Langmuir 1996,11, 3255-3260
3255
Surface Configuration Change Observed for Teflon-PFA on Immersion in Water Hirotsugu Yasuda,*$tTsumuko Okuno,*Yuko Saws,* and Takeshi Yasuda# Department of Chemical Engineering and Center for Surface Science and Plasma Technology, University of Missouri-Columbia, Columbia, Missouri 65211, and Department of Home Economics, Mukogawa Women$ University, Ikebiraki, Nishinomiya 663, Japan Received January 10, 1995. I n Final Form: May 3, 1995@ The oxygen atoms in a dry film of a copolymer of tetrafluoroethylene (97 mol %) and perfluoropropyl vinyl ether (3 mol %) are hardly detectable by ESCA because the atomic percent of oxygen in the copolymer is small and oxygen atoms are not likely to be found in the outer region of a dry surface. When the film is immersed in liquid water, however, a conspicuous 01,peak appears on the ESCA spectrum, and its intensity increases with the immersion time. The advancing and receding contact angles measured by the sessile droplet method deviate from those for a dry film progressively with water immersion time. When the surface tension of water is lowered by adding a small amount (0.01%)of a surfactant (Scourol 400) or ethanol (50 vol %), the surface configuration change decreases according to the decrease in the surface tension of the liquid. The maximum extent of the surface configuration change is correlated to the differential surface tension (the difference between the solid surface tension and the liquid surface tension). The greater the differential surface tension, the larger the extent of surface configuration change that occurs to minimize the interfacial tension.
Introduction According to the concept described by Langmuir in 1938,l the surface properties of a solid are determined by the surface configuration (spatial arrangement of atoms at the interface) rather than the configuration ofmolecules which constitute the greater surface region. In other words, whether a polymer surface is hydrophilic or hydrophobic cannot be predicted by the presence or absence of hydrophilic moieties in the molecules but is determined by whether or not the hydrophilic moieties are located at the interface. In recent years, it has been recognized that the surface of a solid, particularly polymeric solid, is very different from what can be anticipated from the bulkcharacteristics of the same material. This discrepancy has been a focal point of the general phenomena recognized by the terms surface dynamics, surface reconstruction, etc., which deal with the difference of chemical and morphological properties of the polymer surface from the corresponding bulk and their changes due to the change of the surrounding medium .2-14
* Author to whom all communication should be addressed. University of Missouri-Columbia. Mukogawa Women's University. Abstract Dublished in Advance A C S Abstracts. J u l "v 1., 1995. (1) Langmuir, I. Science 1938, 87, 493. (2) Ray, B. R.; Anderson, J. R.; Scholz, J. J. J.Phys. Chem. 1968,62, +
@
"on LLLU. 4
(3) Cuthrell, R. E. J. Appl. Polym. Sci. 1967, 11, 1495. (4) Hollv. F. J.: Refoio. M. F. J . Biomed. Mater. Res. 1975. 9. 315. (5) Basikin, A,'; Niskno, M.; Ter Minassian-Saraga, L. J.'Colloid Interface Sci. 1976, 54, 317. (6) Yasuda, H.; Sharma, A. K.; Yasuda, T. J . Polym. Sci.: Polym. Phys. Ed. 1981, 19, 1285. (7) Gagnon, D. R.; McCarthy, T. J. J . Appl. Polym. Sci. 1984, 29, A225
(8) Lavielle, L.; Schultz, J. J. Colloid Interface Sei. 1985, 106, 438. (9) Ruckenstein, E.; Gourisankar, S.V. J.Colloid InterfaceSci. 1985, 107, 488.
(10)Ikada, Y.; Matsunage, T.; Suzuki, M. Nippon Kugaku Kuishi 1985, 1079.
(11)Taru, Y.; Takaoka, K. Kobunshi Ronbunshu, 1986,43,361. (12)Holmes-Farley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G. M. Lungmuir 1987, 3, 799. (13) Takahara, A,; Higashi, N.; Kunitake, T.; Kajiyama, T. Mucromolecules 1988, 21, 2443. (14)Cross, E. M.; McCarthy, T. J. Macromolecules 1990,23, 3916.
