polymer interfaces — Thin films of Al on polyacrylic acid and polyethylene

Applied Surface Science 27 (1986) 199-213 North-Holland, Amsterdam

199

XPS STUDIES OF METAL/POLYMER INTERFACES - THIN F I L M S O F AI O N P O L Y A C R Y L I C A C I D A N D P O L Y E T H Y L E N E

Benjamin M. D E K O V E N a n d Patrick L. H A G A N S The Dow Chemical Compap~v, Central Research Inorganic Materials and Catalysis l ~ b (CRIMCL), 1776 Building, Midland, M I 48674, USA

Received 26 June 1986; accepted for publication 1 July 1986

A reactivity experimental study of AI with CH 2 and COOH functionalities in polymers is performed using polyacrylic acid (PAA) and low density polyethylene (LDPE). The AI metal is deposited on the polymer surfaces using in-situ sputter deposition. Using X-ray photoelectron spectroscopy (XPS), an A1 oxide-carbide complex is identified at the PAA/AI interface while an AI carbide-like species is observed at the LDPE/AI interface. These conclusions are based on carefully referenced binding energy measurements of the C(ls), O(ls), and Al(2p) core electron levels. Near surface XPS studies involving solvent cast PAA films indicate that they are CH 2 rich, suggesting that a larger than statistical number of carboxylic acid groups are pointing away from the surface towards the bulk. PAA powder, however, is found to have near stoichiometric composition at the surface. The conclusions regarding Al/polymer reactivity agree well with recent literature for other metal/polymer interfaces.

1. Introduction

W e have recently started a research p r o g r a m a i m e d at u n d e r s t a n d i n g the chemistry occurring at the m e t a l / p o l y m e r interface. The results from these studies will have an i m p a c t on the u n d e r s t a n d i n g of a d h e s i o n [1], metal o x i d e / p o l y m e r interfaces, and the chemical b o n d i n g in p o l y m e r - m e t a l composites. In o r d e r to effectively study these types of interfaces using electron spectroscopies two key requirements must be met: (1) thin layered (less than fifty hngstrSms) samples of metal on p o l y m e r (or vice versa) must be p r e p a r e d and, (2) a suitable e x p e r i m e n t a l technique to " s e e " the synthesized interface m u s t be chosen. XPS (X-ray p h o t o e l e c t r o n spectroscopy) has been d e m o n strated to be an ideal technique for studying these types of interfaces through the e x a m i n a t i o n of core level lineshapes [2]. XPS has been a key tool in the e l u c i d a t i o n of the physics a n d chemistry of p o l y m e r surfaces [3]. W i t h XPS, the " v i e w i n g " d e p t h is typically 50 ,~ [4] which is roughly three times the m e a n free p a t h of a C ( l s ) p h o t o e l e c t r o n in a polymer. O n l y several reports involving m e t a l / p o l y m e r interfaces have recently a p p e a r e d in the literature. B u r k s t r a n d has investigated m e t a l / p o l y m e r interfaces of Cu [5,6], N i [2,7], 0 1 6 9 - 4 3 3 2 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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and Cr [2,7] on polystyrene [8,9] and a series of oxygen containing polymers including oxygen plasma treated polymer surfaces [2,7,8]. More recent work has focused on XPS core level studies of Pd sputter [10] deposited onto polyester film and core and valence band studies of A1 evaporated onto polyimide [11]. To the best of our knowledge the reactivity of A1 towards the carboxylic acid functionality on a polymer surface has not been examined. This paper presents the results of preliminary studies aimed at understanding chemical aspects of the m e t a l / p o l y m e r interface. Details of the preparation of the m e t a l / p o l y m e r interface are also given. Initial experiments have focused on the interaction of A1 metal with polyacrylic acid (PAA) and a low density polyethylene (LDPE). PAA has the m o n o m e r unit ( C H 2 - C H C O O H )n which is the simplest polymer having a carboxylic acid group in the polymer backbone. The PAA polymer surface has been previously studied by XPS [12]. LDPE is perhaps the simplest carbon containing polymer since the repeat unit is ( - C H z C H 2 ),,. A comparison of both polymers with respect to their reaction toward A1 should allow the reactivity of C H 2 groups and C - C linkages to be differentiated from that of C O O H functional groups in polymers. In this work an in-situ sputter deposition source is used to deposit controlled amounts of A1 metal onto the polymer surface. In-situ sputter deposition offers a unique way of preparing the interfaces by allowing for the possibility of deposition of metal into the near surface region without exposure of the metal to air. Below is presented results of preliminary studies involving thin A1 layers on PAA and LDPE.

