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Photo-oxidation products of polyetherimide ULTEM determined by MALDI-TOF-MS. Kinetics and mechanisms
Polymer Degradation and Stability 80 (2003) 459–476 www.elsevier.com/locate/polydegstab
Photo-oxidation products of polyetherimide ULTEM determined by MALDI-TOF-MS. Kinetics and mechanisms Sabrina Carroccioa, Concetto Puglisia, Giorgio Montaudob,* a
Istituto per la Chimica e la Tecnologia dei Materiali Polimerici, Consiglio Nazionale delle Ricerche. Viale A. Doria, 6-95125 Catania, Italy b Dipartimento di Scienze Chimiche, Universita’ di Catania. Viale A. Doria, 6-95125 Catania, Italy Received 13 November 2002; received in revised form 6 January 2003; accepted 11 January 2003
Abstract Poly 2,2-bis4-(3,4-dicarboxyphenoxy) phenylpropane dianhydride-1,3-phenylendiamine copolymer (ULTEM) was subjected to photo aging in the attempt to find evidence on the structure of the species formed in the oxidative degradation. The oxidation was followed as a function of the exposure time by MALDI and SEC/MALDI techniques. The SEC curves showed extensive degradation, with the formation of low molar mass oligomers having different end groups. Valuable structural information on the photooxidized ULTEM species was extracted from the MALDI spectra of the photo-oxidized ULTEM. These showed the presence of polymer chains containing acetophenone, phenyl acetic acid, phenols, benzoic acid, phthalic anhydride and phthalic acid end groups. The mechanisms accounting for the formation of photo-oxidation products involve several simultaneous reactions: (1) photo-cleavage of methyl groups of the N-methyl phthalimide terminal units; (2) photoxidative degradation of the isopropylidene bridge of BPA units; (3) photo-oxidation of phthalimide units to phthalic anhydride end groups: (4) hydrolysis of phthalic anhydride end groups. The kinetic behaviour of all the species detected is in agreement with the predictions of the reaction mechanisms hypothesized. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: MALDI-TOF; Polyethereimide; Photo-oxidative degradation; Mechanisms of photo-oxidation; Mass spectrometry
1. Introduction Aromatic polyimides are an important class of engineering thermoplastics possessing useful physical properties such as superior thermal stability, high modulus, toughness, and chemical resistance. These polymers have become increasingly important and are used in a variety of applications such as coatings, adhesives, composites and moulded components. In many cases polyimides are subjected to severe environments including exposure to ultraviolet radiation. This is particularly true for coatings applications where prolonged exposure to sunlight might be expected. In the literature, there are reports on the thermal behaviour [1,2] of heat-resistant polyimides but relatively little is known about their photochemical behaviour. * Corresponding author. Tel.: +39-095-339926; fax: +39-095221541. E-mail address: [email protected] (G. Montaudo).
Molecules formed in the photo-oxidation of many polymeric materials are often very reactive, do not accumulate, and are present only in minor amounts among the reaction products. Therefore, conventional analytical techniques may prove inadequate in establishing the structure of the photo-oxidation products. Modern mass spectrometry offers the opportunity to analyse trace amounts of chemical species and also to explore the finest structural details in polymeric materials [3–5]. Matrix assisted laser desorption ionisation–time of flight–MS (MALDI) provides mass-resolved spectra, which allow the detection of quite large molecules even in complex mixtures. The MALDI spectra originating from ions of intact polymer chains show enough resolution to allow the structural identification of oligomers up to 30,000 Da and above [3–8]. The study of polymer degradation by MALDI [9–12] involves the collection of MALDI spectra at different times and/or temperatures to observe the structural changes induced by heat, light under inert and/or oxidizing atmosphere. The partially
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00030-2
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degraded polymer sample can be analysed directly, and the recorded MALDI spectrum arises from a mixture of non-degraded and degraded chains. This opens new vistas in studying polymer degradation and deserves careful exploration, due to the relevance of these phenomena in everyday practice. We have recently reported [9,10] on the thermal and oxidative degradation of Polycarbonate (PC) and on the products of thermal oxidation of Nylon 6 [12], using MALDI as the main analytical technique. On these occasions, we have remarked the surprising high amount of structural information that can be extracted from the analysis of MALDI spectra of thermal or thermal oxidized polymers. We have now investigated the molecular species produced during the photo-oxidation of Poly 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride-1,3-phenylendiamine copolymer (ULTEM) [13,14] trade name of GE plastics by performing the photo oxidation by exposure at 60 C in atmospheric air. The photo-oxidation was followed as a function of the exposure time by, MALDI, and SEC/ MALDI.
Table 1 Molecular weight distribution data of ULTEM samples as a function of exposure time Temperature ( C)
Exposure time
Mwa
Mna
Dc
60
0 144 216 394 524 648 888 1032b
14285 11700 11418 12150 12577 11215 10575 8759
11747 7377 8166 7155 7585 8370 7490 4502
1.2 1.5 1.4 1.6 1.6 1.3 1.4 1.9
a b c
Obtained by SEC–MALDI method (see Section 2). This sample showed a 12% of CHCl3 insoluble residue. D=(Mw/Mn) Dispersity Index.
aromatic polymers. The remaining layer of film is therefore unaffected by the radiation, causing uncertainty in the molar mass determination. In fact Mn and Mw values reported in Table 1 do not show a constant decrement with the exposure time, although the general trend can be clearly seen. 2.3. SEC analysis and molar mass determinations
2. Experimental 2.1. Materials Basic materials were commercial products appropriately purified before use. Poly (2,2-bis4-(3,4-dicarboxyphenoxy) phenylpropane dianhydride-1,3-phenylendiamine copolymer (ULTEM) and 2-(4 hydroxyphenilazo)benzoic acid (HABA) were obtained from Sigma-Aldrich Chemical Co (Italy) and used as supplied. 2.2. Photo-oxidative degradation of ULTEM The photo-oxidation was performed on films of ULTEM with a uniform thickness of 100 mm. The films were obtained by casting from 2% CHCl3 solution. Photoxidative degradation of ULTEM was carried out on a QUV PANEL apparatus at 60 C with continued exposure to UV radiation (UVA 340 lamps). At least two separate films were analysed at each exposure time. At higher exposure times the sample forms a residue insoluble in CHCl3 (Table 1). It is likely that extensive photo-oxidation occurs in only about 10 mm or less of the film near the exposed surface as it does for many
The analyses were performed on a Waters 600A apparatus, equipped with five Ultrastyragel columns (7.8300 mm) (in the order 105, 103, 500, 104 and 100 A˚ pore size) connected in series, and a Waters R401 differential refractometer. A 60 ml of a CHCl3 ULTEM solution (0.5%) was injected and eluted with CHCl3 at flow rate of 1 ml/min. The molar masses of ULTEM samples were determined by the SEC/MALDI method [4–7]. The original ULTEM sample was fractionated to collect several equal volume fractions of about 0.165 ml each (corresponding to 12 drops). The molar mass of each collected fraction was determined by MALDI– TOF. The absolute calibration curves, obtained by plotting the log molar mass of SEC fractions as a function of the corresponding elution volume, allowed the calculation of the average molar masses of the unexposed and photo-oxidized ULTEM samples, using the Polymer Labs Caliber software. The data obtained are reported in Table 1. 2.4. MALDI-TOF analysis Matrix assisted laser description ionization time of flight (MALDI–TOF) mass spectra were obtained using a Voyager-DETM STR instrument, equipped with a nitrogen laser emitting at 337 nm with a 3 ns pulse width and working in positive ion mode. The accelerating voltage was 20–25 kV, the grid voltage and delay time (delayed extraction, time lag) were optimized for each sample to obtain the higher molar mass values.
