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Electrografting of Preformed Aliphatic Polyesters onto Metallic Surfaces Xudong Lou, Christine Je´roˆme, Christophe Detrembleur, and Robert Je´roˆme* Center for Education and Research on Macromolecules (CERM), University of Lie` ge, Sart-Tilman, B6, B-4000, Lie` ge, Belgium Received August 13, 2001. In Final Form: January 3, 2002 Preformed aliphatic polyesters bearing pendent acrylate groups, poly(4-acryloyloxy--caprolactone)co--caprolactone) [poly(ACL-co-CL)], were grafted onto metallic surfaces by a cathodic electrochemical process. Content of acrylates must exceed a lower limit for them to be adsorbed on the cathode with the proper orientation and for the grafting to be successful. The strongly adhering films were analyzed by IR reflection-absorption spectroscopy and scanning electron microscopy. As a result of the known miscibility of poly--caprolactone with poly(vinyl chloride) (PVC), the pregrafting of poly(ACL-co-CL) imparts strong adhesion to PVC topcoats.
Metal coating by polymer films is of great interest for applications such as corrosion protection and surface functionalization. The main limitation of this approach is the poor adhesion of the organic coating onto the metal and a poor aging resistance of the metal/polymer interface. A few years ago, great progress was made by Lecayon et al.,1 who claimed that an organic polymer could be grafted onto a usual metal by an electropolymerization process. For the past few years, we have been interested in the electrografting of insulating polymer films onto metallic substrates (Ni, Fe).2-6 These films are designated as “grafted films”, because they have the unique property of remaining attached to the electrode surface even when they are prepared and kept in a solvent in which the polymer is highly soluble. This electrografting reaction is successfully achieved in an appropriate organic solvent with acrylic monomers, such as acrylonitrile and ethyl acrylate.3-6 This process proceeds via an electrochemical initiation by the transfer of one electron from the metal to the monomer followed by the chemical propagation of the active species with formation of polymer chains on the metal surface (Scheme 1).3 Up to now, only traditional acrylates have been used for electrografting onto metallic surfaces. This paper aims at reporting the first example of the electrografting of a preformed polymer bearing pendent acrylate groups. Aliphatic polyesters, such as polylactones, polylactides, and polyglycolides, are biodegradable, biocompatible, and permeable to many drugs, which makes them excellent candidates as components in drug delivery systems, biodegradable sutures, resorbable protheses, chemotherapy, and galenic formulations.7 Because metals and alloys, for example, steel, titanium, and gold alloys, are * To whom correspondence should be addressed. Fax: +32-43663497. Tel: +32-4-3663461. E-mail:
[email protected]. (1) Boizau, C.; Lecayon, G. La Recherche 1988, 19, 888. (2) Je´roˆme, R.; Mertens, M.; Martinot, L. Adv. Mater. 1995, 7, 807. (3) Baute, N.; Teyssie´, Ph.; Martinot, L.; Mertens, M.; Dubois, Ph.; Je´roˆme, R. Eur. J. Inorg. Chem. 1998, 1711. (4) Mertens, M.; Calberg, C.; Martinot, L.; Je´roˆme, R. Macromolecules 1996, 29, 4910. (5) Mertens, M.; Calberg, C.; Baute, N.; Je´roˆme, R.; Martinot, L. J. Electroanal. Chem. 1998, 441, 237. (6) Crispin, X.; Lazzaroni, R.; Geskin, V.; Baute, N.; Dubois, Ph.; Je´roˆme, R.; Bre´das, J. L. J. Am. Chem. Soc. 1999, 121, 176. (7) Vert, M.; Feijen, J.; Albertsson, A. C.; Scott, G.; Chiellini, E. In Biodegradable Polymers and Plastics; Royal Society of Chemistry: London, 1992.
