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Surface Initiated Polymerization of Styrene from a Carboxylic Acid Functionalized Polypyrrole Coated Electrode Ste´phane Roux, Anne-Sophie Duwez,† and Sophie Demoustier-Champagne*,‡ Unite´ de Physique et de Chimie des Hauts Polyme` res, Universite´ Catholique de Louvain (UCL), Place Croix du Sud, 1-1348 Louvain-la-Neuve, Belgium Received May 28, 2002. In Final Form: October 18, 2002 Surface initiated radical polymerization (SIRP) is an elegant and efficient method to produce polymer chains attached to surfaces with a high graft density. Growth of polymer chains from a gold surface requires the formation of a self-assembled monolayer (SAM) of thiols functionalized by reactive groups (alcohol or carboxylic acid) in order to covalently bind free radical initiator species. However, thiol desorption induced by the temperature imposed for polymerization impedes the growth of polymer chains. We propose therefore to coat a gold electrode with a polypyrrole (PPy) derivative containing carboxylic acid pendant groups. The resulting conducting polymer layer was shown to be strongly attached to the surface. Grafting of the initiator (azo compound) and SIRP of styrene were successfully performed on this organic layer, as demonstrated by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF SIMS), and contact angle measurements. The roughness, the morphology, and the surface uniformity of the films were investigated by atomic force microscopy (AFM). Finally, cyclic voltammetry experiments were carried out to study the influence of the polystyrene layer on the electrochemical behavior of the conducting polymer.
Introduction The functionalization of a surface by polymers allows an accurate tuning of the substrate surface properties.1-3 Among several procedures,4,5 surface initiated radical polymerization (SIRP), thoroughly described by Prucker and Ru¨he,6-8 is one of the most efficient for coating various substrates (silica particles, gold plates). SIRP proceeds in three steps: surface activation, initiator grafting to the surface, and polymerization. Surface activation depends on its nature and aims at the binding (covalent or electrostatic) of the initiator. It consists of modifying the substrate surface to fix an anchorage site for the chemical and physical binding of the initiator. In the case of a gold substrate, anchorage sites are generated by the formation of a SAM of alcohol or carboxylic acid functionalized thiols.9-11 An alcohol or carboxylic acid moiety ensures the covalent immobilization of the initiator by a condensation reaction. However, the use of thiol SAMs displays major drawbacks revealed by Huang et al.12 They reported that free radical polymerization from gold surfaces is * Corresponding author. Phone: 00 32 10 47 35 60. Fax: 00 32 10 45 15 93. E-mail:
[email protected]. † Postdoctoral researcher of the Belgian National Fund for Scientific Research. ‡ Research associate of the Belgian National Fund for Scientific Research. (1) Cates, M. E.; Brooks, J. T. In Polymer and Surfaces Interfaces; Feast, W. T., Munro, H. S., Richards, R. W., Eds.; Wiley: Chichester, 1993; p 49. (2) Milner, S. T. Science 1991, 251, 905. (3) Uyama, Y.; Kato, K.; Ikada, Y. Adv. Polym. Sci. 1998, 137, 3. (4) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (5) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436. (6) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (7) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602. (8) Prucker, O.; Ru¨he, J. Langmuir 1998, 14, 6893. (9) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembled Monolayers; Academic Press: Boston, 1991. (10) Ista, L. K.; Mendez, S.; Pe´rez-Luna, V. H.; Lo´pez, G. P. Langmuir 2001, 17, 2552. (11) Hyun, J.; Chilkoti, A. Macromolecules 2001, 34, 5644.
