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Novel Ways of Covalent Attachment of Poly(ethylene oxide) onto Polyethylene: Surface Modification and Characterization by XPS and Contact Angle Measurements E Ä . Kiss,*,† J. Samu,‡ A. To´th,§ and I. Berto´ti§ Departments of Colloid Chemistry and Organic Chemistry, Eo¨ tvo¨ s University, POB 32, Budapest 112, H-1518 Hungary, and Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budao¨ rsi u´ t 45, H-1112 Budapest, Hungary Received July 17, 1995. In Final Form: November 15, 1995X Procedures involving the surface halogenation of polyethylene (PE) by wet chemical reactions or radio frequency plasma treatments, followed by covalent bonding of poly(ethylene oxide) (PEO), have been developed. The two-step wet chemical halogenation comprised the chlorination of PE by SO2Cl2 and the transhalogenation by AlBr3 or NaI. For the single-step direct bromination of PE, either the PE substrate was exposed to a glow discharge plasma obtained from CHBr3 vapor or Ar plasma activated PE was reacted with bromoform. The Br or I functional groups were reacted with PEO solution or with PEO melt. XPS and contact angle studies were used to characterize the surfaces during the different stages of the procedures. Wettability as well as surface free energies compared for various PEO-covered surfaces show that the higher the surface amount of PEO, the lower the contact angle (corresponding to higher surface free energy). It has been established that a relatively small amount of PEO proved to be sufficient to render the PE substrate hydrophilic when it had been already hydrated. Both contact angle study and analysis of surface composition suggest that a more homogeneous surface could be obtained by treatment involving plasma bromination in contrast to those produced by wet chemistry. The relative amount of ether type carbon, derived from XPS, has been proposed to be a general parameter for comparison of surface energetics of various PEO-grafted surfaces irrespective of the nature of the substrate.
Introduction Poly(ethylene oxide) (PEO) has been widely used as an efficient stabilizer of various colloidal dispersions for several decades. These nonionic water-soluble polymer molecules provide steric stability by adsorption onto the surface of particles in foods, paints, cosmetics, and pharmaceutical products.1,2 PEO is also a material of growing importance in biomedical applications like controlling pharmacodynamics, affecting immunogenicity, and developing separation techniques and enzyme or polymer surface modification.3 The great attention PEO is receiving is due to its favorable combination of properties such as solubility in water as well as in several organic solvents, its nonionic character, the lack of toxicity, and its availability in a wide range of molecular weights.4 In addition, special properties of PEO such as chemical inertness, insensitivity to changes in ionic conditions of solution, and low protein adsorption made PEO particularly suitable for use as a steric stabilizer in aqueous media including body fluids. According to recent studies, short PEO chains incorporated into lipid bilayer prolong in vivo circulation time of liposomes, due to the steric barrier created which prevents their close approach to other surfaces.5 The fact that many foreign surfaces adsorb proteins when exposed to blood or other protein-containing body fluids may lead to undesired processes in vivo in the case †
Department of Colloid Chemistry, Eo¨tvo¨s University. Department of Organic Chemistry, Eo¨tvo¨s University. § Hungarian Academy of Sciences. X Abstract published in Advance ACS Abstracts, February 15, 1996. ‡
(1) Napper, D. H. In Colloidal Dispersions; Goodwin, J. W., Ed.; Royal Society of Chemistry: London, 1982. (2) Tadros, Th. F.; Vincent, B. J. Colloid Interface Sci. 1979, 72, 505. (3) Harris, J. M., Ed. Poly(ethylene glycol) Chemistry; Plenum Press: New York, 1992. (4) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (5) Kuhl, T. L.; Leckband, D. E.; Lasic, D. D.; Israelachvili, J. N. Biophys. J. 1994, 66, 1479.
