Bonding Mechanisms at Buried Interfaces between Carboxylic

Article pubs.acs.org/JPCC

Bonding Mechanisms at Buried Interfaces between Carboxylic Polymers and Treated Zinc Surfaces P. Taheri,†,‡ M. Ghaffari,§ J. R. Flores,⊥ F. Hannour,⊥ J. H. W. de Wit,†,‡ J. M. C. Mol,‡ and H. Terryn*,†,§ †

Materials Innovation Institute (M2i), Mekelweg 2, 2628 Cd Delft, The Netherlands Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands § Department of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ⊥ Development and Technology, Tata Steel Research, PO Box 10.000, 1970 CA IJmuiden, The Netherlands ‡

ABSTRACT: The interfacial bonding properties of carboxylic polymers on a Zn substrate have been investigated. Poly(methyl vinyl ether-alt-maleic acid monobutyl ester) and cured propoxylated bisphenol A fumarate unsaturated polyester were applied on a set of differently treated Zn samples. The buried metal−polymer interface was studied by polymer removal and evaluation of the residue layers on Zn surfaces representing the metal−polymer interface region. Additionally, the interfacial bondings were mimicked by adsorption of the representative carboxylic monomers, i.e., succinic and myristic acids. The differently treated Zn surfaces were found to be capable of adsorption of the carboxyl functionality of the polymers, resulting in formation of carboxylates. A comparison of the interfacial bondings by the residue layers of the polymers with those formed due to the molecular adsorption showed comparable adsorption mechanisms. Additionally, it was found that the polymer−metal interfacial bonding density mainly depends on the Zn surface hydroxyl fraction, while Zn oxide semiconductor properties play an important role when a curing process occurs during the polymer interaction with Zn surfaces.

1. INTRODUCTION Galvanizing has been used to protect steel parts from corrosion for many years.1 Polymer coatings are often applied on galvanized surfaces to increase the corrosion resistance of the system.2 However in practice, due to the presence of defects in the polymer coatings, an imperfect barrier to water, oxygen, and/or ion penetration is obtained, possibly leading to a cathodic delamination process originating from these defects.3−5 The cathodic delamination propagation at the polymer−metal interface is closely related to the interfacial bonding properties between the polymer coating and metallic substrate.6−8 The metal surface composition of the interfacial metal oxide is a pivotal factor affecting the polymer−metal interfacial bonding properties. Various surface compositions can be obtained through different surface treatments. However, some studies showed that also parameters other than the surface composition change due to the treatments.9,10 It was shown that the Zn oxide resistance (Rox) decreases dramatically due to the chemical surface treatments. Additionally, application of an external potential during the treatments decreases the surface roughness due to the oxide removal. Moreover, treatments under different conditions may lead to formation of various dopants, e.g., Zn+ metal ions, oxygen vacancies, substitutional defects, and surface states, in the oxide structures. These variations may affect the polymer−metal interfacial bonding properties. © 2013 American Chemical Society

A number of recent studies have reported a charge transferred to polymers by successive contacts to different metals.11−13 Some studies showed that initiation of curing is proposed to occur via metal−monomer charge transfer to maintain the electroneutrality within the polymer phase.14,15 Moreover, an external potential during the application of polymer coatings on metal surfaces has recently attracted much attention due to its beneficial adhesion effects. Our previous study showed a substantial effect of application of an external potential in the adsorption configuration of carboxylic compounds on Zn surfaces.16 These results imply the impacts of metal oxide electronic and semiconductor behavior on the interfacial bonding properties. In addition to metal surface beneficial treatments, polymer coatings can be functionalized to enhance the adhesion of the polymer to the metal surface.17,18 Adhesives are functional polymers, which are often used in automotive and construction industries for many applications, ranging from the bonding of metals or composites to materials for aerodynamic surfaces.19 As adhesives, carboxylic resins such as polyesters are often used in industrial polymer coatings because of their desirable structural and interfacial performances.20,21 In addition to the industrial applications, carboxylic groups have attracted much attention because of their importance to understand the Received: September 20, 2012 Revised: January 14, 2013 Published: January 17, 2013 2780

dx.doi.org/10.1021/jp3093572 | J. Phys. Chem. C 2013, 117, 2780−2792

The Journal of Physical Chemistry C

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fundamental adhesion mechanisms between metal surfaces and polymers.22−24 Therefore, examination of their interfacial functional properties assists in developing beneficial polymer and metal surface modification procedures for several applications. Although the characterization of the interaction between metals and polymer coatings is an important goal of surface science, the study of the exact interfacial region is difficult due to the relatively high thickness of the polymer coatings. Early efforts to understand the metal−polymer interface resulted in defining the model compound systems by adsorption of the representative functional molecules to mimic the interface region. The molecular-sized polymer compounds on metal surfaces have been extensively investigated using an array of surface analysis techniques, such as Fourier transform infrared spectroscopy (FTIR), surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopy (XPS). Li et al.25 utilized SERS to compare the interface between a phenylene and model gold and silver electrodes. Using XPS, the ability of poly(acrylic acid) as an adhesive on oxidized metal surfaces was investigated and a salt-bridge intermolecular reaction mechanism was suggested.26 Moreover, Alexander et al.27 investigated the interfacial interactions between plasmapolymerized acrylic acid (PPAA) and an oxidized aluminum surface by means of XPS and FTIR. They correlated the interfacial bonding properties to the acid−base interactions between PPAA and aluminum substrate as well. Yamabe28 also verified a strong ionic interaction between various metal surfaces and the polymer carboxylic functional group. Our previous work modeled the interfacial bondings between copolymerized polyolefin polymers and Zn substrate by adsorption of succinic acid, succinic anhydride, and myristic acid molecules.16 To verify the information derived from the models, the experimental emphasis has developed toward realistic scenarios of polymer−metal interfaces. One approach is a destructive method based on analysis of the interfacial region after the polymer removal from the metal surface.29 The interfacial bonding characterization between the Zn substrate as the major compound formed on galvanized steel and carboxylic polymers, i.e., polyesters, as a widely used resin in industrial coatings, is the objective of the present work. Two types of polyesters are selected to evaluate the effects of functional groups and curing procedures in the interfacial bonding mechanism and density. In this case, the interface region is reached by pulling off the polymer coatings from Zn surfaces. On the other hand, the interface region is modeled by adsorption of the representative carboxylic monomers, and the effects of Zn surface treatment on the interfacial properties are evaluated.

