Langmuir 1995,11, 4173-4176
Brewster Angle Microscopic Observation of Monolayers for Polyamic Acid Tertiary Amine Salts at the Air-Water Interface Koji Hirano and Hiroyuki Fukuda* Nagoya Municipal Industrial Research Institute, 4-41, Rokuban 3-chome, Atsuta-ku, Nagoya 456, Japan Received September 28, 1994. In Final Form: June 12, 1995
Introduction The organic ultrathin films fabricated by using the Langmuir-Blodgett (LB) technique have received much attention. The characteristics of LB films are affected by the behavior of the monolayers at the air-water interface. Although it has been generally accepted that the states of the monolayer could be estimated from the surface pressure-area (x-A) isotherm, the recent development of evaluation techniques including both indirect and in situ observation has revealed that the real states of the monolayer could not be always explained by the classical two-dimensional model for the states of a monolayer. As the monolayers are transferred onto a substrate in the case of indirect observation methods such as bright-field] and dark-field2electron microscopies, electron diffraction a n a l y ~ i s ,replica ~ image observation by transmission electron mi~roscopy,~,~ and atomic force microscopy,6-sthe deposited monolayers sometimes rearrange their orientation. The fluorescence microscopy is the most convenient method among in situ observations of the monolayer^.^-^^ It has been pointed out, however, that the addition of a fluorescent amphiphilic probe occasionally caused some problems. Recently, Brewster angle microscopy (BAM) has been developed to permit an in situ visualization of the monolayer without the addition of any probe molecules. I 2 - l 7 Kakimoto et al. reported the preparation of LB films for polyimide by means of a "precursor method", consisting of the preparation ofthe LB films for polyamic acid tertiary amine salts (the precursor LB films), followed by chemical imidization with elimination of tertiary amine.ls We revealed that the behavior of monolayers for polyamic acid tertiary amine salts was influenced by the number of alkyl chains of tertiary amine and the orientation ofthe (1)Kajiyama, T.; Umemura, K.; Uchida, M.; Oishi, Y.; Takei, R. Bull. Chem. SOC.Jpn. 1989,62,3004. (2)Itoh, T.;Tsujii, Y.; Fukuda, T.; Miyamoto, T.; Ito, S.; Asada, T.; Yamamoto, M. Langmuir 1991,7, 2803. (3)Schoondorp,M. A.; Schouten, A. J.;Hulshof, J. B. E.; Feringa, B. L.;Oostergetel, G.T. Langmuir 1992,8 , 1817. (4)Iriyama, K.; Araki, T. Chem. Lett. 1990,1189. (5) Iwahashi, M.; Kikuchi, K.; Achiba, Y.; Ikemoto, I.; Araki, T.; Mochida. T.: Yokoi. S.: Tanaka. A,: Irivama. K. Langmuir 1992.8.2980. (6)Schwartz, D:K:; ViswanathanjR.;Zasadziniki, J. A. N.'J.'Phys. Chem. 1992,96,10444. (7)Peltonen, J. P. K.; He, P.; Rosenholm, J. B. J . A m . Chem. SOC. 1992,114,7637. ( 8 ) Kato, T.: Matsumoto. N.: Kawano, M.; Suzuki. N.: Araki. T.: Irivama. K. Thin Solid Films 1994.242.'223: ?9)Losche, M.; Sackmann, E.; Mohwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983,87,848. (10)Tieke, B.; Weiss, K. J . Colloid Interface Sci. 1984,101, 129. (11)Chi, L.F.;Johnston, R. R.; Ringsdorf, H. Langmuir 1991,7, 2323. ____ (12)Honig, D.;Mobius, D. J . Phys. Chem. 1991,95,4590. (13)Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991,62, 936. (14)Honig, D.;Mobius, D. Thin Solid Films 1992,210,64. (15) HBnon, S.; Meunier, J. Thin Solid Films 1992,210, 121. (16)Lefevre, D.;Porteu, F.; Balog, P.; Roulliay, M.; Zalczer, G.; Palacin, S. Langmuir 1993,9,150. (17)Byrd, H.; Pike, J. K.; Talham, D. R. Thin Solid Films 1994,242, 100.
