pubs.acs.org/Langmuir © 2009 American Chemical Society
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Grafting of Poly(acrylic acid) onto an Aluminum Surface Fabienne Barroso-Bujans,*,† Rosalia Serna,‡ Eva Sow,† Jose L. G. Fierro,§ and Michael Veith†,
Institute for Inorganic Chemistry, University of Saarland, 66041 Saarbruecken, Germany, ‡Laser Processing Group, Instituto de Optica, CSIC, Serrano, 121, 28006 Madrid, Spain, §Instituto de Cat alisis y Petroleoquı´mica, CSIC, Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain, and Institute for New Materials (INM), 66123 Saarbruecken, Germany )
†
Received February 11, 2009. Revised Manuscript Received June 24, 2009 Grafting of poly(acrylic acid) (PAA) onto an aluminum surface was successfully achieved by free radical polymerization of acrylic acid using typical radical initiators, benzoyl peroxide and 2,20 -azobisisobutyronitrile. Both spotlike and brush morphologies were achieved. A complete coverage of PAA on an aluminum surface was then achieved by using a thermal chemical vapor deposition process. The PAA thickness was determined by ellipsometry and the superficial chemical composition by X-ray photoelectron spectroscopy (XPS). Grazing angle Fourier transform infrared (FTIR) spectroscopy confirmed the presence of carboxylic acid groups on the surface, and the contact angle measurements revealed a decreasing free surface energy of aluminum due to the polymer surface covering.
1. Introduction The chemical modification of surfaces has been studied widely for several decades. More recently, there has been growing interest in surface nanostructuring because of its technological applications in areas such as biomaterials, microelectronics, and medicine, where, for example, the surface is modified to promote interaction with cells.1-3 Furthermore, such surfaces could include a number of chemical functionalities introduced from inorganic and organic surfaces. Metallic surfaces coated with polymer layers appear to be particularly suited from the point of view of versatility and biocompatibility. To improve the durability of the polymer on the metallic surface, traditional methods for covalently anchoring polymers to oxide surfaces of the metals were developed. Such methods include the “grafted from” with initiators4-7 or monomers8,9 initially anchored to the surface or the “grafted onto”, where a preformed polymer is covalently linked to the surface.8,10 A great variety of polymers have been successfully anchored to inorganic surfaces by such methods. In particular, poly(acrylic acid) (PAA) has been grafted via reversible addition-fragmentation chain-transfer (RAFT) living polymerization onto TiO2 nanoparticles,4 by electrochemical *To whom correspondence should be addressed. Present address: Centro de Fı´ sica de Materiales CSIC-UPV/EHU, P° Manuel de Lardizabal 3, 20018 San Sebastian, Spain. Fax: þ34 943015270. E-mail:
[email protected]. (1) Valsesia, A.; Colpo, P.; Manso Silvan, M.; Meziani, T.; Ceccone, G.; Rossi, F. Nano Lett. 2004, 4, 1047. (2) Kindermann, M.; George, N.; Johnsson, N.; Johnsson, K. J. Am. Chem. Soc. 2003, 125, 7810. (3) Song, J.; Chen, J.; Klapperich, C. M.; Eng, V.; Bertozzi, C. R. J. Mater. Chem. 2004, 14, 2643. (4) Hojjati, B.; Sui, R.; Charpentier, P. A. Polymer 2007, 48, 5850. (5) Hojjati, B.; Charpentier, P. A. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 3926. (6) Zheng, G.; Sto1ver, H. D. H. Macromolecules 2002, 35, 6828. (7) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 2001, 123, 7497. (8) Barroso-Bujans, F.; Fierro, J. L. G.; Veith, M. J. Colloid Interface Sci. 2007, 314, 160. (9) Rong, M. Z.; Ji, Q. L.; Zhang, M. Q.; Friedrich, K. Eur. Polym. J. 2002, 38, 1573. (10) Popat, K. C.; Mor, G.; Grimes, C. A.; Desai, T. A. Langmuir 2004, 20, 8035. (11) de Giglio, E.; Cometa, S.; Cioffi, N.; Torsi, L.; Sabbatini, L. Anal. Bioanal. Chem. 2007, 389, 2055. (12) Wu, T.; Gong, P.; Szleifer, I.; Vlcek, P.; Subr, V.; Genzer, J. Macromolecules 2007, 40, 8756.
