Attachment of Polymers to Organic Moieties Covalently Bonded to Iron

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Attachment of Polymers to Organic Moieties Covalently Bonded to Iron Surfaces Alain Adenier,§ Eva Cabet-Deliry,† Thierry Lalot,‡ Jean Pinson,*,† and Fetah Podvorica† Laboratoire d’Electrochimie Mole´ culaire, Universite´ Paris, 7 Denis Diderot, Unite´ Mixte Universite´ Paris 7 CNRS 7591, 2 Place Jussieu, 75251, Paris Cedex 05, France; Laboratoire de Synthe` se Macromole´ culaire, Universite´ Paris, 6 Pierre et Marie Curie, Unite´ Mixte Universite´ Paris 6 CNRS 7610, 4 Place Jussieu, 75252, Paris Cedex 05, France; and ITODYS, Universite´ Paris, 7 Denis Diderot, associe´ au CNRS (UPRESA 7086), 1 rue Guy de la Brosse F 75005 Paris, France Received February 12, 2002. Revised Manuscript Received July 29, 2002

This paper describes two different methods, based on the electrochemical reduction of diazonium salts, for the strong binding of polymers to an iron surface. The first method involves the electrochemical reduction, on an iron electrode, of the diazonium salt of 4-aminobenzophenone. This permits the covalent attachment of 4-benzoylphenyl moieties on the surface. Polystyrene (PS) is then deposited on the modified iron surface and can be covalently bonded to the 4-benzoylphenyl groups under irradiation. Another method involves the attachment of carboxyphenyl groups to the iron surface by reduction of the diazonium salt of 4-aminobenzoic acid and the further attachment of poly(1,2-propanediyl fumarate) to the phenylcarboxylate functions through ionic bonds with Mg2+ ions. The barrier properties of the polymeric films are characterized by electrochemistry and by the protection they provide against corrosion.

Introduction Organic coatings of metallic substrates, particularly steel, are important industrial processes.1 These processes involve the deposition of a polymer on the metal sheet, either from a liquid (roll-coating), from a solid polymeric sheet (roll-bonded cladding), or from a powder (cataphorese). A large variety of polymers are used for this purpose, and they provide protection against corrosion to the metal and add to the aesthetic aspect of the finished material. In these processes, only weak bonds are formed between the metal and the organic coating. There are generally two ways of getting stronger bonds between the metal and the organic coating: in the first one chemical bonds are created between the oxide surface and the organic group, whereas in the second one bonds are formed between the metal itself and the organic molecule. In this paper we will describe an electrochemical method which permits, through the reduction of diazonium salts, attachment of polymer to a steel surface with covalent metal-carbon bonds. An important group of coupling agents is that of the silanes;2,3 the silanols groups react with the OH func* To whom correspondence should be addressed. Phone: +33-14427-2801. Fax: +33-1-4427-7625. E-mail: [email protected]. † Laboratoire d’Electrochimie Mole ´ culaire. ‡ Laboratoire de Synthe ` se Macromolećulaire. § ITODYS. (1) Plankaert, R. Surface Coating. In Ullmann’s Encyclopedia of Industrial Chemistry, Vol A25, 5th ed.; VCH: Weinheim, Germany, 1994; p 170. (2) Ro¨sch, L.; Peter, J.; Reitmeier, R. Silicon Compounds. In Ullmann’s Encyclopedia of Industrial Chemistry, Vol A24, 5th ed.; VCH: Weinheim, Germany, 1994; p 21. (3) Ogarev, V. A.; Selector, S. L. Prog. Org. Coat. 1992, 31, 135.

tions present on the oxidized metal surface. Bonding is therefore to metal oxides and not to the metal itself. This is, for example, the case of plasma vapor deposition of organic thin films.4,5 In a hexamethyldisiloxane/O2 plasma polymers are formed which are connected to the surface by Fe-O-Si bonds.6 Other methods have been published which permit the electrochemical attachment (through covalent bonds) of polymers directly on metallic surfaces. A reduction process leading to the covalent attachment of a polymer to an iron surface had been developed as early as 1982 by Lećayon and co-workers at the Commissariat a` l’Energie Atomique (CEA, France).7-18 The process developed by the CEA involves the electrochemical reduction of an activated acrylic monomer on a metal (nickel, iron, etc.) surface. The radical anion obtained under anhydrous conditions is responsible for the reaction with the metal and for the propagation of the polymerization. A thin layer (2-10 µm)14 of polymer is covalently attached to the surface; this film as well as its mechanism of formation have been thoroughly investigated.7-22 At the same time, a thicker layer of polymer is only deposited on the surface, and it can be removed by rinsing. Interestingly, it was also found14 that cleavage of the metal polymer bond can be achieved at more negative potential. Polymeric layers bonded to (4) Boenig, H. V. Plasma Chemical Vapor of Organic Thin Films. In Advances in Low-Temperature Plasma Chemistry, Technology, Applications; Boenig, H. V., Ed.; Technomic Publishing Company, Lancaster, PA, 1984; p183. (5) d’Agostino, R. Plasma Processing of Polymers; Academic Press: New York, 1997. (6) Vautrin-Ul, C.; Boisse-Laporte, C.; Benissad, N.; Chausse´, A.; Leprince, P.; Messina, R. Prog. Org. Coat. 2000, 38, 9.

