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Surface Modification of Polycarbonate and Poly(ethylene terephthalate) Films and Membranes by Polyelectrolyte Deposition L. Dauginet, A.-S. Duwez,† R. Legras, and S. Demoustier-Champagne*,‡ Universite´ catholique de Louvain, Unite´ de Physique et de Chimie des Hauts Polyme` res, Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium Received September 18, 2000. In Final Form: March 14, 2001 The surface modification by chemical or physical adsorption of a polyelectrolyte (polyallylamine, PAH) was established using hydrolyzed polycarbonate (PC) and poly(ethylene terephthalate) (PET) thick and thin films. Contact angle data indicated changes in the hydrophilicity of PET film surfaces in terms of the chemical treatment. XPS analysis showed that the immobilization of PAH on PET and PC films was effective and that the amount of chemically or/and physically bounded PAH depends on the pH of the electrolyte solution used for the adsorption. Similar treatments have been successfully applied to polymeric track-etched membranes. These modified membranes have been used as templates for the synthesis of polypyrrole (PPy) nanotubules. FE-SEM analysis showed that much thicker PPy tubules were obtained in modified membranes than in virgin ones. This indicates that the surface chemistry of the pore walls plays an important role in the morphology of the nanostructures synthesized within the pores.
* To whom correspondence should be addressed. Tel: 0032 10 473560. Fax: 0032 10 451593. E-mail:
[email protected]. † Senior Research Assistant of the Belgium National Funds for Scientific Research (F.N.R.S.). ‡ Research Associate of the Belgium National Funds for Scientific Research (F.N.R.S.).
have been developing a synthetic route to chemically modify polycarbonate (PC) and PET surfaces with the objective of preparing nanoporous polymeric track-etched membranes (PTM) with controllable hydrophilicity and charge state (negative or positive) surfaces. The general procedure used to prepare PTM consists of first bombarding a polymer film with heavy energetic ions. This leads to the formation of damages, called tracks into the material. Then, after a sensitization of the damaged areas by UV irradiation, the sample undergoes a selective chemical etching in order to reveal the tracks. This treatment leads to the production of micro- or nanofiltration membranes containing randomly distributed pores of uniform surface diameter.6 We7-9 and others10-12 are using this kind of polymeric membrane as a template to prepare nanomaterials, in particular conducting polymer (polypyrrole, PPy, and polyaniline, PANi) nanostructures. When PPy or PANi are chemically or electrochemically synthesized within the pores of particle track-etched membranes, the nascent polymer is preferentially deposited as thin layers on the pore walls, leading to the formation of tubules, whereas solid wires are produced in the case of metal electrodeposition. One possible explanation for the formation of polymeric tubules is the existence of electrostatic interactions between the growing polycationic polymer and the anionic sites on the pore walls. It is thus interesting to study how the surface chemistry of the pore walls of PC and PET membranes can influence the morphology of the polymers synthesized within the pores. We report here the surface modification of PET and PC membranes using a process based on polyelectrolyte adsorption. As the
(1) Ward, W.; McCarthy, T. J. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Kroschwitz, J. I., Eds.; John Wiley and Sons: New York, 1989; suppl. vol. pp 674-689. (2) Decher, G. In Comprehensive Supramolecular Chemistry (Templating, Self-Assembly and Self-Organization); Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (3) Decher, G. Science 1997, 277, 1232. (4) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. In Supramolecular Polymers; Ciferri, A., Ed.; Marcel Dekker: New York, 2000; pp 505-564. (5) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78.
(6) Ferain, E.; Legras, R. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 97. (7) Duchet, J.; Legras, R.; Demoustier-Champagne, S. Synth. Met. 1998, 98, 113. (8) Stavaux, P.-Y.; Demoustier-Champagne, S. Chem. Mater. 1999, 11, 829. (9) Delvaux, M.; Duchet, J.; Stavaux, P.-Y.; Legras, R.; DemoustierChampagne, S. Synth. Met. 2000, 113, 275. (10) Martin, C. R. Science 1994, 266, 1961. (11) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (12) Cepak, V. M.; Martin, C. R. Chem. Mater. 1999, 11, 1363.
