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n-Alkanoic Acid Monolayers on 316L Stainless Steel Promote the Adhesion of Electropolymerized Polypyrrole Films Galit Shustak,†,‡ Abraham J. Domb,‡ and Daniel Mandler*,† Department of Inorganic and Analytical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, and School of Pharmacy, The Hebrew UniVersity of Jerusalem, Jerusalem 91120, Israel ReceiVed February 17, 2006. In Final Form: April 11, 2006 The formation of a self-assembled monolayer significantly promotes the adhesion of electrodeposited polypyrrole on stainless steel. The monolayer affects the nucleation and growth mechanism of polypyrrole as a result of its hydrophobic nature. This was confirmed by analyzing current-time transients of the initial stages of electropolymerization and was in agreement with AFM images.
Considerable efforts are aimed at the development of “molecular adhesion promoters” that improve the linkage between metal substrates and organic coatings.1-4 The adhesion between hydrophobic organic films and hydrophilic metal surfaces is expected to be insufficient for applications such as coating of implantable medical devices. Such coatings are designed for increasing the biocompatibility of the implant as well as for accommodating drugs. Stainless steel is the most widely used metal for implantable medical devices, owing to its corrosion resistance and superior mechanical properties.5 The biocompatibility of a stainless steel implant can be significantly improved by modifying its surface with organic molecules or polymers.6-9 Because of the increased need for drug-eluting implants,10 adherent, uniform (1 to 2 µm), flexible coatings that can easily be loaded with a variety of drugs are desired. Coating implants, such as stents, with polymer solution either by dipping or spraying provides thick coatings (∼15 µm) with limited uniformity and adherence. In a recent publication,11 we reported on the electropolymerization of pyrrole derivatives onto stainless steel, which resulted in an improved coating with respect to uniformity and thickness. Electropolymerization is an appealing approach for surface coating that offers numerous advantages.11,13 The polymeric film, which is electrochemically deposited under mild conditions, usually forms homogeneous layers with controllable thickness. * Corresponding author. E-mail:
[email protected]. Tel: +9722-658-5831. Fax: +972-2-658-5319. † Department of Inorganic and Analytical Chemistry. ‡ School of Pharmacy. (1) Reinartz, C.; Furbeth, W.; Stratmann, M. Fresenius’ J. Anal. Chem. 1995, 353, 657. (2) Stratmann, M. AdV. Mater. 1990, 2, 191. (3) Bascom, W. D. Macromolecules 1972, 5, 792. (4) Sathyanarayana, M. N.; Yaseen, M. Prog. Org. Coat. 1995, 26, 275. (5) Helsen, J. A.; Breme, H. J. In Metals as Biomaterials; Wiley: New York, 1998. (6) Harm, U.; Burgler, R.; Furbeth, W.; Mangold, K.-M.; Juttner, K. Macromol. Symp. 2002, 187, 65. (7) Stratmann, M.; Volmeruebing, M. Appl. Surf. Sci. 1992, 55, 19. (8) Ruan, C. M.; Bayer, T.; Meth, S.; Sukenik, C. N. Thin Solid Films 2002, 419, 95. (9) Marsh, J.; Scantlebury, J. D.; Lyon, S. B. J. Appl. Polym. Sci. 1996, 59, 897. (10) Wieneke, H.; Sawitowski, T.; Wnendt, S.; Fischer, A.; Dirsch, O.; Karoussos, I. R.; Erbel, R. Hertz 2002, 6, 518. (11) Weiss, Z.; Mandler, D.; Shustak, G.; Domb, A. J. J. Polym. Sci., Part A 2004, 42, 1658. (12) Shustak, G.; Domb, A. J.; Mandler., D. Langmuir 2004, 20, 7499. (13) (a) Domb, A. J. Electropolymerization Coating of Stents to PreVent Thrombosis and Restenosis; PCT WO 01/39813 A1. (b) Domb, A. J.; Mandler, D.; Shustak, G.; Danziger, I. Modified ConductiVe Surfaces HaVing ActiVe Substances Attached Thereto and Uses Thereof; patent pending.
