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Investigating the degradation of thermoplastics by thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR)
Polymer Testing 15 (1996) 75-89 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0 142-94 1S/96/$ I 5.oO
0142-9418(95)00025-9 ELSEVIER
TEST METHOD Investigating the Degradation of Thermoplastics by Thermogravimetry-Fourier Transform Infrared Spectroscopy (TG-FTIR)* P. Moulinik,” R. M. Paroli,tb Z. Y. Wang,?” A. H. Delgado,b A. L. Guen,” Y. Qi” & J.-P. Gao” Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada KIS 5B6 “Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR6 “Ottawa-Carleton
(Received
1 May 1995; accepted 21 June 1995)
ABSTRACT Themogravimetry-Fourier
transform infrared spectroscopy (TG-FTIR) has been used
to study the thermal degradation Poly(arylene
ether)s
containing
of three
types of synthetic thermoplastics:
an acenaphthylene
moiety, poly(arylene
ether
ketone)s prepared as analogues of the commercial polymer poly(ether ether ketone) (PEEK), and aromatic polyimides synthesized by a single-step solution polymerization. TG-FTIR was found to be useful in providing important structural information about the poly(arylene ether)s and polyimides, which was not apparent through conventional NMR or FTIR techniques. Spectra obtained using air as a purge for insoluble and soluble poly(arylene ether)s 1 revealed that residual sulfur remained in both polymers after preparation through a reaction with Lawesson’s reagent, Comparison of TGMN-FTIR results of new poly(arylene ether ketone)s 2 with previously reported data on degradation studies of PEEK obtained with TG-mass spectrometry (TG-MS) showed that both polymers had similar degradation patterns, with phenol being a major degradation product. TG-FTIR studies on polyimides 3 and 4 synthesized by one-step solution polymerization demonstrated that it was possible to determine the imidization degree by monitoring the intensity of the water absorption band
*Presented in part at the 40th Canadian Spectroscopy Conference, 1994. tAuthors to whom correspondence should be addressed. 75
Halifax,
N.S., August
8-10.
76
P. Moulinic? et al.
at 179.5 cm-’ as a function of the TG temperature during a weight-loss occurring near 300°C. The FTlR spectra of both polyimides studied in this work showed that carbon monoxide was a significant degradation product. Other degradation products detected for polyimide 3 were ammonia, isocyanic acid and phenyl isocyanate. FTIR peaks consistent with hydrogen cyanide also appeared for polyimide 3.
1 INTRODUCTION
Thermogravimetry (TG) is a thermoanalytical technique where the weight of a sample is monitored during a linear heating program under a constant purge of inert gas or air. For some materials, such as polymers, loss of impurities, (i.e. residual solvent or starting material) or thermal degradation can be observed as weight-losses in the samples during the TG experiment. TG can be used to assess the purity or thermal stability of a polymer and is now a widely used tool in polymer science. A limitation to this technique, however, is that it does not identify the gases which are being evolved during the associated weight-losses. In TG-FTIR, the purge gas used for TG acts as a carrier for the gaseous degradation products. These products are passed through a heated transfer line to a FTIR gas cell. Thus, the FTIR spectra of gases evolved during a TG experiment can be recorded as a function of the TG temperature and interpreted to identify degradation products. Many examples on the use of TGFTIR in identifying degradation products from various materials are available in the literature.ld Thermal stability may be an important factor when considering applications and, therefore, knowledge of the degradation productions helps in understanding the behaviour of the material. In this work, TG-FTIR has been used to determine the products evolved for new synthetic thermoplastics before and during thermal degradation. Three types of thermoplastic polymers were studied: poly(arylene ether)s, poly(arylene ether ketone)s, and aromatic polyimides. The structures for these polymers are given in Fig. 1. Although TG-FAIR is mainly used in the identification of thermal degradation products it can also be used to infer important structural information about the polymers. Poly(arylene ether)s 1 prepared by reacting polymer 5 with Lawesson’s reagent were tested by TG-FTIR for the presence of residual sulfur. Similarly, polyimides 3 and 4 were tested for the completeness of imidization through TG-FTIR experiments. The degradation products from new poly(arylene ether ketone)s 2 prepared as analogues of commercial poly(ether ether ketone) (PEEK) were also investigated.
Degradafion
77
of fhermoplasfics
polymer
laQ&I’
\’
1
1;
/ \
-
\/
‘\
/
-
3
3
n
d BP = 3,3’-, b) BP = 3,3’-,
2
A=
-+
c) BP = 4,4’-,
3
n
4
Fig. 1.
New synthetic polymers analyzed by TG-FTIR
2 EXPERIMENTAL 2.1 Polymer preparation The preparation of the polymers studied in this work have been reported previously.“-7 The information sought out from TG-FTIR experiments resulted from the TG/DTA thermograms obtained for each polymer. Particular attention was given to the poly(arylene ether)s and polyimides since both exhibited TG weight-losses occurring prior to overall thermal degradation. These weight-losses could provide valuable structural information. 2.1.1. Poly(arylene etherjs Soluble and insoluble polymers 1 were obtained from prepolymer 5 upon treatment with Lawesson’s reagent (2,4-Bis(4-methoxyphenyl)-1,3-dithia-2,4diphosphetane-2,4-disulfide) in dilute and concentrated solutions, respectively . During TG experiments, a sulfur-like smell emanated from the sample. TG-PTIR was then used to confirm the presence of sulfur on the polymer.