Table 1. Experimental Parameters Used for the Pretreatments of the Zn Substrate sample code sample sample sample sample sample

1 2 3 4 5

solution

pH

temp (°C)

potential (V)

duration (min)

0.05 M HCl deionized water 1 M Na2CO3 deionized water 0.2 M H3BO3 + 0.1 M NaOH

1.9 6.6 11.5 6.6 12.3

25 65 25 25 25

− − − − 0.8

30 30 30 30 30

Figure 1. Structure of oxide/hydroxyl on Zn surfaces. Blue, zinc; red, oxygen; white, hydrogen.

oxide/hydroxyl on Zn surfaces. Hydroxyls partially cover the surface. All of the measurements discussed in this work were repeated three to five times to check the reproducibility of the results and to obtain mean values and standard deviations. To mimic the interfacial bonding properties of the carboxylic polymers, succinic and myristic acid molecules were adsorbed on the untreated and differently treated Zn samples. The adsorption of the model compound molecules was conducted in tetrahydrofuran (THF) as the supportive solution including 0.1 wt % of the molecules for 30 min. Afterward, the samples were rinsed by THF for 5 s to remove the nonadsorbed molecules. Figure 2 schematically shows the structures of succinic acid and myristic acid molecules used to model the interfacial bonding structure.

Figure 2. Molecular structures of (a) succinic acid and (b) myristic acid as the studied model compounds. Grey, carbon; red, oxygen; white, hydrogen.

2. EXPERIMENTAL SECTION The substrate used in this work was commercially pure Zn sheets (99.95%) supplied by Goodfellow. The samples were mechanically grinded with SiC paper and then polished with different grades of diamond paste (9, 6, 3, 1, and 0.025 μm) in subsequent steps. Then the samples were cleaned ultrasonically in ethanol and water for 2 min and dried under a stream of compressed nitrogen. Subsequently, various sets of differently treated Zn substrates were prepared to obtain different surface oxide/hydroxyl fractions according to the experimental parameters summarized in Table 1. Then they were rinsed once with deionized water for 5 s and dried with compressed nitrogen gas. Figure 1 schematically shows the presence of

Two types of carboxylic polymers were applied on the differently treated Zn samples. Polymer A was a 50 wt % poly(methyl vinyl ether-alt-maleic acid monobutyl ester) dissolved in ethanol. Polymer B propoxylated bisphenol A fumarated unsaturated polyester dissolved in styrene was generated by curing with a liquid methyl ethyl ketone peroxide (MEKP). Figure 3 schematically shows the structures of the polymers and curing agents. Formation of polymer layer A was conducted through solvent evaporation. Free films were obtained by casting the polymers on a Teflon sheet. Thin films of polymers A and B (around 100 μm) were deposited onto the differently treated Zn samples by spin-coating. 2781



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polymer application, infrared backgrounds were obtained from freshly treated Zn samples, and the final measurements were recorded versus these backgrounds. The FTIR spectrum of the bare substrate was conducted versus a clean gold background. To quantitatively analyze the surface compositions with and without the residue layers, XPS analysis was conducted with a PHI 1600/3057 instrument using an incident X-ray radiation (Mg K α1,2 = 1253.6 eV). The vacuum pressure was approximately 5 × 10−9 Torr. The spectra were shifted to set the C−C/C−H components of the C1s peak at a binding energy of 284.8 eV to correct the sample charging.30 The curve fitting and decomposition were performed by PHI Multipak V8.0 software. Figure 4a shows the O1s peak of the Zn samples resolved into three subpeaks located around 530, 531.5, and 533 eV. The subpeaks can be due to the contribution of O2−, OH−, and COx components, respectively.30,31 Figure 4b shows the hydroxyl fractions on the untreated and differently treated Zn samples obtained from the O1s peak. The hydroxyl fraction increases gradually from sample 1 to 5. To calculate the dopant concentration (ND), Mott−Schottky analysis was performed.32,33 The electrochemical impedance spectroscopy (EIS) measurements were conducted in a conventional three-electrode cell using an EG&G 273 potentiostat. The reference was a saturated calomel electrode (SCE), connected to the main compartment through a salt bridge. A Pt wire coupled with a capacitance of 10 nF was employed parallel with the reference electrode to reduce the phase shift induced by the reference electrode at high frequencies.34 A flat platinum plate was utilized as counter electrode (CE), and the Zn sample was used as the working electrode. The measurements were performed in a borate buffer solution including 0.075 M Na2B4O7·10H2O + 0.3 M H3BO3, pH = 8.4, and the exposed area was approximately 0.78 cm2. The measurements were performed in the 10 kHz to 10 mHz frequency range with 10 frequency points per logarithmic decade and a potential step size of 100 mV. The experimental impedance data were fitted to an appropriate equivalent circuit using Z-view software.

