Plasma-Oxidized Polystyrene: Wetting Properties ... - ACS Publications

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Langmuir 2000, 16, 8194-8200

Plasma-Oxidized Polystyrene: Wetting Properties and Surface Reconstruction Ch. C. Dupont-Gillain, Y. Adriaensen, S. Derclaye, and P. G. Rouxhet* Unite´ de chimie des interfaces, Universite´ catholique de Louvain, Belgium Received March 3, 2000. In Final Form: June 22, 2000 The surface of oxygen-plasma-treated polystyrene (PSox) was investigated using X-ray photoelectron spectroscopy (XPS), streaming potential measurements and a dynamic study of the wetting properties at different pH (Wilhelmy plate method). The PSox surface is functionalized with various oxygen-containing groups, including carboxyl functions, and must be viewed as covered by a polyelectrolyte which swells depending on pH. The wetting hysteresis, its evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upon emersion; the retained water may evaporate progressively and allow macromolecule compaction and/or reorientation. Modification of the PSox surface upon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissolution of PSox macromolecular chains, as indicated by adsorption of released fragments on a check PS sample placed nearby. However, the concentration of functionalized molecules at the surface of water-aged PSox was still sufficient to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faster in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plasticizing effect of water. Hydrophobicity recovery upon aging in air was reversed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swelling.

Introduction Polymer materials are often surface treated to change the chemical composition, increase the surface energy, or modify the surface topography while keeping the bulk properties unchanged.1-3 Typical examples are the oxidation of polyolefins to improve paint adhesion2 and the elaboration of biomaterials.4-6 Plasma discharges in oxygen are often used for that purpose; they are known to produce new oxygen-containing chemical functions at the surface and to increase the wettability.7 Oxygen-plasma treated polymers evolve upon exposure to air.8,9 Their wettability generally decreases with time, sometimes even going back to that of the untreated polymer.10-15 In some cases, this may be explained by deposition of a contamination layer on the high-energy * Address for correspondence. Place Croix du Sud, 2/18 B-1348 Louvain-la-Neuve, Belgium. E-mail: [email protected]. (1) Wu, S. Polymer Interface and Adhesion; Marcel Dekker, Inc.: New York, 1982. (2) Brewis, D. M.; Briggs, D. Polymer 1981, 22, 7. (3) Fourche, G. Polym. Eng. Sci. 1995, 35, 968. (4) Hubbell, J. A. Trends Polym. Sci. 1994, 2, 20. (5) Lhoest, J.-B.; Detrait, E.; Dewez, J.-L.; van den Bosch de Aguilar, P.; Bertrand, P. J. Biomater. Sci., Polym. Ed. 1996, 7, 1039. (6) Dewez, J.-L.; Lhoest, J.-B.; Detrait, E.; Rouxhet, P. G.; Bertrand, P.; Van Den Bosch De Aguilar, P. International Patent Application PCT/BE/00104. (7) Ameen, A. P. Polym. Degrad. Stab. 1996, 51, 179. (8) Petrat, F. M.; Wolany, D.; Schwede, B. C.; Wiedmann, L.; Benninghoven, A. Surf. Interface Anal. 1994, 21, 402. (9) Clouet, F.; Shi, M.; Prat, R.; Holl, Y.; Marie, P.; Le´onard, D.; De Puydt, Y.; Bertrand, P.; Dewez, J.-L.; Doren, A. J. Adhes. Sci. Technol. 1994, 8, 329. (10) Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Interface Sci. 1989, 132, 504. (11) Morra, M.; Occhiello, E.; Gila, L.; Garbassi, F. J. Adhes. 1990, 33, 77. (12) Murakami, T.; Kuroda, S.; Osawa, Z. J. Colloid Interface Sci. 1998, 200, 192. (13) Murakami, T.; Kuroda, S.; Osawa, Z. J. Colloid Interface Sci. 1998, 202, 37. (14) Terlingen, J. G. A.; Gerritsen, H. F. C.; Hoffman, A. S.; Feijen J. J. Appl. Polym. Sci. 1995, 57, 969. (15) Briggs, D.; Rance, D. G.; Kendall, C. R.; Blythe, A. R. Polymer 1980, 21, 895.