0743-7463/95/2411-3255$09.0010
The surface reconstruction depends on the reference state from which the change takes place, and if one cannot define the reference state, the surface reconstruction cannot be dealt with in a generic sense. This problem was indeed found with the moderately hydrophilic copolymer of ethylenelvinyl alcohol. "he reference state depends on the history of a sample, and the change cannot be reproduced without precise knowledge of the history of a ~ a m p 1 e . l ~ According to the view that a polymeric surface is an ever-changing entity depending on the surrounding medium,16the restructured surface is not necessarily the final one to stay; i.e., restructuring of once restructured surface or multiple repeated restructuring occurs with highly perturbable polymeric surfaces. The change of the surface, therefore, can be expressed by the change of the surface configuration. In our recent studies dealing with the change of the characteristics of the polymeric surfaces due to the change of the surrounding medium from air to liquid water, the appearance or disappearance of a specific atom such as fluorine or oxygen was used to investigate the change of the surface configuration in a semiquantitative manner. In some cases, fluorine-containing moieties were introduced on the surface of the polymers, which do not contain fluorine atoms, by plasma surface treatment, and the amount of fluorine atoms was followed by ESCA.l5-l9In the case of the copolymers of ethylene and vinyl alcohol, the change of the oxygen atom concentration profile in the surface region was followed without the plasma surface tagging pr0~edure.l~ The general trends found in our previous studies could be viewed as follows, which seem to be in accordance with many observations expressed in various terms in the literature. 1. Polymer Surfaces Kept in Dry Condition. When ~~
~
~~
(15) Yasuda, T.; Miyama, M.; Yasuda, H. Langmuir, 1994,10, 583. (16) Yasuda, H.; Charlson, E. J.;Charlson, E. M.;Yasuda, T.; Miyama, M.: Okuno. T. Lunemuir 1991. 7. 2394. i17) Yasuda, T.;cbkuno, T.; Yoahida, K.; Yasuda, H. J. Polym. Sci., Phys. Ed., 1988,26, 1781. (18) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. J.Polym. Sci., Phys. Ed. 1988,26, 2061. (19) Yasuda, T.; Okuno, T.; Miyama, M.;Yasuda, H. Langmuir 1992, 8, 1425.
0 1995 American Chemical Society
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3256 Langmuir, Vol. 11, No. 8, 1995
a polymer is kept dry, all polar groups contained in the polymer molecule tend to move away from the interface, and consequently, the interface is populated with relatively hydrophobic moieties of the polymer molecule. The surface state of a polymer is rich in (relatively)hydrophobic moieties or the portion of the polymer. A. If a surface modification is applied to make the surface hydrophilic (high surface energy), the introduced hydrophilic moieties tend to migrate into the deeper section of the surface state while the surface is kept dry. Such a migration occurs in the direction to minimize the interfacial tension of the polymer-air interface. This process has been recognized as the "decay of the surface modification effect". B. If a surface modification is applied to make the surface more hydrophobic (low surface energy), such as the cases of waterproofing treatments, no significant change in the surface configuration occurs as long as the surface is kept dry, simply because the surface tension of the polymer surface is lowered by the surface modification and, consequently, the interfacialtension of the polymerair interface is already minimized. 2. Immersion of a Dry Polymer in Liquid Water. When a dry polymer film is immersed into liquid water, an opposite process to what is described above takes place. Hydrophilic groups migrate to the interface, and hydrophobic moieties move to the inner part of the surface state of a polymer. The main driving force for these movements can be considered to be the specific(attractiveor repulsive) interaction force between the moieties and water. The greater the interaction is, the greater is the driving force for the surface configuration change. In the presence of both hydrophilic and hydrophobic moieties, the migrations of the moieties occur in the direction to minimize the interfacial tension. Ifthe surface tension of a polymer is low, a greater extent of the surface configuration change is observed. Water immersion will make a polymer surface more hydrophilic than the dry film. A. If a surface modification is applied to make the surface hydrophilic (high surface energy), the change due to water immersion is minimal, and the decay of surface modification is generally not observed. B. If a surface modification is applied to make the surface more hydrophobic (low surface energy), a significant extent of the decay of hydrophobicity is observed because the surface modification created a large difference in surface tension of polymer and liquid water. 3. Emersion or Drying of a Wet Polymer Surface. When a polymer surface which is kept in liquid water is emerged from liquid water and kept under dry conditions, the reverse process to what is described in subsection 2 occurs. This phenomenon is identical to what is described in subsection 1above except that the starting point or the reference point is different. However, when this process is viewed as the recovery process of the decay of hydrophobicity which took place when the sample was immersed in water the first time, there are significant differences in both the range ofchange and the extent of recovery. This discrepancy is due to the fact these processes are different processes driven by different driving forces. The quantitative comparison ofthe rates and the extent of changes are further complicated by additional phenomena such as imbibition of liquid water, diffusion of water molecules, subsequent swelling or plasticizing of the polymer by absorbed water, etc. The swelling, for instance, would greatly enhance the ease of migration of moieties due to the attained high mobility of polymer molecules,while the driving force (specificinteraction with water) might remain the same.