78
P. Moulinit! et al.
TG-FTIR experiments, using an air purge, were run on samples after the ringclosing reaction with Lawesson’s reagent. 2. I .2 Poly(arylene ether ketone)s Poly(arylene ether ketone)s 2a-c have been synthesized as structural derivatives of the commercial thermoplastic PEEK. Due to the biphenyl structure these new polymers have higher glass-transition temperatures (T,) and greater thermal stabilities than PEEK. Polymers 2ax were studied by TG-FTIR in order to compare their thermal degradation behaviours to that of PEEK. 2. I .3 Polyimides The polyimide formation consists of two steps involving an initial reaction between a diamine and a dianhydride, followed by a ring-closing imidization.” This two-step imidization process is shown in Scheme 2 for polyimide 4, for which both steps were carried out in a single solution polymerization. TGFTIR was used to assess the completeness of imidization. 2.2 TG-FTIR
scans
Samples in powder form, weighing 10-15 mg, were used for these experiments. The samples were placed in a Pt pan and run in a Seiko TG/DTA 320, from ambient temperature to 800°C using a heating rate of lO”C/min. A flow rate of 100 ml/min of nitrogen or air was used for each run. A Nicolet TGA-IR accessory was used to transport the evolved gases from the TG/DTA to a Nicolet 800 FTIR spectrometer. This accessory provided a heated transferline from the TG/DTA to the FTIR spectrometer and oven for the FTIR gascell to prevent condensation of the gases evolved from the TG. The transferline temperature was kept between 230-240°C while the oven for the gas cell was maintained at 260°C. A gas cell (10 cm length x 5 cm diameter) containing a mirror and a KBr window was placed in the TGA-IR oven. All FTIR spectra were collected with 8 cm-’ resolution. Backgrounds were collected when the TG/DTA temperature was between 100-I 10°C. Acquisition of TGFTIR spectra were started once the TG/DTA temperature reached 115°C. FI’IR spectra were collected at a rate of 12 spectra per min (i.e. per 1O’C) through the remainder of the TG/DTA scan. Maxima present in the 1st derivative of the TG thermograms (known as the DTG) were used to locate FTIR spectra of interest. Gram-Schmidt Reconstruction (GSR) plots (which describe the total detector response during the collection of FTIR spectra) were also used to locate FTIR spectra containing products from TG weight-losses. GSR plots are similar to DTG plots, although relative signal intensities may differ because of varying spectra responses for different compounds. Spectra chosen for analyses corresponded to local (or
Degradation of thermoplastics
79
80
P. Moulinit? et al.
pdymic
Scheme 2
acid
Reaction scheme for the formation of polyimide 4.
absolute) maxima in the DTG curves and Gram-Schmidt plots. Water peaks, which obscured many of the degradation product absorption bands, were removed by spectral subtraction using a TG-FTIR spectrum from the same experiment which contained water. The spectrum used for subtraction was collected early in each experiment (typically at 12515OOC) and corresponded to a stable mass region in the TG/DTA curves. The evolution of individual decomposition products was constructed by measuring the baseline-corrected intensity of a known absorption band as a function of the TG/DTA temperature.
3 RESULTS AND DISCUSSION 3.1 Poly(arylene
ether)s
3.1. I Presence of residual sulfur The preparation of polymer 1 was achieved by chemical modification of polymer 5 using Lawesson’s reagent (Scheme 1). This reaction was carried out in either dilute or concentrated solutions, yielding soluble and insoluble 1, respectively. Both polymers had intense red colours, which suggested the presence of the acenaphthylene moiety in their backbones.5 The acenaphthylene formation is believed to go through an intermediate containing a cyclic disulfide, as shown in Scheme 1. It has been shown, however, that this reaction scheme is sensitive to reaction conditions, with conditions a and b yielding soluble and insoluble materials, respectively. It is believed that the insoluble behaviour of material lb results from cross-linking with sulfur. Various pathways to cross-linking with sulfur are possible, including the well-known vulcanization process. The thermograms for both materials
81
Degradation of thermoplastics
showed a two-step weight loss profile with about 10% mass loss near 250°C (Fig. 2). A sulfur-like smell also emanated during this weight loss, suggesting the presence of residual sulfur in both polymers 1. The insolubility and the weak TR intensities of vibrations of sulfur groups, however, made NMR and FTIR methods unsuitable for confirming the presence of residual sulfur in these materials. Using TG-FTIR with a nitrogen carrier gas provided no useful information regarding the identity of the degradation products. Again, a strong sulfur-like smell was given off during the first weight-loss. When air was used as a carrier gas, the TG-FTIR spectra for soluble and insoluble polymers 1 displayed absorptions characteristic of SOi (Fig. 3). This can be attributed to a reaction between oxygen (present in the carrier gas) and residual sulfur in the polymer. This reaction can occur either as sulfur is released from the polymer at high temperature or with sulfur on the polymer itself. A plot of FTIR band intensity versus temperature for SOz (1377 cm-‘) is given in Fig. 4. From this plot, it is apparent for both soluble and insoluble polymers 1, that SO, is only given off during the first weight-loss. Hence, it is possible to confirm that the first weight-loss for 1 corresponds to the loss of residual sulfur. Although soluble polymer 1 had an intense red colour and was soluble in most organic solvents, TG-FTIR studies have shown that sulfur is present.
8
Soluble -----_
0 F
268.5C
2*. 345.9c - \ 94.3%
530.6C 3 81.746 \ \ \
25
I 180
I 335
I 480
l--_ I 645
800
Temp C (Heating) Fig. 2.