surface produced by the plasma treatment.16,17 However, contamination has been ruled out in many studies, which rather attribute the aging behavior to reorganizations taking place in the near-surface;10-19 such reconstruction consists either in the reorientation of functional groups or in the diffusion of functionalized macromolecules into the bulk. The surface of polymers should thus not be seen as a rigid plane, but as reorganizing according to the environment.20-23 The driving force for surface reconstruction observed on plasma-treated polymers is the excess free energy associated with the interface between the material and the phase in contact.13,20,21 However, the excess free energy associated with the interface between the plasma-modified layer and the bulk polymer is important as well.10,14,15 Since the mobility of macromolecular chains increases with temperature, surface reorganization is strongly enhanced upon heating.10,12,14,15,18 The surface modifications of polystyrene (PS) following oxygen-plasma treatment have been investigated, mainly using X-ray photoelectron spectroscopy (XPS), time-offlight secondary ion mass spectrometry (ToF-SIMS) and contact angle measurements. A wide range of oxygencontaining functions were detected at the surface after treatment8,17,24,25 and the aromatic rings were identified (16) Paynter, R. W. Surf. Interface Anal. 1999, 27, 103. (17) Gerenser, L. J. In Plasma Surface Modification of Polymers; Strobel, M., Lyons, C., Mittal, K. L., Eds; VSP Publishing: Utrecht, 1994; p 43. (18) Occhiello, E.; Morra, M.; Garbassi, F.; Johnson, D.; Humphrey, P. Appl. Surface Sci. 1991, 47, 235. (19) Strobel, M.; Lyons, C. S.; Strobel, J. M.; Kapaun, R. S. J. Adhes. Sci. Technol. 1992, 6, 429. (20) Yasuda, T.; Okuno, T.; Yoshida K. J. Polym. Sci. B 1988, 26, 1781. (21) Holly, F. J.; Refojo M. F. J. Biomed. Mater. Res. 1975, 9, 315. (22) Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Interface Sci. 1992, 149, 84. (23) Morra, M.; Occhiello, E.; Garbassi, F. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP Publishing: Utrecht, 1993. (24) Hopkins, J.; Wheale, S. H.; Badyal, J. P. S. J. Phys. Chem. 1996, 100, 14062.

10.1021/la000326l CCC: $19.00 © 2000 American Chemical Society Published on Web 09/19/2000

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as the reactive sites.8,17,25 Etching of the surface was found to be more pronounced after plasma discharges in oxygen compared to other gases.8,17 Consequently, low-molecularweight fragments may be present at the oxygen-plasmatreated polystyrene (PSox) surface; these fragments can be removed by solvents12,13 and are thought to facilitate the surface reconstruction of PSox upon aging.17 Hydrophobicity recovery of the PSox surface aged in air was shown to be enhanced by lowering the molecular weight (Mw) of the polymer: while only reorientation of polar groups could occur with a high Mw, diffusion of macromolecules into the bulk was also observed at low Mw.18 The molecular mobility of the PSox chains was shown to be enhanced compared to that of PS chains; this was attributed to an increased amount of chain ends and a lower Tg.12 The aim of this study was to investigate the surface organization of PSox compared to that of PS. While the chemical changes created by the plasma treatment have already been identified, not much attention has been paid to the structure of the PSox surface. As plasma-treated surfaces are often intended to be used in liquid environments, the dynamic study of wetting and the evaluation of the surface electrical properties are combined with XPS to gain a better understanding of the structure of the PSox surface in water. Moreover, the evolution of the surface organization of PSox upon aging in different environments is examined. Materials and Methods (1) Sample Preparation. The PS samples were cut in Petri dishes (UV-sterilized, Merck-Belgolabo, Belgium), cleaned by ultrasonication in 2-propanol (analytical grade, VEL, Belgium) and dried under a nitrogen flow. PSox was obtained using low-pressure radio frequency (13.56 MHz) plasma discharge in oxygen (>99.9999% pure from Air Liquide, Belgium) in a barrel reactor (Chemprep 130, Chemex, United Kingdom). Prior to sample loading and gas admission, the reactor chamber was evacuated down to 7 × 10-5 bar; it was cleaned by allowing oxygen to flow through the system during 3 min and performing a plasma discharge. After introduction of the samples and evacuation of the chamber down to 7 × 10-5 bar, the plasma dicharge was performed during 30 s at a power of 50 W, under an oxygen flow (working pressure ) 7 × 10-4 bar). After the discharge, the samples were left in contact with the flowing gas during 10 min. Freshly prepared samples of PSox (PSoxfresh) were stored in a desiccator containing P2O5 and analyzed within 24 h following the treatment. For aging, samples of PSox were stored 8-13 days at room temperature, either (i) in a desiccator containing P2O5 (PSoxRH5%), which ensures a relative humidity of about 5%; (ii) in a closed vessel containing a saturated solution of Na2CO3 (PSoxRH95%), which ensures a relative humidity of about 95%; or (iii) in water (PSoxwater) (purified with a Milli-Q plus system from Millipore, hereafter referred to as Milli-Q). The samples aged in water were quickly dried under a nitrogen flow before XPS analysis but were examined without drying with the Wilhelmy plate method. (2) Surface Characterization. X-ray Photoelectron Spectroscopy. XPS spectra were recorded using a SSX-100 spectrometer (model 206 from Surface Science Instruments) equipped with an aluminum anode (10 kV; 20 mA) and a quartz monochromator. Charge stabilization was achieved using an electron flood gun set at 6-8 eV and placing a grounded nickel grid 2-3 mm above the sample surface. The analyzed area was 1.4 mm2, and the pass energy was 50 eV for individual peak analysis and 150 eV for survey analysis. The direction of photoelectron collection made an angle of 55° with the normal to the sample surface. The order of peak analysis was C1s, O1s, C1s again, and survey scan. No modification of the C1s peak shape was noted (25) Lianos, L.; Parrat, D.; Hoc, T. Q.; Duc, T. M. J. Vac. Sci. Technol. A 1994, 12, 2491.