Scheme 1
used in surgery for fracture correction, bone/articular replacement, dental replacement, and so forth,8 modification of metal surfaces with functional aliphatic polyesters is of interest for promoting bioadhesion, as a lubricant in articulates, and so forth. One efficient way to functionalize aliphatic polyesters is the ring-opening polymerization (ROP) of properly functionalized lactones.9-13 4-(Acryloyloxy)--caprolactone (ACL) is a representative example of functional -caprolactone. ACL is actually a dual monomer, which can be polymerized by two independent mechanisms, that is, ROP of the lactone moiety and atom transfer radical polymerization of the acrylate.9 The ring-opening (co)polymerization of this functional -caprolactone has been initiated by Al(Oi-Pr)3 in toluene at 25 °C, with formation of aliphatic polyesters bearing a pendent acrylate functionality, with controlled molecular weight and narrow molecular weight distribution. Because random copolymers of ACL and -caprolactone are nothing but multifunctional macroacrylates, the question was addressed whether they could be substituted for traditional low molecular weight acrylates in the cathodic electrografting process. In case of success, the molecular characteristics of the chains chemisorbed on the conducting substrates (8) Klee, D.; Ho¨cker, H. Adv. Polym. Sci. 1999, 149, 1. (9) Mecerreyes, D.; Humes, J.; Miller, R. D.; Hedrick, J. L.; Detrembleur, C.; Lecomte, P.; Je´roˆme, R.; San Roman, J. Macromol. Rapid Commun. 2000, 21, 779. (10) Detrembleur, C.; Mazza, M.; Halleux, O.; Lecomte, P.; Mecerreyes, D.; Hedrick, J. L.; Je´roˆme, R. Macromolecules 2000, 33, 16. (11) Trollsås, M.; Lee, V. Y.; Meccereyes, D.; Lo¨wenhielm, P.; Mo¨ller, M.; Miller, R. D.; Hedrick, J. L. Macromolecules 2000, 33, 4619. (12) Tian, D.; Dubois, Ph.; Grandfils, C.; Je´roˆme, R. Macromolecules 1997, 30, 406. (13) Mecerreyes, D.; Miller, R. D.; Hedrick, J. L.; Detrembleur, C.; Je´roˆme, R. J. Polym. Sci., Polym. Chem. Ed. 2000, 38, 870.
10.1021/la011289g CCC: $22.00 © 2002 American Chemical Society Published on Web 03/07/2002
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Table 1. Current Intensity at the Potential of Peak I in Relation to the Acrylate Concentration for Poly(ACL) and Ethyl Acrylate, in 0.05 M TEAP Solution in DMF; ν ) 20 mV s-1 poly(ACL)
Figure 1. Voltammetry of poly(ACL) on a steel electrode in a 0.05 M TEAP solution in DMF, ν ) 20 mV s-1. (A) s, [acrylate] ) 1.0 M; - - -, [acrylate] ) 2.0 M. (B) 1, first scan; 2, second scan.
would be predetermined, particularly the molecular weight. Indeed, at a rather low content of ACL, most of the acrylate units would react at the cathode before having a chance to participate in a chain reaction. The pendent acrylates would thus be anchoring groups rather than actual monomers. To test the validity of this concept, three types of functional aliphatic polyesters were prepared, that is, homopoly(4-(acryloyloxy)--caprolactone) [poly(ACL)], poly(ACL-co-CL) random copolymer, and poly(caprolactone) (PCL) with one acrylate end-group (macromonomer). All these chains were terminated by reaction with iodomethane (CH3I) for them to be end-capped by a methyl ether rather than by a hydroxyl group that might compete with the acrylates in the electrochemical process (Scheme 2). PCL was end-capped by one acrylate group by reaction of the living chains initiated by aluminum isopropoxide, by acryloyl chloride. Voltammetric curves were reported for solutions of the acrylate-containing polyesters in dry dimethylformamide added with tetraethylammonium perchlorate (TEAP, 0.05 M). Steel was used as a cathode, and the acrylate concentration was in the 0.5-3.0 M range. When the acrylate concentration is changed from 0.5 to 2.0 M, two reduction peaks are observed for poly(ACL) as was the case for acrylonitrile and ethyl acrylate3 (Figure 1A). Peak I and peak II (shoulder) are commonly referred to as a passivation peak and a diffusion peak, respectively.
