Langmuir 1994,lO,3887-3897
3887
Dynamic Contact Angles and Contact Angle Hysteresis of Plasma Polymers J.-H. Wang,? P. M. Claesson,S J. L. Parker,$ and H. Yasuda*pt Department of Chemical Engineering a n d Center for Surface Science & Plasma Technology, University of Missouri-Columbia, Columbia, Missouri 65211, and Laboratory for Chemical Surface Science, Department of Chemistry, Royal Institute of Technology, S.100 44 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, S-11486 Stockholm, Sweden Received March 2, 1994. I n Final Form: July 5, 1994@ Dynamic advancing and receding contact angles and the contact angle hysteresis for different plasma polymers deposited on microscope cover glasses were investigated by the Wilhelmy plate method. The hysteresis loops observed by this dynamic method show two major factors: (1)the meniscus change at the transition region; (2) the wettability change of solid surfaces. The first factor is due to the fact that three-phase contact line does not move in the transition region while the water level is forced to rise up or lower down. The second factor is due to the change of surface state of the solid. Moreover, this factor can be related to the “stepchange”at the buoyancy slope of a hysteresis loop when the wetting line is moved from a “prewetted”area to a “dry“area of the same sample and also to the gap between the first and second immersion loops. Both factors are caused by the interactionforces between solid surface and water molecules at the solidlliquid interface. Plasma polymer of the tetrafluoroethylene(TFE)showed the largest hysteresis loop primarily due to the significant change of surface state aRer water immersion. Plasma polymer of hexamethyldisiloxane (HMDSO) showed the smallest hysteresis loop due to the minimal change in the surface state after water immersion. Water contact angles of plasma polymers were clearly dependent on plasma conditions manifested by the discharge energy density.
Introduction There is growing evidence that the surface configuration of polymeric materials (spatial arrangement of atoms at the surface) changes in response to a change in surrounding environment. “Reconstruction” and “reorientation” of surface molecules are often used to describe the change of surface characteristics due to the change of the surroundingmedium. Our recent indicated that such a change does not necessarily require long range segmental motion (conformation change) or migration of large segments but can be achieved by relatively simple short range motion such as rotational motion of segments that are at the surface. Because such a change can be accomplished without conformational change of a macromolecule,the term “surfaceconfiguration”has been used in our previous papers. Accordingto the concept of surfacestate equilibration, reconstruction or reorientation is always taking place and reconstruction of the reconstructed structure and so forth occur depending on the change of surrounding medium. In this context, the terms “reconstruction” and “reorientationncould be misleading in that such a reconstructed or reoriented structure could be interpreted as the more stable structure to stay. One of the most obvious surface characteristics that is affected by the environment is the wetting properties. The hysteresis effect observed between advancing and receding contact angles can be due to a multitude of effect^^-^ like (i) surface roughness, (ii)chemical hetero-
* To whom correspondence should be addressed. + University
of Missouri-Columbia.
* Royal Institute of Technologyand Institute for Surface Chem-
geneity, (iii) surface deformation, (iv)surface-configuration change, and (v) adsorption and desorption. From the molecular point of view, however, the most important factor is the surface-configuration change. Langmuir‘s statement’in 1938can be rephrased by using the term “surface configuration” as follows. The surface properties of a polymeric surface are determined by the surface configurationbut not by the chemical configuration of the macromolecule. What kinds of atoms and ligands are in a particular macromolecule does not determine the surface properties, but what kinds of atoms and ligands are actually existing at the top surface does determine the surface properties. According to the principle of surface state equilibration,’P2 when a water droplet is placed on a polymeric surface in the sessile droplet contact angle measurement, or a surface is immersed into water such as the case of the Wilhelmy plate contact angle measurement, the surface configuration of the polymer in contact with liquid water should change following the change of surrounding medium from air to water. It is considered that such a change in surface configuration and its reversibility within the time scale of contact angle measurement will contribute to the hysteresis of contact angles (discrepancy between advancing and receding contact angles) and cannot be ignored under any circumstance. In the sessile droplet contact angle measurement, the advancing contact angle refers to the (near) equilibrium contact angle at the three-phase contact line representing waterlairidry surface, and the receding contact angle should represent the equilibrium contact angle at the contact line representing waterlairlwetted surface. With a polymeric surface, however, the contact line in the receding process often does not recede according to the