10.1021/cm0211397 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/08/2002

Covalent Bonding of Polymers to Iron Surfaces

platinum electrodes can also be obtained by electrochemical oxidation of N-vinyl-2-pyrrolidone.23,24 Electrooxidation of diamines such as ethylenediamine on a metallic (platinum, gold, or aluminum) electrode furnishes a polyethyleneimine coating.25,26 In this case, it is likely that a carbon-metal bond is formed but no proof has been obtained so far. Other methods lead to the formation of a polymeric layer on metals but without covalent attachment. Electrooxidation of phenols leads to the deposition of polyphenol27 on Pt electrodes. Deposition of conducting polymers28 (such as polypyrrole29-33) on engineering metals (iron)34-38 by electrooxidation also has been achieved under carefully designed conditions, as the monomer must be oxidized faster than the metallic substrate. (7) Lećayon, G.; Bouizem, Y.; Le Gressus, C.; Reynaud, C.; Boiziau, C.; Juret, C. Chem Phys. Lett. 1982, 91, 506-510. (8) Deniau, G.; Lećayon, G.; Viel, P.; Hennico, G.; Delhalle, J. Langmuir 1992, 8, 267. (9) Viel, P.; de Cayeux, S.; Lećayon, G. Surf. Interface Anal. 1993, 20, 468. (10) Bureau, C.; Defranscheschi, M.; Delhalle, J.; Deniau, G.; Tanguy, J.; Lećayon, G. Surf. Sci. 1994, 311, 349. (11) Tanguy, J.; Viel, P.; Deniau, G.; Lećayon, G. Electrochim. Acta 1993, 38, 175. (12) Tanguy, J.; Deniau, G.; Auge, C.; Zalczer, G.; Lećayon, G. J. Electroanal. Chem. 1994, 377, 115. (13) Tanguy, J.; Deniau, G.; Zalczer, G.; Lećayon, G. J. Electroanal. Chem. 1996, 417, 175. (14) Deniau, G.; Lecayon, G.; Bureau, C.; Tanguy J. In Protective Coatings and Thin Films; Pauleau, Y., Barna, P. B., Eds.; Kluwer Academic: Amsterdam, The Netherlands, 1997; pp 265-278. (15) Bureau, C.; Deniau, G.; Viel, P.; Lećayon, G. Macromolecules 1997, 30, 333. (16) Deniau, G.; Thome, T.; Gaudin, D.; Bureau, C.; Lećayon, G. J. Electroanal. Chem. 1998, 451, 145. (17) Charlier, J.; Bureau, C.; Lećayon, G. J. Electroanal. Chem. 1999, 465, 200. (18) Viel, P.; Bureau, C.; Deniau, G.; Zalczer, G.; Lećayon, G. J. Electroanal. Chem. 1999, 470, 14. (19) Jerome, C.; Geskin, V.; Lazzaroni, R.; Bredas, J. L.; Thibault, A.; Calberg, C.; Bodart, I.; Mertens, M.; Martinot, L.; Rodrigue, D.; Riga, J.; Jeroˆme, R. Chem. Mater. 2001, 13, 1656. (20) Baute, N.; Martinot, L.; Jeroˆme, R. J. Electroanal. Chem. 1999, 472, 83 and references therein. (21) Mertens, M.; Calberg, C.; Baute, N.; Jeroˆme, R.; Martinot, L. J. Electroanal. Chem. 1998, 441, 237. (22) Calberg, C.; Mertens, M.; Jeroˆme R.; Arys, X.; Jonas, A. M.; Legras, R. Thin Film Solids 1997, 310, 148. (23) Doneux, C.; Caudano, R.; Delhalle, J.; Leonard-Stibbe, E.; Charlier, J.; Bureau, C.; Tanguy, G.; Lećayon, G. Langmuir 1997, 13, 4898. (24) Calberg, C.; Kroonen, D.; Mertyens, M.; Jeroˆme, R.; Martinot, L. Polymer 1998, 39, 23. (25) Herlem, G.; Goux, C.; Fahys, B.; Dominati, F.; Gonç alves, A.M.; Mathieu, C.; Sutter, E.; Trokourey, A.; Penneau, J. F. J. Electroanal. Chem. 1997, 435, 259. (26) Herlem, G.; Reybier, K.; Trokourey, A.; Fahys, B. J. Electrochem. Soc. 2000, 147, 597. (27) McCarley, R. L. J. Electrochem. Soc. 1990, 137, 218C. (28) Lu, W. K.; Basak, S.; Elsenbaumer, R. L. Corrosion Inhibition of Metals by Conductive Polymers. In Handbook of Conductive Polymers; Skotherm, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1988; Chapter 31, p 881. (29) Cheung, K. M.; Bloor, D.; Stevens, G. C. Polymer 1988, 29, 1709. (30) Troch-Nagels, Winand, R.; Weymeersch, A.; Renard, L. J. Appl. Electrochem. 1992, 22, 756. (31) Schirmeisen, M.; Beck, F. J. Appl. Electrochem. 1989, 19, 401. (32) Beck, F.; Michaelis, R.; Scholten, F.; Zinger, B. Electrochim. Acta 1994, 39, 229. (33) Otero, T. F.; Angulo, R. J. Appl. Electrochem. 1992, 22, 369. (34) Ferreira, C. A.; Aeiyach, S.; Delamar, M.; Lacaze, P. C. J. Electroanal. Chem. 1990, 284, 351. (35) Ferreira, C. A.; Aeiyach, S.; Aaron, J. J.; Lacaze, P. C. Electrochim. Acta 1996, 41, 1801. (36) Krstajic´, B. N.; Grgur, B. N.; Jovanovic´, S. M.; Vojnovic´, M. V. Electrochim. Acta 1997, 42, 1685. (37) Su, W.; Iroh, J. O. Electrochim. Acta 1997, 42, 2685. (38) Fraoua, K.; Aeiyach, S.; Aubard, J.; Delamar, M.; Lacaze, P. C.; Ferreira, C. A. J. Adhes. Sci. Technol. 1999, 13, 517.

