Langmuir 1994,10, 583-585
583
Effect of Water Immersion on Surface Configuration of an Ethylene-Vinyl Alcohol Copolymer Takeshi Yasuda and Masayo Miyama Department of Home Economics, Mukogawa Women's University, Nishinomiya, Hyogo 663, Japan
Hirotsugu Yasuda* Center for Surface Science and Plasma Technology, University of Missouri-Columbia, Columbia, Missouri 65211 Received August 4, 1993. In Final Form: December 3 , 1 9 9 9 Ethylene-vinyl alcohol copolymer (44 mol % ethylene) containsvery hydrophilic -OH groups randomly distributed in very hydrophobic hydrocarbon co-monomer units. The hydrophilicityof surface of a film, therefore, is determined by how many-OH groups are actually located on the surface (surfaceconfiguration). Contact angle of water and O/C analysis by ESCA were used to investigate how surface configuration of film changes as a consequence of immersion of a film in water. The take-off angle dependence of ESCA analysis indicates that many -OH groups are buried in the inner region of the top surface in a dry state. Samples immersed in water (and freeze-dried)show that -OH groups are brought to the top region of the surface and increase the O/C ratio at the top region. The contact angle of water also decreases with water immersion. Because of this surface-configuration change caused by contacting with liquid water, a pronounced hysteresis effect (between advancing and receding contact angles) is also observed.
Introduction Change of surface configuration (actual distribution of moieties at the surface) of polymeric surfaces due to the change of surrounding medium from air to liquid water has been demonstrated by using plasma labeling (incorporation of hydrophobic fluorine containing moieties on the top surface region) in previous studies.'" Such a surface configuration change occurs even during the process of contact angle measurement using a sessile droplet and causes a pronounced hysteresis effect in measurements of advancing and receding angles of water.6 A gelatin gel, which consists of highly hydrophilic denatured protein molecules and a large volume fraction of water, shows high contact angle of water (e.g., 90°) at the gel-air interface. Holly and Refojo6 proposed that hydrophilic moieties in a hydrogel are largely buried in the bulk side of a gel at the gel-air interface, but such a preferred orientation of hydrophilic groups could be easily changed by immersing age1in water. Since then numerous studies dealing with this phenomenon (surface reconstruction) appeared in literature.'j1° In the previous studies which used a CF4 plasma surface labelingtschnique, it was possibleto investigate the kinetic aspect of surface-configuration change because extremely hydrophobic moieties are implanted on the top region of the surface state of a polymer which is considerably less hydrophobic than the implanted moieties. In many Abstract published in Advance ACSAbstracts, February 1,1994. (1) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. J. Polym. Sci., Part B Polym. Phys. 1988,26,1781. (2)Yasuda, T.; Yoshida, K.; Okuno, T.; Yasuda, H. J. Polym. Sci., Part B: Polym. Phys. 1988,26, 2061. (3) Yasuda, H.; Charbon, E. J.; Charbon, E. M.; Yasuda, T.;Miyama, M.; Okuno, T. Langmuir 1991, 7, 2394. (4) Yaeuda, T.: Miyama, M.; Yasuda, H. Langmuir 1992,8, 1425. (6) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (6) Ratner, B. D.; Weathersby, P. K.; Hoffman, A. S.; Kelly, M. A.; Scharpen, L. H. J. Appl. Polym. Sci. 1978,22, 643. (7) Gagnon,D. R.; McCarthy,T. J. J..Appl. Polym. Sci. 1984,29,4335. ( 8 ) Lavielle, L.; Schultz, J. J. Collozd Interface Sci. 1985, 106, 438. (9) Ruckenstein, E.;Gourieankar,S.V. J. Colloid Interface Sei. 1985, 0
107,488.
(10) Ikada, Y.;Mataunaga,T.;Suzuki, M. NipponKagakuKaishi 1985,
1079.
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polymeric surfaces,however, surface-configurationchange may not involve such a pronounced change which can be easily followed by experimental methods. In this study, the same techniques (water contact angle measurement and ESCA analysis of surface) were applied to an ethylene-vinyl alcohol copolymer film. It is our intention to demonstrate that the surface-configuration change due to the change of surrounding medium indeed occurs with polymeric films which are not labeled by CF4 plasma surface treatment.
