Langmuir 1991, 7, 444-445
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Adsorption Kinetics of Water-Soluble Polymers onto a Spread Monolayer Kenjiro Miyano' and Kazuya Asano Department of Applied Physics, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Masatsugu Shimomura Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received October 22, 1990. In Final Form: January 14, 1991 The technique of ATR (attenuated total reflection) has been used for the study of the adsorption kinetics of anionic polymers onto a monolayer of cationic amphiphiles. This is, to the best of our knowledge, the first in situ measurement of the polymer adsorption onto a Langmuir film. The polymers are adsorbed in two steps: the adsorbance of the polymer was monomolecular initially and increased with adsorption time, until a final equilibrium plateau was reached. The equilibrium adsorbance increased with molecular weight of a polymer, which indicates that a significant portion of the polymer is in the loop or the tail form.
Introduction There has been a growing interest in the fabrication of LB (Langmuir-Blodgett) films with the polymeric backing layers adsorbed from the The polymers dissolved in the subphase are electrostatically adsorbed onto the spread monolayer. The reinforcement facilitates transfer of otherwise intractable monolayers onto a substrate. For example, Higashi et ale2succeeded in forming LB films of a fluorocarbon amphiphile with polymer PSSK (potassium poly(styrenesu1fonate)). It is known that the monolayer of the amphiphile alone is hard to deposit on a substrate. The composition of the LB film thus formed was determined2 by XPS (X-ray photoelectron spectroscopy) to be a 1:l polyion complex of fluorocarbon amphiphile and PSS-. Moreover, the X-ray diffraction shows3 that the polymer layer is very thin, essentially a monolayer thick. Although, in these works, it has been tacitly assumed that the adsorbed polymer layer was also monomolecular a t the monolayer-solution interface, it is unlikely, from the statistical point of view? that the polymer is adsorbed from the solution without tails or loops. In order to elucidate the state of the adsorbed polymer layer a t the monomer-solution interface, we have performed an in situ ATR measurement.
Experimental Section The water-solublepolymer used in this experiment was PSSK, whose repeat unit is shown in Figure la. Three types of samples were employed, whose average degrees of polymerization were 3500(1),10 500 (2),and42000(3),respectively. Theywere kindly donated by Tokuyama Soda Co. The monolayer-formingmaterial was DOABr (dioctadecyldimethylammonium bromide) (Figure lb), purchased from Sogo Chemicals. In order to examine the effect of a noncationic amphiphile, stearic acid was used. All materials were used as received. The ATR was measured in the Kretchmanngeometry as shown in Figure 2. In brief, P-polarized light is used to create an SPP (surface plasmon polariton) at the interface between the metal (silver) and the monolayer. This process is monitored by recording the reflectivity from the interface as a function of the incident angle. (1) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (2) Higashi, N.; Kunitake, T. Chem. Lett. 1986, 105. (3) Takahara, A.; Morotomi, N.; Hiraoka, S.;Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1989, 22, 617. (4) Papenhuijzen, J.; Van Der Schee, H. A.; Fleer, G . J. J. Colloid Interface Sci. 1985, 104, 540.
0743-7463/91/2407-0444$02.50/0
(b) DOABr
(a) PSSK
Figure 1. Chemical structures of PSSK and DOABr. prism A
4 micro-tube pump waste reservor Figure 2. Experimental arrangement. The reflectivity drops sharply at the angle &pp, where the SPP is excited. This angle is governed by the distribution of the dielectric constant of the medium in contact withmthe metal. We measured the shift of the angle Ospp as the polymer was adsorbed. The theor9 predicts that the adsorbed layer of the dielectric constant, np,and the thickness, Ad, shifts the dip by A8 = aAdAn, where An = np- n, (n,is the index of refraction of the solution)and a is a numerical constant. Therefore, knowing the value of np, it is possible to deduce the adsorption layer thickness from the position of the dip. Data were recorded every 0.025' and, by curve fitting, the dip could be determinedto0.01O. The first step was to record the reflectance curve from a silver evaporated prism in air as a function of the incident angle in order to determine the complex optical constant e and the thickness of the silver film. The typical value was e = -15.5 + 0.63i for the optical constant and 520 8, for the thickness, and the refractive index of the prism was 1.845 for the He-Ne laser light. Next, DOABr was spread on pure water and compressed to a surface pressure of 40 mN/m, which corresponded to one DOA+molecule per 50 &. The prism was brought into contact with the monolayer, and the reflectivity was recorded again to get reference data without adsorbed layers. The subphase pure water was then exchanged with polymer solution by a microtube pump. The ATR signal was measured (5) Zhang, Y.;LBvy, Y.; Lorlergue, J. C. Surf. Sci. 1987, 184, 214.
