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Hydrophilic (Hydrogen-Bonding) Polystyrene Surface by Substrate-Induced Surface Segregation of Benzene Groups Oleg N. Tretinnikov† B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 70 Prospekt F. Skariny, Minsk 220072, Belarus Received December 22, 1998. In Final Form: November 24, 1999 The measurement of contact angle on polystyrene (PS) films solvent cast on acid-cleaned glass substrates was used in conjunction with the Lifshitz-van der Waals/acid-base approach in order to investigate the surface functional-group compositions as well as the physical nature and the strength of intermolecular interactions between the film surfaces and various testing liquids. The static contact angle for water observed on the air-side surface of the films (80°) was consistent with the literature. However, the glassside surface of the films was found to be hydrophilic with a water contact angle of 62°. This hydrophilic character was shown to arise from (i) the substrate-induced surface segregation of benzene groups in the film casting process with consequent formation of benzene-rich PS surface and (ii) the relatively strong hydrogen-bond-accepting (electron-donating) character of the aromatic rings of polymer. The strength of hydrogen bond between the water molecule and the aromatic moiety was estimated to be g10.6 kJ/mol.
Introduction The surface segregation of functional groups is a process which occurs in multifunctional polymers as a result of the surface’s drive to attain its lowest energy state by maximizing the surface concentration of polar or nonpolar functional groups depending on the polarity of contacting phase.1-6 This process (also known as surface reorientation, surface restructuring, or surface functional-group accumulation) has recently been shown to play an important role in various phenomena occurring at polymer surfaces and interfaces, including contact angle hysteresis,7 tacticity-dependent wettability,8 and phase separation in architecturally asymmetric blends.9 Besides, this process may have important practical implications. For instance, simple casting of a polymer melt or solution against a substrate of given polarity can, in principle, yield a polymer surface which is dominated by the specific functional groups and, as the polymer solidifies, permanently retains them in various contacting media.8,10 This paper demonstrates that polystyrene (PS)sa polymer which has been commonly regarded as a hydrophobic material11-14sdisplays hydrophilic surface characteristics,15 when solution-cast against a highly polar glass substrate. This hydrophilic character is shown to arise from (i) the substrate-induced surface segregation of benzene groups with consequent formation of benzene†
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(1) Pchelin, V. A.; Korotkina, I. I. Zh. Fiz. Khim. 1938, 12, 50-58. (2) Reardon, J. P.; Zisman, W. A. Macromolecules 1974, 7, 920-923. (3) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315326. (4) Carre, A.; Gamet, D.; Schultz, J.; Schreiber, H. P. J. Macromol. Sci.sChem. 1986, A23, 1-18. (5) Andrade, J. D., Ed. Polymer Surface Dynamics; Plenum Press: New York, 1988. (6) Tretinnikov, O. N. J. Adhes. Sci. Technol. 1999, 13, 1085-1102. (7) Tretinnikov, O. N.; Ikada, Y. Langmuir 1994, 10, 1606-1614. (8) Tretinnikov, O. N. Langmuir 1997, 13, 2988-2992. (9) Tretinnikov, O. N.; Ohta, K. Langmuir 1998, 14, 915-920. (10) Lai, Y. C.; Friends, G. D. J. Biomed. Mater. Res. 1997, 35, 349356. (11) Ellison, A. H.; Zisman, W. A. J. Phys. Chem. 1954, 58, 503-506. (12) Wu, S. J. Polym. Sci. 1971, C34, 19-30. (13) Qin, X.; Chang, W. V. J. Adhes. Sci. Technol. 1995, 9, 823-841. (14) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982.
