Infrared studies of carbons. XIII The oxidation of polycarbonate chars

Carbon Vol. 28. No 6. pp. 855-865. Rimed in Great Britain.

19‘33 Copyright 0

INFRARED STUDIES OF CARBONS. THE OXIDATION OF POLYCARBONATE

o@w6223/90s3.00+.on 1990 Pcrgamon Press plc

XIII* CHARS

A. S. POLITOU, C. MORTERRA and M. J. D. Lowt Department of Chemistry. New York University. 4 Washington Place, New

York,

NY 10003. U.S.A. (Received 8 December

1989; accepted in revised form 13 February

1990)

Abstract-IR photothermal beam deflection spectroscopy was used to record IR spectra of chars, produced by the in vacua pyrolysis of a polycarbonate resin (PC), at several stages of oxidation. The high-temperature chars seem to be more susceptible to oxidation than the low-temperature PC chars. This behavior is quite unlike that of coals. cellulose based chars, and other carbons. Oxidation of the low temperature chars proceeds mainly through attack on the hydrocarbonaceous side chains and the elimination of carbonate functionality. accompanied by an extensive formation of benzophenone, lactones. and acidic carbonylic species. However, the chars obtained at temperatures higher than 590°C. when polyaromatic domains first form to a significant extent. exhibit the normal oxidation behavior found with chars derived from several types of precursors. Key Words-Infrared

spectra, polycarbonate chars.

char oxidation. photothermal spectroscopy, carbon,

carbon oxidation. 1. INTRODUCTlON

Bisphenol-A polycarbonate resins. because of their unique combination of excellent electrical and mechanical properties. have found extensive application from aeronautics to the food industry. Their most outstanding feature is thermal stability. which makes their processing at temperatures as high as 340°C very simple. They have also been used as precursors of polymeric carbons and carbon films, and lately that seems to attract considerably growing attention as a solution to the recycling of PC[l-31. Our understanding of the mechanism of degradation and carbonization of these polymers is minimal. however. An IR spectroscopic study of the charring of PC resins was recently carried out and information about the degradation mechanism, as well as the nature and composition of the chars produced at various temperatures in vacua, was obtained; a detailed description can be found elsewhere[4]. One of the interesting observations made then was the striking difference in carbonization behavior between PC and other carbons such as cellulose-derived chars and coals. With these some carbonization occurs at low temperatures of pyrolysis and gradually grows with increasing temperature[S-61. The residues produced by pyrolyzing PC at temperatures as high as 500°C. however, were very sparsely if at all carbonized, although an extensive series of branching and crosslinking reactions had taken place during their formation. Essentially, the skeletal structure of the PC precursor was retained, and the char consisted of a system of aromatic nuclei held apart by hydrocar___ ~ ._. -. ---. ‘Part XII: ref 4. tTo whom inquiries should be directed.

---

bonaceous bridges and linkages of the ether and ester type. However, the char framework collapsed completely and extensive carbonization occurred in a surprisingly narrow temperature interval near 590°C. The chars obtained above that temperature were spectroscopically quite similar to other high-temperature carbons. In fact, a similar pattern of carbonization has been found in the case of only one other char among the ones examined so far, namely, the char derived from pyrolysis of a phenol-formaldehyde resin(71. It was considered useful to examine the oxidation pattern of those quite atypical PC chars, mainly because the behavior of carbonaceous materials toward oxygen is greatly determined by the extent of carbonization; so the oxidation pattern can provide us with a better insight into the carbonization mechanism itself. It is also well known that the chemical and physical properties of those materials can be significantly affected by oxidation. We have, therefore. recorded spectra of several PC chars at various stages of oxidation, and describe the results. 2. EXPERIMENTAL

PC chars obtained at various pyrolysis temperatures have been described in detail elsewhere[4]. The abbreviations used previously in both text and figures have been retained (i.e., the symbol PC of each sample is followed by a suffix indicating the temperature of pyrolysis); all samples were pyrolyzed in vacua. After the char had been obtained and its IR spectrum recorded, the temperature was changed to a predetermined oxidation temperature TO, and pure oxygen was introduced into the cell at a pressure of 155 torr, thus simulating under clean and controlled