(18)Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986,823.
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resulting precursor LB films was dominated by the characteristics of monolayers. l9 Further we reported that the d-spacing of LB film for polyamic acid tertiary amine salt having double chains increased with the increase of the deposition pressure.20 Then, we have focused our efforts on the in situ evaluation of monolayers for polyamic acid tertiary amine salts at the air-water interface. In this paper, the aggregation structure of the monolayers for polyamic acid tertiary amine salts containing the different number of alkyl chains at the various surface pressures was discussed on the basis of the BAM observation.
Experimental Section General Methods. Unless stated otherwise,all reagents and
chemicals were obtained from a commercial source and used without further purification. Polyamic acid (1) was prepared from pyromellitic anhydride purified by sublimation and diaminophenyl ether recrystallized from tetrahydrofuran. O-Hexadecanoyldimethylethanolamine (2a), O,O'-dihexadecanoylmethyldiethanolamine (2b), and 0,0,0-trihexadecanoyltriethanolamine (2c)were synthesized according to the previously reported method.Ig Monolayer Preparation and Observation. The monolayers were obtained by spreading N,N-dimethylacetamidebenzene (1:l(v/v)) solutions containing polyamic acid tertiary amine salts, 3 (Scheme 11, onto pure water (Milli-QSP grade). The concentrations of solutions of 1 and 2a-c were 1 mmol/L. Both solutions were mixed in the ratio of 1:2 by volume, just before the cast. The n-A isotherms were measured by a computer-controlled film balance (the moving-wall type equipment, Nippon Laser and Electronics Lab., model NL-LB150MW). This film balance had severalbenefitswhich a conventional film balance does not possess.21The rate ofthe compression was 8 cm2/minand the temperature of the subphase was kept at 20 "C.
The observation of monolayers at the air-water interface was carried out under compression by the Brewster angle microscope (Nippon Laser and Electronics Lab., model NL-EMM633). The microscope system was mounted on the movingwall type film balance. An incident laser light with p-polarization was used under the Brewster angle of the water subphase. The reflected beam was detected, and the resulting picture was stored with a video recorder. The BAM micrographs shown in this paper were taken from a TV screen by using video printer.
Results and Discussion The measurement of the n-A isotherms of monolayers for polyamic acid tertiary amine salts, 3, was carried out by using the moving-wall type film balance. There was one difficulty, that is, the Wilhelmy plate was pushed off the water surface by a monolayer during compression in the case of measurements for 3b and 3c. If a long length Wilhelmy plate was employed to prevent pushing the plate out of the water, their isotherms were successfully obtained. As shown in Figures 1-3, they were somewhat different from those previously obtained by using a normal rectangular type film balance.20 The n-A isotherms of 3b and 3c showed a gentle rise of surface pressure after the region of steep rise. Therefore, the so-called collapse pressure, defined as the peak of the n-A isotherm, was not observed. On the contrary, the monolayer of 3a exhibited a collapse pressure. These results suggest that 3b and 3c formed extremely rigid monolayers consisting of densely packed alkyl chains due to the compression. On the other hand, the monolayer for 3a could not form (19)Hirano, K.; Sato, M.; Fukuda, H.; Kakimoto, M.; Imai, Y. Langmuir 1992,8,3040. (20)Hirano, K.; Nishi, Y.; Fukuda, H.; Kakimoto, M.; Imai, Y.; Araki, T.;Iriyama, K. Thin Solid Films 1994,244,696. (21)Nishikata, Y.; Komatsu, K.; Kakimoto, M.; Imai, Y. Thin Solid Films 1992,210,29.