Experimental Section Materials. Sample films of a copolymer of ethylene-vinyl alcohol (44 mol % ethylene, thickness 30 pm, no surface treatment)were provided by Nippon Gosei Kagaku, KK, Japan. Film were cleaned by acetone and kept in a vacuum desiccator with silica gel before the experimenta. Water Immersion of Films. Purified water (by reverse osmosisand ion exchange)was used and the temperatureof water was controlled (k0.2 "C). In order to avoid the influence of adsorbed water and also change of surface configuration that might occur during the drying process, film,which was immersed for a predetermined time, was frozen by liquid nitrogen for 60 min and freeze-dried at -110 to -113 O C for over 120 min. Contact Angle Measurement with Water Immersed Films. Measurement of contact angle of water immersed films was carried out in a temperature and humidity controlled room (20 O C , 65% relative humidity). A water droplet of 2.7 p L was placed on a surface and a picture of the droplet was taken by a camera. A water droplet was added every 10 s for 5 times (total 13.5 pL) to measure an advancing contact angle. The contact angle was measured using an enlarged image of the developed fiim. By this method, an accurate measurement of contact angle can be measured with controlled advancing rate. Measurement of Advancinga n d ~ d i nContact g Angles of Water. Filmswere conditionedin atemperature and humidity controlled room (20 "C,65% relative humidity) for 3 h before the measurement. A Kyowa Contact Angle Measuring Device, Model CA-A (KyowaKaimen Kagaku, KK, Japan),was used for the measurement of the contact angle of water. A droplet was added every 20 8 and the procedure was repeated 10 times (total 27 pL). Then 2.7 p L of water was withdrawn every 20 s for the measurement of receding contact angle. 1994 American Chemical Society
584 Langmuir, Vol. 10, No. 2, 1994
Yasuda et al.
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Following a generally accepted technique of changing the take-off angle of measurement for the study of the depth-profile of atoms near the surface, the change of O/C ratios due to the water immersion was investigated by changing the take-off angle from 90' to 1 5 O . The results are shown in Figure 4 as a plot of 01JC1, against sin CY, where CY is the take-off angle of ESCA measurement. In a dry film, the concentration of 0 is lower at the top layer and more 0 exists in the deeper region of the surface. This implies that -OH groups tend to be buried in the inner part of the top surface region rather than being exposed at the surface. The concentration of 0 at the top surface (represented by data points at a = 15";the lowest sin CY)increases as a function of immersion time. After 240min of immersion, the depth profiie of 0 atoms changes totally, and the highest concentration is observed at the top surface. The values of 01JC1, measured at a = 1 5 O are shown as a function of temperature of water (60min of immersion) in Table 1. The surface oxygen contents of waterimmersed samplesare higher than that for the dry sample regardless of the temperature of the water. An appreciable increase of the surface oxygen concentration, represented by ESCA measurement at 15' take-off angle, with immersion time is observed with samples immersed in 20 and 40 OC water as shown in Figure 5. The increase of surface oxygen observed in this study can be viewed as a consequence of complex phenomena
Effect of Water Immersion on Surface Configuration 0.35 I
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Langmuir, Vol. 10,No. 2, 1994 585 O/C at the inner layers. It is quite clear that the surface configuration of ethylene-vinyl alcoholcopolymer changes by contacting liquid water at the surface. According to the concept ofthe "equilibration of surface states"?*4the surface configuration of such a copolymer is always changing according to the conditions of surrounding medium, such as temperature and relative humidity. Therefore, the surface configuration of a copolymer depends on the history of a sample, and consequently it is very difficult to establish a reference state for a copolymer in a generic sense. For example, the values of advancing contact angle vary from sample to sample depending on the history of a sample. Even in such a case, the general trend described in this study seems to be found in general cases.
Conclusion The surface configuration change due to the change of surrounding medium observed with the previously reported plasma labeled samples is also found with a copolymer of ethylene-vinyl alcohol without any surface labeling. In this case, -OH groups buried in the inner part of the surface in a dry state is brought out to the surface region on contact with liquid water. It is quite clear that such a change in the surface configuration caused by contacting a surface with liquid water is one of the major cause of hysteresis effect in advancing and receding contact angles observed with polymeric surfaces, which might not be seen with other types of materials such as metals, ceramics, and glasses.
