In Situ Study of Buried Metal–Polymer Interfaces Exposed to an

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In Situ Study of Buried Metal−Polymer Interfaces Exposed to an Aqueous Solution by an Integrated ATR-FTIR and Electrochemical Impedance Spectroscopy System P. Taheri,*,†,‡ J. H. W. de Wit,†,‡ H. Terryn,†,§ and J. M. C. Mol‡ †

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 ‡

ABSTRACT: This study investigates the interfacial bonding variations of carboxylic polymers on Zn surfaces upon exposure to an aqueous solution by means of an integrated ATR-FTIR and electrochemical impedance spectroscopy (EIS) analysis in Kretschmann geometry. In situ ATR-FTIR probes water molecules reaching Zn surfaces and subsequent interfacial bonding degradation at the hidden polymer−metal interfaces. Furthermore, EIS characterizes the polymer film capacitance and resistance determining the exposed polymer bulk and metal−polymer interfacial changes. Consequently, the integrated ATR-FTIR and EIS system is used to resolve the processes of polymer saturation with water and interfacial bonding degradation complementarily and simultaneously. Additionally, two types of carboxylic polymers, i.e., a polyester and a cured unsaturated polyester, are applied on a set of differently pretreated Zn samples to evaluate the effects of the curing process and surface pretreatments on water transport within the polymer structure and subsequent interfacial bonding degradation. Furthermore, Zn surface hydroxyl fractions are calculated by means of X-ray photoelectron spectroscopy (XPS) and correlated to the interfacial bonding degradation process. The results show that the hydroxyl fraction is a determining factor in accomplishing proper initial and durable interfacial bonding. Additionally, the polymer curing process plays an important role in barrier performance against water.

1. INTRODUCTION Polymer coatings often are applied on metal surfaces to protect them against corrosion media.1 For durable protection, organic coatings must provide an adequate barrier to water, oxygen, and/or ion penetration and a high resistance to interfacial bonding degradation.2,3 However, due to the presence of defects in organic coatings, water and ions penetrate into the coatings. This forms a corrosive environment in the metal− polymer interfacial region causing a subsequent metal−polymer disbonding process. Cathodic delamination is a common failure mode of coated zinc caused by alkalinity at the interface as a result of cathodic activity underneath the coating. Moreover, the interfacial polymer−metal bonds can be degraded by formation of a stable bonding between the metal oxide and the water.4,5 Studying the metal−polymer interactions is crucial to understand the interfacial bonding mechanisms, the waterinduced interfacial changes, and the role of corrosive constituents on the interfacial stability. However, interfacial polymer−metal bonds are hidden and difficult to reach due to the relatively high polymer thickness. Efforts to understand the metal−polymer interface resulted in the development of the socalled ATR-FTIR Kretschmann geometry.6 Figure 1 schematically shows the described method to study the buried polymer−metal interfaces. In this case, an electric field passes through a thin metal film deposited on an internal reflection © 2013 American Chemical Society

Figure 1. Schematic ATR-FTIR experimental method to analyze the buried polymer−metal interface based on Kretschmann geometry.

element (IRE). Consequently, if the metal layer is coated with a polymer film, the electric field reaches the metal−polymer interface. We have utilized the ATR-FTIR Kretschmann Received: September 17, 2013 Published: September 19, 2013 20826