Introduction Chemistry at polymer surfaces and interfaces is central to many polymer materials applications. The practical approaches to polymer surface modification are corona discharge treatment, plasma modification, surface graft polymerization, and chemical modification.1 In recent years, the self-organization of polymers has been increasingly explored for the preparation of well-defined surfaces and interfaces. The most recent of the self-organization techniques is the so-called layer-by-layer deposition method developed by Decher2,3 and others.4 It is a simple, relatively fast, environmentally benign, and potentially economical process. The basic process involves dipping a charged substrate (e.g., anionic) into a dilute aqueous solution of a cationic polyelectrolyte and allowing the polymer to adsorb and reverse the charge of the substrate surface. The positively charged coated substrate is rinsed and dipped into a solution of anionic polyelectrolyte that adsorbs and recreates a negatively charged surface. Sequential adsorptions of cationic and anionic polyelectrolytes allow the construction of multilayer films. McCarthy et al.5 have recently reported that layer-bylayer deposition of polyelectrolytes onto a poly(ethylene terephthalate) (PET) film is a suitable method for polymer surface modification. On the basis of these results, we
10.1021/la001333c CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001
Surface Modification of PC and PET Films
chemical and physicochemical characterizations of the pore walls of the membranes are very difficult, model systems have been studied in order to validate the method. PET and PC thick films (films used as starting materials for the preparation of polymeric track-etched membranes) and thin PC and PET supported films have been functionalized following the same procedure and carefully characterized by X-ray photoelectron spectroscopy (XPS) and by measurement of water contact angle. Ellipsometry measurements have also been performed on the thin supported polymeric films. Finally, the effects of the modification have been indirectly studied through its influence on the morphology of the conductive polymer nanostructures obtained by the template method in the modified membranes. Experimental Section Chemicals and Materials. 1,4-Dioxan (HPLC grade Lab Scan), H2SO4 (98% Merck), H2O2 (27%, Vel), LiClO4 (Janssen Chemica), FeCl3 (Merck), and poly(allylamine hydrochloride) (PAH (Mn ) 50 000-65 000), Aldrich) were used without any prior purification. Pyrrole (99% acros) was purified immediately before use by passing it through a microcolumn constructed from a Pasteur pipet, glass wool, and activated alumina. Milli-Q water (18 MΩ cm) was used to prepare all the aqueous solutions. The pH of the solutions for electrolyte adsorption was adjusted with HCl or NaOH. Self-supported 23 µm thick PET films (Mylar from Dupont de Nemours) and 25 µm thick PC films (Lexan-A films from General Electric) were used as starting materials for chemical modification and for the preparation of nanoporous particle track-etched membranes. Thin supported polymer films prepared by spincoating were also used as model systems in this study. PC (Lexan 145 from General Electric) and poly(ethylene terephthalate-coisophthalate) copolymer (PETI), with a terephthalic to isophthalic unit ratio (T/I) of 60/40, were dissolved in 1,4-dioxan (20 mg/mL) and spin-coated onto silicon wafers at 3000 rpm. Prior to their use, silicon wafers were cleaned by immersion for 20 min in a piranha solution [1:1 v/v of H2SO4 and H2O2] and then thoroughly rinsed with Milli-Q water. To improve the adhesion of the films onto the Si wafers, polymer films were annealed for 4 h under vacuum (6 × 10-5 Torr) at a temperature above their Tg (100 °C for PETI and 170 °C for PC). Surface Modification of Films. Thick PET and PC films were first hydrolyzed using 2 M aqueous NaOH for 30 min at 70 °C, subsequently immersed in an acetic acid solution for 15 min, and finally rinsed with Milli-Q water and air-dried. Thin supported PC and PETI films