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Fig. 1. MALDI–TOF isotopic resolution mass spectra of cyclic oligomers of ULTEM unexposed sample obtained in reflectron mode (a) 1208 Da (dimer) and (b) 1801 Da (trimer) (Table 2). The upper curves are the experimental traces, whereas the lower curves are calculates.
The laser irradiance was maintained slightly above threshold. Samples for the MALDI analyses were prepared by mixing appropriate volumes of the matrix solution (HABA, 0.1 M in THF/CHCl3) and polymer solution (2 mg/mL in CHCl3) to obtain a 1:1 or 1:3 ratio (sample/ matrix)v/v. 1 ml of a 0.1 M solution of sodium trifluoroacetate (NaTFA) in THF was added to aid cationisation. A 1 ml of each sample/matrix mixture was spotted on the MALDI sample holder and dried slowly to allow matrix crystallization. The relative amount of species reported in Figs. 7–10 was obtained as the ratio IA/IT where IA is the sum of the peaks intensity of each species in the mass range 1800–5000 Da and IT correspond to the sum of the intensity of all peaks appearing in the same mass range. The MALDI–TOF spectra were recorded in linear mode (mass resolution 1000 M/M) and in reflectron mode (resolution 2000 up to the mass range 2200 Da). The spectra in reflectron mode allow the mass isotopic resolution to be achieved. Due to the complexity of the MALDI spectra of the photo-oxidized ULTEM samples (Figs. 3–5), isotopic resolution helps considerably in peak assignment. The comparison of the relative intensities of isotopic peaks corresponding to oligomers of increasing molar mass is of special interest. In Fig. 1 the experimental/calculated matches for two MALDI peaks of cyclic oligomers, appearing at 1208 and 1801Da (Table 2) in the spectra of the ULTEM samples recorded in reflectron mode are reported. At
least two separate MALDI spectra were obtained at each exposure time. The standard deviation of MALDI–TOF peaks intensity was 8%. Two isotopic clusters are shown in Fig. 1a and b, corresponding to the ULTEM cyclic dimer and trimer (Table 2). It can be clearly seen that the M+1 peak of the cyclic trimer is more intense than the M peak (Fig. 1b), whereas the M+1 peak of the cyclic dimer is less intense with respect to the M peak (Fig. 1a), in agreement with what expected from the structural assignments. Therefore, the comparison of the experimental versus calculated isotopic peak intensities for oligomers of increasing molar mass allows structural assignments in cases where peaks might belong to nearly isobar structures.
3. Results and discussion ULTEM films were subjected to photo aging treatment by using QUV PANEL test in atmospheric air up to 1032 h (Table 1). The longest exposure times caused the formation of portions insoluble in CHCl3. The samples were then filtered, in order to have a soluble portion suitable for SEC and MALDI–TOF analyses. The GPC traces of some photo-oxidized ULTEM samples are shown in Fig. 2. The degradation is already evident after 216 h of exposure and data shown in Table 1 indicate that degradation is fairly extensive, producing a steady reduction of the molar mass and a significant increase of the polydispersity.
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Fig. 2. GPC traces of ULTEM sample photo-oxidized at 60 C for 0, 216, 888 and 1032 h.
Fig. 3 shows the MALDI–TOF spectrum of the unexposed ULTEM sample in the mass range 1200– 12,000 Da, together with an expanded portion from 1800 to 2600 Da (inset). The peaks belong to six different mass series, corresponding to sodiated macromolecular ions. At low molar mass, the most intense peaks in Fig. 3 are due to cyclic oligomers at m/z 1801+n 592.6 Da (mass series A, Table 2), whereas at higher molar masses the most prominent peaks appear at m/z 2169+n 592.6 Da (mass series B, Table 2), corresponding to oligomers with phenyl-phthalimide end groups at both ends. A third mass series, appearing in Fig. 3 at m/z 1949+n 592.6 Da, can be assigned to oligomers of type C, Table 2, corresponding to sodiated ions terminated with phenyl–phthalimide at one end and phthalic– anhydride groups at the other end. The fourth mass series at m/z 1962+n 592.6 Da corresponds to macromolecular ions bearing phenylphthalimide at one end and N-methyl-phthalimide
terminal groups at the other end. (mass series D, Table 2). The last two peaks series, appearing with low intensity in Fig. 3, are due, respectively, to macromolecular ions with N-methyl phthalimide end groups (mass series E, Table 2), and to N-methyl phthalimide/phthalic–anhydride terminal group (mass series F, Table 2). As discussed earlier (Section 2), spectra obtained by using the MALDI instrument in reflectron mode allow peak isotopic resolution. Fig. 4a shows the MALDI spectra of the unexposed ULTEM sample recorded in linear and reflectron mode, in the mass range 1350–1380 Da. The peak labelled C in the spectrum recorded in linear mode is actually constituted of two peaks, corresponding to different oligomers. The first peak appearing in the reflectron spectrum in Fig. 4a at m/z 1354 is assigned to an ULTEM oligomer having NH–phthalimide at one end and phenyl-phthalimide at the other end (mass series G, Table 2), whereas peak C appears with its isotopic
463
Fig. 3. MALDI–TOF spectrum of ULTEM sample in the mass range 1200–11,000 Da.