Langmuir, Vol. 16, No. 21, 2000 8195 indicating that the samples did not undergo degradation nor contamination during analysis.26 The binding energy scale was set by fixing the component of the C1s peak due to carbon only bound to carbon and hydrogen at 284.8 eV. A Shirley-type nonlinear background subtraction was used.27 Intensity ratios were converted into molar concentration ratios by using the sensitivity factors proposed by the manufacturer (Scoffield emission cross sections, variation of the electron mean free path according to the 0.7 power of the kinetic energy, constant transmission function). Atomic Force Microscopy (AFM). AFM images (10 × 10 µm2) were recorded on PS and PSoxfresh using an Autoprobe CP microscope (Park Scientific Instruments, CA) equipped with silicon tips integrated into triangular levers (Park Scientific Instruments, CA, USA; nominal spring constant ) 0.16 or 0.24 N/m). The observations were performed either in air or in water (Milli-Q; pH ) 5.6). The scan rate was 1 Hz and the imaging force was kept as low as possible. Streaming Potential Measurement. The zeta potential ζ, which is the electric potential at the plane of shear between a charged surface and a liquid, was measured using the streaming potential method. The instrument was purchased from the Laboratory for Physical and Colloid Chemistry, Wageningen Agricultural University, The Netherlands (for detailed description, see refs 28 and 29). Two identical sample plates (2.5 cm × 7.5 cm) were clamped parallel to each other and spaced by a 100 µm Teflon gasket, forming the capillary system. On each side of this system, a platinum electrode was lodged in a small compartment. Two Pyrex bottles, directly connected to the electrode compartments, were used as solution reservoirs; they were provided with a nitrogen inlet which allowed varying the overpressure provoking the liquid displacement. The experiments were performed at room temperature in KNO3 (analytical grade, UCB, Belgium) 10-2 M solutions; pH was adjusted using KOH (analytical grade, Janssens, Belgium) and HNO3 (analytical grade, Merck-Belgolabo, Belgium) 10-1 M solutions. For each pH value, the experiment was conducted using at least five different pressures; at each pressure, a minimum of three potential measurements were performed. For each pair of measured pressure and potential, ζ was then deduced as detailed elsewhere.30 A complete ζ vs pH curve was obtained using the same sample plates (about 7 different pH values; 1.5 days, including equilibration at each pH). Study of the Wetting Dynamics. Wetting with water (Milli-Q, pH ) 5.6) and with KOH (pH ) 11.0) and HNO3 (pH ) 3.0) 10-3 M solutions was investigated using the Wilhelmy plate method, with a DCA 322 equipment from Cahn Instruments (Cerritos, CA). The measurements were performed at room temperature. The water container was closed with a lid, pierced with a small hole for the suspension wire; this ensured a relative humidity of 95% 1 cm above the water surface. Repeated cycles of immersion and emersion (advancing - receding) were performed at a speed of 50 µm/s and the contact angles (θadv, θrec) were recorded. Therefore, the effect of buoyancy was corrected, the position scale X referring to the distance between the bottom of the plate and the position of the three-phases contact line as described before.31 In certain cases, a pause was made between two successive immersion-emersion cycles. The surface tension of the KOH and the HNO3 solutions was not significantly different from that of pure water.

Results AFM observations in air and in water showed that the PS surface presents depressions of about 10 nm depth and about 1.5 µm diameter, separated by a distance of the (26) Dengis, P. B.; Gerin, P. A.; Rouxhet, P. G. Colloids Surfaces B 1995, 4, 199. (27) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (28) Van der Linden, A. J.; Bijsterbosch, B. H. Colloids Surf. 1989, 41, 345. (29) Elgersma, A. V.; Zsom, R. L.; Lyklema, J.; Norde, W. Colloids Surf. 1992, 65, 17. (30) Rouxhet, P. G.; Doren, A.; Dewez, J.-L.; Heuschling, O. Prog. Org. Coat. 1993, 22, 327. (31) Tomasetti, E.; Rouxhet, P. G.; Legras, R. Langmuir 1998, 14, 3435.

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Figure 2. Variation of the streaming potential ζ as a function of pH: (0) PS; PSoxfresh, first by (b) decreasing and then by (2) increasing the pH.