were hydrolyzed under milder conditions. They were immersed in a 2 M NaOH aqueous solution at room temperature for 5 min and then rinsed with water and air-dried. Electrolyte adsorptions were carried out at room temperature in open beakers containing unstirred PAH solutions at two different pHs. First procedure: PC and PET (or PETI) films were immersed in a 0.02 M PAH aqueous solution for 20 min at pH 8 and then rinsed with Milli-Q water and air-dried. Second procedure: PC and PET (or PETI) films were immersed in a 0.02 M PAH aqueous solution for 1 h at pH 11.5. Films were then removed from the electrolyte solution, rinsed three times with water, and introduced into an aqueous solution at pH 2.2 for 30 min. After rinsing with Milli-Q water, the samples were air-dried. Membrane Preparation. PET and PC particle track-etched membranes were prepared by the usual procedure described elsewhere.6 Thick polymer films were irradiated with energetic heavy ions (Ar9+) in a cyclotron and then UV-irradiated to increase the selectivity of the further chemical etching. Etching was performed with a NaOH (2 M) aqueous solution at a controlled temperature (T ) 70 °C) for a time up to 1 h and subsequently immersion for 15 min in an acetic acid solution to stop the hydrolysis. After the etching, the membranes were copiously washed with distilled water at 70 °C, air-dried, and stored between two siliconated paper sheets before characterization and further chemical modifications.
Langmuir, Vol. 17, No. 13, 2001 3953 Surface Modification of Membranes. Electrolyte adsorptions were carried out at room temperature in open beakers containing unstirred PAH solutions at two different pHs. First procedure: PC and PET membranes were immersed in a 0.02 M PAH aqueous solution for 20 min at pH 8 and then rinsed with Milli-Q water and air-dried. Second procedure: PC and PET membranes were immersed in a 0.02 M PAH aqueous solution for 1 h at pH 11.5. Membranes were then removed from the electrolyte solution, rinsed three times with water, and introduced into an aqueous solution at pH 2.2 for 30 min. After rinsing with Milli-Q water, the samples were air-dried. Polypyrrole Synthesis. PPy has been chemically and electrochemically synthesized within the pores of virgin and chemically modified PC and PET membranes following the procedures described elsewhere.7,8 Chemical Synthesis. The membrane was used as a dividing wall in a two-compartment cell. In the first compartment, an aqueous pyrrole solution (0.5 M) was added and allowed to diffuse through the membrane for 20 min prior to the introduction of the oxidant reagent (FeCl3 0.5 M) in the second compartment. The monomer and the oxidant reagent diffuse toward each other through the pores of the membrane and react to yield the polymer. Polymerization was allowed to proceed at room temperature for the desired length of time (5 or 30 min). The membrane was then removed from the cell and rinsed several times with Milli-Q water. Electrochemical Synthesis. A layer of Au serving as electrode has been evaporated onto one side of the membrane. Electroplating was performed, at room temperature, in a conventional one-compartment cell with a Pt counter electrode and an Ag/ AgCl reference electrode. A 0.1 M pyrrole/0.1 M LiClO4 solution and a voltage of +0.8V were used for the electropolymerization. Characterization Techniques. The surface chemical composition of treated and untreated samples was determined by XPS using a SSI X probe (SSX 100/206) spectrometer from Fisons, operating at pressures 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). Peak fitting to experimental data was carried out using a mixed Gaussian (80%)-Lorentzian (20%) line shape and a minimum number of peaks consistent with a reasonable fit to the raw data and the molecular structure of the polymer. Advancing type contact angles of water were measured on treated and untreated samples at room temperature, using the sessile drop technique and an image analysis system. Measurements of the pore size at the surface of nanoporous particle