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Fig. 4. MALDI–TOF mass spectra obtained in linear (left side) and reflectron mode (right side) in the mass range 1350–1380 Da of (a) unexposed ULTEM sample and (b) photo-oxidized for 216 h.
distribution at m/z 1355, 1356, 1357. In linear mode, peaks C and G appear unresolved because of the low resolution (Fig. 4a). The reflectron spectrum in Fig. 4a shows also peak D (Table 2) with isotopic distribution (peaks at 1368, 1369 and 1370 Da). The MALDI spectrum of the ULTEM sample photooxidized at 60 C for 216 h is reported in Fig. 5. The spectrum shows a considerably increased number of peaks with respect to the original sample, indicating that photo-oxidation reactions have occurred, producing polymer degradation and oligomers containing several new end groups. The structural assignments of the additional peaks appearing in the MALDI spectrum of the photo-oxidized ULTEM sample are reported in
Table 3. The peak identification also has been performed with the help of the isotopic resolution spectra. For instance, in Fig. 4b peaks at m/z 1356 and 1374 Da appear broad and unresolved in linear mode, whereas the spectrum in reflectron mode allows peaks corresponding to the species G, C, D, H, G1, C1 to be distinguished (Table 3). The comparison of the experimental versus calculated isotopic peak intensities for oligomers of increasing molar mass (see Fig. 1), was also used in making structural assignments in cases where peaks might belong to nearly isobar structures. The spectral evidence and detailed analysis supporting the structural assignments in Tables 2 and 3 are omitted here for brevity. The expanded portion of the spectrum
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Fig. 5. MALDI–TOF mass spectrum of ULTEM sample photoxidized at 60 C for 216 h.
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Fig. 6. MALDI–TOF mass spectra in the mass range 1800–2400 of ULTEM sample photo-oxidized (a) for 524 h, and (b) 1032 h.
in Fig. 5 shows over 20 mass peaks, which have been assigned to oligomers with different structure, formed during the photo-oxidation of ULTEM (Tables 2 and 3). Fig. 6a shows an expanded portion of the MALDI spectrum of an ULTEM sample photo-oxidized at 60 C for 524 h, and Fig. 6b reports that of a sample photooxidized for 1032 h. Both spectra show an increased number of peaks with respect to the sample exposed for 216 h (reported in the inset of Fig. 5), indicating that the photo-oxidation has proceeded further, producing still new oligomers (Tables 2 and 3). The relative intensity of the peaks appearing in the MALDI spectra reported in Figs. 3, 5 and 6, respectively, undergoes marked change as a function of the exposure time, and this effect nicely allows correlating the peak intensity of each species (Tables 2 and 3) to the kinetics of the photo-oxidation. The kinetic behaviour of the species generated, proved to be an important feature of our study, allowing discerning the main oxidation pathways that our MALDI data were indicating.
According to the structure of the major oxidation products, four photo-oxidation processes can be postulated: (1) photo-cleavage of methyl groups of the N-methyl phthalimide terminal units; (2) photoxidative degradation of the isopropylidene bridge of BPA units; (3) photo-oxidation of phthalimide units to phthalic anhydride end groups: (4) hydrolysis of phthalic anhydride end groups. Let’s examine first the photo-cleavage process of methyl groups of the N-methyl phthalimide terminal units. Oligomers D and F (Fig. 3, Table 2), having N-methyl phthalimide end groups, appear after exposure with a drastically reduced peak intensity (Figs. 5 and 6). This effect is due to a specific degradation reaction most likely occurring by loss of the N-methyl group [Eq. (1)], which produces chains terminated with N–H phthalimide groups. In fact, peaks K and L in Figs. 5 and 6, are generated by this photo-oxidation process, and correspond to chains terminated with N–H phthalimide groups (Table 3).
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467
Fig. 7. Relative amount versus exposure time of species Y, M and F as obtained from the MALDI spectra of photo-oxidized ULTEM sample.
The kinetic behaviour of oligomers of type F is shown in Fig. 7, where it can be seen that the abundance of this species approaches zero after 524 h exposure, in agreement with the photo-oxidation mechanism hypothesized earlier. The second oxidation pathway involves the photoxidative degradation of the isopropylidene bridge of BPA units [15–17]. The oxidation is thought to occur through the formation of a hydroperoxide intermediate, which may decompose by two parallel pathways yielding several products (Scheme 1). Peaks M and R appearing in Figs. 5 and 6, have been identified as chains bearing acetophenone units end groups, and peaks of type L to oligomers terminated with benzyl alcohol units (Scheme 1, Table 3). Further oxidation of these terminal groups leads to the formation of species O, Y, N, P, Q and M, which have benzoic acid and phenylacetic acid units at the chain ends (Scheme 1, Table 3). The reaction mechanism in Scheme 1 also predicts the formation of species with phenol as terminal groups. Such species (oligomers K, Table 3), are actually found in the spectra in Figs. 5 and 6.
All six kinds of end groups predicted by this reaction scheme have been detected in the MALDI spectra discussed above; a remarkable success. The abundance of oligomers Y and M (Scheme 1, Table 3), shows (Fig. 7) a rapid increment with the exposure time and then nearly levels off. This behaviour can be accounted for assuming the occurrence of further degradation processes leading to smaller molecules that are most likely lost at the film surface (erosion process). Because of the surface erosion the amount of oxidised oligomers in the MALDI spectra (Figs. 5 and 6) increases with the exposure time up to about 75% of all the peaks appearing in the spectra. A further pathway, related to the photoxidative decomposition of the bisphenol A isopropylidene bridges along the ULTEM chains, is revealed by the presence of peaks A1, A2, B1, B2, B3, C1, C2, C3, D1, E1, G1, H1, O1, X1 (Figs. 4b, 5, 6 and Table 3). These new species are hypothesized to originate from oligomers A, B, C, D, G, H, O, X, respectively, by an oxidative process which introduces one or more oxygen atoms into the isopropylidene bridges along the main chain, as depicted in Scheme 2. More specifically, the
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Fig. 8. Relative amount versus exposure time of cyclic oligomers A, A1and A2 (Tables 2 and 3) as obtained from the MALDI spectra of photooxidized ULTEM sample.
Fig. 9. Relative amount versus exposure time of linear oligomers B, B1, B2, and B3 (Tables 2 and 3) as obtained from the MALDI spectra of photooxidized ULTEM sample.
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Fig. 10. Relative amount versus exposure time of species C, U and N as obtained from the MALDI spectra of photo-oxidized ULTEM sample.
