- Email: [email protected]
polyethylene terephthalate interface
S ?aTMTIIE S|T ILS ELSEVIER
Synthetic Metals 67 (1994) 97-101
A joint theoretical and experimental study of the aluminium/ polyethylene terephthalate interface A. Calderone a, R. Lazzaroni a'l, J.L. Brddas a, Quoc Toan Le b, J.J. Pireaux b ~Service de Chimie des Matdriaux Nouveaux, Universitd de Mons-Hainaut, place du Parc 20, 7000 Mons, Belgium bLaboratoire Interddpartemental de Spectroscopie Electronique, Facultds Universitaires Notre-Dame de la Pair, 5000 Namur, Belgium
Abstract
The aluminium/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum-chemical calculations on short molecular model systems interacting with a few aluminium atoms. These results are compared to experimental X-ray photoelectron spectroscopy data collected during the early stages of interface formation. Keywords: Interface, Polyethylene terephthalate; Aluminium
1. Introduction As polymers are widely used in complex multilayered systems, the nature of their interface with other materials plays a major role in determining the mechanical, electrical and chemical properties of the whole system. In particular, the adhesion between a polymer and a metal layer depends on the chemical interactions occurring at the molecular level at the metal/polymer interface. The interfaces between polymers and aluminium, which is one of the most technologically important metals, have been extensively investigated over the past few years, mostly in the context of the application of macromolecular materials in electronics and packaging [1,2]. In the present study, we use a combined theoretical and experimental approach to investigate the early stages of formation of the interface between aluminium and polyethylene terephthalate (PET). On the theoretical level, we perform quantum-chemical calculations on a model molecule consisting of a segment of the PET chain capped with appropriate end groups. After preliminary calculations on the isolated molecule, one or two aluminium atoms are introduced in the surroundings of the molecule to follow the formation of the organometallic complex. In parallel, the early stages of the tChercheur qualifi~ du Fonds National de la Recherche Scientifique,
0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02216-L
interface formation are investigated with X-ray photoelectron spectroscopy (XPS). The evolution of the core levels of the polymer is followed as an aluminium layer is gradually deposited onto its surface. From the comparison of the experimental data with the theoretical calculations, we derive the most probable structural models for the A1/PET interface°
2. Theoretical and experimental procedures The model system we consider is the acetoxyethyl methyl terephthalate molecule (AEMT; Fig. 1), interacting with two aluminium atoms. Using the MOPAC 6.0 package [3], we have first carried out full geometry optimizations on the molecular system with and without aluminium atoms present, at the Hartree-Fock MNDO (modified neglect of diatomic overlap) semi-empirical H
C~C C--C~ H/
~//0 .4c--c ~H
H ~C-
M
? Fig. 1. Molecular structure of AEMT,
98
A. Calderone et aL / Synthetic Metals 67 (1994) 07-101
level. We have then evaluated the relative stability of different configurations of the complex from their binding energy (BE), defined as the difference in total energies between the complex and the sum of the total energies of the molecule and two aluminium atoms. In order to estimate the distribution of the electron density, a Mulliken population analysis has been performed. For the XPS studies, the substrate was a commercial biaxially oriented PET film (Mylar, 12 /xm, Du Pont de Nemours). Room temperature, in situ, aluminium evaporation was performed onto the PET substrate in the XPS preparation chamber by using a Knudsen cell equipped with a pyrolytic boron nitride crucible in the 3-5X10 -9 Torr range. The evaporation rate was calibrated by a quartz crystal monitor; the typical deposition rate was about 3/~/min. XPS measurements were carried out at room temperature, with a Surface Science Instruments SSX-100 system using a monochromatized A1 K~ X-ray source (1486.6 eV). The operating pressure during analysis was in the low 10 - 9 Torr range. The reference binding energy was set at 284.6 eV for the C-C component of the C(ls) peak.
. . " •-
(f)
'..•.....'......'..-.."..-'
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.' ....-"
•
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3. Results and discussion
,'•.