hindered by the lack of stability of the alkanethiol monolayer. Free radicals in solution accelerate the desorption of thiols from the gold surface, and desorbed alkanethiols appear to serve as efficient chain-transfer reagents that impede styrene polymerization.13,14 To overcome this problem, Huang et al.12 used mercaptopropyltrimethoxysilane to form SAMs on gold with enhanced stability. The hydrolysis generates indeed a stable reticulated layer on which a siloxane terminated initiator could be covalently immobilized by condensation reaction. Although Huang et al.12 demonstrated the efficiency of their procedure, this method requires a multistep synthesis of the initiator and still involves the formation of thiol SAMs on gold, which are relatively fragile under the use of elevated temperature imposed by most radical polymerizations. In this paper, we report on an alternative method for the formation of thiol SAMs as an anchor layer to fix the initiator. It consists of the electrodeposition of a functionalized conducting polymer. This procedure is very attractive, since electrochemical techniques are appropriate to quickly and easily yield adherent and well-defined homogeneous films of intrinsically conducting polymers with a desired thickness.15,16 Among them, N-functionalized polypyrroles (PPy) are probably the most promising ones because of the facile synthesis of the monomers and the polymers and because of the great polymer stability.17 Moreover, these polymers and their behavior are thor(12) Huang, W.; Skanth, G.; Baker, G. L.; Bruening, M. L. Langmuir 2001, 17, 1731. (13) Bamford, C. H. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New York, 1988; Vol. 13, p 743. (14) Mahabadi, K. H.; O’Driscoll, K. F. Makromol. Chem. 1977, 178, 2629. (15) Street, G. B. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; p 265. (16) Diaz, A. F. In Organic Electrochemistry, an Introduction and a Guide; Baizer, M. M., Lund, H., Eds.; Marcel Dekker: New York, 1983; p 1363. (17) Deronzier, A.; Moutet, J.-C. Coord. Chem. Rev. 1996, 147, 339.
10.1021/la0205001 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/14/2002
Surface Initiated Polymerization of Styrene Chart 1. Monomer 2CA and Polymer Poly(2CA) (Reduced State)
oughly studied.18-20 We propose therefore the formation of a thin poly(3-(pyrrol-1-yl)propanoic acid) (further named poly(2CA), Chart 1) layer to immobilize the initiator (2,2′azobis(2-amidinopropane) dihydrochloride, ABAP) onto the surface. The presence of COOH groups in poly(2CA) should allow the grafting of ABAP. This paper describes the thermal radical polymerization of styrene initiated from a poly(2CA) layer deposited onto gold plates. Each step of the surface functionalization was followed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Materials were also characterized by cyclic voltammetry (CV), time-of-flight secondary ion mass spectrometry (TOF SIMS), and contact angle measurements. Experimental Section Materials. 3-(Pyrrol-1-yl)propanoic acid (2CA) was synthesized from alkaline hydrolysis of commercially available cyanoethylpyrrole.21,22 Styrene was purified by extracting three times with 0.1 M aqueous sodium hydroxide (NaOH) solution and twice with milli-Q water (18 MΩ) and drying over anhydrous sodium sulfate (Na2SO4) before polymerization. 2,2′-Azobis(2-amidinopropane) dihydrochloride (ABAP, ACROS), 1-ethyl-3-(3-dimethylamino)propylcarbodiimide (EDC, Aldrich), and pentafluorophenol (PFP, Aldrich) were used as received. Preparation of Gold Substrates. Substrates used for SIRP experiments were composed of glass plates coated by gold. To ensure a good adhesion, the glass surfaces were carefully cleaned in a piranha solution (H2O2/H2SO4, 1:1 v/v) for 20 min and dried at room temperature. Warning: Piranha solution should be handled with extreme caution because of the violent exothermic reaction occurring when exposed to organic material. Glass substrates were then coated by a thin chromium adhesion layer (20 Å) and by a thicker gold layer (200 nm). In the case of thickness measurements, patterned electrodes were elaborated thanks to the use of a removal mask during gold deposition. Delimited zones of the glass substrate were not covered by gold. Before electrosynthesis of poly(2CA), gold electrodes were immersed overnight in aqueous sulfuric acid solution, rinsed three times with milli-Q water, and dried with a nitrogen gas stream. (18) Handbook of Conducting Polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998. (19) Zotti, G. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: Chichester, 1997; Vol. 2, p 137. (20) Park, S.-M. In Handbook of Organic Conductive Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: Chichester, 1997; Vol. 3, p 429. (21) Maeda, S.; Corradi, R.; Armes, S. P. Macromolecules 1995, 28, 2905. (22) Roux, S.; Audebert, P.; Pagetti, J.; Roche, M. New J. Chem. 2000, 11, 877.