ethyl acrylate
[acrylate]
0.5 M
1.0 M
1.5 M
2.0 M
0.5 M
1.0 M
1.5 M
Ip1 (µA)
610
690
700
850
500
400
300
Actually, peak I is the electrochemical signature for the grafting of an insulating polymer onto the cathode. In this respect, Table 1 compares the intensity of peak I (Ip1) for poly(ACL) and ethyl acrylate, at different acrylate concentrations. As previously reported for the ethyl acrylate electrografting,3 an increase in monomer concentration results in smaller Ip1 at constant scanning rate (20 mV s-1). Indeed, polymerization initiation is an electrochemical process in which adsorbed monomers participate. In contrast, chain propagation, and thus deposition of an insulating film, is a chemical process as fast as the local monomer concentration is high. Whenever propagation is faster, fewer electrons are transferred from the cathode to the adsorbed monomers and Ip1 decreases. Ip1 is thus controlled by the chain growth kinetics. In the case of poly(ACL), the reverse observation is noted, that is, the intensity of peak I increases with the acrylate concentration, which indicates that an increasing number of acrylates participate in the electrochemical reaction under the same experimental conditions. The adsorption of the acrylate units which are now attached to a preformed polymer (no spacing group, see Scheme 2) is perturbed because of restraints exerted by the polymer backbone. On the assumption that the adsorbed acrylate units cannot polymerize because they are too far away and not mobile enough, the electron exchange (and thus Ip1) is controlled by the number of chains adsorbed which increases with concentration. However, as soon as the cathode is completely covered by poly(ACL) chains, any further increase in the concentration of these chains in solution perturbs the chain adsorption. This adsorption has to change from an ideal multianchoring adsorption to the extreme tethering of the chains as a result of increasingly stronger competition of the chains for adsorption when their population is increased. This perturbation is important to the point where no grafting occurs anymore when the concentration of the acrylates of poly(ACL) is 3.0 M and higher. Then, the situation is dominated by the chain entropy and chain solvation rather than by the adsorption of the acrylate units. This extreme case is supported by the absence of grafting of the PCL chains end-capped by one acrylate unit, whatever the chain concentration. Attached to a chain of 2000 molecular weight, one acrylate
Electrografting of Preformed Aliphatic Polyesters
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Figure 2. Fourier transform infrared spectra of (A) electrografted poly(ACL-co-CL) film (acrylate concentration ) 1 M), (B) electrografted poly(ACL) film (ibid.), and (C) poly(ACL) film after peeling.
