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The direct observation of ultrathin polystyrene films adsorbed from cyclohexane onto mica
The Direct Observation of Ultrathin Polystyrene Films Adsorbed from Cyclohexane onto Mica 1 HIROSHI TERASHIMA Institute of Applied Physics, University of Tsukuba, Sakura, Ibaraki 305, Japan Received May 21, 1986; accepted July 30, 1986 This.paper describes an experimental study of ultrathin films of polystyrene adsorbed from cyclohexane at the '0-temperature (307.5 K) onto molecularly smooth surfaces of mica. The amount adsorbed is determined directly using a microbalance (sensitivity -~ 1 0 - 6 g wt) so that adsorbances on the order of 0.2 mg m -2 can be detected on mica sheets of,area 5 × 10-3 m 2. The films may then be floated offthe mica onto a water surface using a suitable marker (Chinese ink) to delineate the shape and area of the transferred film. These results are used to discuss the way in which film structure may vary with the adsorbance of polymer on mica. © 1987AcademicPress, Inc. INTRODUCTION
The structure of polymer layers adsorbed on a solid surface has been extensively studied using a variety of techniques, but generally only at the solid-liquid interface (1). There have been few studies of the polymer films obtained when such adsorbed layers are allowed to collapse onto the adsorbing substrate upon removal of the liquid; yet, such ultrathin films may have considerable technological importance (2-4), and their study may afford insight into the process of the adsorption itself. Recently we reported microbalance studies of the adsorbance r of polymers onto cleaved muscovite mica (5); our measured values of r were in good agreement with those determined independently using an optical technique (6, 7). Because muscovite mica can be cleaved to be smooth on a molecular scale over several square centimeters in area (8, 9), there are many advantages in using fleshly cleaved mica as an adsorbing substrate. For example, the exact surface topology is known and therefore Part of this material was presented at the 5th International Conference on Surface and Colloid Science and the 59th Colloid and Surface Science Symposium, Clarkson University, Potsdam, NY, June 24-28, 1985; Paper No. 39.
the area available for adsorption equals the geometrical area of the mica sheets. In addition, mica has been used extensively over the past several years in studies of surface-surface forces with adsorbed polymers (6, 7, 10-14) so that high value is placed on an independent approach for the study of polymers adsorbed onto the mica surface. In this paper we describe the isolation and study of the ultrathin polymer films formed when polystyrene, adsorbed from solution in cyclohexane at the 0-temperature (307.5 K) onto molecularly smooth mica sheets, is allowed to collapse onto the mica surface upon evaporation of the solvent. We use a microbalance to measure the amount of polymer adsorbed; we then describe a novel technique for imaging the adsorbed films by floating them off onto a water surface covered with a suitable marker (Chinese ink, a stabilized colloidal dispersion of carbon black). We use the results to discuss the structure and coherence of the films thus formed. EXPERIMENTAL
Materials. The linear polystyrene used was a standard sample with a narrow distribution
of molecular weight, supplied by Toyo Soda 523 0021-9797/87 $3.00
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
Copyright © 1987 by Academic press, Inc. All rights of reproduction in any form reserved.
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HIROSHI T E R A S H I M A
Co., Ltd.; the molecular weight is 775,000 and the ratio of the weight average to the number average molecular weight Mw/Mn is 1.01. Cyclohexane, chosen as the 0-solvent, was dried over sodium and distilled prior to use. The mica used as an adsorbing substrate was Indian Ruby Mica, supplied by C. M. Rajgarhia (Giridih, India), grade No. 4 and quality C1 and SS (clear and slightly stained). Adsorption experiments. To determine the adsorbance of polystyrene on mica surfaces at different stages of adsorption, it was measured as a function of time by a microbalance technique. A fused quartz torsion microbalance was constructed for this purpose (5). Its performance is as follows: the balance beam to which a small mirror is fastened is suspended by two fine quartz fibers stretched horizontally; the diameter of the fibers is around 40 #m. A sample mica sheet is hung from one end of the beam while the other end is counterbalanced. An increase in weight resulting from the polymer adsorption onto the mica causes the balance beam to be deflected. The deflection is observed by reflecting a light from the small mirror onto a scale 2 m away. To examine whether the deflection is in equilibrium, the balance beam is swung after the image of light comes to a standstill and its position on the scale is noted; this procedure is repeated several times. The displacement of the image of light on the scale by 1 mm was calibrated by known weights made of tungsten wire (10 um in diameter) so as to correspond to the change in weight of 1.07 × 10-3 mg. As mica sheets of 5 × 10-3 m 2 in total area are used and the uncertainty in the scale reading is +0.5 mm, the displacement by 1 mm corresponds to a change in adsorbance by 0.2 + 0.1 mg m -2. The procedure of the measurement is as follows. Mica sheets of ca. 0.5 mm in thickness were cut square (50 × 50 ram) and cleaved in air down to some 10 #m in thickness. Freshly cleaved mica sheets, after a short rinse in pure cyclohexane (this rinse is necessary to ensure the "zero" position as described later), were immersed in a polymer solution, which was Journal of Colloid and Interface Science,
Voi. 117, No. 2, June
1987
stirred gently throughout the immersion. After a prescribed time of adsorption, the mica sheets were rapidly transferred (so as to minimize solvent evaporation) to a large volume of pure cyclohexane and kept there for over 30 min to rinse out the free polymer solution remaining on the mica surface; the pure cyclohexane was also stirred. The mica sheets were then picked out from the cyclohexane and air-dried. The weight of each mica sheet was measured by the microbalance both before and after the adsorption treatment. The difference in weight, divided by the surface area of the mica sheet, gives the adsorbance. The 30-min rinsing time was shown to be sufficient to completely wash away free (unadsorbed) polymer from the mica surface. This is clearly shown in Fig. 1, where the excess weight of polymer remaining on mica surface is plotted as a function of rinsing time. Six sheets of mica were immersed at the same time in a polymer solution of 0.13 mg ml-I for 18 h and then transferred to pure cyclohexane for rinsing. They were withdrawn from the cyclohexane after different rinsing times. In par-
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FIG. 1. The excess weight of mica sheet divided by the surface area, i.e., the a m o u n t of polystyrene remaining per unit area of mica sheet as a function of the duration time of the rinse in cyclohexane. Six sheets of mica, onto which polystyrene was adsorbed from a solution of0.13 m g ml -~ at the 0-temperature (307.5 K) for 18 h, were transferred to cyclohexane and withdrawn after different duration times of rinse. Six different marks correspond to each of the mica sheets. One of t h e m ( e ) was multiply rinsed; the excess weights determined at every rinse are plotted by open circles (©) as a function of the total time of the rinse.