track-etched membranes were carried out using a FE-SEM Digital scanning microscope 982 Gemini from Leo. After chemical or electrochemical synthesis of PPy within the pores, the PC/PPy composites were immersed in dichloromethane in order to dissolve the PC membrane. The nanotubules were then recovered from the solution by filtration through a silver membrane and analyzed using a FE-SEM microscope. PC/PET composites were fractured in liquid nitrogen, and the fracture was analyzed by FE-SEM. The thin film thickness was controlled by ellipsometric measurements carried out with a Digisel rotating compensator ellipsometer from JobinYvon/Sofie Instruments. The ellipsometer works with a single wavelength of 6328 Å (He-Ne laser). A model consisting of an isotropic film on a flat substrate has been used to fit the Ψ and ∆ measurements. The refractive index of silicon was taken to be 3.881. The refractive indexes of the polymer films were fixed to 1.61 for PC and 1.563 for PETI. The TOF-SIMS spectrometer consists of a time-of-flight SIMS microprobe-microscope (Charles Evans and Associates). The surface was bombarded by a pulsed 69Ga+ primary ion beam (15 keV, 5 kHz). The Ga+ beam is rastered over a 0.01 mm2 area. The pressure in the analysis chamber was ∼7 × 10-10 Torr. The secondary ions produced are accelerated up to an energy of 3 keV and deflected by three electrostatic analyzers in order to compensate their initial energy and angular distributions. Charge neutralization was achieved using a pulsed electron flood gun of 24 eV.
Results and Discussion The surface modification by chemical or physical adsorption of a polyelectrolyte (PAH) was first established
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using PC and PET thick (25 and 23 µm, respectively) and thin (ca. 100 nm thick spin-coated films onto Si wafers) films submitted to a hydrolytic treatment in order to display a surface with a chemical composition close to that of the track-etched membrane pore walls. Indeed, the “track-etching” process used to prepare polymeric membranes from PC and PET films involves a hydrolysis step to transform the ion tracks into nanopores of welldefined dimensions. Moreover, the hydrolyzed surface presents a significantly higher degree of surface functionality due to the creation of new hydrophilic chainends by selective cleavage of the ester or carbonate bonds. The hydrolyzed PET surfaces contain a mixture of carboxylic acid and alcohol functional groups. The hydrolysis of PC creates phenoxide chain-ends and releases CO2. Native Films Analysis. The native PC and PET thick and thin films were characterized by XPS. The chemical shifts in C1s and O1s core level spectra of all these samples were in good agreement with the molecular structure of PC and PET. The observed C1s electron binding energy shifts from 284.5 eV for (-C6 H4-) to 285.0 eV for (CH3C-CH3), to 286.2 eV for (-Φ-O-), and to 290.4 eV for (-O-(CdO)-O) in PC and from 284.6 eV for (-C6 H4-) to 286.5 eV for (-COOCH2-) and to 288.8 eV for (-COOCH2-) in PET. These binding energy values correspond nicely to literature values.13 The O1s area (not shown) is made up of two well-resolved shaped peaks in a ratio of 1:1 in PET and 1:2 in PC. The one at the lower electron binding energy is assigned to (O-CdO) at 531.6 eV in PET and to (-O-(CdO)-O) at 532.3 eV in PC. The higher energy oxygen peak is attributed to the (-O-Cd O) group at 533.2 eV in PET and to (-O-(CdO)-O) at 533.9 eV in PC. Finally, the C/O atomic ratio calculated from integration of the areas of the C1s and O1s peaks after application of a linear background subtraction and correction for atomic sensitivity factors was equal to 2.5 for PET and 5.0 for PC. These values are in perfect agreement with the theoretical ratios. Functionalized Films Characterization. The various functionalized film surfaces prepared either by simple hydrolysis (NaOH 2 M, 25 °C, 5 min for thin supported films; NaOH 2 M, 70 °C, 30 min for thick films) or by hydrolysis followed by adsorption/reaction of a polyelectrolyte at pH ) 8 (0.02 M PAH, 