of contact lenses or catheters or in vitro in clinical diagnostic procedures. Some polyurethanes with oligomers or polymers of ethylene oxide on their surfaces were described as being resistant to the adsorption of proteins and cells when contacted with biofluids.6-8 This behavior is attributed to the decrease of the electrostatic and hydrophobic interactions between the surface layer composed of nonionic and hydrophilic PEO and the molecules expected to be adsorbed. To exploit the potential offered by this special property of PEO, different surface treatment methods including adsorption, copolymerization, or grafting techniques have been developed9-19 to prepare PEOcovered surfaces. These methods contribute substantially to reveal the possible applications of such surfaces especially as biomaterials and biocompatible, nonthrombogenic and biosensor surfaces.20-23 (6) Gregonis, D. E.; Chen, C. M.; Andrade, J. D. In Hydrogels for Medical and Related Applications; Andrade, J. D., Ed.; ACS Symposium Series 31; American Chemical Society: Washington, DC, 1973; p 88. (7) Nagaoka, S.; Mori, Y.; Takiuchi, H.; Yokota, K.; Tanzawa, H.; Nishiumi, S. Polym. Prepr. 1983, 24, 67. (8) Ratner, B. D.; Paynter, R. W. In Polyurethanes in Biomedical Engineering; Planck, H., Egbers, G., Syre, R., Eds.; Elsevier: Amsterdam, 1984. (9) Go¨lander, C.-G.; Jo¨nsson, S.; Vladkova, T.; Stenius, P.; Eriksson, J.-Ch. Colloids Surf. 1986, 21, 149. (10) Herren, B. J.; Shafer, S. G.; van Alstine, J.; Harris, J. M.; Snyder, R. S. J. Colloid Interface Sci. 1987, 115, 46. (11) Kiss, E Ä .; Go¨lander, C.-G.; Eriksson, J. C. Prog. Colloid Polym. Sci. 1987, 74, 113. (12) De L. Costello, B. A.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1988/1989, 34, 301. (13) Kiss, E Ä .; Go¨lander, C.-G. Colloids Surf. 1990, 49, 335. (14) Desai, N. P.; Hubbell, J. A. Biomaterials 1991, 12, 144. (15) Brink, C.; O ¨ sterberg, E.; Holmberg, K.; Tiberg, F. Colloids Surf. 1992, 66, 149. (16) Go¨lander, C.-G.; Herron, J. N.; Lim, K.; Claesson, P.; Stenius, P.; Andrade, J. D. In Poly(ethylene glycol) Chemistry; Harris, J. M., Ed.; Plenum Press: New York, 1992; p 221. (17) Gombotz, W. R.; Guanghui, W.; Horbett, T. A.; Hoffman, A. S. In Poly(ethylene glycol) Chemistry; Harris, J. M., Ed.; Plenum Press: New York, 1992; p 247. (18) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (19) Kiss, E Ä .; Berto´ti, I. Prog. Colloid Polym. Sci. 1994, 97, 21.
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Although some methods of immobilization of PEO molecules onto glass, polymers, or other supports have been known for years, the development of PEO-grafting methods is still continuing, with the aim of improving the decisive properties of the PEO layer such as stability and chemical homogeneity. Another crucial point of biological relevance is the coverage of the surface by PEO molecules. Determination or interpretation of this quantity is a challenging task, considering the ability or capacity of surface analytical techniques and also the difference in the structure of the layer under dry and wet conditions. Furthermore, even the need of complete coverage is a subject of discussion. In connection with that, the importance of the nature of the substrate and the real chemical composition of the surface layer involving the uncovered part of the support have been pointed out. In order to avoid protein adsorption and platelet activation, a surface composed exclusively of PEO was found to be essential.24 On the other hand, a theoretical study25 suggested that there was an optimum combination of protein size, polymer size, and polymer density for protein resistance of polymergrafted surfaces. A simulation of molecular dynamics showed that an intermediate packing density of the PEO chains is needed for protein resistance and polymer fluidity.26 Further alteration and improvement of the strategy to immobilize PEO molecules onto a given substrate is necessitated by the introduction of new experimental techniques in the field of characterization of the properties of the surface PEO layers. An understanding of the special behavior of PEO surfaces under conditions other than vacuum (in contact with liquids, protein, surfactant, or electrolyte solutions, model or real body fluids, flowing medium) is essential in the aspect of biocompatibility. For example (to mention a few), investigation of protein adsorption in situ by ellipsometry requires PEO surfaces prepared on silicon wafer substrate.27 In order to study the surface forces acting between PEO layers, a derivative of PEO was adsorbed or coupled covalently to a mica surface.28,29 The PEO coupling was adapted to polystyrene support for studying the mechanism of surface-mediated serum complement activation30 or extending the application of PEO-covered material into the HPLC.31 The aim of our work was to prepare a stable and homogeneous PEO-grafted surface using a method which involves immobilization of PEO molecules on a substrate free from other chemical components unfavorably affecting the properties of the surface layer. A novel means of chemical attachment of PEO on polyethylene, consisting of two or three steps, namely, halogenation of the PE surface by wet chemical or plasma chemical treatment (20) Da Costa, V.; Brier-Russell, D.; Salzman, E. W.; Merrill, E. W. J. Colloid Interface Sci. 1981, 80, 445. (21) Go¨lander, C.