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Table 2 Structural assignments of sodiated ions appearing in the MALDI-TOF spectra of original sample Series mass
Oligomer structures
MNa+
A
1208 1801
B
2169
C
1356 1949
D
1369 1962
E
2334
F
2347
G
1355
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Table 3 Structural assignments of sodiated ions appearing in the MALDI–TOF spectra of photo-oxidized ULTEM sample Series mass
Oligomers structures
MNa+
H
1374 1967
I
1989
K
2056
L
2069
M
1846
N
2085
O
2099
P
2103
(continued on next page)
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Table 3 (continued) Series mass
Oligomers structures
MNa+
Q
2117
R
2303
U
2352
V
2370
X
2388
Y
1861
A1 B1 C1 G1 D1 E1 H1 O1 U1 X1
1817 2185 1371 1370 1978 2350 1983 2115 2368 2404
A2 B2 C2 U2
1833 2201 1981 2384
B3 C3
2217 1997
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Scheme 1.
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methylene radical initially formed, might directly yield an hydroperoxide intermediate (Structure I, Scheme 2), and its subsequent decomposition may lead to Structure II. Alternately, the methylene radical may undergo rearrangement to a benzyl radical, and the decomposition of the benzyl hydroperoxide III may lead to Structure IV (Scheme 2). Since structures II and IV are isobaric, the mass spectrometric analysis is not able to distinguish them. The kinetic profile of the ULTEM cyclic oligomers (peak A) is shown in Fig. 8, where it can be noticed that the abundance of this species drastically decreases with the exposure time. In Fig. 8 are also included the kinetic profile of peak A1, belonging to cyclic oxidised species
(Table 3, Scheme 2), which increase steadily, whereas the intensity of peak A2 increases to slower extent. Analogously, the abundance of the ULTEM linear oligomers (peak B), reported in Fig. 9, is seen to decrease noticeably with the exposure time. In this case, the profiles of the oxidized species, i.e. peaks B1, B2 and B3, also increase. As discussed earlier, a third oxidation process is also noticeable. Photo-oxidation of phthalimide units to phthalic anhydride end groups is also occurring, as revealed by the presence of peaks of type U and V (Figs. 5 and 6, Table 3), corresponding to oligomers with phthalic-anhydride end groups at both ends. The appearance of peaks of type U and V, as well as the
Scheme 2.
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relevant increment of peaks of type C and E, which also contain phthalic-anhydride end groups (Tables 2 and 3), might be explained through the reaction pathways shown in Scheme 3, which has been already proposed in the literature [18] for N-phenyl phthalimide. The first step in Scheme 3 implies the cleavage of the C–N bonds to yield a diradical, which reacts with oxygen to give a peroxide intermediate. The subsequent decomposition of the latter leads to the formation of oligomers with phthalic anhydride terminal groups [18]. The kinetic behaviour of selected peaks bearing phthalic anhydride terminal groups (species C, U and N) is reported in Fig. 10. It can be noted that, after an initial increment of oligomers U, N and C formed during the photo-cleavage of phthalimide units (Fig. 10, Tables 2 and 3), the relative amount of species remains almost constant. The presence of peaks H, I P, Q, V and X (Fig. 6, Table 3) corresponding to species containing phthalic acid end groups, suggests a fourth photo-oxidation pathway that involves the hydrolysis of phthalic anhydride end groups. Noticeably, peaks P, Q, V and X are derived from hydrolysis of photo-oxidation products N, O and U formed at lower exposure time. This indicates that the hydrolysis of phthalic anhydride end groups easily occurs during the photoxidative processes (Scheme 3). Our data indicate that the scission of the ULTEM chain occurs preferentially at the isopropylidene bridges and at the phthalimide units, according to Schemes 1–3. The kinetic behaviour of the main photo-oxidation products suggests that the two cleavage processes occur simultaneously in the exposed films. No evidence is found supporting the rupture of the diphenylether units along the ULTEM chains. The presence of further oxidation products, like ring oxidation [15–17], phenol and nitro or nitroso phthalimide end groups [18], was not clearly detectable in the MALDI spectra of exposed films. This might be due to the crowding of numerous species in the spectra, which causes a relatively high noise level. Alternately, lower molar mass species might be lost, because of the mass limit of MALDI-TOF technique (below 1000 Da).
molar mass of the ULTEM, with formation of acetophenone, benzyl alcohol, benzoic acid, phenols, phenylacetic acid, phthalic-acid, phthalic anhydride, N–H phthalimide groups at the oligomers chain ends. The rupture of the ULTEM chain occurs at the isopropylidene bridges and at the phthalimide units, according to Schemes 1–3. Scission of the diphenylether units is not observed. The structural analysis of the photo-oxidation products provided by the MALDI spectra presented earlier, allowed drawing a detailed map of the photo-oxidation mechanisms of ULTEM (Schemes 1–3) for the first time. The presence of all the oligomers listed in Tables 2 and 3 had been never revealed before. Although the MALDI peak intensity is only a semi-quantitative measure of the relative abundance of the corresponding oligomers, the formation of the oxidation products as a function of exposure time could be easily tracked from the analysis of the MALDI spectra.
4. Conclusions Summarizing our results, it should be remarked that a large amount of valuable structural information on the photo-oxidation products of ULTEM has been extracted from our MALDI data. The peculiarity of the our approach consists in using a non averaging technique, such as mass spectrometry, which allows the detection and monitoring of each oligomer during the oxidation process. The data collected show that the photo-oxidative degradation produces a significant reduction of the
475
Scheme 3.
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The kinetic data on the photo-oxidation proved to be particularly useful, since the kinetic behaviour of all the oligomers identified was found to be in agreement with the predictions of the reaction mechanisms hypothesized.
Acknowledgements Partial financial support from the Italian Ministry for University and for Scientific and Technological Research and from the National Council of Research (CNR, Rome) is gratefully acknowledged. Many thanks are due to Mr. R. Rapisardi and Mr. G. Pastorelli for their continuous and skilful technical assistance.