The organic coating can be grafted with various functional groups that improve biocompatibility and modify its mechanical and chemical properties. In addition, complex shapes of stainless steel surfaces, such as grids and coils, can be coated uniformly. However, the performance of such application is often limited by the poor adhesion of polypyrrole to the stainless steel surface. Here, we report a significant improvement of this conducting polymer film coating by applying carboxylic acid self-assembled monolayer (SAM) onto the metal surface prior to pyrrole electropolymerization (Scheme 1). Most previous studies utilize alkanethiol- or organosilane-functionalized pyrrole or aniline14-19 to increase the adhesion of polypyrrole and polyaniline, respectively, to the surface. Recently, we reported on the electrochemically induced formation of n-alkanoic acid SAMs adsorbed on 316L stainless steel substrates.12 It was demonstrated that modifying the stainless steel surface with a medium-chainlength fatty acid (decanoic acid) monolayer did not block electron transfer, which allowed the driving of subsequent electrochemical reactions at the monolayer/solution interface. Here, we describe the effect of such a carboxylic acid monolayer on the electropolymerization of pyrrole. We find that the monolayer has a significant impact on both the adhesion of the deposited polypyrrole as well as on its morphology. The latter was attributed to the effect of the monolayer on the polypyrrole nucleation and growth mechanism. In a typical experiment, pyrrole was electropolymerized on a stainless steel plate (40 × 9 mm) or disk (3 mm diameter) electrodes in an acetonitrile solution containing 10 mM distilled pyrrole monomer and 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) by applying a constant potential while recording the current versus time (i.e., chronoamperometry). A potential step from 0 V to a potential between 1.2 and 1.7 V versus the Ag/AgBr reference electrode (0.448 V versus ferroceneferrocenium (Fc/Fc+)20) was typically applied. A graphite rod was used as an auxiliary electrode. A decanoic acid monolayer was assembled by electrochemically cycling the stainless steel plate between -0.8 and 1.2 V versus Ag/AgBr in an acetonitrile solution containing 0.1 mM decanoic acid and 0.1 M TBATFB. (See ref 11 for further details.) The adhesion of the polymer film (14) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (15) (a) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (b) Peanaskey, J. S.; McCarley, R. L. Langmuir 1998, 14, 113. (16) Noble, C. H.; McCarley, R. L. J. Am. Chem. Soc. 2000, 122, 6518. (17) McCarley, R. L.; Willicut, R. J. J. Am. Chem. Soc. 1998, 120, 9296. (18) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (19) Peanaskey, J. S.; McCarley, R. L. Langmuir 1998, 14, 113. (20) Gritzner, G.; Kuta, G. J. Pure. Appl. Chem. 1984, 56, 461.
10.1021/la060460r CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006
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Scheme 1. Steps in the Formation Process of an Adherent Organic Coating on Stainless Steela
a Step I involves the formation of a decanoic acid self-assembled monolayer on 316L stainless steel, and step II comprises the electropolymerization of pyrrole on the monolayer.
Figure 1. Cross-cut tape adhesion test following the D-3359-02 ASTM standard test (method B). Photos show the plate (left) and the adhesive tape (right) after peeling: (A) bare stainless steel plate coated with polypyrrole and (B) stainless steel plate where polypyrrole was electrodeposited onto a palmitic acid SAM.
was determined using cross-cut adhesive tape following the D-3359-02 ASTM adhesion standard test (method B). According to this method, the film coating is divided into small squares (∼5 × 5 mm2) using a sharp object. Then, tape (Permacel 99, Permacel, New Jersey) is pressed onto the coating and stripped off. The ratio between the number of squares remaining on the stainless steel plate and the total number of squares is defined as the adherence factor.21 Figure 1 shows an optical micrograph of bare and modified plates after the adhesion test. We found that the electropolymerization of pyrrole on a bare stainless steel plate resulted in an adherence factor smaller than 0.05, which means that the film was almost completely removed from the surface (Figure 1A). However, when electropolymerization was conducted in a (21) Annual Book of ASTM Standards; D-3359-02 ASTM Adhesion Standard Test, Method B; American Society for Testing and Materials, 2002; Vol. 15.09, pp 391-394.