TG thermograms for polymer la and lb in nitrogen.
P. Moulinit2 et al.
82
Fig. 3.
TG-FHR
100
spectra for first mass loss in air for soluble and insoluble polymers and reference infrared spectrum of SO;?.
150
200
250
300
350
400
Temperature (“C) Fig. 4.
SOz absorbance of 1377 cm-’ versus temperature for polymers 1.
450
Degradationof thermoplastics
83
3.1.2Degradation products The thermograms of these polymers (shown in Fig. 2) show that the onset of overall sample degradation occurs near 480°C. Based on the TG-FTIR of these polymers performed in N2 it can be seen that phenol is the dominant degradation product. Figure 5 shows the reference spectrum for phenol and the TG-FJXR spectra at 575 and 540°C for soluble and insoluble polymers 1, respectively. This suggests a simple chain scission degradation mechanism, in which cleavage occurs at the ether and quaternary carbon linkages, yielding phenol. No other degradation products were visible in the TG-FTIR spectra. It can be expected, however, that other possible products from a random scission reaction such as 9,9_diphenylfluorene or acenaphthene would not be detected during TG-FTIR experiments due to their high boiling points. The boiling points of acenaphthene and 9,9-diphenyl fluorene, for example, are 279 and 295OC, respectively, and would thus not be sufficiently volatile to pass through the TGA-IR accessory (i.e. they condense). For both these polymers, a red-brown deposit was observed at the junction of the TG furnace and transfer line, suggesting that a conjugated product, such as acenaphthene, had precipitated before passing through the transfer line.
Fig 5.
TG-FTIR spectra for phenol and polymers 1.
84
P. MoulinG et al.
3.2 Poly(arylene ether ketone)s Degradation of PEEK has been studied previously using TG-MS, a related technique in which the evolved gases are passed from the TG to a massspectrometer.” In these experiments, Day and co-workers found that PEEK exhibited random chain scission upon degradation. Phenol was found to be the predominant degradation product with smaller amounts of benzene and dibenzofuran also being detected. Thus, the degradation products of PEEK were related to the connector groups present in the polymer repeat unit. The onsets of degradation for polymers 2a, 2h and 2c occurred at 497, 489 and 512”C, respectively. The TG-FTIR spectra corresponding to the degradation products for polymers 2a-c gave infrared absorption bands which correlated well with carbon monoxide and phenol. In addition, polymers 2a and 2c gave good correlation with the spectra for 4-propylphenol and isopropyldiphenol (Bisphenol A; BPA), based on weak non-aromatic C-H absorbances between 2900 and 2975 cm-‘. The TG-FTIR spectrum for polymers 2a-c at 500, 545 and 53YC, respectively, along with reference spectra for 4-propylphenol, BPA, phenol and carbon monoxide are shown in Fig. 6. These degradation products are closely related to the connector groups in the polymer, suggesting that the degradation of these polymers is also a scission reac-
3500
Fig. 6.
TG-FTIR
37W
2x0
xc4
lyxl
Imo
5.m
spectra for degradation poly(ary1 ether ketone) and of 4-propylphenol, bisphenol A, phenol and carbon monoxide.
Degradation
of thermoplastics
85
tion. Scission at either the ether linkages or car-bony1 bonds in these polymers can yield carbon monoxide or phenol as degradation products. Thus, these polymers have similar degradation behaviour to PEEK. Cleavage may also occur at the C-C single bonds for polymers 2a and 2c. Polymers 2a-c also contain biphenyl connector groups. It was expected that absorption bands corresponding to biphenyl groups in the TG-FTIR spectra would help corroborate the premise of a random chain-scission mechanism. The absence of these species, however, may be due to condensation of the product before reaching the gas cell, since it has a high boiling point (254255°C). The TG-FUR spectra collected for polymers 2a-c showed no evidence for the evolution of benzene during thermal degradation. In the TGMS experiments carried out on PEEK, it had been found that although the benzene was the second most abundant degradation product, its concentration was very weak compared to phenol, with the peak ion count for phenol exceeding that of benzene by a factor of approximately 20. Thus, although benzene was not observed in these experiments, its concentration may have been below the detection limit of the apparatus.1° 3.3 Polyimides 3.3.1 Determining completeness of imidization by TG-FTIR Polyimides are well-known for their high thermal stability, high strength, and high resistance to solvents. Polyimides are difficult to prepare in a single-step by solution polymerization because of their poor solubility. New polyimides 3 and 4 have been recently prepared by a single solution polymerization step.6 The TG curves for these polyimides are given in Fig. 7, below. Polyimide 3 showed no weight losses occurring before degradation, according to TG experiments. Imidization for polymer 3 was therefore believed to be complete. Polyimide 4, however, had two weight losses prior to degradation (Fig. 7). Although these polyimides have higher solubility than other polyimides, conventional NMR or FTIR measurements could not provide conclusive evidence as to the presence of carboxylic OH groups in the polymer backbone. Carboxylic OH absorptions are characteristic of an amic acid intermediate. TG-FTIR study of polymer 4 revealed that the first weight loss occurring near 147°C corresponded to a slight loss of water (absorbed from atmospheric moisture), while the second, occurring near 315°C corresponded to a loss of methanol. Figure 8 shows the TG-FIIR spectrum taken near 375°C for polymer 4, with the reference spectrum for methanol. Figure 9 shows a profile of the evolution of water and methanol as a function of TG temperature by showing baseline corrected intensities for the 1795 cm- ’ and 1064 cm-’ absorbance bands as a function of TG temperature. During the first weight loss near 147°C an increase in the water absorption
P. Moulinit
86
et al.