Langmuir, Vol. 19, No. 2, 2003 307 Synthesis of Poly(2CA) on Gold Electrodes and Electrochemical Characterization. Electrochemical synthesis of poly(2CA) and CV experiments were performed in an electrochemical cell fitted with a saturated calomel reference electrode (SCE), a gold electrode (1 cm2), and a platinum counter electrode. The electrochemical equipment was a potentiostat EG&G Princeton Applied Research 273A connected to a personal computer. The solvent was spectroscopic grade acetonitrile (Aldrich). Lithium perchlorate (Acros) was added as the supporting electrolyte salt. The concentration of the monomer 2CA for polymer synthesis was 10-2 mol‚L-1. All films were prepared potentiostatically at 1.080 V (vs SCE) during 20 s (Q ) 25 mC‚cm-2) in degassed acetonitrile containing 0.1 M LiClO4. Film cycling was performed in degassed acetonitrile with 0.1 M LiClO4 at a scan rate of 50 mV‚s-1. Covalent Grafting of Free Radical Initiator (ABAP) on Poly(2CA). Poly(2CA) coated gold substrates obtained by the above-described method were immersed in 2-propanol solution containing 0.1 M EDC and 0.2 M PFP for 90 min. The reaction between COOH groups of the polymer and PFP, assisted by EDC, is expected to yield pentafluorophenyl esters at the surface of the substrate.11 Samples were rinsed in 2-propanol solution and dried with a nitrogen gas stream. The covalent grafting of ABAP onto the substrate was performed by immersion of this pentafluorophenyl ester derivatized surface in 0.1 M ABAP dissolved in a mixture of methanol and water (1:1 v/v) for 2.5 h. Samples were washed with THF and dried with a nitrogen gas stream. SIRP of Styrene. ABAP grafted sample was placed in a deoxygenated Schlenk tube. A degassed mixture of styrene and toluene (1:1 v/v) was transferred into the Schlenk tube. Polymerization was carried out by heating the Schlenk tube (90 °C), under nitrogen atmosphere, for 12 or 24 h. Afterward, samples were immersed in toluene, dried, and SOXHLET extracted for 24 h to remove physisorbed polystyrene. Polystyrene formed in solution was precipitated in cold methanol, filtered, washed with cold methanol, and dried under reduced pressure at 60 °C. To estimate the influence of thermal polymerization of styrene without initiator, a degassed mixture of styrene and toluene (1:1 v/v) was heated during 24 h. The resulting polystyrene was submitted to the same treatement as previously described. X-ray Photoelectron Spectroscopy. The surface chemical composition was determined at each step of the functionalization by XPS using a SSIX probe (SSX 100/206) spectrometer from Fisons, operating at a pressure in the low 10-8 Torr range, equipped with an aluminum anode and a quartz monochromator. Spectra were recorded at a takeoff angle of 35° (angle between the plane of the sample surface and the entrance lens of the analyzer) and with a pass energy of 150 eV. The theoritical analyzer resolution expected with that setting is 1.5 eV. Intensity ratios were converted into atomic ratios by using the sensitivity factors proposed by the manufacturer (Scoffield photoemission cross sections, variation of the electron mean free path according to the 0.7th power of the kinetic energy, constant transmission function). Curve fitting has been done using a GaussianLorentzian (85%-15%) linear combination and a linear background. Contact Angle Measurements. Contact angles of water were measured on Au/poly(2CA), Au/poly(2CA)-PFP, Au/poly(2CA)ABAP, and Au/poly(2CA)-PS at room temperature, using the sessile drop method and an image analysis of the drop profile. The water droplet volume was 0.6 µL (purified by MilliQ Plus system from Millipore). Each determination was performed by averaging the results obtained on at least 10 droplets. Atomic Force Microscopy. AFM images were obtained in contact mode with an Autoprobe instrument (Park Scientific Instrument). V-shaped cantilevers with theoretical normal spring constants of about 10 N/m were used in this study. The thickness of each coating was estimated by depth profiles obtained by AFM measurements performed on patterned electrodes (glass surface not totally covered by a gold layer). As poly(2CA) and therefore polystyrene grew only on gold film, the thickness of each layer was deduced from the variation of the height between glass substrate and gold top surface after each step of the functionalization. Time-of-Flight Secondary Ion Mass Spectrometry. The measurements by TOF SIMS were carried out with a Charles
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Scheme 1. Modification of Gold Surface for SIRP of Styrene
Figure 1. CV of poly(2CA) recorded in 0.1 M LiClO4/CH3CN at a scan rate of 50 mV‚s-1.