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[5] Montaudo G, Garozzo D, Montaudo MS, Puglisi C, Samperi F. Macromolecules 1995;28:7983. [6] Montaudo MS, Puglisi C, Samperi F, Montaudo G. Rapid Commun Mass Spectrom 1998;12:519. [7] Puglisi C, Samperi F, Carroccio S, Montaudo G. Rapid Commun Mass Spectrom 1999;13:2260 2268. [8] Carroccio S, Rizzarelli P, Puglisi C. Rapid Commun Mass Spectrom 2000;14:1513. [9] Puglisi C, Carroccio S, Montaudo G. Macromolecules 2002; 35:4297–305. [10] Puglisi C, Samperi F, Carroccio S, Montaudo G. Macromolecules 1999;32:8821. [11] Weidner St, Ku¨hn G, Friedrich J, Schro¨der H. Rapid Commun Mass Spectrom 1996;10:40. [12] Chionna D, Puglisi C, Samperi F, Carroccio S, Turturro A, Montaudo G. Macromol Rapid Commun 2001;22:524. [13] Takekoshi T, Kochanowski JS, Manello JS, Webber MJ. J Polym Sci Polym Symp 1986;74:93. [14] Takekoshi T, Kochanowski JS, Manello JS, Webber MJ. J Polym Sci Polym Chem Ed 1985;23:1759. [15] Hawkins L. Polymer degradation and stabilization. Berlin: Springer–Verlag; 1984. [16] Factor A. In: Clough RL, Billingham NC, Gillen KT, editors. Polymer durability. Degradation, stabilization, and lifetime prediction. Advances in Chemical Series 249. Washington DC: ACS; 1996 (references quoted therein). [17] Factor A, Chu ML. Polym Degrad Stab 1980;2:203. [18] Hoyle CE, Anzures ET, Subramanian P, Nagarajan R, Creed D. Macromolecules 1992;25:6651.
Photo-oxidation products of polyetherimide ULTEM determined by MALDI-TOF-MS. Kinetics and mechanisms Sabrina Carroccioa, Concetto Puglisia, Giorgio Montaudob,* a
Istituto per la Chimica e la Tecnologia dei Materiali Polimerici, Consiglio Nazionale delle Ricerche. Viale A. Doria, 6-95125 Catania, Italy b Dipartimento di Scienze Chimiche, Universita’ di Catania. Viale A. Doria, 6-95125 Catania, Italy Received 13 November 2002; received in revised form 6 January 2003; accepted 11 January 2003
Abstract Poly 2,2-bis4-(3,4-dicarboxyphenoxy) phenylpropane dianhydride-1,3-phenylendiamine copolymer (ULTEM) was subjected to photo aging in the attempt to find evidence on the structure of the species formed in the oxidative degradation. The oxidation was followed as a function of the exposure time by MALDI and SEC/MALDI techniques. The SEC curves showed extensive degradation, with the formation of low molar mass oligomers having different end groups. Valuable structural information on the photooxidized ULTEM species was extracted from the MALDI spectra of the photo-oxidized ULTEM. These showed the presence of polymer chains containing acetophenone, phenyl acetic acid, phenols, benzoic acid, phthalic anhydride and phthalic acid end groups. The mechanisms accounting for the formation of photo-oxidation products involve several simultaneous reactions: (1) photo-cleavage of methyl groups of the N-methyl phthalimide terminal units; (2) photoxidative degradation of the isopropylidene bridge of BPA units; (3) photo-oxidation of phthalimide units to phthalic anhydride end groups: (4) hydrolysis of phthalic anhydride end groups. The kinetic behaviour of all the species detected is in agreement with the predictions of the reaction mechanisms hypothesized. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: MALDI-TOF; Polyethereimide; Photo-oxidative degradation; Mechanisms of photo-oxidation; Mass spectrometry
1. Introduction Aromatic polyimides are an important class of engineering thermoplastics possessing useful physical properties such as superior thermal stability, high modulus, toughness, and chemical resistance. These polymers have become increasingly important and are used in a variety of applications such as coatings, adhesives, composites and moulded components. In many cases polyimides are subjected to severe environments including exposure to ultraviolet radiation. This is particularly true for coatings applications where prolonged exposure to sunlight might be expected. In the literature, there are reports on the thermal behaviour [1,2] of heat-resistant polyimides but relatively little is known about their photochemical behaviour. * Corresponding author. Tel.: +39-095-339926; fax: +39-095221541. E-mail address: [email protected] (G. Montaudo).
Molecules formed in the photo-oxidation of many polymeric materials are often very reactive, do not accumulate, and are present only in minor amounts among the reaction products. Therefore, conventional analytical techniques may prove inadequate in establishing the structure of the photo-oxidation products. Modern mass spectrometry offers the opportunity to analyse trace amounts of chemical species and also to explore the finest structural details in polymeric materials [3–5]. Matrix assisted laser desorption ionisation–time of flight–MS (MALDI) provides mass-resolved spectra, which allow the detection of quite large molecules even in complex mixtures. The MALDI spectra originating from ions of intact polymer chains show enough resolution to allow the structural identification of oligomers up to 30,000 Da and above [3–8]. The study of polymer degradation by MALDI [9–12] involves the collection of MALDI spectra at different times and/or temperatures to observe the structural changes induced by heat, light under inert and/or oxidizing atmosphere. The partially
0141-3910/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0141-3910(03)00030-2
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degraded polymer sample can be analysed directly, and the recorded MALDI spectrum arises from a mixture of non-degraded and degraded chains. This opens new vistas in studying polymer degradation and deserves careful exploration, due to the relevance of these phenomena in everyday practice. We have recently reported [9,10] on the thermal and oxidative degradation of Polycarbonate (PC) and on the products of thermal oxidation of Nylon 6 [12], using MALDI as the main analytical technique. On these occasions, we have remarked the surprising high amount of structural information that can be extracted from the analysis of MALDI spectra of thermal or thermal oxidized polymers. We have now investigated the molecular species produced during the photo-oxidation of Poly 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride-1,3-phenylendiamine copolymer (ULTEM) [13,14] trade name of GE plastics by performing the photo oxidation by exposure at 60 C in atmospheric air. The photo-oxidation was followed as a function of the exposure time by, MALDI, and SEC/ MALDI.
Table 1 Molecular weight distribution data of ULTEM samples as a function of exposure time Temperature ( C)
Exposure time
Mwa
Mna
Dc
60
0 144 216 394 524 648 888 1032b
14285 11700 11418 12150 12577 11215 10575 8759
11747 7377 8166 7155 7585 8370 7490 4502
1.2 1.5 1.4 1.6 1.6 1.3 1.4 1.9
a b c
Obtained by SEC–MALDI method (see Section 2). This sample showed a 12% of CHCl3 insoluble residue. D=(Mw/Mn) Dispersity Index.
