Langmuir 1994,10, 1606-1614
1606
Dynamic Wetting and Contact Angle Hysteresis of Polymer Surfaces Studied with the Modified Wilhelmy Balance Method Oleg N. Tretinnikovt and Yoshito Ikada* Research Center for Biomedical Engineering, Kyoto University, 53 Kawahora-cho, Shogoin, Sakyo-ku, Kyoto 606, Japan Received June 7,1993. I n Final Form: February 25,1994" The dynamic wetting behavior of poly(tetrafluoroethy1ene)(PTFE),polyethylene (PE),polypropylene (PP),poly(ethy1eneterephthalate) (PET), nylon 6, poly(ether urethane) (PU), poly(viny1alcohol) (PVA), and cellulose was studied by the Wilhelmy balance technique at speeds of immersion from 1to 50 mm/min. The Wilhelmy method was modified so as to determine contact angles without extrapolation of the loop to the zero immersion depth, employing a rectangular flat sample having a rectangular hole. This modification of the method allowed us to determine the advancing and receding contact angles on the very narrow sample area close to the lower (first) and the upper (second) sample-hole boundaries, O1 and 02, respectively. The interaction time of the sample part located at the lower boundary with the wetting liquid (water) was twice as long as that of the upper boundary. No differencewas observed between the advancing contact angles measured at the lower and the upper parts of the sample (Omv,1=OADV,~) for all the polymers, displaying that the dried polymer surfaces had no difference in wettability along the sample length. However, the lower part of the sample became more hydrophilic than the upper part during the wetting measurement for PET, PU, nylon 6, PVA, and cellulose, resulting in the difference between the receding contact angles (OREC 1 < OREC,Z). The effect was attributed to the time-dependent surface reorientation of hydrophilic and hydrophobic groups, occurring upon immersion of the polymer sample in water. A close correlation was observed between the hysteresis of the contact angle and the underwater surface reconstruction of polymers: the strongest hysteresis corresponds to the greatest wettability gradient generated by the time-dependent reorientation process. However, even when the effect of reorientation was zero (PTFE, PE, and PP) or very low (cellulose),the observed hysteresis was still as high as 27'. The contribution of the surface reorientation of polar groups to the observed hysteresis was estimated to amount to 0-25', depending on the chemical structure of the polymer investigated. The speed of the sample immersion had no detectable effect on the wettability of PTFE, PE, and PP. On the other hand, the advancing contact angle on PET, PU, and nylon 6 increased while the receding contact angle decreased, as the immersion speed became higher. This behavior may be accounted for by referring to a model of macromolecular dynamics at the three-phase boundary.
Introduction One of the most sensitive methods that provide information on the outermost polymer surfaces of a few angstroms is contact angles measurement.' From contact angle measurements we can get a deep insight into the properties characteristic of polymer surfaces, such as homogeneity, roughness, cleanliness, hydrophilicityhydrophobicity balance, reorientation of surface groups, et~.~93Numerous methods are currently available for measuring contact angles, including capillary rise, Du Nuoy ring, sessile drop or captive bubble, Wilhelmy balance, laser goniometry, and others.P6 They can be divided into two classes: (1) Static contact angle measurement-a measurement at the solid/liquid interface which is not in motion. The conventional sessile drop and captive bubble techniques are the examples. (2) Dynamic contact angle
* To whom correspondence should be addressed. + Permanent address: Institute of Physics, Academy of Sciences of Belarus, 70 Prospekt Skaryna, Minsk 220072, Belarus. e Abstract published in Advance ACS Abstracts, April 15,1994. (1)Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (2)Good, R. J. In Surface and Colloid Science;Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979;Vol. 11. (3)Andrade, J. D.; Smith, L. M.; Gregonis, D. E. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New Yo&, 1985; Vol. 1, Chapter 7. (4) Neumann, A. W.; Good, R. J. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979;Vol. 11. (5)Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; John Wiley and Sons: New York, 1976;Chapter 1. (6)Uyama, Y.; Inoue, H.; Ito, K.; Kishida, A.; Ikada, Y. J. Colloid Interface Sci. 1991,141, 275.
measurement-a measurement where the liquid front is in motion with respect to the solid surface. Thus, for instance, the Wilhelmy balance method is as a rule a dynamic measurement, although the motion of liquid can be stopped to change to a static measurement.'~8 There is an essential difference between the static and the dynamic measurements because of the effect of the velocity of the liquid-solid-vapor line on the dynamic contact angle.9-13 Eliott and Riddifordgstudied the forced spreading of water between parallel plates of polyethylene and of siliconized glass. At very low velocities the dynamic contact angle remained equal to the static one, but a t a certain critical velocity (about 1mm/min) the contact angle started to rapidly vary and then level off. They ascribed this behavior to a relaxation phenomenon of the water molecules occurring at the three-phase boundary line, in accordance with ideas previously proposed by Hansen and Miotto.lo Andrade et al.3concluded that, at low velocities with normal low-viscosity liquids on relatively rigid (7)Gregonis, D. E.; Hsu, R.; Buerger, D. E.; Smith, L. M.; Andrade, J. D. In Macromolecular Solutions; Seymour, R. B., Stahl, G. A., Eds.; Pergamon Press: New York, 1982. (8)Hsieh, Y.-L.; Wu, M.; Andres, D. J. Colloid Interface Sci. 1991, 144,127. (9)Elliott, G. E. P.; Riddiford, A. C. J. Colloid Interface Sci. 1967,23, 389. (10)Hansen, R. S.; Miotto, M. J.Am. Chem. SOC.1957,79, 1765. (11)Newcombe, G.; Ralston, J. Langmuir 1992,8, 190. (12)Johnson, R. E., Jr.; Dettre, R. H.; Brandreth, D. A. J. Colloid Interface Sci. 1977,62,205. (13)de Gennes, P. G. Rev. Mod. Phys. 1985,57,827.