--

x55

856

A. S. Po~trou et al.

conditions the oxidative properties of the atmosphere; the temperature was kept constant in the presence of oxygen for 30 min. After a brief degassing at T,,, the sample was cooled to room temperature and within one hour or less an IR spectrum was recorded. A sample oxidized in this manner is further identified by appending to its code the oxidation temperature in parentheses. For example, the code PC440(400) refers to a PC sample pyrolyzed in vacua at 440°C and oxidized at 400°C for 30 min. For simplicity, only the oxidation temperature is used to label spectra in the various figures. Spectra of chars produced by pyrolysis at a certain temperature and then exposed to air at room temperature (nominally 25°C) are labeled 25 in the figures. IR Fourier transform photothermal beam deflection spectroscopy (PBDS) was used to record the spectra at 8 cm-’ resolution using 2000 scans. The principle of the method and various sampling techniques have been described elsewhere[8-101. PBDS yields the single-beam spectrum, S, of a sample. A pseudo-double-beam spectrum, S/S,, is then computed by ratioing S against the spectrum, S,,, of a standard absorber (a Pt black or a reference high-temperature carbon). When spectra are shown in a series their ordinates are displaced to avoid overlap. All spectra were normalized, and the ordinate of the spectrum with the highest absorption has been used as the ordinate of all spectra within a series shown in each figure. 3. RESULTS AND DISCUSSION Oxidations were carried out with PC samples charred at 440,490,590, and 700°C. Those temperatures were selected because they represent characteristic stages in the PC degradation and carbonization process as follows: (a) 440°C is the temperature at which the first clear spectroscopic signs of transformations in both the carbonate groups and isopropylidene linkages became apparent. At this stage the thermal cross-linking and branching began, as evidenced by the dramatic decrease in the intensity of -CH,-related bands and the development of bands ascribable to -CH2 and, on the other hand, by the decline of the -OCOO- band and the appearance of a new vibration associated with the ester C-0 group. There were the first indications of the formation of a polyenic system, but there were no indications of polyaromatization at this stage, and the char is believed to consist of aromatic nuclei held apart by bridges of the diary1 ether and ester type and methylene linkages which were created at this temperature, while there were still a few residual methyl groups. (b) 490°C is the temperature at which the branching and cross-linking induced by heat was complete. More specifically, all the -CH, groups present in the parent polymer have been eliminated and replaced by -CH- saturated and unsatured linkages, the ar-

omatic C-H stretching absorptions grew significantly, the carbonate group was no longer present, and the ester and ether functionahties had increased in intensity. Briefly, the pyrolysis near 490°C resulted in an entirely cross-linked network of predominantly aromatic character consisting of small, discrete aromatic entities linked by ether, ester, and C-H bridges. Very little polyaromatization seemed to be present in chars generated at that temperature. (c) 590°C is a very important temperature for the pyrolysis of PC because it marks the onset of extensive carbonization. This is evidenced by the sharp growth of the IR continuum absorption, as well as by the complete elimination of all hydrocarbonaceous groups present initially or created in the course of pyrolysis. This is the temperature at which the spectrum of the char first assumed the “standard” profile of all intermediate temperature chars[ 111. (d) 700°C is the temperature at which only traces of residual surface structures of the completely carbonized material could be observed spectroscopically because there were very few of them and because the continuum of the carbon absorption built up significantly and extended throughout the midIR spectral region, and the vibrational component of the spectrum had decreased drastically in intensity[ll]. 3.1 The oxidation of PC440 No changes in the appearance and the spectra of the char obtained from PC pyrolysis at 440°C were observed when it was exposed to oxygen at temperatures as high as 220°C. However, the spectra changed markedly after treatment with oxygen at 240°C and the char was characterized by a darkening in color. which increased gradually with increasing oxidation temperature. A significant weight loss was observed on oxidation at 400°C which must indicate the initiation of a slow burn-off. Oxidation at temperatures slightly higher than 440°C (i.e., than the temperature at which the char was produced in vacua) led to the complete gasification of the char. Series of normalized, compensated spectra of the PC440 chars oxidized at several temperatures are shown in Figs. 1 through 4. The observed spectral changes can be summarized as follows: (i) An O-H stretching band was originally present in the spectrum of the nonoxidized PC440 residue, relatively sharp and well defined and centered at 3560 cm-‘; that band is believed to be due to the phenohc groups produced in the course of pyrolysis of PC[4]. Upon oxidation at 240°C it showed a distinct broadening and shifting to lower wavenumbers (Fig. 1). That broad band extended through the 35603230 cm-’ region, reached its maximum intensity with PC440(320), and persisted throughout the oxidation temperature range with its maximum shifted to the 3450-3360 cm -I range. (ii) There was a significant decrease of the overall absorption in the C-H stretch region, accompanied by several rearrangements in the intensities of the

