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New Route for the Elaboration of Polyolefin Surfaces Bearing Azo Molecules N. Delorme,†,‡ J.-F. Bardeau,‡ D. Nicolas-Debarnot,† A. Bulou,‡ and F. Poncin-Epaillard*,† Laboratoire Polyme` res, Colloı¨des et Interfaces (CNRS, UMR 6120), Universite´ du Maine, Avenue O. Messiaen, 72085 Le Mans Cedex 09, France, and Laboratoire de Physique de l’Etat Condense´ (CNRS, UMR 6087), Universite´ du Maine, Avenue O. Messiaen, 72085 Le Mans Cedex 09, France Received November 29, 2002. In Final Form: March 20, 2003 A new route for the elaboration of the polypropylene surface directly modified by 4-amino-4′nitroazobenzene (Disperse Orange 3) was studied. The synthesis consists of a cold carbon dioxide-plasma treatment followed by a mixed anhydride reaction and Disperse Orange 3 fixation. The quantity of active acid introduced by the plasma treatment was estimated to be 0.7 groups/nm2 by fluorescence labeling with thionine acetate. The azo-modified surfaces were characterized by Fourier transform infrared, resonance Raman, and X-ray photoelectron spectroscopies. Confocal resonance Raman spectrometry has shown that Soxhlet extraction is effective to eliminate diffused molecules from the material. It was shown that the azo dye molecules are chemically fixed onto the polymer surface.
Introduction Polymeric materials that are able to undergo reversible photochemical transformations are interesting for material engineering. Azobenzene derivatives are well-known photosensitive chromophores because of their reversible trans-cis isomerization (Figure 1) by irradiation with UV or visible light.1 When azobenzene derivatives isomerize, the molecular dimensions and absorption spectra are modified.1 Variations of the molecular dimensions lead to the modification of the dipolar moment, which varied from 0 to 3 D during the isomerization of trans-azobenzene into cis-azobenzene.2 Thus, the protocol of surface wettability should be possible and seems to be of high practical interest.3 Recently, a large number of research groups have studied the properties of polymer materials containing azobenzene derivatives because of their various potential applications.4-7 Natansohn et al.8-10 prepared an azo-dye-containing polymer with reversible optical storage properties. Kaino11 synthesized a poly(methyl methacrylate) (PMMA) co-methacrylate ester of a bis-azo* Corresponding author: E-mail, fabienne.poncin-epaillard@ univ-lemans.fr. † Laboratoire Polyme ` res, Colloı¨des et Interfaces, Universite´ du Maine. ‡ Physique de l’Etat Condense ´ , Universite´ du Maine. (1) Rau, H. In Photoisomerization of azobenzenes; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. 2, p 119. (2) Zollinger, H. Azo and diazo chemistrysaliphatic and aromatic compounds; Interscience Publishers, Inc.: New York, 1961. (3) Ishihara, K.; Okazaki, A.; Negishi, N.; Shinohara, I.; Kataoka, K.; Sakurai, Y. J. Appl. Polym. Sci. 1982, 27, 239. (4) Rocha, L.; Dumarcher, V.; Malcor, E.; Fiorini, C.; Denis, C.; Raimond, P.; Geffroy, B.; Nunzi, J.-M. Synth. Met. 2002, 127, 75. (5) Kucharski, S.; Janik, R.; Szkodzinska, J. Colloids Surf., A 2002, 198-200, 359. (6) Pan, Q.; Zhang, Z.; Fang, C.; Shi, W.; Gu, Q.; Wu, X. Mater. Lett. 2001, 50, 284. (7) Sarkar, N.; Sarkar, A.; Sivaram, S. J. Appl. Polym. Sci. 2001, 81, 2923 (8) Ho, M. S.; Natansohn, A.; Rochon, P. Macromolecules 1995, 28, 6124. (9) Xie, S.; Natansohn, A.; Rochon, P. Macromolecules 1994, 27, 1489. (10) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268. (11) Kaino, T. J. Opt. A: Pure Appl. Opt. 2000, 2, R1.