ULTRATHIN POLYMER FILMS
ticular, one of them was multiply rinsed; i.e., the treatment of rinsing for a short time followed by drying was repeated, and the excess weight obtained at every rinsing treatment was plotted against the total time of rinse as shown by open circles in Fig. 1. It is seen that unadsorbed polymer is washed away within several minutes and polymer adsorbed firmly on mica surfaces without further desorption remains for up to more than 20 h. Figure 2 shows the experimental results of the adsorbance P measured as a function of time at a solution concentration of 0.005 mg m1-1 and at the 0-temperature of 307.5 K. The axis of abscissa is on a logarithmic scale to cover the wide range of the time of adsorption, while a linear plot (inset to Fig. 2) shows quasiplateau behavior. The polystyrene films labeled P, Q, R, S, and T in Fig. 2 were removed from mica surfaces and observed visually by TIME (hours) 10 100
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TIME (s) FIG. 2. Adsorbance I' of polystyrene (mol vet 775,000), adsorbed from a cyclohexane solution of 0.005 mg m1-1 onto mica at the 0-temperature (307.5 K), as a function of time. Mean thickness d was calculated assuming the bulk density to be 1.05 g cm -3. The letters P, Q, R, S, and T denote the adsorbances of the removed films shown in Figs. 5a, 6a, 6b, 8, and 9, respectively. The inset shows a linear plot of F versus time.
525
a method explained in the next section. To show the mean thickness of the removed films, the ordinate is also marked in nanometers; the mean thickness is calculated assuming the bulk density to be 1.05 g c m -3 (15). We note in passing that a similar increase in P on a loglinear plot has been observed by other workers (16, 17).
Visual observation of polystyrene films. Polystyrene films adsorbed on a mica surface are floated off the mica onto a water surface and the shape and area of the removed polystyrene films are observed visually. Polystyrene films less than some nanometers in mean thickness formed in the initial stages of adsorption are too thin to be visible to the naked eye. To avoid this difficulty, a simple technique has been developed for visualizing such ultrathin polystyrene films. The key point of this method is to spread a film of Chinese ink on a surface of distilled water prior to the removal of polystyrene films. As soon as a drop of Chinese ink is touched to the water surface, the ink spreads rapidly over the water surface to form a film. The area and thickness of the ink film can be controlled by adjusting the amount of ink. Since Chinese ink is a colloidal dispersion of fine lampblack powder stabilized by a glue of which the main component is gelatin, the ink film consists of a two-dimensional array of fine carbon particles, which are mobile on the water surface; the diameter of the carbon particles, namely the thickness of ink film, is on the order of 0.1 tzm (18). A mica sheet bearing polystyrene film, of which a comer is held in tweezers, is inserted vertically into the ink film as shown in Fig. 3a. The polystyrene films are floated off onto the water surface from both sides of the mica sheet. The mica sheet must be immersed sufficiently slowly so that the removed polystyrene film is not broken; this is the most delicate part of the procedure. The removed polystyrene film pushes aside the carbon particles to make a clear region in the ink film, which corresponds to the shape of the polystyrene film (Fig. 3b). The pattern of the clear region can be conveniently copied down on Journal of Colloid and Interface Science, VoL 117,No. Z June 1987
526
HIROSHI TERASHIMA INSERTION LINE FILM OF CHINESE INK ONjWATER
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MICA SHEET IN WATER
FIG. 3. Schematic illustration of the method of removing adsorbed polystyrene films from a mica surface. (a) A mica sheet beating adsorbed polystyrene films is inserted into water where the surface is covered by a film of Chinese ink. The polystyrene film is floated off onto the water surface to form a clear region in the ink film as the mica sheet enters the water. (b) After the mica sheet sinks in the water, there remain clear regions showing the existence of polystyrene films on the water surface.
paper (a piece of Japanese paper for calligraphic use is especially suitable) if the paper is applied gently on the water surface covered with the ink film. In this way, we can determine the shape and area of the removed polystyrene films from the pattern of the clear region on the paper. A preparatory experiment. The change in the weight of the mica sheet was measured when it was immersed in polymer-free cyclohexane and distilled water. This measurement was carried out, first, to ensure the zero position of the adsorption measurement, i.e., the position of the image of light on the scale corresponding to the case where no polymer adsorption takes place, and second, to check the
removal of adsorbed polystyrene films from mica surfaces. The result is shown in Fig. 4, where the positions of the image of light projected on the scale are plotted in the sequence of the following measurements. A sheet of freshly cleaved mica was hung from the microbalance (position labeled A in Fig. 4). The mica sheet was immersed in polymer-free cyclohexane for 7 min, then withdrawn, and dried (position B). This treatment was carded out once more. The weight of the mica sheet was found to be unchanged (position C). This means that the zero position is ensured once mica sheets are rinsed with cyclohexane prior to the polymer adsorption. After the above measurement, the
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FIG. 4. Change in weight of a mica sheet shown by the reading of the microbalance scale noted in sequence after (A) cleavage, (B) immersion of the mica sheet in polymer-free cyclohexane, (C) subsequent immersion in the cyclohexane, (D) rinse in water, (E, F) subsequent rinses in water, (G) adsorption of polymer, and (H) removal of adsorbed polymer films from the mica sheet.