solution containing 1 mM decanoic acid in addition to the pyrrole (0.1 M), the adherence factor increased to about 0.4, suggesting that incorporating a fatty acid in the course of the polymerization increases the adhesion of the polypyrrole to the stainless steel. Moreover, the adherence factor was further increased to more than 0.65 as a result of coating the stainless steel plate by a decanoic acid SAM (Figure 1B) prior to electropolymerization. It should be noted that most of the film endured a second standard adhesion test. The significant effect of the decanoic acid monolayer on the adhesion of polypyrrole to stainless steel alludes to differences in the polypyrrole/monolayer interface, which required the application of additional and more quantitative tools. The initial stages of polymerization can readily be studied using currenttime transients. Typical i-t curves of pyrrole electropolymerization on bare and decanoic acid monolayer-modified stainless steel disk electrodes are presented in Figure 2A and B. Each graph shows a series of transients where the final potential was systematically increased. Chronoamperometry is sensitive to the kinetics of deposition22-25 and therefore discloses the effect of the substrate on polypyrrole growth and nucleation. Moreover, theoretical analysis of i-t curves has been well established.26-32 Both graphs (Figure 2A and B) show some similarities; namely, after a short current decay due to charging, the current increases to a maximum and then decreases. Such behavior is typical for the electropolymerization of conducting polymers36-40 where (22) Harrison, J. A.; Thirsk, H. R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Academic Press: New York, 1971; Vol. 5, pp 67-103. (23) Harrison, J. A.; Thirsk, H. R. In A Guide to The Study of Electrode Kinetics; Academic Press: London, 1972; pp 115-135. (24) Levie, de R. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. E., Eds.; John Wiley: New York, 1984, Vol. 13, p 1. (25) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. In Instrumental Methods in Electrochemistry; Kemp, T. J., Ed.; John Willy: New York, 1985; pp 283-316 (26) Bosco, E.; Ranjarajan, S. K. J. Electroanal. Chem. 1982, 134, 213. (27) Abyaneh, M. Y. J. Electroanal. Chem. 2002, 530, 82. (28) Abyaneh, M. Y.; Fleischmann, M. J. Electroanal. Chem. 2002, 530, 89. (29) Abyaneh, M. Y. J. Electroanal. Chem. 2002, 530, 96. (30) Fletcher, S. J. Electroanal. Chem. 2002, 530, 105. (31) Abyaneh, M. Y.; Fleischmann, M. J. Electroanal. Chem. 2002, 530, 108. (32) Fletcher, S. J. Electroanal. Chem. 2002, 530, 119. (33) Miller, L. L.; Zinger, B.; Zhou, Q. X. J. Am. Chem. Soc. 1987, 109, 2267.