0 1991 American Chemical Society
Letters
Langmuir, Vol. 7, No. 3, 1991 445
incident angle (degree)
Figure 3. Typical ATR signal-curve a, recorded with the monolayer on the surface of pure water; curve b, taken after passing the 10-5 unit mol/L solution of PSSK-3 for 50 min; curve c, taken 1 h after the pump was turned off. 1.4
1.0
time (hour)
Figure 4. Time development of dip-shift (adsorbance of polymer) and half-width (the nonuniformity of the adsorbed layer) of SPP resonance. The microtube pump was turned on a t time = 0. The lines are drawn to guide the eye. from time to time to follow the adsorption kinetics. The microtube pump pushed 20 mL of solution per minute and the volume of the trough was about 150 mL. The flow of the polymer solution may create the shear in the vicinity of the monolayer. But the shear rate would be about 0.1 s-l, too small to alter the adsorption kinetics. It has been reported: for example, that the shear rate of 3 s-1 is required to affect the kinetics of the polymer adsorption. It took about 1 min to complete one ATR measurement. Therefore, as will be shown below, the change of the thickness of the layer was small during one scan.
Results and Discussions The results of typical ATR measurements are illustrated in Figure 3. The leftmost dip (marked a) was recorded with the monolayer on the surface of pure water. The microtube pump was then turned on and 10" unit mol/L solution of PSSK-3 started passing through the trough. The middle curve (b) was taken 50 min later. After 1h, the pump was turned off. We considered that further pumping would not have altered the results. The rightmost curve (c) was taken 1 h after the pump was turned off. In order to see the time evolution of the ATR signal, the angle and the width of the dip for PSSK-2 are plotted in Figure 4. The curve is composed of three parts (1-3). In part 1, the dip shifts to a higher angle accompanied by a slight increase of the width. They both seem to come to a plateau. In part 2, both the angle and the width start to rise again, followed by a saturation in part 3. We interpret this behavior as follows. In part 1, the polymers begin to be adsorbed to the monolayer, forming (6) McGlinn, T. G.; Kuzmenka, D. J.; Granick, S. Phys. Reu. Lett. 1988, 60, 805.
a uniform sheet. Taking the index of refraction of PSSK as 1.56 f 0.01 (determined by the Becke line methodT), the thickness of the sheet a t the plateau is 6 A. The adsorbed film must be uniform in thickness, for the width increases little. Considering that the density of PSSK is about 1g/cm3 and that the molecular weight per monomer unit is 222, a monolayer of PSSK would be 7 A thick, which is in reasonable agreement with the observed thickness of the adsorbed layer. The initial uniform sheet is, thus, nearly monomolecular. The subsequent stage (part 2) is strongly marked by the rapid increase in the width. The thickness is certainly more than monomolecular. This indicates that the polymers attach to the surface already covered with a polymer sheet. The mechanism of further polymer attachment is not clear. There may be cationic sites sparsely left in the monolayer for the polymer adsorption, leading to the loops and tails. Since the position of the dip is insensitive to the local details of the adsorbant conformation, loops and tails contribute much to the effective thickness of the adsorbed layer. For example, one additional molecule per 600 A X 600 A area would be enough to result in an average thickness of 18 A, which was the final thickness of the polymer layer in this case. With loops and tails, the thickness is not uniform, which leads to a strong scattering of the SPP and thus an increase of the width of the ATR signal. In the final stage (stage 3), no more sites are available and thus no further adsorption. That the adsorption beyond the monolayer thickness is not caused by simple aggregation or entanglement was confirmed by passing pure water through the trough after stage 3. After 50 min, no change in dip angle was seen. This final thickness was 16 A for PSSK-1,lSA for PSSK-2, and 33 A for PSSK-3. It was also shown that the attachment or aggregation of polymers to the interface does not occur when positive charge is absent in the monolayer. We have observed no change in the ATR signal in a stearic acid monolayer after passing unit mol/L PSSK solution for 50 min beyond the resolution of our measurement. The ATR method has already been applied for the study of adsorption kinetics of polymers from a solution onto a metalic ~ u r f a c e . In ~ ~ previous ~ studies, the adsorbed polymers retain the random coil form, resulting in thick layers. In contrast, in the case of adsorption of strong polyelectrolyte, inter- and intramolecular electrostatic repulsion favors flat configuration of adsorbed layers. Nonetheless, the adsorbed polymer layer in the final stage was thicker than a monolayer. This contradicts the observations in LB f i l m ~ . ~Considering a the apparent twostep adsorption, one could argue that the molecules in the first layer and those in the subsequent layers are in different states, so that only the first layer survives the LB transfer process. Further work to distinguish the molecules in the first layer from the rest is clearly needed.
Acknowledgment. We have benefited very much from the discussions with Professor K. Okano and the suggestions by Dr. A. Tomioka. This work was supported in part by a Grant for International Joint Research Project from NEDO, Japan, and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture. (7) Hartshorne, N. H.; Staut, A. Crystals and the Polarising Microscope; Arnold: London, 1960. (8)Tassin, J. F.; Siemens, R. L.;Tang, W. T.; Hadziioannou, G.; Swalen, J. D.; Smith, B. A. J. Phys. Chem. 1989,93, 2106. (9) Loulergue, J. C.; LBvy,Y.; Allain, C. Macromolecules 1986,18,306.