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..... -'.....',.•--.,
"-'...--'--.•'"
"'"'
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3.1. X P S measurements
The C(ls) photoemission signal of clean PET exhibits four distinct components (Fig. 2(a)). The main component corresponds to the carbon atoms of the phenyl ring (284.6 eV); the shoulder at 286.1 eV is assigned to the carbon singly bonded to oxygen C-O, while the component at 288.6 eV is attributed to the ester group O-C = O. The experimental intensity ratio of these three components is 59.5:22.0:18.5, in good agreement with the expected stoichiometry (60:20:20). The low-intensity peak appearing at higher binding energy (291.0 eV) is attributed to a 7r-Tr* 'shake-up' process. In the O(ls) signal of clean PET (Fig. 3(a)), the doubly bound oxygen corresponds to the peak located at 531.4 eV; the singly bound oxygen appears at 533.0 eV and a 'shake-up' peak is observed around 538 eV. The evolution of the C(ls) core-level spectra as a function of aluminium deposition is shown in Fig. 2. We observe a dramatic decrease in the ester signal intensity, clearly indicating that the ester groups are the preferential sites of interaction with aluminium. In addition, a new spectral component appears on the low binding energy side of the C(ls) signal, which extends to around 282 eV for the thickest aluminium layer. We also observe that the 7r-Tr* shake-up satellite, which persisted after the first deposition step, completely vanishes after about 1.5 A aluminium coverage (Fig. 2(c)). Upon aluminium deposition, the O(ls) emission
"'.
(b)
...........
..............
..."
.. %..•
"/
si \ (~)
. . . . . . . . . . . . . . . .
294.0
290.0
,,."
286.0
.............
/ 1 \
,',, .
•...
/
,,,,
\__.,._
282.0
BINDING ENERGY (eV) Fig. 2. XPS C(ls) spectra of the AI/PET interface: (a) clean polymer; (b) 0.5 A,; (c) 1.5 .~,; (d) 3.0 A,; (e) 4.5/~; 9.0 ]k aluminium coverage.
exhibits a line broadening and a shoulder appears on the low binding energy side of the peak (Fig. 3). This new component is located at about 530.2 eV, i.e., 1.1-1.2 eV lower than the doubly bound oxygen. Again, the 7r-Tr* shape-up peak decreases in intensity after the first metallization step and completely disappears after further deposition. The energy difference between the new component at low binding energy in the O(ls) spectrum and the metallic component in the Al(2p) peak is about 457.5 eV, i.e., 2 eV lower than the value found in aluminium oxide/aluminium [4]. Therefore the
A. C a l d e r o n e et al. I Synthetic Metals 67 (1994) 97-101 H
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/H
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/\c--c \ \c~C
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,, ,
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i
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(a) i
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Fig. 4. Structures of the three most stable AI2/AEMT complexes, as optimized with the M N D O theoretical method.
,
533.0
i
529.0
BINDING ENERGY (eV) Fig. 3. XPS O(ls) spectra of the AI/PET interface: (a) clean polymer; (b) 0.5 ~; (c) 1.5 /k; (d) 3.0/~; (e) 4.5 ~; 9.0 ~ aluminium coverage.