Synthesis and Characterization of Poly(2CA). A detailed description of the electrosynthesis of poly(2CA) and its electrochemical behavior was reported elsewhere.22 The average value of the charge (Q) passed during the electrolysis is around 25 mC. According to Faraday’s law (eq 1), this charge corresponds to a polymer layer thickness (e) of around 90 nm.
e)
Evans and Associates TFS-4000 MMI TOF SIMS using a 69Ga+ (15 keV) liquid metal source. The Ga+ beam is rastered over a 175 µm × 175 µm surface area. The pressure in the analysis chamber was 7 × 10-10 Torr. The secondary ions produced were accelerated up to an energy of 3 keV and deflected by three electrostatic analyzers in order to compensate their initial energy and angular distributions. All spectra were acquired during 5 min.
Results and Discussion We implemented SIRP onto carboxylic acid functionalized PPy (poly(2CA)) on gold using a sequential approach to attach a free radical initiator to the surface followed by polymerization of styrene, as follows (Scheme 1): (1) a PPy layer, presenting terminal carboxylic acid groups, was formed by electropolymerization of 2CA onto gold, (2) the COOH groups in the poly(2CA) layer were converted into pentafluorophenyl esters by reaction with PFP/EDC, (3) a commercially available amine-terminated free radical initiator (ABAP) was fixed by reaction in solution with the pentafluorophenyl ester groups,and (4) styrene was polymerized at the surface by free radical polymerization. Each step (from the deposition of poly(2CA) to the SIRP of styrene) of the procedure was characterized by XPS, AFM, CV, and contact angle measurements.
Q(MPy + δMClO4) FA(2 + δ)F
(1)
MPy and MClO4 are the masses of the 2CA repeat unit and of the doping perchlorate anion, respectively, δ is the doping level,22 F is the relative density of the polymer, A is the electrode area, and F is the Faraday constant. The thickness of poly(2CA) estimated by topographic profiles deduced from AFM measurements was 146 ( 15 nm. These values are slightly larger than those calculated by Faraday’s Law (90 nm) but are in the same magnitude range. After electrolysis the gold electrode became light green and the resulting film displayed a characteristic CV response (Figure 1). Poly(2CA) films were analyzed by XPS (Figure 2a). The experimental atomic ratios are consistent with the expected stoichiometric values (Table 1). The XPS C 1s spectrum of poly(2CA) shows two main peaks at about 284.5 and 288.9 eV (Figure 3a). The shape of the peak centered on 284.5 eV (broad and asymmetric) is typical of polypyrrole.23-28 Although this C 1s main structure does not show clear shoulders or splitting, the peak envelope obviously does not originate from a single line. Curve fitting is shown in Figure 3. Some of the fitting components can include contributions from different species whose chemical shifts are too close in comparison with the instrumental resolution (typically 1.5 eV). The initial parameters used to fit the spectral features (number of (23) Pfluger, P.; Street, G. B. J. Chem. Phys. 1984, 80, 544. (24) Eaves, J. G.; Munro, H. S.; Parker, D. Polym. Commun. 1987, 28, 38. (25) Atanasoska, L.; Naoi, K.; Smyrl, W. H. Chem. Mater. 1992, 4, 988. (26) Malitesta, C.; Iosito, I.; Sabbatini, L.; Zambonin, P. G. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 629. (27) Glidle, A.; Swann, M. J.; Hadyoon, C. S.; Cui, L.; Davis, J.; Ryder, K. S.; Cooper, J. M. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 131. (28) Tourillon, G.; Jugnet, Y. J. Chem. Phys. 1988, 89, 1905.
Surface Initiated Polymerization of Styrene
Figure 2. XPS survey spectra of (a) poly(2CA), (b) poly(2CA)PFP, (c) poly(2CA)-ABAP, and (d) poly(2CA)-PS (12 h).
components and their energy) were chosen on the basis of previous XPS works on PPy23-28 and poly(acrylic acid).29,30 The results are summarized in Table 2. The peaks appearing at 284.1 and 284.9 eV correspond to the β and (29) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons Ltd.: Chichester, 1992. (30) Ko¨hler, L.; Gourdin, D.; Sporken, R.; Grigorov, K.; Riga, J.; Caudano, R. Polym. Int. 1998, 47, 474.