aromatic polymers. The remaining layer of film is therefore unaffected by the radiation, causing uncertainty in the molar mass determination. In fact Mn and Mw values reported in Table 1 do not show a constant decrement with the exposure time, although the general trend can be clearly seen. 2.3. SEC analysis and molar mass determinations
2. Experimental 2.1. Materials Basic materials were commercial products appropriately purified before use. Poly (2,2-bis4-(3,4-dicarboxyphenoxy) phenylpropane dianhydride-1,3-phenylendiamine copolymer (ULTEM) and 2-(4 hydroxyphenilazo)benzoic acid (HABA) were obtained from Sigma-Aldrich Chemical Co (Italy) and used as supplied. 2.2. Photo-oxidative degradation of ULTEM The photo-oxidation was performed on films of ULTEM with a uniform thickness of 100 mm. The films were obtained by casting from 2% CHCl3 solution. Photoxidative degradation of ULTEM was carried out on a QUV PANEL apparatus at 60 C with continued exposure to UV radiation (UVA 340 lamps). At least two separate films were analysed at each exposure time. At higher exposure times the sample forms a residue insoluble in CHCl3 (Table 1). It is likely that extensive photo-oxidation occurs in only about 10 mm or less of the film near the exposed surface as it does for many
The analyses were performed on a Waters 600A apparatus, equipped with five Ultrastyragel columns (7.8300 mm) (in the order 105, 103, 500, 104 and 100 A˚ pore size) connected in series, and a Waters R401 differential refractometer. A 60 ml of a CHCl3 ULTEM solution (0.5%) was injected and eluted with CHCl3 at flow rate of 1 ml/min. The molar masses of ULTEM samples were determined by the SEC/MALDI method [4–7]. The original ULTEM sample was fractionated to collect several equal volume fractions of about 0.165 ml each (corresponding to 12 drops). The molar mass of each collected fraction was determined by MALDI– TOF. The absolute calibration curves, obtained by plotting the log molar mass of SEC fractions as a function of the corresponding elution volume, allowed the calculation of the average molar masses of the unexposed and photo-oxidized ULTEM samples, using the Polymer Labs Caliber software. The data obtained are reported in Table 1. 2.4. MALDI-TOF analysis Matrix assisted laser description ionization time of flight (MALDI–TOF) mass spectra were obtained using a Voyager-DETM STR instrument, equipped with a nitrogen laser emitting at 337 nm with a 3 ns pulse width and working in positive ion mode. The accelerating voltage was 20–25 kV, the grid voltage and delay time (delayed extraction, time lag) were optimized for each sample to obtain the higher molar mass values.
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Fig. 1. MALDI–TOF isotopic resolution mass spectra of cyclic oligomers of ULTEM unexposed sample obtained in reflectron mode (a) 1208 Da (dimer) and (b) 1801 Da (trimer) (Table 2). The upper curves are the experimental traces, whereas the lower curves are calculates.
The laser irradiance was maintained slightly above threshold. Samples for the MALDI analyses were prepared by mixing appropriate volumes of the matrix solution (HABA, 0.1 M in THF/CHCl3) and polymer solution (2 mg/mL in CHCl3) to obtain a 1:1 or 1:3 ratio (sample/ matrix)v/v. 1 ml of a 0.1 M solution of sodium trifluoroacetate (NaTFA) in THF was added to aid cationisation. A 1 ml of each sample/matrix mixture was spotted on the MALDI sample holder and dried slowly to allow matrix crystallization. The relative amount of species reported in Figs. 7–10 was obtained as the ratio IA/IT where IA is the sum of the peaks intensity of each species in the mass range 1800–5000 Da and IT correspond to the sum of the intensity of all peaks appearing in the same mass range. The MALDI–TOF spectra were recorded in linear mode (mass resolution 1000 M/M) and in reflectron mode (resolution 2000 up to the mass range 2200 Da). The spectra in reflectron mode allow the mass isotopic resolution to be achieved. Due to the complexity of the MALDI spectra of the photo-oxidized ULTEM samples (Figs. 3–5), isotopic resolution helps considerably in peak assignment. The comparison of the relative intensities of isotopic peaks corresponding to oligomers of increasing molar mass is of special interest. In Fig. 1 the experimental/calculated matches for two MALDI peaks of cyclic oligomers, appearing at 1208 and 1801Da (Table 2) in the spectra of the ULTEM samples recorded in reflectron mode are reported. At
least two separate MALDI spectra were obtained at each exposure time. The standard deviation of MALDI–TOF peaks intensity was 8%. Two isotopic clusters are shown in Fig. 1a and b, corresponding to the ULTEM cyclic dimer and trimer (Table 2). It can be clearly seen that the M+1 peak of the cyclic trimer is more intense than the M peak (Fig. 1b), whereas the M+1 peak of the cyclic dimer is less intense with respect to the M peak (Fig. 1a), in agreement with what expected from the structural assignments. Therefore, the comparison of the experimental versus calculated isotopic peak intensities for oligomers of increasing molar mass allows structural assignments in cases where peaks might belong to nearly isobar structures.
3. Results and discussion ULTEM films were subjected to photo aging treatment by using QUV PANEL test in atmospheric air up to 1032 h (Table 1). The longest exposure times caused the formation of portions insoluble in CHCl3. The samples were then filtered, in order to have a soluble portion suitable for SEC and MALDI–TOF analyses. The GPC traces of some photo-oxidized ULTEM samples are shown in Fig. 2. The degradation is already evident after 216 h of exposure and data shown in Table 1 indicate that degradation is fairly extensive, producing a steady reduction of the molar mass and a significant increase of the polydispersity.
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Fig. 2. GPC traces of ULTEM sample photo-oxidized at 60 C for 0, 216, 888 and 1032 h.
Fig. 3 shows the MALDI–TOF spectrum of the unexposed ULTEM sample in the mass range 1200– 12,000 Da, together with an expanded portion from 1800 to 2600 Da (inset). The peaks belong to six different mass series, corresponding to sodiated macromolecular ions. At low molar mass, the most intense peaks in Fig. 3 are due to cyclic oligomers at m/z 1801+n 592.6 Da (mass series A, Table 2), whereas at higher molar masses the most prominent peaks appear at m/z 2169+n 592.6 Da (mass series B, Table 2), corresponding to oligomers with phenyl-phthalimide end groups at both ends. A third mass series, appearing in Fig. 3 at m/z 1949+n 592.6 Da, can be assigned to oligomers of type C, Table 2, corresponding to sodiated ions terminated with phenyl–phthalimide at one end and phthalic– anhydride groups at the other end. The fourth mass series at m/z 1962+n 592.6 Da corresponds to macromolecular ions bearing phenylphthalimide at one end and N-methyl-phthalimide
terminal groups at the other end. (mass series D, Table 2). The last two peaks series, appearing with low intensity in Fig. 3, are due, respectively, to macromolecular ions with N-methyl phthalimide end groups (mass series E, Table 2), and to N-methyl phthalimide/phthalic–anhydride terminal group (mass series F, Table 2). As discussed earlier (Section 2), spectra obtained by using the MALDI instrument in reflectron mode allow peak isotopic resolution. Fig. 4a shows the MALDI spectra of the unexposed ULTEM sample recorded in linear and reflectron mode, in the mass range 1350–1380 Da. The peak labelled C in the spectrum recorded in linear mode is actually constituted of two peaks, corresponding to different oligomers. The first peak appearing in the reflectron spectrum in Fig. 4a at m/z 1354 is assigned to an ULTEM oligomer having NH–phthalimide at one end and phenyl-phthalimide at the other end (mass series G, Table 2), whereas peak C appears with its isotopic
463
Fig. 3. MALDI–TOF spectrum of ULTEM sample in the mass range 1200–11,000 Da.