istry. Abstract published in Advance ACS Abstracts, September 15, 1994. (l)Yasuda,H.;Charlson,E.J.;Charlson,E.M.;Yasuda,T.;Miyama, M.; Okuno, T. Dynamics of surface property change in response to (4)Johnson, R.E.,Jr.; Dettre, R. H. Wettability and contact angles. changes in environmental conditions. Langmuir 1991,7 , 2394. Su$. Colloid Sci. 1969,2,85-153. (2)Yasuda,T.;Miyama,M.;Yasuda, H. Dynamicsofsurfaceproperty (5)Chen,Y.L.;Helm,C. A;Israelachvili,J.N. Molecularmechanisms change in response to changes in environmental conditions. 2. associated with adhesion and contact angle hysteresis of monolayer Comparison of changes in air and in liquid water. Langmuir 1992,8, surfaces. J. Phys. Chem. 1991,95,10736-10747. 1425. (6)Berg,J. C.Wettability;Surfacantscienceseries49;MarcelDekker, (3)Yasuda, T.;Miyama, M.; Yasuda, H. Effect of water immersion Inc.: New York, 1993. of surface configuration of an ethylene-vinyl alcohol copolymer. (7)Langmuir, I. Science 1958,87, 493. Langmuir 1994,10,583. @
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3888 Langmuir, Vol. 10, No. 10, 1994
volume of water, and the lower contact angle, which decreases with the volume of the droplet and diminishes to zero contact angle, is observed on the receding process. In such a case, the lower value of the receding contact angle is caused largely by the flattening of the sessile droplet .a Therefore, the term “receding contact angle” in sessile droplet method applied for many polymeric surfaces is misleading unless the receding of the contact area is confirmed, and a constant contact angle is observed on the receding process. In a recent study dealing with Teflon, agar gel, and gelatin gel: a constant (independent of the droplet volume) receding contact angle of water was observed only with the surface of Teflon. Hysteresis effect between the advancing and the receding contact angles can be viewed as an indication of the extent of the change in the surface properties caused by the wetting. In the Wilhelmy plate method, dynamic (nonequilibrium) contact angles are measured while a surface is moved either in or out of a liquid. Generally, the contact angle deduced on immersion is called the advancing contact angle, O,, and that on emersion is called the receding angle, 0,. The difference, AB, is called the contact angle hysteresis. In the Wilhelmy plate method, the threephase contact line is indeed forced to recede. In this case, the receding contact angle refers to the dynamic emerging contact angle at the three-phase contact line representing liquid waterlairlwetted surface. However, the meaning of “receding”is that the line is moving toward the wetted surface. Likewise, the advancing contact angles refer to the dynamic immersing contact angles at the three-phase line representing liquid waterlairlsurface in contact with air. The term “advancing” means that the contact line is moving toward the surface in contact with air, which can be either (1)never-wetted surface or (2)previously wetted surface. Because of the possibility of repeating immersion and emersion in a preprogrammedmanner, the Wilhelmy plate method seems to be ideally suited for the investigation of the extent of surface change caused by contacting the surface with liquid water. In this study, the method was applied to different plasma polymers which were deposited on thin microscope cover glasses. The thicknesses of deposited layers are of the order of 20-50 nm. At this level of coating thickness, the surface of the coated glass plate is as smooth as the uncoated one under observation by scanning electron microscopy (SEMI. Generally, the hysteresis will be negligible when the roughness is below 0.1 pm, or when the heterogeneous phase is smaller than 0.1pm.9 The plasma polymer films are generally highly cross-linked and insoluble in water. The good plasma polymer film not only has homogeneous properties but also has good adhesion to the substrates. The swelling of a plasma polymer layer is closely related t o the delamination of the film. Conversely, an ultrathin plasma polymer film with a good wet adhesion does not swell. Therefore, the hysteresis attributed to the effects such as roughness, heterogeneity, and deformation of a surface can be considered to be minimized (if not totally eliminated) in this study, and the effect due to the surfaceconfiguration change could be singled out in this approach. Experimental and Method Glass Slide. The glass slides were square microscope cover glasses having a typical width of 22 mm and thickness of 0.153 f0.002 mm. Prior to plasma deposition, they were cleaned with (8)Yasuda, T.;Okuno, T.; Yasuda, H. Contact angles of Teflon, agar gel, and gelatin gel. Langmuir, in press. (9) Wu,S. Polymer interfaceandadhesion;MarcelDekker,Inc.: New York, 1982; Chapter 8.