rich PS surface and (ii) the relatively strong hydrogenbonding character of the aromatic moiety. Experimental Section Polymer Film Preparation. Commercially available, predominantly syndiotactic PS (tacticity, 73% rr, 15% rm, and 12% mm triads, Mn ) 2.4 × 105) was used in this study. The polymer was first purified by precipitation and then dissolved in toluene at a concentration of 2 mass %. Glass substrates (Pyrex glass Petri dishes) were soaked in a chromic acid cleaning solution for 2-3 h, washed in running water for at least 15 min, thoroughly rinsed with double-distilled water, and dried in a clean oven at 120 °C for 1 h. The high polarity of the substrates thus prepared was confirmed by measuring the water contact angle, which was found to be 5-10° and to be consistent with the literature.7,14 In one case, the substrate was subsequently soaked in 1 N NaOH overnight, wiped with a filter paper, and dried. The polymer films with a thickness in the range 15-20 µm16 were cast on the freshly prepared substrates by slowly evaporating the solvent at 25 °C in a dust-free environment. For the main part of this study, the residual solvent was removed by drying films on the substrates at 75 °C for 10 h in a clean oven. The films were then scored around the edges and floated onto the surface of doubledistilled water, from which they were picked up, blotted with filter paper and stored over desiccant until required. Some of the as-cast films were first separated from the substrates and then dried in the free-standing form. Contact Angle Measurements. The liquids used for contact angle measurements were glycerol, s-tetrabromoethane, and diiodomethanesall of purity higher than 99%sand doubledistilled water. Static contact angles17 of PS films against these liquids were measured by the sessile drop method using a Rame´Hart model A-100 contact angle goniometer at 22 ( 2 °C and about 65% relative humidity. These were determined 20-40 s after application of the liquid drop, except for diiodomethane (15) We use the quantitative definition of terms “hydrophobic” and “hydrophilic” proposed recently by Vogler (Vogler, E. A. Adv. Colloid Interface Sci. 1998, 74, 69-117) on the basis of a rigorous analysis of the structure and hydrogen-bonding interactions of water at solid surfaces. The surfaces exhibiting a water contact angle θ > 65° are defined as hydrophobic surfaces and those having θ < 65° as hydrophilic ones. (16) The film thickness measurements were made using a profilometer with a sensitivity of (0.5 µm. (17) A static contact angle is defined here as the contact angle measured shortly after a drop of test liquid has stopped self-spreading along the solid surface and reached its equilibrium shape.
10.1021/la981749e CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000
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(see below). The drops (3 µL) were applied with a Gilmont Microliter syringe. They had spherical shape, and their contact angles did not change significantly in 2 min for water, glycerol, and s-tetrabromoethane. In the case of diiodomethane, the contact angle rapidly decreased because the liquid reacted with the polymer soon after the liquid/solid contact was established. Therefore, the contact angle reported for diiodomethane was measured immediately after application of the drop. All reported values are the average of at least six measurements taken at different locations of the film surface. Standard deviation was typically about (1°. X-ray Photoelectron Spectroscopic (XPS) Measurements. The XPS spectra were obtained with an ES-2401 instrument utilizing monochromic Mg KR radiation. Samples of PS films were analyzed at ambient temperature in ∼10-6 Pa vacuum. The spectra were recorded using a pass energy of 50 eV. The photoelectron escape angle was 45°. Determination of Surface Energy. The surface energy was determined for the air-side and glass-side surfaces of the PS films from the contact angle data using the Lifshitz-van der Waals/acid-base (LW/AB) approach developed by van Oss, Chaudhury, and Good.18-20 This method yields the solid surface energy (γS) as the sum of the two components, γSLW and γSAB (i.e., γS ) γSLW + γSAB), associated with electrodynamic Lifshitz-van der Waals (LW) interactions and (Lewis) acid-base interactions (AB), respectively. The component γSAB is a combination of the electron-acceptor (γS+) and the electron-donor (γS-) parameter of the surface energy: γSAB ) 2(γS+γS-)1/2. The relationship between the work of adhesion between liquids and solids, WSL, the equilibrium contact angle, θ, and the surface tension components of liquids and solids is given by the modified YoungDupre´ equation
WSL ) γL(1 + cos θ) ) 2xγSLWγLLW + 2xγS+γL- + 2xγS-γL+ (1) where addition of the subscript “L” denotes the surface energy components and parameters of the test liquids. Thus, from contact angle measurements with three different liquids of known surface energy components and parameters, the solid surface energy can be determined by solving a system of three equations in the form of eq 1 with respect to the surface energy components and parameters of the solid. If one of the acid-base parameters of the solid surface energy is negligibly small, and the other parameter is rather large, the surface is called monopolar. For a monopolar surface possessing, for example, basic properties (γS- . 0 and γS+ ≈ 0), eq 1 becomes