In all cases, the surface configuration change occurs in the direction to minimize the interfacial tension between the polymer and liquid water. Therefore, it is anticipated that the greater the interfacial tension is, the greater is the extent of surface configuration change. The overall change in the surface configuration can be viewed as the product of two major factors, i.e., the chain mobility and the driving force. This is an analogous situation to the diffusive transport of a small molecule in a polymer matrix. The flux J can be given by J = D dcldx, where D is the diffusion constant (related to the mobility of polymer segments) and dddx (concentration gradient) is the driving force. If the presence or absence of liquid water contacting a polymer surface changes the mobility of the polymer segments, the influence of the driving force term may be overwhelmed by the change of the surface state, e.g., swelling. Experimental data pertinent to this situation are being presented elsewhere as a separate paper. On the other hand, if one could find a system in which water does not influence the mobility of the polymer segments, it could be possible to clearly see the role of the interfacial tension for the surface configuration change for such a polymer. In this paper, we present data pertinent to such a case.
Experimental Section Polymer Film. A copolymer of tetrafluoroethylene-perfluoropropyl vinyl ether (neoflon-PFA, Daikin Industry) was used for this study. The films were provided by courtesy of Daikin Industry. The thickness of film was 50 pm. Films were stored in a desiccator with silica gel. For the water immersion study, 1.5-cm x 1.0-cmspecimens were used. The copolymer is similar to teflon-PFA by DuPont. Experimental Procedure. An identical method previously reported16-19was used in this study. Dry films were immersed in water, a water-ethanol (50-50 vol %) mixture, or 0.01% aqueous solution of a surfactant (Scourol400, Kaoh Chemical) at 40 "C for preset periods of time and then freeze-dried immediately in order to remove water molecules from the surface without changingthe surface configurationthat is attained during the immersion process. ESCA analysis and the measurement of the contact angle of water by the sessile droplet method were carried out using the freeze-dried samples. Contact angles were measured in a temperature/humiditycontrolled room (20 "C, 60%) by a contact angle meter (Kyowa Interface Science Co., Ltd.). "he temperature of water used for contact angle measurement was equilibrated at the room temperature before the measurement. The surface tensions of the liquids (at 40 " C ) were measured according to Du Nouy's method by using a Shimazu surface tension meter. A platinum wire ring was cleaned by flame before the measurement. Water for immersion as well as for contact angle measurements was purified by the combination of reverse osmosis and ion exchange (electrical resistivity = 11.1MQ cm).
Results and Discussion The change of the ESCA peaks (FI,, Cis, and 01,)due to the water immersion is shown in Figure 1. A conspicuous 01, peak develops as a function of water immersion time. The appearance of the 01, peak is not seen when an unexposed sample of the same film is immersed in a ethanoywater (50/50 volume ratio) solution as shown in Figure 2. When a small amount of a surfactant is added in water (0.01%Scourol solution),the appearance of the 01, peak is significantly less than in pure water. The ratios of 01JC1, as a function of immersion time are compared in Figure 3. These data indicate that the small amount of oxygen atoms present in the ether linkages is located away from the top layer of the film and is hardly seen by ESCA (90" take-off angle). When a film is immersed in water, however, the oxygen atoms are pulled out to the interface.
Configuration Change for Teflon-PFA in Water FlS
Langmuir, Vol. 11, No. 8, 1995 3257 0 1s
ClS
A
water
immersion time
A
lOmin
no immersion 896
892
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300
290
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542
538
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Binding Energy (eV) Figure 1. Change of ESCA peaks on immersion of a film in water.