I --
----w-w__
Polymer 3
v-s_
611.OC 96.68
_r \,
t 147.1 c
100
I 240
i
315.7 c
I 380
I 520
I 660
Temp C (Heating) Fig. 7.
Fig. 8.
TG thermograms for polyimides 3 and 4.
TG-FTIR spectrum for polyimide 4 at 375’C; with reference infrared spectrum for methanol.
Degradation of thermoplastics
87
Temp (“C)
Fig. 9. Absorption intensities for water and methanol absorption bands for polyimide 4.
is apparent. During the second weight loss, the water signal remains relatively stable as the methanol absorption increases. Since the weight loss associated with water (i.e. near 147°C) is only -l%, it can be concluded that although nearly 12% of the sample weight had been lost prior to degradation, imidization is complete for this polymer. The remaining 11% being accounted for by the loss of methanol. The presence of methanol in these polymers may be explained from the fact that these polymers are isolated by adding the reaction mixtures (which contains the polymer in solution) to methanol. During this step, the final polymer is precipitated as an insoluble fiber in methanol. The presence of solvents in these materials is usually minimized by vacuum drying at -100°C for several days. Hydrogen bonding between methanol OH groups and imide C=O groups or trapping of the solvent during precipitation may cause the solvent to evaporate at higher temperatures. 3.3.2. Thermal degradation products The TG-FTIR spectra of polyimides 3 and 4 at 495 and 61YC, respectively (Fig. 10). Also shown in Fig. 10, are reference spectra which showed good correlation with the degradation spectra. Carbon monoxide was a degradation product common to both polymers 3 and 4. Peaks present in the TG-FTIR spectrum at 615°C for polyimide 3 also showed good correlation to absorbance bands for ammonia aniline, hydrogen cyanide, isocyanic acid, and phenyl isocyanate. The presence of these compounds in the TG-FTIR spectra suggests that degradation occurs through a ring-opening mechanism. In addition to a ring-opening degradation mechanism, evidence was also obtained suggesting a chain scission mechanism, with cleavage occurring between the benzene and imide connector groups. An
88
Fig. 10.
P. Modink!
et al.
TG-FTIR for polyimides 3 and 4 with reference spectra for HCN, CO and NH,, aniline, isocyanic acid, phenyl isocyanate.
intense band near 1750 cm-’ in the TG-FTIR of polymer 3 is consistent with the absorbance of an imide C=O group. Although no reference spectrum was available to unambiguously assign this band, it is believed that it may be attributed to an imide compound originating from cleavage at the tertiary amine. Thus, polyimides can undergo degradation through two pathways.
4 SUMMARY It has been shown that TG-FTIR is a valuable tool in the synthesis of polyimides and poly(arylene ether)s. In addition to providing the identity of degradation products, the technique can be used to infer information as to the structure of the polymer. Based on the results shown here, TG-FTIR provided valuable information pertaining to the preparation of new synthetic poly(arylene ether)s and polyimides. The presence of residual sulfur on a poly(ary1 ether) and the completeness of imidization were both confirmed using this technique. For these polymers, the information provided by TGFTIR was not available through characterization with conventional FTIR or NMR. TG-FTIR was also useful for confirming that poly(arylene ether
Degradationoffhewnoplasfics
89
ketone)s prepared as PEEK analogues exhibited similar degradation behaviour with their respective decomposition products being closely related to the connector groups in the polymer. TG-FTIR spectra of degradation products from polyimides indicated that both chain scission and ring-opening degradation mechanisms occur in these polymers.
ACKNOWLEDGEMENTS We wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fond pour la Formation de Chercheurs et 1’Aide a la Recherche du Quebec (FCAR) and Carleton University for financial assistance in this research.
REFERENCES 1. Bowley, B., Hutchinson, E. J., Gu, P., Zhang, M., Pan, W.-P. & Nguyen, C., Thermochimica Actu, 200 (1992) 309. 2. Zhang, Q., Pan, W.-P. & Lee, W. M., Thermochimica Actu, 226 (1993) 115. 3. Johnson, D. J., Compton, D. A. C., Cass, R. S., Canale, P. L., Thermochimicu Actu, 230 (1993) 293. 4. Paroli, R. M. & Delgado, A. H., Applications of TG-FTIR in the characterization of weathered sealants. In ACS Symposium Series Volume: Hyphenated Techniques in Polymer Characterization, eds T. Provder & M. Urban, American Chemical Society, Washington, DC, May 1994. 5. Le Guen, A., M.Sc. thesis, Carleton University, Ottawa, Ontario, 1994. 6. (a) Qi, Y. & Wang, Z. Y., Macromolecules, 27 (1994) 625 (b) Gao, J. P. & Wang, Z. Y., J. Polym. Sci. Part A: Polym. Chem. in press (1995). 7. Moulinie, P., Wang, Z. Y. & Paroli, R. M., submitted to J. Polym. Sci. Part A: Polym. Chem. 8. Saunders, K. J., Organic Polymer Chemistry, Chapman and Hall, 2nd edn, New York, 1988, p. 216. 9. Saunders, K. J., ibid. 8, p. 451. 10. Day, M., Cooney, J. D. & Wiles, D. M., J. Analytical and Appl. Pyr., 18 (1990) 163. 11. Colthup, N. B., Daly, L. H. & Wiberly, S. E. Introduction to Znfrured and Ruman Spectroscopy, 3rd edition, Academic Press, Toronto, 1990, p. 323.