-G.; Kiss, E Ä . J. Colloid Interface Sci. 1988, 121, 240. (22) Brinkman, E.; Poot, A.; Beugeling, T.; Van Der Does, L.; Bantjes, A. Int. J. Artif. Organs 1989, 12, 390. (23) Horbett, T. A. Cardiovasc. Pathol. 1993, 2, 137. (24) Merrill, E. W. In Poly(ethylene glycol) Chemistry; Harris, J. M., Ed.; Plenum Press: New York, 1992; p 199. (25) Jeon, S. I.; Lee, J. H.; Andrade, D. J.; de Gennes, P. G. J. Colloid Interface Sci. 1991, 142, 149. (26) Lim, K.; Herron, J. N. In Poly(ethylene glycol) Chemistry; Harris, J. M., Ed.; Plenum Press: New York, 1992; p 29. (27) Lin, Y. S.; Hlady, V.; Go¨lander, C.-G. Colloids Surf., B 1994, 3, 49. (28) Claesson, P. M.; Go¨lander, C.-G. J. Colloid Interface Sci. 1987, 117, 366. (29) Claesson, P. M.; Cho, D. L.; Go¨lander, C.-G.; Kiss, E Ä .; Parker, J. L. Prog. Colloid Polym. Sci. 1990, 82, 330. (30) Ekdahl, K. N.; Nilsson, B.; Go¨lander, C.-G.; Elwing, H.; Lassen, B.; Nilsson, U. R. J. Colloid Interface Sci. 1993, 158, 121. (31) Bayer, E.; Rapp, W. In Poly(ethylene glycol) Chemistry; Harris, J. M., Ed.; Plenum Press: New York, 1992; p 325.
Kiss et al. Table 1. Experimental Conditions of Preparation of PEO-Grafted PE Surfacesa sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
wet chemical treatment
plasma treatment
A A+B A+B A + B* A + B* A+D A+D
PEO coupling
C C* C C* E E F F G G G* G* G*
C C C H I
a A: Sulfochlorination by SO Cl . B: Halogen exchange by 2 2 bromination with AlBr3 in CS2. B*: As in B, but with a higher amount of AlBr3. C: Coupling of PEO of M ) 1900 to the halogenated surface in benzene. C*: As in C, but with PEO of M ) 4000. D: Transiodination by NaI. E: Treatment in streaming bromoform plasma, p1 ) 0.2 mbar, t1 ) 1 min; then staying at p2 ) 10-3 mbar, t2 ) 1 h. F: Treatment in argon plasma, p1 ) 0.1 mbar, t1 ) 1 min; then staying in streaming bromoform, p2 ) 0.2 mbar, t2 ) 1 h. G: As in F, but t2 ) 24 h. G*: As in G but in dark space region of plasma. H: As in C but in acetonitrile. I: Coupling of melted PEO (M ) 1900).
and reaction of the halogen functions with PEO in order to achieve stable covalent grafting of the latter to PE (a readily available inert, pure substrate material), is reported here. Wettability and surface free energy characterization of the PEO-grafted surfaces have been investigated and were related to the surface composition determined by XPS. Experimental Section Starting Materials. Low-density PE plates (TVK, Hungary) were used as substrate materials for PEO grafting. Smooth and clean PE surfaces were prepared by melting (at 120 °C) of the additive free polymer pressed between glass plates followed by an ultrasonic rinse in ethanol for 10 min. Monomethoxypoly(ethylene oxide) powder (PEO), M ) 1900, Mw/Mn ) 1.08 (Sigma, Deisenhofen, Germany), was generally used. In certain cases bifunctional poly(ethylene oxide) (Fluka, Buchs, Switzerland) with hydroxyl groups at both ends of the chain, M ) 4000, was also used. All other chemicals used in either surface preparation or wettability measurements were of analytical grade. The purity of wetting liquids was also checked by surface tension measurements. Sample Preparation. The types of chemical treatment and the experimental conditions of sample preparation are summarized in Table 1. Halogenation of PE by Wet Chemical Method. PE was sulfochlorinated by SO2Cl2 in carbon tetrachloride in the presence of azobisisobutyronitrile (AIBN) as a catalyst.32 SO2Cl2 (2 mL) was dissolved in 50 mL of carbon tetrachloride containing 0.0082 g of AIBN (treatment A in Table 1). Following a 5 min induction period the PE samples were placed into the solution and treated at 55-60 °C for 14 min. After reaction the PE plate was rinsed thoroughly with carbon tetrachloride and, in order to further reduce the amount of organic and water-soluble comtamination adsorbed on the PE surface, the following rinsing procedure was performed: 5 min in acetonitrile, 5 min in a 1:1 v/v mixture of acetonitrile and water, 5 min in water. Then the samples were dried in a vacuum desiccator for 24 h at room temperature. In order to obtain a more reactive component for PEO coupling the chlorinated surface was subjected to a halogen exchange (32) Ford, M. C.; Waters, W. A. J. Chem. Soc. 1951, 1851.