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Figure 2. Current-time transients of the electrodeposition of polypyrrole on (A) decanoic acid-modified and (B) bare stainless steel disk electrodes. The curves correspond to final potentials of 1.2, 1.3, 1.4, 1.5, and 1.6 V vs Ag/AgBr. Table 1. Sum of the Squares of the Vertical Deviations, R2, Obtained by Fitting the Current-Time Transients Shown in Figure 1 with the 3D Nucleation and Growth Theoretical Models (Equations 1 and 3) potential (V vs Ag/AgBr)
3D progressive (R2) (DA-modified SS)
3D instantaneous (R2) (DA-modified SS)
3D progressive (R2) (bare SS)
3D instantaneous (R2) (bare SS)
1.4 1.5 1.6 1.7
0.036 0.64 0.91 0.95
0.86 0.83 0.89 0.88
0.68 0.81 0.85 0.88
0.72 0.80 0.88 0.95
the enhancement of the current is due to nucleation, whereas the current decay is a result of nuclei overlapping and polymer growth. As the potential is made more positive, the driving force for electropolymerization is also increased, which results in a steeper increase of the current. This is attributed to faster nucleation. A more detailed inspection of both cases points to two main differences between the bare and the decanoic acid-modified surface. Whereas the current transients of a bare stainless steel surface show two maxima (where the ratio between the first and the second increases with potential), these of a modified surface exhibit only one maximum. The second difference can be seen in the modified electrode transients that change abruptly by stepping the potential to more than 1.4 V. The shape of the transient is indicative of the nucleation and growth mechanism.22-25 It is well accepted that the nucleation and growth of metal deposition22-32 can each be divided into two principal mechanisms, namely, instantaneous and progressive nucleation and 2D and 3D growth. Recently, it has been reported that the kinetics of conducting polymer electrodeposition can be treated similarly.33-40 This is not trivial because the mechanism of metal deposition involves the reduction of metal ions at the electrode surface, whereas the deposition of conducting polymers proceeds through the formation of radical cations, which produce oligomers in solution via several coupling and deprotonation steps. Typically, the deposition of oligomers commences at a chain length between six to eight units. The instantaneous nucleation of conducting polymers is characterized by the immediate formation of a constant number of nuclei as opposed to progressive nucleation in which the growth of nuclei, which continues throughout the polymerization, is faster than their formation. In the 3D growth model, the nuclei growth rate is (34) Yang, R.; Dalsin, K. M.; Evens, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1990, 94, 6117. (35) Caple, G.; Wheeler, B. L.; Swift, R.; Porter, T. L.; Jeffers, S. J. Phys. Chem. 1990, 94, 5639. (36) Cossement, D.; Plumier, F.; Delhalle, J.; Hevesi, L.; Mekhalif, Z. Synth. Met. 2003, 138, 529. (37) Hwang, B. J.; Santhanam, R. Lin, Y. L. Electroanalysis 2003, 15, 115. (38) Li, F.-B.; Albery, W. J. Langmuir 1992, 8, 1645. (39) Bade, K.; Tsakova, V.; Schultze, J. W. Electrochim. Acta 1992, 37, 2255. (40) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 229.
essentially equal or comparable in the parallel and perpendicular directions with respect to the electrode surface, whereas in 2D growth the nuclei grow faster laterally. These models have been formulated;22-39 however, because 2D models showed poor fitting to the experimental data, only the 3D equations are presented: 3D growth, progressive nucleation
() (
){
[
( ) ]}
(1)
( )]}
(2)
tm i 2 t ) 1.2254 1 - exp -2.3367 im t tm
2
2
3D growth, instantaneous nucleation
() (
){
[
tm i 2 t ) 1.9542 1 - exp -2.3367 im t tm
2
where i and t are the current and time, respectively, and im and tm correspond to the maximum current and time, respectively. An analysis of the choroamperometry data was carried out using a linear least-squares regression fitting method and is summarized in Table 1. An inspection of the polymerization at a bare stainless steel electrode shows a gradient change in the transients as the driving force increases (i.e., the potential is made more positive). Fitting the various curves to the theoretical 3D instantaneous and progressive nucleation (eqs 1,2) clearly shows a better fit to the instantaneous mechanism as the potential increases. This is expected24-25 because increasing the driving force for polymerization should increase the rate of nucleation. Surprisingly, the stainless steel modified surface shows a significantly different behavior, thereby increasing the driving force results in transients that fit much better with 3D progressive nucleation. Evidently, this cannot be explained by the effect of overpotential on the rate of electron transfer and therefore on nucleation. Such an anomaly has been reported by Lin and coworkers37-39 in the case of the electropolymerization of pyrrole on HOPG and has been attributed to the hydrophobic interactions between the monomers/oligomers and the surface. We believe that hydrophobic interactions between the pyrrole and the SAM are also responsible here for the prevailing 3D progressive nucleation mechanism. As opposed to a bare electrode, the methyl-
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Figure 3. AFM images of pyrrole electrodeposited on (A, C) bare and (B, D) palmitic acid SAM-modified stainless steel plates. Electropolymerization was carried out at (A, B) 1.6 V and (C, D) 1.2 V vs Ag/AgBr in a solution containing 10 mM pyrrole and 0.1 M TBATFB.