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polymerization onto titanium substrates,11 with gradual variation of chain grafting densities on flat silica,12 and as hyperbranched PAA onto both gold surfaces13 and gold-coated porous alumina supports.14 PAA is a well-known biocompatible polymer, used as a polyelectrolyte in different biomedical applications.15,16 Moreover, the carboxylic groups present in its backbone allow it to be functionalized with bioactive molecules such as drugs.17,18 The aim of this work is to develop new procedures for obtaining a PAA-grafted aluminum surface based on the vinyl modification of the substrate. The vinyl groups introduced through a silane coupling agent participated in the polymerization of acrylic acid and acted as points of anchoring to the substrate. The polymerization was conducted by free radical polymerization using thermal and UV light initiation. A second procedure was thermal chemical vapor deposition (CVD), where polymerization occurs in the hot vinyl-modified substrate. The surfaces were characterized by X-ray photoelectron spectroscopy (XPS), grazing angle Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact angle measurements.
2. Experimental Section 2.1. Materials. Aluminum discs with a diameter of 32 mm
and a thickness of 2 mm were polished in a Struers polisher8 to yield mirror-like surfaces. Comparative nonpolished disks were also used to produce rough surfaces. Allyltrimethoxysilane (ATS), benzoyl peroxide, and 2,20 -azobisisobutyronitrile (AIBN) were purchased from Aldrich. Pentane and tetrahydrofuran (THF) were dried by distillation from sodium ketyl benzophenone and stored under a nitrogen atmosphere with a sodium wire pressed into the solvent. Acrylic acid (AA) was distilled to remove trace amounts of inhibitors. (13) Bergbreiter, D. E.; Kippenberger, A. M. Adv. Polym. Sci. 2006, 198, 1. (14) Nagale, M.; Kim, B. Y.; Bruening, M. L. J. Am. Chem. Soc. 2000, 122, 11670. (15) Ulbricht, M. Polymer 2006, 47, 2217. (16) Lee, K. Y.; Yuk, S. H. Prog. Polym. Sci. 2007, 32, 669. (17) Kitaeva, M. V.; Melik-Nubarov, N. S.; Menger, F. M.; Yaroslavov, A. Langmuir 2004, 20, 6575. (18) Jones, D. S.; Muldoon, B. C. O.; Woolfson, A. D.; Andrews, G. P.; Sanderson, F. D. Biomacromolecules 2008, 9, 624.
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2.2. Silane Treatment of the Al Surface. Aluminum surfaces were activated with 0.1 M NaOH (forming Al-OH functions) for 10 min in an ultrasonic bath, followed by rinsing. We carefully removed superficial water by washing the mixture with dried pentane under a nitrogen atmosphere and then drying it under vacuum for 3 min. The Al disks were dipped in 0.1 vol % toluene solutions of ATS under an inert atmosphere and then heated at 60 °C for 1 h. Finally, the silane-coated Al disks were washed with dry pentane for 10 min in an ultrasonic bath and dried under vacuum. The final coatings were designated as Al-V (aluminum grafted with vinyl functions). 2.3. Grafting of Poly(acrylic acid). Acrylic acid was polymerized in situ at the Al surface by (a) thermal initiation of benzoyl peroxide, (b) ultraviolet light-induced decomposition of AIBN, and (c) a chemical vapor deposition (CVD) process. The nonpolished Al was used as a substrate in the solution polymerizations (a and b) and the polished Al in the CVD method (c). The polymerization procedures were performed as follows. (a) The AlV target was dipped in 20 mL of toluene containing 45 mg (0.5 mol %) of benzoyl peroxide and heated to 70 °C under a nitrogen atmosphere. Subsequently, a mixture of acrylic acid (2.3 mL) with toluene (2 mL) was added dropwise to the vessel in which Al targets were present. The reaction mixture was heated (70 °C) for 1 h. The Al target was removed from the solution and cleaned in ethanol for 10 min in an ultrasonic bath three times. (b) The Al-V target was dipped into a solution of acrylic acid (2.6 mL) and AIBN (15 mg, 0.25 mol %) in toluene (20 mL) contained in a quartz vessel under a nitrogen atmosphere at room temperature. Then, the solution was irradiated with ultraviolet light at 366 nm for 2 h. The target was cleaned as described above. (c) The CVD experiments were conducted in a horizontal glass tube reactor.19 Acrylic acid (1 mL) was fluxed and guided to an inductively heated (150 °C) substrate (Al-V) by applying reduced pressure (2.4 mbar). Polymerization of acrylic acid on the hot substrate resulted in the grafting of poly(acrylic acid) to the vinylmodified Al surface. These samples were designated as Al/PAA/ CVD. 2.4. Characterization of Aluminum Surfaces. Chemical