857

Infrared studies of carbons

various C-H stretching vibrations as shown in Fig. 1 and, in an expanded form, in Fig. 2. At T,,= 24CK, the -C-H band at 2900 cm-’ was completely eliminated, and the =C-H stretching band at 3020 err-’ was greatly decreased in intensity. The -CH2related bands at 2927 and 2847 cm-‘, as well as the weak =CH2 stretch at 3078 cm-l, became significantly weaker and the -CH, absorptions at 2970. 2874, 1366, and 1200 cm-’ were very little if at all affected. The aromatic C-H stretch at 3051 cm-” also declined in intensity. At To,-=320°C however, the -CHX stretching band underwent a dramatic decrease in intensity and the rest of the peaks also kept declining. Only traces of the aliphatic bands were present at oxidation temperatures higher than 320°C and the only band left in the C-H stretch region was the aromatic C-H stretch centered at 3070 cm -‘.

uo 3im 320

240

25 31’00

3600

2doo

2&O

frvenumbers Fig. 2. Expanded C-H stretch region of PC440 oxidized at several temperatures.

440

420

400 360

340 320

280 240

3700

3150

3200

25 2MiO 2’ 10

Wawnumbsm Fig. 1. The oxidation of PC44O. Compensated, normalized spectra in the O-H and C-H stretch region. The number beside each spectrum indicates the oxidation tem~rature in “C.

(iii) Oxidation at 240°C led to several spectral modifications of the double-bond range of the “fingerprint” region (Figs. 3 and 4). These included the formation of a weak band at 1844 cm ‘, the decrease in the relative intensity of the -OCOOmode at 1778 cm-‘, the substantial increase of the side band at 1736 cm-‘, and the increase and/or development of a band at 1670 cm I. The aromatic ring stretch at 1609 cm-’ appeared to be unaffected by oxidation at 240°C. Oxidation at temperatures higher than 300°C resulted in a progressive growth of the 1844 cm- ’ and its shift to 1848 cm-’ band, the decline of the 1778 cm -’ band (-OCOO- stretch), and its possible coexistence and overlap with a band at 1782 cm-‘. which replaced the carbonate absorption at T,, = WC and kept growing. At the same time, the band at 1736 cm- ’ , after an initial increase at T,,, = 320°C. started to decline in intensity and shifted to higher frequency (1744 cm I). At 400°C it had been entirely replaced by an absorption at 1755 cm I. The 1670 cm-’ band, on the other hand, maintained a constant relative intensity and showed signs of decrease only in the spectrum of PC~(4~)(i.e.. when burn-off slowly started). There was also a detectable change in the case of the 1609 cm-’ band, which broadened appreciably, indicating an increasing degree of conjugation. but even that occurred at high oxidation temperature (Figs. 3,4, spectrum Labeled 440). (iv) Oxidation at increasing temperatures brought about the elimination of the 1,6substituted ring stretching band at 1504 cm-‘, which was completed on oxidation at 320°C (Fig. 3). The component at 1015 cm-‘, as well as the ones near 1110 and 1087 cm-‘, decreased progressively and more smoothly until, in the spectrum of ~~(~), they all be-