Chem. Mater., Vol. 14, No. 11, 2002 4577 Scheme 1

However, it was possible to obtain in this way a protection of iron similar to that provided by classical industrial treatments of steel. A large amount of work has been devoted to the deposition of self-assembled monolayers (SAMs) for the protection of metals;39 for example, n-dodecanethiol on polycrystalline nickel.40 These monolayers have also been used for the further binding of polymers: with polybithiophene films on titanium electrodes adhesion of the polymer was weak, but could be improved through a pretreatment with phenylalkanethiols;41 ω-(N-pyrrolyl)alkanethiols42,43 could be adsorbed on the gold surface through the thiol group and the pyrrolyl groups could then be polymerized elctrochemically, similar reactions were achieved with aniline attached to a hexanethiol group.44,45 Undec-10-ene-1-thiol could be adsorbed on a gold surface through the SH group and the terminal double bonds could be polymerized by γ-ray irradiation.46 However, there are many obstacles for the practical utilization of these SAMs; for example, thiol compounds bind to coinage or noble metals, but their affinity to engineering metals such as steel is very limited.47 An interesting and very efficient way of attaching polymers to iron surfaces has been described by Van Alsten:48 self-assembled monolayers of alkyl-R,ωbisphosphonic acids on common engineering metals could be prepared, it was then possible to establish ionic bonds between the free phosphonate end, Zn2+, and a carboxylic-terminated tetrafloroethylene. This method has permitted the construction of metal/SAM/polymer assemblies of surprising durability which provided an efficient protection of iron against corrosion. As shown recently, it is possible to attach organic groups to an iron surface by electrochemical reduction of aromatic diazonium salts49,50 (Scheme 1). The organic groups are strongly bonded as they resist prolonged rinsing in an ultrasonic bath in various solvents. They can be removed either by mechanical polishing or by corrosion of the iron surface in acidic medium. The grafting of these groups has been demonstrated47 by cyclic voltammetry, capacity measurements, IRRAS or PMIRRAS, RBS, and XPS (which permits the observation of the metal-carbon bond). As diazonium salts are (39) Rohwerder, M.; Stratmann, M. MRS Bull. 1999, 24, 43. (40) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle Langmuir 1997, 13, 2285. (41) Mekhalif, Z.; Delhalle, J.; Lang, P.; Garnier, F. Synth. Met. 1998, 96, 165. (42) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (43) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759. (44) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 1983. (45) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 1993. (46) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113. (47) Nozawa, K.; Nishihara, H.; Aramaki, K. Corros. Sci. 1997, 39, 1625. (48) Van Alsten, J. G. Langmuir 1999, 15, 7605-7614. (49) Adenier, A.; Bernard, M. C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541-4549. (50) Chausse´, A.; Chehimi, M. M.; Karsi, N.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2002, 14, 392.

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easily prepared from aromatic amines, many of which are commercially available, this method provides a convenient way for the modification of iron surfaces by organic groups bearing a large variety of substituents. The mechanism of this attachment reaction involves the aryl radicals obtained upon reduction of the diazonium salt. One of the advantages ensuing from the radical character of this reaction is that it does not require specially dry conditions. Another advantage is that it can be performed without exclusion of oxygen, as the electrochemical reduction of diazonium salts takes place at potentials close to 0 V/SCE, i.e., at potentials positive to the reduction of oxygen; however, in the case of iron, reactions are performed in absence of oxygen to prevent oxidation of the surface during the experiments. These covalently bonded layers have been shown to provide some protection against corrosion.50 However, because this protection is insufficient for practical purposes, we have tried to attach polymers to the surface-bonded aryl groups. In addition to protection of the surface, many applications can be thought of for surface-attached polymers in the biomedical field, in the field of adhesion or lubrication, and in the field of sensors. In this paper we will describe two examples showing that it is possible to use these functionalized aryl groups attached to the iron surface to further bind polymers. Experimental Section Acetonitrile was from Merck (Uvasol) and used without any further purification, H2S04 was from Prolabo (Titrinorm), and tetrabutylammonium tetrafluoroborate was from Fluka. The synthesis of 4-carboxy benzenediazonium59 and 4-benzoyl benzenediazonium tetrafluoroborate59 [1H NMR (200 MHz, DMSO): δ (ppm) 8.85 and 8.23 (AA′BB′ spectrum, 4H, aromatic group bearing the diazonium), 7.5-7.8 (m, 5H, aromatics)] from commercial amines (Aldrich) has been described previously. Iron electrodes were made from either 1-mm diameter iron wire (99.99% Johnson-Matthey) imbedded in epoxy resin in a glass tube (used for cyclic voltammetry) or from 3-mm diameter mild steel disks in a Teflon holder. The disks were obtained from large mild steel plates kindly provided by Sollac society. Analysis was as follows: Fe, 95.68%; C, 0.31%; Mn, 2.03%; P, 0.05%; S, 0.13%; N, 0.56%; Si, 0.10%; Cu, 0.07%; Ni, 0.18%; Cr, 0.30%; Sn, 0.01%; and Al, 0.58%. Glassy carbon electrodes were prepared from 3-mm diameter carbon rods (Tokai) in the same way as the iron wire electrodes. Before use, all the electrodes were carefully polished with 1-µm diamond paste and carefully rinsed in an ultrasonic cleaner in deoxygenated acetone to prevent, as much as possible, the formation of oxides on the iron surface. The disks were then placed immediately in a solution of deoxygenated ACN, 2 mM solution of the diazonium salt, and 0.1 M NBu4BF4, and connected to a potentiostat (Versastat II from EGG) as the working electrode. The reference electrode was a saturated calomel and the counter electrode was a platinum wire. The potential was maintained for 5 min at a potential of 200 mV negative to the voltammetric peak of the diazoniums. The disk was then rinsed thoroughly for 10 min in an ultrasonic bath in acetone to remove any diazonium salt or electrolyte. Polarization resistance obtained, as usual, from the slope of the I ) f(E) curve at the rest potential by scanning at 0.1 mV/s with the same electrochemical equipment. The films of polyester were prepared from 600 mg of poly(1,2-propanediyl fumarate) and 130 mg Mg(acac)2 dissolved in 400 mg of THF, and 20m µL of this solution was deposited on the disk (eventually after addition of 180 mg of styrene and 6 mg of AIBN). The film thickness was estimated from the amount of polymer deposited, and thinner films were obtained by diluting the solution.