Figure 1. C1s peaks of (- -) PS and (s) PSoxfresh. The different chemical functions contributing to the signal are indicated. The ordinate scale was adjusted to obtain the maximum of the main peak at the same height.

order of 4 µm; the root-mean-square roughness measured on 10 × 10 µm2 images was about 3 nm. Between these depressions, the surface showed a positive relief made by lines (about 5 nm high, mesh of the order of 500 nm) which are clearly due to scratches of the mold; the rms roughness measured on 2 × 2 µm2 areas devoid of depression was about 1.8 nm. The surface relief of PSox was similar to that of PS. The O/C molar concentration ratio determined by XPS is 0.01 and 0.26 for PS and PSoxfresh, respectively. The position of the O1s peak shifts from 533.3 eV for PSoxfresh and PSoxRH5% to 533.1 eV for PSoxRH95% and to 532.7 eV for PSoxwater, while the full width at half-maximum is about 2.5 eV. Typical C1s peaks are shown in Figure 1; they are similar to those presented in the literature.32,33 The main peak at 284.8 eV is due to carbon atoms only bound to carbon and hydrogen (C-(C,H)) and the broad satellite peak at 291.4 eV is a shake-up peak due to π f π* transitions of the aromatic ring.33 After plasma treatment, new contributions appear in the intermediate binding energy domain, which may be due32-34 to alcohol or ether (C-O, 286.1-286.8 eV), aldehyde or ketone (CdO, 287.1288.1 eV), carboxylate (OdC-O-, near 288.0 eV), ester or carboxylic acid (OdC-O, 288.6-289.2 eV), and finally, carbonate (O-(CdO)-O, 290.0-290.3 eV) functions. Broadening of the left-hand side of the main C-(C,H) component may also be due to carbon in alpha position with respect to carbon doubly bound to oxygen (OdCC-(C,H), 285.2-285.6 eV). Figure 2 presents the variation of ζ as a function of pH for PS and PSoxfresh. The confidence interval (95% probability) was below 0.8 mV. For PSoxfresh, the measurements were first performed by decreasing the pH, starting from pH 10.7, and then by increasing the pH, without changing the sample plates. The last point was obtained 3 days after the plasma treatment. The isoelectric point was pH (32) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA 300 Database; John Wiley and Sons: Chichester, 1992. (33) Dewez, J.-L.; Doren, A.; Schneider, Y.-J.; Rouxhet, P. G. Biomaterials 1999, 20, 547. (34) Gerin, P. A.; Dengis, P. B.; Rouxhet, P. G. J. Chim. Phys. 1995, 92, 1043.

4.1 for PS and about pH 2.5 for PSoxfresh. At high pH, ζ reached a plateau at about -30 mV for PS. The plateau was at -57 mV and -47 mV for PSoxfresh initially and after 3 days of analysis, respectively. The surface of PSoxfresh is thus more negatively charged compared to PS; this difference decreases when PSox is left under water. For PS, the surface electrical charge may originate from oxygen-bearing functions of impurities. Note that, according to the diffuse double layer model (Debye-Hu¨ckel approximation), a surface charge density as low as 0.05 µmol/m2, i.e., 0.03 site/nm2, may be responsible for a ζ potential of 20 mV.35 The variation of cos θ during repeated immersionemersion cycles, measured with the Wilhelmy plate method at pH 3.0, 5.6, and 11.0, is shown in Figure 3 for PS and PSoxfresh. The slight excess of cos θ with respect to 1, observed in certain cases, may be attributed to imprecision of the measurement of the sample perimeter. For PS, the shape of the hysteresis loop remained constant upon repeated cycles; θadv and θrec were not affected by the pH and were equal to 97° and 78°, respectively. The wetting behavior of PSoxfresh was strongly dependent on the pH; it changed upon repeating cycles and was affected by pauses between cycles. While θrec was always equal to zero, θadv recorded during the first immersion was equal to 40°, 36° and 30° at pH 3.0, 5.6 and 11.0, respectively. When a second cycle was performed immediately after the first one, θadv remained close to zero, whatever the pH. If the emersed sample was subject to a pause before being immersed again (cycles 3 to 5), the behavior became dependent on pH. At pH 11.0, θadv remained close to zero even after a pause of 30 min. At pH 3.0 after a pause of 5 min, θadv progressively shifted to the value observed during the first cycle; when a longer pause was used, θadv became higher than the value obtained in the first cycle (θadv ) 55° for the fifth cycle). The behavior at pH 5.6 was intermediate between those at pH 11.0 and pH 3.0. C1s peaks obtained by XPS on PSoxRH5%, PSoxRH95%, and PSoxwater are shown in Figure 4a, b and c respectively and compared to that of PSoxfresh. While the peak was almost unchanged after aging in a dry atmosphere, the components due to carbon bound to oxygen decreased appreciably after aging in a humid atmosphere or in water. In coherence with these observations, the O/C molar ratio, equal to 0.26 for PSoxfresh, was 0.25 for PSoxRH5% and dropped to 0.18 and 0.14 for PSoxRH95% and PSoxwater, respectively. (35) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997.