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Fig. 4. MALDI–TOF mass spectra obtained in linear (left side) and reflectron mode (right side) in the mass range 1350–1380 Da of (a) unexposed ULTEM sample and (b) photo-oxidized for 216 h.
distribution at m/z 1355, 1356, 1357. In linear mode, peaks C and G appear unresolved because of the low resolution (Fig. 4a). The reflectron spectrum in Fig. 4a shows also peak D (Table 2) with isotopic distribution (peaks at 1368, 1369 and 1370 Da). The MALDI spectrum of the ULTEM sample photooxidized at 60 C for 216 h is reported in Fig. 5. The spectrum shows a considerably increased number of peaks with respect to the original sample, indicating that photo-oxidation reactions have occurred, producing polymer degradation and oligomers containing several new end groups. The structural assignments of the additional peaks appearing in the MALDI spectrum of the photo-oxidized ULTEM sample are reported in
Table 3. The peak identification also has been performed with the help of the isotopic resolution spectra. For instance, in Fig. 4b peaks at m/z 1356 and 1374 Da appear broad and unresolved in linear mode, whereas the spectrum in reflectron mode allows peaks corresponding to the species G, C, D, H, G1, C1 to be distinguished (Table 3). The comparison of the experimental versus calculated isotopic peak intensities for oligomers of increasing molar mass (see Fig. 1), was also used in making structural assignments in cases where peaks might belong to nearly isobar structures. The spectral evidence and detailed analysis supporting the structural assignments in Tables 2 and 3 are omitted here for brevity. The expanded portion of the spectrum
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Fig. 5. MALDI–TOF mass spectrum of ULTEM sample photoxidized at 60 C for 216 h.
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Fig. 6. MALDI–TOF mass spectra in the mass range 1800–2400 of ULTEM sample photo-oxidized (a) for 524 h, and (b) 1032 h.
in Fig. 5 shows over 20 mass peaks, which have been assigned to oligomers with different structure, formed during the photo-oxidation of ULTEM (Tables 2 and 3). Fig. 6a shows an expanded portion of the MALDI spectrum of an ULTEM sample photo-oxidized at 60 C for 524 h, and Fig. 6b reports that of a sample photooxidized for 1032 h. Both spectra show an increased number of peaks with respect to the sample exposed for 216 h (reported in the inset of Fig. 5), indicating that the photo-oxidation has proceeded further, producing still new oligomers (Tables 2 and 3). The relative intensity of the peaks appearing in the MALDI spectra reported in Figs. 3, 5 and 6, respectively, undergoes marked change as a function of the exposure time, and this effect nicely allows correlating the peak intensity of each species (Tables 2 and 3) to the kinetics of the photo-oxidation. The kinetic behaviour of the species generated, proved to be an important feature of our study, allowing discerning the main oxidation pathways that our MALDI data were indicating.
According to the structure of the major oxidation products, four photo-oxidation processes can be postulated: (1) photo-cleavage of methyl groups of the N-methyl phthalimide terminal units; (2) photoxidative degradation of the isopropylidene bridge of BPA units; (3) photo-oxidation of phthalimide units to phthalic anhydride end groups: (4) hydrolysis of phthalic anhydride end groups. Let’s examine first the photo-cleavage process of methyl groups of the N-methyl phthalimide terminal units. Oligomers D and F (Fig. 3, Table 2), having N-methyl phthalimide end groups, appear after exposure with a drastically reduced peak intensity (Figs. 5 and 6). This effect is due to a specific degradation reaction most likely occurring by loss of the N-methyl group [Eq. (1)], which produces chains terminated with N–H phthalimide groups. In fact, peaks K and L in Figs. 5 and 6, are generated by this photo-oxidation process, and correspond to chains terminated with N–H phthalimide groups (Table 3).
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467
Fig. 7. Relative amount versus exposure time of species Y, M and F as obtained from the MALDI spectra of photo-oxidized ULTEM sample.
The kinetic behaviour of oligomers of type F is shown in Fig. 7, where it can be seen that the abundance of this species approaches zero after 524 h exposure, in agreement with the photo-oxidation mechanism hypothesized earlier. The second oxidation pathway involves the photoxidative degradation of the isopropylidene bridge of BPA units [15–17]. The oxidation is thought to occur through the formation of a hydroperoxide intermediate, which may decompose by two parallel pathways yielding several products (Scheme 1). Peaks M and R appearing in Figs. 5 and 6, have been identified as chains bearing acetophenone units end groups, and peaks of type L to oligomers terminated with benzyl alcohol units (Scheme 1, Table 3). Further oxidation of these terminal groups leads to the formation of species O, Y, N, P, Q and M, which have benzoic acid and phenylacetic acid units at the chain ends (Scheme 1, Table 3). The reaction mechanism in Scheme 1 also predicts the formation of species with phenol as terminal groups. Such species (oligomers K, Table 3), are actually found in the spectra in Figs. 5 and 6.
All six kinds of end groups predicted by this reaction scheme have been detected in the MALDI spectra discussed above; a remarkable success. The abundance of oligomers Y and M (Scheme 1, Table 3), shows (Fig. 7) a rapid increment with the exposure time and then nearly levels off. This behaviour can be accounted for assuming the occurrence of further degradation processes leading to smaller molecules that are most likely lost at the film surface (erosion process). Because of the surface erosion the amount of oxidised oligomers in the MALDI spectra (Figs. 5 and 6) increases with the exposure time up to about 75% of all the peaks appearing in the spectra. A further pathway, related to the photoxidative decomposition of the bisphenol A isopropylidene bridges along the ULTEM chains, is revealed by the presence of peaks A1, A2, B1, B2, B3, C1, C2, C3, D1, E1, G1, H1, O1, X1 (Figs. 4b, 5, 6 and Table 3). These new species are hypothesized to originate from oligomers A, B, C, D, G, H, O, X, respectively, by an oxidative process which introduces one or more oxygen atoms into the isopropylidene bridges along the main chain, as depicted in Scheme 2. More specifically, the
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Fig. 8. Relative amount versus exposure time of cyclic oligomers A, A1and A2 (Tables 2 and 3) as obtained from the MALDI spectra of photooxidized ULTEM sample.
