow feedback control in the event of exothermic reactions. We considered that such exotherms are less likely to be a problem with 5-10-mg samples and placed the thermocouple inside the reaction tube, adjacent to the sample pan. We were also concerned about possible catalytic effects of the thermocouple metals. DTA experiments indicated that 4 mg of poly(methy1 methacrylate) (PMMA) heated at 5 "C/min (endotherm) did not give a significant deviation of sample temperature from the set ramp rate (less than 0.5 "C); 10 mg of epoxy resin, cured at 10 "C/min (exotherm) gave a 1.5 OC maximum deviation. It is probable that these represent the maximum temperature variations to be expected, so the results from our EGA system are more likely to be comparable with DTA than DSC. However, sample temperature variations would need to be considered when dealing with more energetically reacting materials. Preliminary experiments with PMMA and the IO-cm, 7.5-mL-volume cell operating at a fixed wavelength indicated that the sample responded faster than the thermocouple to the heat input during the energized cycle of the controller. In Figure 1,the spikes on the broad evolution envelope represent a rapid local 2-3" increase in temperature with corresponding increase in evolution of monomer. This phenomenon was not observed when the multipass cell was used, since the sampling intervals were much greater. It was considered to be undesirable, however, and all subsequent work was done with the thermocouple outside the tube, adjacent to the sample. The temperature increase is "over-run" caused by the thermal inertia of the system, exacerbated by the slow thermocouple response. The discrimination level of the controller was 1 OC, and the relatively high energy input required to reach the upper temperature limits resulted in a larger cycle above and below the set ramp rate at lower temperatures than at higher ones. By placement of the thermocouple between the heat source and the pyrolysis tube, faster response was obtained, and the temperature ramp inside the tube was more uniform, though lagging slightly behind the furnace temperature. Such a lag is to be expected
under dynamic heating conditions in the presence of a physical barrier between heat source and detector (or sample). The difference can be corrected for by measuring the temperature inside the pyrolysis tube, as we have done, but it is more important that the system is reproducible so that valid comparisons between samples can be made. The transfer lines and cell were left at room temperature in all our experiments. This decision was originally made to simplify the system, but it turns out that the geometry of our dynamic system tends to produce stable aerosols of higher molecular weight materials that are readily detected and identified. The results are sufficiently reproducible for our purposes, and very little contamination of the cell has occurred. Most of the transfer line (all but 5 cm) was inside the spectrophotometer cell compartment, which stabilized at about 32 OC during operation. Saturation by small molecules such as water has not been observed in the continuous analysis system, as the samples are generally too small. In stopped flow work, with larger samples, we have not examined materials that yield amounts of water large enough to saturate the cell atmosphere; this would need to be considered for such materials. The spectra for continuous scan work were acquired with an OBEY file as SCAN X (between user specified limits), DIFF (subtracts previously recorded empty cell background stored in memory Y), and SAVE* (writes the corrected spectrum to disk and assigns a file number that is automatically incremented with each cycle). This program is run as a continuous loop, since the number of scans required cannot be predicted; it is terminated as required, with BREAK. Further OBEY files were compiled to extract data in the form of absorbance differences between selected peak positions and suitable base-line reference points. These data were then transferred to a VAX 11/780 computer for treatment with the resident graphics program. Reproducibility of evolution profiles may be gauged from Figure 2, which shows monomer evolution from pyrolysis of PMMA. The three analyses were performed on the same sample of PMMA powder, on three different days. Noise level under our normal operating conditions was about 0.1% at 40% T, equivalent to -0.005 A . Stopped flow work was performed by using a 20-cm cell equipped with a bypass line to allow rapid flushing between analyses. Secondary reactions were sometimes observed, which can provide information not readily obtained by the continuous analysis method. In the stopped flow work, most of the liquid pyrolysate condenses in the cooler part of the pyrolysis tube and does not normally enter the cell. Materials such a polyethylene (PE) could cause considerable contamination of either type of cell, but we would not normally examine PE by EGA. Other polyolefins such as elastomers have not given significant contamination.
RESULTS AND DISCUSSION Stopped flow Analysis. In this technique the sample (-0.2 g) is heated in a static atmosphere in steps of 50 or 100 "C. The gases evolved during these heating steps are all swept into the gas cell a t the end of each step so that the spectrum is a composite sample. It is sometimes useful for analyses in
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Flgure 3. HCI evolution profiles, 10 "C/min in nitrogen: (-) sample, (- -) PVC, (- - -) chlorinated rubber.