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Figure 4. C1s peaks of PSox aged in (-) different media: (a) PSoxRH5%, (b) PSoxRH95%, and (c) PSoxwater. Each peak is compared to the one of a reference (- -) PSoxfresh sample. The ordinate scale was adjusted to obtain the maximum of the main peak at the same height. Figure 3. Variation of cos θ as a function of the three-phases contact line position X during repeated immersion-emersion cycles for (a) PS and (b) PSoxfresh at pH 3.0, 5.6, and 11.0. On PSoxfresh, no pause was used between cycles 1 and 2; a pause of 5, 10, and 30 min was used after cycles 2, 3 and 4, respectively.

A summary of O/C molar ratios obtained by XPS and of θadv of the first wetting cycle (θadv.1) obtained using the Wilhelmy plate method is presented in Table 1. Figure 5 presents the variation of cos θ measured during successive immersion-emersion cycles at pH 3.0, 5.6, and 11.0 on PSoxRH5%, PSoxRH95%, and PSoxwater. The wetting behavior of PSoxRH5% was similar to that of PSoxfresh; however, θadv of the first cycle was noticeably higher. After aging in humid atmosphere (PSoxRH95%), θadv of the first cycle became still higher whatever the pH. At pH 3.0, θadv of the second cycle was close to the value observed for the first cycle, instead of remaining close to zero as observed for PSoxfresh and PSoxRH5%. When PSox was aged in water (PSoxwater), the samples were still wet when the analysis began, thus provoking an overestimation of their weight and shifting the experimental cos θ scale. It may be assumed that the real value of θrec was 0°, as observed for PSoxfresh, PSoxRH5%, and PSoxRH95%; then θadv of the first cycle was also equal to zero. Accordingly, θadv at pH 3.0 was equal to 57° for the second cycle and increased slightly after pauses. At pH 5.6 and 11.0, wetting was complete for all cycles. One may note that θadv.1 was always higher at pH 3.0 compared to pH 11.0 (Table 1). Moreover, the value of θadv at pH 3.0 for the 5th cycle, i.e., after a pause of 30 min, was about the same (55° - 65°) for all oxidized samples (Figures 3 and 5). The possible release of macromolecular chains from PSox into water was investigated by the following procedure. Two samples of PSoxfresh were partially immersed in Milli-Q water together with a PS check sample (sample size: approximatively 1 cm × 2 cm; water volume

Table 1: O/C Molar Ratios Measured by XPS and θadv of the First Wetting Cycle Obtained at Different pH Using the Wilhelmy Plate Method on PS, PSoxfresh, and PSox in Different Conditions θadv.1 (deg)a sample

O/C

pH 3.0

pH 5.6

pH 11.0

PS PSoxfresh PSoxRH5% PSoxRH95% PSoxwater

0.01 ( 0.02a 0.26 ( 0.02a 0.25(0.003)b 0.18(0.009)b 0.14(0.006)b

98.3 ( 3.8 39.9 ( 6.0 50.0 ( 2.2 64.8 ( 6.1 c

96.5 ( 2.1 35.7 ( 6.1 50.0 ( 6.6 65.6 ( 5.8 c

97.7 ( 1.4 29.9 ( 6.4 36.0 ( 10.8 58.8 ( 6.7 c

a Average value on at least three measurements and confidence interval at 95%. b Average value on two measurements and interval between the results. c Not determined due to the use of wet samples.

) 16 mL; room temperature); after 6 days, the check sample was removed from water, quickly dried under a nitrogen flow and immediately analyzed using the Wilhelmy plate method. A PS reference sample was prepared using the same procedure but in absence of PSox samples. The results are presented in Figure 6. Comparison between Figures 6a and 3a shows that the wetting properties of the reference sample were not modified by a prolonged stay in water. However, Figure 6b shows that the check sample became more hydrophilic when introduced in water together with PSox. While the advancing contact angle decreased slightly compared to native PS (θadv ) 88° compared to 97°), the receding contact angle was much more affected by the treatment (θrec ) 47° for PS immersed in the presence of PSox, instead of θrec ) 78° for native PS). This indicates that the surface of the check sample was partially covered by molecules originating from PSox, resulting in a less hydrophobic surface. The amount of adsorbed PSox chains was too low to produce a XPS signal different from that of native PS, which already showed an O/C molar ratio of 0.01. On the other hand, the surface

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Figure 5. Variation of cos θ as a function of the three-phases contact line position X during repeated immersion-emersion cycles for (a) PSoxRH5%, (b) PSoxRH95%, and (c) PSoxwater at pH 3.0, 5.6, and 11.0. No pause was used between cycles 1 and 2; a pause of 5, 10, and 30 min was used after cycles 2, 3 and 4, respectively.