terminated monolayer creates a hydrophobic environment, thereby enhancing the formation of more nucleation sites for polypyrrole growth. Finally, the transients show also that the tm (eqs 1 and 2), which defines the time where nuclei overlap, of pyrrole electrodeposition on modified stainless steel (tm(1.7V) ) 1.6 s) is shorter than on a bare surface (tm(1.7V) ) 3.8 s) at potentials >1.4 V. This is in accordance with the above finding, by which the monolayer facilitates the nucleation of polypyrrole. It is well documented that the nature of the nucleation mechanism has a significant impact on the morphology of the deposit.37-40 Specifically, an instantaneous mechanism is likely to result in a rougher surface than a progressive nucleation because of the formation of a constant number of nuclei that grow and eventually overlap, forming a bumpy structure. However, during progressive nucleation nuclei are constantly formed, filling the surface with different sizes of nuclei, making it smoother. Indeed, a remarkable correlation between choroamperometry and surface morphology was found by AFM. AFM images of polypyrrole electropolymerized on bare and palmitic acid SAM-modified stainless steel plates under a potential step of 1.6 and 1.2 V versus Ag/AgBr are shown in Figures 3A and B and 3C and D, respectively. In view of the fact that polypyrrole is electropolymerized on bare stainless steel at 1.6 V via an instantaneous nucleation mechanism, it is not surprising that the roughness factor is fairly high (Rq ) 199 nm). However, the roughness factor of polypyrrole that was electropolymerized on a palmitic acid SAM-modified surface under the same potential is ca. 6 times smaller (Rq ) 32 nm). Conversely, the roughness of polypyrrole films electropolymerized on bare and modified surfaces at 1.2 V is comparable (Rq ) 48 and 25 nm, respectively), which is in complete agreement with the transients alluding to a similar nucleation mechanism.
In conclusion, a simple and cost-effective method has been developed for the preparation of an adherent, thin, homogeneous polymer coating on 316L stainless steel surfaces. More specifically, we can control the polymer coating roughness and adhesion by changing the applied potential and by forming a carboxylic acid SAM prior to pyrrole electrodeposition. The decanoic acid SAM is not sufficiently dense and thick to block electron transfer, which allows us to use electrochemistry as the driving force for further film deposition (e.g., via electropolymerization). Furthermore, the end group of the decanoic acid SAM, which can be modified, provides the crucial molecular interactions with the coating and as such serves as an adhesion promoter. In this specific case (i.e., for attaching polypyrrole), we find that a hydrophobic adhesion promoter is required; however, the end group of the decanoic acid can be replaced by other functions (e.g., amino groups) if, for example, negatively charged nanoparticles are to be attached to the stainless steel surface.41 In addition, the adhesion-promoting film has molecular dimensions, and thus the overlayer polymer film acquires the surface morphology (as can be seen in Figure 3B). These findings are important because they provide detailed insight for a better understanding and utilization of electropolymerization as a means of coating medical devices by thin, uniform, adherent films. Acknowledgment. This research is partially supported by the European Community (BBMO, contract number LSSB-CT2005-005199) and the Israel Science Foundation. The Krueger Center for Nanoscience and Nanotechnology at the Hebrew University and Elutex Ltd are acknowledged. LA060460R (41) Shustak, G.; Domb, A. J.; Mandler, D., to be submitted for publication.
