396
Langmuir 2004, 20, 396-401
Hydrogel Characteristics of Electron-Beam-Immobilized Poly(vinylpyrrolidone) Films on Poly(ethylene terephthalate) Supports Dorit Meinhold,† Ruediger Schweiss,† Stefan Zschoche,† Andreas Janke,† Angela Baier,† Frank Simon,† Helmut Dorschner,† and Carsten Werner*,†,‡ Institute of Polymer Research Dresden & The Max Bergmann Center of Biomaterials, Hohe Strasse 6, D-01069 Dresden, Germany, and University of Toronto, Department of Mechanical and Industrial Engineering, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada Received July 25, 2003. In Final Form: October 28, 2003 A novel strategy for the preparation of thin hydrogel coatings on top of polymer bulk materials was elaborated for the example of poly(ethylene terephthalate) (PET) surfaces layered with poly(vinylpyrrolidone) (PVP). PVP layers were deposited on PET foils or SiO2 surfaces (silicon wafer or glass coverslips) precoated with PET and subsequently cross-linked by electron beam treatment. The obtained films were characterized by ellipsometry, X-ray photoelectron spectroscopy, infrared spectroscopy in attenuated total reflection, atomic force microscopy (AFM), and electrokinetic measurements. Ellipsometric experiments and AFM force-distance measurements showed that the cross-linked layers swell in aqueous solutions by a factor of about 7. Electrokinetic experiments indicated a strong hydrodynamic shielding of the charge of the underlying PET layer by the hydrogel coatings and further proved that the swollen films were stable against shear stress and variation of pH. In conclusion, electron beam cross-linking of preadsorbed hydrophilic polymers permits a durable fixation of swellable polymer networks on polymer supports which can be adapted to materials in a wide variety of shapes.
Introduction The prevention of nonspecific binding of proteins to surfaces is very often essential for the performance of demanding products such as medical devices and biosensors.1-3 Surface grafting of the uncharged, water soluble polymers poly(ethylene glycol) (PEG),4-9 poly(vinylpyrrolidone) (PVP),10,11 or poly(vinyl alcohol) (PVA)12 is frequently applied for that purpose. These modifications were reported to strongly attenuate adsorption of proteins and the adhesion of cells. To maintain the protein-resistant features over longer time periods, high coverage polymer layers and good mechanical and chemical stability of the coatings are required. Commonly this is accomplished by a chemical attachment of the polymer chains to the surface. The use of silane13-15 or alkylthiol16-18 coupling
chemistry is most straightforward. However, it is limited to inorganic surfaces such as gold or glass, while for practical use deposition on polymer supports is very often desirable. In this field, different approaches as grafting to plasma-modified polymer surfaces,19-21 deposition by plasma polymerization of vinyl-substituted monomers,22 coupling to activated surfaces,23 or photografting24 were reported in the recent past. Also, interpenetrating networks with poly(ethylene oxide) (PEO) were described to serve that purpose.25 Cross-linking of the polymers further enhances the stability and leads to structures which are often referred to as hydrogels. In this paper we present a novel and versatile approach to prepare PVP hydrogel coatings on poly(ethylene terephthalate) (PET) surfaces which can be adapted for
* To whom correspondence may be addressed: e-mail, werner@ ipfdd.de; fax, ++49 351 4658 533. † Institute of Polymer Research Dresden & The Max Bergmann Center of Biomaterials. ‡ University of Toronto.