characterization of modified aluminum surfaces was performed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). XPS spectra were recorded using an Escalab 200R spectrometer provided with a hemispherical analyzer, operated in a constant pass energy mode and unmonochromatized Mg KR X-ray radiation (hν = 1253.6 eV) operated at 10 mA and 12 kV. The binding energies (BE) were referenced to the C1s peak at 284.9 eV. Data processing was performed with XPS peak. The spectra were decomposed with the least-squares fitting routine provided with the software, with a Gauss/Lorentz product function and after subtracting a Shirley background. Atomic percentage values were calculated from the peak areas using sensitivity factors provided with the data system and background subtraction. Infrared spectra were recorded on a Bio-Rad 165 FTIR spectrometer at a specular reflectance angle of 75° and using perpendicular polarized light. The spectra were recorded at 2 cm-1 resolution, and 200 scans were recorded. The morphology of modified aluminum surfaces was studied by scanning electron microscopy (SEM) in a scanning electron microscope (CAMSCAN S4) with Si(Li) semiconductor detectors and thin windows (Cameca and Noran) and with an atomic force microscope (AFM) (Veeco diMultiMode V) using both tapping and contact modes under ambient conditions, equipped with a commercial Si cantilever with a 40 N/m force constant and a 2030 nm tip radius. AFM image processing was conducted with WSxM 4.0 Develop 12.2 from Nanotec Electronica S.L.20 The polymer effective thickness in the mirror-like polished samples was determined using a variable-angle spectroscopic ellipsometer (19) Veith, M.; Kneip, S. J. Mater. Sci. Lett. 1994, 13, 335. (20) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705-1.
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Article with a J. A. Woollam rotating polarizer. The ellipsometric parameters were measured in the wavelength range from 400 to 800 nm at angles of incidence of 60°, 65°, and 70° to ensure a consistent and accurate determination of the substrate effective optical constants and of the polymer thickness. The optical model for the analysis of the ellipsometry measurements is a system with flat and parallel interfaces composed by a substrate, intermediate layers, and the polymer layer. It is assumed that the effective substrate on which the polymer is attached is the vinyl-modified Al surface. The optical properties of the pristine Al and Al-OH substrates were also determined so that the surface modifications could be followed after the different treatments. The substrate properties were determined assuming a semi-infinite medium with the optical properties of Al as a starting point.21 The measurements of contact angles were performed at room temperature (20 °C) using a contact angle meter goniometer equipped with software for drop-shape analysis. Three different liquids (Milli-Q water, ethylene glycol, and diiodomethane) were used. The drop image was recorded with a video camera and digitized. Each contact angle is the average value of 20 measurements. Surface energy components were determined from contact angle measurements using the Liftshitz-van der Waals (LW) acid-base approach (three-liquid acid-base method) proposed by van Oss et al.22,23 van Oss et al. divided the surface tension into different components, i.e., the Liftshitz-van der Waals (LW), acid (þ), and base (-) components: qffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1Þ þ γAB ¼ γLW þ 2 γiþ γiγi ¼ γLW i i i where i denotes either the solid or the liquid phase. The acid-base component (γAB i ) takes into account electron-donor (γi ) and þ electron acceptor (γi ) interactions. The following expression was given for solid-liquid systems: 1 þ cos θ γL ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi LW γLW γSþ γL- þ γS- γLþ S γL þ
ð2Þ
where the three components of the surface free energy of the solid, þ γLW S , γS , and γS , can be determined from the contact angle measurements of three testing liquids with known surface tension components. θ is the measured contact angle, and subscripts L and S refer to liquid and solid, respectively. According to van Oss’s claim,24 one apolar liquid, i.e., diiodomethane (γL =50.8 mJ/m2, þ 2 γLW L =50.8 mJ/m , and γL =γL =0), and two polar liquids, i.e., þ 2 LW water (γL=72.8 mJ/m , γL =21.8 mJ/m2, and γL =γL =25.5 mJ/ 2 m2) and ethylene glycol (γL=48.0 mJ/m2, γLW =29.0 mJ/m , γL = L þ 2 2 47.0 mJ/m , and γL =1.92 mJ/m ), comprised one of the optimal combinations for test liquids and were employed in this study.