new O(ls) feature does not correspond to aluminium oxide; it could be attributed to another form of oxidized aluminium, probably an O-A1 bond in the A1/PET complex. 3.2. Theoretical results
Depending on the initial configuration, the MNDO geometry optimization leads to different stable structures for the A12/AEMT complex, each of these corresponding to a local energy minimum (Fig. 4). In all cases, the aluminium atoms interact preferentially with the oxygen atoms of the C = O groups. System A, in which each doubly bound oxygen is attacked by a metal atom, is found to be the most favourable situation (BE, 155.6 kcal/mol), systems B and C being 15.9 and 19.4 kcal/mol less stable, respectively. The possibility of
interaction with the phenyl part of the molecule has been also considered; this kind of configuration appears to be much less favourable (45-60 kcal/mol) relative to those described above. Thus, in view of these results, the first aluminium atoms are expected to interact with the ester groups, while the interaction with the phenyl rings could only appear at higher metal coverage, i.e., when all available ester sites have been attacked. Upon aluminium bonding, the C = O linkage is found to lose its double-bond character. In complexes A and C, the C = O bond lengths increase from 1.226 to 1.325 ~ , which is typical of single C-O bonds. In system B, the C = O bond becomes even longer (1.446 A) as the two metal atoms attack the same oxygen atom. When one aluminium atom binds to a C = O oxygen, the second metal atom can interact simultaneously with both the same oxygen and the carbon atom bonded to it, as observed in structure B. This three-membered structure between aluminium, carbon and oxygen atoms also appears in system C; in that case, the oxygen singly bound to the carbon of the ester group is also involved. In both B and C situations, the carbon of the ester group becomes tetravalent (sp3-hybridized) after complexation. Note that in structure A, the phenyl ring, which is initially aromatic, is driven to a fully quinoid
100
A. Calderone et al. / Synthetic Metals 67 (1994) 97-101
character after complexation (this can explain the vanishing of the ~--~-* shake-up peak in the C(ls) spectrum). The optimized A1-C and AI-O distances are short: 1.90 and 1.60-1.62 ~, respectively. These values are very similar to the A1-C and A1-O bond lengths observed experimentally in organometallic molecules: 1.96/~ for AI-C in trimethylaluminium [5], 1.76 A for A1-O in a tetrahedral coordination [6]. These results are consistent with covalent bond formation between aluminium atoms and the oxygen or carbon atoms of the PET model molecule (note that we have obtained similar theoretical results for the aluminium/polyimide system [7]). From the Mulliken population analysis, we observe that each metal atom loses between 0.4 and 0.6[e I to the molecule. After bonding with aluminium, the net atomic charge on the doubly bound oxygens increases by 0.2[e[ in structures A and B, and 0.31el in C. In system A, the remaining part of the transferred charge accumulates on the ring (0.4]e] globally, but with weak modifications on each individual carbon atom), with a minor increase on the carbons of the ester functions (0.1[el). Due to the interaction with both aluminium atoms, the charge on the ester carbons is strongly affected in systems B and C, in which it increases by 0.6[el and 0.4]e[, respectively. 3.3. Discussion
The XPS core-level data point to a strong modification of the ester groups of PET upon aluminium deposition; this is consistent with the theoretical results which show that the most favourable Al/PET interaction occurs when the metal atoms bind to the ester group. The observed chemical shifts can be interpreted in terms of the calculated changes in charge densities. In clean PET, the ester carbon atom is 'electron poor', hence, its 4 eV chemical shift upward relative to the aromatic carbons. Accordingly, the charge calculated on that site is +0.49[el, to be compared with about -0.05[el on the aromatic carbons. When aluminium is deposited on PET, the ester component of the C(ls) spectrum gradually disappears, which means that the photoelectronic intensity associated with that carbon has been shifted to another part of the spectrum, most probably inside the main envelope, where it cannot be distinguished from the other components. In structure A, where the aluminium atoms bind only to the doubly bound oxygens, the change in the charge of the ester carbon remains small (about -0.11el). That carbon atom is thus still positive and only a slight shift of the ester component would be expected in the C(ls) spectrum. In contrast, in the case of structures B and C, where aluminium atoms also bind to the ester carbon, we calculate a very strong increase of the electron density on that carbon: - 0.56 and - 0.44H, respectively.