Adenier et al. Scheme 2

IRRAS spectra were recorded with a Magna-IR 860 IR-TF (Nicolet Instruments) equipped with a reflection accessory at 80° and a 13-mm diameter mask. To improve the signal-overnoise ratio, each spectrum was recorded from 512 acquisitions with a high sensitivity detector (spectral resolution 4 cm-1). The amount of carbonyl groups having reacted in FeBzPstyrene is estimated in the following way. In the FeBzPstyrene film one can measure ∆Tν(CdO)/∆Tν(803 cm-1) ) (99.975 - 99.8875)/ (99.975 - 99.65) ) 0.268, where ∆T represents the difference in transmittance units between the baseline and the peak. In the case of FeBz, one obtains ∆Tν(CdO)/∆Tν(803 cm-1) ) (0.11)/ (0.275) ) 0.40. From these data it is possible to estimate the percentage of carbonyl groups which have reacted as follows: % CdO ) 100 [(1 - 0.268)/0.4] ) 33% in good agreement with the value obtained by cyclic voltammetry. AFM images were obtained with a Nanoscope III (Digital Instruments) in tapping mode with a silicon nitride cantilever at a resonance frequency of 204 ( 10 kHz (Olympus Optical Co) at a scan rate of 12 Hz. The average surface roughness, expressed in nm, was calculated using the manufacturer’s software according to

Ra )

1 LxLy

∫ ∫ Lx

0

Ly

0

|f(x,y)| dx dy

where Lx and Ly are the dimensions of the surface and f(x,y) is the surface relative to a calculated flat plane based upon the surface data that has equal volume above and below the plane.

Results Attachment of Polystyrene to an Iron Surface. The reaction (as evidenced by the experiments described below) is shown in Scheme 2. The first step consists of grafting a benzophenone group on the iron surface which is prepared to be as free as possible from surface oxides. This is done by electrochemical reduction on the iron electrode of 4-benzoylbenzene diazonium tetrafluoroborate. Its cyclic voltammogram (iron electrode, acetonitrile (ACN) + 0.1 M NBu4BF4) is presented in Figure 1b (Figure 1a is the background current), which shows a broad irreversible cathodic wave located at Ep ) -0.15 V/SCE. The height of this wave decreases to nearly nothing on the second scan which is the usual signal of the grafting of the aryl radical49,50 produced during this reduction. After electrolysis, the electrode was rinsed in acetone in an ultrasonic bath to remove any unreacted diazonium salt and transferred into a deoxygenated solution containing only ACN and 0.1 M NBu4BF4. The voltammogram recorded in this medium


Chem. Mater., Vol. 14, No. 11, 2002 4579

Figure 1. Cyclic voltammetry of an iron electrode (d ) 1 mm) (a) in a solution of ACN and 0.1 M NBu4BF4, (b) after addition of 4-benzoyl benzenediazonium tetrafluoroborate (2 mM). Scan rate: v ) 0.2 V s-1, reference SCE.

presents a reduction wave at Ep ) -1.87 V/SCE located at the same potential as that of benzophenone on an iron electrode (Figure 2). This experiment clearly indicates that 4-benzoylphenyl groups have been attached to the electrode (FeBz) and could not be removed by vigorous rinsing. The attachment of this group was confirmed by infrared spectroscopy (Figure 3). The main characteristics of the spectra of benzophenone and FeBz are summarized in Table 1. Obvious similarities between these two spectra point to the presence of benzoylphenyl groups on the surface, particularly the presence of the carbonyl band, but also of in-plane CH bending vibrations. However, differences can be noticed between the two spectra. The aromatic CdC stretching vibrations become very weak in FeBz, but most interesting is the presence of the 762 cm-1 band in the reference spectrum of benzophenone, as it is typical of out-of-plane CH deformation of monosubstituted benzene and indicates the presence of five adjacent hydrogens. It is associated with a band at 698 cm-1 which indicates that two different substituents (hydrogen and benzoyl group) are positioned para to each other. These two bands become very small in the spectrum of FeBz and two new bands appear at 805 and 859 cm-1 indicative of a 1,4-disubstituted benzene ring. This is in agreement with multilayer 4-benzoylphenyl groups attached to the surface as shown in Scheme 1 and with the AFM measurements described below. In addition, the absence of any significant band in the 2300-2130 cm-1 range where the NtN stretching could be expected indicates that the reduction of the diazonium group has occurred. The second step of the process consists of attaching polystyrene chains to the grafted benzoylphenyl groups. We followed the photochemical reaction used previously51 by C. W. Frank for the attachment of polystyrene to silica in which benzophenone was first attached to silica, polystyrene was then deposited and bonded under irradiation, and the unattached polystyrene was then removed in a Soxhlet in toluene for 20 h. The attachment of the carbonyl of 4-benzoylphenyl groups to polystyrene follows a mechanism which has been previously investigated.52 Irradiation of benzophenone pro-

Figure 2. Cyclic voltammetry on an iron electrode (d ) 3 mm) (a) in a solution of ACN, 0.1 M NBu4BF4, and benzophenone (c ) 2 mM), (b) after modification with 4-benzoylphenyl groups in ACN and 0.1 M NBu4BF4. Scan rate: v ) 0.2 V s-1, reference SCE.