3. Results and Discussion Grafting of vinyl groups onto Al surfaces was achieved by the reactions depicted in Scheme 1. The activation of “pristine Al” (p-Al) (which still has an oxide layer on the aluminum) with NaOH is a step necessary for the introduction of hydroxyl functional groups onto the surface. This reaction induces physical changes to the surface, as the mirror-like surface of pristine Al becomes dull after the basic treatment. Close inspection of the samples with an AFM and ellipsometric measurements revealed physical changes. The AFM images clearly show an increase in superficial roughness after the basic treatment, erasing the polish tracing observed for p-Al (21) Handbook of optical constants of solids III; Palik, E. D., Ed.; Academic Press: New York, 1998. (22) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (23) van Oss, C. J.; Jub, L.; Chaudhury, M. K.; Good, R. J. J. Colloid Interface Sci. 1989, 128, 313. (24) Giese, R. F., van Oss, C. J., Eds. Surfactant Science Series: Colloid and surface properties of clays and related minerals; Marcel Dekker: New York, 2002; Vol. 105, p 119.
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Figure 1. AFM images (4 μm 4 μm) of pristine Al (p-Al) and base-treated Al (Al-OH) surfaces in contact mode. The first column shows the topography images, the second column the derivative images, and the third column the cross sections from the topography images. The mirror-like polished surfaces were used in this analysis. Scheme 1. (a) Grafting of Vinyl Groups onto an Aluminum Surface by Reaction of ATS with the Hydroxy-Activated Substrate and (b) Grafting of Poly(acrylic acid) by Radical Polymerization of Acrylic Acid on Vinyl-Modified Aluminuma
a X represents fragments coming from the initiator decomposition. The bonding of silicon to aluminum through three oxygen bonds is only a borderline case, and other bondings are possible (see the text).
(Figure 1). p-Al shows a deep scratch from polishing, but the rest of the surface is very flat with a height oscillation of ∼5 nm. In contrast, the height of the Al-OH sample oscillates several nanometers across the traced line. These observations are confirmed with the root-mean-square (rms) roughness measurements taken in a 4 μm 4 μm area. The rms roughness increases from 4 nm in p-Al to 31 nm in Al-OH. Additionally, the ellipsometry analysis clearly showed different effective refractive index and extinction coefficient spectra for both surfaces (Figure 2), indicating that different chemical and morphological surfaces were obtained after treatment. The decrease in the extinction coefficient in the full wavelength range is especially significant. Simulation with a two-layer model (aluminum and aluminum oxide) suggests the formation of an aluminum oxide layer with a thickness of ∼8 nm. Therefore, in addition to the formation of Al-hydroxyl bonds, it seems that there is further growth of the aluminum oxide layer, which can be explained by partial condensation of Al-OH 9096 DOI: 10.1021/la900518s
groups. Also, the possibility that some dissolved dioxygen participated in the oxidation of Al by alkali cannot be excluded. The next step was the “silanization” of the surface. As it is known, the presence of water molecules in silanization reaction mixtures is critical since it induces the condensation of siloxane groups and produces multisiloxane layers.25 For that reason, water was carefully removed from the solvent to favor the formation of a monosiloxane layer. The measured optical constants (Figure 2) revealed that, after the vinyl-modified surface was formed, the optical properties of the surface changed again, resembling those of the original pristine Al surface. Further investigations are required to clarify this result; nevertheless, it suggests that, when the hydroxyl groups were reacted with the methoxysilane groups (Scheme 1a), the underlying aluminum (25) Boerio, F. J.; Armogan, L.; Cheng, S. Y. J. Colloid Interface Sci. 1980, 73, 416.