As a consequence, the charge of the ester carbon found in those complexes is close to those of the aromatic carbons (about -0.051el). This is consistent with the large chemical shift observed experimentally and suggests that the reacted species contribute to the main C(ls) peak. Therefore, it appears that the formation of complexes B and C better accounts for the evolution of the C(ls) spectrum, even though they are calculated to be slightly less stable than structure A. The O(ls) XPS spectra are also consistent with the presence of complexes A, B and C. In all three cases, the charge density on the doubly bound oxygen atom is calculated to increase significantly (between - 0.22[e[ and - 0.271el) upon interaction with aluminium. This increase most probably corresponds to the appearance of the new component around 530.2 eV in the O(ls) spectrum (Fig. 3). The low binding energy component of the C(ls) spectrum might be due to the formation of complexes between aluminium and phenyl rings. In these systems, the carbon sites where the A1-C bonding takes place become markedly negative (around -0.451e1). As a consequence, the corresponding photoelectronic signal is expected to be shifted out of the main peak towards lower binding energies, which would give rise to the intensity observed around 282-283 eV. Since these complexes are much less stable than those where aluminium binds to the ester groups, they are not expected to appear during the very first steps of the metal deposition. They could only be formed when all the ester groups available for aluminium bonding have reacted.
Acknowledgements Studies of metal/conjugated polymer interfaces in Mons are supported by the Belgian Government 'P61e d'Attraction Interuniversitaire en Chimie et Catalyse' and 'Programme d'Impulsion en Technologic de l'Information' (Contract SC/IT/22), FNRS/FRFC and the European Commission SCIENCE Programme (Project 0661 POLYSURF).
References [1] P.S. Ho, P.O. Hahn, J.W. Bartha, G.W. Rubloff, F.K. LeGoues and B.D. Silverman, J. Vac. Sci. Technol. A, 3 (1985) 739; M. Bou, J.M. Martin and T. Le Mogne, Appl. Surf Sci., 47 (1991) 149. [2] J.J. Pireaux, P. Bertrand and J.L. Br6das (eds.), Polymer-Solid Interfaces, Institute of Physics, Bristol, 1992; E. Sacher, J.J. Pireaux and S.P. Kowalczyk (eds.), Metallization of Polymers, ACS Symposium Series, Vol. 440, American Chemical Society, Washington DC, 1990.
A. Calderone et aL / Synthetic Metals 67 (1994) 97-101 [3] J.J.P. Stewart, MOPAC 6.0, Quantum Chemistry Programme Exchange, QCPE #455, Department of Chemistry, Indiana University, Bloomington, IN. [4] B.R. Strohmeier, Surf. Interface Anal., 15 (1990) 5. [5] A. Almenningen, S. Havorsen and A. Haaland, Acta Chem. Scand., 25 (1971) 1937.
101
[6] L.S. Dent Glasser and R. Giovanoli, Acta Crystallogr., Sect. B. 28 (1972) 760. [7] S. Stafstr6m, M. Boman and J.L. Br6das, in S.P. Kowalczyk (ed.), Handbook of Polymer Metallization, Marcel Dekker, New York, 1994, in press.
Synthetic Metals 67 (1994) 97-101
A joint theoretical and experimental study of the aluminium/ polyethylene terephthalate interface A. Calderone a, R. Lazzaroni a'l, J.L. Brddas a, Quoc Toan Le b, J.J. Pireaux b ~Service de Chimie des Matdriaux Nouveaux, Universitd de Mons-Hainaut, place du Parc 20, 7000 Mons, Belgium bLaboratoire Interddpartemental de Spectroscopie Electronique, Facultds Universitaires Notre-Dame de la Pair, 5000 Namur, Belgium
Abstract
The aluminium/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum-chemical calculations on short molecular model systems interacting with a few aluminium atoms. These results are compared to experimental X-ray photoelectron spectroscopy data collected during the early stages of interface formation. Keywords: Interface, Polyethylene terephthalate; Aluminium
1. Introduction As polymers are widely used in complex multilayered systems, the nature of their interface with other materials plays a major role in determining the mechanical, electrical and chemical properties of the whole system. In particular, the adhesion between a polymer and a metal layer depends on the chemical interactions occurring at the molecular level at the metal/polymer interface. The interfaces between polymers and aluminium, which is one of the most technologically important metals, have been extensively investigated over the past few years, mostly in the context of the application of macromolecular materials in electronics and packaging [1,2]. In the present study, we use a combined theoretical and experimental approach to investigate the early stages of formation of the interface between aluminium and polyethylene terephthalate (PET). On the theoretical level, we perform quantum-chemical calculations on a model molecule consisting of a segment of the PET chain capped with appropriate end groups. After preliminary calculations on the isolated molecule, one or two aluminium atoms are introduced in the surroundings of the molecule to follow the formation of the organometallic complex. In parallel, the early stages of the tChercheur qualifi~ du Fonds National de la Recherche Scientifique,