A. S. POUTOUet al.

858

440 420

400 360 340 320

280 240 25 0

1650

1300

950

600

Wavenumbera

on the side chains of the PC440 residue[4], decreased significantly at T,, = 24O”C, and was eliminated at higher temperatures (Fig. 3). (viii) A new band at 925 cm-’ formed upon oxidation at 240°C and kept growing as the oxidation temperature was increased. In the PGMO(320) spectrum another absorption at 906 cm-l became appreciable in intensity and ended up the predominant one in that region. (ix) Finally, in the “wagging” region of the spectrum, oxidation seemed to cause the most dramatic spectral modifications, including a marked decrease in the intensity of the overall absorption, the complete elimination of the already reduced -OCOOdeformation band at 887 cm-‘, and the progressive decrease of the aromatic C-H deformation bands at 833 and 752 cm-’ (Fig. 3). The IR spectra of the oxidized PC440 char are very complex and in some cases poorly resolved, so that a complete and detailed band assignment and identification of all the functionalities created and destroyed during oxidation is not feasible. Still, some of the major oxygenated species formed, and the transformations those species underwent during the course of the oxidation process, can be traced on the basis of a careful analysis of the spectra obtained. A description and, if possible, an interpretation of the general trends of the oxidation process will be attempted in the following discussion: (a) Among the hydrocarbonaceous groups present in the char, the C-H groups are the most vulnerable, while the -CH3 groups are the most resistant to oxidation, as shown by the evolution of the bands in the 3100-2800 cm-’ spectral region and also of

Fig. 3. The “fingerprint” region of compensated spectra of PC440 oxidized at several temperatures.

came part of a broad, almost structureless absorption covering the 1400-1000 cm-’ region. The evolution of those bands reflects the destruction of the parasubstituted aromatic system. (v) The bands at 1465, 1360, and 1196 cm-‘, all three related to the isopropyl structure, started declining at 240°C and ceased to exist on oxidation at 320°C (Fig. 3). (vi) It was not easy to identify all of the species absorbing in the crowded and narrow 1300-1000 cm-’ range that were created and lost on oxidation, especially as the oxidation progressed and the band system gradually broadened and lost its structure. However, careful examination of the 1300-1150 cm-’ range shows that there must have been a rearrangement of the C-O stretches at 7’,, = 32o”C, amounting to the elimination of the -OCOO- related bands at 1236 and 1166 cm-’ and their replacement by a multiplet of absorptions at 1242, 1211, and 1175 cm-‘, which broadened up with increasing oxidation temperature, centering finally near 1261 and 1238 cm- I. (vii) A band at 957 cm-‘. assigned previously to the C-H deformation in the poiyenic system created

0

!: tn

Fig. 4. Expanded double bond stretch region of oxidized PC440.