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Figure 2. Effective optical constants, indices of refraction (filled symbols), and extinction coefficients (empty symbols) as a function of the wavelength for pristine Al (p-Al), base-treated Al (Al-OH), and vinyl-modified Al (Al-V). The mirror-like polished surfaces were used in this analysis.
oxide layer was also removed to some extent. Therefore, the optical constants of the Al-V surface resemble that of the pristine Al because the newly formed siloxane layer on the Al has a thickness in the subnanometer range and has a negligible influence on the optical properties of the Al surface. The binding sites in Scheme 1a may clearly be different from those sketched in the figure as mono- and bidentate fashions together with the tridentate fashion are also possible. Grafting polymerization of acrylic acid onto the Al-V surface (Scheme 1b) was conducted via free radical mechanisms by three processes, thermally induced, UV light-induced, and the CVD process. There are two possible mechanisms that would covalently attach PAA molecules to the Al surface. First, the thermally decomposed benzoyl peroxide initiates the polymerization, creating active centers in both the surface and the dissolved monomer. Vinyl groups attached to the surface propagate the reaction as the acrylic acid molecules approach the surface. Second, because of the limited mobility of grafted vinyl groups, the monomer would not propagate the reaction and terminate by coupling with a near macroradical. The polymerization of acrylic acid initiated by UVinduced decomposition of AIBN should follow a mechanism similar to that of the thermally induced decomposition of benzoyl peroxide. However, SEM images (Figure 3) revealed spotlike or brush morphologies of the grafted polymer initiated by temperature or UV light, respectively. Additionally, the SEM images suggest that the aluminum surface was not completely covered by the PAA film on either thermally induced or UV light-induced samples. A similar procedure was used by Namkanisorn et al.26 to graft polystyrene onto aluminum foil. They reported the formation of a strong polymer-aluminum joint by studying the fracture of the interface. However, the authors added the silane directly to the monomer solution instead of creating a thin silane layer prior to the polymerization reaction and found that their method offered better reproducibility than the other. In the case of the polymerization of acrylic acid, the direct mixture of the monomer with the silane should be avoided to prevent possible esterification reactions.27,28 The formation of the polysiloxane layer in a step prior to polymerization is an adequate methodology for PAA as demon(26) Namkanisorn, A.; Ghatak, A.; Chaudhury, M. K.; Berry, D. H. J. Adhes. Sci. Technol. 2001, 15, 1725. (27) Sumrell, G.; Ham, G. E. J. Am. Chem. Soc. 1956, 78, 5573. (28) Ansell, R. J.; Barrett, S. A.; Meegan, J. E.; Warriner, S. L. Chem.;Eur. J. 2007, 13, 4654.
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Figure 3. SEM micrographs of PAA grafted on Al-V (nonpolished surface) by (A and B) thermal decomposition of benzoyl peroxide (Al/PAA/BP) and (C and D) UV light-induced decomposition of AIBN (Al/PAA/AIBN). Arrows point to the polymer grafted onto the Al surface.
strated here. However, the radical polymerization on the silanized surface seems not to be suitable for obtaining a polymer film. In contrast, the chemical vapor deposition (CVD) technique resulted in the deposition of a PAA film on the substrate. Acrylic acid, used as a precursor of the PAA film, was volatilized through a vacuum chamber and targeted onto the hot Al-V plate. After the sample had been cooled, it was washed in ethanol to remove the free monomer from the surface. A control experiment was conducted by targeting the acrylic acid flow into a nonsilanized surface. The obtained sample exhibited contact angles similar to those of the former substrate, indicating that no polymer was chemically bonded to the surface. The CVD polymerization on the silanized Al surface promoted the grafting and coating of aluminum with a 22 nm thick PAA film. This thickness was determined by ellipsometry measurements using the previously determined optical constants of the Al-V substrate (Figure 2) and a refractive index of 1.56 for PAA. The fit to the experimental data is significantly improved (a 15% decrease in the mean square error) when an intermediate layer between the substrate and the polymer consisting of a mixture of PAA (61.5) and Al (38.5) with a thickness of 2 nm is included. Note that in this case the model is as follows: Al-V/[Al(61.5%)-PAA(38.5%)/PAA. The optical constants of the mixed layer were determined using a Bruggeman effective medium model approximation. The fact that the interpretation of the ellipsometric measurements needed this intermediate layer in the analysis suggests that there has been a reaction at the interface and can also be consistent with the existence of a nanometric roughness of the Al substrate. SEM images of the PAA film deposited by CVD (Figure 4) showed a flat surface, where spots or branches do not emerge. Although this experiment was conducted on the mirror-like polished surface, the soft appearance suggests the recovering of the surface following the superficial morphology of the substrate. Additionally, after every SEM image capture, a square burned area appeared (image not shown), which indirectly indicates the existence of an organic superficial layer. Height AFM images of this sample (Al/PAA/CVD) recorded in the tapping mode (Figure 5) revealed a morphology similar to that obtained in the SEM micrographs. The height image in the derivative mode DOI: 10.1021/la900518s
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Figure 4. SEM micrographs of PAA grafted onto Al-V by CVD (Al/PAA/CVD). A mirror-like polished surface was used in this analysis.