2. Experimental All polymer surfaces and p o l y m e r / m e t a l interfaces reported here are investigated using X-ray photoelectron spectroscopy (XPS). The instrument used in these studies is commercially available (Vacuum Generators, ESCALAB II) and uses a hemispherical analyzer in an ultra-high vacuum (UHV) chamber for photoelectron kinetic energy measurements. For these studies an unmonochromatized A1 K a X-ray source is used and the analyzer is operated at 50 eV pass energy (1.40 eV resolution) unless otherwise stated. Digital XPS spectra are obtained using the VG 5000 data system. The instrument also is equippped with a custom UHV material/thin film preparation chamber and a rapid sample entry lock. The latter allows for sample transfer from atmosphere into UHV in minutes. Both U H V chambers have a base pressure of 2 × 10 1o Torr. PAA (Aldrich Chemical Co., powder) polymer films are solvent cast onto a metal sample stub (VG Instruments standard nickel stub) using methanol (5 g PAA in 100 ml methanol) having a liquid thickness of several millimeters. The "liquid" is then air oven dried at 60°C for 1 h to remove the solvent. LDPE

B.M. DeKot,en, P.L. Hagans / XPS studies of metal/polymer interfaces

201

films (Dow Chemical Co., No. 681) having a thickness of 1 mm are mounted directly onto a sample stub with copper wire wrapped around a groove on the side of the stub. The polymer films are turbomolecular pumped in the rapid sample entry lock for several hours before insertion into the UHV system. The m e t a l / p o l y m e r interfaces are made in the preparation chamber using in-situ metal sputter deposition. An Ar + ion beam (5 eV, 200 /~A, VG Instruments model AG21) is directed at a circular target disk which is divided into three sectors and capable of holding three different metal foils. The polymer is situated at an angle of 80 ° with respect to the target surface normal allowing for interception of the ejected metal particles. There is no direct line of sight between the Ar + ion beam and the polymer films. AI, Cu, and Au foils (Alpha, 99.999% purity) are spot-welded to the sputter target disk and sputter cleaned for 30 min prior to the onset of deposition on the polymer surface. Unless otherwise mentioned all depositions were done using identical Ar + beam conditions for 15 min.

3. Results

3.1. Polyacryfic acid (PAA) In fig. l a is shown the XPS survey spectrum for a PAA film between 0-600 eV binding energy recorded using 0.5 eV/step. The dominant spectral features indicated in fig. la are the C(ls) and O(ls) lines. Even on this low resolution scan two features in the C(ls) region corresponding to saturated hydrocarbon (lower binding energy) and carboxylic acid functionalities (COOH) are observed. The O(ls) region, however, does not show features corresponding to C=O and O H bonds presumably because of hydrogen bonding effects [12]. In fig. l b the resulting spectrum following the deposition of a thin A1 layer on PAA is shown. Both spectra in fig. 1 are shown normalized to the O(ls) intensity. The presence of A1 is evident by the appearance of the Al(2s) and Al(2p) lines (see fig. lb). Note also the dramatic change in the carbon to oxygen signal intensity compared to fig. la. A close inspection of fig. l b also indicates a dramatic reduction of the signal intensity originating from the carboxylic acid carbon functionalities of PAA polymer seen in fig. la. There is a possibility that some fraction of this signal decrease is due to polymer degradation during the metal sputter deposition. At present the magnitude of this effect is not known and is currently being investigated by comparison with evaporated metal [13]. Figs. 2a and 2b are the results of scans of the O(ls) region before and after deposition of A1 on the PAA surface recorded at 0.05 eV/step. The low intensity features indicated in fig. 2 by ' x l ' and 'x2' are the AI X-ray K a satellites. In comparing figs. 2a and 2b there is a dramatic change in the

B.M. DeKot~en, P.L. ftagans / XPS studies of metal/polymer interfaces

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binding energy of the O(ls) peak position before and after deposition of AI on the PAA surface. The dashed line in fig. 2a aids in illustrating this. For the PAA polymer before deposition of AI, the O(ls) core level is at 533.5 eV. This is achieved by referencing the CH 2 carbon in PAA to 285.0 eV as done by Clark [14]. Following deposition of AI and a small amount of Au to allow for core level referencing (5% compared to AI), there is a noticeable decrease in

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the peak position of the O(ls) line to 530.9 eV which is significantly less than the O(ls) line for A1203 (see table 1). The Au(4f) bands measured at 84.1 eV binding energy can easily be seen in fig. 4. This shift in O(ls) binding energy indicates that a reaction between A1 and PAA has occurred and that the reaction product is not simply A1203.