Wang et al. ethanoywater solution by ultrasonic cleaner, thoroughly rinsed, and then dried in the air. Plasma Reactors and Plasma Deposition. Two types of plasma reactors are used to prepare the samples. The HMDSO plasma polymer deposition is carried out in a large cylindricalshape plasma reactor. Power is supplied through two external ring-shape copper electrodesby a 300-kHz radio frequencypower generator. This reactor system has been described extensively previously.1° The TFE and methane (CH4) plasma polymers were prepared in a capacitivelycoupled glass bell-jar reactor which uses parallel internal magnetron electrodes. (The adhesion of these plasma polymers deposited on the glass surface by the first reactor was not good enough and plasma polymer layers peeled off on water immersion.) Permanent magnets are arranged on the back side of each electrode in a circular configuration. Power is supplied to the electrodesby a 40-kHz audio frequencypower supply made by Advanced Energy. The plasma reactor was described previously in detail e1sewhere.l’ A magnetron glow discharge was used to ensure good adhesion and the minimal swelling of the film. Monomers are fed in through a tube of which outlet is located above the electrodes facing the bell-jar wall. Monomer continuously flows in to the reactor and is fed to the glow discharge zone in between two electrodes by diffusion. Gaseous byproducts and unreacted monomer are continuously pumped out of the reactor through a hole placed below the electrodes. The system pressure is measured usingapressure transducer (Vacuum General). The monomer flow rate is controlled by a MKS mass flow controller unit. Sample substrates are mounted on a circular disk that rotates in the interelectrode space. Rotation of the substrate through the glow improves the uniformity and reproducibility of the film. In order to obtain a similar thickness of depositedfilms, the deposition time was adjusted from one kind of monomer to the other. All samples after plasma polymer deposition were stored in a vacuum desiccatorprior to contact angle measurement in order to prevent any dust particle contamination. The Wilhelmy Plate Apparatus. The dynamic contact angle was measured by a fully computer controlled and automatic tensiometer, Sigma 70 (KSVinstruments, Ltd., Finland). There are three measuring ranges, 0.25, 2.5, and 25 mN giving a resolution of 0.05,0.1, and l.OpN, respectively. The calibration is automatic. The built-in micro-electrobalanceis ultrasensitive and capable of characterizing the wetting properties of single fibers with thicknesses of only a few micrometers. The liquid was poured in a beaker and placed inside the closed test chamber, that protects the sample and liquid from airborne particles and draft.. Immersion and withdrawal ofthe glass slides are accomplished by raising and lowering the beaker stage. The speed of the stage can be varied from 0.1 t o 40 m d m i n . In the present work, the speed was fixed at 5 m d m i n . Data points (including location, total force, temperature) were collected and stored into a file by computer a t the rate of 1 data seffs. The balance is reset to zero after the plate is installed each time. The immersion depth is calculated from the speed of immersion starting from the reference point which is automatically reset to zero when the plate is first contacted with the surface of the water level for each measurement. Water Purification. The water used in the experiments was first prepurified by decalcification, prefiltration, and reverse osmosis. The final purification was carried out by a modified Milli-Q unit, which included two mixed-bed ion exchangers, an activated charcoal cartridge, a 0.2-pm in-line filter cartridge, and a final 0.2-pm filter. The filter was a Zetapore product, whereas all other cartridges were from Millipore. The surface tension of water a t 24.5 “Cis 71.9 f 0.2 mN/m measured by Du Nouy ring method with the same instrument. The water is discarded and changed after each sample measurement for preventing any possible contamination. The measurements are all performed at temperatures 24.5 f 0.5 “C.The temperature (10) Parker, J. L.;Cho, D. L.; Claesson, P. M. Plasma modification of mica: forces between fluorocarbon surfaces in water and a nonpolar liquid. J . Phys. Chem. 1980,93,6121. (11)Ho,C.-P.; Yasuda, H. Coatings and surface modification by methane plasma polymerization. J.Appl. Polym. Sci. 1990,39,1541.