duces a singlet state which transforms into a triplet state of biradical character. This species abstracts a hydrogen atom from a methylene group of polystyrene to produce two radicals which recombine. Films a few nanometers thick have been obtained on silica by this process.51 Following this procedure, a thin layer (about 100 nm) of polystyrene was deposited on top of the modified iron surface. This was accomplished by spin coating (2000 rotations per minute) a solution of polystyrene (M ) 34 000, 10 mg/mL in toluene). Irradiation51,52 of the coated iron was performed at 340 nm for 70 min, and unattached polystyrene was removed by ultrasonication in toluene for 30 min. In a blank experiment it was ascertained that irradiation of polystyrene merely deposited on the surface does not modify this surface. We checked the attachment of polystyrene groups by cyclic voltammetry. Figure 4A shows, for comparison, the voltammogram of a carbon electrode modified by 4-benzoylphenyl-groups (CBz) and by 4-benzoylphenyl-polystyrene (CBzPstyrene) in a solution of (51) Prucker, O.; Naumann, C. A.; Ru¨he, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766-8770. (52) Dorma`n, G.; Prestwich, G. D. Biochemistry 1994, 33, 56615673.

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Figure 3. Infrared spectra of (a) benzoylphenyl groups attached to the iron surface FeBz and (b) of the benzophenone (Aldrich spectrum). Table 1. IR Spectra of Benzophenone and FeBz in cm-1 benzophenone

FeBz

1657 1596, 1574, 1443 1311, 1275 1200-1000 762, 698

1659 1317, 1278 1200-1000 859, 805

ACN and 0.1 M NBu4BF4. The height of the wave of 4-benzoylphenyl groups has decreased in height but not completely disappeared, which shows that the reaction between the benzoylphenyl group and polystyrene does indeed takes place but that not all of the carbonyl groups react under irradiation with the polystyrene chain (see below). Comparison of the height of the two waves (before and after attachment of polystyrene) indicates that about 40% of the carbonyl groups have reacted with polystyrene (only unreacted benzoyphenyl groups are electrochemically active). Similar voltammograms (Figure 4B) are obtained on iron; in this case, (FeBz and FeBzPstyrene) the decrease of the wave is about 30%, a value which compares favorably with that obtained for carbon. Attachment of polystyrene in FeBzPstyrene can also be demonstrated by vibrational spectroscopy (using a glass plate coated with iron by sputtering). We first recorded the spectrum of a ∼100nm polystyrene (M ) 34 000) film deposited on iron (FePstyrene). This spectrum and that of FeBzPstyrene are presented in Figures 5 and 6, and the main characteristics are summarized in Table 2. The spectrum of polystyrene deposited on iron is in good agreement with a reference spectrum (Aldrich) including a band at 1717 cm-1. The spectrum of FeBz-

CdO stretching aromatic ring stretching in plane CH bending aromatic CH rocking out-of-plane CH deformation for monosubstituted benzenes out-of-plane CH deformation for 1,4-disubstituted benzenes

Pstyrene presents a series of bands which signal the presence of polystyrene on the surface. (i) Two wide weak bands at 3053 and 2930 cm-1 are analogous to the methylenic and aromatic CH stretching vibrations of polystyrene (Figure 6). The band at 2930 cm-1 is particularly demonstrative of the attachment of polystyrene, as benzophenone does not present any absorption in this region. (ii) Several bands of FeBzPstyrene are also observed in the film of polystyrene: two bands at 1717 and 1603 cm-1; bands at 1493 and 1451 cm-1 which become weak in the spectrum of FeBzPstyrene, while the strong band at 1146 cm-1 becomes of medium intensity and shifts to 1140 cm-1. Three bands of particular interest are at (i) 1665 cm-1 assigned to unreacted carbonyl groups, (ii) 704 cm-1 assigned to outof-plane CH vibrations of monosubstituted benzene in substituted diphenylmethanol (-C(polystyrene)(OH)C6H5) groups and phenyl groups of polystyrene, and (iii) 803 cm-1 observed in the spectrum of attached benzoylphenyl groups independently of the reaction of the carbonyl groups. The intensity of this band in FeBz and FeBzPstyrene permits the evaluation of the amount (33%) of reacted carbonyl groups (Experimental Section), in good agreement with the electrochemical measurements.


Figure 4. Cyclic voltammetry on (A) a glassy carbon electrode (d ) 3 mm) and (B) an iron electrode (d ) 1 mm) in ACN and 0.1 M NBu4BF4: (a) untreated, (b) modified with 4-benzoylphenyl groups FeBz, and (c) modified with 4-benzoylphenyl groups and further treated with polystyrene FeBzPester (see text). Scan rate: v ) 0.2 V s-1, reference SCE.

We also examined by atomic force micropscopy (AFM) the surface of iron before and after modification. Figure 7a-c show the surface of an iron disk carefully polished with 1-µm diamond paste untreated (a), modified with benzoylphenyl groups FeBz (b), and after attachment of polystyrene FeBzPstyrene (c). The untreated iron surface presents a rather smooth surface (rugosity R ) 0. 90 nm) with the largest groves (due to diamond grains) approximately 4 nm deep. After grafting benzoylphenyl groups (FeBz) the image shows that the grafting is rather uniform; there are a few dark spots on the image which correspond to places of lower height which could be ungrafted. These dark ungrafted spots could correspond to places where small impurities were deposited during the grafting reaction and then removed during ultrasonic rinsing. The groves keep the same deepness. However there is an increase of the rugosity (R ) 1. 08 nm) and if one considers that the dark spots correspond to underivatzed places it is possible to obtain a mean value of 6.8 nm for the thickness of the organic layer. This points to a multilayer of benzoylphenyl groups as the height of a benzoylphenyl group standing