Journal of Colloid and InterfaceScience, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS mica sheet was rinsed in distilled water for a few seconds. The weight decreased by 3.4 mg m -2 (position D). Such a decrease was also observed for other mica sheets whenever they were rinsed in water, but its amount differed among different samples. We do not know the cause for this decrease at the present. In the usual removing procedure, therefore, it is impossible to examine the completeness of the removal of polymer films by measuring the change in the weight of the mica sheets. The rinse in water was then repeated for this mica sheet. As seen from a comparison between positions E and F, the weight of the mica sheet did not change during the repetition of the additional rinse in water. As a test for removal, the polystyrene films were adsorbed onto this mica sheet under the conditions of a solution concentration of 0.005 mg m1-1, a duration time of adsorption of 16 h, and a rinse in pure cyclohexane for 1 h. After measurement of adsorbance (position G), the polystyrene films were removed. The weight of the mica sheet removed from the water and dried was found to return its original value although the positions were scattered accidentally in this case (position H). RESULTS AND DISCUSSION
Imaging of polystyrene films on a water surface. Figure 5a shows a photograph of the paper on which the pattern of the clear region was transferred from the water surface covered with an ink film; this pattern corresponds to a polystyrene film of adsorbance 1.3 ___0.1 mg m -2, labeled P in Fig. 2. The mica sheet was inserted vertically into the water along the dotted line drawn in Fig. 5a. Two separate clear regions are seen on both sides of the dotted line. For the sake of comparison, the mica sheet from which the polystyrene film was removed was placed over the pattern at the left; as seen in Fig. 5a, the shape and area of the clear region closely mimic those of the mica sheet. It is notable that the irregular shape of the mica sheet appears again in the pattern.
527
Because of this connection, it is instructive to describe the observation of another polystyrene film ofadsorbance around 2 mg m -2. After this film was floated off onto a water surface, an edge of the clear region was pushed lightly by the tip of clean tweezers. The whole clear region was found to move in parallel with the direction of the push; the clear region behaved like a thin solid film floating on water. These qualitative observations, while they cannot directly give detailed information on the structure of the film, do suggest that it is continuously connected over the entire adsorbed area. In addition, it may be of interest to show the pattern of the clear region obtained in the case where no polymer is adsorbed on a mica surface. A freshly cleaved mica sheet was inserted vertically into an ink film on water. Immediately after the mica sheet sank below the surface of the water, a narrow clear region of a few millimeters in width appeared along the line of insertion and began to change to a vortexlike shape owing to the flow of water (Fig. 5b). Very similar patterns were also observed in the case where a cleaved mica sheet was rinsed in pure cyclohexane and air-dried prior to insertion into ink film, implying that cyclohexane evaporates essentially completely (leaving no residue) from the bare mica surfaces during the drying process. Polystyrene films in the initial stages of adsorption. T h e present method may give some information on the initial structural state of adsorbed polystyrene films, in particular the adsorbance at which adsorbed polystyrene molecules form a continuous film. The clear region shown in Fig. 6a corresponds to a polystyrene film obtained at an adsorption time of 30 s, labeled Q in Fig. 2. Although the adsorbance of this film was below the detection limit of the microbalance, it was estimated from the data shown in Fig. 2 at somewhat less than 0.2 mg m -2. The dotted line drawn across the clear region shows the line along which the mica sheet was inserted. On both sides of this line, clear regions Journal of Colloid and Interface Science, Vol. ! 17, No. 2, June 1987
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HIROSHI TERASHIMA
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FIG. 5. (a) The pattern of clear region corresponding to a polystyrene film of 1.3 _+ 0.1 mg m - 2 in adsorbance. The mica sheet from which the polystyrene film was removed was placed on the printed paper over the pattern at the left. The dotted line shows the position where the mica sheet was inserted normal to the water surface. (b) The pattern of clear region appearing in the ink film in case where no polymer was adsorbed on mica. Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS
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FIG. 6. The pattern of clear region corresponding to polystyrene films at adsorbances of (a) less than 0.2 mg m -2 and (b) 0.4 + 0.1 mg m -2. The mica sheets used were placed over the pattern. The dotted line indicates the position of insertion of mica sheets into water. Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
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HIROSHI TERASHIMA
are seen to appear. But, in contrast to the pattern shown in Fig. 5a, they are not separated into two square regions but are incorporated into one region. This pattern is clearly different from that shown in Fig. 5b where no polymer is adsorbed, suggesting that s o m e polystyrene molecules are floated off onto the water surface. A possible explanation of the origin of the appearance of such a clear region as shown in Fig. 6a may be as follows: In the initial stage of adsorption, i.e., Q in Fig. 2, isolated polystyrene molecules are adsorbed randomly on the surface, but do not cover it entirely. When the adsorbed film is floated off onto the water surface, the polystyrene molecules are desorbed individually from the mica surface to form a two-dimensional surface film hemmed in by the ink film and subsequently (under the slight counterpressure of the ink film) coalesce to each other on the surface of the water. The clear region in Fig. 6b corresponds to a polystyrene film of adsorbance 0.4 + 0.1 mg m -2 labeled R in Fig. 2: two separate clear regions appeared on both sides of the line of insertion of the mica sheet. The mica sheet from which the polystyrene film was removed was placed over the pattern at the right transferred to the paper. It is seen that the pattern keeps the original shape of the mica as in Fig. 5a. These observations suggest that the polystyrene molecules are built into a continuously connected film at the stage where the adsorbance attains a value F = 0.4 mg m -2, though we cannot say what the detailed film structure
a
_
is. The area of the clear region was found to be reduced to 80% of that of the mica (see the pattern on the right-hand side in Fig. 6b). The cause of the reduction in area will be considered briefly below. Very crudely, one may hypothesize that in the initial stages polymer molecules from the solution are uniformly adsorbed to occupy adjacent "cells" on the mica surface, each of radius Rg (radius of gyration) as indicated in Fig. 7a. Eventually all such "cells" become occupied (corresponding to F = 0.6 mg m -2 for this polymer, where tool wt 775,000 and Rg = 25 nm (19)). Upon drying the surface polymer molecules presumably collapse (Fig. 7b) and a continuously connected meshlike network is formed (where continuity is provided by overlap and entanglements between neighboring segments in adjacent cells). The small shrinkage of the film ('~20% relative to the size of the original mica) can then be accommodated by a uniform compression of the meshlike structure. A more marked effect indicating the meshlike structure is shown in Fig. 8. In this case, a sufficient amount of Chinese ink was added so as to cover the whole surface of the water in the trough. A polystyrene film of 0.6 + 0.1 mg m -E, labeled S in Fig. 2, was then floated off as described previously. As the mica sheet was inserted into the water along the dotted line drawn in Fig. 8, the clear region extended in the direction normal to the plane of the mica sheet. As seen from Fig. 8, the clear region shrank considerably in the direction nor-
q
21:~__ FIG. 7. Two-dimensional array of (a) the random coils of polystyrene and (b) their collapsed state. The surface of adsorbing substrate is partitioned into closely packed circular cells of which the radius is equal to the radius of gyration, and the random coils of polystyrene are shared out one by one to each of the cells.