Langmuir, Vol. 10,No. 10,1994 3889
Dynamic Contact Angles of Plasma Polymers effect on liquid surface tension is negligible in this small temperature range. The Wilhelmy Plate Method. The Wilhelmy plate method was first developed by Wilhelmyin 1863. This method is suitable for both static and dynamic contact angle measurements. The measured force acting on a thin plate which is partially immersed in a liquid can be represented by the force balance equation
+
F = Mg - QgtHd PyL COS 8
(1)
where F is the measured force on an electrobalance, M is the mass of the plate, g is the gravitational acceleration, is the liquid density, t is thickness of the plate, H is the width of the plate, d is the immersion depth, P is the plate perimeter, y~ is the liquid surface tension, and 8 is the contact angle at the liquidsolid-air three phase contact line. Before the contact angle measurements, the sample (hanging under an electrobalance) is located at least 5 mm above the water level surface, and the electrobalance is reset to zero at this time. Hence, the measured force is given by
Then the force measured by the balance, F,, is only the differencebetween the buoyancy force and interfacial interaction force. Experimentally, the surface of the sample was scanned by moving the three-phase contact line forward from bottom of the sample to a certain height of the sample and then backward to its original position. (A typical Wilhelmy plate force loop for a plasma polymer coated thin glass slide is shown in Figure 2. A detail discussion of this figure will be given later.) Since the contact angle is obtained by force measurement, the sensitivity is very high. Unlike the sessile drop techniques, the accuracy ofthe Wilhelmyplate method can be better than 0.5”and without human reading uncertainty. For the sessile drops and goniometer method, the uncertainty of contact angle is usually f2-5” due to the uncertainty involved in establishing the tangent line.
Results and Discussion Change of Meniscus on Immersion and Emersion Processes. When a thin plate is partially immersed into a liquid vertically, the liquid meniscus either rises (6 90’)or is depressed (0 > 90’)along the vertical wall of the plate. The plasma polymers investigated in this study are all quite hydrophobic; hence, the liquid meniscus is depressed when the plate is immersed. A schematicdescription of the changingmeniscus shape at the contact line within a complete wetting cycle is shown in Figure 1,which depicts the immersion of a sample plate while the actual measurement is done by raising the water level. In the immersion process, the bottom edge of the plate initially contacts with the liquid, and the shape of meniscus changes with the immersion depths. In this transition region (i.e., parts a-c in Figure l), the shape of the meniscus does not fully develop and the liquid is continuously depressed by the plate. The three-phase contact line does not move until the force exceeds the adhesion tension between the (wet) solid and liquid water while the water level is continuously forced to rise (see part d in Figure 1). This minimum tension is a “yield point” and was called the adhesion tension of immersion Ti by Guastalla,12 above which the. contact line starts to move with respect to the sample surface. This adhesion force is mainly due to the strong interaction forcesbetween the surface of plasma polymer and the adjacent water molecules at the solid/liquid interface. As the withdrawal process is started, the three-phase contact line does not move (see parts d and e of Figure 1) until the force exceeds the adhesion tension of emersion z, between the solid/liquid interface (see part f i n Figure 1). However, the adhesion tension in the withdrawal _
_
_
~
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(12)Guastalla, J. Recent work on surface activity, wetting, and dewetting. J. Colloid Sci. 1966,11,623-636.
Figure 1. A schematic description of changing meniscus shape at solid/liquid interface when the plate in immersed into (a-d) or withdrawal from (d-f) the water.
Table 1. Preparation Conditions of Plasma Polymers ~
sample C2F4-1 C2F4-2 CH4-1 CH4-2 CH4-3
HMDSO
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6 130 838 4180 16800 49
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process usually does not have the same magnitude as that in the immersion process unless the surface is initially completely wettable by the liquid (i.e., zero contact angle). During this transition region, the contact angle is decreasing and the shape of meniscus is changing, but the measured force is increasing (due to the higher adhesion tension). Note that during the receding contact angle measurement, the contact line is moved in the opposite direction through the “prewetted surface”, which has previously interacted with water molecules for a different time depending on the position within an immersed portion. Force Loops. In our experiments, dynamic contact angles were measured on a number of different plasma polymers. The preparation conditions of these plasma polymers are listed in Table 1. Figure 2 shows hysteresis of the measured force; i.e., the force measured on immersion is less than that measured on emersion. Most surfaces (e.g., polymeric or plasma polymer surfaces) show such hysteretic behavior. In order to see the effect of the duration of water contact, each sample investigated in this study was taken through two successivewetting runs. In the first run,see Figure 2a, the bottom edge of sample was moved from 5 mm above the water surface to 10 mm below (A B C), then withdrawn to 5 mm below the water level (C D E), then immersed again to 10 mm depth (E F G), and finally withdrawn to 5 mm above the water surface (G H I J). Note that the data points within 5 mm below to 5 mm above water level in the h a l withdrawal and point J are not shown in Figure
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Wang et al.