0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02216-L
interface formation are investigated with X-ray photoelectron spectroscopy (XPS). The evolution of the core levels of the polymer is followed as an aluminium layer is gradually deposited onto its surface. From the comparison of the experimental data with the theoretical calculations, we derive the most probable structural models for the A1/PET interface°
2. Theoretical and experimental procedures The model system we consider is the acetoxyethyl methyl terephthalate molecule (AEMT; Fig. 1), interacting with two aluminium atoms. Using the MOPAC 6.0 package [3], we have first carried out full geometry optimizations on the molecular system with and without aluminium atoms present, at the Hartree-Fock MNDO (modified neglect of diatomic overlap) semi-empirical H
C~C C--C~ H/
~//0 .4c--c ~H
H ~C-
M
? Fig. 1. Molecular structure of AEMT,
98
A. Calderone et aL / Synthetic Metals 67 (1994) 07-101
level. We have then evaluated the relative stability of different configurations of the complex from their binding energy (BE), defined as the difference in total energies between the complex and the sum of the total energies of the molecule and two aluminium atoms. In order to estimate the distribution of the electron density, a Mulliken population analysis has been performed. For the XPS studies, the substrate was a commercial biaxially oriented PET film (Mylar, 12 /xm, Du Pont de Nemours). Room temperature, in situ, aluminium evaporation was performed onto the PET substrate in the XPS preparation chamber by using a Knudsen cell equipped with a pyrolytic boron nitride crucible in the 3-5X10 -9 Torr range. The evaporation rate was calibrated by a quartz crystal monitor; the typical deposition rate was about 3/~/min. XPS measurements were carried out at room temperature, with a Surface Science Instruments SSX-100 system using a monochromatized A1 K~ X-ray source (1486.6 eV). The operating pressure during analysis was in the low 10 - 9 Torr range. The reference binding energy was set at 284.6 eV for the C-C component of the C(ls) peak.
. . " •-
(f)
'..•.....'......'..-.."..-'
,••
..... ""•'-d.:
(e)
• ,..'''",....," -
• ...•,..,....................••.
""-. ....... ,
...°..
(d)
...
.' ....-"
•
......... ........... ... .........
"'... -....
...-
3. Results and discussion
,'•.
""•. 12)
..... -'.....',.•--.,
"-'...--'--.•'"
"'"'
"..
3.1. X P S measurements
The C(ls) photoemission signal of clean PET exhibits four distinct components (Fig. 2(a)). The main component corresponds to the carbon atoms of the phenyl ring (284.6 eV); the shoulder at 286.1 eV is assigned to the carbon singly bonded to oxygen C-O, while the component at 288.6 eV is attributed to the ester group O-C = O. The experimental intensity ratio of these three components is 59.5:22.0:18.5, in good agreement with the expected stoichiometry (60:20:20). The low-intensity peak appearing at higher binding energy (291.0 eV) is attributed to a 7r-Tr* 'shake-up' process. In the O(ls) signal of clean PET (Fig. 3(a)), the doubly bound oxygen corresponds to the peak located at 531.4 eV; the singly bound oxygen appears at 533.0 eV and a 'shake-up' peak is observed around 538 eV. The evolution of the C(ls) core-level spectra as a function of aluminium deposition is shown in Fig. 2. We observe a dramatic decrease in the ester signal intensity, clearly indicating that the ester groups are the preferential sites of interaction with aluminium. In addition, a new spectral component appears on the low binding energy side of the C(ls) signal, which extends to around 282 eV for the thickest aluminium layer. We also observe that the 7r-Tr* shake-up satellite, which persisted after the first deposition step, completely vanishes after about 1.5 A aluminium coverage (Fig. 2(c)). Upon aluminium deposition, the O(ls) emission
"'.