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS
531
FIG. 8. The pattern of clear region corresponding to a polystyrenefilm of 0.6 + 0.1 mg m-2, which was removed under the condition where the whole surface of water in the trough was covered with Chinese ink film. The mica sheet indicates the place where a clear region would appear if the polystyrenefilm were not shrunk. The meaning of the dotted line is the same as before. mal to the dotted line, while remaining essentially unchanged parallel to it. This deformation probably occurred because the m o v e m e n t of the ink film, as the polymer film floated off, was resisted by the wall of the trough, and the effective resulting surface pressure (normal to the dotted line) then led to considerable compression of the polymer film. The mica sheet placed over the pattern shown in Fig. 8 indicates the original size of the polystyrene film. A closer observation of the pattern reveals that the original shape, especially the corner shape, of the mica sheet is retained "affinely" in the clear regions despite the large deformation. This supports the idea that the deformation is attributable to the compression of the meshlike polystyrene film normal to the plane of the mica sheet as it is slowly immersed in the water rather than to the buckling and doubling up of the film as
it pushed against the ink film. Figure 9 shows what happens when a m u c h thicker film of adsorbed polystyrene ( r = 3.4 mg m -2, labeled T in Fig. 2) is floated off under the same conditions: no distortion comparable to that in Fig. 8 is observed, suggesting that the meshlike structure has become considerably more closely packed. CONCLUSIONS AND FUTURE WORK We have shown that it is possible to create ultrathin polymer films by allowing polymer layers adsorbed at a solid-liquid interface to collapse on the adsorbing substrate upon removal of the solvent. By floating these films off the solid substrate onto a water surface covered with a film of Chinese ink consisting of stabilized carbon black particles, we were able to isolate and to directly "view" such films Journal of Colloid and Interface Science, V ol. 117, No. 2, June 1987
532
HIROSHI TERASHIMA
FIG. 9. The pattern of clear region corresponding to a polystyrene film of 3.4 +__0.1 nag m -2, which was removed under the same condition as that shown in Fig. 8.
for the first time. Our results suggest that, for the adsorbance of polystyrene (mol wt 775,000) onto mica from cyclohexane at the 0-temperature, (i) polymer adsorbed at adsorbances P < 0.2 mg m -2 is not continuously connected when collapsed on the surface, (ii) for F > 0.4 mg m -2 continuously connected films are formed when the adsorbed layer is collapsed on the mica surfaces, (iii) the films, when floated off onto water, appear to have an altinely deformable two-dimensional meshlike structure for F = 0.4 or 0.6 mg m -2 and to be (qualitatively) considerably more rigid at higher F values. The present technique of removing polystyrene films adsorbed on mica was developed originally in the hope of preparing the specimens for electron microscope observation. This attempt was not successful because the Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
polystyrene films concerned were too thin and fragile to be mounted on electron microscope grids. An ingenious method for making the specimens is still sought. Further studies for acquiring information relative to the strength and structure characteristics of these ultrathin films are currently in progress. ACKNOWLEDGMENTS The author is grateful to Professor J. Klein for encouragement and for valuable advice on both the experiment and the preparation of the manuscript, to Professor D. Tabor for useful remarks on the draft, and to Professor S. Fujiwara for encouragement and for critical reading of the draft. REFERENCES 1. Rickert, S. E., Balik, C. M., and Hopfinger, A. J., Adv. Colloid Interface Sci. 11, 149 (1979). 2. Vincett, R. S., and Roberts, G. G., Thin Solid Films 68, 135 (1980). 3. Tredgold, R. H., and Winter, C. S., Thin Solid Films 99, 81 (1983).
ULTRATHIN POLYMER FILMS 4. Larkins, G. L., Jr., Thompson, E. G., Ortiz, E., Burkhart, C. W., and Lando, J. B., Thin Solid Films 99, 277 (1983). 5. Terashima, H., Klein, J., and Luckham, P. F., in "'Adsorption from Solution" (R. H. Ottewill, C. H. Rochester, and A. L. Smith, Eds.), p. 299. Academic Press, London/New York, 1983. 6. Klein, J., Nature (London) 288, 248 (1980); J. Chem. Soc. Faraday Trans. 1 79, 99 (1983). 7. Israelachvili, J. N., Tirrell, M., Klein, J., and Almog, Y., Macromolecules 14, 204 (1984). 8. Tolansky, S., "Multiple-Beam Interferometry of Surfaces and Films," p. 123. Dover, New York, 1970. 9. Bailey, A. I., and Courtney-Pratt, J. S., Proc. R. Soc. London Ser. A 227, 500 (1955). 10. Klein, J., and Luckham, P. F., Nature (London) 300, 429 (1982); Macromolecules 17, 1041 (1984). 11. Luckham, P. F., and Klein, J., Colloids Surf. 10, 65
12. 13. 14. 15. 16. 17. 18. 19.
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(1984); J. Chem. Soc. Faraday Trans. 1 80, 865 (1984). Klein, J., and Luckham, P. F., Nature (London) 308, 836 (1984). Almog, Y., and Klein, J., J. ColloidlnterfaceSci. 106, 33 (1985). Luckham, P. F., and Klein, J., Maeromoleeules 18, 721 (1985). Brandrup, J., and Immergut, E. H. (Eds.), "Polymer Handbook," p. V-59. Wiley, New York, 1975. Grant, W. H., Smith, L. E., and Stromberg, R. R., Disc. Faraday Soc. 59, 209 (1975). Takahashi, A., Kawaguchi, M., Hirota, H., and Kato, Y., Macromolecules 13, 884 (1980). Terada, T., Yamamoto, R., and Watanabe, T., Proc. Imp. Acad. 10, 10 (1934). Brandrup, J., and Immergut, E. H. (Eds.), "Polymer Handbook," p. IV-40. Wiley, New York, 1975.