Chem. Mater., Vol. 14, No. 11, 2002 4581

up on the iron surface should be 0.95 nm (as measured from molecular models). An important change is obtained after attachment of polystyrene (and thorough ultrasonic rinsing in toluene); small white particles are observed which increase the rugosity up to R ) 3.31 nm. Observation of a section analysis shows that the height of the white particles is of the order of 10 nm. These images therefore confirm the attachment of a multilayer of benzoylphenyl groups and then of polystyrene on the surface of iron, it is indeed unlikely that the small white particles observed on the AFM images could be only deposited, as they would not have resisted ultrasonic rinsing in toluene. Then we characterized the compactness and the isolating properties of the films by the way in which they block the electron transfer and protect the surface of mild steel against corrosion. For this purpose we recorded the reversible voltammogram of phenazine, a compound which presents a fast electron transfer on carbon, and observed its modification from a bare iron surface to a modified surface. On a carefully polished iron electrode permanently maintained under argon, in ACN + 0.1 M NBu4BF4, phenazine presents a reversible voltammogram (E° ) -1.23 V/ECS) with ∆Ep ) 247 mV (at 0.2 V s-1). After attachment of benzoylphenyl groups (FeBz) the cathodic and anodic peaks are shifted far apart (∆Ep ) 422 mV at 0.2 V s-1). Little change is observed after attachment of polystyrene FeBzPstyrene. From ∆Ep it is possible to obtain the heterogeneous electron-transfer rate constant, k0 ) 9 × 10-4 cm s-1, for the clean electrode and, k0deriv ) 1.8 × 10-4 cm s-1, for the modified electrode. This value can be compared to the values measured by McCreery53 in H2O/ 40% methanol for substituted phenothiazines on carbon electrodes modified with organic layers formed by electrochemical reduction of diazonium salts. Assuming the formation of monolayers, k0 could be correlated with the thickness of the layer (d), a correlation which points to a tunneling mechanism [k0deriv ) k0 exp(-βd)] for the long-range electron transfer with a tunneling parameter β ) 0.22 Å-1, this value is in reasonable agreement with previously determined values for layers of similar structures. Comparing our values for k0 and k0deriv with d ) 67.8 Å (obtained from AFM measurements) we find β ) 0.024 Å. This value is approximately 10 times smaller than that obtained by McCreery53 but close to the value (0.013 Å-1) found by McDermott54,55 for 200-Å thick layers obtained by reduction of 4-N,N-diethylbenzene diazonium on carbon electrode. This last value, as well as that determined above on an iron electrode, clearly indicate that electron transfer pathways other than tunneling are available in such layers. The fact that attachment of polystyrene does not change significantly the rate of heterogeneous electron transfer is easily understood by looking at the AFM image which shows that polystyrene is very unevenly dispersed on the surface and that part of the surface is covered only with benzoylphenyl groups. We have shown previously50 that attachment of organic groups with hydrophobic substituents (long alkyl chains, C4, C12, C16; or fluoro substituents, F, CF3, (53) Yang, H. H.; McCreery, R. L. Anal. Chem. 1999, 71, 4081. (54) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (55) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947.

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Figure 5. Infrared spectra of (a) a polystyrene film (M ) 34 000) on an iron surface (deposited on glass) FePstyrene and (b) of the same surface with attached polystyrene FeBzPstyrene (see text).

Figure 6. Infrared spectra of (a) benzophenone (Aldrich spectrum), (b) a polystyrene film (M ) 34 000) on an iron surface FePstyrene, and (c) FeBzPstyrene.

OCF3, C6F13) reduced the corrosion of iron and mild steel. We examined in the same way the FeBz and FeBzPstyrene assemblies. Tafel plots were obtained in 0.1 N H2SO4 for Fe, FeBz, and FeBzPsty-

rene from which corrosion currents densities icorr were obtained; the values were respectively 570, 400, and 150 µA cm-2. The value on untreated iron is similar to what was previously obtained,50 but benzoylphenyl groups (FeBz) bring a lower protection than previously investigated groups such as dodecylphenyl: 100 µA cm-2. Further attachment of polystyrene (FeBzPstyrene) does not increase the protection very much. These results can be explained in the case of benzoylphenyl groups (FeBz) by the polar character of the attached molecule. In acidic solution, the carbonyl groups are protonated and serve as a relay for the corrosion of iron. Polystyrene of FeBzPstyrene is too unevenly spread on the surface to bring any further protection. From the above experiments it clearly appears that benzoylphenyl groups can be attached to the surface by reduction of the corresponding diazonium salt, and that the obtained layer (FeBz) is a multilayer as already observed by McDermott on HOPG54 and glassy carbon.55 These multilayers are most likely formed by reaction of electrogenerated aryl radicals with already attached benzoylphenyl groups through SH homolytic aromatic substitution.56-58 This multilayer is of uniform thickness; it is quite compact as it blocks the direct electron transfer of an organic molecule, but due to its polar character it does not block the access of protons (56) Williams, G. H. Homolytic Aromatic Substitution; Pergamon: New York, 1960. (57) Hey, D. H. Advances in Free Radical Chemistry; Williams, G. H., Ed.; Logos Academic: London, 1967; Vol II, pp 47-86. (58) Nonhebel, D. C.; Walton, J. C. Free Radical Chemistry; Cambridge University Press: Cambridge, 1974; pp 417-469.