(13) Tseng, Y.-C.; McPherson, T.; Yuan, C. S.; Park, K. Biomaterials 1995, 16, 963. (14) Lee, S. W.; Laibinis, P. E. Biomaterials 1998, 19, 1669. (15) Jo, S.; Park, K. Biomaterials 2000, 21, 605. (16) Sheth, S. R.; Leckband, D. E. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 8399. (17) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426. (18) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 18, 8862. (19) Kingshott, P.; Thissen, H.; Griesser, H. J. Biomaterials 2002, 23, 2043. (20) Fujimoto, K.; Inoue, H.; Ikada, Y. J. Biomed. Mater. Res. 1993, 27, 1559. (21) (a) Beyer, D.; Knoll, W.; Ringsdorf, H.; Wang, J.-H.; Timmons, R. B.; Sluka, P. J. Biomed. Mater. Res. 1997, 36, 181. (b) Nitschke, M.; Menning, A.; Werner, C. J. Biomed. Mater. Res. 2000, 50, 340. (c) Vasilets, V. N.; Werner, C.; Hermel, G.; Pleul, D.; Nitschke, M.; Menning, A.; Janke, A.; Simon, F. J. Adhes. Sci. Technol., B 2002, 16, 1855. (22) Wu, Y. J.; Timmons, R. B.; Jen, J. S.; Molock, F. E. Colloids Surf., B 2000, 18, 235. (23) Alcantar, N. A.; Aydil, E. S.; Israelachvili, J. N. J. Biomed. Mater. Res. 2000, 51, 343. (24) (a) Thom, V. H.; Jankova, K.; Ulbricht, M.; Kops, J.; Jonsson, G. Macromol. Chem. Phys. 1998, 12, 2723. (b) Thom, V. H.; Altankov, G.; Groth, T.; Jankova, K.; Jonsson, G., Ulbricht, M. Langmuir 2000, 16, 2756. (25) Desai, N. P.; Hubbell, J. A. Macromolecules 1992, 25, 226.
(1) (a) Ratner, B. D. J. Biomed. Mater. Res. 1997, 27, 837. (b) Andrade, J. D.; Hlady, V. Adv. Polym. Sci. 1986, 79, 1. (2) Proteins at Interfaces, Physicochemical and Biochemical Studies; Brash, J. L., Horbett, T. A., Eds.; American Chemical Society: Washington, DC, 1987; pp 1-33. (3) Norde, W. Adv. Colloid Interface Sci. 1986, 25, 267. (4) Poly(ethyleneglycol) Chemistry; Harris, J. M., Ed.; Plenum: New York, 1992. (5) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; de Gennes, P. G. J. Colloid Interface Sci. 1991, 142, 149. (6) Kidane, A.; Lantz, G. C.; Jo, S.; Park, K. J. Biomater. Sci., Polym. Ed. 1999, 10, 1089. (7) Prime, K. L.; Whitesides, G. J. Am. Chem. Soc. 1993, 115, 10714. (8) Currie, E. P. K.; Norde, W.; Cohen Stuart, M. A. Adv. Colloid Interface Sci. 2003, 100-102, 205. (9) Zaman, A. Colloids Polym. Sci. 2000, 278, 1187. (10) (a) Rovira-Bru, M.; Giralt, F.; Cohen Y. J. Colloid Interface Sci. 2001, 235, 70. (b) Higa, O. Z.; Rogero, S. O.; Machado, L. D. B.; Mathor, M. B.; Lugao, A. B. Radiat. Phys. Chem. 1999, 55, 705. (11) Robinson, S.; Williams, P. A. Langmuir 2002, 18, 8743. (12) Barrett, D. A.; Hartshorne, M. S.; Hussain, M. A.; Shaw, P. N.; Davies, M. C. Anal. Chem. 2001, 73, 5232.
10.1021/la0353531 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/20/2003
Hydrogel Characteristics of Films
the surface modification of materials in a wide variety of shapes (foils, fibers, tubes, or films). We chose PVP as the hydrogel polymer because of its high molecular weight and its proven biocompatibility10 and utilized electron beam treatment for immobilization. The chemical structure of the hydrophilic polymer layers was thoroughly characterized by means of X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The characteristics of the coated samples upon exposure to aqueous salt solutions (swelling, electrosurface properties) were studied by in situ ellipsometry, atomic force microscopy (AFM), and streaming potential/current experiments. Materials and Methods Sample Preparation on PET Foils and Silicon Surfaces. Solvents and chemicals were used without further purification. Poly(vinylpyrrolidone) PVP K90 was purchased from Fluka (Deisenhofen, Germany). Commercially available silicon wafers (Sico Wafer GmbH, Jena, Germany) and glass coverslips were used as substrates for film preparation. The substrates were cleaned in an ultrasonic bath for 30 min in ethanol and for 30 min in distilled water, respectively, then freshly oxidized in a mixture of aqueous solutions of ammonia (Acros Organics, Fairlawn, NJ) and hydrogen peroxide (Merck, Darmstadt, Germany), and thereafter pretreated in a gas-phase reaction with hexamethyldisilazane (Fluka) to provide a hydrophobic surface. The PET model surfaces were prepared by spin coating (RC5, Suess Microtec, Garching, Germany) from 0.5% wt/wt solution in hexafluoro-2-propanol (Merck). PET foils of 0.5 mm thickness were purchased from Goodfellow (Bad Nauheim, Germany). The foils were