Langmuir 1994,10, 2766-2773
2766
Surface Forces between Plasma Polymer Films John L. Parker,t*fPer M. Claesson,*>+ Jenn-Hann Wang,tp§ and H. K. Yasudag Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, S.100 44 Stockholm, Sweden, Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden, and Center for Surface Science and Technology, College of Engineering, W2008 Engineering Building East, University of Missouri-Columbia, Columbia, Missouri 65211 Received February 18, 1994. I n Final Form: May 23, 1994@ The surface properties of two different plasma polymer coatings were investigated by means of ESCA, wetting studies, and surface force measurements. The plasma polymer films formed from hexamethyldisiloxane (HMDSO) monomers were found to be very smooth. This made it possible to measure the attractive van der Waals interaction between HMDSO-coated surfaces in air. When the HMDSO-coated surfaces were immersed in water, they initially appeared strongly hydrophobic with an advancing and receding contact angle of 109”and 98”,respectively. Nevertheless,no long range “hydrophobic”interaction was observed. The HMDSO coating became slightly more hydrophilic when kept in an aqueous phase for a prolonged time, indicating migrationheorientation of polar groups to the surface. Surface force measurements in water clearly demonstratedthe appearanceof electrostatic charges and extending polymer tails as a result of a prolonged exposure to water. Plasma polymer coatings of CzF4 or CzF6 deposited on top of a plasma polymer of CH4 were also studied. The forces between such a polymer and a glass surface are on the first approach in air characteristic of a van der Waals force. However, the attraction increases as the surfaces are brought together repeatedly. The increase in the long range attraction is a result of contact electrification due to electron transfer from glass to the fluorocarbon surface. It was found that in aqueous environment the C2F4 plasma polymer surface is significantly less smooth and less stable than the HMDSO plasma polymer surface.
Introduction Thin polymeric layers have a number of useful and important applications, for example, as adhesion promoters and biocompatible or protective coatings. Understanding the physical-chemical surface properties of these films is central to enhancing their efficacy in applications. One class of thin polymeric layers is plasma po1ymers.l This type of coating can be prepared from many organic compounds (many of them normally unreactive) by exposing them to a high-energy radio frequency electric field a t low pressure. One important advantage of this technique over many other coating methods is that it is a dry technique. As a result costly, time-consuming, and polluting wet processes can be avoided. As very thin films can be deposited, the amount ofmaterial needed for coating a substrate is also very low and hardly any waste material is produced making the process environmentally friendly. One characteristic of the plasma polymer process, which can be both advantageous and a hindrance, is that the properties of the plasma polymer depend both on the monomer used and on the production parameters2z3(power input, monomer flow rate, pressure, and to some extent reactor design and position of the sample in the reactor). The fact that coatings with very different properties can be produced even when using simple monomers (e.g.,CZF4 and CH4) means that in order to obtain the best result for a particular application a considerable amount of research is needed to establish the correct coating conditions.
Hence, it is extremely important to have the scientific tools for characterizing the properties of the polymers obtained. Spectroscopicmethods such as ESCA and SIMS provide accurate information about chemical composition of the surface. However, these techniques alone do not provide sufficient information on the stability of the coatings and on the type of intermolecular forces that are important for interactions between the polymer coating and the environment. A knowledge of the intermolecular forces is particularly important in adhesion and biocompatibility applications, whereas highly stable films are essential for use as a protective coating. In this report we use two methods, the surface force t e ~ h n i q u e ~and - ~ the Wilhelmy balance t e ~ h n i q u e to ,~ obtain information which complements that obtained by various types of spectroscopy. We have deduced the types and strength of intermolecular forces acting between two plasma polymer coatings as well as between one plasma polymer coated surface and one uncoated surface. Of particular importance for processes and products in dry air is contact electrification.* A phenomena that is conveniently studied with the surface force t e ~ h n i q u e . ~ Another important aspect of our studies is concerned with changes in the plasma polymer coatings that occur as a result of a change in the environment. It is shown that detailed information on such changes in plasma polymer films can be obtained by combining surface force and wetting studies. (4)Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans.
t Royal Institute of Technology and Institute for Surface Chem-
istry. On leave from the Department of Applied Mathematics, Research School of Physical Sciences, G.P.O. Box 4,Canberra, ACT 2601,Australia. 6 University of Missouri-Columbia. Abstract published in Advance A C S Abstracts, J u l y 15,1994. (1)Yasuda Plasma Polymerization;AcademicPress: New York, 1985. (2)Yasuda, H.; Hirotsu, T. J . Polym. Sci. 1978,16,743. (3)Cark, D.T. Pure Appl. Chem. 1982,54,415. @
0743-7463/94/2410-2766$04.5QI0
1 1978,74,975. ( 5 ) Parker, J . L.; Christenson, H. K.; Ninham, B. W.Reu.Sci. Instrum.
1990,60,3135. (6)Parker, J. L. In preparation. (7)Adamson, A. W. Physical Chemistry of Surfaces; John Wiley and Sons, Inc.: New York, 1990. (8) Charlson, E. M.; Charlson, E. J.; Burkett, S.; Yasuda, H. IEEE Trans. Electr. Insul. 1992,i24, 1144. (9)Horn, R. G.;Smith, D. T. Science 1992,256,362. (10)Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Tidblad, J.; Blomberg, E. J . Colloid Interface Sci. 1990,134,449.