0142-9418(95)00025-9 ELSEVIER
TEST METHOD Investigating the Degradation of Thermoplastics by Thermogravimetry-Fourier Transform Infrared Spectroscopy (TG-FTIR)* P. Moulinik,” R. M. Paroli,tb Z. Y. Wang,?” A. H. Delgado,b A. L. Guen,” Y. Qi” & J.-P. Gao” Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada KIS 5B6 “Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR6 “Ottawa-Carleton
(Received
1 May 1995; accepted 21 June 1995)
ABSTRACT Themogravimetry-Fourier
transform infrared spectroscopy (TG-FTIR) has been used
to study the thermal degradation Poly(arylene
ether)s
containing
of three
types of synthetic thermoplastics:
an acenaphthylene
moiety, poly(arylene
ether
ketone)s prepared as analogues of the commercial polymer poly(ether ether ketone) (PEEK), and aromatic polyimides synthesized by a single-step solution polymerization. TG-FTIR was found to be useful in providing important structural information about the poly(arylene ether)s and polyimides, which was not apparent through conventional NMR or FTIR techniques. Spectra obtained using air as a purge for insoluble and soluble poly(arylene ether)s 1 revealed that residual sulfur remained in both polymers after preparation through a reaction with Lawesson’s reagent, Comparison of TGMN-FTIR results of new poly(arylene ether ketone)s 2 with previously reported data on degradation studies of PEEK obtained with TG-mass spectrometry (TG-MS) showed that both polymers had similar degradation patterns, with phenol being a major degradation product. TG-FTIR studies on polyimides 3 and 4 synthesized by one-step solution polymerization demonstrated that it was possible to determine the imidization degree by monitoring the intensity of the water absorption band
*Presented in part at the 40th Canadian Spectroscopy Conference, 1994. tAuthors to whom correspondence should be addressed. 75
Halifax,
N.S., August
8-10.
76
P. Moulinic? et al.
at 179.5 cm-’ as a function of the TG temperature during a weight-loss occurring near 300°C. The FTlR spectra of both polyimides studied in this work showed that carbon monoxide was a significant degradation product. Other degradation products detected for polyimide 3 were ammonia, isocyanic acid and phenyl isocyanate. FTIR peaks consistent with hydrogen cyanide also appeared for polyimide 3.
1 INTRODUCTION
Thermogravimetry (TG) is a thermoanalytical technique where the weight of a sample is monitored during a linear heating program under a constant purge of inert gas or air. For some materials, such as polymers, loss of impurities, (i.e. residual solvent or starting material) or thermal degradation can be observed as weight-losses in the samples during the TG experiment. TG can be used to assess the purity or thermal stability of a polymer and is now a widely used tool in polymer science. A limitation to this technique, however, is that it does not identify the gases which are being evolved during the associated weight-losses. In TG-FTIR, the purge gas used for TG acts as a carrier for the gaseous degradation products. These products are passed through a heated transfer line to a FTIR gas cell. Thus, the FTIR spectra of gases evolved during a TG experiment can be recorded as a function of the TG temperature and interpreted to identify degradation products. Many examples on the use of TGFTIR in identifying degradation products from various materials are available in the literature.ld Thermal stability may be an important factor when considering applications and, therefore, knowledge of the degradation productions helps in understanding the behaviour of the material. In this work, TG-FTIR has been used to determine the products evolved for new synthetic thermoplastics before and during thermal degradation. Three types of thermoplastic polymers were studied: poly(arylene ether)s, poly(arylene ether ketone)s, and aromatic polyimides. The structures for these polymers are given in Fig. 1. Although TG-FAIR is mainly used in the identification of thermal degradation products it can also be used to infer important structural information about the polymers. Poly(arylene ether)s 1 prepared by reacting polymer 5 with Lawesson’s reagent were tested by TG-FTIR for the presence of residual sulfur. Similarly, polyimides 3 and 4 were tested for the completeness of imidization through TG-FTIR experiments. The degradation products from new poly(arylene ether ketone)s 2 prepared as analogues of commercial poly(ether ether ketone) (PEEK) were also investigated.
Degradafion
77
of fhermoplasfics
polymer
laQ&I’
\’
1
1;
/ \
-
\/
‘\
/
-
3
3
n
d BP = 3,3’-, b) BP = 3,3’-,
2
A=
-+
c) BP = 4,4’-,
3
n
4
Fig. 1.
New synthetic polymers analyzed by TG-FTIR
2 EXPERIMENTAL 2.1 Polymer preparation The preparation of the polymers studied in this work have been reported previously.“-7 The information sought out from TG-FTIR experiments resulted from the TG/DTA thermograms obtained for each polymer. Particular attention was given to the poly(arylene ether)s and polyimides since both exhibited TG weight-losses occurring prior to overall thermal degradation. These weight-losses could provide valuable structural information. 2.1.1. Poly(arylene etherjs Soluble and insoluble polymers 1 were obtained from prepolymer 5 upon treatment with Lawesson’s reagent (2,4-Bis(4-methoxyphenyl)-1,3-dithia-2,4diphosphetane-2,4-disulfide) in dilute and concentrated solutions, respectively . During TG experiments, a sulfur-like smell emanated from the sample. TG-PTIR was then used to confirm the presence of sulfur on the polymer.