0743-7463/94/2410-l606$04.50/00 1994 American Chemical Society
Dynamic Wetting and Contact Angle Hysteresis surfaces, there is no difference between the static and the dynamic contact angle. However, Good2 found that the dynamic contact angle on Teflon is still different from the static contact angle for liquids such as decane a t speeds as low as 0.1 mmlmin. Recently, Newcombe and Ralstonll studied forced and spontaneous movements of water between silica surfaces of varied hydrophobicity and found a linear dependence of the difference between the cosines of the dynamic and static angles on the velocity which was varied from 120 to 600 cmlmin. They interpreted the observed behavior in terms of surface friction. Another interesting finding obtained by them is a linear relationship between the cosines of the dynamic and static contact angles a t a constant velocity, which, a t the same time, was different among the surfaces hydrophobized with different agents. From the data obtained, Newcomble and Ralston concluded that the molecular orientation and surface inhomogeneity affected the dynamic contact angle. For velocities of less than 120 cm/min no analysis could be made due to imprecise data, but they suggested that precise and accurate data would be obtained if the Wilhelmy balance technique was used. Contact angle measurements with the Wilhelmy balance method were carried out by Jonson et a1.12 on fluoropolymers and siliconized glass a t speeds of immersion between 0.02 and 25 mm/min. Large velocity effects observed with other techniques were not seen when the Wilhelmy method was used. Moreover, the effect of velocity was not noticed on any homogeneous surfaces. They ascribed the dependence of contact angles on the velocity to surface heterogeneity, The dynamic aspects of wetting have undergone a renaissance since the mid-19809, thanks to an excellent review on the topic of wetting by de Gennes.l3 Both static and dynamic measurements can be carried out in two different manners. In one case, a liquid front advances or is caused to advance over a solid. In the other, a liquid recedes or is caused to recede over a solid surface. Advancing or receding is in progress in the dynamic measurement, while it must be stopped immediately prior to reading in the static measurement. The contact angles measured in these manners are the advancing and the receding contact angles, respectively. The difference between the advancing and the receding contact angles is called the contact angle hysteresis. It is commonly observed for most of solidlliquid pairs with nonzero contact angles? especially on polymer s u r f a ~ e s .There ~ are two classes of hysteresis: thermodynamic and kine ti^.^ Thermodynamic hysteresis on a clean surface is thought to be caused by surface roughness and surface heterogeneity.l"l8 Kinetic hysteresis is usually accounted for by time- or rate-dependent processes such as swelling, penetration of liquid into the surface region, and surface reorientation of functional group^.^^^ Because so many factors are involved in contact angle hysteresis, elucidation of the mechanism is difficult to make. Even in the simplest case of optically smooth and nondeformable surfaces of homopolymer solids the hysteresis has not as yet been answered in spite of numerous studies.1g-22Jonson et a1.12 have considered surface heterogeneity as the primary cause (14) Johnson, R. E., Jr.; Dettre, R. H. Adu. Chem. Ser. 1964,43, 112. (15) Johnson, R. E., Jr.; Dettre, R. H. J.Phys. Chem. 1964,68,1744. (16) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972,38, 341. (17) Eick, J. D.;Good,R. G.;Neumann, A. W. J. Colloid Interface Sci. 1975, 53, 235. (18) Joanny, J. F.; de Gennes, P. G. J. Chem. Phys. 1984,81,552. (19)Penn, L. S.;Miller, B. J. Colloid Znnterface Sci. 1980, 78, 238. (20) Good,R. J.; Kotsidas, E. D. J. Colloid Interface Sci. 1978,66,360. (21) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285.