0 1994 American Chemical Society
Surface Forces between Plasma Polymer Films
Langmuir, Vol. 10,No.8,1994 2767
Table 1. Plasma Deposition Conditions power
sample
mass flow rate (SCCM)
(W)
pressure (mTorr)
treatment time (min)
CzF4 on CHI CH4 HMDSO CHI
4.58 8.34 2.22 4.58
52 68 13 52
22.7 45.9 19.0 22.7
8 1.2 2.0 20
Table 2. Peak ESCA Intensities sample glass C2F4 on CHI HMDSO CHI
C 3.2 46.1 56.2 80.7
F
Si 31.2
52.1 24.4 1.4
0 65.6 1.8 19.4 17.9
radius (a)depends on the applied force (F)and the interfacial energy ( y ) as
where R is the radius of the spheres, E is the Young's module, about 6 x 1O1O(N/m2)for borosilicate glass,'* andn is the Poissons ratio, about 0.2 for borosilicate glass.18 The interfacial energy is according to the JKR theory related to the adhesion force F(0) as
(3)
Table 3. Surface Force Measurements in Dry Air substrate 1 glass glass glass HMDSO CzF4 on CH4
substrate 2 HMDSO methane CZF4 on CH4 HMDSO C2F4 on CH4
adhesion (mN/m) 154-162 19-21 90-95 125 5.5
surface displacement (nm) -4.1 -1.0 -2.8 -3.5 -0.4
Experimental Section Plasma Polymer Preparation. Plasma polymer deposition was carried out in a radio frequency plasma reactor consisting of a large glass vessel (length 65 cm, inner diameter 15 cm). The power was supplied through two external Cu-electrode rings. The plasma reactor system has been described extensively previously.2 The details of the preparation conditions are given in Table 1. The thickness of the films was measured by ellipsometry. Films 50 nm thick were deposited for HMDSO and CHQmonomers and the thickness obtained by ellipsometry was confirmed by measuring the film thickness with interferometry using mica substrates and a conventional surface force apparatus4 In order to enhance the adhesion between fluorocarbon-containing films and the glass substrate a -10 nm thick coating made from CH4 monomers was first deposited on the substrate. The chemical composition of the plasma polymer surface was analyzed by means of ESCA (Kratos AXIS-HS).The atom percentages, measured directly after preparation are given in Table 2. Surface Force Measurements. Surface forces were measured with a new type of surface force apparatus which has been used in only a few previous studies.l1JZ One surface is mounted at the end of a bimorph force sensor and the other is mounted at the end of a piezo electric tube. The bimorph is enclosed in a Teflon sheath mounted inside a small chamber (volume 10 mL) clamped to a translation stage. The coarse position of the piezo electric tube and the upper surface is controlled by this translation stage. A complete description of the apparatus will be given in a forthcoming publication.6 The surfaces are pushed together with the piezo tube, and the response from the bimorph is recorded. When in contact, the motion ofthe piezo tube is transmitted directly t o the sensor and this straight line is used to calibrate the sensitivity of the bimorph. Using this procedure it is not possible to define the separation with respect to the contact between the two surfaces prior to plasma treatment. As a result it is not possible t o determine the thickness of the polymer layer and this was carried out using FECO interferometry of plasma films deposited on mica.13 We also note that when the surfaces are in adhesive contact, they deform. This deformation is, in contrast to the deformation due to the comparatively small repulsive force experienced away from ~ o n t a c t , significant. ~J~ The distances referred to in the figures are relative to the position ofthe deformed surfaces. The actual surface separation between the undeformed surfaces away from contact is smaller than that indicated in the figures. The actual deformation for a given adhesion force can be estimated using l ~ central displacement (6)andthe contact the JKR t h e ~ r y . ' ~ -The
-
(11)Parker, J. L.; Yaminsky, V.; Claesson, P. M. J . Phys. Chem. 1993,97,7706. (12)Parker, J. L.; Rutland, M. W. Langmuir 1993,9, 1965. (13)Israelachvili, J. N. J . Colloid Interfate Sci. 1973,44,259. (14)Parker, J. L.; Attard, P. J . Phys. Chem. 1992,96, 10398.
When it is assumed that the thin plasma polymer layer does not affect the effective elasticity of the system, one can calculate the central displacement. Adding the (negative)central displacement to the distances given in the figures provides a first-order correction to the data to give the force vs distance curve for undeformed surfaces. The magnitude of the central displacement under a zero applied load calculated from eqs 1-3 is given in Table 3. Results obtained from a more sophisticated theory indicate that these calculations may overestimate the actual surface deformation somewhat. l7 Under conditions when the gradient of the force exceeds the spring constant of the bimorph force sensor, the mechanical system becomes unstable and a jump to the next stable region is observed. For this reason a jump inward occurs close to contact as the surfaces approach each other, and also out from contact when the surfaces are separated. Glass spheres for surface force measurements are prepared by cutting a 3 cm length of a 2 mm diameter glass rod. The rod is then cleaned with ethanol and the end melted in an oxygen gas burner until a molten droplet of glass is formed with a radius of2 mm. Two surfaces were, after surface modification, mounted in the apparatus and aligned with the centers of both spheres as close to parallel with the axis of motion of the piezo electric tube as possible. Water was purified with a Millipore Milli-Q PLUS 185 water purification system and deaerated just before use. Wetting Studies. The wetting behavior and stability of the plasma polymer coatings were studied with the Wilhelmy plate method7 using either microscope glass slides or muscovite mica as substrate. The results were not affected by the choice of substrate material. The force (F)on the plate in addition to the weight of the plate in air was measured with a high accuracy balance (KSV Co., Finland). The force has two additive components, the buoyancy force (4)
and the capillary force
FJL = yLv COS e
(5)
where L,, Ly,and L GZ U , are the thickness, the width, and the perimeter of the plate, respectively, e is the density of the liquid (the density of airis neglected),g is the gravitational acceleration, 2 is the immersion depth, y ~ vis the interfacial tension of the V 0 is normally liquid, and 0 is the contact angle. The term ~ L cos referred to as the wetting tension or the adhesion tension.