cleaned in an ultrasonic bath for 15 min in acetone and subsequently rinsed with distilled water. After being cleaned, the foils were dried at 80 °C for 1 h. The silicon wafer and glass coverslips with PET model surfaces were coated by spin coating (2000 rpm/s, 60 s) from PVP solutions (2% wt/wt in methanol) and the PET foils by dip coating in the corresponding solutions. Electron Beam Treatment. The electron beam irradiation was performed with an accelerator ELV-2 (Budker Institute of Nuclear Physics, Novosibirsk, Russia).26,27 The accelerator was operated in the energy range from 0.6 to 1.5 MeV, the beam power maximum was 20 kW, and the beam current maximum was 25 mA. The samples were irradiated on a step-by-step basis (single dose 20 kGy, total dose 100 kGy) in a saturated humid N2 atmosphere at ambient temperature and pressure in a closed bag. ATR-FTIR Spectroscopy. Infrared spectra from native and coated PET foils were recorded by means of the ATR technique. For the ATR-FTIR measurements a Bruker IFS 66 spectrometer (Bruker, Karlsruhe, Germany) and a Golden Gate single reflection diamond ATR unit (L.O.T.-Oriel, Darmstadt, Germany) was used. Photoelectron Spectroscopy. XPS studies were carried out by means of an Axis Ultra photoelectron spectrometer (Kratos Analytical, Manchester, U.K.). The spectrometer was equipped with a monochromatic Al KR1,2 X-ray source of 300 W at 20 mA, the radiation of which was monochromated by a quartz crystal monochromator. The information depth of the XPS method corresponds with the mean free path of the electrons in the material under investigation. In the case of polymer samples the information depth of XPS is not more than 8 nm. The kinetic energy of photoelectrons was determined using a hemispherical analyzer with a constant pass energy of 160 eV for survey spectra and 20 eV for high-resolution spectra. During all measurements, electrostatic charging of the sample was avoided by means of a low-energy electron source working in combination with a magnetic immersion lens. Later, all recorded peaks were shifted by the same value which was necessary to set the C 1s peak to 285.00 eV. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity (26) Dorschner, H.; Jenschke, W.; Lunkwitz, K. Nucl. Instrum. Methods Phys. Res., Sect. B 2000, 161-163, 1154. (27) Dorschner, H.; Lappan, U., Lunkwitz, K. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 139, 495.
Langmuir, Vol. 20, No. 2, 2004 397 Table 1. Quantitative Evaluation of XPS Data Showing the Elemental Ratios of Untreated and Electron-Beam Treated PVP
[N]:[C] [O]:[C] [N]:[O]
PVP untreated
PVP electron-beam treated
0.146 0.148 0.989
0.149 0.128 1.170
factors and the spectrometer transmission function. The highresolution spectra were analyzed by means of the spectra deconvolution software (Kratos Analytical). Free parameters of component peaks were their binding energy (BE), height, full width, and the Gaussian-Lorentzian ratio. AFM. Atomic force microscopy (AFM) was performed with a NanoScope IV-Dimension 3100 microscope (Veeco, Santa Barbara, CA). The measurements in air were carried out in tapping mode with ARROW NC sensors (NanoWorld, Neuchaˆtel, Switzerland) (frequency ) 261 kHz, spring constant ) 42 N/m). To investigate the layer system and the swelling properties, the PVP/PET layers on glass slides were locally removed by scratching with a needle resulting in a two step profile of the layers. After this procedure it was possible to investigate the swollen state in water in contact mode with POINTPROBE sensors (NANOSENSORS, Neuchaˆtel, Switzerland) (spring constant 0.2 N/m). Ellipsometry. Ellipsometry was performed with a variable angle scanning ellipsometer (VASE, Woollam Co. Inc., Lincoln NE, 44 wavelengths between 428 and 763 nm) equipped with a rotating analyzer type at three angles of incidence: 65°, 70° and 75°. For the swelling measurements (angle of incidence 68°), a cell for liquid media with a 10-3 M KCl solution at three pH values (pH 3, 7, and 9) was used. For the variation of the pH, 1 N HCl or 1 N KOH was added. To analyze the data, an optical five-layer system (see Table 1) was assumed. All polymer layers were modeled with Cauchy functions. Electrokinetic Measurements. Electrokinetic measurements were performed by streaming potential and streaming current measurements using the in-house developed microslit