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Langmuir 1996, 12, 1391-1394
1391
Surface-Configuration Change of CF4 Plasma Treated Cellulose and Cellulose Acetate by Interaction of Water with Surfaces Takeshi Yasuda,† Tsumuko Okuno,† Kaori Tsuji,† and Hirotsugu Yasuda*,‡ Department of Home Economics, Mukogawa Women’s University, Ikebiraki, Nishinomiya, 663, Japan, and Department of Chemical Engineering and Center for Surface Science and Plasma Technology, University of MissourisColumbia, Columbia, Missouri 65211 Received February 27, 1995. In Final Form: November 20, 1995X Surface-configuration change due to the change of contacting medium from (dry) air to liquid water and also from (dry) air to a high relative humidity air was investigated for cellulose and cellulose acetate. CF4 plasma treatment was used to tag the surface, and the change of the ESCA F(1s) signal was followed as a function of the (contact) time after the change of contacting phase. In the water immersion mode, fluorine atoms on cellulose (crystallinity 70-75%) decreased to a plateau value (approximately 9% of the dry sample) within 2 min but that on cellulose acetate (degree of substitution 2.45 and amorphous) decreased to approximately 60% in 15 min. The decrease of fluorine atoms was also observed when films were exposed to a high relative humidity but at much slower rates (roughly 1/10000 of the water immersion case). On the exposure of 10 days, cellulose reached a level of approximately 75%, and cellulose acetate roughly 85%. The decline observed for cellulose was found to be linearly proportional to the moisture uptake (weight percent) of film. The data indicated that the swelling of a polymer plays a passive but significant role in the surface-configuration change of hydrophilic polymers when the surface is exposed to high relative humidity or immersed in liquid water.
Introduction In a previous study dealing with surface-configuration change of a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether (a similar copolymer to TeflonPFA) on water immersion, in which water has the minimal effect on the bulk properties of the polymer, the role of the interfacial tension was demonstrated by lowering the liquid surface tension of water by addition of a small amount of a surfactant or ethanol.1 The system represented an ideal case in which the interaction of (liquid) water and the polymer surface, with respect to the changes of physicochemical properties of polymer solid, is negligibly small. It was also pointed out that the force, which causes the rearrangement or migration of a specific moiety or an atom, is the attractive or repulsive interaction force between the moiety and water molecules, and the differential surface tension, which is the difference between surface energy of the two contacting phases before the environmental change is made, is the driving force for the surface configuration change which minimizes the interfacial tension. The gas or vapor phase was considered as the phase with zero surface energy because no coherent condensed phase exists. In contrast to such an ideal situation for very hydrophobic polymers, the interaction of water molecules with hydrophilic polymers influences the surface state of a polymer strongly, and the surface state cannot be defined without designating the activity of water in the contacting medium. Therefore, when liquid water contacts the dry surface of a hydrophilic polymer, the change of surface state in terms of physicochemical parameters, such as the degree of swelling, may overwhelm the surfaceconfiguration change. In such a case, water vapor changes * Author to whom correspondence should be addressed. † Mukogawa Women’s University. ‡ University of MissourisColumbia. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Yasuda, H.; Okuno, T.; Sawa, Y.; Yasuda, T. Langmuir, 1995, 11, 3255.
the surface state, as recognized by the water sorption or the moisture regain, while the surface contacts the zero surface energy phase. The surface-configuration change may be considered to be minimal for polymer molecules in the crystalline phase. It was demonstrated that the change is proportional to the amorphous content for poly(ethylene terephthalate) films.2 It is, therefore, interesting to examine whether the crystallinity or the extent of water-polymer interaction plays the dominant role in the change of surface configuration when the contacting phase is changed. The interaction of water molecules with polymer molecules at the surface does not necessarily require contact with the condensed (liquid) phase. If the mobility of polymer segments increases with the sorbed water, a significant extent of surface-configuration change could take place even with the change of relative humidity in the contacting air phase. It should be recognized, however, that such a change of surface configuration is not an interfacial phenomenon in a strict sense. In this paper, we have chosen two model samples, cellulose (highly hydrophilic and highly crystalline) and cellulose acetate (amorphous and moderately hydrophilic), and examined the influence of the differential surface tension and that of swelling of samples on the surface configuration change by extending the same experimental procedure for F-tagged surfaces previously employed.2 Experimental Section Materials Used. Cellulose film (thickness 22 µm) and cellulose acetate (degree of substitution 2.45) film (thickness 40 µm) were provided courtesy of Daicell Chemical Co., Japan. The crystallinity of cellulose film was estimated to be 70-75% by X-ray analysis. Cellulose acetate did not show crystalline peaks and can be considered amorphous. Water for immersion as well as for contact angle measurements was purified by the combination of reverse osmosis and ion exchange (electrical resistivity