Results Forcesbetween Plasma Polymersin Air. The forces measured between two glass spheres coated with two different plasma polymers are shown in Figure 1. The long-range forces are purely attractive. The measured (15)Johnson, K.L.;Kendall,K.; Roberts,A. D. Proc. R . SOC.London, Ser. A 1971,324,301. (16)Horn, R. G.;Israelachvili, J. N.; Pribac, F. J. Colloid Interface Sci. 1987,115, 480. (17)Attard, P.;Parker, J. L. Phys. Rev. A 1992,46, 7959. (18)Kirk Othmer Encyclopedia of Chemical Technology;John Wiley & Sons, New York, 1980;Vol. 11,p 830.
Parker et al.
2768 Langmuir, Vol. 10, No. 8, 1994
al
0.5
h
E
2 E
v
$ -1
J 0
I 10
I
I
20
30
Separation (nm) I
0.5
-0.5
.,E
-0.8 I
-1
bl
:
0
30
Figure 2. Force normalized by radius between one plasma
polymer coated glass sphere and one bare glass sphere in air as a functionof separation. The plasma polymers were obtained from HMDSO (filledsquares),CH4 (unfilledsquares),and CzF4 aRer first depositing a layer of CHI (unfilleddiamonds). Sample preparation details are provided in Table 1.
t 0
20
10
Separation (nm)
10
20
30
Separation (nm)
Figure 1. Force normalized by radius of curvature between plasma polymer coated glass spheres in air as a function of separation. The plasma polymer was made from HMDSO monomers, results for two separate sets of surfaces are shown (a),andfromCzF4afterfirstdepositingalayer ofC&(b). Sample
preparation details are provided in Table 1.
interaction is in both cases consistent with a nonretarded van der Waals interaction which is given by
FIR = -A/12D2
(6) 0
Due to the surface deformation in contact, it is difficult to determine the exact effective Hamaker constant (A) for the plasma polymers by fitting forces calculated using eq 6 to the measured interaction. However, when one uses the JKR theory to estimate the surface deformation, correct the distances shown in Figure 1for this deformation, and finally fit the measured interaction with eq 6, one finds that the effective Hamaker constant is about (1-2) x J. If the surface deformation is neglected, the fitted Hamaker constant will be overestimated and J. We note one obtains values in the range (2-3) x that the apparent oscillatory character of the forces at large separations is not reproducible but due to noise caused by external vibrations. These problems are more pronounced in air than in liquids due to the lower viscosity. For the CzF4 plasma polymer surface coating, in contrast to for the other coatings, the “jump” into contact is not very distinct. The adhesion force between the surfaces, measured on separation, is in the case of C2F4 about 5.5 mN/m and in the case of HMDSO about 125 mN/m (Table 3). This indicates that the contact area in the case of C2F4 is, on the molecular scale, less smooth than for HMDSO. The long-range forces measured on the first approach between one bare glass sphere and one glass sphere coated with a plasma polymer are also attractive and well described by a van der Waals interaction (Figure 2). The adhesion force between the glass surface and a plasma polymer coated surface is considerably larger than that between two plasma polymer surfaces (Table 3). However,
100
200
300
400
500
Separation (nm) Figure 3. Force normalized by radius between one glass sphere coated with CzF4 after first depositing a layer of CHI (Table 1) and one bare glass sphere in air as a function of separation.
The force curves were measured repeatedly directly after each other. The second to the fiRh force curve are shown in this figure, and the first one is shown in Figure 2. the adhesion is still low, particularly in the case of the methane plasma polymer. This again reflects that the surfaces are not perfectly smooth.