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Table 1 Measured core level peak binding energies for polymer surfaces and species formed at thc metal/polymer interfaces ~,1 Material

C( 1s)

O( 1s)

AI(2p)

Au(4f:,, )

PAA PAA/A1 LDPE LDPE/A1 A1 ~1 A120~ ,-I

285.0 b) 289.0 283.4 285.0 283.5

532.8 530.9

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84.1 ')

73.3 72.9 75.4

84 1

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531.1 d) 532.0

") Measured binding energies reported in cV, + (1.3 cV error. bl Determined by Clark and Thomas [121 for PAA only. ~1 Agrees with Anthony and Seah [17] and ref. [19] on absolute binding energies. dl Impurity from ambient or A1 sputter target as discussed in text. ") Measured by J.J. Kester in our laboratory using the same VG instrument; AI metal has 10()' oxygen contamination.

More insight can be gained regarding the species formed during reaction of A1 with PAA by e x a m i n i n g the C(ls) and Al(2p) core level regions. Figs. 3a and 3b show the C(ls) before a n d after deposition of A1, respectively. Note the d r a m a t i c change in the C(ls) region before and after reaction. The features in fig. 3a due to C H 2 a n d C O O H carbon have completely changed following deposition of A1 as seen in fig. 3b. The data in fig. 3b suggest the formation of an A1 carbide-like species based on the shift of the C ( l s ) peak position b i n d i n g energy to a value less than that for aliphatic carbon (see table 1). This shift in the C(ls) b i n d i n g is consistent with other XPS studies of metals and their carbides [15,16]. The XPS regional scan for the Al(2p) level for PAA is presented in fig. 4a. The Al(2p) peak position b i n d i n g energy is observed to be greater than that for clear A1 metal, but less than that for AlzO~. Table 1 summarizes the b i n d i n g energy changes before and after deposition of A1 onto PAA. The measured b i n d i n g energies are corrected for charging and this is considered more thoroughly in the discussion (section 4). There is strong evidence suggesting that an A1 o x i d e - c a r b i d e species is formed on the surface of the PAA polymer. We have also conducted preliminary studies involving Cu and Au on the PAA surface. The details will be published elsewhere [13], but a brief m e n t i o n is appropriate here to support the findings with AI. Although no XPS spectra are shown the relevant m e a s u r e m e n t s are summarized briefly below. For both C u and A u a C H z (aliphatic) c a r b o n at 285.0 eV is present following AI metal deposition in contrast to the carbidic carbon formed following A1 depositon. As in the case of AI, the C O O H functionality is dramatically reduced for both A u and Cu. Surprisingly, no evidence for the formation of Cu oxide or carbide

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species is found based on the metallic appearing lineshape of the Cu(2p) region. This supports the extreme reactivity of A1 compared to Cu and Au on PAA surfaces.

B.M. DeKot,en, P.L. ttagans / XPS studies of metal/polymer interface.~

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3.2. Low density polyethylene (LDPE) An identical series of deposition experiments was carried out using LDPE for several reasons. L D P E was selected due to the lack of oxygen in the monomer repeat unit, thus allowing a direct comparison of reactivity between

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CH 2 and C O O H groups towards A1 to be made. LDPE is also chosen in order to check whether the deposited A1 layer reacts with the vacuum chamber ambient gases and to be certain that A1 metal is deposited instead of A1203. The latter was carried out since the surface of the metal sputter target could not be analyzed by XPS. Fig. 5a shows an intensity versus binding energy spectrum for a L D P E film. The C(ls) to O(ls) ratios in fig. 5a indicate only a

208

B.M. DeKot~en, P.L. Hagans / XPS studies of rnetal/polvmer interfaces

few atomic percent oxygen species c o n t a m i n a t i o n is present at the L D P E surface. In fig. 5b are the results following A1 sputter deposition carried out in a n identical m a n n e r as the AI deposition onto P A A (compare with fig. lb). Both spectra in fig. 5 are shown normalized to the C ( l s ) intensity. C o m p a r i s o n of fig. l b with fig. 5b indicates that for A1 on PAA no less than 80% of the

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B.M. DeKoven, P. L ttagans / XPS studies of metal/polymer interfaces

209

oxygen present on the surface is obtained from PAA and not from oxygen on the AI foil sputter target or from the vacuum ambient. High resolution scans indicate that the AI also reacts with the L D P E surface and that a carbidic-like carbon species is formed. Results for the C(ls) region for L D P E surface are shown in fig. 6. There is clear evidence for carbide formation as evidenced by the C(ls) peak position change from 285.0 to 283.5 eV. Comparison of the Al(2p) region of LDPE to that for PAA in fig. 4b indicates that the peak position for Al(2p) is significantly lower than that measured for AI reaction with PAA (see table 1).