Figure 5. Height AFM image (top left) in tapping mode of Al/PAA/CVD and its derivative image (top right). Depth profile (bottom left) across the green line and roughness histogram (bottom right) from the height AFM image. The mirror-like polished surface was used in this analysis.
helps in the visualization of the topography. The softer surface shown in these images compared with the Al-OH roughness (Figure 1) seems to indicate the superficial polymer covering. However, the depth profile and histogram of the height AFM image (Figure 5) indicate a persistent roughness in the surface (rms = 32 nm). The existence of hills from few nanometers to hundreds of nanometers in height indicates that the polymer film follows the superficial morphology of the substrate and cannot fill the recessed areas of the substrate. This superficial roughness of the polymer-covered aluminum substrate will certainly affect the spectroscopic detection of the polymer. Some commentaries are addressed below. To determine the nature of superficial atoms, chemical characterization of the modified Al surfaces was conducted via XPS. 9098 DOI: 10.1021/la900518s
C, Al, and O were detected in all samples, and Si was detected in those which were modified with allyltrimethoxysilane (Tables 1 and 2). Carbon contamination was detected in pristine aluminum (p-Al) and in activated aluminum (Al-OH) substrates, as previously reported.8 Therefore, the carbon signal coming from that contamination is probably superposed in “silanized” and polymer-covered surfaces to the carbon peaks arising from chemically bound molecules. The C 1s peak was fitted in p-Al, Al-OH, Al-V, and Al/PAA samples to two components according to the peak assignment used by Hiura et al.29 (Figure 6). The most intense peak at 284.9 eV is assigned to C-C and C-H bonds, and the less
(29) Hiura, H.; Ebbesen, T. W.; Tanigaki, K. Adv. Mater. 1995, 7, 275.
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Table 1. Binding Energies (electronvolts) of Core Electrons of Aluminum-Functionalized Substrates binding energy (eV) (relative intensity) C 1s
Al 2p
p-Al
284.9 (80) 286.3 (20) Al-OH 284.9 (78) 286.2 (22) Al-V 284.9 (82) 286.3 (18) a 284.9 (67) Al/PAA/BP 286.3 (21) 288.6 (12) 284.9 (74) Al/PAA/AIBNa 286.3 (20) 288.7 (6) Al/PAA/CVD 284.9 (65) 286.5 (23) 288.8 (12) a Non polishedsurfaces.
Si 2p
72.2 (28) 74.6 (72) 72.3 (10) 74.7 (90) 72.2 (20) 74.6 (80) 74.9 74.3 72.2 (20) 74.6 (80)
O 1s
-
532.1
-
532.3
102.2
532.3
102.3 (66) 103.4 (34)
532.3
102.0 (86) 103.4 (14)
532.3
102.3
532.2
Table 2. Surface Atomic Composition of Al Substrates Determined by XPS C (atom %) Al (atom %) Si (atom %) O (atom %) p-Al 2.7 Al-OH 3.1 Al-V 37.4 62.3 Al/PAA/BPa Al/PAA/AIBNa 66.3 Al/PAA/CVD 59.8 a Nonpolished surfaces.