electrokinetic setup (MES).28,29 This technique allows for a precise determination of streaming potential and streaming current measurements in microchannels consisting of the planar surfaces along with the determination of surface conductivity. The polymer layers were prepared on polished borofloat glass slides or silicon wafers and aligned parallel to each other forming a microslit channel of height (or sample distance) h. Further details of this technique can be found in the corresponding literature.28 The streaming potential (ES) and the streaming current (IS) were measured by applying pressure ramps up to 200 mbar in 50 mbar steps and in both directions of the slit channel. The zeta potential (ζ) can be calculated from ES and IS by means of the Helmholtz-Smoluchowski equations (eqs 1 and 2) with respect to the slopes (dES/dp) and (dIS/dp)
ζ(ES) )
[
]( ) ( )
η 2Kσ λ∞ + 0r h
ζ(IS) )
dES dp
dIS η L ‚ ‚ 0r b ‚h dp
(1) (2)
where η and are the viscosity/dielectric constants of the liquid, λ∞ is the bulk electrolyte conductivity, and Kσ is the surface conductivity. The length L and the width b of the channel (or sample distance) are predetermined by the size of the sample slides (L ) 20 mm, b ) 10 mm). As revealed in eq 2, the zeta potential obtained from streaming current is independent from the surface conductance contributions and thus yields the effective zeta potential.
Results and Discussion PVP layers were spin-coated or cast-coated on the polymer support and cross-linked afterward by electron (28) Werner, C.; Ko¨rber, H.; Zimmermann, R.; Dukhin, S. S.; Jacobasch, H. J. J. Colloid Interface Sci. 1998, 208, 329. (29) Zimmermann, R.; Dukhin, S. S.; Werner, C. J. Phys. Chem. B 2001, 105, 8544.
398
Langmuir, Vol. 20, No. 2, 2004
Meinhold et al.
Scheme 1. Preparation of a PVP Hydrogel Layer on a PET Surface by Electron Beam Cross-Linking
Chart 1. Assignment of the Elemental Components of PVP
beam treatment to prevent displacement of the adsorbed polymers in aqueous solutions (see Scheme 1). While the use of electron beam treatment to induce free-radical crosslinking was studied in earlier works for PVP and PEO solutions,30-32 we put focus on the application of the electron beam treatment for immobilizing and crosslinking of thin preadsorbed PVP films on the surface of PET bulk materials. Previous studies reported that the efficiency of electronbeam-induced cross-linking reactions is strongly dependent on the crystallinity/packing of the cross-linked polymers and on the environmental humidity.32 The importance of the presence of water for the success of the crosslinking had been attributed to the formation of two wellknown ring-conserving macroradicals of PVP (bearing radicals in one of the C positions indicated in Chart 1, respectively) due to the reaction with the initially formed OH radicals, which were found to be more important as compared to the similarly formed hydrated electrons and hydrogen atoms.32 In accordance with those findings, the preparation of stable PVP hydrogel films on top of PET was only successful in our hands in the presence of humid nitrogen atmospheres. Irreversibly bound PVP layers of 50-60 nm thickness were obtained by electron-beam cross-linking of spin coated films of about 200 nm initial thickness; i.e., a significant fraction of PVP was attached to the surface or to other, surface-bound PVP molecules and was not removed upon repeated rinsing with aqueous solutions. FTIR Spectroscopy. Figure 1 shows the ATR-FTIR spectra of the native PET surface (1), the surface of PVPcoated PET (2), and immobilized PVP on PET surface by electron beam irradiation after several washing steps with water. The information depth of the used golden gate ATR unit was about 1.18 µm at 1700 cm-1. The PET spectrum (1) shows the strong antisymmetric carbonyl stretching band at 1713 cm-1 of the esters and two supplementary ester bands at 1246 and 1099 cm-1. All these vibrations (30) Chapiro, A. Radiat. Phys. Chem. 1995, 46, 159. (31) Rosiak, J. M.; Ulanski, P.; Pajewski, L. A.; Yoshii, F.; Makuuchi, K. Radiat. Phys. Chem. 1995, 46, 161. (32) Rosiak, J.; Olejniczak, J.; Pekala, W. Radiat. Phys. Chem. 1990, 36, 747. Miranda, L. F.; Lugao, A. B.; Machado, L. D. B.; Ramanathan, L. V. Radiat. Phys. Chem. 1999, 55, 709. Rosiak, J. M.; Yoshii, F. Nucl. Instrum. Methods Phys. Res., Sect. B 1999, 151, 56.