Figure 1. Photoisomerization of azobenzene.
dye derivative that can be used as a third-order nonlinear optical polymer waveguide. Other groups12,13 have studied the properties of an azo chromophore (Disperse Red 1; DR1) spin-coated with PMMA. However, none of these routes seem compatible with the use of bare commercial polypropylene (PP), a low-cost and easily recycled material. Indeed, the introduction of the azo-dye-containing polymer in a polyolefin material may involve a modification of the bulk properties of the polyolefin, as was shown by Xie et al.,9 who observed an increase in the glass temperature transition of a polystyrene (from 100 to 110 °C) blended with an azo-dye-containing polymer. Furthermore, the spin-coating process cannot be easily applied to PP because of its low solubility in most of the organic solvents. The first evidence of a surface grafting of an azo functionality onto polyethylene was shown by Sarkar et al.14 In a previous paper,15 we have shown the possibility of preparing a PP surface bearing grafted azo polymer chains. DR1 was transformed into an acrylic monomer and then grafted onto the PP surface modified by cold CO2-plasma treatment. In this article, we report on the preparation of a PP surface directly modified by 4-amino4′-nitroazobenzene (Disperse Orange 3, DO3; Figure 2). Such a synthesis route allows the introduction of the azo (12) Buffeteau, T.; Lagugne-Labarthet, F.; Pe`zolet, M.; Sourisseau, C. Macromolecules 1998, 31, 7312. (13) Wang, G.; Gan, F.; Wang, J.; Yang, L.; Wang, G.; Xu, Z. J. Phys. Chem. Solids 2002, 63, 501. (14) Sarkar, N.; Bhattacharjee, S.; Sivaram, S. Langmuir 1997, 13, 4142. (15) Poncin-Epaillard, F.; Beunet, J.; Bulou, A.; Bardeau, J.-F. J. Polym. Sci., Part A 2001, 39, 3052.
10.1021/la026926x CCC: $25.00 © 2003 American Chemical Society Published on Web 05/16/2003
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covalent bond between carboxylic acid and the amine function of the Disperse Orange molecule. Because the absolute number of surface groups is low and the bulk polymer has to be distinguished from the surface, a set of very sensitive surface techniques has been used to characterize the modified material. Spectrochemical titration and X-ray photoelectron measurements gave evidence of plasma modification, whereas the grafting of the azo molecules was determined by Fourier transform infrared (FT-IR) spectrometry, micro-Raman spectrometry under resonance conditions, and X-ray photoelectron spectroscopy (XPS) measurements. Experimental Section
Figure 2. Preparation of the DO3-fixed PP surface.
molecules onto a PP material without modifying its bulk properties because only the material surface is modified.16 Because of the poor reactivity of polyolefins, the functionalization of the PP surface is necessary before the fixation of the DO3 molecules. Polyolefin surface functionalization has been the subject of a variety of studies: several groups have described the solution-state oxidative method,17 entrapment functionalization,18 or plasma treatments.19 In this work, a CO2 plasma is used to introduce carboxylic acid functions on the surface of the polymer. The interest of such functionalization is that plasma introduces chemical reactive species only onto the surface, with little etching or cross-linking,20 allowing the bulk properties of the materials to remain unchanged, contrary to chemical oxidative treatments.21 Furthermore, although the present study deals with the modification of the PP surface properties, the preparation route can be applied to other polyolefins, such as polyethylene or polystyrene surfaces, by modifying the plasma parameters (power of the discharge, distance from the exciting source, and time of treatment). It is well-known that to be efficient, the reactions employed to modify the groups at a polymeric surface must proceed in high yields because the separation and purification of the products are difficult.22 The mixed anhydride method23 was chosen precisely to introduce a (16) Hollander, A.; Wilken, R.; Behnisch, J. Surf. Coat. Technol. 1999, 116-119, 788. (17) Tao, G.; Gong, A.; Lu, J.; Sue, H.-J.; Bergbreiter, D. E. Macromolecules 2001, 34, 7672. (18) Bergbreiter, D. E.; Walchuk, B.; Holtzman, B.; Gray, H. N. Macromolecules 1998, 31, 3417. (19) Carrino, L.; Moroni, G.; Polini, W. J. Mater. Proc. Technol. 2002, 5584, 1. (20) Me´dard, N.; Soutif, J.-C.; Poncin-Epaillard, F. Surf. Coat. Technol. 2002, 160, 197. (21) Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M. J. Am. Chem. Soc. 1977, 99, 4736. (22) Rasmussen, J. R.; Bergbreiter, D. E.; Whitesides, G. M. J. Am. Chem. Soc. 1977, 99, 4746. (23) Vaughan, J. R.; Osato, R. L. J. Am. Chem. Soc. 1952, 74 (3), 676.