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R carbon species, respectively. Their binding energy splitting is close to the expected splitting of 0.9 eV for polypyrrole.23-28 The line at 285.5 eV originates from the CH2 units situated on the pendant group and next to the COOH and pyrrole functions. The two next structures at 286.4 and 287.7 eV are more difficult to assign. Their origin is controversial and seems to depend on the preparation conditions. They are usually referred to structural disorder, such as Rβ linkages. Malitesta et al.26 have assigned these peaks to CsN+ (polaron) and CdN+ species (bipolaron), in agreement with the work of Eaves et al.24 The torsion angle between monomeric units is wellknown to exist in PPy bearing pendant groups (steric effects), which breaks the π electron delocalization and tends to localize the positive charges during the doping process.28 The result is the appearance of an asymmetry toward high binding energy. These attributions are consistent with the presence of shoulders in the N 1s spectrum (not shown) at 401 and 402.7 eV, corresponding to N atoms existing in more positive environments.31 Since the N 1s core level should not be sensitive to structural disorder,28 we are in favor of the second interpretation for the appearance of these two peaks at 286.4 and 287.7 eV. Several satellites appearing between 286 and 290 eV contribute also to the appearance of this broad asymmetry. They are attributed to π-π* shake-up transitions.23,25,26,28 The doping process has been shown to lead to a uniform extraction of electrons from the π bonding band with an extension of the π and π* bands toward the Fermi level.28 The structure situated at 289.0 eV can be attributed to carboxylic groups (COOH) and confirms the presence of these groups at the surface of the film. As ulterior polymerization of styrene is carried out at 90 °C, we have controlled that poly(2CA) was not affected by such a heating. Gold substrates covered by poly(2CA) were immersed in toluene at 90 °C for 24 h. No color change and no major change in the electrochemical behavior were observed, indicating that the polymer film was not removed under these conditions. XPS data confirmed the presence of poly(2CA) on the gold plate after the treatment: no gold peak appeared on the survey spectrum, no change of the C 1s spectrum shape could be detected, and the film composition was very close to that of the freshly synthesized poly(2CA) (Table 1). Activation of COOH Groups and Covalent Grafting of ABAP. To bind the largest possible amount of initiator (ABAP) to the surface, COOH groups were activated by EDC and PFP. The reaction between PFP and COOH moieties is expected to yield fluorinated ester groups on the surface. The efficiency of the esterification reaction is revealed by XPS thanks to the presence of fluorine atoms at the surface (Figure 2b). The structure that grows at 287.9 eV (Figure 3b, Table 2) is characteristic of the carbon atoms belonging to the pentafluorophenyl ring.27,32 This peak includes also the carbon atoms of the ester functions. A structure characteristic of COOH groups (289.0 eV) still appears, indicating that a part of the carboxylic acid functions is intact (Figure 3b, Table 2). Assuming a statistical distribution of N and F species along the axis perpendicular to the surface, a rough estimation of the conversion rate of COOH groups into fluorinated esters can be obtained. Assuming a complete derivatization of the COOH groups, the calculated atomic (31) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135. (32) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr. NIST X-ray Photoelectron Spectroscopy Database 20, Version 3.2; Standard Reference Data Program of the National Institute of Standards and Technology; USA, 2000.
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Table 1. Atomic Compositions Obtained from the Area of Core Level Photoemission Peaks Corrected by Sensitivity Factors (meas), or from Stoichiometric Ratios (calc) O/C poly(2CA)a poly(2CA)b poly(2CA)-PFP poly(2CA)-ABAP poly(2CA)-PS a
F/C
N/C
N/F
meas
calc
meas
calc
meas
calc
0.28 0.34 0.17 0.20 0
0.29 0.29 0.15 0.066 0
0 0 0.17 0.08 0
0 0 0.39 0 0
0.13 0.11 0.15 0.17 0
0.14 0.14 0.077 0.47 0
meas
calc
0.87 2.61
0.20
After electrosynthesis. b After 24 h in toluene at 90 °C.
Figure 3. XPS spectra of the C 1s core level of poly(2CA) after (a) electrosynthesis, (b) reaction with PFP, (c) grafting of ABAP, and (d) SIP of styrene for 12 h.