Figure 3. Schematic structures of (a) poly(methyl vinyl ether-altmaleic acid monobutyl ester) (polymer A), (b) methyl ethyl ketone peroxide (MEKP), (c) styrene, and (d) propoxylated bisphenol A fumarate unsaturated polyester (polymer B). Grey, carbon; red, oxygen; white, hydrogen.

Polymers A and B, respectively, were dried and cured at 40 °C for 18 h. The formed polymer layers A and B were pulled off from Zn surfaces by means of an Instron 1122 testing machine operating at a crosshead speed of 0.05 mm/min. To evaluate the metal surface after the polymer removal, field emission scanning electron microscopy (FE−SEM) observations were performed using a Jeol JSM-7000F FE-SEM. The acceleration voltage was 5 kV, and the working distance was 10 mm. To characterize the residue interfacial films, a ThermoNicolet Nexus FTIR apparatus equipped with a mercury cadmium telluride liquid-nitrogen cooled detector and a nitrogen-purged measurement chamber was used. Prior to the

Figure 4. (a) Typical XPS, O1s peak fittings of the untreated sample and (b) hydroxyl (OH−) fraction of the untreated and differently pretreated zinc samples. 2782



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Figure 5. (a) Typical Bode plot of the Zn oxide obtained at OCP. (b) Representative equivalent circuit of the Zn oxide/electrolyte system used for the EIS data fitting.

Table 2. EIS Fitting Data of the Untreated Pure Zn Sample at Different Potentials E (VSCE) −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 relative error (%)

Re (KΩ·cm2)

Rct (KΩ·cm2)

QH (10−6·sn.Ω−1·cm−2)

nH

CH (μF·cm−2)

Rox (MΩ·cm2)

Qsc (10−6·sn.Ω−1·cm−2)

nsc

Csc (μF·cm−2)

1.43 1.43 1.49 1.49 1.49 1.49 1.49 1.48 1.48 1.48 0.2−0.8

24.0 9.2 17.3 10.7 12.8 14.0 14.2 15.5 7.9 9.6 8−13

78.5 14.6 10.4 12.1 10.3 90.0 72.9 60.4 3.6 1.8 0.6−4

0.89 0.91 0.92 0.92 0.92 0.99 0.93 0.94 1.00 1.00 0.02−0.1

8.24 14.94 10.75 12.13 10.37 9.00 7.17 5.95 36.01 18.63 −

146.300 0.016 0.007 0.011 0.009 0.010 0.008 0.007 0.026 0.203 0.001−4

203.4 25.9 64.0 30.5 32.5 27.8 32.1 32.4 4.8 4.5 0.2−3

1.00 0.93 1.00 1.00 1.00 1.00 1.00 1.00 0.94 0.94 0.02−0.09

22.91 9.61 26.74 8.13 8.38 6.80 6.06 18.27 5.04 4.78 −

Figure 5a shows the typical Bode plot of the pure Zn sample performed after 10 min of immersion in the borate buffer solution. Figure 5b shows the equivalent circuit used to fit the experimental data consisting of two parallel capacitance and resistance elements in series with the electrolyte resistance (Re) presenting the charge transfer resistance (Rct), Helmholtz double layer capacitance (CH), space charge capacitance (Csc), and oxide electrical resistance (Rox).35,36 Considering the nonideal behavior of the capacitive elements, they were assumed as constant phase elements (CPE) in the models. The resistive elements and the CPE parameters, Q and n, of the untreated pure Zn sample determined through a fitting procedure are presented in Table 2. These data are used to calculate space charge pseudocapacitance (Csc) proposed by Mansfeld37 and Helmholtz double layer pseudocapacitance (CH) according to the procedure explained elsewhere.38,39 CH and Csc values obtained from EIS data were used to build up the Mott−Schottky curves.40,41 Figure 6 shows a typical Mott− Schottky curve of the Zn oxide in which (1/Ctot)2 is plotted against the applied potential. The dopant concentration (ND) was estimated from the slope of the linear part of the (1/Ctot)2 versus voltage curve.36

Figure 6. Typical Mott−Schottky plot of the Zn oxide.

image at low and high magnifications obtained for the substrates after the polymer coating removal. Figure 7a and 7b shows the interface between the coated area and the part from which the polymer coating is removed. The absence of remaining bulk coatings on the Zn surfaces after the polymer removal verifies the fact that any probable residue layer may represent the metal−polymer interface. FTIR measurements also are conducted on the bare regions to verify the absence of bulk polymer on metal surfaces. These regions were carefully chosen to conduct the following qualitative and quantitative measurements.