WSL ) γL(1 + cos θ) ) 2xγSLWγLLW + 2xγS-γL+
(2)
The two unknown surface properties of a monopolar basic surface, γSLW and γS-, can be determined from a set of two equations in the form of eq 2. Since a hydrogen bond is a type of acid-base bond, the above general treatment of interfacial acid-base interactions holds for the interfacial hydrogen-bonding interactions.21 It should be noted that the LW/AB approach has some limitations, which must be borne in mind to avoid erroneous results and conclusions when the method is applied in practice. The calculation results depend on the choice of the test liquids. Hollander22 and Della Volpe et al.23,24 showed that this effect is caused partly by mathematical instabilities of the LW/AB model. (18) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927-941. (19) van Oss, C. J.; Good, R. J. J. Macromol. Sci.sChem. 1989, 26, 1183-1203. (20) van Oss, C. J. In Polymer Surfaces and Interfaces II; Feast, W. J., Munro, H. S., Richards, R. W., Eds.; Wiley: New York, 1993; pp 267-290. (21) Lee, L.-H. Langmuir 1996, 12, 1681-1687. (22) Hollander, A. J. Colloid Interface Sci. 1995, 169, 493-496. (23) Della Volpe, C.; Siboni, S. J. Colloid Interface Sci. 1997, 195, 121-136. (24) Della Volpe, C.; Deimichei, A.; Ricco, T. J. Adhes. Sci. Technol. 1998, 12, 1141-1180.
To overcome this problem, Della Volpe et al. proposed a multiluquid approach in which the surface characteristics of a solid are derived from contact angle data for a number of test liquids using the least-squares method. On the other hand, Hollander’s study implied that it is equally effective to use only three test liquids, including an apolar liquid and a pair of polar liquids, provided the difference of the γL-/γL+ ratios of the polar liquids is large. Since the latter requirement is best fulfilled for the pair water-glycerol, these liquids employed at the same time (together with an apolar liquid) should yield the most consistent results. Indeed, recent publications strongly suggest that using only water, glycerol, and diiodomethane (a virtually apolar liquid) yields nearly the same results as those derived from the multiliquid method.25,26 It is not yet possible to establish the ratio of acidic and base components of liquid surface tension. This necessitates introducing an arbitrary γL-/γL+ value for one liquid as a reference. The arbitrary scale of acid-base strength (usually based on the ratio 1:1 for water) does not allow the acidic and basic components of a given surface to be compared directly. It allows, however, one to compare the basic or acidic components of different surfaces.23,24 Furthermore, the γL-/γL+ ratio does not impinge on the calculated values of the acid-base components of the surface energy and work of adhesion.20,21 Determination of the Strength of Hydrogen Bond. According to Fowkes et al.,27,28 the hydrogen-bonding contribution to the work of adhesion can be correlated to the molar enthalpy of the interfacial hydrogen-bonding interaction using the formula
WSLHB ) -fnHB∆HHB
(3)
where f is a constant for converting enthalpies to free energies and nHB is the number of moles of interacting functional groups per unit area on the solid surface. In the Lifshitz-van der Waals/ acid-base approach, the hydrogen-bonding contribution to the work of adhesion is given by the expression18,20
WSLHB ) 2(xγS+γL- + xγS-γL+)
(4)
Combining eqs 3 and 4 yields the following expression for the enthalpy of the hydrogen-bonding interaction between a monopolar basic (hydrogen-bond-accepting) solid surface and a liquid:
-∆HHB ) 2xγS-γL+/fnHB
(5)
Equation 5 holds for PS because the latter is a monopolar basic polymer.27,28
Results and Discussion Contact Angles. The surface tension components and parameters of liquids used for contact angle determination are listed in Table 1. Also included in Table 1 are the static contact angles observed with these liquids on the PS films solvent-cast against acid-cleaned glass substrates. θair and θglass refer to the contact angles on the polymer surfaces contacted with air and glass substrate in the film casting process, respectively. As is seen, for each testing liquid, the contact angle on the air-side surface was always higher than that on the glass-side surface. Furthermore, the difference between θair and θglass (∆θ) varied with the wetting liquid; that is, ∆θ increased from 5° for diiodomethane and tetrabromoethane to 8° for glycerol and reached a markedly high value of 18° for water. Another feature of the wettability of PS by water (25) Wu, W.; Giese, R. F., Jr.; van Oss, C. J. Langmuir 1995, 11, 379-382. (26) Michalski, M. C.; Hardy, J.; Saramago, B. J. V. J. Colloid Interface Sci. 1998, 208, 319-328. (27) Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3-7. (28) Fowkes, F. M.; Kaczinski, M. B.; Dwight, D. W. Langmuir 1991, 7, 2464-2470.