Langmuir, Vol. 10, No. 5,1994 1607 (a)
vapor
i liquid
't
dl
d,
dmax- "
Figure 1. (a) Sample geometry and notation for contact angle determinationand (b)a typical forceimmersion (emersion)depth recording by the modified Wilhelmy balance method.
of hysteresis on Teflon and methacrylate fluoropolymer. The data obtained by Penn and Mille+ on a number of polymer solids were consistent with the theory which includes the surface chemical heterogeneity as the main source of hysteresis. Good and Kotsidas20 found that the hysteresis of the contact angle on polystyrene is the combined effect of a number of simultaneous causes such as roughness, heterogeneity, orientation, etc. Yasuda et suggested that large hysteresis observed for polymers containing hydrophilic functional groups is mainly due to the high rotational mobility of macromolecules a t the surface, which is much higher than in the bulk of polymer. Baszkin et a1.22investigated the temperature effect on the wettability of oxidized polyethylene films for different polar liquids and found that an increase of chain mobility leading to redistribution of external polar groups initially located a t the solidlair interface took place a t the same temperature as the beginning of the melting transition in the bulk polymer. I t followsthat both the effect of velocity on dynamic wetting and the sources of hysteresis on polymer surfaces are still to be answered. In an attempt to understand the wetting hysteresis, we studied the wetting behavior of various polymers including poly(tetrafluoroethy1ene) (PTFE), polyethylene (PE), polypropylene (PP), poly(ethy1ene terephthalate) (PET), nylon 6, poly(ether urethane) (PU), poly(viny1 alcohol) (22) Baszkin, A.; Nishino, M.; Ter Minassian-Saraga, L. J. Colloid Interface Sci. 1976, 54, 317.
Tretinnikou and Ikada
1608 Langmuir, Vol. 10, No. 5, 1994
(PVA), and cellulose. We measured their contact angles by the Wilhelmy balance technique, which was elaborated so as to determine the contact angle without extrapolation of the loop to the zero immersion depth, employing a rectangular flat sample having a rectangular hole. For such a sample, the contact angle could be determined from the jump of the measured force occurring at the samplehole boundary. Experimental Section Materials. Films of PTFE, PE, PP, PET, PU, PVA, and cellulose with thicknesses ranging from 20 to 100 pm, as well as nylon 6 fibers with a diameter of 0.45 mm were used in this study. They were supplied by their manufacturers in Japan as follows: PTFE (Daikin Inc., Osaka, Japan), PE (Aicello Inc., Toyohashi, Japan), PP (Gunze Ltd., Osaka, Japan), PET (Teijin Co. Ltd., Tokyo, Japan), nylon 6 (Toray Inc., Tokyo, Japan), PU (Nissho Corp.,Osaka, Japan), PVA (AicelloInc., Toyohashi, Japan), and cellulose (Nagoya Gurabia Inc., Nagoya, Japan). All the films and fiberswere purifiedby Soxhlet extraction for 24 h with methyl alcohol and stored over desiccant under a reduced pressure until use. A 20-mg sinker was fashioned from a small piece of platinum wire and cleaned in chromic acid prior to use. Apparatus. An apparatus (Automated System for Dynamic Contact Angle Measurement, ST-1S type) manufactured by Shimadzu Inc., Kyoto, Japan, was used in the present work. All measurements were performed at 25 "C and 60% relative humidity. We used doubly distilled water as a wetting liquid, having a surface tension of 72.1 mN/m which was checked with the same instrument using an Ar plasma cleaned mica. Classical Wilhelmy Balance Technique. When a thin solid plate is partially immersed in a liquid, the liquid either rises or depresses along the vertical wall of the plate, thus exerting a force on the plate. The vertical component of this attractive force is the wetting force (Fw): F, = TLVpCOS e
(1)
where y ~ isv the surface tension of the liquid, P is the perimeter of the plate, 0 is the contact angle between the liquid and solid interface. The measured force on an electrobalance is the sum of gravitational, buoyancy, and interfacial forces: F = F,
+ M g- F b
over the wholesample length. If the surfaceenergy is not uniform, the obtained value is a contact angle averaged over a wide sample area and hence has little significance. Another problem occurs when the Wilhelmy technique is applied to thin films and fibers. In such cases a sinker must be attached to the bottom of the sample for its straightening to overcome the liquid's surface tension, preventing immersion of the sample into the liquid.6," It is very difficult and, sometimes, impossible to precisely keep the sinker buoyancy. Since this buoyancy may be the same order of magnitude as the wetting force on the sample, it must produce a considerable error in determining the contact angle? Modified Wilhelmy Method. We propose here a modified method which can avoid the above-mentioned limitations of the Wilhelmy technique. Let us consider a rectangular flat sample (e.g., a thin polymer film)having a rectangular hole inside it, and a sinker on ita bottom as illustrated in Figure la. First the sample is partially immersed in the wetting liquid and fixed in a static state. We start with the immersion depth corresponding to the static state (point A in Figure lb) which is considered as a zero depth in the dynamic wetting experiment. A t the beginning of the experiment, the system passes from the static to dynamic wetting state (from A to B in Figure lb). Once the dynamic equilibrium is established and the liquid front is below the first sample-hole boundary (d < d l ) , the wetting force is (6)
where P = 2(H + t) and H is the sample width. The subscript ADV denotes advancing wetting measurement. Under the same conditions, eqs 3 and 4 give the buoyancy and the measured force:
where F b 9 is the sum of the buoyancy of the sinker and that of the initidly immersed part of the sample. When the liquid front approaches the lower sample-hole boundary (d = dl - 6, 6