3. RESULTS AND DISCUSSION 3.1. SEM Analysis of the Coated and Bare Zn Substrate. To ensure that no bulk polymer remains on Zn surfaces, SEM pictures were taken from Zn surfaces after the polymer coating removal. Figure 7 presents a typical SEM 2783



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compounds on Zn surfaces and bare Zn sample, residue, and free polymer coatings. Figure 8a shows the infrared spectra of succinic acid and myristic acid model compounds adsorbed on the Zn substrate. The presence of the bands at 1443, 1459 cm−1 and 1592, 1541 cm−1, together with the absence of a carbonyl band around 1700 cm−1, indicates that the adsorption process resulted in the formation of carboxylate salts coordinatively bonded to the surfaces.42−44 The peaks at 1443, 1459 cm−1 and 1592, 1541 cm−1 can be assigned to symmetric νs(COO−) and asymmetric νas(COO−) carboxylate stretching vibration bands, respectively.10,45 The variation of peak position may originate from different interfacial structures, resulting in altered bonding strengths.46 It is known that a carboxylate ion can coordinate to metals in three basic ways: monodentate, chelating, and bridging bidentate structures.47 Because every coordination has a specific νs(COO−) and νas(COO−) peak position, the peak separation can be used to determine the coordination type of the formed carboxylates. In this case, the difference between the asymmetric νas(COO−) and the symmetric νs(COO−) stretch

Figure 7. SEM images of the coated and Zn surfaces after polymer coating (a) A and (b) B removal; the bottom regions of the central images are coated and the top regions are bare (scale = 10 μm for central images, scale = 1 μm for side images).

3.2. Metal−Polymer Interfacial Bonding Structure. To explore the polymer structures and interface bonding mechanisms, FTIR measurements are conducted on different samples. Figure 8 shows FTIR spectra of the adsorbed model

Figure 8. FTIR spectra of (a) succinic acid (1) and myristic acid (2) molecules adsorbed on the untreated Zn samples and (b) bare Zn sample (1), residue polymer film A (2), free polymer film A (3), residue polymer film B (4), and free polymer film B (5). 2784



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Table 3. FTIR Stretching Vibration Peak Positions (cm−1) of the Free Film Polymer A and the Untreated and Differently Treated Zn Samples after Contact with Polymer A peak assignment

polymer a

sample 1

sample 2

untreated sample

sample 3

sample 4

sample 5

ν(OH) [OH stretching] ν(CH) [CH stretching] ν(CO) [CO stretching] γ(CH3) [CH3 twisting] δ(OH···O) [OH bend, coupled] ω(CH3) [CH3 wagging] ν(C−O) [C−O stretch, coupled] ν(C−C) [C−C stretching] ν(C−O) [C−O stretching] ν(CH) [CH stretching] νas(COO−) [COO− asymmetric stretch] νs(COO−) [COO− symmetric stretch] Δν(COO−)

3570−3650 3250−3260 1753 1473 1405 1380 1312 1123 1037 884 −

3600−3650 3200−3300 1752 − − − 1313 1124 − 867 1630

3600−3650 3200−3300 1735 − − − 1315 1122 − 878 1617

3600−3650 3200−3300 1728 − − − 1333 1123 − 887 1627

3600−3650 3200−3300 − − − − 1313 1123 − 862 1654

3600−3650 3200−3300 − − − − 1322 1122 − 878 1620

3600−3650 3200−3300 − − − − 1330 1124 − 871 1654

−

1437

1437

1431

1447

1437

1437

−

193

180

196

207

183

217

be detected in the FTIR spectrum of the residue film A on the Zn substrate. On the other hand, two new bands around 1627 cm−1 and 1431 cm−1 appear in the spectrum of the residue polymer compared to that of the free film. These are assigned to the asymmetric νas(COO−) and symmetric νs(COO−) stretching vibration bands, respectively, indicating the formation of carboxylates in the interface region.42−44 The two characteristic adsorption bands are both associated with ion interactions between polymer A and the Zn surface, specifying that the most likely mode of interaction between the Zn and polymer A is through carboxyl groups. The probability of the metal polymer interaction can be evaluated by determination of the isoelectric point (IEP) of a hydrated metal oxide surface, which is considered as its acid− base character.57 IEP can be calculated as the pH of an aqueous solution in which the solid surface exists in a state of electrical neutrality. A low IEP value indicates an acidic surface while a high value suggests a basic one.58 It is known that the Zn surface is hydrated in ambient air and covered with ZnO and Zn(OH)2 components.59 As the mean IEP value of these compounds is 9.5, the Zn surface is considered as a basic substrate in view of the chemical reactivity during the interfacial interactions.60 Therefore, the Zn surface is expected to react with the organic acidic functionality of the polymer, i.e., COOH, and form the carboxylate. The adsorption of COOH may be attributed to interaction between R-COOH and OH on the Zn surface. The adsorption of polymer A through the carboxyl functional group takes place by an ion exchange mechanism as represented below:

bands can be used to identify the type of coordination bonds.27,47 The frequency shifts of the carboxylate stretchings, Δν(COO) = (υas(COO−) − υs(COO)), are 180 and 82, respectively, for the adsorbed succinic acid and myristic acid molecules, indicating a bridging bidentate coordination state for the model compounds studied. This assignment is in agreement with those of others, studying the chemisorption of carboxylic acids on metallic oxide layers.46,48,49 However, the lower peak separation value obtained for the adsorbed myristic acid compared with that of succinic acid molecules is due to a higher bonding strength obtained for myristic acid on Zn samples for which a higher energy level is required to stretch the group. This can be correlated to a higher van der Waals force between the longer chain of myristic acid molecules and the Zn surface, increasing the dipole force between oxygen and Zn atoms.50,51 The FTIR spectrum of the bare Zn sample was collected versus a clean gold background (Figure 8b). The presence of the bands at 1624 cm−1 can be ascribed to the carbonyl stretching vibrations of carbonates.48,49,52 It is known that upon exposure of the metal substrates to ambient air, several organic contaminants form on the hydroxylated oxide surface, giving rise to carbonaceous contamination.53 A comparison of the compound wavenumbers formed on Zn surfaces due to the residue polymer films A and B with those of the hydrocarbons presented on the bare Zn sample suggests the replacement of the carbonaceous contamination due to the metal−polymer interaction. Table 3 shows an overview of the stretching vibration peak positions, as obtained for the free film and residue polymer A on differently treated samples. Table 3 and Figure 8b demonstrate that most of the FTIR peaks of free film A disappear in the residue film spectra on the metal surface, showing that the polymer was subjected to a compositional change in the interface region due to the interaction with the Zn substrate. The spectrum of the free film polymer A displays a strong characteristic band at about 1753 cm−1, which is assigned to the CO stretching vibration of the dimerized carboxylic acid group.54−56 The polymeric structure (Figure 3) and the high FTIR intensity of CO stretching band show that the carbonyl group is the predominant structure of polymer A. However, a negligible carbonyl vibration band can