It was observed that when a bare glass surface is repeatedly brought into contact with a surface coated with a plasma polymer of C2F4 on CH4, the long-range attractive force increases in magnitude for each approach (Figure 3). This additional attraction is due to a Coulombic attraction caused by charge transfer between the two surface^.^^^ We also note that when the plasma polymer surface and the glass surface are left apart overnight, the charge disappears and the long-range force is once again consistent with a van der Waals interaction. It was observed that the adhesion force between the plasma polymer and the glass surface was independent of the number of times the surfaces were brought into contact. At short separations the van der Waals interaction is more important than the electrostatic attraction. The largest
Surface Forces between Plasma Polymer Films
Langmuir, Vol. 10, No. 8, 1994 2769
20
0.3
i
0.2 .+ h
E
01
2
g -20-
h
\E o
z
d LL
-60
0
attractive force prior to the van der Waals jump into adhesive contact is --0.4 mN/m whereas the adhesion between these two surfaces is -90-95 mN/m. The electrostatic attraction varies rather slowlywith distance, and as a result the weak electrification which occurs during each contact is not expected to significantly affect the adhesion between the surfaces. Plasma Polymers in Aqueous Solutions,HMDSO. The force acting on a thin glass plate coated with a HMDSO plasma polymer was measured as a function of the immersion depth using the Wilhelmy plate method. (The weight in air is subtracted from the measured total force.) When the HMDSO-coated surface was immersed into water, it experienced a repulsive force corresponding to a negative wetting tension (Figure 4). The advancing contact angle is >goo. Due to the change in the buoyancy force, the total force on the plate becomes increasingly repulsive (negative) as the immersion depth (2)increases. The contact angle can be estimated by extrapolating the wetting tension to Z = 0, a t which point F/L = YLV cos 8. In the case of water on HMDSO, the initial advancing contact angle is 109". When the direction of the plate is reversed, the threephase line initially remains stationary,resulting in a less negative wetting tension (adhesion tension) and a reduction in the contact angle. Once a critical value of the contact angle has been reached (the receding contact angle) the three-phase line again moves along the surface, resulting in an increase in FIL in accordance with the buoyancy force. An extrapolation of this part of the wetting tension curve to zero immersion depths gives the receding contact angle of 98". The HMDSO plasma polymer coated surfaces were left in water for various periods of time and the wetting behavior reanalyzed with the Wilhelmy plate technique. It was found that the surfaces became less hydrophobic with increasing time in water (Figure 4). Both the advancing and the receding contact angles were affected, with the largest decrease occurring for the receding contact angle. The forces acting between two glass surfaces coated with a HMDSO plasma polymer directly after immersion in water are displayed in Figure 5. On separation from contact, a n adhesion force of 150 mN/m is measured. From JKR theory the central displacement ofthe surfaces under such a force is estimated to be about -3.9 nm. It was noted that after the surfaces had jumped out, at a separation of about 1.1 pm, a vapor cavity remained between the surfaces. However, when the surfaces were separated further,the cavity disappeared completely.This
20
40
60
80
100
Separation (nm) Figure 5. Normalized force between two glass spheres coated with a HMDSO plasma polymer as a function of surface separation. The forces were measured in pure water (unfilled squares)and after increasing the pH to 2 (unfilledcircles)."he smooth solid line is calculated DLVO forces, assuming interaction at constant charge with the plane of charge located at D = 3.9 nm (YO = 25 mV, K - ~= 120 nm, A = 0.5 x J).
is consistent with theoretical expectationslg and previous experiments with strongly hydrophobic surfaces.20-22 The long-range interaction is due to a weak repulsive double-layer force. The surface potential was estimated by fitting the measured forces with those calculated from the nonlinear Poisson-Boltzmann equation and an attractive nonretarded van der Waals interaction. I t was found that for a surface-solution interface located at D = 3.9 nm, the surface potential was estimated to be 27 mV, corresponding to an area per charge of 960 nm2. (If the displacement of the surfaces caused by the elastic deformation under the action of the strong adhesion force was neglected, i.e. the surface-solution interface was assumed to be a t D = 0, the surface potential obtained was about 40 mV.) The sign of the surface potential is unknown; however considering that HMDSO plasma polymer films contain a large amount of oxygen, it is likely that the very small (but measurable) charge arises from ionization of a few hydroxyl or acid groups and would therefore be negative. At D = 10-15 nm, the measured force is steeper and slightly more repulsive than expected from DLVO theory. This indicates that a steric force component may be present. Once the surfaces have come less than 10 nm from the adhesive contact position, they are pulled into a strong adhesive contact. The force barrier is located a t roughly the separation predicted by the DLVO theory considering the uncertainty in the Hamaker constant and the location of the undeformed surfaces. The repulsive double-layer force disappears as the pH is decreased to 2 by addition of HC1. The adhesion force is lower than that in pure water, about 100 mN/m, and no cavitation was observed when the surfaces were separated from contact. The measured adhesion force would, according to JKR theory, result in a surface deformation of about -3.0 nm. A weak repulsive force, most likely of steric origin, remains in the range D = 1015 nm (corresponding to a separation between undeformed ~
(19) Yushchenko, V. S.;Yaminsky,V. V.; Shchukin, E. D. J.Colloid Inteq5ace Sci. 1983, 96, 307. (20) Pashley, R. M.;McGuiggan,P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229, 1088. (21) Christenson, H. K.; Claesson, P. M. Science 1988,239, 390. (22) Claesson, P. M.; Christenson, H. K. J.Phys. Chem. 1988, 92, 1650.
Parker et al.