78
P. Moulinit! et al.
TG-FTIR experiments, using an air purge, were run on samples after the ringclosing reaction with Lawesson’s reagent. 2. I .2 Poly(arylene ether ketone)s Poly(arylene ether ketone)s 2a-c have been synthesized as structural derivatives of the commercial thermoplastic PEEK. Due to the biphenyl structure these new polymers have higher glass-transition temperatures (T,) and greater thermal stabilities than PEEK. Polymers 2ax were studied by TG-FTIR in order to compare their thermal degradation behaviours to that of PEEK. 2. I .3 Polyimides The polyimide formation consists of two steps involving an initial reaction between a diamine and a dianhydride, followed by a ring-closing imidization.” This two-step imidization process is shown in Scheme 2 for polyimide 4, for which both steps were carried out in a single solution polymerization. TGFTIR was used to assess the completeness of imidization. 2.2 TG-FTIR
scans
Samples in powder form, weighing 10-15 mg, were used for these experiments. The samples were placed in a Pt pan and run in a Seiko TG/DTA 320, from ambient temperature to 800°C using a heating rate of lO”C/min. A flow rate of 100 ml/min of nitrogen or air was used for each run. A Nicolet TGA-IR accessory was used to transport the evolved gases from the TG/DTA to a Nicolet 800 FTIR spectrometer. This accessory provided a heated transferline from the TG/DTA to the FTIR spectrometer and oven for the FTIR gascell to prevent condensation of the gases evolved from the TG. The transferline temperature was kept between 230-240°C while the oven for the gas cell was maintained at 260°C. A gas cell (10 cm length x 5 cm diameter) containing a mirror and a KBr window was placed in the TGA-IR oven. All FTIR spectra were collected with 8 cm-’ resolution. Backgrounds were collected when the TG/DTA temperature was between 100-I 10°C. Acquisition of TGFTIR spectra were started once the TG/DTA temperature reached 115°C. FI’IR spectra were collected at a rate of 12 spectra per min (i.e. per 1O’C) through the remainder of the TG/DTA scan. Maxima present in the 1st derivative of the TG thermograms (known as the DTG) were used to locate FTIR spectra of interest. Gram-Schmidt Reconstruction (GSR) plots (which describe the total detector response during the collection of FTIR spectra) were also used to locate FTIR spectra containing products from TG weight-losses. GSR plots are similar to DTG plots, although relative signal intensities may differ because of varying spectra responses for different compounds. Spectra chosen for analyses corresponded to local (or
Degradation of thermoplastics
79
80
P. Moulinit? et al.
pdymic
Scheme 2
acid
Reaction scheme for the formation of polyimide 4.
absolute) maxima in the DTG curves and Gram-Schmidt plots. Water peaks, which obscured many of the degradation product absorption bands, were removed by spectral subtraction using a TG-FTIR spectrum from the same experiment which contained water. The spectrum used for subtraction was collected early in each experiment (typically at 12515OOC) and corresponded to a stable mass region in the TG/DTA curves. The evolution of individual decomposition products was constructed by measuring the baseline-corrected intensity of a known absorption band as a function of the TG/DTA temperature.
3 RESULTS AND DISCUSSION 3.1 Poly(arylene
ether)s
3.1. I Presence of residual sulfur The preparation of polymer 1 was achieved by chemical modification of polymer 5 using Lawesson’s reagent (Scheme 1). This reaction was carried out in either dilute or concentrated solutions, yielding soluble and insoluble 1, respectively. Both polymers had intense red colours, which suggested the presence of the acenaphthylene moiety in their backbones.5 The acenaphthylene formation is believed to go through an intermediate containing a cyclic disulfide, as shown in Scheme 1. It has been shown, however, that this reaction scheme is sensitive to reaction conditions, with conditions a and b yielding soluble and insoluble materials, respectively. It is believed that the insoluble behaviour of material lb results from cross-linking with sulfur. Various pathways to cross-linking with sulfur are possible, including the well-known vulcanization process. The thermograms for both materials
81
Degradation of thermoplastics
showed a two-step weight loss profile with about 10% mass loss near 250°C (Fig. 2). A sulfur-like smell also emanated during this weight loss, suggesting the presence of residual sulfur in both polymers 1. The insolubility and the weak TR intensities of vibrations of sulfur groups, however, made NMR and FTIR methods unsuitable for confirming the presence of residual sulfur in these materials. Using TG-FTIR with a nitrogen carrier gas provided no useful information regarding the identity of the degradation products. Again, a strong sulfur-like smell was given off during the first weight-loss. When air was used as a carrier gas, the TG-FTIR spectra for soluble and insoluble polymers 1 displayed absorptions characteristic of SOi (Fig. 3). This can be attributed to a reaction between oxygen (present in the carrier gas) and residual sulfur in the polymer. This reaction can occur either as sulfur is released from the polymer at high temperature or with sulfur on the polymer itself. A plot of FTIR band intensity versus temperature for SOz (1377 cm-‘) is given in Fig. 4. From this plot, it is apparent for both soluble and insoluble polymers 1, that SO, is only given off during the first weight-loss. Hence, it is possible to confirm that the first weight-loss for 1 corresponds to the loss of residual sulfur. Although soluble polymer 1 had an intense red colour and was soluble in most organic solvents, TG-FTIR studies have shown that sulfur is present.
8
Soluble -----_
0 F
268.5C
2*. 345.9c - \ 94.3%
530.6C 3 81.746 \ \ \
25
I 180
I 335
I 480
l--_ I 645
800
Temp C (Heating) Fig. 2.
TG thermograms for polymer la and lb in nitrogen.