FI s
Cls
0 1s
Elhanol/Water (50/50) soh o ;o m in ;l; time
~
1Omin
no immersion 696
692
6ee
300
290
280
542
538
534
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Binding Energy (eV)
Figure 2. Change of ESCA peaks on immersion of a film in ethanollwater (50/50) solution.
This change occurs in a relatively short time and reaches the plateau value in approximately 10 min. The change of surface configuration due to the water immersion is small due to the very small vinyl ether content of the copolymer. However, the change c a n be detected by the measurement of the contact angles. Figures 4 and 5 depict the change of cos 8 and the contact area of a sessile droplet respectively with the volume of the droplet. These data seem to be important to elucidate the cause of surface configuration change due to the change of surrounding medium. A particularly important feature is that the major liquid involved in all cases is water, but with significantly different surface tensions. In order to see the role of interfacial tension in the surfactant
configurationchange, it is worth examining the nature of the interaction taking place at the level of surfacefliquid as well as at the level of watedpolymer molecules. Liquid WaterBurface Interaction vs the Molecular Interaction of Water with Polymer. Small molecules (termed as solvent) interact with polymer molecules in different degrees: some dissolve polymers, some swell polymers, and some are merely sorbed in polymer with no significant alteration of the physical properties of the polymers. The enthalpy term of the interaction can be represented by the Flory-Huggins interaction parameter. When a solid polymer is in contact with a solvent (water in this study),of course,the molecular interaction plays a key role in the liquidsurface interaction. However, the interfacial phenomena generally
3258 Langmuir, Vol. 11, No.8, 1995
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0 Water (40’C) h
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0 0.01% Scourol s o h . (40.C) A Ethanol/Water(50/50) s o h . (40’C)
5’0F 0.0
‘
1
20.0
0.0
TIME
40.0 (min)
Figure 3. Change of ESCA 0&1, ratios as a function of immersion time in water, 0.01% Scourol solution,and ethanoY water (50/50) solution. 0.4 .
Water immersion time Omln: 0 (Adv.) 0 (Res.)
0.3 L
SOmin:A(Adv.) A(i7.c.)
0.2 .
0.0
10.0
20.0
30.0
Volume o f Water Drop (pi)
Figure 4. cos B as a function of the droplet volume for dry and water-immersed (60-min) films. 8: advancing or receding contact angle of water. 250.0
Water immersion time h
N
E E v 0
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librium) sorption of water vapor in a polymer solid is a good measure to judge the degree of hydrophilicity or hydrophobicity of the polymers. A hydrophobic polymer generally shows a very small value ofthe solubility (grams ofwatedgram of solid),which is independent of the activity of water, and a hydrophilic polymer has a large value which also increases with the activity. In this respect, polymers of perfluorocarbons and hydrocarbons are hydrophobic. However, the “hydrophobic”polymer does not mean that liquid water cannot wet the surface or that water cannot be sorbed in a polymer. With hydrophobic polymers, spontaneous wetting of the surface with water does not occur, and water molecules are sorbed by utilizing the free volume of a solid polymer without expanding its size (no swelling occurs). In a practical sense, polymers which have weight percent solubility values of less than 1%can be considered very hydrophobic. Due to an extremely high cohesive energy density of water, the interaction between water and polymer molecules is much greater than those for substances with low cohesive energy density. The solubility coefficients of water in polymers are 2-5 orders of magnitudes greater than those of oxygen, which has a similar molecular size but much smaller cohesive energy density. This is also true for very hydrophobic polymers, but the water sorbed in those polymers does not solvate polymer molecules, and thus, the physical properties of polymers are not significantly influenced by the sorbed water. The physical adsorption of water molecules would undoubtedly occur even with a very hydrophobic polymer, and most solubility values include the contribution of physical adsorption on the surface. When the extent of sorptiodadsorption is large, the contribution of adsorption can be generally neglected. When the extent of sorption/ adsorption is small, such as the case of hydrophobic polymer, the contribution of adsorption can be potentially large. However, conceptional and experimentally verifiable distinction of sorption and adsorption becomes extremely difficult, implying that the contribution of both is very small. The physical adsorption of water on a hydrophobic surface could be interpreted as the hydration of a hydrophobic surface. It is important to recognize, however, that such a change is a part of the change of the surface state, and it is very unlikely that a thin layer of water (identical to that in the bulk phase of liquid water) is present as a hydration layer. Furthermore, the overall influence of the sorbed and adsorbed water on a hydrophobic polymer is much smaller than that for hydrophilic polymers. The term “interaction”or “hydration”is generally used to describe any level of interaction or hydration, and therefore, these terms should not be interpreted in a too literal sense. Interfacial Tension for Polymer Surfaces. In the classical treatment of the interfacial tension, the force balance a t the three-phase contact line (liquid-surfaceair) is established, and the interfacial tension, y s ~is, given by the surface tension of the solid, ys, and the surface tension of the liquid, y ~ as ,
50,0 0.00.0
10.0
20.0
30.0
Volume o f Water Drop ( P I )
Figure 5. Contact area of a sessile droplet as a function of the droplet volume for dry and water-immersed (60-min)films.