R‐COOH + ZnxOH → R‐COO‐Zn2 + (x − 2)Zn + H 2O

(1)

On the basis of reaction 1, the higher the surface hydroxyl fraction, the more carboxylate that forms on the surface. The extent of interaction between polymers and the metal surface depends on metal and polymer parameters. The FTIR results of differently treated Zn samples after the interaction with polymer A reveal the presence of an OH peak around 3600 cm−1. This peak may originate from OH functionality of the polymer or metal surface, showing that OH on the Zn surface or in the polymer has not reacted completely in the interfacial region. On the other hand, the appearance of carbonyl FTIR 2785



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Table 4. FTIR Stretching Vibration Peak Positions (cm−1) of the Free Film Polymer B and the Untreated and Differently Treated Zn Samples after Contact with Polymer B peak assignment

polymer B

sample 1

sample 2

untreated sample

sample 3

sample 4

sample 5

ν(CO) [CO stretching] ν(CC) [CC ring stretching] ν(CC) [CC ring stretching] ω(CH3) [CH3 wagging] ν(C−O) [C−O stretch, coupled] ω(CH2) [CH2 wagging] ν(C−C) [C−C stretching] ν(C−O) [C−O stretching] ν(CH) [CH stretching] νas(COO−) [COO− asymmetric stretch] νs(COO−) [COO− symmetric stretch] Δν(COO−)

1741 1517 1465 1365−1435 1315 1266 1191 1056 884 − − −

1734 1512 1452 − 1300 1288 1181 1090 832 1611 1419 192

1752 1512 1458 − 1301 1281 1185 1071 871 1617 1420 197

1745 1509 1450 − 1303 1275 1122 1081 885 1612 1421 191

1741 1508 1457 − 1306 1273 1124 1076 873 1617 1419 198

1752 1512 1451 − 1298 1275 1123 1082 867 1617 1420 197

1735 1508 1447 − 1299 1271 1122 1080 871 1617 1420 197

peaks around 1700 cm−1 of the samples with a low hydroxyl fraction (Table 3) proves that the CO functional group remains partially unreacted in the interface when there is not enough hydroxyl fraction available. The same result was observed for adsorption of succinic acid molecules on the Zn samples with a low hydroxyl fraction.16 Table 3 clearly shows that the peak positions are reasonably comparable for all samples and in agreement with the peak positions observed due to the adsorption of the succinic acid model compound. The frequency shifts, Δν(COO) = (νas(COO−) − νs(COO)), of different samples vary between 180 and 217 cm−1 depending on the sample type. The frequency shift variations may arise from differences in the relative concentration of functional groups, the bonding strength, and orientation effects. These values indicate a bridging bidentate coordination state in which two C−O bonds of the carboxylate COO− become equivalent and the carboxylate is coordinated to two cations on the metal surface.22,27,28 This is in good agreement with the configuration of model compounds adsorbed on Zn surfaces. However, the frequency shift values, Δν(COO), obtained for the interfacial bonds of polymer A are higher than those of model compounds, showing the presence of a weaker interaction between the polymer and metal surface. This might be correlated to the polymeric constraint weakening the interfacial bondings formed due to the residue polymer A compared to the model compounds. It is known that the COO− anion exhibits the resonance structure rather than the characteristic peak of dissociative carbonyl.58 Therefore, both carbon−oxygen bonds in COO− are assumed to be equivalent in the complete resonance structure. The symmetric stretch transition moment is along the bisector of each oxygen−carbon−oxygen group, and the asymmetric stretch moment is perpendicular to the symmetric stretch moment, i.e., the transition moments for the symmetric and asymmetric stretching modes are parallel to and perpendicular to the 2-fold axis, respectively.58 The adsorption of the model compounds and polymer B shows that the adsorption peak of symmetric carboxylate stretching vibration νs(COO−) is stronger than the asymmetric carboxylate stretching vibration νas(COO−). However, for the residue polymer A, the intensity of the symmetric carboxylate stretching mode is weaker than that of the asymmetric carboxylate vibration band. This can be correlated to the fact that the 2-fold axis of the carboxylic group inclines to a certain degree from the normal to the surface rather than completely