2770 Langmuir, Vol. 10,No. 8,1994
1 . i E 0.3 0'4
2
5
3 0.2
1: 1
h
E
2E
v
Day 1
r
I
-0.1 I 0
0.1
10
20
30
40
0.01
50
Separation (nm)
Figure 6. Normalized force between two glass spheres coated with a HMDSO plasma polymer as a function of surface
separation. The forces were measured in an aqueous solution at pH 2 immediately after the addition of HC1 (unfilledcircles) and 1 day later (filled squares). The solid line is calculated DLVO forces assuming interaction at constant charge with the plane of charge located at D = 3.0 nm (YO= 13 mV, K - ~= 5.5 nm, A = 0.5 x J). surfaces of 7-12 nm). Once this small force barrier has been overcome, a strong attractive force pulls the surfaces into contact. The wetting studies indicated that the surface properties of the HMDSO plasma polymer surfaces change with time of immersion. The surface forceswere also found to change with the time the surfaces were kept in the aqueous phase. The forces measured immediately after changing the pH to 2 are compared with those observed 1day later in Figure 6. Clearly, the repulsive force increases strongly with the time the surfaces have been left in the solution. The measured forces can only partly be explained in terms of a repulsive double-layer force, and we conclude that a significant stericforce componentis present. The adhesion force, 70 mN/m, is lower than that observed immediately after reducing the pH. From JKR theory we estimate the central surface displacement due to the attractive force to be -2.4 nm. At this point the measuring chamber was flushed with pure water, diluting the acidic solution by a t least a factor of 100. At this lower electrolyte concentration and higher pH, the electrostatic repulsive double-layer force completely dominates over the steric force component a t distances above about 5 nm. At small surface separations a strong attractive force pulls the surfaces into contact (Figure 7). The adhesion force was found to be 70 mN/m, the same a s before dilution. An increase in ionic strength to 0.1 M, by addition of KBr, reduces the electrostatic force. In this case it is found that the repulsive force is much more long-range than the expected double-layer force. Hence, a steric force component is now predominant a t large separations. C a s on C&. The wetting tension curve for a plasma polymer of CzFe deposited on top of a layer ofCH4 is shown in Figure 8. (A qualitatively similar behavior was observed for CzF4 on CH4.) On immersion, the force on the plate is strongly negative, corresponding to an advancing contact angle of about 120". The wettinghysteresis is considerably larger than for the HMDSO plasma polymer, and the receding contact angle is below 90". When the receding motion is reversed and the surface immersed a second time, immediately after the first immersion, it is observed that the force is less negative. Hence, the surface appears
Separation (nm)
Figure 7. Normalized force between two glass spheres coated with a HMDSO plasma polymer as a function of surface separation. The forces were measured after the surfaces had been immersed in a solution at pH 2 for 1day and then rinsed with pure water (filled triangles). The forces after addition of 0.1 M KBr (filled diamonds)are also shown. The solid line is forces calculated using DLVO theory, assuming interaction at constant charge and the plane of charge located at D = 2.4 nm. The parameters used are YO= 26 mV, K - ~= 12 nm, and A = 0.5 x J.
'-
-60
I
c
\ -5
0
5
10
15
20
Depth (mm) Figure 8. Force acting on a glass plate coated with a plasma polymer of CzFs on CHI normalized by the perimeter length as a function of the immersion depth in water. Results for the initial wetting cycle, after 2 h of immersion in water, and after 7 h of immersion in water are shown.
less hydrophobic. A further decrease in hydrophobicity takes place when the surface is left in water for a prolonged time. The wetting tension curves observed after 2 h and 7 h of immersion in water are shown in Figure 8. The reduction in wetting tension with time is about the same for the advancing and the receding situation, and in both cases much larger than that for the HMDSO plasma polymer. The force acting between two glass spheres coated with a plasma polymer of CzF4 deposited on a plasma polymer layer of CH4 across water was measured after 2 h of immersion in the aqueous phase. The force was strongly repulsive a t distances below about 60 nm, and the repulsive force increases with decreasing surface separation. The forcewas hysteretic and the distance dependence of the force was not consistent with that of a repulsive double-layer force, and we conclude that the measured force is of steric rather than electrostatic nature and that these plasma polymer coatings are unstable in water and have a rough surface on the molecular scale.
Surface Forces between Plasma Polymer Films
Langmuir, Vol. 10, No. 8, 1994 2771
Discussion
Van der Waals andAdhesion Forces. The attractive forces measured between the plasma polymer coated surfaces in air are consistent with a van der Waals force. The estimated Hamaker constants were found to be about (1-2) x J. The expected Hamaker constant can be calculated from the dielectric response functions of the materiabZ3 A good estimate of the Hamaker constant can often be obtained when the refractive index and the static dielectric constant of the surfaces (and the intervening medium) are Based on such calculations, the Hamaker constant for Teflon interacting across air has been estimated to be about 3.8 x J and for J.24We note that the value hydrocarbon about 5 x of the Hamaker constants obtained from force measurements are lower than expected. This indicates that the plasma polymer-air interface are not smooth and not welldefined on the molecular scale relative to the contact a t D = 0 in our Figures 1and 2. Instead, it is likely that a considerable amount of air remains in clefts and voids of the plasma polymer surface which lowers the effective Hamaker constant. According to JKR theory the surface energy for identical perfect surfaces is related to the measured adhesion force normalized by the radius (FIR)between two equal spheres by eq 3. For real surfaces one has to consider that the contact is not perfect. This can formally be done by adding a term, yss, that describes the excess energy associated with surfaces in contact. This gives for two spheres
FIR = 1.5n(y - yss)
(7)
yss will increase with increasing surface roughness and its value depends not only on the surfaces but also on the surrounding medium. The measured adhesion forces can be converted to surface energies using eq 3. The values obtained for HMDSO and CzF4are 26.5 and 1.2 mN/m, respectively. For comparison the surface energy of hydrocarbons is about 20-25 mN/m and for PTFE 18.3 Clearly, the surface energy obtained for the HMDSO plasma polymer is not unreasonable, indicating that yss is small and the plasma polymer surface smooth. In contrast, the value for the surface energy for the C2F4 plasma polymer obtained from eq 3 is clearly unreasonably low. This shows that the excess energy associated with the contact between the surfaces is large which strengthens our conclusion that the C2F4 plasma polymer surface is rough on the molecular scale. We note that if surface flattening was not considered, the numerical factor in eqs 3 and 7 would change from 1.5 to 2. Hence, the calculated surface energies would be smaller. The uncertainty in this numerical factor is an important obstacle when trying to convert adhesion force measurements using the surface force technique into surface e n e r g i e ~ . ~ ~ , ~ ' Forces due to Contact Electrification. The transfer of charges from one surface to another upon contacting two different materials with each other is a general phenomenon. By measuring the current flow upon separating different materials from each other, it has been possible to characterize the triboelectric properties of a range of polymers and plasma polymer coatings.8 For plasma polymers it was found that fluorine-containing (23)Hough, B.D.; White, L. R. Adu. Colloid Interface Sci. 1980,14, 3.