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which only small amounts of evolved gas are expected and no secondary reactions occur, for example, desorption of retained solvent at low temperatures. It can also be used when fast scan facilities are not available. We were able to detect traces of residual curing agent in epoxy systems by this method (8). Polyurethanes based on poly(tetramethy1ene adipate) give very characteristic breakdown products of polyadipate into COz, tetrahydrofuran (THF), and cyclopentanone,while those based on poly(ecapro1actone) yield no carbonyl compounds, and THF only if 1,4-butanediolis present as a chain extender. Some unsaturated hydrocarbons appear but have not been investigated. There is no evidence of the isocyanate in the gas spectra. In the analysis of cross-linked PMMA used for aircraft canopies, samples with different cross-linking agents can be distinguished. One sample yielded methanol between 200 and 250 "C, while another sample did not. The methanol probably arises from an inter- or intramolecular reaction that eliminates methanol, since the temperature is above that for evolution of retained methanol, which occurs between 100 and 200 "C. These samples could not be distinguished by 13C NMR spectroscopy (CP-MAS). Accelerated cure of epoxy resin with thiol-terminated polyethers cannot be readily be identified from the spectrum of the cured material, but the pyrolysis gases contain carbonyl sulfide derived from the thiol. It is possible that the sulfur is evolved as CS2,but this reacts with water to form COz,COS, and HzS. The latter has such weak absorption that it is not observed. No CS2is detected after the water vapor spectrum is subtracted. A rigid poly(viny1 chloride) (PVC) based foam is used in composite ships' hull construction, and a sample with anomalous thermal properties was found to differ from normal material by the absence of retained Freon 11blowing agent, as shown by EGA-IR a t 150 "C. This material was subsequently found to contain a phosphate ester plasticizer that made the cell walls permeable. Continuous Flow Analysis. Identification of the polymeric binder in a highly filled, weathered paint fragment was an early use of EGA-IR. Transmission IR had failed to identify the binder, which was known to be either PVC or chlorinated rubber, and a relatively crude experiment, at 10 "C/rnin, gave an evolution profile for HC1 that coincided with that obtained from chlorinated rubber under the same conditions (Figure 3). Reactions between HC1 and carbonate and oxide fillers to yield COz and water were also observed. Pyrolysis of cross-linked and non-cross-linked PMMA at 5 "C/min yielded some interesting results. Some samples gave monomer below 300 "C, in small amounts, and in reproducibly separate events. This has been attributed to the lower thermal stability of various end groups, compared with that of the ideal, saturated end group (9). The main monomer evolution event peaked at a higher temperature in the cross-linked
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Flgure 4. Monomer evolution from PMMa, 5 "C/min (1310 cm-'): un-cross-linked (PLEX 201), (-- -) cross-linked (PLEX 55).
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Figure 5. Evolution profiles from cross-linked PMMA (PLEX 55), 5 "C/min: 1-( monomer (1310 cm-I), (- - - -) C02(2360 cm-I), (- -) methanol (1050 cm-I).
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Flgure 6. Spectra of gases evolved from continuous flow pyrolysis of polyether urethane at (A) 300 OC (XIO) and (E) 330 "C (X4).
material than in the un-cross-linked sample, and in general, less monomer was obtained from the copolymers (Figure 4). Methanol and COz were evolved from some of the cross-linked systems, with the evolution profiles being dependent on the cross-linked copolymer (Figure 5). Further work on these systems is under way. Analysis of polyurethanes was characterized by the appearance of compounds associated with decomposition of the urethane linkage around 300 "C. Isocyanate, COz, amide, and amine absorptions were all observed (Figure 6). This was rather surprising, since most of these compounds have relatively low vapor pressure and would not be expected to be detected. Some base-line shift above 2000 cm-l was observed, attributable to the scattering effect of particulates in the cell. It seems probable that the geometry of the system and the relatively high flow rates has produced stable aerosols of the higher boiling components. This idea is supported by the
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Flgure 7. Evolved gas spectrum at 400 "C from urethane based on
Figure 8. Evolution profiles from polyether urethane based on poly(tetramethylene oxide) (poly THF): (-) poly THF (1I 10 cm-'), (- - - -) COP (2360 cm-'), (- - - -) isocyanate (2265 cm-I).
polyadipate, continuous flow analysis. appearance of an aerosol of e-caprolactone from a sample based on polycaprolactone. The spectrum is indistinguishable from that of the liquid, and the base-line shift increases as the concentration in the cell increases. This also gives a ready identification of polycaprolactone systems that is not given by the stopped flow method. Further support for the existence of an aerosol as opposed to condensation of the liquid on the cell optics is given by the decay profile after the maximum rate has been reached. The profile matches those of known gaseous components, e.g., carbon dioxide or methyl methacrylate (compare Figures 4 and 8) and is close to the natural decay profile of the cell at the same gas flow rate. Condensates of high molecular weight species would not be expected to evaporate so rapidly. Cyclopentanone and THF are still detected in the pyrolysis gases from polyadipate systems, but in lower yield, and a second carbonyl band is observed near 1735 cm-l (Figure 7 ) . Base-line shifts associated with the appearance of this band suggest it is from an aerosol, and the absence of characteristic C-0 absorptions elsewhere in the spectrum suggests a ketone. The appearance of this carbonyl compound is an indication that complete decomposition of the polyadipate chains into THF, COz, and cyclopentanone is relatively slow, and the residence time (