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Covalent Attachment of PEO onto Polyethylene reaction. Brominated and iodinated PE surfaces were prepared by transhalogenation of chlorinated PE samples. For this reason the well-known halogen exchange procedure33 using AlBr3 in an inert solvent was adapted to surface reaction (treatment B* in Table 1). Care was taken to avoid even traces of moisture. The reaction was fast, even at room temperature. In our case 0.37 g of AlBr3 was added to 40 mL of dry CS2, and the mixture was stirred. The chlorinated PE plate, with a 10 cm2 average surface area, was immersed into the reaction mixture for either 2 or 5 min. After the reaction the PE samples were rinsed with CS2 and benzene prior to drying in a vacuum desiccator. Transbromination was also performed with a smaller amount of AlBr3 (0.032 g), which was reacted with the chlorinated PE surface for 10 min (treatment B). Transiodination of chlorinated PE samples34 was performed in acetone saturated with NaI while the solvent was refluxed for 1 h (treatment D). Bromination of PE by Radiofrequency (RF) Plasmas. PE samples from 9 to 17 (Table 1) were treated in various RF plasmas (treatments E-G, and G*) in a home-made, inductively coupled plasma reactor (frequency, 13.56 MHz; power, 150 W; contact time, 1 min). In order to minimize the eventual posttreatmenttype oxidation effect, the samples were kept in the plasma reactor for at least 1 h after treatment; then, after contact with the ambient air, they were introduced into the XP spectrometer. Sample 9 was treated in a plasma obtained from flowing bromoform vapor at a static pressure of CHBr3 of about 0.2 mbar (treatment E). After treatment it was kept in the reactor under vacuum (10-3 mbar) for 1 h. Samples 11 and 13 were treated in Ar plasma at about 0.1 mbar. After treatment they were kept in flowing bromoform vapor at about 0.2 mbar, for 1 h or 24 h (treatment F or G). Sample 15 was treated in the “dark space” region of the Ar plasma. Then it was exposed to flowing bromoform vapor for 24 h (treatment G*). Coupling of PEO to the Halogenated PE Surfaces. PEO was reacted with the brominated or iodinated surfaces according to the classical Williamson’s ether synthesis.35 The method given by Harris et al.36 was adapted here, the brominated or iodinated surfaces being reacted with dissolved PEO in the following way. The hydroxyl-terminated PEO was reacted with potassium tertbutoxide to yield the corresponding alkoxide derivative in completely moisture free conditions. Coupling of 0.5 g of PEO in 15 mL of dry benzene in the presence of 0.036 g of t-BuOK was carried out at 25 °C for 40 h (treatment C, samples 4, 7, 10, 12, 14). Bifunctional PEO was reacted under the same conditions (treatment C*, samples 6 and 8). In treatment H (sample 16) acetonitrile was used as a reaction medium instead of benzene. The coupling of PEO to plasma-brominated surfaces was also performed in the absence of solvent (treatment I, sample 17). Dehydrated PEO melt was reacted with the solid surface at 110 °C for 1 h. Upon completion of the coupling, the samples were washed in the solvent or thoroughly rinsed in water following the melt PEO reaction. Surface Characterization by XPS. XPS measurements were performed by a KRATOS XSAM800 type spectrometer, using Mg KR1,2 (E ) 1253.6 eV) radiation. The X-ray source was operated at 15 kV and 10 mA. Spectra were recorded in the fixed analyzer transmission mode of the electron energy analyzer (pass energy ) 40 eV). The pressure in the sample analysis chamber was about 10-9 mbar. If not stated otherwise in the text, a photoelectron takeoff angle of 0° (i.e., angle normal to the surface) was applied. Data acquisition and processing were performed by a SUN Sparc Station IPX computer, using the VISION 2000 program. The overview spectra were taken between 50 and 1300 eV with 0.5 eV steps, while the detailed spectra of the peaks of interest (O 1s, C 1s, Cl 2p, Br 3d, etc.) were recorded with an energy step of 0.1 eV. Referencing was done to saturated hydrocarbon (C 1s ) 285.0 eV). The overlapping peaks were (33) Ponret, C. C. R. Hebd. Seances Acad. Sci. 1900, 130, 1191. In Rodd, E. H. Chemistry of Carbon Compounds; Elsevier: Amsterdam, 1951; Vol. 1, Part A, p 275. (34) Wohl, A. Ber. Dtsch. Chem. Ges. 1906, 39, 1951. In Rodd, E. H. Chemistry of Carbon Compounds; Elsevier: Amsterdam, 1951; Vol. 1, Part A, p 275. (35) Vogel, A. I. J. Chem. Soc. 1948, 616. (36) Harris, J. M.; Struck, E. C.; Case, M. G.; Paley, M. S.; Yalpani, M.; van Alstine, J. M.; Brooks, D. E. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 341.
