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Langmuir 1999, 15, 6400-6404
Surface Evaluation of Plasma-Modified Polysulfone (Udel P-1700) Films Marek Bryjak,* Irena Gancarz, and Gryzelda Poz´niak Institute of Organic and Polymer Technology, Wroclaw University of Technology, 50-370 Wroclaw, Poland Received May 29, 1998. In Final Form: April 15, 1999 The paper presents a new method for the evaluation of surface properties. Polysulfone films modified by plasma treatment in carbon dioxide, nitrogen, and vapors of n-butylamine were taken as test samples. Surface concentrations of acidic or basic functionalities were estimated by contact angle measurements. Aqueous solutions of HCl and NaOH with various pH values were used. The method of surface evaluation is based on fitting the γLcosΘ versus pH (or pOH) relationship with a polynomial and finding its inflection point. The pH (or pOH) values at this point are recalculated into apparent surface concentrations of basic (or acidic) functionalities by means of Gibbs’ and Langmuir’s equations. It was found that surfaces contain acidic functionalities (0.15 µmol/m2) when polysulfone was modified in carbon dioxide and basic functionalities (2.50 µmol/m2) for butylamine plasma. Modification with N2 resulted in the creation of an amphoteric surface.
Introduction Alteration of surface properties of conventional polymers opens new perspectives for their technical use. Among dozens of examples, such as modification of surface conductivity, permeability, biocompatibility, printability, and alteration friction properties,1-3 one may point out some extra advantages arising from changes in the surface properties of ultrafiltration membranes. Surface-hydrophilized membranes do not foul to the same extent as their unmodified analogues. Hence, the separation run may last much longer.4-9 Taking into account the economy of ultrafiltration, the longer the time of a membrane’s service, the more profitable the processes. An additional advantage of surface modification is a chance to modify the membrane surface without any alteration of the bulk material properties. One of the convenient ways to modify a surface is a plasma treatment. More details concerning plasma modification of polymers can be found in our previous paper.10 The literature review, supported by 70 references, reveals the growing interest in surface modification performed by means of a plasma action. It is worth mentioning that so far it has been possible to predict the character of surfaces only.11 One could determine whether the obtained surface is neutral, basic, acidic, or even amphoteric. The suggested criterion of such evaluation was focused on (1) Yasuda, H. Plasma Polymerization; Academic Press: New York, 1985. (2) Piglowski, J.; Gancarz, I.; Staniszewska-Kus, J.; Paluch, D.; Szymanowicz, M.; Konieczny, A. Biomaterials 1994, 15, 909. (3) Yasuda, H.; Gazicki, M. Biomaterials 1982, 3, 68. (4) Higuhi, A.; Kogo, H.; Nakagawa, T. J. Appl. Polym. Sci. 1992, 46, 449. (5) Bryjak, M.; Trochimczuk, W. Angew. Makroml. Chem. 1993, 207, 111. (6) Bryjak, M.; Trochimczuk, W. Angew. Makromol. Chem. 1993, 208, 173. (7) Bryjak, M.; Gancarz, I. Angew. Makromol. Chem. 1994, 219, 117. (8) Poz´niak, G.; Bryjak, M.; Trochimczuk, W. Angew. Makromol. Chem. 1995, 233, 23. (9) Bryjak, M.; Pozniak, G.; Trochimczuk, W. Angew. Makromol. Chem. 1992, 200, 93. (10) Gancarz, I.; Pozniak, G.; Bryjak, M. Eur. Polym. J., in press. (11) Shahidzadeh-Ahmadi, N.; Arefi-Khonsari, F.; Amouroux, J. J. Mater. Chem. 1995, 5, 229.
changes of work of adhesion calculated for various aqueous solutions of HCl or NaOH. When work of adhesion is not pH dependent, the surface is considered nonionic. When the pH value of a solution rises and work of adhesion increases, the surface has acidic character. When work of adhesion drops, there is evidence of its basic character. The decrease of work followed by its gradual increase shows the amphoteric nature of the surface. Sometimes, such characterization of the modified surface does not satisfy the researcher who wants to know the progress of the modification. We believe that a suitable tool is now available. The technique presented may be considered as the first attempt to launch such a method. Theory Fowkes, in his last scientific paper,12 presented a simple method for the evaluation of surface basicity. The calculations are based on a dynamically developed interpretation of surface properties using acid-base (AB) interactions.13-17 A small addition of an acidic component to a van der Waals liquid is reflected in a change in the Gibbs surface excess (Γ2). This may be correlated to a change in the interphase solid-liquid tension (γSL) and finally to a change in contact angle (Θ) by means of the YoungLaplace equation. Fowkes’ restriction of the method regarded the selection of the acidic component’s concentration. The solutions should have the same surface tension as the solvent (γL). As a result, diiodomethane was taken as the solvent and phenol as the solute. The phenol concentration was changed to 10 mM, and the surface tension of solution was still unchanged. (12) Fowkes, F. M.; Kaczinski, M. B.; Dwight, D. W. Langmuir 1991, 7, 2464. (13) Fowkes, F. M. J. Adhesion Sci. Technol. 1990, 8, 669. (14) van Oss, C. J.; Good, R. J.; Chaudhury, M. K. Langmuir 1988, 4, 884. (15) Vrbanac, M. D.; Berg, J. C. In Acid-Base Interactions; Mittal, K. L., Anderson, H. R., Eds.; VSP: Ultrecht, 1991; pp 67-78. (16) Good, R. J.; Spirvasta, N. M.; Islam, M.; Huang, H. T. L.; van Oss, C. J. In Acid-Base Interactions; Mittal, K. L., Anderson, H. R., Eds.; VSP: Ultrecht, 1991; pp 79-89. (17) Whitesides, G. M.; Biebuyck, H. A.; Folkers, J. P.; Prime, K. L. In Acid-Base Interactions; Mittal, K. L., Anderson, H. R., EDs.; VSP: Ultrecht, 1991; pp 229-241.