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Table 1. Surface Energy Components and Parameters29 (in mJ/m2) and Static Contact Angles (in deg) for the Test Liquids on the Air-Side (θair) and Glass-Side Surfaces (θglass) of PS Films liquid
γL
water (W) glycerol (GL) diiodomethane (DM) 1,1,2,2-tetrabromoethane (TBE)
72.8 64.0 50.8 49.7
γLLW
γL+
γL-
θair θglass ∆θ
21.8 25.5 25.5 80 34.0 3.9 57.4 71 50.8 0.70a 0 33c 49.7 0.37b 0 28
62 63 28c 25
18 8 5 5
a From ref 30. b Our data (ref 31). c The solid surface was attacked by the liquid.
Table 2. Effect of Substrate Treatment and Film Heating on Water Contact Angles (in deg) of PS Films system
θair
θglass
∆θ
film cast on acid-cleaned glass substrate film cast on acid-cleaned glass substrate; then heated at 75 °C on the substrate film cast on acid-cleaned glass substrate; then separated from the substrate and heated at 75 °C film cast on acid-cleaned + NaOH-treated glass substrate
80 80
61 62
19 18
Figure 1. XPS C1s and O1s spectra for (a) the air-side and (b) the glass-side surface of a PS film.
81
63
18
81
77
4
appeared smooth and showed no difference in the surface morphology when they were examined under a light microscope (1000× magnification). Furthermore, in our earlier studies, the PS films appeared totally amorphous in IR transmission measurements as well as in the IRATR spectra measured at the air-side and the glass-side surfaces of the films.37,38 Accordingly, surface roughness and surface crystallinity could not be responsible for the observed difference between the contact angles on the opposite film surfaces. The surface chemistry of PS films was studied with X-ray photoelectron spectroscopy (XPS). Figure 1 shows the XPS C1s and O1s spectra for the airside and the substrate-side surface of a PS film cast on chromic acid cleaned glass and subsequently annealed in air at 75 °C for 10 h. A low-intensity O1s signal observed for the both surfaces is indicative of slight oxidation of the polymer. The oxygen peaks correspond to atomic percentages of 1.5 and 2.0% for the air side and the glass side of the film, respectively. Obviously, this difference in the content of oxygen between the opposite film surfaces is too low to cause an observable difference in their wetting behavior. Moreover, it seems very unlikely that this trace amount of incorporated oxygen would have an appreciable effect on the wettability of PS. We conclude therefore that the only factor responsible for the observed low-wettability/high-wettability character on opposite surfaces of the PS films is the difference in the surface functional-group composition. In PS, there are two types of functionalities present: the methylene groups (-CH2-) forming the polymer backbone, and the benzene rings (-C6H5) existing as substitutes on the principal chain. The methylene group is apolar, whereas the benzene ring is polar. Accordingly, in the film casting process, PS tends to construct a -CH2- surface at the polymer-air interface and a -C6H5 surface at the polymer-glass interface, in a drive to minimize the interfacial free energy. As the solvent evaporates the difference in functionalgroup surface composition is frozen in the film formed, with consequent higher wettability of the glass-facing surface as compared to that of the air-facing surface. Thus, the marked difference between θair and θglass (∆θ ) 18°) of PS films cast against acid-cleaned glass is readily explained. However, the fact that the contact angle difference vanishes in casting the polymer on the glass treated with
was that the glass-side surface appeared rather hydrophilic, displaying a contact angle of 62°. Such a low value for the water contact angle on this polymer had not been previously reported. In contrast, the contact angle result for water on the air-side surface (80°) was in good agreement with the literature data.32 Table 2 shows the water contact angles observed on the as-cast PS film as well as on the films subsequently annealed in air at 75 °C for 10 h on the glass substrate or in the free-standing form. As can be seen, all three films display the contrasting hydrophobic-hydrophilic character on the opposite surfaces and have