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Two types of carboxylic polymers, namely, polymer A and B, were applied on the differently pretreated deposited Zn layers. Polymer A was a 50 wt % poly(methyl vinyl ether-alt-maleic acid monobutyl ester) dissolved in ethanol. Formation of polymer layer A was conducted through solvent evaporation. The absence of lateral bonding among the structure of polymer A is expected to provide a poor resistance against water ingress. Polymer B was propoxylated bisphenol A fumarated unsaturated polyester dissolved in styrene and generated by curing with a liquid methyl ethyl ketone peroxide (MEKP). For the curing of the system B, a free-radical chain growth cross-linking reaction has been proposed.22 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 cross-linking between polymer B chains is formed by the activated styrene. Thin films of polymers A and B of, respectively, 100 and 30 μm in thickness were applied onto the differently treated Zn layers by spin coating. Subsequently, polymers A and B, respectively, were dried and cured at 40 °C for 18 h. In the case of exposure, the relatively low thicknesses of the polymer systems applied on Zn surfaces are expected to reduce the time required for water molecules to reach the metal−polymer interface. A Thermo-Nicolet Nexus FTIR apparatus equipped with a mercury−cadmium−telluride liquid-nitrogen cooled detector and a nitrogen-purged measurement chamber with a Harrick Seagull multipurpose reflection accessory were used. Prior to the polymer application, infrared backgrounds were obtained from pretreated deposited Zn layers on an IRE. The samples were exposed to a borate buffer solution including 0.075 M Na2B4O7·10H2O + 0.3 M H3BO3 and the final measurements were recorded versus the backgrounds. The measurements were performed with an incident beam angle of 65°, using ppolarized radiation. For nonabsorbing media, with a negligible imaginary refractive index, the depth of IR penetration can be estimated.23 However, Nakao et al.24 argued that the depth of penetration is considerably decreased in the presence of a deposited metal layer. Consequently, the exact penetration depth is not clear. Electrochemical impedance spectroscopy (EIS) measurements were conducted simultaneously with the ATR-FTIR measurements in a conventional three-electrode cell using an EG&G 273 potentiostat and a Solartron frequency response analyzer 1255. The reference was a saturated calomel electrode (SCE), connected to the main compartment through a salt bridge. A flat platinum plate was utilized as counter electrode (CE), and the deposited zinc layer was used as the working electrode. A Pt wire coupled with a capacitance of 10 nF was employed in parallel with the reference electrode to reduce the phase shift induced by the reference electrode in high frequencies.25 The measurements were performed in the 10 kHz−10 mHz frequency range with 10 frequency points per logarithmic decade and sinusoidal voltage of 10 mV. To quantitatively analyze the differently pretreated deposited Zn layers, XPS analysis was conducted with a PHI 1600/3057 instrument using an incident X-ray radiation (Mg Kα1,2 = 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 C 1s peak at a binding energy of 284.8 eV to correct the sample charging.26 The curve fitting and decomposition were done by PHI Multipak V8.0 software. Figure 2 shows typical XPS, O 1s peak fittings of the deposited zinc substrate. The O 1s peak is resolved into three

geometry to in situ probe the interfacial bond making between carboxylic-based polymers and differently pretreated Zn surfaces as well as the polymerization process at the interface.7 FTIR also can be used to provide information about the water transport, degradation of organic coatings, and corrosion processes.8 On the other hand, with electrochemical impedance spectroscopy (EIS) the polymer film capacitance, resistance, and charge-transfer processes at the interface can be studied.9−11 This provides the opportunity to in situ monitor the electrolyte transport in polymer films applied on metals, as well as corrosion and other processes at the metal surface. Consequently, an integrated ATR-FTIR Kretschmann and EIS system enables one to obtain a comprehensive overview of the polymer−metal system combining the information of the vibrational bonding and electrochemical properties. Integrated ATR-FTIR Kretschmann geometry and EIS have been used successfully to provide complementary information on polymer saturation with water molecules and disbonding processes.12 Carboxylic functionalities are often used in polymers to increase the adhesion between the polymer coatings and metal substrate.13−16 Additionally, carboxylic groups have attracted much attention because of their importance to understand the fundamental adhesion mechanisms between polymer and metal.17−19 On the other hand, the interaction of carboxylic functionalities depends on the metal surface chemistry. A metal surface modification is expected to affect the adsorption process. In this case, Valtiner et al.20 realized formation of a stable bonding between carboxylic macromolecules and (0001) ZnO, while Nicholas et al.21 showed that the bonding strength of the formed carboxylate is different on (0001) and (1010) ZnO crystals. Therefore, examination of the interfacial functional properties assists in elucidating the fundamentals of the interfacial bonding properties and developing beneficial polymer and metal surface modifications. This work investigates the influence of the curing and zinc pretreatment on the metal−polymer interfacial region during exposure to an electrolyte solution using an integrated in situ ATR-FTIR and EIS system. Carboxylic polymer coatings were used, and the composition and curing process were controlled for a proper and consistent prediction and interpretation of the interfacial bonding variations. Moreover, Zn surfaces were subjected to various treatments prior to polymer application and examined using XPS. The coated Zn surfaces were exposed in a borate buffer solution to evaluate the polymer saturation with water and interfacial bonding degradation process.