10.1021/la980628b CCC: $18.00 © 1999 American Chemical Society Published on Web 08/17/1999
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For the above restrictions Fowkes et al.12 obtained
Γ2 )
δ(λL cos Θ) 1 δ(λL cos Θ) 1 ) RT δ ln c2 2.303RT δ log c2
(1)
The Langmuir linearization then allowed them to calculate the surface concentration of the acidic or basic functionality (Γm):
c2 1 1 ) + c Γ2 ΓmKeq Γm 2
(2)
where Keq is the equilibrium constant for adsorption. The third-order polynomial function used to describe the γL cos Θ vs log c2 correlation seems to be the best compromise. Taking this procedure as the method for “surface titration”, we have tried to adapt it to the estimation surface of concentrations of acidic or basic functionalities created during plasma treatment. However, several assumptions have to be made. The first of them states that when any of the functional groups on a surface becomes ionizable, the surface free energy alters and the measured contact angle (or much better cos Θ) changes. The second assumption deals with a liquid used in the contact angle measurements. In the case of Fowkes’ paper,12 diiodomethane was chosen. Here, in this paper, we use water as the solvent. One may note that the surface tension of aqueous solutions of HCl or NaOH or their dispersive and polar components remains constant for various concentrations.11 This phenomenon holds for solutions of pH ranging from 1 to 14, according to Huttinger.18 Hence, it is not difficult to draw the following conclusion: the dispersive component of surface tension is not pH-dependent. The pH values of solutions express only concentrations of hydronium or hydroxide ions. Thus, when changes in pH values affect surface energetics,11 they do so through the polar component. Simultaneously, Huttinger’s observation18 holds the surface tension constant. In the context of van Oss’s theory,16 the polar component of an aqueous solution equals
) 2(λxi λXi )1/2 λAB i
(3)
where superscripts x and X denote the Bro¨nsted acid and base components of the surface free energy for the ith solution, respectively. The condition of a constant γAB value means that any efforts to increase the acidic component must be compensated by a decrease in the basic component. Thus, it is possible to use a large number of test liquids: aqueous solutions of HCl or NaOH. For these liquids, numerical values for dispersive and polar components remain constant but the character of the AB component is pH-dependent. The third assumption is also connected with the selection of the testing liquids. Water molecules can interact with surface groups, forming ions or Hbonding. They may also screen the existing functionality on the interphase and limit the proton-transfer reactions (the reader who wants more details should refer to ref 17). Such a situation appears in each real system when a polymer surface is exposed to contact with water. In agreement to Whitesides et al.,17 we wish to note that the surface acidity (or basicity) of a solid can be easily characterized by the properties of a solution in contact with the solid. It is convenient to find the pH value of a solution for which the interphase moieties are half(18) Huttinger, K. J.; Hohmann-Wien, S.; Kreker, G. J. Adhes. Sci. Technol. 1992, 6, 317.