20 min) and pH ) 11.5 (0.02 M PAH, 1 h + reprotonation in a solution at pH ) 2.2 for 30 min) were characterized by water contact angle measurements and XPS analysis. Thanks to their very smooth and planar surfaces, the thin spin-coated films have also permitted us to follow, by ellipsometry, the evolution of their thickness at the different stages of the chemical modification. It was observed that the hydrolysis treatment led to a thickness decrease of 10 Å for supported PC films and of 45 Å for supported PETI films. A thickness increase of 25 Å was measured on PC and PETI supported films after the adsorption of PAH at both pHs. The water contact angle measurements gave qualitative information about surface hydrophilicity and its change after the successive treatments. The results are summarized in Table 1. Very slight modifications of the hydrophilicity of PC surfaces were observed (Table 1, entries 1-4). However, the hydrolysis treatment leads to an increase of the hydrophilicity of PET film surfaces (Table 1, entry 2). Adsorption of PAH at pH 8 induces a significant decrease of the hydrophilic character of PET surfaces (Table 1, entry 3). θw increases indeed from 58°
(for the hydrolyzed PET films) to 90° (after adsorption of PAH). Finally, contact angle measurements carried out after PAH deposition at pH 11.5 onto the hydrolyzed PET films reveal a decrease of the hydrophilicity of the surface compared to the hydrophilicity of the hydrolyzed films (Table 1, entry 4). Nevertheless, the variation of the mean contact angle is not so pronounced as that for PET treated with PAH at pH 8. The hydrophilicity decrease of the PET surface after PAH adsorption/or reaction is rather surprising. Two possible explanations can be given for this effect. First, the polycation adsorption just compensates the negative charges present on the hydrolyzed surface but the positive charges are insufficient to reverse the surface charge state and the surface is just neutralized. A second possible explanation is that if there is overcompensation of the negative charges, the NH3+ pendant groups are not exposed at the surface but are oriented to the inner part of the film, and consequently the film surface presents aliphatic groups and not polar charged groups (Figure 1). Hydrolyzed PET and PETI surfaces give similar XPS spectra as compared to the native films, whereas XPS spectra of hydrolyzed PC films show spectacular modifications in the O1s region. As mentioned before, the O1s area is made up of two peaks at 532.3 and 533.9 eV with a 1:2 ratio. After hydrolysis, the O1s peaks become nearly symmetric (Figure 2). This modification results from three different effects happening during the hydrolysis: (1) a decrease of the number of carbonate functions [(-O-(Cd O)-O) at 532.3 eV]; (2) a decrease of the single-bounded oxygen [(-O-(CdO)-O) at 533.9 eV]; (3) the creation of phenoxide chain-ends which present a O1s peak at the same energy as the carbonate group (532.3 eV). Because of its pH dependence, the adsorption of PAH onto hydrolyzed PET and PC has been carried out at two different pHs. The pKa of PAH is estimated to be ∼10.6 based on a comparison to monoamines, and those of benzoic acid and phenol are 4.20 and 9.98, respectively.14 At pH 8, PAH is mainly in its protonated form and the hydrolyzed PET surface is more negatively charged than the PC one. Therefore, electrostatic interactions between the substrate
(13) Briggs, D.; Beamson, G. In High-resolution XPS of organic polymers; Wiley: Chichester, 1992.
(14) Harris, D. C. In Quantitative Chemical Analysis; Freeman and Company: New York, 1987; pp 729-738.
Table 1. Contact Angles of Aqueous Solutions on Modified and Unmodified PC and PET Films entry
treatment
1 2 3 4
native film hydrolyzed film hydrolyzed film + PAH at pH 8 hydrolyzed film + PAH at pH 11.5 + reprotonation at pH 2.2
PC θw (deg)
PET θw (deg)
81.7 78.9 83.3 84.1
69.7 58.1 90 81.5
Figure 1. The two possible orientations of the NH3+ groups at the PET surface leading to a hydrophobic surface (a) or to a hydrophilic surface (b).