4. Discussion

Charging is observed to occur during the course of XPS studies of the polymer films and m e t a l / p o l y m e r interfaces. The magnitude of this effect is between 0.5 and 3.5 eV. The carbon in CH 2 bonds is set to 285.0 eV based on several studies [2,14]. This allows referencing of spectra for the pure PAA and L D P E films. To reference the atomic core levels following primary metal deposition a small amount of Au is co-deposited (less than 10% atomic concentration compared to A1) and referenced to 84.1 eV based on previous studies [17,18] and an unpublished ASTM-E42 committee report [19]. The hemispherical analyzer has been calibrated for energy measurements using both Ag and Cu [17]. There is also the possibility that extra atomic relaxation effects could influence the above referencing procedure in that the Au(4fT/2) level could be about 0.5 eV greater since the referencing is based on a fairly thin layer [20]. Based on these studies, however, this effect would only enhance our conclusions regarding the species formed during A1 metal reaction. No attempt is made to either correct for or measure possible relaxation effects at the present time. The values in table 1 listed for A1 and A1203 are from measurements on this instrument. These values are within 0.3 eV of previously published values for these materials [21]. The binding energies listed in table 1 all have errors +0.3 eV. Integration of the spectra shown in figs. 2a and 3a was carried out to obtain information regarding the relative amounts of carbon and oxygen at the surface of PAA films before A1 deposition. These results are presented in table 2 for total carbon, C H 2 carbon, C O O H carbon and total oxygen in the near surface region. Using an escape depth of 20 .A for the photoelectrons and an experimental exit angle near 0 °, the data are probably representative of the topmost 50 A of the polymer surfaces. Data is shown for P A A / H e O , P A A / M e O H , and PAA powder compared with expected ratios based on the monomer unit and data from Thomas and Clark [12]. The relative sensitivity of O to C was measured for our instrument using B, C, and O intensity ratios from BaC and B203 and determined to be 3.0 (+0.3) [13]. Within our

210

B.M. DeKoven, P.L. llagans / XPS studies of metal/po6~mer interfaces'

Table 2 Measured integrated intensity ratios for the various C and O species in the PAA polymer for both solvent cast films and the powder '~} PAA/H20 (60°C)

P A A / M e O H P A A / M e O H PAA powder PAA h} Expected ,,/ (60°C) (25°C5

C ( C O O H ) / O total 0.62 (0.10) 0.62 (0.10} C t o t a l / O total 2,40 (0.305 2.50 (0.405 C ( C O O H ) / C (CH2) 0,35 (0.05) 0.32 (0.05)

0.69 (0.10) 2.90 (0.40) 0.32 (0.055

0.52 (0.08) 1.69 (0.25) 0.45 (0.07)

0.49 1.58 {).46

0.50 1.50 0.5(}

~ + 15% relative error based on ability to determine the instrumental C / O sensitivity factor using B4C and B203 as discussed in text: errors in parentheses. b} Measurements from ref. [12]. ,9 Based on the PAA monomer unit.

experimental error the C ( C O O H ) / O ratio agrees well with published data on the PAA polymer [12] and expectations based on the stoichiometry of the monomer unit. The data shown for solvent cast PAA films, however, does not agree with the PAA powder, previously published PASA work, and the expected values. There is evidence for an enhanced CH 2 carbon species at the surface of all the prepared PAA films. For the P A A / M e O H films this cannot be due to solvent trapped in the near surface region since this would actually make the C / O ratio be less than the expected value of 1.5. The similarity of the ratios for P A A / H 2 0 films also supports this observation. If H 2 0 w e r e trapped, then the C ( C O O H ) / O and C / O ratios would also be lower than the expected values. The same argument applies to adsorbed CO of CO 2 if either were present, Also, X-ray beam degradation of the polymer during the spectral acquisition time is unlikely since the measured values for the PAA powder are very near the expected values. We believe that the film production process leaves the surface CH 2 rich, thereby implying that a larger than average number of C O O H groups are pointing away from the surface. Similar results were obtained for PAA films cast from M e O H and air dried at 25°C resulting in nearly identical observations reported above for P A A / M e O H air oven dried at 60°C. The results of these studies of AI deposition onto both PAA and LDPE clearly show evidence for reactivity. Comparison of the binding energy for the C(ls), O(ls), and Al(2p) peaks for PAA/A1 with known literature and measured values for A1 metal and AI203 indicate that AI is in an oxidized state and that both oxygen and carbon have reacted with the deposited AI. An estimate of the A i / O ratio for A1 reactions with PAA is very near 2/3. For PAA reacting with AI, however, O(ls) and Al(2p) binding energies are significantly lower than those expected for pure A I 2 0 3 . Combined with the observed carbidic-like carbon, this suggests that formation of an A1 oxide-carbide species on the surface of the PAA polymer has occurred. Both the C H 2 and C O O H groups appear to be very reactive toward A1 metal. The