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Langmuir, Vol. 12, No. 6, 1996 1653 resolved by the peak synthesis method, applying Shirley-type background subtraction and peak profiles with 60% of Gaussian and 40% of Lorentzian contributions. The C 1s peaks were decomposed into four components (C1 to C4) at binding energies (BE) of 285.0, 286.5, 287.9, and 289.3 eV. In the case of oxidized surfaces (with low halogen contents) these components correspond to hydrocarbon, CsO, CdO and/or OsCsO, and carboxyl-type functional groups, respectively.37 Wettability Measurements. Contact angles were measured by the sessile drop technique, on a Rame´-Hart contact angle goniometer. Advancing and receding angles were obtained by increasing or decreasing the drop volume until the three-phase boundary moved over the surface. The reported values are the average of at least 8 drops placed and measured on different parts of the sample surface. Standard deviation is presented with experimental data in the figures. The typical error is (3°. Surface free energy values as a sum of dispersion and polar components were calculated from contact angles using the harmonic mean method according to the equation derived by Wu.38 Testing liquids were water and formamide. Since the hydration or solvation of the surface PEO layer is of main importance in the wettability behavior, advancing and receding contact angle data were used separately in the calculations.39 The difference between surface energy values related to advancing and receding modes reflects the alteration in the state of the surface due to contact with liquids.
Results and Discussion Surface Composition. The surface composition (atomic ratios of the constituents) of PE after various wet chemical and plasma treatments and that of the PEO films were determined by XPS. The surface of the starting PE substrate contains only a small amount of oxygen (∼3 atom %) as a detectable impurity. Chlorination. The surface chlorination of PE by sulfuryl chloride, similarly to the bulk radical chain chlorination,32 led to preferential attachment of chlorine. Minor amounts of sulfur, oxygen, nitrogen, and silicon (below 1 atom %) have also been detected and can originate from the chemicals applied and the reaction vessel. Here we have to point out that in spite of the considerable effort invested we were not able to couple PEO to a chlorinated surface. This is why we proposed the exchange of chlorine to bromine or iodine, expecting to provide an effective means of forming an ether-type linkage of PEO to a PE surface. The order of reactivity of the halide leaving groups is I- > Br- > Cl- . F-. This order is opposite to that of the electronegativity and is dominated by the strength of the bond to carbon. Transhalogenation. Bromine or iodine appeared in easily measurable quantities in the spectra after transbromination or transiodination. It is known that the carbon-halogen bonds under prolonged X-ray exposure suffer partial degradation40 where the major mechanism of degradation is dehydrohalogenation. This is why special care was taken to record the bromine and chlorine lines. In order to assess the effect of this degradation on our quantitation of brominated samples, the time-dependent spectra of the Br 3d region were recorded at 15 min intervals. Figure 1 shows the elimination of bromine during the longest exposure (45 min); e.g., data acquisition time is about 10%, without any change of its chemical environment. In order to reduce this small error the Br 3d line was recorded at the beginning of the data acquisition cycle, so reducing the bromine loss to 3%. (37) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Wiley: Chichester, 1992. (38) Wu. S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982; p 178. (39) Ruckenstein, E.; Gourisankar, S. V. J. Colloid Interface Sci. 1986, 109, 557. (40) Chang, H. P.; Thomas, J. H., III. J. Electron Spectrosc. Relat. Phenom. 1982, 26, 203.
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Kiss et al. Table 2. Surface Composition of the PEO-Grafted PE Surfaces as Determined by XPS (atom %) atom % sample no. O 1s N 1s K 2p C 1s Cl 2p S 2p P 2p Si 2p Al 2p Br 3d 4 6 7 8 10 12 14 16 17
Figure 1. Time-dependent XPS spectra of the Br 3d region of a bromoform plasma treated PE sample recorded at 15 min intervals.
On the pristine PE (sample 1) no chlorine was detected. On the chlorinated samples the chlorine content was less than 5 atom %. After transbromination only traces of chlorine (