Figure 6. Variation of cos θ as a function of the three-phases contact line position X during two successive immersionemersion cycles for PS pretreated with water alone (a ) reference sample) and in the presence of PSoxfresh samples (b ) check sample). The arrows indicate the position of the water level on the PS plates during the aqueous pretreatment (pretreated area, left-hand side; non pretreated area, righthand side).

tension of water having contained PSox was not significantly different from that of pure water. Discussion In an idealized wetting process, the solid surface is considered as smooth, homogeneous, non deformable, and

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interacting with the liquid only at the surface plane. These are conditions to measure an equilibrium contact angle. However, in real systems, one or several of these conditions are not encountered; this leads to a contact angle hysteresis, the advancing contact angle being different from the receding contact angle.36,37 Four different kinds of hysteresis loop sequences were encountered in this study, depending on material and pH: (i) θadv and θrec keep the same value during one immersion or emersion event and also from cycle to cycle. This is the case for PS at all pH values (Figure 3) and for PSoxRH95% and PSoxwater at pH 3.0 (Figure 5). (ii) θrec and θadv are equal to zero with the possible exception of θadv.1. This is the case for PSoxfresh and PSoxRH5% at pH 11.0, and for PSoxwater at pH 5.6 and 11.0 (Figures 3 and 5). (iii) θrec of all cycles is equal to zero; θadv.1 is different from zero. For the subsequent cycles, θadv varies as a function of sample position, and the slope is influenced by the duration of the pause following the previous emersion. This is the case for PSoxRH5% at pH 5.6 and PSoxRH95% at pH 5.6 and 11.0 (Figure 5). (iv) Same as III; however, θadv tends to shift from θrec to θadv.1 and even higher when the sample is submitted to pauses of increasing duration at emersion. This is the case for PSoxfresh at pH 3.0 and 5.6 and for PSoxRH5% at pH 3.0 (Figures 3 and 5). PS (hysteresis loop sequence I) is expected to be rigid (Tg ) 100 °C, elasticity modulus ) 3200 N/mm2)38 and not to be solvated by water. Surface deformation and macromolecule solvation thus cannot explain the wetting hysteresis. Andrade and co-workers37 mentioned that the effect of roughness below the 0.1 µm level was negligible, while more recent studies point out that topography may even play a role at the molecular scale.39,40 Actually, the influence of roughness should also be dependent on the lateral size of the surface irregularities. The network of lines observed by AFM may be responsible for the hysteresis. Moreover, the chemical homogeneity of the surface may be altered by contamination, sterilization process, or residues of the polymerization process, which are also responsible for the oxygen content (O/C ) 0.01) as well as for the observed ζ potential. The same explanations are also applicable to PSoxRH95% and PSoxwater at pH 3.0. The hysteresis loop sequences of type II observed on PSox at high pH may be explained taking into account the results obtained by XPS and streaming potential measurements. The appearance of C1s contributions above 288.0 eV following plasma treatment (Figure 1) and the more negative character of PSox compared to PS (Figure 2) indicates that the PSox surface bears polyanions at high pH. Deprotonation of the carboxylic functions provokes a repulsion between adjacent polymer chains and their close interaction with water; θrec is equal to zero and, once the surface has been wetted, θadv is also equal to zero. However, θadv.1 measured on dry PSoxfresh and PSoxRH5% is significantly different from zero; this indicates that, during the first immersion, the water meniscus (36) Johnson, R. E.; Dettre, R. H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Interscience: New York, 1969; Vol. 2, p 85. (37) Andrade, J. D.; Smith, L. M.; Gregonis, D. E. In Surface and Interfacial Aspects of Biomedical Polymers Andrade, J. D., Ed.; Plenum: New York, 1985; Vol. 1, p 249. (38) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley and Sons: New York, 1989. (39) Imabayashi, S.-I.; Gon N.; Sasaki T.; Hobara D.; Kakiuchi T. Langmuir 1999, 14, 2348. (40) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.