38.1 36.6 23.0 3.9 2.7 3.2
9.5 8.9 8.9 7.4
59.2 60.3 30.1 24.9 22.1 29.6
intense one at 286.3 eV has been often assigned to C-OH.29,30 Polymer-modified surfaces (Al/PAA) exhibited a third peak at 288.6-288.8 eV, which arises from CdO groups of PAA.30,31 As evidence, the increase in the number of superficial carbon atoms in the polymer-modified surfaces also suggests the presence of the polymer. It is worth noticing that it is not possible to compare the carbon areas of the polymer-modified surfaces since the aluminum substrates have different roughnesses. The aluminum signal is observed in all XPS spectra. Al 2p spectra have been fitted to two components: the first component at 72.2-72.3 eV corresponds to metallic Al, and the second one at 74.3-74.7 eV is associated with Al oxide/hydroxide.32 Those samples partially covered with the polymer suffered an oxidation by exposure to air, leading to an oxide layer thicker than 2 nm. Thus, the corresponding signal to metallic aluminum substrate in Al/PAA/BP and Al/PAA/AIBN samples was not detected by XPS. The observation of an Al 2p signal of the substrate covered by a very thin layer of the polymer (Al/PAA/CVD) can be interpreted in terms of the presence of some fissures in the film and/or some residual roughness of the substrate surface. As the XPS signal is attenuated exponentially with the thickness, no signal could be recorded for a uniformly developed film with a thickness somewhat above 3 nm. Therefore, the only reason aluminum would be detected in the polymer-covered surface is that some Al atoms are uncovered as a consequence of the irregular surface topography or even that some nanocracks developed on the grown film, thus making feasible photoelectron emission from the naked substrate. (30) Boehm, H. P. Carbon 2002, 40, 145. (31) Martı´ nez, M. T.; Callejas, M. A.; Benito, A. M.; Cochet, M.; Seeger, T.; Anson, A.; Schreiber, J.; Gordon, C.; Marhic, C.; Chauvet, O.; Fierro, J. L. G.; Maser, W. K. Carbon 2003, 41, 2247. (32) Franquet, A.; Biesemans, M.; Terryn, H.; Willem, R.; Vereecken, J. Surf. Interface Anal. 2006, 38, 172.
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Figure 6. C 1s core level spectra of p-Al, Al-OH, Al-V, and Al/ PAA obtained by thermal decomposition of benzoyl peroxide (Al/ PAA/BP), UV light-induced decomposition of AIBN (Al/PAA/ AIBN), and the CVD process (Al/PAA/CVD).
Figure 7. FTIR spectra of the aluminum surface grafted with PAA (Al/PAA/BP, Al/PAA/AIBN, and Al/PAA/CVD) and PAA deposited by solvent casting on an aluminum substrate.
The peak appearing in the Si 2p region can be attributed to the adsorption of organosilane on the substrate. Thus, the Si 2p signal at a binding energy of 102.2-102.3 eV detected in the silanized surface (Al-V) and in samples subjected to polymer grafting arises from metal-O-Si-R linkages.33 A small component placed at 103.4 eV in Al/PAA/BP and Al/PAA/AIBN samples indicates the presence of a certain proportion of O-Si-O linkages resulting from both branched species like RSi(OMe)2-O-SiR(OMe)O-Si(OMe)R-. The presence of Me-O-Si-R structures in all (33) Pleul, D.; Frenzel, R.; Eschner, M.; Simon, F. Anal. Bioanal. Chem. 2003, 375, 1276.
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Barroso-Bujans et al. Table 3. Contact Angle and Surface Tension Components of Nonmodified and Modified Aluminum Surfaces (polished substrates) contact angle (deg)
target p-Al Al-OH Al-V Al/PAA/CVD Al/PAA-Naþ/CVD
water
ethylene glycol
diiodomethane
(mJ/m2) γLW S
2 γS (mJ/m )
2 γþ S (mJ/m )
72.2 82.2 80.0 94.3 90.3
66.0 65.5 68.2 75.8 71.9
45.3 47.1 47.1 51.8 52.4
36.8 35.8 35.8 33.2 32.9
20. 8 9.0 12.6 3. 6 5.0
0 0 0 0 0
polymer-grafted surfaces by different methods indicates that the siloxane layer is preserved during polymer grafting. FTIR spectra in reflectance mode are very useful for characterizing thin superficial layers. In spite of the low signal intensity encountered in PAA-modified surfaces (Figure 7), the presence of a signal at 1727 cm-1 (CdO stretching vibration) in Al/PAA/BP, Al/PAA/AIBN, and Al/PAA/CVD samples suggests the existence of PAA carboxyl groups. It is worth noting that surface roughness is one of the drawbacks in using grazing angles since it not only increases the spectral noise but may result in inadvertent sampling of a surrounded area. Therefore, roughness probably caused the low signal-to-noise ratio in these samples, the ratio being higher in the uncompleted covered substrates because of both