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Langmuir, Vol. 17, No. 13, 2001 3955
Figure 2. XPS spectra of the O1s core level recorded from PC films before (a) and after (b) hydrolysis.
Figure 3. Possible reactions between the hydrolyzed PET surface and the polycation (PAH) at low pH (left) or at high pH (right). Table 2. Atomic Percentages Obtained by XPS Analysis of Modified Thick PET Films O1s peak proportion (%)
N1s peak proportion (%)
treatment
%C
531.8 eV
533.4 eV
total amount
399 eV
401 eV
total amount
C/O
hydrolyzed film hydrolyzed film + PAH at pH 8 hydrolyzed film + PAH at pH 11.5 + reprotonation at pH 2.2
75 73 72
12 14 13
13 10 7
25 24 20
1.5 1
1.5 2
3 3
3.0 3.0 3.6
Table 3. Atomic Percentages Obtained by XPS Analysis of Modified Thick PC Films O1s peak proportion (%)
N1s peak proportion (%)
treatment
%C
532.3 eV
533.9 eV
total amount
399 eV
401 eV
total amount
C/O
hydrolyzed film hydrolyzed film + PAH at pH 8 hydrolyzed film + PAH at pH 11.5 + reprotonation at pH 2.2
82 78 80
10 12 6
7 4 6
17 16 12
2 3
1 2
3 5
4.8 4.9 6.7
and the polycation are mainly expected, but there should be less electrostatic interaction between PC and PAH. At higher pH (pH ) 11.5), PAH is in its neutral state and adsorbed to the charged PET and PC surfaces as a more collapsed coil than when protonated; this should lead to a higher nitrogen content. Moreover, under these conditions the formation of covalent links between the polymeric surface and PAH may occur. This leads to the formation of amide groups on PET and carbamate groups on PC. As the carbonate group is more reactive with an amine than an ester group, the proportion of covalent links between PAH and PC should be higher than with PET. Figure 3 resumes the possible reactions between the hydrolyzed PET surface and the polycation PAH in the two experimental conditions. Analysis by XPS of PET and PC films that were hydrolyzed and modified by adsorption of PAH at pH 8 show the presence of 3% of nitrogen atoms on their surface (Tables 2 and 3). These values are in good agreement with the results obtained by McCarthy et al.5 for PET modi-
fications under similar experimental conditions. Two peaks of equivalent intensity (Table 2) are present in the N1s region of the modified PET films (Figure 4): the first one located at 399 eV can be attributed to an amine or amide group and the second peak at 401 eV corresponds to the protonated amine. In PC modified film spectra, the N1s peak intensities are not identical but within a ratio of 2:1 (Table 3). For both polymers, a slight decrease of the single-bounded oxygen peak (at 533.2 eV for PET and at 533.9 eV for PC) is also observed. The decrease of the C-O peak intensity, combined with the presence of two N1s peaks, suggests that at pH 8 some covalent links (amide for PET and carbamate for PC) between the polycation and the substrates are formed, simultaneously to the formation of electrostatic interactions. Analysis by XPS of PET and PC films that were hydrolyzed and modified by adsorption/reaction of PAH at pH 11.5, followed by desorption for 30 min at pH ) 2.2, shows the presence of 3% and 5% of nitrogen atoms on their surface, respectively (Tables 2 and 4). The formation
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Figure 4. XPS spectrum of the N1s core level recorded from a hydrolyzed PET film on which a PAH layer has been adsorbed at pH 8. Table 4. Characteristics of PET and PC Membranes membrane PET PET PC PC
mean pore size (nm)
pore density (pores/cm2)
500 540 100 350
1.6 × 108 1.6 × 108 1 × 108 1 × 108