Fig. 9. Relative amount versus exposure time of linear oligomers B, B1, B2, and B3 (Tables 2 and 3) as obtained from the MALDI spectra of photooxidized ULTEM sample.
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Fig. 10. Relative amount versus exposure time of species C, U and N as obtained from the MALDI spectra of photo-oxidized ULTEM sample.
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Table 2 Structural assignments of sodiated ions appearing in the MALDI-TOF spectra of original sample Series mass
Oligomer structures
MNa+
A
1208 1801
B
2169
C
1356 1949
D
1369 1962
E
2334
F
2347
G
1355
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471
Table 3 Structural assignments of sodiated ions appearing in the MALDI–TOF spectra of photo-oxidized ULTEM sample Series mass
Oligomers structures
MNa+
H
1374 1967
I
1989
K
2056
L
2069
M
1846
N
2085
O
2099
P
2103
(continued on next page)
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Table 3 (continued) Series mass
Oligomers structures
MNa+
Q
2117
R
2303
U
2352
V
2370
X
2388
Y
1861
A1 B1 C1 G1 D1 E1 H1 O1 U1 X1
1817 2185 1371 1370 1978 2350 1983 2115 2368 2404
A2 B2 C2 U2
1833 2201 1981 2384
B3 C3
2217 1997
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Scheme 1.
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methylene radical initially formed, might directly yield an hydroperoxide intermediate (Structure I, Scheme 2), and its subsequent decomposition may lead to Structure II. Alternately, the methylene radical may undergo rearrangement to a benzyl radical, and the decomposition of the benzyl hydroperoxide III may lead to Structure IV (Scheme 2). Since structures II and IV are isobaric, the mass spectrometric analysis is not able to distinguish them. The kinetic profile of the ULTEM cyclic oligomers (peak A) is shown in Fig. 8, where it can be noticed that the abundance of this species drastically decreases with the exposure time. In Fig. 8 are also included the kinetic profile of peak A1, belonging to cyclic oxidised species
(Table 3, Scheme 2), which increase steadily, whereas the intensity of peak A2 increases to slower extent. Analogously, the abundance of the ULTEM linear oligomers (peak B), reported in Fig. 9, is seen to decrease noticeably with the exposure time. In this case, the profiles of the oxidized species, i.e. peaks B1, B2 and B3, also increase. As discussed earlier, a third oxidation process is also noticeable. Photo-oxidation of phthalimide units to phthalic anhydride end groups is also occurring, as revealed by the presence of peaks of type U and V (Figs. 5 and 6, Table 3), corresponding to oligomers with phthalic-anhydride end groups at both ends. The appearance of peaks of type U and V, as well as the
Scheme 2.
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relevant increment of peaks of type C and E, which also contain phthalic-anhydride end groups (Tables 2 and 3), might be explained through the reaction pathways shown in Scheme 3, which has been already proposed in the literature [18] for N-phenyl phthalimide. The first step in Scheme 3 implies the cleavage of the C–N bonds to yield a diradical, which reacts with oxygen to give a peroxide intermediate. The subsequent decomposition of the latter leads to the formation of oligomers with phthalic anhydride terminal groups [18]. The kinetic behaviour of selected peaks bearing phthalic anhydride terminal groups (species C, U and N) is reported in Fig. 10. It can be noted that, after an initial increment of oligomers U, N and C formed during the photo-cleavage of phthalimide units (Fig. 10, Tables 2 and 3), the relative amount of species remains almost constant. The presence of peaks H, I P, Q, V and X (Fig. 6, Table 3) corresponding to species containing phthalic acid end groups, suggests a fourth photo-oxidation pathway that involves the hydrolysis of phthalic anhydride end groups. Noticeably, peaks P, Q, V and X are derived from hydrolysis of photo-oxidation products N, O and U formed at lower exposure time. This indicates that the hydrolysis of phthalic anhydride end groups easily occurs during the photoxidative processes (Scheme 3). Our data indicate that the scission of the ULTEM chain occurs preferentially at the isopropylidene bridges and at the phthalimide units, according to Schemes 1–3. The kinetic behaviour of the main photo-oxidation products suggests that the two cleavage processes occur simultaneously in the exposed films. No evidence is found supporting the rupture of the diphenylether units along the ULTEM chains. The presence of further oxidation products, like ring oxidation [15–17], phenol and nitro or nitroso phthalimide end groups [18], was not clearly detectable in the MALDI spectra of exposed films. This might be due to the crowding of numerous species in the spectra, which causes a relatively high noise level. Alternately, lower molar mass species might be lost, because of the mass limit of MALDI-TOF technique (below 1000 Da).
molar mass of the ULTEM, with formation of acetophenone, benzyl alcohol, benzoic acid, phenols, phenylacetic acid, phthalic-acid, phthalic anhydride, N–H phthalimide groups at the oligomers chain ends. The rupture of the ULTEM chain occurs at the isopropylidene bridges and at the phthalimide units, according to Schemes 1–3. Scission of the diphenylether units is not observed. The structural analysis of the photo-oxidation products provided by the MALDI spectra presented earlier, allowed drawing a detailed map of the photo-oxidation mechanisms of ULTEM (Schemes 1–3) for the first time. The presence of all the oligomers listed in Tables 2 and 3 had been never revealed before. Although the MALDI peak intensity is only a semi-quantitative measure of the relative abundance of the corresponding oligomers, the formation of the oxidation products as a function of exposure time could be easily tracked from the analysis of the MALDI spectra.
4. Conclusions Summarizing our results, it should be remarked that a large amount of valuable structural information on the photo-oxidation products of ULTEM has been extracted from our MALDI data. The peculiarity of the our approach consists in using a non averaging technique, such as mass spectrometry, which allows the detection and monitoring of each oligomer during the oxidation process. The data collected show that the photo-oxidative degradation produces a significant reduction of the
475
Scheme 3.
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The kinetic data on the photo-oxidation proved to be particularly useful, since the kinetic behaviour of all the oligomers identified was found to be in agreement with the predictions of the reaction mechanisms hypothesized.
Acknowledgements Partial financial support from the Italian Ministry for University and for Scientific and Technological Research and from the National Council of Research (CNR, Rome) is gratefully acknowledged. Many thanks are due to Mr. R. Rapisardi and Mr. G. Pastorelli for their continuous and skilful technical assistance.
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