P. Moulinit2 et al.
82
Fig. 3.
TG-FHR
100
spectra for first mass loss in air for soluble and insoluble polymers and reference infrared spectrum of SO;?.
150
200
250
300
350
400
Temperature (“C) Fig. 4.
SOz absorbance of 1377 cm-’ versus temperature for polymers 1.
450
Degradationof thermoplastics
83
3.1.2Degradation products The thermograms of these polymers (shown in Fig. 2) show that the onset of overall sample degradation occurs near 480°C. Based on the TG-FTIR of these polymers performed in N2 it can be seen that phenol is the dominant degradation product. Figure 5 shows the reference spectrum for phenol and the TG-FJXR spectra at 575 and 540°C for soluble and insoluble polymers 1, respectively. This suggests a simple chain scission degradation mechanism, in which cleavage occurs at the ether and quaternary carbon linkages, yielding phenol. No other degradation products were visible in the TG-FTIR spectra. It can be expected, however, that other possible products from a random scission reaction such as 9,9_diphenylfluorene or acenaphthene would not be detected during TG-FTIR experiments due to their high boiling points. The boiling points of acenaphthene and 9,9-diphenyl fluorene, for example, are 279 and 295OC, respectively, and would thus not be sufficiently volatile to pass through the TGA-IR accessory (i.e. they condense). For both these polymers, a red-brown deposit was observed at the junction of the TG furnace and transfer line, suggesting that a conjugated product, such as acenaphthene, had precipitated before passing through the transfer line.
Fig 5.
TG-FTIR spectra for phenol and polymers 1.
84
P. MoulinG et al.
3.2 Poly(arylene ether ketone)s Degradation of PEEK has been studied previously using TG-MS, a related technique in which the evolved gases are passed from the TG to a massspectrometer.” In these experiments, Day and co-workers found that PEEK exhibited random chain scission upon degradation. Phenol was found to be the predominant degradation product with smaller amounts of benzene and dibenzofuran also being detected. Thus, the degradation products of PEEK were related to the connector groups present in the polymer repeat unit. The onsets of degradation for polymers 2a, 2h and 2c occurred at 497, 489 and 512”C, respectively. The TG-FTIR spectra corresponding to the degradation products for polymers 2a-c gave infrared absorption bands which correlated well with carbon monoxide and phenol. In addition, polymers 2a and 2c gave good correlation with the spectra for 4-propylphenol and isopropyldiphenol (Bisphenol A; BPA), based on weak non-aromatic C-H absorbances between 2900 and 2975 cm-‘. The TG-FTIR spectrum for polymers 2a-c at 500, 545 and 53YC, respectively, along with reference spectra for 4-propylphenol, BPA, phenol and carbon monoxide are shown in Fig. 6. These degradation products are closely related to the connector groups in the polymer, suggesting that the degradation of these polymers is also a scission reac-
3500
Fig. 6.
TG-FTIR
37W
2x0
xc4
lyxl
Imo
5.m
spectra for degradation poly(ary1 ether ketone) and of 4-propylphenol, bisphenol A, phenol and carbon monoxide.
Degradation
of thermoplastics
85
tion. Scission at either the ether linkages or car-bony1 bonds in these polymers can yield carbon monoxide or phenol as degradation products. Thus, these polymers have similar degradation behaviour to PEEK. Cleavage may also occur at the C-C single bonds for polymers 2a and 2c. Polymers 2a-c also contain biphenyl connector groups. It was expected that absorption bands corresponding to biphenyl groups in the TG-FTIR spectra would help corroborate the premise of a random chain-scission mechanism. The absence of these species, however, may be due to condensation of the product before reaching the gas cell, since it has a high boiling point (254255°C). The TG-FUR spectra collected for polymers 2a-c showed no evidence for the evolution of benzene during thermal degradation. In the TGMS experiments carried out on PEEK, it had been found that although the benzene was the second most abundant degradation product, its concentration was very weak compared to phenol, with the peak ion count for phenol exceeding that of benzene by a factor of approximately 20. Thus, although benzene was not observed in these experiments, its concentration may have been below the detection limit of the apparatus.1° 3.3 Polyimides 3.3.1 Determining completeness of imidization by TG-FTIR Polyimides are well-known for their high thermal stability, high strength, and high resistance to solvents. Polyimides are difficult to prepare in a single-step by solution polymerization because of their poor solubility. New polyimides 3 and 4 have been recently prepared by a single solution polymerization step.6 The TG curves for these polyimides are given in Fig. 7, below. Polyimide 3 showed no weight losses occurring before degradation, according to TG experiments. Imidization for polymer 3 was therefore believed to be complete. Polyimide 4, however, had two weight losses prior to degradation (Fig. 7). Although these polyimides have higher solubility than other polyimides, conventional NMR or FTIR measurements could not provide conclusive evidence as to the presence of carboxylic OH groups in the polymer backbone. Carboxylic OH absorptions are characteristic of an amic acid intermediate. TG-FTIR study of polymer 4 revealed that the first weight loss occurring near 147°C corresponded to a slight loss of water (absorbed from atmospheric moisture), while the second, occurring near 315°C corresponded to a loss of methanol. Figure 8 shows the TG-FIIR spectrum taken near 375°C for polymer 4, with the reference spectrum for methanol. Figure 9 shows a profile of the evolution of water and methanol as a function of TG temperature by showing baseline corrected intensities for the 1795 cm- ’ and 1064 cm-’ absorbance bands as a function of TG temperature. During the first weight loss near 147°C an increase in the water absorption
P. Moulinit
86
et al.