Journal of Colloid and InteoCaceScience, Vol. 117, No. 2, June 1987
The structure of polymer layers adsorbed on a solid surface has been extensively studied using a variety of techniques, but generally only at the solid-liquid interface (1). There have been few studies of the polymer films obtained when such adsorbed layers are allowed to collapse onto the adsorbing substrate upon removal of the liquid; yet, such ultrathin films may have considerable technological importance (2-4), and their study may afford insight into the process of the adsorption itself. Recently we reported microbalance studies of the adsorbance r of polymers onto cleaved muscovite mica (5); our measured values of r were in good agreement with those determined independently using an optical technique (6, 7). Because muscovite mica can be cleaved to be smooth on a molecular scale over several square centimeters in area (8, 9), there are many advantages in using fleshly cleaved mica as an adsorbing substrate. For example, the exact surface topology is known and therefore Part of this material was presented at the 5th International Conference on Surface and Colloid Science and the 59th Colloid and Surface Science Symposium, Clarkson University, Potsdam, NY, June 24-28, 1985; Paper No. 39.
the area available for adsorption equals the geometrical area of the mica sheets. In addition, mica has been used extensively over the past several years in studies of surface-surface forces with adsorbed polymers (6, 7, 10-14) so that high value is placed on an independent approach for the study of polymers adsorbed onto the mica surface. In this paper we describe the isolation and study of the ultrathin polymer films formed when polystyrene, adsorbed from solution in cyclohexane at the 0-temperature (307.5 K) onto molecularly smooth mica sheets, is allowed to collapse onto the mica surface upon evaporation of the solvent. We use a microbalance to measure the amount of polymer adsorbed; we then describe a novel technique for imaging the adsorbed films by floating them off onto a water surface covered with a suitable marker (Chinese ink, a stabilized colloidal dispersion of carbon black). We use the results to discuss the structure and coherence of the films thus formed. EXPERIMENTAL
Materials. The linear polystyrene used was a standard sample with a narrow distribution
of molecular weight, supplied by Toyo Soda 523 0021-9797/87 $3.00
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
Copyright © 1987 by Academic press, Inc. All rights of reproduction in any form reserved.
524
HIROSHI T E R A S H I M A
Co., Ltd.; the molecular weight is 775,000 and the ratio of the weight average to the number average molecular weight Mw/Mn is 1.01. Cyclohexane, chosen as the 0-solvent, was dried over sodium and distilled prior to use. The mica used as an adsorbing substrate was Indian Ruby Mica, supplied by C. M. Rajgarhia (Giridih, India), grade No. 4 and quality C1 and SS (clear and slightly stained). Adsorption experiments. To determine the adsorbance of polystyrene on mica surfaces at different stages of adsorption, it was measured as a function of time by a microbalance technique. A fused quartz torsion microbalance was constructed for this purpose (5). Its performance is as follows: the balance beam to which a small mirror is fastened is suspended by two fine quartz fibers stretched horizontally; the diameter of the fibers is around 40 #m. A sample mica sheet is hung from one end of the beam while the other end is counterbalanced. An increase in weight resulting from the polymer adsorption onto the mica causes the balance beam to be deflected. The deflection is observed by reflecting a light from the small mirror onto a scale 2 m away. To examine whether the deflection is in equilibrium, the balance beam is swung after the image of light comes to a standstill and its position on the scale is noted; this procedure is repeated several times. The displacement of the image of light on the scale by 1 mm was calibrated by known weights made of tungsten wire (10 um in diameter) so as to correspond to the change in weight of 1.07 × 10-3 mg. As mica sheets of 5 × 10-3 m 2 in total area are used and the uncertainty in the scale reading is +0.5 mm, the displacement by 1 mm corresponds to a change in adsorbance by 0.2 + 0.1 mg m -2. The procedure of the measurement is as follows. Mica sheets of ca. 0.5 mm in thickness were cut square (50 × 50 ram) and cleaved in air down to some 10 #m in thickness. Freshly cleaved mica sheets, after a short rinse in pure cyclohexane (this rinse is necessary to ensure the "zero" position as described later), were immersed in a polymer solution, which was Journal of Colloid and Interface Science,
Voi. 117, No. 2, June
1987
stirred gently throughout the immersion. After a prescribed time of adsorption, the mica sheets were rapidly transferred (so as to minimize solvent evaporation) to a large volume of pure cyclohexane and kept there for over 30 min to rinse out the free polymer solution remaining on the mica surface; the pure cyclohexane was also stirred. The mica sheets were then picked out from the cyclohexane and air-dried. The weight of each mica sheet was measured by the microbalance both before and after the adsorption treatment. The difference in weight, divided by the surface area of the mica sheet, gives the adsorbance. The 30-min rinsing time was shown to be sufficient to completely wash away free (unadsorbed) polymer from the mica surface. This is clearly shown in Fig. 1, where the excess weight of polymer remaining on mica surface is plotted as a function of rinsing time. Six sheets of mica were immersed at the same time in a polymer solution of 0.13 mg ml-I for 18 h and then transferred to pure cyclohexane for rinsing. They were withdrawn from the cyclohexane after different rinsing times. In par-
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FIG. 1. The excess weight of mica sheet divided by the surface area, i.e., the a m o u n t of polystyrene remaining per unit area of mica sheet as a function of the duration time of the rinse in cyclohexane. Six sheets of mica, onto which polystyrene was adsorbed from a solution of0.13 m g ml -~ at the 0-temperature (307.5 K) for 18 h, were transferred to cyclohexane and withdrawn after different duration times of rinse. Six different marks correspond to each of the mica sheets. One of t h e m ( e ) was multiply rinsed; the excess weights determined at every rinse are plotted by open circles (©) as a function of the total time of the rinse.