cannot be interpreted by virtue of molecular level parameters describing the bulk behavior. The wetting of a polymer surface is characterized by the interfacial tension between the liquid and the surface. The contact angle at the three-phase line is a good measure of the wettability of the surface. However, a high cntact angle of water on a polymer surface does not uniquely relate to the hydrophobicity of the polymer molecules which constitute the surface. A high contact angle merely indicates that the surface configuration of the surface attained in the environment just before a droplet was placed was in such a way that the contact angle is high but does not mean that the polymer molecules which constitute the surface are hydrophobic in nature. A good example of this situation is that a very high (sessile droplet) contact angle is observed with a highly hydrated hydrogel, e.g., of gelatin,22while the polymer is known to be highly hydrophilic. Whether a polymer is hydrophilic or hydrophobic may be judged by the physicochemical phenomena which can be described by the interaction parameter. The (equi-
YSL
= Y s - Y L cos 6
(1)
where e is the contact angle of the liquid at the threephase contact line. In the classical treatment of surface tensions, it is intuitively assumed that the surface tension of a solid, ys, can be assigned as a surface constant. In a practical sense, eq 1is valid if the surface tension of the solid does not change after contact with the liquid is made.
Configuration Change for Teflon-PFA in Water
An empirical method to estimate the surface tension of a solid is Zisman’s plot (cos 8 as a function of y ~to) obtain the critical surface tension of wetting. In the absence of specific interaction between the surface and the liquids used for the measurement of the contact angles, the critical contact angle of wetting can be accurately estimated and its value can be used as the surface tension of the surface. However, if a surface interacts with liquids used for the contact angle measurement to the extent that the surface tension is altered, Zisman’s plot deviates from the ideal linear relationship. In a strict sense, the plot is applicable only to unperturbable surfaces with which liquid contacts do not alter surface configuration. With hydrophilic polymers, such as gelatin, various nonideal surface characteristics have been reported.20-22 The surface configuration change of a polymer surface in contact with liquid water is concerned with this specific question of whether or not the surface tension of the polymer changes by contact with liquid water. If ys changes by the interaction of water with polymer molecules, ~ S Lalso changes accordingly even when cos 8 remains unchanged. It has been found that a water droplet placed on a hydrophobic surface did not roll down when the surface is tilted to the vertical position while the contact angle remains over 90°.21 The large discrepancy between advancing and receding contact angles of a sessile droplet on a gelatin surface is caused by the fact that the contact area of the water droplet does not decrease as the volume of water is decreased.22 These observations indicatethat a strong attractive force is created between water and the surface under the droplet and also that the decreased interfacial tension does not influence the force balance at the three-phase line. This implies that Young‘s equation given by eq 1only applies at the three-phase line, and y s in ~ the equation does not represent the interfacial tension between the surface and liquid water. In many cases, the rate of contact line advancement is much faster than that ofthe surface configuration change, and a reasonably accurate estimate of the surface energy of the polymer surface based on the value of the contact angle that can be made from the advancing contact angle. The fact that a constant advancing angle (independent of droplet volume, in the case of sessile droplet method) can be obtained indicates that such a requirement &e., the time scale of advancing the contact line is much smaller than the time scale of surface configuration change) can be met in many cases. It should be noted, however, that this is not a generally acceptable assumption, and if the polymer/water interaction is strong, an accurate measurement of the contact angle becomes extremely difficult, and the contact angle loses its meaning as a material constant because the value depends on the time scale of measurement and also on the history of a sample. In dealing with the surface configuration change of a polymer surface, it seems to be important to reexamine definitions and the validity of intuitive assumptions. In this paper, the following viewpoints (adopting concepts expressed in the literature) are taken into consideration throughout the discussion of the subject. These are as follows: 1. A solid surface can be recognized only as an interface. In this strict sense, only interfacial properties of a solid surface can be considered, and generic surface properties of a solid, particularly of a polymer, cannot be considered. (20) Baier, R. E.; Zisman, W. A. Adv. Chem. Ser. 1978,145, No. 4, 1.55.