vertical to the surface. In this case, the FTIR results show that the COOH functionality present in the metal−polymer interfacial structure is nearly fully deprotonated. Consequently, the polymeric constraint is expected to lead to such inclination of the carboxylic acid group. On the other hand, polymer B and the Zn surface interaction was accompanied by the curing process. This provides the polymer more structural freedom in the interface area and helps the molecules at the interface to have a preferential conformation and reactivity with Zn adsorption sites. Li et al.58 showed that the interfacial orientation of the carboxylic functionalities is more perpendicular than those of small carboxylic acid molecules for which a configuration approximately vertical to the metal surface is proposed. Table 4 shows an overview of the stretching vibration peak positions, as obtained for the free and residue polymer films B on differently treated Zn samples. Dimer CC functionality in the spectra originated from the chain and phenyl structure of polymer B. The presence of CC in the residue films indicates that phenyl rings are stable during the curing and adsorption procedures. Our previous work16 conducted on the adsorption of the carboxylic model compounds showed that the ringopening of carboxylic rings is promoted through the catalytic activity of the H-terminated Zn surface and C−O−C functionality of the carboxylic ring. However, the absence of the O terminus stabilizes the phenyl structure and prevents the ring-opening procedure during the adsorption of polymer B. Like polymer A, the residue polymer B on metal surfaces shows the presence of asymmetric νas(COO−) and symmetric νs(COO−) stretching vibration bands around 1612 cm−1 and 1450 cm−1, respectively, indicating the formation of carboxylate structures in the interface region probably through a selfdissociation of carbonyl (CO). The broad symmetric carboxylate band is probably due to the coupling of this peak with CC ring and CH3 wagging stretching bands on the Zn surface. As shown in Table 4 the frequency shift values, Δν(COO), obtained for the interfacial bonds of polymer B are between 191 and 198 cm−1, indicating a bridging bidentate coordination state like those observed for the residue polymer A and the model compounds. For the curing of polymer B, a free-radical chain-growth cross-linking reaction is proposed.61 Figure 9 schematically shows the curing and adsorption procedures of polymer B applied on Zn surfaces. In this case, the alkenyl function (C C) present in the chains of polymer B and styrene dissociate and become activated by MEKP radicals. Subsequently, a cross2786



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C1s, O1s, and Zn3p peaks. The FTIR results indicate that the observed C and O peaks presumably originate from the residue polymers, while the native oxide under the residue coatings contributes to the Zn peaks. The presence of the Zn peak indicates that the thickness of residue films is less than the XPS sampling depth. Figure 11 shows C1s and O1s peak fittings of the bare Zn sample and residue, removed, and free polymer films A and B.

Figure 9. Typical curing and adsorption of polymer B on Zn surface. Blue, zinc; grey, carbon; red, oxygen; white, hydrogen.

linking between polymer B chains is formed by the activated styrene. On the other hand, R-COO− functions of polymer B interact with Zn ions (Znx+), resulting in the adsorption by an ion exchange mechanism: R‐COO− + 2Zn x + → R‐COO−Zn2 x +

(2)

Unlike polymer A, the presence of carbonyl functionality (CO) is obvious in FTIR spectra of the residue polymer films B. This may originate from either the nondeprotonated carboxyls (COOH) remaining unabsorbed after interaction with Zn surfaces due to the scarcity of the adsorption sites on Zn metal surfaces or carbonyls present in the main chain. 3.3. Quantitative Metal−polymer Interfacial Analyses. Figure 10 shows the typical XPS survey spectra of the residue polymers A and B on Zn samples. The spectra clearly show

Figure 11. (a) C1s and (b) O1s peak fittings of the (1) bare Zn sample, (2) residue, (3) removed, and (4) free polymer film A, (5) residue, (6) removed, and (7) free polymer film B.

Figure 10. Typical XPS survey spectra of the residue polymer films (a) A and (b) B on Zn samples. 2787



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Figure 12. XPS, (COO−)/Zn3p relative peak area values of (a) succinic acid and myristic acid model compounds and (b) residue polymer film A vs the Zn surface hydroxyl fraction.

removed film is subjected to substantial structural changes due to the adsorption. This implies a substantial consumption of carbonyl with metal sites in the interfacial region. Figure 11b shows XPS, O1s peak fittings of the bare Zn sample and free, removed, and residue polymer films A and B. The O1s peaks of the removed and free films are fitted to C− O−C/O−CO and O−CO subpeaks.68,69 Moreover, O1s spectra collected from the residue films on Zn surfaces are fitted to Zn oxides, C−O/CO, and O−C−O− functional groups.70−74 The O1s spectrum of the bare Zn sample is fitted to O2−, OH−, and COx.27,45 A comparison of the O1s peak fitting of the bare Zn sample with that of the residue polymers A and B shows that the hydroxyl and carbonates are replaced by carboxyl and carboxylate, respectively. It can be considered that the formation of the carboxylic group was accompanied by the consumption of the hydroxyl fraction and the transformation of the carbonates. 3.3.1. Metal−Polymer Interfacial Mode. To evaluate the interfacial structures of Zn surfaces and polymers A and B, O(C−O,CO)/C(C−O,CO) peak area ratios are calculated from O1s and C1s peak fittings. The results show that the O(C−O,CO)/C(C−O,CO) ratio for the bare Zn surface is 0.51, while the ratio is 1.1 for the residue film A, 0.27 for the removed film A, and 0.74 for the free film A. On the other hand, the O(C−O,CO)/C(C−O,CO) ratio is 0.72 for the residue film B, 0.68 for the removed film B, and 0.22 for the free film B. Different O(C−O,CO)/C(C−O,CO) ratios indicate the variation of the polymer structures. The large variation of O(C−O,CO)/C(C−O,CO) ratios for the residue and free films A shows a substantial structural change of the interface compared with that for the bulk polymer. Moreover, different O(C−O,CO)/C(C−O,CO) ratios for free film A compared with that for removed film A shows that the pulled-off film represents the interfacial functionality as well. On the other hand, the negligible CO subpeak in the C1s spectrum of the residue film A (Figure 11a) shows that minor polymer chains remained on the Zn surface and that the residue layer originates mainly from the carboxylates. These factors indicate an adhesive interfacial mode between the Zn surface and polymer A. On the other hand, the close values of O(C−O,CO)/ C(C−O,CO) ratios for the residue and removed film B, i.e., 0.71 and 0.68, respectively, as well as the presence of phenyl rings in the residue layer B (Figure 8b), indicate that the residue layer possesses the same polymeric structure as the