ULTRATHIN POLYMER FILMS
ticular, one of them was multiply rinsed; i.e., the treatment of rinsing for a short time followed by drying was repeated, and the excess weight obtained at every rinsing treatment was plotted against the total time of rinse as shown by open circles in Fig. 1. It is seen that unadsorbed polymer is washed away within several minutes and polymer adsorbed firmly on mica surfaces without further desorption remains for up to more than 20 h. Figure 2 shows the experimental results of the adsorbance P measured as a function of time at a solution concentration of 0.005 mg m1-1 and at the 0-temperature of 307.5 K. The axis of abscissa is on a logarithmic scale to cover the wide range of the time of adsorption, while a linear plot (inset to Fig. 2) shows quasiplateau behavior. The polystyrene films labeled P, Q, R, S, and T in Fig. 2 were removed from mica surfaces and observed visually by TIME (hours) 10 100
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TIME (s) FIG. 2. Adsorbance I' of polystyrene (mol vet 775,000), adsorbed from a cyclohexane solution of 0.005 mg m1-1 onto mica at the 0-temperature (307.5 K), as a function of time. Mean thickness d was calculated assuming the bulk density to be 1.05 g cm -3. The letters P, Q, R, S, and T denote the adsorbances of the removed films shown in Figs. 5a, 6a, 6b, 8, and 9, respectively. The inset shows a linear plot of F versus time.
525
a method explained in the next section. To show the mean thickness of the removed films, the ordinate is also marked in nanometers; the mean thickness is calculated assuming the bulk density to be 1.05 g c m -3 (15). We note in passing that a similar increase in P on a loglinear plot has been observed by other workers (16, 17).
Visual observation of polystyrene films. Polystyrene films adsorbed on a mica surface are floated off the mica onto a water surface and the shape and area of the removed polystyrene films are observed visually. Polystyrene films less than some nanometers in mean thickness formed in the initial stages of adsorption are too thin to be visible to the naked eye. To avoid this difficulty, a simple technique has been developed for visualizing such ultrathin polystyrene films. The key point of this method is to spread a film of Chinese ink on a surface of distilled water prior to the removal of polystyrene films. As soon as a drop of Chinese ink is touched to the water surface, the ink spreads rapidly over the water surface to form a film. The area and thickness of the ink film can be controlled by adjusting the amount of ink. Since Chinese ink is a colloidal dispersion of fine lampblack powder stabilized by a glue of which the main component is gelatin, the ink film consists of a two-dimensional array of fine carbon particles, which are mobile on the water surface; the diameter of the carbon particles, namely the thickness of ink film, is on the order of 0.1 tzm (18). A mica sheet bearing polystyrene film, of which a comer is held in tweezers, is inserted vertically into the ink film as shown in Fig. 3a. The polystyrene films are floated off onto the water surface from both sides of the mica sheet. The mica sheet must be immersed sufficiently slowly so that the removed polystyrene film is not broken; this is the most delicate part of the procedure. The removed polystyrene film pushes aside the carbon particles to make a clear region in the ink film, which corresponds to the shape of the polystyrene film (Fig. 3b). The pattern of the clear region can be conveniently copied down on Journal of Colloid and Interface Science, VoL 117,No. Z June 1987
526
HIROSHI TERASHIMA INSERTION LINE FILM OF CHINESE INK ONjWATER
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CLEAR REGION
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POLYSTYRENE FILM
MICA SHEET IN WATER
FIG. 3. Schematic illustration of the method of removing adsorbed polystyrene films from a mica surface. (a) A mica sheet beating adsorbed polystyrene films is inserted into water where the surface is covered by a film of Chinese ink. The polystyrene film is floated off onto the water surface to form a clear region in the ink film as the mica sheet enters the water. (b) After the mica sheet sinks in the water, there remain clear regions showing the existence of polystyrene films on the water surface.
paper (a piece of Japanese paper for calligraphic use is especially suitable) if the paper is applied gently on the water surface covered with the ink film. In this way, we can determine the shape and area of the removed polystyrene films from the pattern of the clear region on the paper. A preparatory experiment. The change in the weight of the mica sheet was measured when it was immersed in polymer-free cyclohexane and distilled water. This measurement was carried out, first, to ensure the zero position of the adsorption measurement, i.e., the position of the image of light on the scale corresponding to the case where no polymer adsorption takes place, and second, to check the
removal of adsorbed polystyrene films from mica surfaces. The result is shown in Fig. 4, where the positions of the image of light projected on the scale are plotted in the sequence of the following measurements. A sheet of freshly cleaved mica was hung from the microbalance (position labeled A in Fig. 4). The mica sheet was immersed in polymer-free cyclohexane for 7 min, then withdrawn, and dried (position B). This treatment was carded out once more. The weight of the mica sheet was found to be unchanged (position C). This means that the zero position is ensured once mica sheets are rinsed with cyclohexane prior to the polymer adsorption. After the above measurement, the
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FIG. 4. Change in weight of a mica sheet shown by the reading of the microbalance scale noted in sequence after (A) cleavage, (B) immersion of the mica sheet in polymer-free cyclohexane, (C) subsequent immersion in the cyclohexane, (D) rinse in water, (E, F) subsequent rinses in water, (G) adsorption of polymer, and (H) removal of adsorbed polymer films from the mica sheet.
Journal of Colloid and InterfaceScience, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS mica sheet was rinsed in distilled water for a few seconds. The weight decreased by 3.4 mg m -2 (position D). Such a decrease was also observed for other mica sheets whenever they were rinsed in water, but its amount differed among different samples. We do not know the cause for this decrease at the present. In the usual removing procedure, therefore, it is impossible to examine the completeness of the removal of polymer films by measuring the change in the weight of the mica sheets. The rinse in water was then repeated for this mica sheet. As seen from a comparison between positions E and F, the weight of the mica sheet did not change during the repetition of the additional rinse in water. As a test for removal, the polystyrene films were adsorbed onto this mica sheet under the conditions of a solution concentration of 0.005 mg m1-1, a duration time of adsorption of 16 h, and a rinse in pure cyclohexane for 1 h. After measurement of adsorbance (position G), the polystyrene films were removed. The weight of the mica sheet removed from the water and dried was found to return its original value although the positions were scattered accidentally in this case (position H). RESULTS AND DISCUSSION
Imaging of polystyrene films on a water surface. Figure 5a shows a photograph of the paper on which the pattern of the clear region was transferred from the water surface covered with an ink film; this pattern corresponds to a polystyrene film of adsorbance 1.3 ___0.1 mg m -2, labeled P in Fig. 2. The mica sheet was inserted vertically into the water along the dotted line drawn in Fig. 5a. Two separate clear regions are seen on both sides of the dotted line. For the sake of comparison, the mica sheet from which the polystyrene film was removed was placed over the pattern at the left; as seen in Fig. 5a, the shape and area of the clear region closely mimic those of the mica sheet. It is notable that the irregular shape of the mica sheet appears again in the pattern.