Infrared studies of carbons

the one at 957 cm-‘. The aromatic C-H groups are also affected very much and at low oxidation temperatures, but their abundance as well as the exceptional stability of the aromatic ring seem to protect them from complete oxidative destruction. In general, the less substituted the aliphatic group, the more susceptible to oxidation it seemed to be, although at high oxidation temperatures all the aliphatic linkages were destroyed. (b) At least four different carbonylic species were formed, absorbing near 1670, 1736-1744, 1720 and 1755, 1782 and 1848 cm-‘, while the one already present in the polymer (i.e., the carbonate group), with characteristic absorptions at 1178, 1236, 1166, and 887 cm-l, underwent rapid deterioration and was absent in the spectrum of the char oxidized at 360°C. Of the newly formed species the one absorbing near 1670 cm-’ can be assigned to a benzophenone and/or quinone structure created probably by oxidative attack to the aliphatic bridges. The selective formation of similar structures upon oxidation was detected in the case of other low temperature chars derived from other CH, bridged aromatic polymers[7], as well as in the photolytic degradation in air of the polycarbonate itself [ 121.The carbonyl groups of the benzophenone structure cannot be solely responsible for the intensity and the broadening of the O-H stretch band. Such carbonyls cannot be involved in a hydrogen bonding system as extensive as the one suggested by the shape and intensity of the perturbation introduced by oxidation in the 36003200 cm-’ region (Fig. 1). If that were the case, the position of the carbonyl absorption should have been affected also, but no indication of such a shift can be found in the IR spectrum (Figs. 3 and 4). Moreover, no correlation seems to exist between the relative intensities of the band at 1670 cm-’ and the O-H stretch band as oxidation temperatures were increased. There is a correlation between the C=O species absorbing at 1736 cm-’ and the extensive hydrogen bonding; the species giving rise to both vibrations is thought to be a carboxylic acid. With all types of ether and hydroxyl functionalities in its immediate vicinity, such a species can be involved in various types of hydrogen bonding, such as hydroxyl-carboxy1 bonds and/or carboxyl-ether bonds, whereas formation of carboxylic dimers is highly unlikely because of the rigidity of the structure. An indication of the existence of the above type of alternate hydrogen bonding is provided by the position of the -C=O absorption at 1736 cm-‘(131. The presence of such a species can also account for the formation at T,, = 240°C of the band at 925 cm-‘, due to the out-of-plane deformation of the O-H group. The bands at 1848 and 1782 cm-’ can be ascribed to the asymmetric and symmetric C-O stretch of five-membered cyclic anhydride-like structures. The positions of those bands, their separation by 66 cm- ’ , their relative intensities, as well as their coexistence

859

and similar trend in growth with the bands at 13001180 and 910 cm-‘, strongly support that assignment[l4]. In addition, that same group of bands has been identified in other low-temperature chars of various types[l5,16]. Those structures first formed at r,, = 24O”C, and they progressively grew and persisted throughout the oxidation range. Finally there was the formation of an additional carbonylic species at T,, = 400°C indicated by the development of two bands at 1755 cm-’ and 1720 cm-‘. These abso~tions can be assigned to lactonelike structures, most likely of the six-membered ring unsaturated type (unsaturated delta lactone). The doublet in these compounds is thought to be due to a Fermi resonance effect[ 141. It shoufd be noted at this point that of the carbonylic species successively formed in the course of oxidation of the PC440 residue, the lactonic, anhydric, and benzophenone-like species were never completely destroyed at high oxidation temperatures, even after the burn-off started, in distinct contrast to the behavior of other chars (e.g., those derived from a phenol-formaldehyde precursor bearing similar structure and ~nctionalities)[l6]. (c) The increasing intensity with increasing oxidation temperature of the bands due to ether-like species in the 1200-1260 cm-’ region shows that the oxidic layer grew progressively. A part of that symmetrical absorption centered at 1260 cm -’ is believed to be caused by the C-O stretch of the pre-existing ether linkages retained in the oxidized char. 3.2 The oxidation of PC490 Some results obtained with the PC490 char are shown in Figs. 5 and 6, The first changes in mass as well as in the IR spectra were brought about by oxidizing PC490 at 25O”C, when it underwent a weight loss of approximately lO%, which increased slowly with increasing oxidation temperature. Oxidation at 46O”C, however, resulted in a residue that weighed less than 40% of the original unoxidized PC490, while oxidation just above 490°C led to the complete burnoff of the char. These observations, as well as a close inspection of the spectra obtained at several oxidation temperatures, reveal that the PC490 pyrolyzate exhibited a behavior similar to that of the PC440 char in some respects but quite different in others. Although there is a significant difference in the properties and functionalities present between the two chars, their oxidation patterns show many analogies. More specifically: (1) Of the hydrocarbonaceous groups present, the residual aliphatic ones (mainly C-H groups), absorbing at 2912 cm-‘, were most susceptible to oxidation, although they seemed to be completely eliminated only at T,, higher than 350°C. On the other hand, the strong aromatic C-H stretch showed the first signs of decline at only 7’,, = 350-400°C. (2) The oxidation of the PC490 char led to the formation of the same oxidized species formed on exposing the PC440 char to oxygen. These species,