practically the same values of θair (80-81°) and θglass (61-63°). The invariability of θglass indicates that the hydrophilic character imposed on the as-cast surface by the acid-cleaned glass was not affected by heating, independently of whether the polymer film was still adhering to the substrate or separated from it. Also shown in Table 2 are the contact angle results for a PS film cast on a glass substrate which, in addition to cleaning with chromic acid, was subsequently treated with 1 N NaOH. As can be seen, the value of θglass (77°) for this film is only slightly reduced from that of θair (81°), indicating that the polymer-substrate interaction causing the surface hydrophilicity of polymer in casting against the acid-cleaned glass was precluded when the NaOHtreated glass was used instead. Interpretation of Wettability Data. The wettability change of a multifunctional polymer surface brought about by changing the polarity of adjacent phase in the surface formation process is known to be associated with the change in surface functional-group composition when no other disturbing factors are present.2-5,8,33 These factors can be surface roughness,34 surface crystallinity,35 or surface oxidation.36 The PS surfaces under investigation (29) Good, R. J. J. Adhes. Sci. Technol. 1992, 6, 1269-1302. (30) Hirasaki, G. J. J. Adhes. Sci. Technol. 1993, 7, 285-322. (31) Tretinnikov, O. N. Macromolecules, submitted for publication, 1999. (32) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302-320. (33) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces, from Physics to Technology; Wiley & Sons: Chichester, England, 1994. (34) Johnson, R. E., Jr.; Dettre, R. H. J. Phys. Chem. 1964, 68, 1744. (35) Schonhorn, H.; Ryan, F. W. J. Phys. Chem. 1966, 70, 3811. (36) Dwight, D. W.; Riggs, W. M. J. Colloid Interface Sci. 1974, 47, 650.
(37) Tretinnikov, O. N.; Zhbankov, R. G. Polym. Sci. USSR (Engl. Transl.) 1990, 32, 2176-2185; Vysokomol. Soedin. 1990, A32, 22732282. (38) Zhbankov, R. G.; Tretinnikov, O. N. Int. Polym. Sci. Technol. 1984, 11, 81-83; Vysokomol. Soedin. 1984, B26, 146-149.
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NaOH (∆θ ) 4°) seems to be inconsistent with the above mechanism according to which every highly polar substrate should cause the polar groups of polymer to segregate to the substrate surface. We believe that, to explain this inconsistency, one has to take into account the nature of polar interactions between the polymer and substrate. Specifically, due to the monopolar basic character of benzene rings,39,40 their surface segregation could probably be driven only by acid-base interactions with some acidic cites on the substrate surface. Since treatment with inorganic acids is known to introduce acidic silanol groups onto the glass surface,41 it is very likely that the acidic silanol groups of the acid-cleaned glass provided sites for the strong attachment and, hence, surface segregation of benzene groups. On the other hand, treatment with NaOH could reduce the substrate acidity and, therefore, diminish the attraction and (consequently) segregation of benzene groups to the polar substrate. The basic (hydrogen-bond-accepting) character of benzene ring accounts also for the fact that the amplitude of wettability difference between the opposite film surfaces (∆θ) varies with the wetting liquid (Table 1). The effect of the formation of interfacial hydrogen bonds is to cause an increase in wettability (a decrease in contact angle). Obviously, this effect must be stronger at the benzenerich glass-side surface as compared to the air-side surface dominated by non-hydrogen-bonding methylene groups. The resultant difference in wettability between the two surfaces must increase with the strength of the specific interfacial interactions and, consequently, with the hydrogen-bonding ability of the contacting liquid. As is evident from the wettability data (Table 1), this behavior was indeed observed; i.e., ∆θ was found to increase in the following order: virtually non-hydrogen-bonding liquids (tetrabromoethane and diiodomethane), moderately hydrogen-bonding liquid (glycerol), and strongly hydrogenbonding liquid (water). Surface Energy from LW/AB Method. First, we test the capability of the LW/AB approach to adequately describe the wettability of PS. Since the only possible source of PS polarity is the benzene ring and benzene is a monopolar electron-donating molecule,39,40 eq 2 should be applicable to the PS surfaces. Furthermore, since the LW component of the surface free energy of a nonpolar or monopolar material is equal to the total surface free energy and the latter for benzene is close to that of a pure methylene surface (28.9 and 30 mJ/m2, respectively),42,43 the γLW component of PS surface should be invariable (within (0.5 mJ/m2) to the fractional surface coverage of benzene and methylene groups. Therefore, the following relation should apply to the air-side and glass-side PS surfaces
γS,airLW ) γS,glassLW
(6)
By applying eq 2 to the air-side and the glass-side surfaces and taking into account eq 6, one can write the difference in the work of adhesion for a probe liquid between the (39) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A., III; Blake, G. A. Science 1992, 257, 942-945. (40) Gotch, A. J.; Zwier, T. S. J. Chem. Phys. 1992, 96, 3388-3401. (41) Fowkes, F. M.; Tischler, D. O.; Wolfe, J. A.; Lannigan, L. A.; Ademu-John, C. M.; Halliwell, M. J. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 547-566. (42) Fowkes, F. M. Ind. Eng. Chem. 1964, 56 (12), 40-52. (43) Zisman, W. A. Adv. Chem. Ser. 1964, No. 43, 1-51.
Figure 2. Difference in the work of adhesion of four wetting liquids with the opposite surfaces of PS films, WSLglass - WSLair ) γL(cos θglass - cos θair), plotted vs (γL+)1/2 (data from Table 1).
opposite surfaces of PS film as
WSLglass - WSLair ) γL(cos θglass - cos θair) ) 2(xγS,glass- - xγS,air-)xγL+ (7) Thus, if the LW/AB approach describes adequately the wetting of this polymer, the results of contact angle measurements plotted as γL(cos θglass - cos θair) vs (γL+)1/2 should yield a straight line going through the origin. In Figure 2, the values of γL(cos θglass - cos θair) are plotted vs the corresponding values of (γL+)1/2 for the four wetting liquids employed here. It can be seen that a straight line going through the origin fits the data very well. Having established that the LW/AB method adequately describes the wettability behavior of PS surfaces, we are in position to apply this method to determine the surface free energy of the film surfaces under investigation. The values of the total surface energy and its LW and AB components and parameters obtained for the air-side and glass-side surfaces of PS films cast against acid-cleaned glass are summarized in Table 3. The calculations were performed first by the use of the “three-liquid” method (eq 1) with the water-glycerol-tetrabromoethane trio. As can be seen from Table 3, the value of electron-acceptor parameter, γS+, was found to be zero for both the air-side and the glass-side surfaces. This result is in full accordance with the fact that this polymer does not contain any electron-accepting moieties. The values of electron-donor parameter (γS,air- ) 6.5 and γS,glass- ) 22.0 mJ/m2) reveal an appreciable basicity of the surfaces, which is due solely to the basic character of aromatic groups. Furthermore, a 3-fold increase in the value of γS- by changing the contacting medium from air to glass indicates clearly that the environmentally driven segregation of the functional groups did actually take place in the film casting process. To validate the results of the “three-liquid” method, additional analyses were performed by the use of the “twoliquid” method, i.e., by solving a system of two equations in the form of eq 2 for the two unknown properties (γSLW and γS-) of the basic monopolar surfaces under investigation. The three test liquids employed in the “three-liquid” method gave three possible liquid pairs that could be used in the “two-liquid” method. The results obtained in this way were essentially independent of the liquid pair used and consistent with the results of the “three-liquid” method. The average over the four γS- values was 6.5 (standard deviation (0.2) and 23.1 (( 1.0) mJ/m2 for the air-side and the glass-side surface, respectively.