3890 Langmuir, VoE. 10, No. 10, 1994
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Figure 2. (a) A typical Wilhelmy plate wetting curve of a plasma polymer sample. This sample was prepared by depositing a 20 nm thick CHI plasma polymer layer on both sides of a microscope cover glass and then fluorinating its surface with CzFsplasma for 1min. Based on the ellipsometry data, the thickness ofthisfluorinatedlayer is about 5 nm. (b)Wetting curves of the same sample, but immersion depth is twice as deep as that in part a. Note that there is a step "cd"in the line %e".
2a (the upper figure). In the second run, see Figure 2b (the lower figure),another two successivecycles (e.g.,third and fourth cycles),were performed immediately after the first run was completed. The procedure is as follows: the same sample was immersed from 5 mm above the water level surface to 15 mm below (a b c d e), then withdrawn to 5 mm below the water surface (e f g), then immersed again to 15 mm depth (g h i), and finally withdrawn to 5 mm above the water level surface (i j k 1). Again, the data points within 5 mm below to 5 mm above water level in the final withdrawal and point 1 are not shown in Figure 2b. In order to see the residual effect of previous wetting on contact angle, in the third and fourth cycles the glass slide was intentionally immersed deeper than at the first two cycles. By doing this, the three-phase contact line is moved from a "prewetted" area (0-10 mm) to a "dry and never-wet" area (10-15 mm) of the same sample. And the location at 10 mm height is the intersection line between those two areas. The measured "step change" (segment cd in Figure 2b) can be related to the nonreversible (within the time frame of the experiment) surface change caused by the water contact which took place in the first and the second cycles. In absence of other factors such as swelling of the film and roughness introduced by the wetting with water, this shift can be mainly attributed to the change of surface state (in most cases the surfaceconfiguration change) due to the water immersion. It is worth noting that the segment "de" on cycle 3 in Figure 2b is a continuation of the slope of segment "BC" on cycle 1in Figure 2a. This is not surprising considering that they both are measured from the initially dry surfaces. The segments BC, FG, DE, and HI in Figure 2a are usually called the buoyancy slopes, which are not parallel to each other in this case. This means that they are not "true"
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Figure 3. Corresponding dynamic contact angles vs depth of immersion calculated from data points shown in Figure 1.The contact angles were calculated by virture of eq 2. buoyancy force slopes which are simply due to the change of immersion depths. The surface (immersed section) continuously interacts with the water at the solid/liquid interface. The system is still in the process of approaching a new equilibrium, and consequently the shape of meniscus (Le., contact angle) is still continuously changing with changing depths. Contact Angle Loops. By use of eq 2 and data sets from recorded files, contact angles of a solid surface can be calculated from any point at a loop of force measurement. Therefore, we can draw a loop of contact angle calculated from its corresponding loop of force measurement. For example, parts a and b of Figure 3 are the corresponding contact angle loops calculated from the data shown in parts a and b of Figure 2. However, when the contact angle is greater than go", the liquid meniscus is depressed along the vertical wall of the plate and the three-phase contact line is actually located below the water level surface. The volume of plate actually immersed in the water is less than the calculated one. Hence, the calculated buoyancy force is slightly larger than the actual value. The uncertainty in contact angle due to the error in calculating the buoyancy force based on immersion depth can be determined using the equation
de =ae 6F
a(m
(3)
where dF is the uncertainty in the calculated buoyancy force. It can be expressed as follows.
6F = QgtHGd
(4)
The value of dd is the difference between recorded immersion depth and location of the three-phase contact line. It is proportional to the amount of degrees of contact angle beyond 90".In the present experiments, the above uncertainty resulted in less than 1"for the contact angles in the range of 90-125". Conventionally, the contact angle hysteresis is given as a simple differencebetween the advancing contact angle
Dynamic Contact Angles of Plasma Polymers 140
Langmuir, Vol. 10, No. 10, 1994 3891
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In other words, the cause ofcontact angle hysteresis cannot be solved without elucidating factors which cause these transitional domains. Fluorocarbon Plasma Polymers. Figures 4 and 5 show the contact angle hysteresis loops of CzF4 plasma polymers prepared at two different conditions (see Table 1). The power density parameter W / F W 3is a very useful index for guiding the characterizations of plasma polymers. The properties of created plasma polymer are highly dependent on those operation factors: power input (W), monomer flow rate (F), and molecular weight of monomer (M).