protonated. For simplicity, the mechanism of surface moiety protonation is not described in detail. It is much better to use a term “apparent interphase acidity” or “apparent interphase basicity” to express tendency of proton transfer between interphase groups and bulk solution. In this way, every interphase moiety is considered in our evaluation. The fourth assumption concerns the curve shape of a contact angle titration. The solution pH value at which surface functionalities are half-protonated (usually the inflection point of the titration curve) is defined by Whitesides’ pK1/2 value.17 Taking the latter as representative for a particular interphase-water system one may find the equivalent surface excess (Γ1/2) as an apparent measure of the surface concentration of ionizable functionalities. The region of pH change should be also defined. When a titration jump appears in the region 1 < pH < 7 it is convenient to perform calculations on the pH scale. When a jump appears at pH > 7, it is suggested to use the pOH scale. In this way, calculations are performed for solutions with the excess H+ or OH- ions, respectively. The evaluation presented below, verified by Fowkes’ calculations,11 should be considered as a rough approach for the estimation of interphase acidity and/or basicity. Materials and Methods Pellets of the polysulfone (Udel P-1700, Union Carbide) were dissolved in dimethylformamide (20 wt %), and the solution was cast on a glass plate. The solvent was evaporated in the absence of moisture, and the polymer was vacuum-dried at 50 °C for 24 h. Obtained polymer films were kept in a desiccator. Plasma modifications in atmospheres of nitrogen, carbon dioxide, and butylamine vapors were performed in a microwave reactor chamber, at a distance of 190 mm from the edge of the plasma. Plasmas were sustained by a 600 W, 2.45 GHz generator, with a 125 Hz pulse frequency and 25% duty cycle. N2 and CO2 gases were pumped over the film. The flow rate was set at 20 cm3/min and the pressure at 0.4 mbar. Butylamine vapors (liquid amine placed directly in the reactor) were used in a mixture with argon (flow rate 10 cm3/min) under a total pressure of 0.8 mbar. Contact angle measurements were performed within 2-4 h after the plasma action. Contact angle measurements were performed by means of a TM 50 system (Technicome) in an environmental control chamber (room temperature and saturated vapor). Prior to the measurements, samples were kept in the chamber for 30 min to allow the vapor to be adsorbed. Solution droplets of about 5 µL volume were placed on the polymer surface. Each contact angle reading was averaged from at least 20 independent measurements. The pH value for the solution used was measured directly after contact angle titration. ATR-FTIR spectra were collected on a Perkin-Elmer system 2000 spectrometer equipped with an ATR device (Ge, 45°). The spectra were taken with 256 scans and 4 cm-1 resolution.
Results and Discussion Taking into account the general aim of our works plasma modification of ultrafiltration membranes10s polysulfone was used as received. Polymer test specimens were obtained from the same solutions as ultafiltration membranes. In that way, any phenomena that appear on the membrane surface should take place also on the surface of the test specimens. Some commercial polymers may contain small amounts of impurities or processing additives, which, transferred to the interface region, might affect the plasma modification treatment. However, it should be emphasized here that this phenomenon takes place also on the surface of ultrafiltration membranes. Evaluation of their surface character allowed us to find the relationship between plasma action and membranes’ performances.
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Figure 1. Effect of solution pH on γL cos Θ for a polysulfone film.
Figure 2. Effect of solution pH on γL cos Θ for a polysulfone film modified in CO2 plasma.
Surface characterization of modified specimens by calculating free energy (harmonic averaging for several liquids) revealed10 alteration of surface properties during the first 2 min of the plasma action. Then interphase tension and both components reached a plateau. For this reason, a 6-min plasma treatment was selected for the test sample modifications. Equation 1 may be easily modified to the form
Γ2 ) -
δ(λL cos Θ) 1 2.303RT δ(pH)
(4)
When the γL cos Θ vs pH dependence is fitted by a polynomial of the third-order, one obtains a convenient tool for further calculation. Zeroing the second derivative of the fitted function allows us to find the inflection point. Then the value of Γ2 (named herein Γ1/2) can be calculated by means of eq 4. The obtained value expresses surface concentration of H+ ions in solution when the polymer surface functionalities are half-protonated. The concentrations of apparent basic sites should be in an equimolar balance. The calculation of the surface excess of OH- ions gives the measure of the apparent surface concentration of acidic functionalities. The following sections present the properties of plasma-modified polysulfone films. The first, unmodified polysulfone, was tested in the presence of ionizable groups. As one sees from Figure 1, there is no evidence for such functionalities when a film is tested in aqueous solutions of pH ranging from 1 to 12. The reason for an increase in surface wettability at pH 13.0 is not clear and will not be considered in this paper. Hence, any alteration of surface energy that appears at pH < 12 should come from functionalities created during the plasma treatment. Experimental relationships between γL cos Θ and pH for polysulfone modified with carbon dioxide, n-butylamine, and nitrogen plasmas are shown in Figures 2-4, respectively. The titration jump shows the creation of ionizable functionalities during plasma action: acidic for polysulfone modified in carbon dioxide, basic for butylamine, and amphoteric for nitrogen. Whereas the two first cases are quite predictable, the last one needs further explanation. It is possible that some traces of oxygen are present in the plasma system. They can originate from used gas and are created during plasma action (ablation of polymer). Some postreactions of surface radicals and
Figure 3. Effect of solution pH on γL cos Θ for a polysulfone film modified in BuNH2 plasma.
Figure 4. Effect of solution pH on γL cos Θ for a polysulfone film modified in N2 plasma.
atmospheric oxygen are plausible also. Finally, the surface of N2-plasma-treated polysulfone contains acidic and basic functionalities.
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Table 1. Polynomial Fittings of Experimental Dataa plasma
>7 7 7