of carbamate groups in PC and of amide groups in PET is indicated by the low oxygen content: C/O ) 3.6 and 6.7 for modified PET and PC surfaces, respectively. It is also indicated by the existence of a N1s peak at a binding energy of 399 eV on samples that were dipped in an acidic aqueous solution (pH 2.2 for 30 min). This treatment indeed leads to the reprotonation of the amine groups present on the polymer films and to the desorption of the PAH that is physically (ionically) adsorbed. Thus, the reprotonation/ desorption step allows us to assign undoubtedly the lowenergy N1s peak (399 eV) to the amide or carbamate function and to estimate the atomic percentage of nitrogen of PAH engaged in covalent bonds. This percentage reaches 3% and 1% for PC and PET, respectively. The higher ratio obtained for PC underlines its stronger reactivity with the amine groups of PAH. Functionalized Nanoporous Membrane Characterization. PC and PET particle track-etched membranes with different pore sizes (Table 4) have been prepared following the procedure described in the Experimental Section. These membranes were treated with PAH, but the hydrolysis (initial step) was omitted because the samples naturally displayed carboxylate (on PET) and phenoxide (on PC) chain-ends resulting from the tracketching treatment. To confirm that PAH was deposited on the entire length of the pore walls, microtome sections of modified and unmodified PC membranes, parallel to the membrane surface, have been characterized by TOFSIMS. The negative ion spectra of the modified sample reveal the presence of PAH. Different fragments containing nitrogen atoms were indeed found: CN- (m/z ) 26), CNO- (m/z ) 42), C3N- (m/z ) 50), and C4N- (m/z ) 62). The unmodified and modified membranes were then used as templates to prepare polypyrrole nanostructures. PPy can be synthesized by oxidative polymerization of the corresponding monomer. This may be accomplished either electrochemically or with a chemical oxidizing agent. In this work, both methods have been used and the morphologies of the different PPy structures obtained within the pores of unmodified and modified membranes were analyzed by FE-SEM. As reported earlier, the chemical and electrochemical growing of PPy within the pores of unmodified PC membranes led to the formation of nanotubules as illustrated in Figure 5. The nascent polymer is preferentially deposited as thin layers on the
Figure 5. FE-SEM image of PPy nanotubules chemically synthesized in a virgin PC membrane.
pore walls and thus produces hollow polymer tubules that run through the entire thickness of the membrane. A possible explanation for the formation of polymeric tubules is the existence of electrostatic interactions between the growing polycationic polymer and the anionic sites on the pore walls. The synthesis within the pores of PET and PC membranes modified by adsorption or reaction of PAH at pH 8 or 11.5 leads, in all cases, to the formation of thicker PPy tubules than within the pores of unmodified membranes (Tables 5 and 6). The thickness increase is homogeneous on the whole length of the nanotubules (Figure 6). This demonstrates that the surface chemistry of the pore walls (in particular, the surface charge state) of the template membrane influences the growing process and the morphology of the PPy nanostructures synthesized within the pores. The largest thickness increase (107%, Table 6, entry 6) appears in modified PC membranes. For this kind of membrane, the increase of PPy tubule thickness is independent of the pH of the PAH solution used to modify the pore walls, whereas for PET membranes, larger increases of nanotubule thickness are observed for PPy grown in membranes treated with a PAH solution at pH 8 (Table 5, entries 3 and 4) than with a PAH solution at pH 11.5 (Table 5, entries 5 and 6). As shown in Tables 5 and 6, longer chemical polymerization times did not change the nanotubule morphology. Conclusions A process based on a two-step sequence involving the hydrolysis of ester or carbonate bonds under basic conditions followed by the adsorption of a polyelectrolyte has been developed and applied to PET and PC films in order to modify the charge state and the hydrophilicity of their surfaces. Various functionalized polymeric film surfaces were prepared either by simple hydrolysis or by hydrolysis followed by adsorption/reaction of PAH at pH ) 8 or 11.5 and characterized by water contact angle measurement and XPS analysis. Very slight modifications of the hydrophilicity of PC surfaces were observed after the different treatments. However, the hydrolysis treatment leads to an increase of the hydrophilicity of PET film surfaces, but the adsorption of PAH at pH 8 and pH 11.5 induced a significant decrease of the hydrophilic character of PET surfaces. XPS spectra of PET films which were hydrolyzed and modified by adsorption of PAH at pH 8 or 11.5 show nearly the same amount of nitrogen atoms on their surface. For PC, the amount of immobilized