Parts a and b of Figure 4 show the hysteresis loops of the CzF4 plasma polymer (W/FM = 6 MJkg) coated glass slide after immersion in water. Note that the average second advancing angle (115")is smaller than the average &st advancing contact angle (119"). Here, we give the average contact angle value because it is not necessarily (13)Yaauda, H.Plasma Polymerization;Academic Press: New York, 1985.
Wang et al.
3892 Langmuir, Vol. 10, No. 10, 1994
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Dynamic Contact Angles of Plasma Polymers 140
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Figure 6. Dependence of the contact angle cycle for plasma polymers of methane on the power of discharge for plasma polymerization.
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in water for 1 h. The average first advancingand receding angles are 102"and 50")respectively. It is worth noting that the difference between the first and second advancing angles is only 1" to 2". This means that the surface of plasma polymer was closer to the equilibrium with water after 1h of immersion. It should be noted that the true equilibrium (interfacial tension equals zero) cannot be attained in most cases, and the issue is how much the interfacial tension can be minimized. Parts e and f of Figure 4 show the hysteresis loops of the same sample, which was heat treated in an oven at 80 "C for 20 min after the sample was immersed in water for an hour and then cooled down to room temperature for 1 h inside a vacuum desiccator. It clearly shows that both the advancing and the receding angles recover toward the original value after the heat treatment. The average first advancing and receding angles are 115" and 63", respectively. This means that a new distribution of fluorine-containingmoieties, which is close to the original one, can be reached at an elevated temperature in the
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Figure 7. Contact angle hysteresis loops for a plasma polymer of CH, prepared under the conditions of 1 sccm and 10 W (a) the initial cycle; (b)the second cycle measurement with a deeper immersion depth; (c and d) the hysteresis loops of the same sample after 1 h of water immersion, (e and f) the hysteresis loops of the same sample after heat treatment (of immersed sample).
Wang et al.
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Figure 8. Contact angle hysteresis loops for a plasma polymer of C& prepared under the conditions of 1 sccm and 50 W: (a)the initial cycle; (b)the second cycle measurement with a deeper immersion depth; (c and d) the hysteresis loops of the same sample after 1h of water immersion, (e and f) the hysteresis loops of the same sample after heat treatment (of immersed sample).
given time. This does not mean that the hydrophobic groups repelled from the top surface region return to the near original location by the heat treatment. According to this principle, the complete recovery by heat treatment can hardly be achieved. The surface-configuration change is often explained by a simplified model such as rotating ligands or migration of moieties. It is more appropriate, however, to consider the equilibration of surface state of polymer with the surrounding medium. Thus, a particular moiety which left the top surface region is very unlikely to return to the original location. Instead, a new step of redistributing moieties starting from the reconstructed structure would occur when the surrounding medium is changed back to the original one. Parts a and b of Figure 5 show the hysteresis loops of high W/FM (=130 MJkg) CzF4 plasma polymer coated glass slide. The average first advancing angle is 125" and second advancing angle is 111". The difference is 14" and bigger than that for the low WIFM sample in Figure 4. It also means that the change of surface configuration
of high WIFM sample is larger than that of low WIFM sample. Note that the receding angles do not reach a constant value since the contact angles are very dependent on the time in water. In parts c and d of Figure 5, the dynamic contact angles are measured after the same sample has been immersed in water for 1 h. The average first advancing contact angle is 105" and the receding angle is within 45-50". The hysteresis is about 60". In parts e and f of Figure 5, after heat treatment, the surface becomes more hydrophobic again. The average first advancing angle is 112.5" and the receding angle is within 50-55". The "step change" shows up again in Figure 5f a t a depth of 10 mm. However, the magnitude of step change in Figure 5f is smaller than that in Figure 5b. This indicates that the surface state ofplasma polymer is dependent on the history ofthe surface with respect to the contact with liquid water. The surface of the high WIFM sample has larger "step change" than the low WIFM sample surface as a result of water immersion. It seems that the surface-configuration change can be related to the "step change" a t a hysteresis
Dynamic Contact Angles of Plasma Polymers
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Figure 9. Contact angle hysteresis loops for a plasma polymer of CHI prepared under the conditions of 1 sccm, 200 W: (a) the initial cycle; (b) the second cycle measurement with a deeper immersion depth; (c and d) the hysteresis loops of the same sample after 1h of water immersion, (e and 0 the hysteresis loops of the same sample after heat treatment (of immersed sample). loop when wetting line is moved from a “prewetted”area to a “dry” area at the same sample. And the magnitude of step change is proportional to the amount of surfaceconfiguration change. Methane Plasma Polymers. Figure 6 shows the relationships between the contact angles, immersion depth, and WIFM parameter for three different CHI plasma polymer samples. It is clearly shown that both the advancing and the receding contact angles decrease with increasing power input. Also, the magnitude of the hysteresis increases with increasing power input. For all three samples, the difference between the average first and second advancing angles is only 2-3”, much smaller than that for the plasma polymer of fluorocarbon. The smaller difference, in comparison to that for plasma polymers of TFE, is due to the fact that the interfacial tension is smaller. Chemically, the difference can be attributed to a small amount of oxygen-containing groups on the surfaces of plasma polymers. These functional groups are formed by oxidative coupling of residual free radicals on the surface after the plasma deposition.