unit is not adsorbed onto the cathode anymore, and the prerequisite for electrografting is no longer satisfied.3,5 There is no basic difference in the electrochemical behavior of the poly(ACL-co-CL) chains compared to that of the poly(ACL) ones, except that the grafting capacity is lost at lower acrylate concentration for the copolymer chains. At acrylate concentrations lower than 2 M, electrografting occurs as assessed by the cathode passivation. Indeed, when the electrolysis is carried out in the potential range of peak I, the current drops down and only a residual current is observed when the potential scan is repeated after intense stirring of the solution between the two scans. This phenomenon is illustrated in Figure 1B for poly(ACL) and is exactly the same for the random copolymer. Whenever the electrolysis is carried out in the potential range of peak II, the cathode is expectedly not passivated, but the acrylate units are polymerized (at least partly) in solution, which results in polymer precipitation consistently with the cross-linking of the multifunctional macromonomer. Because of the aforementioned tethering of the chains at high enough (>2 M) acrylate concentration, the passivation peak is not observed anymore for poly(ACL-co-CL); only a slight decrease in the current intensity is noted when the cathodic scan is repeated after a hold at -1.9 V (thus at the potential of peak I). IR spectroscopy was used to confirm the deposition of polyester films in a good solvent for them. Figure 2A,B shows the spectra for the poly(ACL-co-CL) and poly(ACL) films electrografted to the electrode. The three main absorptions at 2948, 1726, and 1167 cm-1 are characteristic of CH2, CdO, and C-O, respectively. The intensity of the peaks is higher for poly(ACL-co-CL) than for poly(ACL). Because the IR spectra were recorded by IR reflectionabsorption spectroscopy (IRRAS) of the grafted substrate, the observation of higher intensity peaks for the copolymer is consistent with films thicker than those for poly(ACL). This conclusion is confirmed by scanning electron microscopy (SEM). Indeed, Figure 3B shows that the film of poly(ACL-co-CL) is thick enough to screen the ridges of the underlying steel when compared to Figure 3C. This is not the case for the poly(ACL) film (Figure 3A) for which the steel roughness is still visible. This difference is thought to originate from the multianchoring of the chains, which leads to poly(ACL) chains lying flat on the cathode, in contrast to the poly(ACL-co-CL) chains that have the opportunity to form loops protruding in solution and
Figure 3. SEM images for (A) electrografted poly(ACL), (B) electrografted poly(ACL-co-CL), and (C) neat steel surface.
possibly trapping nonadsorbed chains by physical entanglement. It is essential to confirm that the polyester films are actually grafted to the electrode (thus strongly adhering) and not merely deposited by precipitation as a result of cross-linking. For this purpose, adherence has been compared for films of poly(ACL) and poly(ACL-co-CL) by peeling tests at 180°. Table 2 shows the experimental data for the neat steel surface used as the cathode (1350 N/m) and for the poly(ACL) and poly(ACL-co-CL) films either electrografted or spin-casted onto steel. Clearly, the standard Scotch tape (3M, acrylic foam 4930) is strongly adhering to the poly(ACL) films that have been electrografted. Indeed, although the peeling energy is higher than for the metal substrate, this value is smaller than the actual polymer/metal adhesion energy, because the poly(ACL) film remains attached to the substrate after peeling. This observation has been systematically confirmed by the IR analysis of the surface before and after peeling (Figure 2C). In sharp contrast, the solvent-cast poly(ACL) films have no adhesion to steel, as testified by
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Table 2. Peeling Tests for Electrografted and Solvent-Cast Polyester Films poly(ACL)a
poly(ACL-co-CL)a
[acrylate]
0.5 M
1.0 M
1.5 M
0.5 M
1.0 M
1.5 M
neat steel
PCL
poly(ACL)b
poly(ACL-co-CL)b
PVCb onto poly(ACL)a
PVCb onto poly(ACL-co-CL)a
adhesion ((10% N/m) steel surface after peeling
2800
2000
2200
370
140
77
1350
100
30
30
100
3700
Oc
O
O
O
O
O
×
×
×
O
a
×d
Electrografted films. b Spin-coated films from a THF solution. c O, polymer remaining on steel.