860

A. S. POL~TOUet nl.

490 460 430 400 350

in the oxidized PC490 char compared with the PC440 char. One should also notice at this point the much lower relative intensity of the O-H auction in respect to the C-H stretching bands, as compared to what was observed in the spectra of the oxidized PC440 chars. This difference is illustrated in Fig. 8, where the spectra shown have been normalized to the same highest intensity. The lower yield of OH groups upon oxidation of the PC490 char parallels the reduced fo~ation of the C===Ogroups assigned to carboxylic species and further supports that assignment. An even more striking difference, however, can be identified, when the IR spectra of the high oxidation temperatures of the two chars are compared. In the case of the PC490 char, oxidation at 430°C resulted in the reappearance of two bands at 2924 and 2860 cm-’ (Fig. S), characteristic of the -CH, group. These bands persisted at high oxidation temperatures. It is reasonable to conclude, therefore, that oxidation at relatively high temperatures brought about an extensive fragmentation of the polymeric

300 250 25

490

460 0

s

430

Fig. 5. The oxidation al PC490. Compensated spectra in the high frequency region. The numbers indicate the oxidation temperature.

however, with the exception of the benzophenonelike structures and of the carboxylic acid, were formed in greater amounts and at lower oxidation temperatures than with PC440. Two sets of spectra of the PC440 and PC490 chars oxidized at similar temperatures, as well as difference spectra in the 1660-1750 cm-’ region, are shown in Fig. 7. The bands due to benzophenone and carboxylic acid C=O stretch are the only ones in that region that have uniformly lower intensity in the PC490 char in both cases of oxidation temperatures, indicating the presence of those types of functionalities in much smaller amounts

2t Wavmnumbem Fig. 6. Compensated spectra of oxidized PC490 in the “fingerprint” region.

I?661

Infrared studies of carbons

M:

Fig. 7. Expanded compensated spectra of oxidized PC440 and PC490 chars and difference spectra in the C=O stretch region.

network and an appreciable destruction of the small aromatic domains formed in the course of pyrolysis at 490°C. possibly followed by chain rearrangement and insertion reactions, leading to the reappearance of saturated H-containing structures. 3.3 Oxidation of the PC590 char The oxidation of PC590 at a temperature as low as 250°C resulted in the formation of the first oxygenated species identifiable in the IR spectrum of the char. There were two new absorptions at 1755 and 1720 cm ’ (Fig. 10) ascribable to lactone-like structures, most likely of the same type as the ones found on the oxidation products of the lower temperature chars. Careful inspection of the spectrum also reveals the presence of a pair of bands at 1848 and 1780 cm-‘, barely discernible as weak shoulders. which can be assigned to five-membered ring cyclic anhydride-like structures; the latter bands grew slightly at higher oxidation temperatures and were destroyed at T,,, higher than 45o”C, concurrently with a remarkable weight loss of 42% (Fig. 10). It seems interesting to contrast this behavior with that of the PC490 char, where the anhydridic structures declined only at T,, = 490°C. after burn-off started and just before complete gasification of the residue. and also with

that of the PC440 char, where the anhydridic bands did not seem to change even at the highest oxidation temperatures. No significant perturbation in the -OH stretching region was caused by the oxidation, indicating that the carboxylic species found with lower temperature chars were most likely formed to a negligible extent, if at all, on oxidizing PC590. The aromatic C-H-related bands gradually declined in intensity with increasing oxidation temperature, illustrating in that fashion the vulnerability to oxidation of the peripheral portion of the aromatic system carrying the CH terminations (Fig. 9). The most severe weight loss took place concurrently with the reappearance in the IR spectrum of aliphatic CH stretching bands of several types (CH2, CHr, and CH), upon exposure of the PC59Ochar to oxygen at 500°C (Fig. 9); the same observation was made in the case of PC490 char, and this phenomenon seems to be more pronounced in the case of the PC590 char. Even though the extinction coefficients of saturated C-H stretching absorptions are known to be higher than those of unsaturated CH, it would hardly be reasonable to suggest that the strong aliphatic C-H bands in the spectrum of PC590(500) are derived from a mere “local” saturation of the previously existing unsaturated C-H bands in PC590(450). This behavior is not typical of the oxidative properties of chars and might indicate that the polyaromatic network newly formed from PC at 590°C is unusally unstable and bears many defects that make it prone to oxidative degradation, brought about by a series of rearrangements and insertions within the highly imperfect polyaromatic system.