2. EXPERIMENTAL SECTION Pure zinc sheets (99.95%) supplied by Goodfellow were deposited on IRE hemispheres made of Ge (25 mm in diameter) by Harrick Scientific Products Inc. by means of a high-vacuum evaporation system, i.e., Balzers, BAE 250, coating system. The film thickness was around 80 nm determined by an in situ quartz crystal microbalance. Subsequently, the deposited films were treated in different aqueous solutions by exposing the Zn-coated face of the IRE to the treatment solutions for 30 min. Sample A was treated in 0.05 M HCl, pH 1.9, sample B deionized water at room temperature, sample C 1 M Na2CO3, pH 11.5, sample D 0.2 M H3BO3 + 0.1 M NaOH, pH 12.3, and sample E deionized water at 65 °C. After the treatments, Zn layers were rinsed thoroughly with deionized water for 2 s to remove the remaining treatment solutions from the surfaces. 20827

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νs(COO−) and asymmetric νas(COO−) carboxylate stretching vibration bands, respectively.7,29,30 Figure 3a exhibits a broad band around 3450 cm−1 assigned to hydroxyl vibrations of water or metal hydroxides which increases with time for polymer A. Additionally, a band at 1650 cm−1 assigned to molecular water increases gradually indicating that water molecules are rapidly transported to the interface through the polymer layer, which also results in a fast increase of the OH stretching band as well. However, the carbonyl stretching vibration ν(CO) at 1760 cm−1 decreases dramatically within the initial 8 h exposure time. On the other hand, symmetric νs(COO−) and asymmetric νas(COO−) carboxylate stretching vibration bands persist for a relatively longer time proving the stability of metal−polymer bondings against the aqueous solution regardless of the carbonyl peak decay. The diminishing trend of carbonyl peak intensities in favor of increasing the water and hydroxyl peak intensities indicates that the unsaturated carboxylic acid functionality, i.e., carbonyl, is not stable in the presence of water. In this case, the carboxylic acid present in the metal−polymer interface is deprotonated with water molecules according to the following reaction31

Figure 2. Typical XPS, O 1s peak fittings of the deposited zinc substrate.

individual subpeaks due to the contribution of O2−, OH−, and COx, which are roughly located around 530, 531.5, and 533 eV with peak areas of 790 c/s, 840 c/s, and 194 c/s, respectively. To obtain a reliable hydroxyl content, it is necessary to correct the O 1s photopeak for contributions from the C−O and O− CO/O−C−O− species.27 The correction of the hydroxyl fraction was done according the procedure described by Wielant et al.28 The results exhibited that hydroxyl fractions of samples A, B, C, D, and E are 6%, 49%, 58%, 66%, and 76%, while the hydroxyl fraction of the untreated sample was 34%.

R‐COOH + H 2O → R‐COO− + H3O+

(1)

Figure 3b shows that the stretching vibration band of OH around 3450 cm−1 increases slightly, while that of polymer B functionalities persists. These indicate the stability of polymer B against the water saturation and interfacial degradation during the whole exposure period. The efficient blocking properties of polymer B compared to that of polymer A originate from the curing reactions providing lateral barriers against the water intrusion, while the solely metal−carboxylic group interaction of polymer A hardly provides sufficient barrier properties. Additionally, the stability of carbonyl stretching vibration ν(CO) of polymer B originates from the curing, increasing the molecular weight and consequently hydrophobic nature of the alkyl chain.32,33 Figure 4 shows the peak intensities at 3450 and 1650 cm−1 corresponding to the stretching vibrations of hydroxides and water molecules versus the exposure time. It can be seen that during the first hour of exposure the peak intensities increase rapidly for polymer A, which is due to the water transport through the polymer network and formation of Zn hydroxyls

3. RESULTS AND DISCUSSION Figure 3 exhibits ATR-FTIR spectra of Zn layers coated with polymers A and B after exposure to the borate buffer solution.