Chem. Mater., Vol. 14, No. 11, 2002 4583

Table 2. IR Spectra of Deposited Polystyrene and of FeBzPstyrene in cm-1 FePstyrene

FeBzPstyrene

3022, 3057, 3080 2848, 2917 1717 1601, 1493, 1451

3022, 3053, 3080 2930 1708 1603 1665 1140 803 704

1146 704

aromatic CH stretching methylenic CH stretching overtone or combination aromatic ring stretching CdO stretching in-plane CH deformation vibration out-of-plane CH deformation of 1,4-disubtituted benzene out-of-plane CH deformation of monosubtituted benzene

to the surface. Further, photochemical attachment of polystyrene chains deposited by spin coating is possible, but it is too unevenly spread on the surface to provide any further barrier, either to electron transfer or to corrosion. Attachment of a Polyester to an Iron Surface. The principle of the construction is shown in Scheme 3. The iron surface is modified with carboxyphenyl groups through the electrochemical reduction of the diazonium salt of 4-aminobenzoic acid, and a mixture of polyester and magnesium ions is deposited mechanically on the surface. It is also possible (provided the polyester possesses unsaturated bonds) to cross-link the polymer by copolymerization with styrene as shown in Scheme 3 (the layer of phenyl carboxylic groups is represented as a monolayer for the sake of simplicity but is likely a multilayer as observed above in the Experimental Section). The attachment of 4-carboxyphenyl groups on the surface of both iron and carbon has been previously described.49,59 The voltammogram of 4-carboxyphenyl diazonium tetrafluoroborate presents a broad irreversible wave located at Ep ) -0.23 V/SCE. The charge passed through the electrode (1.50 µC/cm2) does not change significantly with the concentration of the diazonium (2.2, 5.0, and 9.0 mM) indicating a passivation of the electrode during the voltammogram at a scan rate of 0.2 V s-1. On the second scan the wave disappears and the charge which is transferred during the scan is only 0.57 mC/cm2 as a consequence of the passivation. The XPS spectrum49 of an underivatized mild steel plate presents C[1s], O[1s], and Fe[2p] peaks at 285, 510, and 710 eV. After derivatization by reduction of 4-carboxybenzene diazonium, the XPS spectrum presents49 a C[1s] peak at 289 eV, and an O[1s] signal fitted with two components at 531 and 532 eV assigned to the carbon and CdO and OH oxygens, respectively. The C[1s] spectrum was fitted with a C-metal component at low binding energy (283.8 eV) that has an intensity comparable to that of COOH. The IRRAS spectrum (Figure 8) of an iron plate modified with phenylcarboxylic groups presents the broad OH stretching band of a carboxylic acid at 3236 cm-1; the CdO band is merged with the aromatic CdC stretching in a broad band ranging from 1580 to 1720 cm-1; the other CdC band is observed at 1422 cm-1 and the C-H in-plane vibration in at 1124 cm-1; the OH out of plane deformation is observed at 950 cm-1 and the out-of-plane CH deformation is at 796 cm-1. The IR spectrum thus confirms the attachment of the phenylcarboxylic group to the iron surface. (59) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveánt, J. M. J. Am. Chem. Soc. 1997, 119, 201-207.

Figure 7. AFM images of (a) an untreated iron surface, (b) FeBz, and (c) FeBzPstyrene, and the corresponding section analysis.

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Scheme 3

Figure 8. Infrared spectrum of an iron disk modified with phenylcarboxylic groups. Table 3. Polarization Resistances Rp sample 200 µm thick

corrosive medium

time (h)

Rp (kΩ cm2)

FeCOOPester FeCOOPester FeCOOPester FeCOOPester steel + polymera steel + polymera steel + polymera

H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N

2 6 24 48 2 24 48

2.66 17.50 70.00 462.00 2.17 5.45 6.06

a

The interaction between carboxylate groups of polyester60-63 and metallic ions has already been observed through the increase of vicosity with time. This reaction,60-65 which is termed ripening, has been investigated with industrial polymers and magnesium oxide,60-63 but also in purely homogeneous medium (THF) with model polymers and soluble ripening agents such as magnesium acetylacetonate Mg(acac)2.65 The central step of the ripening mechanism consists of the formation and aggregation of magnesium carboxylates in ionic areas, with entanglement of attached polyester chains.65 (60) Alvey, F. B. J. Polym. Sci. 1971, 9, 2233 (61) Vansco-Smercsanyi, I.; Szilagyi, Y. J. Polym. Sci. 1974, 12, 2155. (62) Burns, R.; Ghandi, K. S.; Hankin, A. G.; Lunskey, B. M. Plast. Polym. 1975, 43, 228. (63) Judas, D.; Fradet, A.; Marećhal, E. Makromol. Chem. 1983, 184, 1129.

Mild steel disk with deposited poly(1,2-propanediyl phthalate).

We reproduced this ripening reaction by changing one of the carboxylic groups of the polyester by a carboxylic group previously attached to the iron surface. To neutralize the carboxylic functions we followed the procedure65 previously described using magnesium acetylacetonate Mg(acac)2 in THF. In a first experiment we prepared a mixture of poly(1,2-propanediyl phthalate) and Mg(acac)2 in THF and we deposited 20 µL of this solution on a previously modified 3-mm diameter mild steel (C 0.3%) disk creating a film 200-µm thick. The preparation (FeCOOPester) was maintained 3 days at 70 °C to effect the ripening. In a second series of experiments we used poly(1,2-propanediyl fumarate) and Mg(acac)2; by heating at 70 °C in the presence of styrene and a radical initiator (azobisisobutyronitrile, AIBN) the polymer undergoes cross-linking and ripening at the same time to give a hard film (FeCOOPesterStyrene) attached to the iron surface. To measure the barrier properties of FeCOOPester we have measured the polarization resistances. As a matter of comparison an untreated mild steel disk presents a polarization resistance of about 66 Ω cm2. It is observed that if the disk is modified with 4-carboxyphenyl groups, there is no increase of the polarization resistance. But as shown in Table 3 the 200-µm film of FeCOOPester increases the polarization resistance Rp to 2.66 kΩ cm2 after preparation of the assembly and up to 462 kΩ cm2 after 48 h (for the moment we have no explanation of this variation with time). The final Rp is much higher in FeCOOPester than when the (64) Judas, D.; Fradet, A.; Marećhal, E. Makromol. Chem. 1984, 185, 2583. (65) Judas, A.; Fradet, A.; Marećhal, E. J. Polym. Sci. 1984, 22, 3309.