Figure 1. ATR-FTIR spectra of (1) the native PET surface, (2) PET coated with PVP, and (3) surface after electron beam irradiation and rinsing, respectively.
are partially hidden under the PVP layer bands in the case of spectrum 2. By comparison of carbonyl bands of PET, we can estimate the thickness of the layer to about 500-700 nm. Nevertheless, the strong amid I vibration of PVP at 1667 cm-1 is observable. The ATR-FTIR spectra of the surface of PVP-coated PET before (2) and after irradiation without washing (not shown) were nearly identical. The spectrum of surface after washing procedure (3) also shows the typical frequencies of PVP, but they are decreased clearly. The thickness of the immobilized PVP layer was about 100200 nm (assumed from the ATR-FTIR data). A significant change of PET or PVP caused from irradiation was not detectable by the ATR-FTIR experiment. X-ray Photoelectron Spectroscopy. The X-ray photoelectron spectra shown in Figure 2 indicate that there are no significant differences between the untreated and the electron-beam-irradiated PVP. Both N 1s spectra contain only one peak indicating the amide nitrogen. Although PVP has only one type of oxygen, the O 1s spectra can be deconvoluted into two component peaks. The component peak G corresponds to the oxygen of the PVP’s amide group. The second component peak H appears from water traces that are usually observed in XPS spectra of polymers which are able to incorporate high amounts of water by swelling processes. From the wide-scan XPS spectra, the surface compositions of the untreated and the electron-beam-treated PVP samples were determined. Table 1 gives the derived elemental ratios. The ratio [N]:[C] is not influenced by
Hydrogel Characteristics of Films
Langmuir, Vol. 20, No. 2, 2004 399 Table 2. Mean Refractive Indices and Ellipsometric Layer Thickness Compared to Layer Thickness Data from AFM Force-Distance Measurements (single sample for exemplification) refractive index thickness, nm thickness, nm n (630 nm) (ellipsometry) (AFM) ambient: H2O PVP swollen PVP PET SiO2
1.33 1.35 1.50 1.59 1.46
365 53 58 129
378 52 57
Figure 3. Typical AFM section analysis of a partially removed PVP/PET layer on a glass slide in water.
Figure 2. XPS spectra of the untreated (left) and the electronbeam treated (right) PVP sample: (a) wide-scan spectrum; (b) N 1s spectrum; (c) O 1s spectrum; (d) high-resolution C 1s spectrum.
the electron-beam treatment of the PVP layer. This indicates that the amide group of the ring system is not involved in the cross-linking reactions of the PVP molecules and the ring system remains stable under electron irradiation. The elemental ratio [O]:[C] of the electronbeam-treated sample is somewhat smaller as compared to the unmodified PVP which may result from the different water contents in the PVP layers. The measurements further show that the elemental ratios [N]:[O] are nearly equal the expected ratio [N]:[O]|ex ) 1. Figure 2(d) shows the high-resolution X-ray photoelectron spectra of the C 1s peak for PVP on PET before and after the electron beam irradiation (and subsequent rinsing of the sample with aqueous solution). The C 1s spectrum of the untreated PVP film (Figure 2(d) left) was deconvoluted into four component peaks representing the saturated hydrocarbons (CxHy, component peak A), the C-N bonds of the amino groups (component peak C), the amide group (N-CdO, component peak D), and the carbon in β position to the amide group (C-(N)CdO, component peak B). The component peak areas [B]:[C]:[D] ) 1:2:1 excellently agree with stoichiometry of the PVP unit. Typical surface contaminations weakly contribute to the component peak A. The C 1s spectrum of the electronbeam-irradiated PVP film is quite similar to the C 1s spectrum of the unmodified PVP film (Figure 2(d) right). Obviously, the fine structure analysis clearly indicates no significant modifications of the pyrrolidone ring. This finding is in accordance with the structure of the PVP macroradicals formed upon irradiation in aqueous environments (see above and compare ref 32) which can be expected to react without any destruction of the PVP ring while a more rigorous excitation required for cross-linking