B.M. DeKoven, P.L. Hagans / XPS studies of metal~polymer interfaces

2l 1

results for the LDPE/A1 interface also show a surprisingly high reactivity of the CH 2 functionality in a polymer. The measured peak position of Al(2p) is nearly equivalent to that for the clean A1 metal. This is consistent with measurements involving other metals and their carbides. The oxygen impurity on the LDPE surface after sputter deposition appears to be in the same chemical state as that found in the PAA/A1 case except that the origin of the oxygen is not known. For the L D P E / A I interface, the O(ls) peak position is less than that for A1203, but very similar to the O(ls) binding energy for AI on PAA. Qualitatively, our observations regarding the nature of the metal polymer interfaces studied here agree with the recent conclusions in the literature regarding species formed at metal/polymer interfaces [2,11]. Even though the actual stoichiometry of the species formed following A1 deposition is unknown, the results are consistent with formation of a metal oxide-carbide-like complexes on PAA and a metal carbide-like complex on LDPE. For PAA/A1 interfaces partial justification of the observation of these species comes from studies of Crowell et al. [22] showing A1203 and carbon formation in the decomposition of simple organic acids on A1(111). Studies involving acetic acid adsorption and decomposition on polycrystalline Fe and Ni shows evidence for metal carbide formation [23]. The investigation of such complexes could be extremely significant in developing models aimed at understanding the important chemical factors involved in adhesion [1]. The carbon lineshape and peak positions measured following A1 reaction qualitatively agrees well with recent high energy metal ion implantation studies of polymers [24]. At the present time the details of these profiles are not totally understood. There is the likely possibility of some polymer damage occurring during the metal sputter deposition process. The distribution of kinetic energies of all secondary particles, n (E), should follow the theoretical relation [ E/(E + 2 E b)3 ], where E b is the binding energy of the atom to the solid [25]. This relation has been verified for 10 keV Ar + ions interacting with an AI target [26]. Based on these studies 75% of all secondary particles, both ions and neutrals, should have kinetic energy less than 25 eV, significantly less than the 5 keV primary Ar + ion beam kinetic energy. Therefore, the A1 metal particles interacting with the polymer in these experiments should have a much lower kinetic energy than the high energy case. These results definitely provide important insight into selecting methods of chemically modifying the outermost layer of a polymer surface.

5. Conclusions and future studies

In conclusion our initial studies show that AI metal is extremely reactive when brought into contact with a polymer surface such as PAA or LDPE.

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B.M. DeKo~,en, P.L. ltagans / XPS studie~ oj metal/po(vmer interfaces

Reaction is observed to occur at both C O and C - C linkages with A1 oxide and carbide-like species being the final product. This is clearly evident from the tabulated binding energies. Our results suggest the importance of investigating other A1 oxide-carbide polymer complexes. Future research will focus on studies of thin films of other metals (e.g. Au, Cu, Fe, Ni, and Cr) on these and other polymers including those with amine and amide type linkages. The effect of having oxygen bound to the metal before deposition will also be examined by vaporizing metal oxides onto the same polymer surfaces. Direct evaporation versus the sputter deposition now used will be compared in order to assess whether beam damage is occurring. The application of other surface spectroscopic probes, e.g., HREELS and UPS, should also be very revealing. We believe that studies of this type can be beneficial in obtaining insight into the nature of the polymer/metal interaction which up to now is so poorly understood.

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[24] P.E. Pehrsson, D.C. Weber, N. Koons, J.E. Compana and S.L. Rose, Mater. Res. Soc. Symp. Proc. 27 (1984) 429. [25] A. Benninghoven, J. Okano, R. Shimizu and H.W. Werner, Eds., Proc. 4th Intern. Conf. on Secondary Ion Mass Spectrometry (SIMS IV) (Springer, Berlin, 1984). [26] R.L. Ingelbert and J.F. Hennequin, in ref. [25], p. 49.