(b)
...........
..............
..."
.. %..•
"/
si \ (~)
. . . . . . . . . . . . . . . .
294.0
290.0
,,."
286.0
.............
/ 1 \
,',, .
•...
/
,,,,
\__.,._
282.0
BINDING ENERGY (eV) Fig. 2. XPS C(ls) spectra of the AI/PET interface: (a) clean polymer; (b) 0.5 A,; (c) 1.5 .~,; (d) 3.0 A,; (e) 4.5/~; 9.0 ]k aluminium coverage.
exhibits a line broadening and a shoulder appears on the low binding energy side of the peak (Fig. 3). This new component is located at about 530.2 eV, i.e., 1.1-1.2 eV lower than the doubly bound oxygen. Again, the 7r-Tr* shape-up peak decreases in intensity after the first metallization step and completely disappears after further deposition. The energy difference between the new component at low binding energy in the O(ls) spectrum and the metallic component in the Al(2p) peak is about 457.5 eV, i.e., 2 eV lower than the value found in aluminium oxide/aluminium [4]. Therefore the
A. C a l d e r o n e et al. I Synthetic Metals 67 (1994) 97-101 H
~". o
9q
/H
A[
/\c--c \ \c~C
0/
/o / C ~
\c :
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c
\ 0
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C-N
H
\ 0
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--
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..•.,..,....-. ....... ... ....... •,......,.
/ 0
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/. 0
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(b)
..............................................
/
1
,, ,
t
i
t.. t t
/
H
'... ..........
\
(a) i
541.0
i
537.0
Fig. 4. Structures of the three most stable AI2/AEMT complexes, as optimized with the M N D O theoretical method.
,
533.0
i
529.0
BINDING ENERGY (eV) Fig. 3. XPS O(ls) spectra of the AI/PET interface: (a) clean polymer; (b) 0.5 ~; (c) 1.5 /k; (d) 3.0/~; (e) 4.5 ~; 9.0 ~ aluminium coverage.
new O(ls) feature does not correspond to aluminium oxide; it could be attributed to another form of oxidized aluminium, probably an O-A1 bond in the A1/PET complex. 3.2. Theoretical results
Depending on the initial configuration, the MNDO geometry optimization leads to different stable structures for the A12/AEMT complex, each of these corresponding to a local energy minimum (Fig. 4). In all cases, the aluminium atoms interact preferentially with the oxygen atoms of the C = O groups. System A, in which each doubly bound oxygen is attacked by a metal atom, is found to be the most favourable situation (BE, 155.6 kcal/mol), systems B and C being 15.9 and 19.4 kcal/mol less stable, respectively. The possibility of
interaction with the phenyl part of the molecule has been also considered; this kind of configuration appears to be much less favourable (45-60 kcal/mol) relative to those described above. Thus, in view of these results, the first aluminium atoms are expected to interact with the ester groups, while the interaction with the phenyl rings could only appear at higher metal coverage, i.e., when all available ester sites have been attacked. Upon aluminium bonding, the C = O linkage is found to lose its double-bond character. In complexes A and C, the C = O bond lengths increase from 1.226 to 1.325 ~ , which is typical of single C-O bonds. In system B, the C = O bond becomes even longer (1.446 A) as the two metal atoms attack the same oxygen atom. When one aluminium atom binds to a C = O oxygen, the second metal atom can interact simultaneously with both the same oxygen and the carbon atom bonded to it, as observed in structure B. This three-membered structure between aluminium, carbon and oxygen atoms also appears in system C; in that case, the oxygen singly bound to the carbon of the ester group is also involved. In both B and C situations, the carbon of the ester group becomes tetravalent (sp3-hybridized) after complexation. Note that in structure A, the phenyl ring, which is initially aromatic, is driven to a fully quinoid
100
A. Calderone et al. / Synthetic Metals 67 (1994) 97-101