Substrate. The 180-µm-thick PP substrate supplied by Goodfellow (England) corresponds to a copolymer of predominantly PP with a small amount of polyethylene, obtained by extrusion and synthesized with a Ziegler-Natta catalyst. It is isotactic and semicrystalline (metastable β form, melting point ) 145 °C). Before any use, the films (2 × 2 cm) were ultrasonically washed in ethanol for 10 min. Cold Plasma Treatment. A tubular quartz sample chamber with dimensions 76 × 500 mm (diameter × length) was used. Power (P) was supplied by a microwave generator (433 MHz). A primary pump and a diffusion pump were used to attempt a 10-6 mbar ultimate pressure in the chamber. The plasma was turned on after 5 min of gas introduction. In the following, d denotes the distance between the bottom of the excitator and the sample, t the treatment time, and F the gas flow. Previous studies in our laboratory15,24 determined that the following parameters constitute a good compromise between the optimal functionalization and minimal degradation for PP: P ) 60 W, d ) 10 cm, t ) 270 s, and F ) 20 sccm (standard cubic centimeters per minute). Surface Carboxylic Acid Titration. The plasma-treated films were placed into a capped glass tube containing an ethanol solution of thionine acetate (5 × 10-8 M). The solutions were then stirred at room temperature for 24 h before the fluorescence titration. The fluorescence titration was performed on a Jobin-Yvon T64000 Raman spectrometer equipped with a cooled chargecoupled device camera, 600-groves/mm grating, and Notch filter for the rejection of excitation. The laser light excitation (λ ) 514.5 nm) was generated with a coherent argon-krypton ion laser, and the spectra were collected with a ×50 microscope objective. Grafting Procedures. The plasma-treated film was first rinsed with acetone to eliminate the degradation products due to the plasma treatment. The treated sample was dried with dry N2 and placed in a capped glass tube with a pivaloyl chloride solution (t-Bu-COCl, 1.2 mL; 99.9%, Aldrich) in anhydrous toluene (5 mL; Aldrich) containing triethylamine (Et3N, 1.4 mL; Aldrich) as an HCl acceptor. During the reaction, the solution was stirred and kept at -5 °C for 3h. The substrate was then quickly rinsed with dry toluene and placed in a capped glass tube containing a 4-amino-4′-nitroazobenzene (DO3) solution (0.05 g, Aldrich) in anhydrous toluene (10 mL) for 15 h. The substrate was finally Soxhlet extracted with acetone in reflux for 7 h, dried with dry N2, and stored under vacuum. Raman Confocal Microspectrometry Analysis. MicroRaman experiments were performed on a Jobin-Yvon T64000 Raman spectrometer. The spectra were collected with a ×100/ 0.95 numerical aperture microscope objective (laser wavelength ) 514.5 nm, excitation power ) 6 mW on the sample). The spot size on the sample is smaller than 2 µm in diameter and depth, thanks to a confocal system (pinhole ) 1 µm). XPS Analysis. XPS spectra were acquired with an ESCA LHS 12 instrument (Leybold) at the Laboratoire de Physique des Couches Minces (Institut des Mate´riaux de Nantes). The photoemission was excited by a monochromatic Mg KR beam at 1253.6 eV. The emission was analyzed at a takeoff angle of 90° (24) Aouinti, M. Caracte´risation de la de´charge microonde de CO2 et e´tude des interactions plasma-polypropyle`ne. Ph.D. Thesis, Universite´ du Maine, Le Mans, France, 2000.
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Figure 3. C1s high-resolution spectrum of PP treated by CO2 plasma (P ) 60 W, d ) 10 cm, t ) 270 s, and F ) 20 sccm). relative to the horizontal, yielding a sampling depth around 10 nm25 due to the mean free path of the electrons. Calibration was conducted on the C1s peak of the C-C bond at 285.0 eV. The curve fitting was performed using Peak Fit 4.0 software (Jandel Scientific). The full width at half-maximum was kept at less than 1.5 when it was possible, and the peak shape was chosen with Gaussian (80%)/Lorentzian (20%) curve fitting.26 FT-IR Analysis. IR spectroscopy analyses were performed on a Bru¨ker IFS 66 spectrometer with a grazing angle unit (IRAS cell, θ ) 70°, resolution ) 2 cm-1, 200 scans, MCT detector). Contact-Angle Measurements. The contact-angle measurements were performed using a Rame´-Hart Inc. goniometer. Five drops of Ultrapure (Millipore) water (2 µL) were used to measure the wettability of the films.