Figures 7-9 show the dynamic contact angles of CH4 plasma polymers prepared at three different power inputs (also see Table 1). For low power input (10 W), see Figure 8, the advancing and receding angles are constant values through 10 mm depth for three different test cases. For medium and high power input (50 and 200 W), see Figures 9 and 10, the advancing angles are still constant values, but the receding angles are changing with depths. The change can be as large as 10”.After the heat treatment, the contact angles increase slightly for all three samples of plasma polymers of methane. HlvIDSO Plasma Polymers. Parts a and b of Figure 10 show the dynamic contact angles of HMDSO plasma samples. Note that the average first advancing and second advancing angles are very close (almost overlap) and the hysteresis is only 10”. Even after 2 h ofwater immersion, see parts c and d ofFigure 10,the advancing contact angles hardly changed, whereas the receding angles decrease 3”. The same sample was then immersed into water for another 22 h. The measurements for sample after 24 h of water immersion are shown in parts e and f of Figure
Wang et al.
3896 Langmuir, Vol. 10, No. 10, 1994
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Figure 10. Contact angle hysteresis loops for a plasma polymer of HMDSO prepared under the conditionsof 2.22 sccm and 13 W (a) the initial cycle; (b)the second cycle measurement with a deeper immersion depth; (c and d) the hysteresis loops of the same sample after 2 h of water immersion; (e and f) the hysteresis loops of the same sample after 24 h of water immersion; (g and h) the hysteresis loops of the sample after heat treatment; (i and j)the hysteresis loops of the sample aRer 7 days of vacuum desiccator storage.
10. The measured average contact angles for the advancing and receding processes are 104"and 88", respectively.
The wetting behavior of the same sample is then studied again after heat treatment. There is no change for either
Dynamic Contact Angles of Plasma Polymers the advancing or the receding contact angles, shown in parts g and h of Figure 10. The same sample was then stored in a vacuum desiccator for 7 days before the next measurement. Finally, after a long storage time, both the advancing and the receding contact angles recover slightly (105”and 90”); see parts i and j of Figure 10. This indicates that the degree and the rate of surfaceconfiguration change are very small for the HMDSO sample. Because the difference between first advancing and second advancing angles is so small, the ESCA data as expected cannot show any detectable difference for samples before and after water immersion. The data obtained by the Wilhelmy plate method for plasma polymers of TMDSO showed the following significant features of the surface: (1)The measurement cycles repeat nearly identically. The influence of the wetting in the first cycle has no net effect on the subsequent cycles. (2) Constant values (independent of the depth of immersion) for advancing and receding contact angles are found. The difference between these two contact angles is small (roughly lo”,compared to 30”for plasma polymers of methane and 60” for plasma polymers of TFE). (3) No “step change” is observed when the depth of immersion was changed after repeated cycles of measurement. All these factors indicate that plasma polymer of HMDSO has a remarkably stable surface configuration with respect to the contact with liquid water. It should be noted that plasma polymers of HMDSO are highly hydrophobic, and there exists a large interfacial tension between liquid water and the solid surface, nearly as great as the case of plasma polymer of TFE. In other words, there exists the great driving force (and the need to minimize the interfacial tension) to modify the surface configuration. The remarkably small contact angle hysteresis can be attributed to the high level of the stability or unperturbability of the surface configuration. Contact Angle Hysteresis Based on Overall Change of Surface State. According to the concept of the “equilibration of surface statesn,1,2the surface state of a material is always changing according to the conditions of surrounding medium, such as nature of material (air, gas, liquid, or solid), temperature, and relative humidity in the case of air. If the surrounding medium is changed, e.g., from air to liquid water, the surface state of a solid will change to attain an equilibrium with the new surrounding medium in order to minimize the interfacial tension under the new conditions. With respect to parameters which describe the surface state, it may need further consideration. Dealing with highly purturbable or mobile surfaces, such as ofpolymeric materials, the surface configuration, which refers to