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Langmuir, Vol. 17, No. 13, 2001 3957
Table 5. Dimension Characteristics of PPy Nanotubes Chemically Synthesized within the Pores of Unmodified and Modified PET Membranes entry
membrane treatment
tpolym (min)
PPy nanotube outer ø (nm)
PPy nanotube thickness (nm)
thickness increase (%)
1 2 3 4 5 6
none none PAH pH 8 PAH pH 8 PAH pH 11.5 + reprotonation at pH 2.2 PAH pH 11.5 + reprotonation at pH 2.2
5 30 5 30 5 30
500 500 540 540 500 500
90 95 157 170 120 120
5.5 75 88 33 33
Table 6. Dimension Characteristics of PPy Nanotubes Chemically or Electrochemically Synthesized within the Pores of Unmodified and Surface Modified PC Membranes entry
membrane treatment
tpolym (min)
PPy nanotube outer ø (nm)
PPy nanotube thickness (nm)
1 2 3 4 5 6 7 8
none PAH pH 8 PAH pH 11.5 + reprotonation at pH 2.2 none none PAH pH 8 PAH pH 11.5 + reprotonation at pH 2.2 PAH pH 11.5 + reprotonation at pH 2.2
50 s 70 s 50 s 5 min 30 min 5 min 5 min 30 min
95 95 95 350 350 350 350 350
18 33 33 40 40 83 80 75
Figure 6. FE-SEM image of PPy nanotubules chemically synthesized in a modified PET membrane.
PAH was larger at pH 11.5 than at pH 8. XPS analysis indicates that covalent links (amide for PET and carbamate for PC) between the polycation and the substrates are formed, simultaneously to the formation of electrostatic interactions at both pHs. XPS data indicates also that the ratio of chemically bounded PAH to physically adsorbed PAH depends on the pH of the electrolyte solution used during the process. Our strategy of PET and PC film surface modification was also successfully applied to membranes. Virgin and modified polymeric membranes were used as templates to prepare PPy nanostructures. FE-SEM analysis showed that the synthesis within the pores of PET and PC
thickness increase (%) 83 83 107 100 88
membranes modified by adsorption or reaction of PAH at pH 8 or 11.5 led, in all cases, to the formation of thicker PPy tubules than within the pores of unmodified membranes. The thickness increase is homogeneous on the whole length of the nanotubules. The surface chemistry of the pore walls (in particular, the surface charge state) thus plays an important role in the morphology of the nanostructures synthesized within the pores. However, other factors should affect the morphology of the nanostructures as PPy solid fibers were never obtained. For example, the solvophobicity of PPy that is completely insoluble in water can also lead to preferential interactions between the growing polymer chains and the pore walls of the template membrane. Finally, another potential application of our modified membranes bearing amino groups may be the anchorage of biologically active molecules capable, for instance, of improving the cell culture. Acknowledgment. S.D.-C. and A.S.D. thank the Belgian National Fund for Scientific Research (F.N.R.S.) for their positions as Research Associate and Senior Research Assistant, respectively. L.D. is a fellow of the “Fonds pour la formation a` la recherche dans l’industrie et dans l’agriculture” (FRIA, Belgium). The authors thank A. M. Jonas, P. Rouxhet, and E. Ferain (UCL) for stimulating discussions and the “Unite´ de chimie des interfaces” (UCL) for access to the XPS facilities. Thanks also go to X. Vanden Eynde and C. Poleunis for SIMS analysis. This work was supported by the “Ministe`re de la Re´gion Wallonne, Direction Ge´ne´rale des Technologies, de la Recherche et de l’Energie” (Convention No. 9914152). LA001333C