the very low energy required to detach the organic film from the metal. The same conclusion can be drawn for the poly(ACL-co-CL) films, although the Scotch tape is less adhering to the organic film itself. In this respect, it must be noted that the adherence of the tape to the electrografted film decreases as the concentration of the acrylate units in solution is higher when the film is deposited. As aforementioned, the conformation of the adsorbed chains must change when their concentration is increased, because the number of actual anchors per chain is expected to decrease, which must influence the interaction with the tape. This effect appears to be more pronounced for chains of a lower acrylate content. The electrografted chains can be considered as a primer coating that could improve the adherence of a topcoat. This concept has been tested in the specific case of poly(vinyl chloride) (PVC), which is known to be miscible with PCL.14 A PVC film has thus been spin-coated onto the electrografted poly(ACL) and poly(ACL-co-CL) films. Quite interestingly, the peeling tests show that the adherence of PVC strongly depends on the structure of the primer film in relation to the chain conformation on the steel surface. When the polyester chains are lying flat on the electrode, they cannot be intimately intermingled with the PVC chains and the adherence is poor. As soon as loops can be formed, the situation changes dramatically and PVC is no longer detached from the metal. As a rule, the experiments reported in this paper demonstrate that preformed insulating polymers can be electrografted onto metals, provided that they contain pendent acrylate groups. The content of these groups must exceed a lower limit for them to be adsorbed on the cathode with the proper orientation and to be reduced electrochemically. If the chain solvation and entropy overcome the chain adsorption, no grafting onto the cathode occurs. This novel strategy has the advantage to chemisorb chains with predetermined characteristics, mainly molecular weight. The content and distribution of the acrylatecontaining units is a way to modulate the structure and thickness of the electrografted film. A typical example is the strong adherence to steel that pregrafted poly(ACLco-CL) chains can impart to PVC. Last but not least, if the polymer backbone is not intrinsically functional (as is the case for PCL), a terpolymer can be prepared. In the series of aliphatic polyesters discussed in this paper, new (14) Eastmood, G. C. Adv. Polym. Sci. 1999, 149, 59.
d
×, polymer removed with the tape.
γ-functional CLs have been recently synthesized and (co)polymerized in a living manner, for example, CL substituted in the γ position by a protected alcohol, a bromide that can be quaternized, and so forth.9-13 Experimental Section ACL was synthesized as detailed elsewhere.9 (Co)polymerization of ACL was carried out at 0 °C in dry toluene. -Caprolactone was previously dried over CaH2. ACL was dried by repeated azeotropic distillation of toluene (three times) just before polymerization. Then, toluene, -caprolactone (in the case of copolymerization), and the required amount of Al(Oi-Pr)3 (1 M solution) were added through a rubber septum with a syringe. After polymerization for 3 h, an excess of CH3I was added. The polymer was recovered by precipitation in cold heptane. The molecular characteristics of the polymers were as follows: for poly(ACL), Mn ) 12 000 and Mw/Mn ) 1.25; for the poly(ACLco-CL) random copolymer (41 mol % of ACL according to the 1H NMR spectrum), Mn ) 15 000 and Mw/Mn ) 1.20; and for PCL with one acrylate end-group (macromonomer), Mn ) 2000 and Mw/Mn ) 1.20. Peeling measurements were carried out at 180° with the acrylic foam 4930 tape from 3M according to the ASTM standards D 3330M-90. The peeling energy was measured with an Instron tensile tester. The tape was kept in contact with the substrate for 24 h before measurement. The electrografting experiments were carried out in a onecompartment cell with a platinum counter electrode in a glovebox under a dry inert atmosphere. Because of the drastic anhydrous conditions needed for electrografting, the use of conventional reference electrodes is precluded and the potentials have to be measured against a Pt pseudo-reference electrode. Therefore, it is delicate to compare the absolute values of the electrochemical potentials for processes occurring in different media. A PAR potentiostat (EG&G, model 273A) was used. All of the polymers were dried by repeated azeotropic distillation of dry toluene (three times). Dimethylformamide (DMF) was dried for 5 days over P2O5, distilled at 60 °C under reduced pressure, and stored under dried nitrogen in a glovebox. Tetraethylammonium perchlorate was dried by overnight heating at 80 °C under vacuum. This technique was detailed elsewhere.3,4
Acknowledgment. The authors are indebted to the “Services Fe´deraux des Affaires Scientifiques, Techniques et Culturelles” for general support to CERM in the frame of the “PAI 4-11: Supramolecular Chemistry and Supramolecular Catalysis”. C.J. is grateful to the “Fonds National de la Recherche Scientifique” for appointment as “Charge´ de Recherche”. LA011289G