(24)Israelachvili, J. N.Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (25)Moy, E.; Neuman, A. W. J . Colloid Interface Sci. 1987,119,296; 1988,122,294. (26)Christenson, H.K.;Claesson,P. M.J. Colloid Interface Sci. lW0, 139,589. (27)Dann, J. R.J . Colloid Interface Sci. 1970,32, 302.
0.01
I 10
I00
loo0
Separation (nm) Figure 9, Force obtained after the second contact between on CzF4 plasma polymer coated surface and a glass surface plotted on a log-log scale. The line illustrates a force curve that decreases with separation as D-2.
polymers were most negative, siloxane-containing polymers were in the middle of the triboelectric series, and amine-containing polymers were most electropositive.8 Oppositely charged surfaces in air attract each other according to Coulombs law taking into account the superposition principle. Hence, the expected distance dependence of the force law between a fured charge distribution can in principle easily be calculated. For instance, the force law between two point charges decays as D-2 whereas the force between two flat infinite and homogeneously charged surfaces is independent on the surface separation. Hence, if the surface separation is much larger than the diameter of the charged region on the surfaces, then the distance dependence of the force due to electrification is close to that between two point charges. At smaller separation one would instead expect a very slow variation of the force with separation. Forces due to contact electrification between a silica surface and a mice ~ u r f a c eand , ~ even between two mica surfacesz8in N2, have recently been reported. Here we observe that the Coulombic force between a C2F4 plasma polymer and a glass surface a t distances below about 300 nm varies much slower than the D-2 force law between point charges (Figure 9). At distances above 400 nm the measured force appears to change more rapidly than D-2. This result is most likely a n artifact due to the difficulty in measuring these rather long-range, weak, and slowly decaying forces. Over the maximum range of the measurement there is no separation where the force is not acting and it is difficult to establish a baseline of zero force. We note that our experimental setup differs from that used by Horn and Smith8in that we do not have any conducting metal layer on the backside of our surfaces which would stabilize the surface charge through image charges. In our measurements the adhesion force was not strongly influenced by the contact electrification. This is opposite to what was observed by Horn and Smith when studying contact electrification between mica and silicae8 The reason for this difference is the much smaller contact area of the surfaces used in this experimental setup. Here we used 2 mm radius glass spheres as substrates whereas Horn and Smith employed e 2 0 mm radius mica or silica surfaces glued onto silica disks with a comparatively soft epoxy glue. This latter method provides a flat contact between the surfaces with a radius of the order of 50 pm. (28)Dunstan, D. E.J. Chem. Soc., Faraday Trans. 1992,88, 759.
2772 Langmuir, Vol. 10,No. 8, 1994 In our experimental setup, which does not use interferometry, it is not possible to measure the actual contact area. However, it can be estimated from JKR theory employing eq 1. For our adhesion value of 90-95 mN/m, the estimated contact radius is 2.1 pm. That the contact area indeed makes a large difference to the measured electrostatic force was demonstrated by the fact that hardly any electrification could be observed between one glass surface and one HMDSO plasma polymer coated glass surface using the method described here. On the other hand, when a soft large radius surface (one mica surface and one HMDSO coated mica surface) was used, employing the interferometric surface force technique also used by Horn and Smith, a considerable electrification force was detected. Wetting Hysteresis and the Effect of the Environment. It is well-known that nearly every surface displays a contact angle hysteresis. Several molecular origins to this phenomenon like surface roughness,29 chemical h e t e r ~ g e n e i t y , deformation ~~,~~ of the solid a t the threephase line,32 adsorptiodde~orption,~~ and reorientation of surface groups34have been identified. It goes beyond the scope of this article to discuss them all, and by simply measuring the contact angles, it is not possible to distinguish which mechanism is most important. For the plasma polymer surfaces under investigation here, suffice it to say that they are chemical heterogeneous on the molecular level (as any plasma polymer) and that the surface groups are able to rotate. From the range of the steric type force observed during surface force measurements, it is seen that the HMDSO surface is very smooth whereas the CzF4 plasma polymer surface is considerably rougher. Hence, it is clear that surface roughness and thus deformation effects are more important for the CzF4 plasma polymer than for the HMDSO plasma polymer, but we cannot conclude that molecular scale roughness and deformation effects are more important reasons for the contact angle hysteresis than, e.g., changes in molecular orientation on the surface. By combining Young's equation and eq 7, one obtains a relation that relates the normalized adhesion force in air to the normalized adhesion force in water, the liquid surface tension, and the contact angle
Parker et al.