ratio N/F should be equal to 0.2. The experimental ratio of 0.87 is significantly higher than the calculated one, indicating that only 25% of the COOH groups are converted into fluorinated esters. However, as the derivatization is supposed to be more effective on the very extreme surface, and as the depth probed by XPS is about four to five monolayers, we can conclude that the conversion rate of the COOH groups of the outer layer into fluorinated esters is high (largely more than 25%): the functionalization is therefore particularly efficient. After reaction with ABAP, the fluorine peak does not disappear but N/F and N/C ratios increase while the F/C ratio decreases (Figure 2c and Table 1). This observation is consistent with the partial derivatization of the surface by the free radical initiator, ABAP. Accordingly, the peak centered on 286.5 slightly increases, in agreement with the disappearance of Cs OsCdO species and the appeareance of CdN species, and the peak corresponding to the carbon atoms of the pentafluorophenyl ring decreases (Figure 3c, Table 2). Polymerization of Styrene from a Functionalized Gold Surface. The polymerization of styrene was carried out at 90 °C under nitrogen for 12 and 24 h in the presence of the ABAP derivatized surface. The heating of the polymerization medium generates a thermal homolytic
cleavage of ABAP, providing two free radicals. As the initiator is bound to the surface by only one end, the second part of the initiator can diffuse into the solution and starts a bulk polymerization of styrene. Consequently, a part of the formed polystyrene is not covalently attached to the substrate (Scheme 1). At the end of the polymerization, the solid sample was removed from the polymerization medium and was SOXHLET extracted for 24 h to remove polystyrene (produced in solution) physisorbed on the surface. The polystyrene present in the solution was recovered by precipitation into cold methanol. Polystyrene in solution originates not only from the initiation by free radicals formed by thermal decomposition of the azo initiator grafted onto the surface but also from thermal induced polymerization.13,33-35 To estimate the influence of the latter on the amount of precipitated polystyrene, a mixture of styrene and toluene was heated in the same conditions but without any initiator. The weight of polystyrene formed in these conditions is very small. It corresponds to around 10% of the total weight (33) Talat-Erben, M.; Bywater, S. J. Am. Chem. Soc. 1955, 77, 3710. (34) Mayo, F. R. J. Am. Chem. Soc. 1968, 75, 1289. (35) Chong, Y. K.; Rizzardo, E.; Solomon, D. H. J. Am. Chem. Soc. 1983, 105, 7761.
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Table 2. Results of the C 1s Curve Fitting binding energy (eV) A B C
284.1 284.9 285.5
D
286.4
E
287.7
F G
289.0 290.7
A B C
283.9 284.8 285.5
D
286.5
E
287.9
F G
289.0 290.1
A B
283.7 284.6
C
285.5
D
286.5
E
288.0
F
288.9
G
290.5
assignment
ref
%
Poly(2CA) Cβ CR CsCOOH CsNR2 CsN+ a shake-up π-π*b CdN+ a shake-up π-π*b COOH shake-up π-π*
23-28 23-28 29, 30 29, 30 24, 26 23, 25, 28 24, 26, 29 23, 25, 28 29, 30 23, 26, 28
26.5 26.7 20.2
Poly(2CA)-PFP Cβ CR CsCOO CsNR2 OdCsOsC CsN+ a shake-up π-π*b CsF OdCsOsC CdN+ a shake-up π-π*b COOH shake-up π-π*
23-28 23-28 29 29, 30 29 24, 26 23, 25, 28 27, 31 29 24, 26, 29 23, 25, 28 29, 30 23, 26, 28
21.5 23.0 19.5
Poly(2CA)-ABAP Cβ CR CH3sC CsCONH CsNR2 C(CH3)2sN CdN CsN+ a shake-up π-π*b CsF OdCsOsC CdN+ a shake-up π-π*b COOH CONHc shake-up π-π*
23-28 23-28 29 29 29, 30 29, 30 29, 30 24, 26 23, 25, 28 27, 31 29 24, 26, 29 23, 25, 28 28, 29 29, 31 23, 26, 28
12.6 34.6
Table 3. m/z, Composition, and Structure of the Main Fragments Revealed by TOF SIMS Spectra of Samples after 24 h of Styrene Polymerization
7.3 2.7 14.1 2.5
8.3 21.2
5.0 1.5
19.5 11.6 12.4
7.9 1.4
a Three electrostatically inequivalent N sites are detected in the N 1s spectrum at about 399.7, 401.0, and 402.7 eV. They have been attributed to CsN (main pyrrole peak), CsN+, and CdN+ species, respectively. b This peak envelope contains also a contribution from π-π* transitions. A series of satellites are known to appear on the high binding energy side of the C 1s peak of polypyrrole, arising from shake-up transitions involving several valence electrons.23,25,28 c This peak envelope contains probably also a contribution from CONH species, situated at a slightly lower binding energy, typically at about 288.6 eV.