concentration heterogeneities and greater roughness. The spectrum of PAA deposited on the Al substrate by solvent casting was added for the sake of comparison. The strong absorption at 1649 cm-1 belonging to the H-O-H deformation mode of physically adsorbed molecular water34 was also detected with the Al/ PAA/CVD sample. The contact angle measurements (Table 3) clearly show that the surface properties change drastically. Note that there is an important contribution of the roughness to the water contact angle, evidenced in the high contact angle value encountered for the nonpolished aluminum substrate (112.9°). Thus, the reaction of the aluminum with sodium hydroxide caused an unexpected increase in the water contact angle, and this result can be attributed to the evolution of roughness after treatment (see Figure 1). Further treatments to this surface caused some variations in contact angles, but they were kept higher than expected. “Silanization” slightly reduced the water contact angle of the AlOH substrate, but the polymer grafting surprisingly increased that value, indicating a more hydrophobic surface. The hydrophobicity of the polymer-covered surface could be the result of both surface roughness35 and oxygen loss by heating. It is also expected that an increase in the PAA thickness decreases the water contact angle,36 which should be confirmed in further experiments. The neutralization of PAA with 0.01 M NaOH (Al/PAA-Naþ/CVD) reduced the water contact angle value, as expected, due to the formation of ionic COO- groups. However, although the encountered values of water contact angles are still high because of (34) Al-Abadleh, H. A.; Grassian, V. H. Langmuir 2003, 19, 341. (35) Hsieh, M. C.; Youngblood, J. P.; Chen, W.; McCarthy, T. J. In Polymer Surface Modification: Relevance to Adhesion; Mittal, K. L., Ed.; VSP: Leiden, The Netherlands, 2000; Vol. 2, p 77. (36) Chang, Y. Ch.; Li, J.; Chen, X. U.S. Patent 6,861,103.
9100 DOI: 10.1021/la900518s
surface irregularities, this result does not exclude the formation of a thin polymer layer as evidenced by ellipsometry. The three-liquid acid-base method provides information about the basic and acidic nature of the modified surfaces in þ terms of γþ S and γS values (Table 3). The γS and γS values suggest þ that all surfaces are essentially basic, as their γS components are close to zero. Thus, grafting of the aluminum surface with PAA led to a decrease in the surface energy and Lewis base component (the tendency to donate a pair of electrons). The ionic form of the polymer, instead, showed a small increase in the Lewis base component compared with the acidic polymer.
4. Conclusions Grafting of poly(acrylic acid) onto an aluminum surface was developed following two different methodologies, one by in situ polymerization (using benzoyl peroxide and AIBN initiators) and the other by chemical vapor deposition. The first processes produce spotlike and brush morphologies of the grafted polymer. However, a PAA layer with a thickness of 22 nm is obtained by the CVD process. The CVD methodology used in our experiments produces a polymer film covalently bonded to the surface through a polysiloxane interfacial layer. This layer contains vinyl functionalities that promote anchorage to the PAA layer. This process should not involve monomer fragmentation in the monomer flux, as in the plasma-enhanced CVD polymerization,37,38 and all the chemical reactions should occur in the hot substrate during collisions. Thermal CVD polymerization also seems an advantageous method for producing linear chains with retained pendant functionalities as reported by Mao et al.39 However, a profound study of the mechanism of thermal CVD polymerization of acrylic acid should still be conducted. To the best of our knowledge, there is no report about the thermal polymerization of acrylic acid by CVD. Therefore, this study could help in the development of the simplest methodologies for obtaining thin polymer films on metallic substrates. Acknowledgment. We thank the “Europ€aisches Graduiertenkolleg” GRK 532, financed by the Deutsche Forschungsgemeinschaft (DFG), Bonn, Germany, for the provision of a postdoctoral grant to F.B.-B. (37) O’Toole, L.; Beck, A. J.; Short, R. D. Macromolecules 1996, 29, 5172. (38) O’Toole, L.; Beck, A. J.; Ameen, A. P.; Jones, F. R.; Short, R. D. J. Chem. Soc., Faraday Trans. 1995, 91, 3907. (39) Mao, Y.; Gleason, K. K. Langmuir 2004, 20, 2484.
Langmuir 2009, 25(16), 9094–9100