Figure 3. ATR-FTIR Kretschmann spectra of the Zn layer coated with polymer (a) A and (b) B exposed to the borate buffer solution at different times.

The spectra collected immediately after the exposure (0 h) show that carbonyl stretching vibration ν(CO) at 1750− 1760 cm−1 corresponding to the undeprotonated carboxylic functionality dominates in the graphs. Additionally, the presence of the bands at 1400−1450 cm−1 and 1600 cm−1 indicates the formation of carboxylates coordinatively bonded to the Zn surfaces. These peaks are assigned to symmetric

Figure 4. Corresponding ATR-FTIR peak intensities of polymer A at (a) 3450 cm−1 and (b) 1650 cm−1 and polymer B at (c) 3450 cm−1 and (d) 1650 cm−1 versus the exposure time. 20828

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Figure 5. Corresponding Nyquist and Bode plots of polymer (a) A and (b) B at different exposure periods.

the peak at 1650 cm−1 solely originates from the water molecules. Consequently, it can be inferred that during the initial 1 h of exposure water ingresses and reaches the interface accompanied by the hydroxyl formation, while within 1−3 h the hydroxyl fraction increases which is responsible for the increase of peak intensity at 3450 cm−1. Figure 5 shows the corresponding EIS Nyquist and Bode plots collected at different exposure times. Previous studies showed that the low-frequency regions of EIS data only contain dielectric information about the metal−polymer interface if the total impedance of the coating is low due to the presence of major defects in the coating.34 Figure 5a shows that during the initial 8 h exposure period of polymer A the Nyquist plots exhibit single arcs indicating the stability of the Zn−polymer A interface as proved by ATR-FTIR results. ATR-FTIR results proved that water reaches the interface upon the exposure,

on the surface. On the other hand, the intensity increase of the band at 3450 and 1650 cm−1 of polymer B shows a slower growth of the peak intensity compared to that of polymer A. As discussed, this is correlated to the curing process providing a lateral barrier to water saturation through the polymer network as well as an increase of the hydrophobic nature. Moreover, our previous study showed a stronger Zn−polymer interfacial bonding for polymer B compared to that of polymer A, proving an adequately stable interface in the presence of an aqueous solution. It can be seen that the increase of the peak intensities occurs in the initial hours of exposure for both vibration bands at 3450 and 1650 cm−1 for polymer A. For the exposure time more than 1 h, the intensities of the 1650 cm−1 peak reach a steady state, while those of peaks at 3450 cm−1 still increase till 3 h. As mentioned, the peak at 3450 cm−1 corresponds to hydroxyls, which can originate from water and Zn hydroxyl fraction, while 20829