120 110
K
bz,
on CH4
60
I 0
5
10
15
20
25
Time in Water (h)
Figure 10. Results from the wetting studies ofHMDSO plasma polymers and of plasma polymer of CzF6 on CH4 summarized as advancing and receding contact angles as a function of immersion time in water.
HMDSO plasma polymer surface can be calculated from the measured adhesion force in air and water. The value obtained is 94",which is close to the receding contact angle. (If instead the numerical factor 2n is used in eq 9, the contact angle is estimated to be 93".) This indicates that the HMDSO plasma polymer surface initially remains smooth upon immersion in water. Again, the data obtained for the CzF4 plasma polymer are not consistent with eq 9, demonstrating the nonideal nature of the surface. A similar analysis of the wetting and adhesion properties of plasma polymer coatings of acrylic acid has previously been carried It was concluded that also these coatings have molecular scale roughness that affects their interaction. The change in wetting behavior of the plasma polymer surfaces when left immersed in water is significant. Our data are summarized in terms of contact angles in Figure 10. Clearly, the surfaces are becoming more hydrophilic with increasing time in contact with water. This indicates that less hydrophobic groups are appearing on the surface (F(air)/l.5nR - yss(air)) - (F(water)/l.5nR due to migration in or simple reorientation of the polymer. yss(water)) = yLv cos 8 (8) These effects will be discussed in more detail in a forthcoming paper.37 The changes in the contact angle In general the excess energy associated with the surface are rather slow and small for the homogeneous and rather contact is not the same in air and water. However, cavities cross-linked HMDSO surface. Surface force measureform around the contact zone for the HMDSO coated ments show that charged groups appear on the surface surfaces, and therefore the contact region will be eswhen left in water. This could partly be due to migration sentially dry and similar to that in air. (Note that the of free radicals in the polymer to the surface and presence of a vapor cavity around the surface contact zone subsequent reaction with water and ionization of hydroxy in water, or the presence of a capillary condensate in air, and carboxy groups. The changes in contact angles does not affect the adhesion force for perfect s u r f a ~ e s . 3 ~ , ~ ~occurring ) for the fluorocarbon plasma polymer surface Hence, for this surface it is reasonable to assume that are more rapid and significant. This is consistent with y d a i r ) y d w a t e r ) and eq 8 is simplified to the less homogeneous and less ideal surface structure of this polymer. F(air)ll.5& - F(waterYl.5nR = yLv cos 8 (9) Forces in Aqueous Solutions. Surface force measurements allow the determination of the important types Equation 9 is also always valid in the JKR approximation of long-range forces as well as adhesion forces. We have for perfect surfaces. From eq 9 the contact angle for the been able to demonstrate that HMDSO plasma polymer surfaces acquire a small surface charge upon a prolonged (29)Dettre, R.H.; Johnsson, R. E.Adv. Chem.Ser. 1964,No. 43,136. exposure to aqueous solutions. The CzF4 plasma polymer (30)Cassie, A. B. D. Discuss. Faraday SOC.1962, 75, 5041. surface is rough on the molecular scale and steric forces (31)Israelachvili, J. N.;Gee, M. L. Langmuir 1989,5, 288. (32)deGennes, P. G. Rev. Mod. Phys. 1986, 57, 827. extending to a separation of 60 nm predominate the (33)Yaminsky, V. V.;Claesson, P. M.; Eriksson, J. C. J. Colloid interaction. A strong rather short range attraction exists Interface Sci. 1993,161, 91. between the HMDSO coated surfaces. The range of this (34)Yasuda, T.;Miyama, M.; Yasuda, H. Langmuir 1994,10,583. (35)Christenson, H.K. J. Colloid Interface Sci. 1988, 121, 170. (36)Cho, D.L.;Claesson, P. M.; Mlander, C. G.; Johansson, K J. (37)Wang, J. H.;Yasuda, H.; Claesson, P. M.; Parker, J. L. In
Appl. Polym. Sci. 1990,41, 1373.
preparation.