Chem. Mater., Vol. 14, No. 11, 2002 4585

Table 4. Polarization Resistances Rp sample 1-3 µm thick

volume deposited/µLa

corrosive medium

Rp (kΩ cm2)

FeCOOPesterStyrene steel + polymera FeCOOPesterStyrene steel + polymera FeCOOPesterStyrene steel + polymera

10 10 20 20 30 30

H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N H2SO4 0.1 N

0.42 0.32 0.43 0.36 0.85 0.68

a Poly (1,2-propanediyl fumarate) + Mg(acac) + styrene + 2 AIBN in THF.

polymer is merely deposited on the steel disk (steel plus polymer). This experiment clearly shows the efficiency of the process: the protection is much higher when the steel surface is pretreated with the diazonium salts than when the polymer is merely deposited on the surface. The protecting effect of the treatment was also observed when the potential of FeCOOPester was swept between -1 and 0.2 V/SCE in 4 N sulfuric acid: there was nearly no increase of the current, whereas in the same medium, on a bare steel disk, the current would be hundreds of times larger. We prepared thin (∼1-3 µm) films of FeCOOPesterStyrene which were left for 72 h at 70 °C. Table 4 shows the resistance polarizations obtained with these films. The resistance of these thin films is, again, larger when the steel has been treated by the process of this paper than if the polymer had only been deposited on the surface, thereby indicating that for similar thickness the resistance is higher when the film is bonded to the surface. This is likely related to a better uniformity of the film and a decrease of the number of defects induced by the organic precoating. To obtain even higher resistances and protection, one should increase the thickness of the film, but ∼200-µm films are so resistant that electrochemical measurements become impossible, even in 5 M sulfuric acid or 6 M hydrochloric acid. In addition, these films are strongly adherent to the surface. If one tries to insert the sharp blade of a knife or a cutter between the film and the metal they cannot be separated. Discussion and Conclusion We had previously demonstrated the possibility of covalently attaching organic groups to the surface of iron. These groups can be used in turn to attach polymers. We have described, as examples, two different methods: the photochemical covalent attachment of styrene and the ionic attachment of polyesters to carboxylic groups. The layer of benzoylphenyl groups is 6.8 nm thick which represents six to seven layers of molecules. In addition, it is likely that only the carbonyl groups on top of the layer react with polystyrene; one should therefore find that, at most, 16% of the carbonyl groups have reacted under photochemical irradiation. As one finds consistently by IR spectroscopy and cyclic voltammetry that 30-40% of the carbonyl groups have reacted, it is likely that some (about 40-16%) have reacted by hydrogen transfer59 with the solvent. We have shown that this first process provides a multilayer of benzoylphenyl groups which efficiently blocks the electron transfer from the metal surface, but does not

block efficiently the transfer of protons to the surface because of the polar character of the carbonyl group. The two tests which have been performed indicate that, despite the relatively high molecular weight, the polystyrene film is not compact enough to bring additional blocking of the electron transfer or protection of the iron surface. To obtain a better protection of the electrode one should obtain a more uniform layer of polymer on top of the benzoylphenyl groups. The layer of polystyrene spin-coated directly on iron appears rough but rather uniform by AFM; the fact that, after grafting, polystyrene appears as a dispersion of small particles is likely related to the fact that most of the chains do not attach to the benzoylphenyl groups either for steric reasons or because the photochemical reaction which must take place through the layer of polystyrene is not efficient enough. The reactive volume of benzophenone52 was approximated by a sphere with a radius of 3.1 Å centered on the oxygen radical, and a hydrogen of polystyrene should be inside this sphere to be abstracted. A way to obtain thicker and more protective layers would be to use, in place of styrene, a polymer which could undergo further polymerization and crosslinking. Besides, this method could be generalized to attach molecules which could provide specific properties to the surface in the biomedical or in the adhesion or lubrication field. Whereas the first method leads to the covalent attachment of polymers to the primary layer, the second method provides an example of the attachment of the polymer through ionic bonds, with the negatively charged surface being attached to the negatively charged polyester through the intermediate magnesium ions. The thickness of the film has been varied from a few µm to about 200 µm. The success of this strategy is demonstrated by the difference observed between the polarization resistances measured (i) on a polymer film only deposited on the surface, and (ii) the same polymer attached to the pretreated surface. This second method provides a good protection of the iron surface against corrosion if the polyester film which is deposited is thick enough. Films of cross-linked polyester that are ∼200µm thick are completely isolating and do not permit any electrochemical measurement. Besides, this film is strongly attached to the surface. This method could be generalized, for example, to the attachment of charged (opposite to the charge of the pretreated surface) polymers to obtained assemblies of multilayered films.66 This paper describes two examples for the attachment of polymer to prefunctionallized metallic surfaces, but many other ways of attaching polymers, either covalent, ionic, or even with hydrogen bonds, can be thought of, for example, formation of diazonium salts on an amino functionalized polymer followed by its electrochemical reduction or attachment of organic groups from which a polymerization can be started. These possibilities are currently under investigation. CM0211397 (66) (a) Sukhorukov, G. B.; Moehwald, H.; Decher, G.; Lvov, Y. M. Thin Film Solids 1996, 284-285, 220. (b) Decher, G.; Lvov, Y.; Schmitt, J. Thin Film Solids 1994, 244, 772. (c) Lvov, Y.; Decher, G.; Moehwald, H. Langmuir 1993, 9, 481.

Attachment of Polymers to Organic Moieties Covalently Bonded to Iron

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