PC490(400) P040(400)

0

!:

PC49O(Zso)

In

PC440(24Q)

PC490

PC440

Fig. 8. Compensated spectra of oxidized PC440 and PC490 chars normalized to the same highest intensity.

A. S. POL~TOUet al.

862

550 530

500 450

one at 1604 cm-‘, and the continuing decline of the aromatic C-H-related bands. No or very little perturbation in the O-H region was introduced by the oxidation, indicating that the formation of carboxylic structures is characteristic of only the low temperature chars. The dauble bond region of the spectra did not show the development of any absorptions above 1800 cm-‘, so that formation of any anhydridic structures can be excluded. There was also no indication of the formation of acetophenone-like structures, which were present in the spectra of the oxidized PC440 char and, to a lesser extent in those of the PC490 char, but not in those of the PC590 char. This supports our suggestion that the acetophenonelike structures are derived from local oxidation of bridge -CHr- groups; these groups are present to an appreciable extent only in low temperature chars. TheSaliphatic C-H species found in some of the

400 350

300

250 25 3450 3200 2050 2

0

0

P

belmmbes8 Fig. 9. IR spectra of PC590 oxidized at the temperatures indicated by the numbers in “C-High

frequency region.

3.4 The oxidation of PC700 Figures 11 and 12 show segments of normalized, compensated spectra obtained after the oxidation of the PC700 residue at various temperatures. New spectral features were already produced on oxidizing PC700 at the surprisingly low temperature of 250°C. including the formation of a doublet at 1760 and 1720 cm-‘, and of a broad and highly symmetric band around 125Ocm-’ (Figs. 12 and 13). Also, there were dramatic decreases in the intensities of both the aromatic C-H stretch near 3050 cm - ’ and the aromatic C-H deformation bands at 884,830, and 750 cm-l. Only a few more changes occurred at higher oxidation temperatures, namely, the progressive increase of the newly formed bands, as well as of the

lrav8Ilumbeml Fig. IO. IR spectra of PCS90 oxidized at the temperatures indited by the numbers in “C-Low frequency region.

Infrared studies of carbons

570

540

500

450

375

863

tensity of the IR continuum absorption (i.e., from the deviation from the baseline of the region above 2000 cm-‘)[7,8,19]. Inspection of the spectra in Figs. 14 and 15 reveals that an enhancement of carbonization upon oxidation is detectable only in the case of a char oxidized at a temperature as high as the one at which it was produced (for example, see spectrum PC490(490) in Fig. 14).

4. CONCLUSIONS

Fig. 16 illustrates and summarizes the oxidation behavior of all the PC chars studied. Based on that and the various IR data obtained, some general conclusions can be drawn, as follows: The low temperature PC chars were relatively resistant to oxidation. This behavior is reminiscent of that of the parent material. Thermo-oxidative processes in polycarbonate polymer were shown to be

260 25 a

Fig. 11. Segments of IR spectra of PC700 oxidized at several temperatures; the O-H and C-H stretch regions.

spectra of the oxidized lower temperature chars were not detected at any stage of the oxidation of PC700, an indication that the carbon derived at 700°C is highly organized, at least much more so than the ones produced at lower temperatures. Burn-off of the char took place on exposure to oxygen just above 570°C. In summary, oxidation of the high-temperature PC700 char led to the destruction of the few peripheral C-H aromatic groups left on the char and to the formation of an oxidic layer made up of double-bond ring species (lactones) and single bonded oxygen bridges, which grew somewhat at high oxidation temperatures. The behavior exhibited by PC700 is typical of a wide variety of intermediate temperature chars[lS-171.