of polystyrene obtained in the presence of the initiator derivatized surface. The polystyrene layer formed from the derivatized gold surface is sufficiently thick to mask the poly(2CA) film, since no nitrogen or oxygen species (present in poly(2CA)) are detected in the XPS survey spectrum and the C 1s related structures disappear (Figures 2d and 3d). The C 1s core level spectrum of the surface after SIRP of styrene (for 12 and 24 h) displays two components situated at binding energies of 284.6 and 291.6 eV (Figure 3d). The first component is characteristic of hydrocarbon species, and the second one can be associated with a shake-up satellite, characteristic of an aromatic ring. The spectral features are those expected for polystyrene.29 But XPS is not the most appropriate tool to identify the chemical nature of the layer formed onto poly(2CA) after the polymerization stage. This layer was therefore characterized by TOF SIMS. The positive ion spectra of the sample after styrene polymerization reveal the presence of
Figure 4. Water contact angle values of poly(2CA), poly(2CA)ABAP, and poly(2CA)-PS.
polystyrene on the surface which was undoubtedly identified by characteristic fragments36,37 (Table 3). Surface Characterization of Polystyrene Layer. Contact angle measurements reveal a modification of the surface wettability after polymerization and are in perfect agreement with the XPS and TOF SIMS data (Figure 4). The surface is becoming largely more hydrophobic after polystyrene synthesis, and the contact angle value measured on the poly(2CA)-PS sample (θ ) 84°) is consistent with the value of the water contact angle on polystyrene reported in the literature.38 The initial poly(2CA) surface exhibited a water contact angle of 49°. AFM experiments showed obviously the surface modification after polymerization (Figure 5). Samples covered by a PS layer appeared smoother than those coated by a poly(2CA) film which was rougher than the naked gold substrate (Figure 5). This was confirmed by roughness values determined by AFM experiments: Rms of the polystyrene layer was largely smaller than the Rms values of the poly(2CA) film and the naked gold surface (Figure (36) Newman, J. G.; Carlson, B. A.; Michael, R. S.; Moulder, J. F.; Hohlt, T. A. Static SIMS Handbook of Polymer Analysis, A Reference Book of Standard Data for Identification and Interpretation of Static SIMS Data; Perkin-Elmer Corporation: USA. (37) Vanden Eynde, X.; Oike, H.; Hamada, M.; Tezuka, Y.; Bertrand, P. Rapid Commun. Mass Spectrom. 1999, 13, 1917. (38) Jarvis, N. L.; Fox, R. B.; Zisman, W. A. In Contact Angles Wettability and Adhesion; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; p 317.
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Figure 5. AFM images and Rms values of (a) an uncoated gold electrode, (b) a poly(2CA) film electrodeposited on a gold electrode and (c and d) a polystyrene layer after (c) 12 h and (d) 24 h of styrene polymerization from an ABAP derivatized poly(2CA) film.
5). After 24 h of polymerization, the polystyrene layer exhibited a very weak value of Rms. Moreover, AFM images showed that polystyrene layers appeared uniform and homogeneous over a wide range (50 µm × 50 µm). The polystyrene layer thickness was estimated by AFM because the strong absorption of the laser light by the poly(2CA) layer impedes its measurement by ellipsometry. As the weight of the precipitated polystyrene generated after 24 h of polymerization (m ) 8.00 g) was twice as large as the one of polystyrene obtained after 12 h of polymerization (m ) 3.95 g), a thicker film was naturally expected for longer polymerization duration. The polystyrene layer was effectively thicker after 24 h of polymerization (184 ( 15 nm) than after only 12 h (86 ( 15 nm). Moreover, it must be pointed out that the precipitated polystyrene weight and the polystyrene layer thickness doubled for a 2-fold polymerization duration. A linear variation of thickness was indeed demonstrated by previous works.8,12 As poly(2CA) was not degraded at 90 °C, the variations of CV response observed for samples obtained after 24 h of polymerization can be attributed to the presence of a polystyrene layer on poly(2CA). The polystyrene layer should indeed behave as a barrier hindering the diffusion of doping species. The electroactivity of the underlying ICP film should thus be greatly affected. But surprisingly, the samples obtained after 12 h of polymerization still exhibited a CV response attributed to the classical redox switching of poly(2CA) (Figure 6a). This could be explained by either a discontinuous polystyrene layer (this case will be discussed later) or an influence of the solvent. Acetonitrile (solvent of the electrolytic solution) probably induces the swelling of polystyrene allowing the diffusion of doping species. However, the swollen polystyrene film limits the accessibility of the perchlorate anions to the electrode. Therefore, the CV response is less reversible and peak intensities are smaller (cathodic peak intensity, Ipc ∼ 25 µA) than those for uncoated poly(2CA) (Ipc ∼ 170 µA)
Figure 6. CV curves of functionalized Au/poly(2CA) after polymerization of styrene for (a) 12 h (thick line) and (b) 24 h (thin line) recorded in 0.1 M LiClO4/CH3CN at the scan rate 50 mV‚s-1.