what atoms and ligands exist at the interface, is an obviously important factor. There is ample evidence that the surface configuration of polymeric surfaces change when surfaces are immersed in liquid water. An inspection of three-dimensional molecular models of the segments of poly(dimethylsi1oxane) and poly(tetrafluoroethylene) suggests that there is a very little possibility to change surface configuration due to the stable preferred conformation of the segment. In other words, no matter how a molecule changes its conformation, the surface configuration does not change much. Accordingly it is anticipated that both poly(dimethylsi1oxane)and poly(tetrafluoroethylene) should have the minimum contact angle hysteresis. However, Wilhelmy plate method applied to Teflon surface shows a significant level of contact angle hysteresis. This suggests that the surface-configuration change is not the sole factor involved in the overall change of the surface state. While the change of surface configuration seems to be
Langmuir, Vol. 10, No. 10,1994 3897 one of the most important factors relevant to the surface state of polymeric surfaces, other factors such as the surface state electron level may play an important role in the cases in which no discernible surface-configuration change occurs. The transfer of surface state electrons and its dependency on their energy level may be explained by the contact electrification of surfaces.l When two dissimilar surfaces contact each other, the transfer of surface state electrons occurs to equilibrate the surface-state electron levels. When two surfaces are separated, each surface retains the equilibrium electron level which has been just attained on the contact, which leads to creation of the static charge, if a material is or both materials are nonconducting. The loss or gain of electrons a t the surface is an important change of the surface state. The transfer of electrons occurs even in the case of contacting liquid water, although its magnitude depends on the separation in the triboelectric series, or the difference of work functions. The creation of surface charge by immersion and emersion of a polymer surface in water has been indeed observed. A polymeric surface which is immersed in water and pulled out, therefore, has a different surface state from that of before the immersion even if no discernible surface-configuration change could occur. Such transfer of electrons to a polymer surface is anticipated to be greatest for PTFE and other fluorinecontaining surfaces because those polymers are situated at the most negative end oftriboelectric series, presumably due to the high electron-withdrawing characteristics of fluorine atom. It is important to understand that in the Wilhelmy plate measurement the advancing and receding contact angles are measured on the same sample surface but with different “surface states”in each cycle ofthe measurement. It is equally important to recognize that the surface state of a polymer is functions of parameters involved in the contacting medium and the contact time. As the surface state changes, the wetting characteristics of the solid surface changes. If there is no surface state change during the immersion and emersion processes (in absence of surface roughness and inhomogeneity), the advancing and receding contact angles must be identical. Therefore, the extent of hysteresis of contact angle is not dependent on how hydrophobic (or hydrophilic) the solid surface initially is, but it is related to how much the surface state qualitatively and/or quantitatively changes during the process of measurement. Within the framework of this study, it can be concluded that the larger the change of the surface state which is caused by contacting liquid water, the greater the hysteresis of the contact angles of water on a polymeric surface. The change of surface state of polymeric surface can be detected by the Wilhelmy plate method in the following aspects of the measurement (1)dependency of the force (or calculated contact angle) on the immersion time; (2) discrepancy between the first immersion and the second immersion data; (3) the separation of the immersion data and the emersion data in a measurement cycle, which is dependent on the magnitude of the transitional domains where the change of meniscus occurs when the sample movement is reversed; (4) the extent of the “step change’’ in the measurement when the depth of immersion is changed after repeated cycle of measurement. Acknowledgment. This study was supported in part by National Science Foundation International Collaboration Project; NSF INT-8722457. The contributions of Mr. Yasuo Matsuzawa and Ms. Yin Lin, who prepared the plasma polymer samples used in this study, are gratefully acknowledged.