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defines a border between solvated and nonsolvated macromolecules. While several authors showed that low-molecularweight fragments produced by the plasma treatment may be extracted by solvents,12,13,41,42 a more direct evidence of the release of PSox polymer chains in water is presented here: the PSox fragments dissolved by water were adsorbed on a neighboring PS check sample, as detected by wetting measurements on the latter (Figure 6). As this adsorption had a strong influence on θrec of the check sample but not on θadv, it is concluded that the check sample surface was only partially covered by PSox fragments, resulting in a heterogeneous surface.37 The decrease of the ζ potential of PSox after 3 days of contact with water (Figure 2) also supports the loss of functionalized chains in water. The variation of θadv as a function of the sample position in the hysteresis loops of type III may not be attributed to the dissolution of oxidized fragments and thus to a variation of the immersion duration. The total time previously spent in water decreases when X increases; a progressive dissolution making the surface more hydrophobic should thus give a slope of opposite sign. The time spent at a certain distance of the water surface, i.e., in contact with a water partial pressure below saturation, increases with X. The increase of θadv as a function of X may thus be attributed to evaporation of a water film retained at the surface and the concomitant compaction of the macromolecules and/or a reorientation of functionalized fragments toward the bulk. This interpretation is in agreement with the effect of pause duration on the hysteresis loop and, in particular, with the absence of hysteresis in cases where no pause was performed between emersion and immersion (cycle 2). The hysteresis loop sequences of type IV differ from those of type III by the fact that increasing the pause duration leads to θadv which becomes less sensitive to X and tends to reach higher values compared to θadv.1. This is found at low pH for surfaces characterized by a high concentration of functionalized fragments (PSoxfresh, PSoxRH5%). It may be attributed to a combination of the process typical of type III (water evaporation) and of the progressive dissolution of functionalized fragments upon repeated wetting cycles. Attribution of the hydrophobicity recovery to adsorption of hydrocarbon contaminants from water may not be considered in the present case. The same wetting experiment was indeed performed with a clean glass slide (results not presented here) and cos θadv remained equal to 1 during all the cycles. The influence of aging conditions may now be discussed in the light of the description of the different hysteresis loop sequences. Aging of plasma-treated polymers in water has been studied by several authors.12,13,19,42 The final state of these polymer surfaces results from the balance of two mechanisms producing opposite effects: (i) the reorganization of the sample surface in contact with the polar medium tends to bring the functionalized polymer chains at the surface and to increase the surface hydrophilicity and (ii) the dissolution of functionalized polymer chains in water leads to a decrease of the surface hydrophilicity. The XPS results obtained here show a decreased O/C ratio (Table 1) and a loss of oxygenated functions (Figure 4c) following aging of PSox in water. This can be attributed to the dissolution of functionalized chains in water as (41) Schamberger, P. C.; Abes, J. I.; Gardella, J. A. Colloids Surf. B 1994, 3, 203. (42) Callen, B. W.; Ridge, M. L.; Lahooti, S.; Neumann, A. W.; Sohdi, R. N. S. J. Vac. Sci. Technol. A 1995, 13, 2023.

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discussed above. The shape of the hysteresis loop at pH 3.0 and its independence with respect to pauses (Figure 5c) indicates that the wetting properties are not controlled by a water evaporation process; the surface wets and dewets such as a conventional solid with defined values of θadv and θrec. However, at pH 5.6 and 11.0, deprotonation of the remaining functionalized chains, which are at the surface or in the near-surface zone, allows a closer interaction with water, and the contact angle, either on advancing or receding, is equal to zero. When PSox is stored in humid air, the surface concentration of oxygen moieties decreases (Table 1, Figure 4b) and the wetting properties are modified compared to PSoxfresh. The value of θadv.1 is higher at all pH, revealing a lower hydrophilicity. As the samples were not put in contact with a solvent before XPS analysis, this may only be attributed to surface reconstruction during aging. The decrease of the O/C ratio from 0.26 to 0.18 shows that the surface reconstruction takes place in a layer of the order of, or deeper than the photoelectron mean free path, equal to 3.8 and 3.2 nm for C1s and O1s in PS, respectively.43 The reconstruction is thus due to diffusion of oxidized macromolecular chains into the bulk. At pH 3.0, the surface wets and dewets such as a conventional solid, with defined values of θadv and θrec. At pH 5.6 and 11.0, deprotonation of the functionalized chains allows a closer interaction with water. A marked hydrophilicity is recovered; however, θadv depends on the time spent in air after the preceding emersion, indicating that it is controlled by the evaporation of a film of water. When PSox is stored in dry air, the surface chemical composition (Table 1, Figure 4a) remains unchanged compared to that of PSoxfresh. The wetting behavior becomes intermediate between that of PSoxfresh and PSoxRH95%. The θadv vs position profiles (Figure 5a) reveals a dependence of the contact angle with respect to the time spent after the preceding emersion, attributed again to water evaporation. θadv.1 is higher at all pH compared to PSoxfresh, indicating that surface reconstruction occurred during aging. However, the O/C ratio did not change significantly compared to PSoxfresh (Table 1). This means that the reconstruction took place in a layer of the order of a few nanometers, suggesting functionalized group reorientation rather than macromolecule diffusion into the bulk. Many authors have pointed out that the driving force for surface reconstruction of plasma-treated polymers is the minimizing of the free energy of the interface between the material and the surrounding medium.13,20,21 This would lead to a larger hydrophobic recovery in the case of aging in dry air compared to humid air due to the presence of an adsorbed water layer in the latter case. However, the contrary was observed. The more important surface reconstruction of PSox in humid air compared to dry air is attributed to a plasticizing effect of water which increases macromolecular chain mobility. The wetting behavior of PSoxfresh resembles that of PSoxRH5%, with the exception of lower θadv.1 which indicates that the functionalized chains are well exposed at the surface. It is known that plasma treatment may provoke crosslinking in the polymer.44-46 However, the major observa(43) Ashley, J. C. IEEE Trans. Nucl. Sci. 1980, NS-27, 1454. (44) Badey, J. P.; Urbaczewski-Espuche, E.; Jugnet, Y.; Sage, D.; Duc, T. M.; Chabert, B. Polymer 1994, 35, 2472. (45) Vallon, S.; Hofrichter, A.; Guyot, L.; Drevillon, B.; KlembergSapieha, J. E.; Martinu, L.; Poncin-Epaillard, F. J. Adh. Sci. Technol. 1996, 10, 1287. (46) Nihlstrand, A.; Hjertberg, T.; Johansson, K. Polymer 1997, 38, 1557.