character after complexation (this can explain the vanishing of the ~--~-* shake-up peak in the C(ls) spectrum). The optimized A1-C and AI-O distances are short: 1.90 and 1.60-1.62 ~, respectively. These values are very similar to the A1-C and A1-O bond lengths observed experimentally in organometallic molecules: 1.96/~ for AI-C in trimethylaluminium [5], 1.76 A for A1-O in a tetrahedral coordination [6]. These results are consistent with covalent bond formation between aluminium atoms and the oxygen or carbon atoms of the PET model molecule (note that we have obtained similar theoretical results for the aluminium/polyimide system [7]). From the Mulliken population analysis, we observe that each metal atom loses between 0.4 and 0.6[e I to the molecule. After bonding with aluminium, the net atomic charge on the doubly bound oxygens increases by 0.2[e[ in structures A and B, and 0.31el in C. In system A, the remaining part of the transferred charge accumulates on the ring (0.4]e] globally, but with weak modifications on each individual carbon atom), with a minor increase on the carbons of the ester functions (0.1[el). Due to the interaction with both aluminium atoms, the charge on the ester carbons is strongly affected in systems B and C, in which it increases by 0.6[el and 0.4]e[, respectively. 3.3. Discussion
The XPS core-level data point to a strong modification of the ester groups of PET upon aluminium deposition; this is consistent with the theoretical results which show that the most favourable Al/PET interaction occurs when the metal atoms bind to the ester group. The observed chemical shifts can be interpreted in terms of the calculated changes in charge densities. In clean PET, the ester carbon atom is 'electron poor', hence, its 4 eV chemical shift upward relative to the aromatic carbons. Accordingly, the charge calculated on that site is +0.49[el, to be compared with about -0.05[el on the aromatic carbons. When aluminium is deposited on PET, the ester component of the C(ls) spectrum gradually disappears, which means that the photoelectronic intensity associated with that carbon has been shifted to another part of the spectrum, most probably inside the main envelope, where it cannot be distinguished from the other components. In structure A, where the aluminium atoms bind only to the doubly bound oxygens, the change in the charge of the ester carbon remains small (about -0.11el). That carbon atom is thus still positive and only a slight shift of the ester component would be expected in the C(ls) spectrum. In contrast, in the case of structures B and C, where aluminium atoms also bind to the ester carbon, we calculate a very strong increase of the electron density on that carbon: - 0.56 and - 0.44H, respectively.
As a consequence, the charge of the ester carbon found in those complexes is close to those of the aromatic carbons (about -0.051el). This is consistent with the large chemical shift observed experimentally and suggests that the reacted species contribute to the main C(ls) peak. Therefore, it appears that the formation of complexes B and C better accounts for the evolution of the C(ls) spectrum, even though they are calculated to be slightly less stable than structure A. The O(ls) XPS spectra are also consistent with the presence of complexes A, B and C. In all three cases, the charge density on the doubly bound oxygen atom is calculated to increase significantly (between - 0.22[e[ and - 0.271el) upon interaction with aluminium. This increase most probably corresponds to the appearance of the new component around 530.2 eV in the O(ls) spectrum (Fig. 3). The low binding energy component of the C(ls) spectrum might be due to the formation of complexes between aluminium and phenyl rings. In these systems, the carbon sites where the A1-C bonding takes place become markedly negative (around -0.451e1). As a consequence, the corresponding photoelectronic signal is expected to be shifted out of the main peak towards lower binding energies, which would give rise to the intensity observed around 282-283 eV. Since these complexes are much less stable than those where aluminium binds to the ester groups, they are not expected to appear during the very first steps of the metal deposition. They could only be formed when all the ester groups available for aluminium bonding have reacted.
Acknowledgements Studies of metal/conjugated polymer interfaces in Mons are supported by the Belgian Government 'P61e d'Attraction Interuniversitaire en Chimie et Catalyse' and 'Programme d'Impulsion en Technologic de l'Information' (Contract SC/IT/22), FNRS/FRFC and the European Commission SCIENCE Programme (Project 0661 POLYSURF).
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