Surface Forces between Plasma Polymer Films force is roughly consistent with that of a van der Waals force. However, the attraction starts to dominate the interaction, as indicated by the inward jump, a t a separation that is nearly independent of the strength of the repulsive force component (Figure 7). The jump inward occurs when the gradient of the total force dFldD just exceeds the spring constant. The fact that the jump distance is very insensitive to the magnitude of the repulsive force component demonstrates that the attractive force increases very rapidly with decreasing surface separation. It may be that in addition to the van der Waals attraction also a force caused by the spontaneous cavitation of the thin metastable aqueous film that separatesthe surfaces is present. From a theoretical point ofview the metastability ofthe thin aqueous phase is well underst~od.’~ That the vapor and the liquid phases coexist a t a small surface separation is also well understood from a theoretical point of v i i e ~ . ~It~should J ~ be pointed out that we also have observed cavitation away from contact for glass surfaces in mercury (unpublished results). Further, cavitation between fluorocarbon and hydrocarbon surfaces in contact in water has also been reported.20-22 Many reports exist about the presence of very long range attractive forces in water, extending in extreme cases to above 100 nm, acting between hydrophobic surfaces obtained by adsorption of surfactant^,^^^^^ LangmuirBlodgett d e p o s i t i ~ n , ~ and ~ , s~i ~l a, n~ a~t i ~ n . ~Proust ~ - ~ et al.&reported the observation of long range attractive forces between plasma polymer surfaces; however these authors did not present any data. In light of this, it is remarkable that no such a long-range “hydrophobic”attraction exists between the very hydrophobic HMDSO coated surfaces. One then has to ask why this is the case, and one cannot avoid the conclusion that the macroscopic contact angle alone cannot be used when predicting the range of the “hydrophobic”interaction between macroscopic surfaces. Recently the importance of bubble nucleation on and cavity formation between hydrophobic surfaces has been demonstrated for the case of glass silanated with some fluorocarbon silanes.44 If bubble nucleatiodcavity formation is the reason for the long-range hydrophobic interaction between all macroscopic surfaces, one has to consider why cavitation does not occur between hydrophobic HMDSO plasma polymer surfaces until they are less than 10nm apart. From the force measurements it is observed that a few hydrophilic and partly charged polymeric chains are present on the HMDSO surface, giving rise to a steric force component. It is conceivablethat these short polymer (38)Sarman, S.;Eriksson, J. C.; Kjellander, R.; Ljunggren, S. Prog. Colloid Polym. Sci. 1987,74, 76. (39) Evans, R. J.Phys.: Condens. Matter 1990,2,8989. (40)Tsao, Y. H.; Evans, D. F.; Wennerstrom, H. Science 1993,262, 547. (41)Claesson, P. M.; Blom, C. E.; Herder, P. C.; Ninham, B. W. J . Colloid Interface Sci. 1986,114, 234-242. (42)Rabinovich,Y. I.;Derjaguin, B. V. Colloids Surf. 1988,30,243oc.
LDl.
(43)Parker, J. L.; Cho, D. L.; Claesson, P. M. J . Phys. Chem. 1989, 93,6121. (44)Parker,J.L.; Attard, P.; Claeson, P. M. Submittedforpublication in J. Phys. Chem. (45)Proust, J.;Perez, E.; Segui, Y.; Montalan, D. J . Colloid Interfuce Sci. 1988,126,629.
Langmuir, Vol. 10, No. 8, 1994 2773 chains effectively prevent the nucleation of bubbles and cavities and hence remove the long range part of the hydrophobicinteraction. In this case no non van der Waals attraction would be expected until the spinodal separation has been reached as suggested by BBrard et aL46 No dipolar domains are expected on HMDSO coated surfaces and the lack of a long range attraction in this case is consistent with the electrostatic explanation put forward by Tsao et al.40 Note, however, that their explanation cannot explain the fact that the long-range attraction observed between silanated glass is very insensitive to the salt concentration.u Hence, if bubbles/ cavitation is not the explanation for the long range force observed between all hydrophobic surfaces, the only other possible but, in our view, less likely interpretation of all data is that several mechanisms are a t play. In some cases bubble nucleation is important,44 in other cases electrostatic forces arising from dipolar domains40or forces due to a propagating water s t r ~ c t u r e , and ~ ’ in cases like for HMDSO coated surfaces, no exceptionally long range attraction exists even though the surfaces are very hydrophobic.
Conclusions The combined use of wetting and surface force measurements has been used for studying surface properties of plasma polymer films. For instance, the creation of electrostatic charges upon contacting a plasma polymer coated surface with a bare glass surface in air has been demonstrated and the resulting force measured. The combination of surface force measurements and wettability measurements demonstrates that plasma polymer layers of HMDSO are very smooth and comparatively stable in aqueous solutions. However, even for HMDSO layers the surface properties change when the environment is changed from air to water. With increasing time in water the surface becomes increasingly charged and decreasingly hydrophobic. Under no circumstance do we observe any long range “hydrophobic”force between these very hydrophobic surfaces. Stable plasma polymer layers made from C2F4 or C2F6 monomers are harder to obtain. In this case a strong decrease in hydrophobicity is observed when the surface is immersed in water. Surface force measurements also demonstrate the presence of a polymeric chains extending away from the plasma polymer surface in water. We conclude that surface force and wetting studies are a very important complement to spectroscopic analyses of plasma polymer surfaces (e.g. ESCA and SIMS) as well as determination of molecular scale surface structures using, e.g., AFM. Acknowledgment. This work was in part supported by a n International Collaborative Research Program, National Science Foundation, USA, No. NSF-INT8722457. J.P. and P.C. acknowledge financial support from the Swedish Natural Science Research Council (NFR). (46)BBrard, D.R.;Attard, P.; Patey, G. N. J . Chem. Phys. 1993,98, 7236. (47)Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J . Chem. SOC., Faraday Trans. 2 1989,85,163.