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Langmuir, Vol. 16, No. 6, 2000 2755
Table 3. Surface Energy Components and Parameters (in mJ/m2) of the Air-Side and Glass-Side Surfaces of PS Films, on the Basis of the Contact Angles (in deg) for Water, Glycerol, and Tetrabromoethane in Table 1 liquids
γS+
γS-
γSAB
γSLW
γS
air side
W-GL-TBE W-GL W-TBE GL-TBE mean (SD)
0.00 0 0 0
6.5 6.5 6.3 6.8 6.5 ((0.2)
0.00 0 0 0
41.1 41.2 41.2 41.1 41.15 ((0.06)
41.1 41.2 41.2 41.1 41.15 ((0.06)
glass side
W-GL-TBE W-GL W-TBE GL-TBE mean (SD)
0.00 0 0 0
22.0 24.2 22.5 23.6 23.1 ((1.0)
0.00 0 0 0
40.5 38.0 40.6 40.3 39.9 ((1.2)
40.5 38.0 40.6 40.3 39.9 ((1.2)
film surface
The knowledge of the electron-donor parameter of the PS surfaces allows us to estimate quantitatively the strength of the benzene ring-water hydrogen bond by the use of eq 5. Taking 0.6 nm as a mean value for the size of PS monomer unit,44,45 the surface concentration of monomer units in PS can be estimated as 4.6 µmol/m2. This gives nHB ) 4.6 µmol/m2, if every monomer unit in the surface region is so oriented that the aromatic group is exposed at the polymer surface, i.e., if the surface fractional coverage of the aromatic group is unity. If we assume that the latter is the case for the glass-side surface of PS film (γs- ) 23.1 mJ/m2), and take (according to Fowkes et al.28) f equal to unity, we obtain from eq 5
-∆HPS-H2OHB )
2x23.1 × 25.5 ) 10.6 kJ/mol 1 × 4.6
Taking into account that the actual value of nHB may be somewhat lower than the assumed 4.6 µmol/m2 (i.e., the polymer surface is not in full cover with the benzene groups) and that f may be lower than unity due to entropic effects at the interface,46,47 the obtained value 10.6 kJ/ (44) Helfand, E.; Tagami, Y. J. Chem. Phys. 1972, 56, 3592-3601. (45) Siqueira, D. F.; Kohler, K.; Stamm, M. Langmuir 1995, 11, 30923096. (46) Vrbanac, M. D.; Berg, J. C. J. Adhes. Sci. Technol. 1990, 4, 255.
mol should be regarded as the lower limit for the strength of hydrogen bond between the benzene group of PS and water. It is worthwhile to note that this result falls within the experimental boundaries (6.8-11.5 kJ/mol) which have recently been established for the energy of the benzene-water hydrogen bond39,40 Furthermore, it reveals that the hydrophilic character of the benzene group of PS is rather strong, e.g., stronger than or comparable to that of the ester group of poly(methyl methacrylate) for which the enthalpy of hydrogen bond with water was reported to be 8.27 kJ/mol.48 In conclusion, when a PS film is cast on acid-treated glass, the benzene groups of polymer segregate at the polymer-glass interface, resulting in the polymer surface, which is enriched in the aromatic moiety and is hydrophilic. This is demonstrated by the low water contact angle (62°) and high electron-donor parameter (23.1 mJ/m2) of the substrate-facing surface as compared to those (80° and 6.5 mJ/m2) of the air-facing surface of the film. The enthalpy of hydrogen bond between the benzene group of PS and the water molecule, estimated as g10.6 kJ/mol, indicates the rather strong hydrophilic character of the aromatic moiety. LA981749E (47) Douillard, J. M. J. Colloid Interface Sci. 1997, 188, 511-515. (48) Chaudhury, M. K. Mater. Sci. Eng. 1996, R16, 97-159.