Results and Discussion CO2-Plasma Treatment. For polyolefins, exposure to a CO2 plasma introduces different types of oxygencontaining functional groups on the uppermost surface of the material.27 Analysis of the high-resolution XPS spectrum of a CO2-plasma-treated PP film (Figure 3) presents five components corresponding to different polar groups, as is described in the literature.26-28 The main contribution, fixed at 285.0 eV, is related to the hydrocarbon structure of the polymer and represents the CsC and CsH bonds. The second peak (285.6 eV) indicates the presence of CsCdO bonds. The components at 286.6 and 287.6 eV can be assigned to CsO and CdO bonds, respectively. Last, the peak at 288.9 eV corresponds to carboxylic acid functions (COOH). Comparison between the areas of the peaks allows for the determination of the relative proportions of the CsO, CdO, and COO groups as 4/2/1, respectively. These proportions are similar to those found by Kuhn29 on a PP surface treated by an O2plasma treatment. However, previous works in our laboratory have shown that the degradation of the PP material is weaker for that treated by CO2 plasma than for that treated by O2 plasma.20 After the CO2-plasma treatment (Table 1), we observed a decrease of 27° for the water contact-angle value between the virgin (θ ) 96°) and the treated (θ ) 69°) samples. This can be explained by the introduction of polar groups (25) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (26) Beamson, G.; Briggs, D. High-Resolution XPS of Organic PolymerssThe Scienta ESCA300 Database; John Wiley & Sons: Chichester, 1992. (27) Me´dard, N.; Soutif, J.-C.; Poncin-Epaillard, F. Langmuir 2002, 18, 2246. (28) Inagaki, N.; Tasaka, S.; Hibi, K. J. Polym. Sci., Part A 1992, 30, 1425. (29) Kuhn, G.; Weidner, S.; Decker, R.; Ghode, A.; Friedrich, J. Surf. Coat. Technol. 1999, 116-119, 796.
Delorme et al.
Figure 4. Fluorescence spectra obtained with the Raman spectrometer and the standard curve of the fluorescence intensity maximum versus the concentration. Table 1. Evolution of the Water Contact Angle during the Fixation Reaction sample
water contact angle (deg)
virgin PP reference plasma-treated PP before washing plasma-treated PP after washing reference PP DO3-fixed PP
96 55 69 95 105
during the plasma treatment.30 Let us point out that the contact angle observed immediately after the plasma treatment was θ ) 55°. The change from 55 to 69° was observed after a quick acetone ultrasonic washing. Surface reorganization of the polymer segments tends to reduce the interfacial energy between the polymer and the phase in contact with it and, thus, induces surface modifications.31 Furthermore, ultrasonic cleaning presumably eliminates the fragments of the polymer chains containing polar groups, as is illustrated by the steric exclusion chromatography analysis of the resulting cleaning solution. These fragments are the result of the degradation process often present in plasma treatment. Indeed, even if surface functionalization is the main process involved in the plasma treatment of polymeric surfaces, degradation and cross-linking can occur.20 The degradation process is characterized by chemical bond scissions involving the formation of low-mass products and a loss of weight after solvent cleaning. However, this process remains weak; the loss of weight was evaluated to be 16 µg‚cm-2, which is in agreement with Aouinti,24 who found a loss of weight of 24 µg‚cm-2 for a PP film treated by CO2 plasma (P ) 60 W, d ) 10 cm, t ) 1 min, and F ) 20 sccm). Thus, the cleaning procedure after the plasma treatment is necessary to prepare the surfaces with effectively attached molecules. Surface Acid Group Titration. To evaluate the number of carboxylic acid groups introduced by the CO2plasma treatment, titration with thionine acetate was performed.32 The optical properties of this molecule (excitation wavelength of 594 nm and emission maximum of 618 nm) allow for a fluorescence titration. The use of the high sensitivity of the Raman spectrometer allows for the completion of the fluorescence titration at a low concentration. A fluorescence signal for thionine acetate concentration down to 10-9 M was observed. A (30) Cui, N.-Y.; Brown, N. M. D. Appl. Surf. Sci. 2002, 7769, 1. (31) Chan, C.-M. Polymer surface modification and characterization; Hanser: New York, 1994. (32) Nedelmann, H.; Weigel, T.; Hicke, H. G.; Muller, J.; Paul, D. Surf. Coat. Technol. 1999, 116-119, 973.