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tion made here is a formation of acidic groups, the deprotonation of which favors solvation and increases macromolecular chain mobility. In the literature, the influence of pH on the static contact angle has been used to investigate the acid-base properties of surface-modified polymers.47,48 It is shown here that the effect of pH is very different on θadv.1 and on θadv measured subsequently. For PSox, θadv.1 refers to a meniscus defining a border between solvated and nonsolvated macromolecules, rather than a separation between wetted and nonwetted states of the same compact solid surface. After a first wetting event at high pH, both the advancing and receding angles are zero except for PSoxRH95%. This is attributed to a swollen layer of polyanions retaining water at the surface and is still observed when the surface concentration of oxidized molecules is considerably reduced by aging in water (PSoxwater, Figure 5c). For PSoxRH95%, the advancing contact angle shows a tendency to differ from zero. The better wetting of PSoxwater compared to PSoxRH95%, despite the lower O/C ratio determined by XPS on the former, may be due to a higher proportion of oxygen in the form of carboxyl, as supported by a relatively larger contribution in the range of 531-532 eV for the O1s peak, and/or to a higher concentration of oxygenated functions at the surface compared to the near surface. All together, it appears that the variation of θ as a function of pH reflects processes much more complex than the change of a compact surface due to deprotonation of acid functions, and is strongly influenced by the surface reconstruction and the wetting history.

a water film which may evaporate progressively and allow macromolecule compaction and/or reorientation and (ii) dissolution of functionalized fragments. Further evidence has been provided regarding the dissolution of functionalized fragments upon storage in water. Accordingly, the PSoxwater surface recovers hydrophobicity when analyzed at pH 3.0. However, the surface concentration of functionalized molecules, which in this case are exposed to the surface, is still sufficient to allow swelling and perfect wetting at pH 5.6 and 11.0. When starting with a dry solid, the water meniscus of the first immersion defines a border between solvated and nonsolvated molecules. Hydrophobicity recovery is much more marked in humid air (relative humidity 95%), where it is attributed to diffusion of oxidized chains into the bulk, compared to dry air (relative humidity 5%), where functional group reorientation only would occur. This is attributed to a plasticizing effect of adsorbed water, increasing the chain mobility. Hydrophobicity recovery is reversed quickly by immersion at pH 5.6 or 11.0, but not at pH 3.0, due to deprotonation and swelling. From a methodological point of view, this study shows that the analysis of wetting in dynamic conditions at different pH values, combined with XPS analysis and streaming potential measurements, is well suited to the investigation of surface-modified polymers and of their surface reconstruction with time in different environments.

Conclusions

Acknowledgment. The authors thank M. Genet for technical assistance with the XPS measurements, E. Tomasetti for valuable discussions concerning wetting experiments and H. De Deurwaerder (Coatings Research Institute, Limelette, Belgium) for access to the plasma reactor. The support of the National Foundation for Scientific Research (F.N.R.S.); of the Federal Office for Scientific, Technical and Cultural Affairs (Interuniversity Poles of Attraction Program); and of the Research Department of the Communaute´ Franc¸ aise de Belgique (Concerted Research Action); as well as the stimulation of COST Action 520 (Biofouling and Materials); are gratefully acknowledged.

The surface of PSox may be viewed as covered by a polyelectrolyte which swells in water at high pH. This behavior was identified in a previous study as the origin of a repulsive interaction with the tip of an atomic force microscope.49 The wetting hysteresis and the sequence of hysteresis loops upon repeated wetting cycles are controlled by two processes, the relative importance of which depends on pH and sample conditioning: (i) retention of (47) Lee, R. L.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (48) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (49) Dupont-Gillain, Ch. C.; Nysten, B.; Hlady, V.; Rouxhet, P. G. J. Colloid Interface Sci. 1999, 220, 163.

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