C1s peaks have been used to calculate the thickness of the residue layers on the untreated samples using the method described by McCafferty et al.,62 assuming that the values of inelastic mean free path (λ) and cross section for photoionization (σ) of C were 2.3 and 0.157, respectively.63,64 Accordingly, thicknesses of the residue layers of polymer A and B are 1.13 and 1.62 nm, respectively. C1s peaks of the residue films A and B compared to those of the bare Zn sample are shifted to higher binding energies, showing a polymeric structural change in the interface region. This can be correlated to the presence of a higher amount of hydrogen bonding between the large carboxyl components that remained on the Zn surfaces after the polymer removal. This verifies the FTIR results indicating that the carboxylic functionalities of the polymers replace the carbonaceous contamination after interaction with Zn metal surfaces. C1s peaks decomposed into CC/C−C/C−H, C−COOX, C−O, CO, and O−C−O − species. Binding energy separations correspond to published values: 0.5 eV for C− COOX, 1.5 eV for C−O, 2.9 eV for CO, and 3.6−6.3 eV for O−C−O− with respect to the C−C/C−H peak.65−67 The C1s peak of the bare sample originates from the carbonaceous contamination, while those of residue, removed, and free polymer films A and B are expected to originate from the polymer functionalities. The peak at 284.8 eV in the peak fitting of polymer B is expected to partially originate from CC components in addition to C−C/C−H due to the presence of phenyl rings. O−C−O− subpeak areas showing amounts of carboxylates formed on the metal surfaces are larger on the residue films A and B than that of the bare sample. This can be correlated to a higher deprotonation affinity of the carboxylic macromolecules stimulated by a larger charge and consequently dipole moment. Additionally, the absence of a O−C−O− subpeak in the C1s spectra of the free films A and B indicates the absence of carboxylates in the structure of the nonadsorbed films as expected. C1s peak configurations of the free film polymers different from those of the residue films demonstrate a polymeric structural change at the interface. This change is expected to originate from the active interaction of carboxyl (COOH) and carbonyl (CO) with Zn surfaces and formation of carboxylates (reactions 1 and 2). Moreover, a comparison of C1s peaks of the free films and the film removed from the Zn surface shows that the CO peak diminished dramatically on the film pulled off from the Zn surface, showing that the 2788



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Figure 13. Residue polymer film B (a) XPS, (COO−)/Zn3p relative peak area values vs the Zn surface hydroxyl fraction, (b) relative FTIR, (νas(COO−) + νs(COO−))/CO peak intensities vs the Zn surface hydroxyl fraction, and (c) XPS, (COO−)/Zn3p relative peak area values vs ND.

Figure 12a shows that the amounts of the formed carboxylates due to the adsorption of the succinic acid and myristic acid model compounds increase with the hydroxyl fraction, indicating that the higher the surface hydroxyl content, the more carboxylate formation on the surface. A comparison of the carboxylates formed due to the adsorption of succinic acid and myristic acid molecules shows a higher deprotonation level as well as an increasing rate of deprotonation by the hydroxyl fraction for myristic acid compared with that for succinic acid molecules. Van den Brand et al.75 and Brogly et al.76 showed that carbonyl oxygen acts as a Lewis base (electron donor) and the proton of the hydroxyl group as a Lewis acid (electron acceptor). Consequently, the charge transfer from the myristic acid adsorption at the metal/electrolyte interface is more than that from the succinic acid adsorption due to the longer molecular chain that results in a higher amount of charge transfer. Figure 12b shows the amounts of the carboxylates formed due to the residue polymer A. The amount of carboxylates formed on Zn surfaces due to the interaction with polymer A increases with the hydroxyl fraction. This is a result of an increase of the adsorption reaction at the metal−polymer interface (reaction 1), proving the involvement of the hydroxyl fraction in the deprotonation level of the carboxylic functionality (COOH) of polymer A. This is in good agreement with the role of the hydroxyl fraction observed for the adsorption of the model compounds, indicating a similar interfacial bonding mechanism. Nevertheless, it is obvious that the amount of carboxylates (COO−) formed on sample 5 is low

removed layer. However, the substantial difference between O(C−O,CO)/C(C−O,CO) ratios of free film B and those of residue and removed films manifest a different polymeric structure of the interface compared with that of bulk polymer B, which can be correlated to the effects of Zn oxide in the polymeric structure of the interface region. These factors imply a cohesive interfacial mode between the Zn surface and polymer B. The adhesive interfacial mechanism of polymer A indicates that the polymer−polymer bonding strength is higher than that of metal−polymer in the interfacial region. However, in the interface of polymer B, polymer−metal bonding is stronger than polymer−polymer bonding. Consequently, the interfacial strength of polymer A is determined by the adhesion between metal substrate and polymer structure, while that of polymer B is defined by the cohesion of the polymer structure. This can be related to the level of curing and the obtained structure of polymer B in the interface, while the adhesion of polymer A is defined by the deprotonation level of the carboxylates formed. 3.3.2. Polymer Interfacial Deprotonation Density. Figure 12 shows the amounts of carboxylates (COO−)/Zn3p formed due to the adsorption of the model compounds and residue polymer film A versus the Zn surface hydroxyl fraction. The XPS, C1s peak fittings are used to determine the atomic concentration of O−C−O− compound presenting adhesion/ cohesion degrees. To subtract the equipment variations affecting the signal intensities obtained, the O−C−O− peak area was divided by the Zn3p peak area. 2789