Polyolefin Surfaces
Figure 5. FT-IR reflectance spectra of (a) CO2-plasma-treated PP and (b) PP after the reaction with pivaloyl chloride.
calibration curve (Figure 4) was established by measuring the maximum fluorescence intensity (λ ) 618 nm) for different concentrated solutions (from 10-9 to 10-5 M). This standard curve was used to measure the concentration of a solution containing the plasma-treated sample and a virgin PP film. The difference in the signals (which is assumed to be proportional to the number of carboxylic acid groups present on the plasma-treated film) gives a concentration of carboxylic acid groups of about 0.7 ( 0.2 sites/nm2. As a comparison, Ivanov et al.33 found a carboxylic acid concentration of 18 sites/nm2 for a chromic acid-treated polyethylene surface. The larger number of acid sites could be attributed to the degradation process, which is greater with a chemical oxidative method than with plasma treatments.22 When the titration experiments and XPS quantitative data are combined, the number of sites present on the plasma-treated surface can be estimated to be 0.7 sites/nm2, 1.4 sites/nm2, and 2.8 sites/nm2 for the COOH, CdO, and CsO groups, respectively. The results of the chemical titration combined with the XPS analysis indicate that the polar groups introduced by the CO2plasma treatment (around 5 sites/nm2) are of a sufficient number to allow postreactions with the DO3 molecules because the maximum packing density on a planar surface for the stearic acid units in a monolayer is 5 sites/nm2.34 DO3-Fixation Reaction. The reaction between the carboxylic functions introduced by the CO2-plasma treatment with pivaloyl chloride was characterized by FT-IR spectroscopy (Figure 5). After the reaction with pivaloyl chloride, the IR spectrum shows three distinctive carbonyl bands associated with the anhydride functions. The bands at 1815 and 1750 cm-1 correspond to the CdO asymmetric and symmetric stretching vibrations of the anhydride function, respectively. The presence of a broad band at 1715 cm-1 is characteristic of the CdO symmetric stretching vibration of carboxylic acid and can be explained by a partial hydrolysis of the anhydride groups. Moreover, an increase of the water contact angle from 69 ( 2 to 95 ( 2° after the reaction with the pivaloyl chloride is observed (Table 1). This result indicates the presence of hydrophobic tert-butyl groups (t-Bu) on the surface (Figure 2). One would expect that the anhydride group would be quickly hydrolyzed during the water contact-angle measurement. However, the high value of the water contact angle indicates that hydrolysis does not occur, which confirms the reaction between the carboxylic acid functions (introduced by the plasma treatment) and pivaloyl chloride. (33) Ivanov, V. B.; Behnish, J.; Holla¨nder, A.; Mehdorn, F.; Zimmermann, H. Surf. Interface Anal. 1996, 24, 257. (34) Kuhn, H.; Mobius, D.; Bucher, H. Techniques of Chemistry; WileyInterscience: New York, 1972.
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Figure 6. Raman spectra of DO3-fixed PP after Soxhlet extraction (a) onto the surface, (b) at a 5-µm depth, and (c) at a 10-µm depth. (V) DO3 vibration bands.
After the activation of the carboxylic acid function by the mixed anhydride method, the DO3 molecules have been fixed through a reaction between the amino group of DO3 and the anhydride group of the PP film to form an amide function. To show the fixation of the DO3 molecules onto the surface, micro-Raman measurements were performed on DO3-fixed PP films after Soxhlet extraction.35 Indeed, under certain experimental conditions, an enhancement of the DO3 signal due to the preresonance Raman effect is expected.36,37 This effect, which occurs when the excitation energy is close to that of the absorption band of the DO3 (λmax ) 397 nm), allows for a huge increase of its signal with respect to the polymer substrate signal. The Raman spectra of the DO3-fixed PP film were performed at different depths of analysis (Figure 6) using the confocal system of the Raman spectrometer and an excitation wavelength of 514.5 nm. The spectrum performed on the surface shows three peaks at 1150, 1400, and 1590 cm-1 associated respectively to the PhsNd, s NdNs, and sCdCs stretching vibrations of the DO3 molecules. This establishes the presence of DO3 in the near vicinity of the surface (depth