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while a gradual decrease of the arc sizes and impedance values implies an ongoing system change. As shown, peak intensities of polymer A at 1650 cm−1 show that the amount of water at the metal and polymer interface becomes relatively constant after the initial 1 h of exposure, while the peak intensities at 3450 cm−1 still increase between 1 and 3 h of exposure time (Figure 4). Consequently, the impedance change in this period is expected to originate from the progressive hydroxyl formation rather than an increase of the water amount at the interface. This is reflected by the scattered Nyquist and Bode plots at low frequencies as a result of the localized chemical alteration. However, a gradual decrease of impedance values till 8 h shows a progressive decrease of polymer coating resistance presumably due to an increase of water content within the polymer structure. The Nyquist plots show that two arcs obtained after 10 h, while the impedance of the Bode plot increases dramatically indicating a change of interfacial mechanisms. On the other hand, ATR-FTIR spectra showed a gradual decrease and subsequently negative vibration bands at 2360 and 2340 cm−1, assigned to CO2 of the polymer functionalities. This may originate from a decrease of the interfacial bonding density. Consequently, it is likely that the polymer film starts an interfacial degradation procedure at 10 h possibly due to formation of corrosion products or the swelling process. From 10 to 70 h, a gradual decrease of the impedance at low frequencies is detected. This indicates a progressive interfacial degradation process resulting in a decrease of the interfacial density giving rise to deeper negative peaks in the ATR-FTIR spectra. The decrease of impedance is followed by formation of relatively constant arcs and impedance values with time indicating a completion of the interfacial degradation process. The plots in Figure 5b show a decrease of impedance values indicating an ongoing decrease in polymer coating resistance due to the electrolyte penetration. However, it can be seen that the plots remain relatively constant after 50 h due to a limited saturation of the polymer with water molecules. EIS results together with the ATR-FTIR spectra indicate that the saturation of polymer B with water molecules is retarded resulting in the interfacial stability during the exposure. The lower water uptake of polymer B than that of A can be correlated to the curing procedure saturating the carbonyl functional group, which in turn increases the polymer stability against water. Moreover, a higher barrier performance of polymer B than that of polymer A is promoted by the curing. The water content in the intact polymer film can be estimated during the initial exposure time from impedance data by assuming the system to act as a pure capacitor at high frequencies.35 Figure 6 shows the capacitance values during the initial 25 h of exposure time, based on the modulus of impedance at 1 × 104 Hz. Polymer A exhibits a gradual capacitance increase exhibiting the water uptake process, while the sudden capacitance increase at ∼10 h of exposure presumably represents the initiation of an interfacial bonding degradation. On the other hand, the lower increasing rate of capacitance values of polymer B demonstrates a diminished water uptake as compared to polymer A. Figure 7 shows the required time for initiation and completion of interfacial bonding degradation of polymer A versus the surface hydroxyl fractions on Zn surfaces obtained through the different treatments. The initiation and completion of interfacial bonding degradation times are obtained through the Nyquist plots where the second arc initiates and the

Figure 6. Capacitance of polymer (a) A and (b) B at 1.104 Hz versus the exposure time.

Figure 7. Required time of polymer A for (▲) initiation (obtained from EIS plots), (■) completion (obtained from EIS plots), and (∗) completion (obtained from ATR-FTIR spectra) of the interfacial bonding degradation process versus the surface hydroxyl fraction.

impedance values become relatively constant, respectively. Additionally, the completion of interfacial bonding degradation is estimated from ATR-FTIR spectra where the carboxylate symmetric and asymmetric vibrational bands, respectively, around 1450 and 1600 cm−1 disappear. It can be seen that the interfacial bonding degradation starts relatively at the same time for the different samples, while the interfacial bonding degradation ends after a longer period for increasing hydroxyl fractions. As discussed, the interfacial bonding density increases with the hydroxyl fraction formed on Zn surfaces through the treatments applied. Consequently, it can be inferred that the interfacial density prolongs the disbonding period expected to increase the coated metal’s resistance against corrosive media. Moreover, the results obtained through EIS and ATR-FTIR show relatively identical increasing trends for completion of the interfacial bonding degradation step with hydroxyl fraction. The observed deviation of FTIR and EIS results for samples B and C may originate from the intense water vibrational bands at 1650 cm−1 masking the asymmetric and symmetric carboxylate vibrational bands. 20830

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4. CONCLUSIONS This work investigates the interfacial bonding degradation of carboxylic polymers and Zn substrate by means of ATR-FTIR and electrochemical impedance spectroscopy (EIS) in a Kretschmann geometry. The results suggest that the setup provides the opportunity to probe the interfacial bonding degradation using Ge as an internal reflection element, while Ö hman et al.36 probed the soaked polymer bulk using ZnSe possessing a lower reflective index than that of Ge. Additionally, The results exhibit that the curing process provides a blocking performance retarding water penetration through the polymer structure. Furthermore, the interfacial bonding degradation procedure starts regardless of the Zn oxide composition, while the time span between the initiation and the end of interfacial bonding degradation is prolonged with the Zn surface hydroxyl fraction and consequently the interfacial bonding density.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +31 15 2784526. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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