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Consequently, it can be inferred that adhesion of polymer A is mainly determined by the hydroxyl fraction, i.e., adsorption sites (Figure 12b). On the other hand, in the case of the residue polymer B, the hydroxyl fraction is not the only factor determining the amount of carboxylates formed on the Zn surface. In this case, the curing procedure is stimulated with the electrons supplied by the metal oxide Fermi level, resulting in an increased amount of adsorbates due to the interaction with polymer B (Figure 13c), while among the carboxylic functionalities presented in the interface region, the level of deprotonation increases by the hydroxyl fraction as shown by the relative FTIR peak intensities (Figure 13b).

despite the high hydroxyl fraction. This can be due to the buffering effects of the solution together with the anodic potential applied for the treatment of this sample to remove wurzite-structured oxides from the surface.9 The oxide removal in turn decreases the surface roughness and consequently the surface area available for the interaction between the Zn surface and polymer A. Figure 13a shows the amounts of carboxylates formed by the residue polymer B versus the Zn hydroxyl fraction. A comparison of Figures 11 and 12a shows relatively similar amounts of carboxylates (COO−)/Zn3p formed due to the adsorption of the both model compounds and the residue polymer films A and B. This indicates that the amount of carboxylates formed is determined mainly by the metal substrate type as far as the reactive functional polymer is similar for all cases, i.e., (COOH). The Zn oxide Schottky barrier probably determines the capacity of the carboxyl adsorption level. In this case, when the polymer compounds are brought into contact with Zn oxides, electrons flow till an equilibrium level blocks further electron flow.77,78 Although a coherent relationship between the amounts of formed carboxylates (XPS, C1s) due to the residue polymer B and the metal surface hydroxyl fraction is not observed (Figure 13a), the FTIR relative (νas(COO−) + νs(COO−))/CO peak intensities increase gradually by the hydroxyl fraction (Figure 13b). This can be correlated to either an increase of the formed carboxylates (νas(COO−) + νs(COO−)) or a decrease of CO functionality as a result of a higher deprotonation level. Both cases prove that the dissociation level increases with the hydroxyl fraction, confirming the importance of the hydroxyl fraction in carboxyl dissociation. However, the untreated sample deviates from this rule where the relative peak intensity value, (νas(COO−) + νs(COO−))/CO, is relatively high despite the hydroxyl fraction. The level of surface reactivity of the untreated Zn sample is lower than that of the treated ones, which presumably is related to the higher oxide resistance (ROX) of the untreated sample.9 Figure 13c shows the amount of formed carboxylates (COO−)/Zn3p due to the residue polymer film B on Zn surfaces versus the dopant concentration (ND). The amount of carboxylates decreases gradually with ND. It is known that Zn oxide is an n-type semiconductor, the electronic properties of which are determined by the dopant type and concentration.9 The importance of dopant type and concentration in determination of the oxide semiconductor properties can be explained by the variation of the Fermi level with respect to the conduction band edge. In this case, metal oxides commonly act as accelerators during the radical cross-linking reactions.79,80 Curing starts with decomposition of MEKP and generation of free radicals by the metal accelerator, providing electrons for the decomposition reaction:

4. CONCLUSIONS In this work, poly(methyl vinyl ether-alt-maleic acid monobutyl ester) and propoxylated bisphenol A fumarate unsaturated polyester dissolved in styrene, cured using a liquid methyl ethyl ketone peroxide (MEKP), were applied on differently treated Zn surfaces. Formation of symmetric ν s (COO − ) and asymmetric νas(COO−) vibration bands in the interfacial region shows the deprotonation of the carboxyl group of the polymers and formation of carboxylates in a bridging bidentate status. The same interfacial bonding configuration was observed due to the adsorption of model compounds on the Zn surfaces. It was also shown that the polymeric constraints lead to an inclination of the formed carboxylate versus the Zn surface. Furthermore, the results show that the amount of bondings formed between the Zn surface and poly(methyl vinyl ether-altmaleic acid monobutyl ester) increased with the adsorption sites, i.e., hydroxyl fraction. The importance of the hydroxyl fraction in the amount of the formed carboxylates was confirmed by the adsorption of the monomers, indicating a successful modeling of the interfacial bonding properties. The interfacial bonding formation of propoxylated bisphenol A fumarate for which a curing process is required was correlated to the metal oxide electronic properties. However, the deprotonation level among the adsorbates increased with the hydroxyl fraction. Additionally, adhesion/cohesion mechanisms depend on the polymer bulk structure rather than the active end group participating in the metal−polymer interaction, i.e., COOH.

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Corresponding Author

*Tel:+ 3226293537; fax:+3226293200; e-mail: Herman. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was carried out under project number MC6.06254 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl).

Zn+ → Zn 2 + + e−

C8H18O6 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2C4 H 9O3·

AUTHOR INFORMATION

(3)

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This polarizes Zn anodically up to its own redox-potential, which depends on the metal oxide Fermi level.81 Consequently, an increase in dopant concentration decreasing the flatband potential results in a lower amount of electrons supplied by the Fermi level and consequently a lower curing level and residue coating. The deviation of sample 2 from this rule may originate from a change in the dopant type, regardless of the concentration, due to the high tempereture applied for the treatment of this sample.

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