(21) Iriyama, Y.; Yasuda, T.;Cho,D. L.; Yasuda, H. J . Appl. Polym. Sci. 1990,39, 249. (22) Yasuda, T.;Okuno, T.; Yasuda, H. Langmuir, 1994,10,2435.
Langmuir, Vol. 11, No. 8, 1995 3259 2. The interfacial properties of a solid polymer are dependent on the conditions under which the surface is equilibrated. The surface configuration of a polymer is a function of the phase which contacts the surface (surrounding medium). 3. The d a c e configuration change occurs when the interfacial equilibrium is broken. In dealing with different phases (materials), we have an inherent problem of not having the identical reference state for the thermodynamic parameters for different phases. In order to simplify the discussion, the following definition and notations are used in this paper. The standard solid surface tension may be defined as the surface tension of a solid in contact with its own vapor only. The phase which has zero surface tension (e.g., vacuum or gases which do not interact with the solid surface) can be considered as the contacting phase in most practical cases, and the standard solid surface tensions designated by y$ can be assigned. Likewise, the standard liquid surface tension may be defined as the surface tension of a liquid in contact with its own vapor and designated by y! (which is often noted . order to further simplify the discussion, let us as y ~ v ) In limit our consideration to a standard temperature only. Then the ideal interfacial tension can be defined as the difference of the surface tension of a solid and the liquid,
-
0 - 0 YSL YL
-
0
- Ys
The ideal interfacial tension for a polymer surface contacting liquid water, yiL, is given by this equation, if water does not change the surface tension of the polymer. As we know, the surface tension of a polymer changes as a consequence of contacting liquid water in many cases. The water-immersed polymer samples often show significantly different advancing contact angles from that for the dry sample. While eq 1 describes the force balance at the threephase boundary line, our major concern is the interface between a polymer and liquid water without the air phase, which may be described by eq 2. The actual surface tension of a polymer surface could differ significantly from y! depending on the conditions of the surrounding medium with which the surface is equilibrated. The surface tension of a polymer surface can be generally given by Ys = Y: + Y:
(3)
where the superscript a refers to a specific conditions of equilibration. When more than one surrounding medium (one at a time) is involved, a can be replaced by the numbers 1,2,etc., to designate the conditions (e.g.,water, oil, biological system, etc.). In the case of a relatively hydrophobic (not very hydrophilic) polymer kept in air or in vacuum, the surface tries to minimize the value of ys (i.e., y: 01, and after sufficiently long time, ys becomes y$. The minimization of ys can be achieved only by the change of the surface configuration,without consideringphysical changes such as swelling or deswelling which require more drastic changes of the surface. The liquid surface tension can be dealt with in a more straightforward manner. Consequently,the interfacial tension may be dealt with the differential surface tension, A(y) (the difference between the surface tension of the polymer and that of the contacting phase):
A(Y) = Ycontacting phase - Ysolid
(4)
Yasuda et al.