3.5 Effects of oxidation on carbonization Oxidation of all chars examined caused several changes, but did not affect the extent of carbonization, as expected. Some representative single-beam, uncompensated spectra of the four PC chars examined, oxidized at several temperatures, are shown in Figs. 14 and 15. Single-beam spectra are useful in providing a qualitative indication of the degree of carbonization; this is obtained from the relative in-

25 I

ld50

lioo

960

Wavenumbem Fig. 12. Segments of IR spectra of PC700 oxidized at several temperatures; the “fingerprint” region.

A. S. Po~rrou et al.

864

B-A

Fig. 15. Single-beam spectra of PC590 and PC700 chars oxidized at several temperatures.

Fig. 13. Expanded compensated spectra of the PC700 char oxidized at two different temperatures and their difference spectrum; the C-O stretch region.

Pc490(490)

Pc49q4&-0) PC490

K4NM)

PC440

Fig. 14. Single-beam spectra of PC440 and PC490 chars oxidized at selected temperatures.

of such minor significance as not to impart noticeable changes in most of its physical properties below 125”C[18]. Oxygen uptake by PC at low temperatures is unusually low, one reason why PC rate is high among thermoplastics in thermo-oxidative stability. This inertness towards oxidation was also observed with the PC chars prepared and studied previously[4] for long periods of exposure to the atnosphere at room temperature; the spectra of chars obtained immediately after their formation were identical to those of the same chars after standing in air for as long as six months. The oxidation of PC chars proceeded mainly through the attack on the hydrocarbonaceous side chains. A variety of carbonylic species, including benzophenone structures, carboxylic acids, lactones, and anhydrides were formed and those were very resistant to oxidative destruction. The main reaction when low-temperature chars were heated in an oxygen atmosphere seemed to be oxidative degradation . In distinct contrast to a wide variety of coais[20] and cellulose-derived carbons[ 151, the higher-temperature PC chars were more reactive to oxidation than the low-temperature ones. That might be due to the difference in the nature of the carbonization process; in the case of cellulose and other carbons, the carbonization began at,, very low temperatures and increased gradually with increasing temperature, leading to a progressively more “regular” and organized network. On the other hand, with PC. the

Infrared studies of carbons

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400

450

933

550

600

650

700

pymlysis tcmpetamre

Fig. 16. Oxidation behavior of PC chars.

10. M. J. D. Low and M. Lacroix, Infrared Phys. 22. 139 (1982). 11. M. J. D. Low and C. Morterra, In Structure and Reactivity of Surfaces (Edited by C. Morterra, A. Zecchina, and G. Costa), p. 601. Elsevier, Amsterdam (1989). 12. D. Bellus. P. Hrdlovic, and Z. Manasek, Polym. Len. 4, l(1966). 13. N. B. Colthup, L. H. Daly, and S. E. Wibberley, fntroduction to Infrared and Roman Spectroscopy,

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Ed. Academic Press. New York (1975). 14. G. Socrates, Infrared Characteristic Group Frequencies. Wiley, New York (1980). 15. C. Morterra. M. J. D. Low, and A. G. Severdia. Cur-

collapse of the char framework and the formation of an extensive but poorly organized polyaromatic network occurred suddenly in a very narrow temperature range near 590°C. The chars obtained at 590°C were still quite atypical, but the ones produced at higher temperatures exhibited the “normal” oxidation behavior found with chars derived from several different types of precursors[lS161.

16. C. Morterra and M. J. D. Low, Langmuir 1,320 (1985). 17. V. A. Garten and D. E. Weiss, Rev. Pure Appl. Chem.

Acknowledgemenf-Support by NSF Grant 8516257 is gratefully acknowledged.

20. A. Cagiga, J. B. Escudera, M. J. D. Low, J. J. Pis, and J. M. D. Tascon, Fuel Proc. Technol. IS. 245 (1987).

bon 22, 5 (1984).

7. 69 (1957).

18. B. D. Gesner and P. G. Kelleher, 1. Appl. Polym. Sci. 13, 2183 (1%9). 19. M. J. D. Low, M. Lacroix, and C. Morterra, Appl. Spectrosc. 36. 582 (1982).

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