I --
----w-w__
Polymer 3
v-s_
611.OC 96.68
_r \,
t 147.1 c
100
I 240
i
315.7 c
I 380
I 520
I 660
Temp C (Heating) Fig. 7.
Fig. 8.
TG thermograms for polyimides 3 and 4.
TG-FTIR spectrum for polyimide 4 at 375’C; with reference infrared spectrum for methanol.
Degradation of thermoplastics
87
Temp (“C)
Fig. 9. Absorption intensities for water and methanol absorption bands for polyimide 4.
is apparent. During the second weight loss, the water signal remains relatively stable as the methanol absorption increases. Since the weight loss associated with water (i.e. near 147°C) is only -l%, it can be concluded that although nearly 12% of the sample weight had been lost prior to degradation, imidization is complete for this polymer. The remaining 11% being accounted for by the loss of methanol. The presence of methanol in these polymers may be explained from the fact that these polymers are isolated by adding the reaction mixtures (which contains the polymer in solution) to methanol. During this step, the final polymer is precipitated as an insoluble fiber in methanol. The presence of solvents in these materials is usually minimized by vacuum drying at -100°C for several days. Hydrogen bonding between methanol OH groups and imide C=O groups or trapping of the solvent during precipitation may cause the solvent to evaporate at higher temperatures. 3.3.2. Thermal degradation products The TG-FTIR spectra of polyimides 3 and 4 at 495 and 61YC, respectively (Fig. 10). Also shown in Fig. 10, are reference spectra which showed good correlation with the degradation spectra. Carbon monoxide was a degradation product common to both polymers 3 and 4. Peaks present in the TG-FTIR spectrum at 615°C for polyimide 3 also showed good correlation to absorbance bands for ammonia aniline, hydrogen cyanide, isocyanic acid, and phenyl isocyanate. The presence of these compounds in the TG-FTIR spectra suggests that degradation occurs through a ring-opening mechanism. In addition to a ring-opening degradation mechanism, evidence was also obtained suggesting a chain scission mechanism, with cleavage occurring between the benzene and imide connector groups. An
88
Fig. 10.
P. Modink!
et al.
TG-FTIR for polyimides 3 and 4 with reference spectra for HCN, CO and NH,, aniline, isocyanic acid, phenyl isocyanate.
intense band near 1750 cm-’ in the TG-FTIR of polymer 3 is consistent with the absorbance of an imide C=O group. Although no reference spectrum was available to unambiguously assign this band, it is believed that it may be attributed to an imide compound originating from cleavage at the tertiary amine. Thus, polyimides can undergo degradation through two pathways.
4 SUMMARY It has been shown that TG-FTIR is a valuable tool in the synthesis of polyimides and poly(arylene ether)s. In addition to providing the identity of degradation products, the technique can be used to infer information as to the structure of the polymer. Based on the results shown here, TG-FTIR provided valuable information pertaining to the preparation of new synthetic poly(arylene ether)s and polyimides. The presence of residual sulfur on a poly(ary1 ether) and the completeness of imidization were both confirmed using this technique. For these polymers, the information provided by TGFTIR was not available through characterization with conventional FTIR or NMR. TG-FTIR was also useful for confirming that poly(arylene ether
Degradationoffhewnoplasfics
89
ketone)s prepared as PEEK analogues exhibited similar degradation behaviour with their respective decomposition products being closely related to the connector groups in the polymer. TG-FTIR spectra of degradation products from polyimides indicated that both chain scission and ring-opening degradation mechanisms occur in these polymers.
ACKNOWLEDGEMENTS We wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fond pour la Formation de Chercheurs et 1’Aide a la Recherche du Quebec (FCAR) and Carleton University for financial assistance in this research.
REFERENCES 1. Bowley, B., Hutchinson, E. J., Gu, P., Zhang, M., Pan, W.-P. & Nguyen, C., Thermochimica Actu, 200 (1992) 309. 2. Zhang, Q., Pan, W.-P. & Lee, W. M., Thermochimica Actu, 226 (1993) 115. 3. Johnson, D. J., Compton, D. A. C., Cass, R. S., Canale, P. L., Thermochimicu Actu, 230 (1993) 293. 4. Paroli, R. M. & Delgado, A. H., Applications of TG-FTIR in the characterization of weathered sealants. In ACS Symposium Series Volume: Hyphenated Techniques in Polymer Characterization, eds T. Provder & M. Urban, American Chemical Society, Washington, DC, May 1994. 5. Le Guen, A., M.Sc. thesis, Carleton University, Ottawa, Ontario, 1994. 6. (a) Qi, Y. & Wang, Z. Y., Macromolecules, 27 (1994) 625 (b) Gao, J. P. & Wang, Z. Y., J. Polym. Sci. Part A: Polym. Chem. in press (1995). 7. Moulinie, P., Wang, Z. Y. & Paroli, R. M., submitted to J. Polym. Sci. Part A: Polym. Chem. 8. Saunders, K. J., Organic Polymer Chemistry, Chapman and Hall, 2nd edn, New York, 1988, p. 216. 9. Saunders, K. J., ibid. 8, p. 451. 10. Day, M., Cooney, J. D. & Wiles, D. M., J. Analytical and Appl. Pyr., 18 (1990) 163. 11. Colthup, N. B., Daly, L. H. & Wiberly, S. E. Introduction to Znfrured and Ruman Spectroscopy, 3rd edition, Academic Press, Toronto, 1990, p. 323.