(Figure 1). As already shown, the SIRP of styrene for 24 h provided a thicker film: in this case, the CV response is actually not well-defined (Figure 6b). The intensity of the reduction peak is very weak, and its shift is large (ca. 600 mV). Moreover, no oxidation peak is detectable in the investigated potential domain. The shape of this CV response arises from a severe restriction of doping species diffusion due to the presence of a thicker and more uniform polystyrene film. The CV response recorded for a sample coated by polystyrene after 24 h of polymerization allows us to rule out the assumption of a patchy coverage, advanced to explain the unexpected electrochemical response of the sample obtained after 12 h of polymerization. The uniformity of the polystyrene layer obtained after 24 h of polymerization demonstrates that the polystyrene layer obtained after 12 h exhibits no discon-
Surface Initiated Polymerization of Styrene
tinuity. Polymerization duration influences indeed the chains’ length but not their density on the substrate surface. The chain density depends on initiation conditions, which remain the same whatever the polymerization duration. Longer chains (resulting from a longer polymerization duration) can obviously mask uncoated zones. Such zones would be easily accessible when the sample is immersed in the electrolytic solution (0.1 M LiClO4/ CH3CN), that is, when the chains are stretched. So, whatever the polymerization duration, CV responses would be the same, since the uncoated area would be identical. Consequently, the CV responses observed after styrene polymerization (Figure 6) actually arise from the restricted diffusion of doping species due to the presence of a continuous polystyrene layer bound to poly(2CA). From this electrochemical study and the AFM observations, we can conclude that styrene polymerization initiated from ABAP functionalized poly(2CA) yields a uniform continuous PS layer. Conclusion In this contribution, we have reported on the successful surface initiated polymerization of styrene from a gold electrode coated with carboxylic acid functionalized polypyrrole. The proposed method is based on a sequential approach to attach a free radical initiator onto the surface. A polypyrrole layer containing carboxylic acid pendant groups was first formed by electropolymerization of 3-(pyrrol-1-yl)propanoic acid onto gold. This layer has been shown to be unaffected by the temperature imposed for the polymerization of styrene. The COOH groups of this layer were converted to pentafluorophenyl esters by reaction with pentafluorophenol assisted by 1-ethyl-3(dimethylamino)propylcarbodiimide. An amine terminated free radical initiator, 2,2′-azobis(2-amidinopropane) dihydrochloride, was then grafted on this surface, and
Langmuir, Vol. 19, No. 2, 2003 313
finally styrene was polymerized by free radical polymerization. TOF SIMS, XPS, AFM, CV, and contact angle measurements performed at each step of the procedure have confirmed the modifications generated at the surface. The proposed method has been shown to allow the growth of a thick, continuous, and smooth polystyrene layer. Up to now, SIRP from gold surfaces required the presence of adsorbed functionalized thiols. Thermal desorption and chain transfer behavior of thiols moderate strongly the efficiency of SIRP. In this paper, we demonstrated that the use of electrodeposited conducting polymers containing reactive pendant groups is a very efficient method for the covalent immobilization of the initiator. The electrochemical synthesis of an anchoring layer offers many advantages such as the accurate control of the layer thickness, the simplicity, and the swiftness of the procedure. The concept, described and demonstrated here with a model system (polymerization of styrene), is a useful method for the formation of a thin film from any polymer that can be synthezised using a radical chain polymerization reaction. Therefore, this method can be employed to tailor surface properties of conducting polymers or to prepare multilayered polymeric materials with covalent binding between each layer. Acknowledgment. S.D.C. and A.S.D. thank the Belgian National Fund for Scientific Research for its financial support. Thanks also go to the “Unite´ de Chimie des Interfaces” (UCL) for access to the XPS facilities. Dr. F. Cleymand and Dr. B. Nysten (POLY, UCL) are thanked for their fruitful help for AFM experiments, and C. Poleunis (Laboratoire de Physico-Chimie et de Physique des Mate´riaux, UCL) is thanked for the polystyrene identification by TOF SIMS. LA0205001