527
Because of this connection, it is instructive to describe the observation of another polystyrene film ofadsorbance around 2 mg m -2. After this film was floated off onto a water surface, an edge of the clear region was pushed lightly by the tip of clean tweezers. The whole clear region was found to move in parallel with the direction of the push; the clear region behaved like a thin solid film floating on water. These qualitative observations, while they cannot directly give detailed information on the structure of the film, do suggest that it is continuously connected over the entire adsorbed area. In addition, it may be of interest to show the pattern of the clear region obtained in the case where no polymer is adsorbed on a mica surface. A freshly cleaved mica sheet was inserted vertically into an ink film on water. Immediately after the mica sheet sank below the surface of the water, a narrow clear region of a few millimeters in width appeared along the line of insertion and began to change to a vortexlike shape owing to the flow of water (Fig. 5b). Very similar patterns were also observed in the case where a cleaved mica sheet was rinsed in pure cyclohexane and air-dried prior to insertion into ink film, implying that cyclohexane evaporates essentially completely (leaving no residue) from the bare mica surfaces during the drying process. Polystyrene films in the initial stages of adsorption. T h e present method may give some information on the initial structural state of adsorbed polystyrene films, in particular the adsorbance at which adsorbed polystyrene molecules form a continuous film. The clear region shown in Fig. 6a corresponds to a polystyrene film obtained at an adsorption time of 30 s, labeled Q in Fig. 2. Although the adsorbance of this film was below the detection limit of the microbalance, it was estimated from the data shown in Fig. 2 at somewhat less than 0.2 mg m -2. The dotted line drawn across the clear region shows the line along which the mica sheet was inserted. On both sides of this line, clear regions Journal of Colloid and Interface Science, Vol. ! 17, No. 2, June 1987
528
HIROSHI TERASHIMA
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FIG. 5. (a) The pattern of clear region corresponding to a polystyrene film of 1.3 _+ 0.1 mg m - 2 in adsorbance. The mica sheet from which the polystyrene film was removed was placed on the printed paper over the pattern at the left. The dotted line shows the position where the mica sheet was inserted normal to the water surface. (b) The pattern of clear region appearing in the ink film in case where no polymer was adsorbed on mica. Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS
529
FIG. 6. The pattern of clear region corresponding to polystyrene films at adsorbances of (a) less than 0.2 mg m -2 and (b) 0.4 + 0.1 mg m -2. The mica sheets used were placed over the pattern. The dotted line indicates the position of insertion of mica sheets into water. Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
530
HIROSHI TERASHIMA
are seen to appear. But, in contrast to the pattern shown in Fig. 5a, they are not separated into two square regions but are incorporated into one region. This pattern is clearly different from that shown in Fig. 5b where no polymer is adsorbed, suggesting that s o m e polystyrene molecules are floated off onto the water surface. A possible explanation of the origin of the appearance of such a clear region as shown in Fig. 6a may be as follows: In the initial stage of adsorption, i.e., Q in Fig. 2, isolated polystyrene molecules are adsorbed randomly on the surface, but do not cover it entirely. When the adsorbed film is floated off onto the water surface, the polystyrene molecules are desorbed individually from the mica surface to form a two-dimensional surface film hemmed in by the ink film and subsequently (under the slight counterpressure of the ink film) coalesce to each other on the surface of the water. The clear region in Fig. 6b corresponds to a polystyrene film of adsorbance 0.4 + 0.1 mg m -2 labeled R in Fig. 2: two separate clear regions appeared on both sides of the line of insertion of the mica sheet. The mica sheet from which the polystyrene film was removed was placed over the pattern at the right transferred to the paper. It is seen that the pattern keeps the original shape of the mica as in Fig. 5a. These observations suggest that the polystyrene molecules are built into a continuously connected film at the stage where the adsorbance attains a value F = 0.4 mg m -2, though we cannot say what the detailed film structure
a
_
is. The area of the clear region was found to be reduced to 80% of that of the mica (see the pattern on the right-hand side in Fig. 6b). The cause of the reduction in area will be considered briefly below. Very crudely, one may hypothesize that in the initial stages polymer molecules from the solution are uniformly adsorbed to occupy adjacent "cells" on the mica surface, each of radius Rg (radius of gyration) as indicated in Fig. 7a. Eventually all such "cells" become occupied (corresponding to F = 0.6 mg m -2 for this polymer, where tool wt 775,000 and Rg = 25 nm (19)). Upon drying the surface polymer molecules presumably collapse (Fig. 7b) and a continuously connected meshlike network is formed (where continuity is provided by overlap and entanglements between neighboring segments in adjacent cells). The small shrinkage of the film ('~20% relative to the size of the original mica) can then be accommodated by a uniform compression of the meshlike structure. A more marked effect indicating the meshlike structure is shown in Fig. 8. In this case, a sufficient amount of Chinese ink was added so as to cover the whole surface of the water in the trough. A polystyrene film of 0.6 + 0.1 mg m -E, labeled S in Fig. 2, was then floated off as described previously. As the mica sheet was inserted into the water along the dotted line drawn in Fig. 8, the clear region extended in the direction normal to the plane of the mica sheet. As seen from Fig. 8, the clear region shrank considerably in the direction nor-
q
21:~__ FIG. 7. Two-dimensional array of (a) the random coils of polystyrene and (b) their collapsed state. The surface of adsorbing substrate is partitioned into closely packed circular cells of which the radius is equal to the radius of gyration, and the random coils of polystyrene are shared out one by one to each of the cells.
Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
ULTRATHIN POLYMER FILMS
531
FIG. 8. The pattern of clear region corresponding to a polystyrenefilm of 0.6 + 0.1 mg m-2, which was removed under the condition where the whole surface of water in the trough was covered with Chinese ink film. The mica sheet indicates the place where a clear region would appear if the polystyrenefilm were not shrunk. The meaning of the dotted line is the same as before. mal to the dotted line, while remaining essentially unchanged parallel to it. This deformation probably occurred because the m o v e m e n t of the ink film, as the polymer film floated off, was resisted by the wall of the trough, and the effective resulting surface pressure (normal to the dotted line) then led to considerable compression of the polymer film. The mica sheet placed over the pattern shown in Fig. 8 indicates the original size of the polystyrene film. A closer observation of the pattern reveals that the original shape, especially the corner shape, of the mica sheet is retained "affinely" in the clear regions despite the large deformation. This supports the idea that the deformation is attributable to the compression of the meshlike polystyrene film normal to the plane of the mica sheet as it is slowly immersed in the water rather than to the buckling and doubling up of the film as
it pushed against the ink film. Figure 9 shows what happens when a m u c h thicker film of adsorbed polystyrene ( r = 3.4 mg m -2, labeled T in Fig. 2) is floated off under the same conditions: no distortion comparable to that in Fig. 8 is observed, suggesting that the meshlike structure has become considerably more closely packed. CONCLUSIONS AND FUTURE WORK We have shown that it is possible to create ultrathin polymer films by allowing polymer layers adsorbed at a solid-liquid interface to collapse on the adsorbing substrate upon removal of the solvent. By floating these films off the solid substrate onto a water surface covered with a film of Chinese ink consisting of stabilized carbon black particles, we were able to isolate and to directly "view" such films Journal of Colloid and Interface Science, V ol. 117, No. 2, June 1987
532
HIROSHI TERASHIMA
FIG. 9. The pattern of clear region corresponding to a polystyrene film of 3.4 +__0.1 nag m -2, which was removed under the same condition as that shown in Fig. 8.
for the first time. Our results suggest that, for the adsorbance of polystyrene (mol wt 775,000) onto mica from cyclohexane at the 0-temperature, (i) polymer adsorbed at adsorbances P < 0.2 mg m -2 is not continuously connected when collapsed on the surface, (ii) for F > 0.4 mg m -2 continuously connected films are formed when the adsorbed layer is collapsed on the mica surfaces, (iii) the films, when floated off onto water, appear to have an altinely deformable two-dimensional meshlike structure for F = 0.4 or 0.6 mg m -2 and to be (qualitatively) considerably more rigid at higher F values. The present technique of removing polystyrene films adsorbed on mica was developed originally in the hope of preparing the specimens for electron microscope observation. This attempt was not successful because the Journal of Colloid and Interface Science, Vol. 117, No. 2, June 1987
polystyrene films concerned were too thin and fragile to be mounted on electron microscope grids. An ingenious method for making the specimens is still sought. Further studies for acquiring information relative to the strength and structure characteristics of these ultrathin films are currently in progress. ACKNOWLEDGMENTS The author is grateful to Professor J. Klein for encouragement and for valuable advice on both the experiment and the preparation of the manuscript, to Professor D. Tabor for useful remarks on the draft, and to Professor S. Fujiwara for encouragement and for critical reading of the draft. REFERENCES 1. Rickert, S. E., Balik, C. M., and Hopfinger, A. J., Adv. Colloid Interface Sci. 11, 149 (1979). 2. Vincett, R. S., and Roberts, G. G., Thin Solid Films 68, 135 (1980). 3. Tredgold, R. H., and Winter, C. S., Thin Solid Films 99, 81 (1983).
ULTRATHIN POLYMER FILMS 4. Larkins, G. L., Jr., Thompson, E. G., Ortiz, E., Burkhart, C. W., and Lando, J. B., Thin Solid Films 99, 277 (1983). 5. Terashima, H., Klein, J., and Luckham, P. F., in "'Adsorption from Solution" (R. H. Ottewill, C. H. Rochester, and A. L. Smith, Eds.), p. 299. Academic Press, London/New York, 1983. 6. Klein, J., Nature (London) 288, 248 (1980); J. Chem. Soc. Faraday Trans. 1 79, 99 (1983). 7. Israelachvili, J. N., Tirrell, M., Klein, J., and Almog, Y., Macromolecules 14, 204 (1984). 8. Tolansky, S., "Multiple-Beam Interferometry of Surfaces and Films," p. 123. Dover, New York, 1970. 9. Bailey, A. I., and Courtney-Pratt, J. S., Proc. R. Soc. London Ser. A 227, 500 (1955). 10. Klein, J., and Luckham, P. F., Nature (London) 300, 429 (1982); Macromolecules 17, 1041 (1984). 11. Luckham, P. F., and Klein, J., Colloids Surf. 10, 65
12. 13. 14. 15. 16. 17. 18. 19.
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(1984); J. Chem. Soc. Faraday Trans. 1 80, 865 (1984). Klein, J., and Luckham, P. F., Nature (London) 308, 836 (1984). Almog, Y., and Klein, J., J. ColloidlnterfaceSci. 106, 33 (1985). Luckham, P. F., and Klein, J., Maeromoleeules 18, 721 (1985). Brandrup, J., and Immergut, E. H. (Eds.), "Polymer Handbook," p. V-59. Wiley, New York, 1975. Grant, W. H., Smith, L. E., and Stromberg, R. R., Disc. Faraday Soc. 59, 209 (1975). Takahashi, A., Kawaguchi, M., Hirota, H., and Kato, Y., Macromolecules 13, 884 (1980). Terada, T., Yamamoto, R., and Watanabe, T., Proc. Imp. Acad. 10, 10 (1934). Brandrup, J., and Immergut, E. H. (Eds.), "Polymer Handbook," p. IV-40. Wiley, New York, 1975.
Journal of Colloid and InteoCaceScience, Vol. 117, No. 2, June 1987