3260 Langmuir, Vol. 11, No. 8, 1995
~
Table 1. Surface Tension of the Liquid (40 "C)Used for Immersion liquid surface tension, dyn/cm water 70.3 0.01% Scourol solution 42.8 50 vol % ethanol solution 28.0
When A ( y ) is positive, the spontaneous wetting of the contacting phase is unlikely to occur. The differential interfacial tension in a general case can be given by
for a polymer-liquid interface
(5) and for a polymer-dry air interface
Surface Configuration Change when a Polymer Is Immersed in Water. The driving force for the surface configuration change is the interaction (attractive or repulsive)force between water and a specificmoiety under consideration. In the case of this study, the attractive force between oxygen atoms in the ether linkages and water and the repulsive force between water and fluorine atoms are considered. Because of the dominance of fluorine atoms in a Teflon-PFA-type polymer, the latter does not contribute to the overall surface configuration change. In the case of moderately hydrophobic (moderately hydrophilic) polymers, the cumulative effects of the attractive force and the repulsive force determine the overall surface configuration change. The maximum extent of such surface configuration changes can be correlated to the magnitude of the differential surface tension, because surface-configuration changes occur in order to minimize the interfacial tension. The larger the differential surface tension is, the greater is the maximum extent of surface configuration change which can be seen when an equilibrium with the new environment is reached. The magnitude of surface configuration change can be seen by the differential surface tension given by eq 5, and the value of yg varies depending on the equilibration conditions of the surface before the new interface was created. Considering that the surface tension of the pure liquid water is close enough to and does not change, the reduction of A(Y)SLcan be achieved mainly by increasing
yi
Y& In the case of perfluorinated polymers, yg is of the order of 19 dyn/cm, and $ r~ 0, because the surface configuration is not influenced by the surrounding medium (air or vacuum). The value of y: for water is of the order of 70 dyn/cm. The differential interfacial tension, therefore, is of the order of 51 dyn/cm. The increase of ys can be achieved either by moving polar moieties toward the top surface, in the case of this study, or moving hydrophobic moieties into the deeper section of the surface state, in the cases of CF4 plasma treated p ~ l y m e r s . ~ J ~ - l ~ In the wetting process, if we decrease y~ of water by adding a small amount of surfactant or alcohol (50vol %), the value of A ( Y ) ~cLa n be reduced significantly. The values of the surface tensions of the liquids used in this study are listed in Table 1.
The differential surface tension, A(Y)SL(assuming y: of the sample to be 19.0 dyn/cm), decreases from 51.3 dyn/ cm for pure water to 23.8 dyn/cm for the surfactant solution and 9 dyn/cm for the ethanol solution. This is a reasonable explanation why the increase of the 01,peak is less for the surfactant solution and practically none for the ethanol solution. In the case of hydrophilic and moderately hydrophilic polymers, yg cannot be assumed to be zero, and the value depends on the conditions of the equilibration. In this case, however, the differential surface tension, A(Y)sL,is smaller than that in the case of the perfluorinated polymers mentioned above, because Y S is significantly larger. Surface Configuration Change in Dry Air. The extent of surface configuration change is dependent on the differential surface tension given by eq 6, and the value of yg varies depending on the equilibration conditions of the surface before the new polymer-air interface was created. Because the value of ys cannot be smaller than y:, the extent of surface configuration change is determined by y;. In the reverse process of a water-immersed CF4 plasma treated polymer surface, the extent of surface configuration change is dependent on the value of y: which was caused by the change of surface configuration on the previous wetting process. A comparison of eq 5 and eq 6 indicates that the difference in the extent of surface configuration changes in these two processes is not the same. In the wetting process, the differential surface tension, A(Y)sL, is defined by eq 5. The values of yg in both processes are much smaller than the value of y t for water. Thus, the extent of the reverse process is much less than the wetting process. Consequently a simple drying of the wetted sample does not cause a significant recovery of the hydrophobicity of the surface. From the viewpoint of the driving force, the repulsive force between fluorine atoms and water molecules is not replaced by the attractive force of the similar magnitude (which might cause a significant recovery) but the repulsive force is merely removed. The reverse process, therefore, may be more appropriately considered as a redistribution process of certain chemical moieties, which is driven by the local differential chemical potential of each species, and the differential surface energy sets its extent. It has been indeed found that the characteristic transition temperature observed for the temperature dependence of the rates of the surface configuration change in the reverse (drying) process is completely different from that for the immersion process. The transition temperature for the reverse process coincides with the corresponding glass transition temperature of the polymer, indicating that the bulk (not surface) properties of a polymer control the rate of the reverse process.18 In the case of a polymer surface treated to make it hydrophilic, the value of ys is significantly large, and its reduction by virtue of surface configuration change is generally observed. A plasma-modified hydrophilic surface often loses its gained hydrophilicity in 2 weeks to 2 month depending on the nature of substrate polymer and of the treatment. Results for teflon-PFA show, because ofthe absence of a strong interaction between the liquid and the polymer, that the extent of surface configuration change due to the change of surrounding medium from air to liquid water is roughly proportional to the differential surface energy, A ( ~ ) s L .The same principle also explains why the surface configuration change is not completely reversible. LA9500134