in the absence of water may lead to a far-going structural change of the PVP, including the opening of the ring and the formation of distinctively basic surface sites due to the formed amine components. The latter effect has been observed in a recent study where thin films of PVP were attached to PET substrates by means of low-pressure plasma treatment in dry argon atmospheres.33 Therefore, the electron beam cross-linking of PVP can be considered as more conserving when compared to low-pressure plasma treatments earlier applied for the same purpose as the latter generates amino groups from the pyrrolidone rings besides the cross-linking reactions.33 Ellipsometric Characterization. Table 2 gives the refractive indices and the layer thickness values obtained from ellipsometric data of thin film substrates on top of silicon wafers (see also Scheme 1). An optical five-layer system was used for the evaluation (bulk silicon/silicon oxide/PET/ PVP/ambient). The layer thickness values of the PVP films increased in water by a factor of about 7 while the mean refractive index of the hydrogel layer was significantly reduced due to the incorporation of water. (Data given in Table 2 are mean values of at least four experiments, the standard deviation was below 5%.) This degree of swelling corresponds to the swelling characteristics of many hydrogels and indicates that the electron beam cross-linking did not strongly interfere with the intrinsic characteristics of the PVP polymer. For all PVP hydrogel layers, the degree of swelling was independent of the solution pH. The latter observation provides a further corroboration that the electron beam cross-linking does not cleave the pyrrolidone rings to produce ionizable groups such as -NH2. AFM Experiments. Contact mode AFM measurements in water confirmed the ellipsometric data. The section analysis given in Figure 3 (repeated three times with different samples) clearly shows the thickness of the different layer components in water. As obvious from Table 2, very similar layer thickness values and degrees of (33) Nitschke, M.; Zschoche, S.; Baier, A.; Werner, C. Submitted for publication in Surf. Coat. Technol.
400
Langmuir, Vol. 20, No. 2, 2004
Meinhold et al. Scheme 2. Simplified Representation of the Electric Potential Profile and Different Locations of the Electrokinetic Shear Plane of an Uncovered PET Surface (x ) 0) and a PET Surface Covered with a Hydrogel Layer of PVP (x ) d) in an Electrolyte Solution (with K-1 ∼ d)
Figure 4. Zeta potentials (obtained from streaming current measurements) vs pH in 3 × 10-4 M KCl (A) and vs KCl concentration (B) of unmodified PET (]) and a 380 nm (swollen state) PVP hydrogel layer (b). Right Axis of (B): Electrokinetic film thickness (×) calculated from eq 4.
swelling were obtained by the AFM experiments for layers having similar dry film thickness. That is, the thickness of the swollen layer is directly related to the dry film thickness which varied from 50 to 60 nm. The thickness of the swollen layers accordingly was found in the range from 375 to 415 nm indicating similar degrees of swelling for different dry film thickness. Electrokinetic Measurements. Figure 4 shows the zeta potentials obtained from streaming current measurements for a pure PET surface (]) and for cross-linked PVP layers (b, thickness 50 ( 5 nm in the dry state, 380 nm in the swollen state) on PET surfaces. The PET films show electrosurface properties characteristic of inert polymer surfaces. Hence, surface charge is mainly established by preferential adsorption of hydroxide ions on the hydrophobic domains of the polymer. The slightly acidic isoelectric point of 3.2 indicates that there is a possible contribution of free COOH groups of the polyester to the surface charge. The high zeta potentials of PET are further typical of hard, nonswelling polymer surfaces without dissociable sites.28,29 Moreover, our data are consistent with previous streaming potential measurements on PET.30 Preferential adsorption of hydroxide ions on PET was also reported previously.31 Evaluation of the surface conductivity Kσ of the PET film obtained by measuring dUS/dp and dIS/dp at varying channel heights as previously reported28,29 (Kσ ) 4.3 nS in 3 × 10-4 M KCl at pH 9.5) underlines the similarity of the substrate to former studies on amorphous fluorinated polymers. According to ref 29, the surface charge densities of fluorpolymers are low. However, most of the counterions are distributed in the diffuse double layer thus yielding high zeta potentials. The same situation should be given here for PET. Conversely, the surfaces with cross-linked PVP on PET exhibit very low zeta potentials (