0743-746319512411-4173$09.00/0 0 1995 American Chemical Society
4174 Langmuir, Vol. 11, No. 10, 1995
5E
60 8o
Notes
t
d
40
C
500 nm 20
1 2 Area of repeat unit (nm2)
0
3
Figure 1. x-A isotherm and BAM micrographs of monolayer for 3a: (a) 0 mN/m; (b) 5 mN/m; (c) 45 mN/m; (d) 50 mN/m. Scheme 1
n HOOCuCOOH
1
a rigid surface, because of the smaller area of two alkyl chains compared with the unit area of polyamic acid.lg Figure 1shows the n-A isotherm of the monolayer for 3a as well as the BAM micrographs at surface pressures of 0,5,45,50 mN/m.22 Before 3a was cast, the image of micrograph was dark, representing only the water surface. After the cast, the BAM micrograph shows the monolayer having many holes with low contrast (Figure la). The brightness of the micrograph was attributed to the strength of reflected light which was based on the refractive index of the monolayer surface. Therefore, the refractive index of the monolayer for 3a might be almost the same as that of the water surface. The holes in the monolayer were gradually diminished with the decrease of surface area. They completely disappeared when the surface pressure began to be detected. The formation of homogeneous monolayer was recognized from the uniform reflection, as shown in Figure lb. The micrograph gradually brightened with the compression (Figure IC). This fact indicated that alkylaminesalts of 3a were packed close to one another in the monolayer at the air-water interface. When the surface pressure reached the so-called collapse pressure determined from the n-A isotherm, it was observed that strong light was reflected from the whole (22)The equidistant and parallel lines seen on all the BAM micrographsare artifactscoming from the hardwareof the BAM system.
monolayer surface. Finally, the monolayer was broken parallel to the direction of the compression (Figure Id). This fact indicated that the refractive index of the monolayer for 3a instantaneously increased, just when the collapse pressure was attained. Figure 2 shows the n-A isotherm for 3c,together with the B A M micrographs of the monolayer at pressures of 0, 5,22, and 30 mN/m.22 Many isolated featureless domains were explicitly observed even a t 0 mN/m (Figure 2a). The isolated domainscould be collected by compression in spite of a surface pressure maintained a t 0 mN/m. The BAM micrograph clearly exhibits the bump of domains and their gathering process (Figure 2b). A monolayer was formed even a t the expanded phase (5 mN/m), as shown in Figure 2c. The monolayer for 3c showed a typical n-A isotherm having condensed-to-expanded phase transition, and the steep rise of surface pressure corresponding to the condensed phase was recognized up to about 50 mN/m. However, a BAM micrograph obviously shows that the monolayer is destroyed perpendicular to the direction of compression at about 22 mN/m, which corresponded to the phase transition (Figure 2d). It was reported that a monolayer of fatty acid on pure water broke a t the socalled collapse pressure when continuously compressed.l4 Nevertheless, a monolayer of fatty acid salts such as barium stearatel and cadmium a r a ~ h i d a t ewhich , ~ formed
Langmuir, Vol. 11, No. 10, 1995 4175
Notes
80
2
0 0 v) I
500 nm
\JC
b
a
I
I
0 k e a of repeat unit (nm2) Figure 2. x-A isotherm and BAM micrographs of monolayer for 3c: (a) 0 mN/m; (b) 0 mN/m; (c) 5 mN/m; (d) 22 mN/m; (e) 30 mN/m.
80
\zed 500 nm
,
0
I\,
I
2 1 Area of repeat unit (nm2)
3
Figure 3. x-A isotherm and BAM micrographs of monolayer for 3b: (a) 0 mN/m; (b) 10 mN/m; (c) 35 mN/m; (d) 47 mN/m; (e) 55 mN/m.
the condensed monolayer, collapsed at a fairly high surface pressure lower than the so-called collapse pressure. With an increase of the rigidity of the monolayer, the collapse of the monolayer actually took place at a comparatively low surface pressure, which indicated that the alkyl chains of 3c were in an extremelyrigid state even in the expanded phase. Although many stripes formed by the collapse of the monolayer were observed (Figure 2e), the surface pressure steeply rose up to about 40 mNlm and then gently rose to about 55 mN/m. The n-A isotherm for 3b and the BAM micrographs of the monolayer a t surface pressures of 0, 10,35,47, and 55 mNlm are shown in Figure 3.22 With the Brewster
angle microscope, large isolated domains were observed at 0 mNlm (Figure 3a). A homogeneous monolayer was formed through both the gathering process and the fusing of the domainsduring compression (Figure 3b), even when the n-A isotherm displayed an expanded phase. When the monolayer changed fiom expandedphase to condensed phase on the n-A isotherm, the monolayer showed a large refractive index, indicating dense packing of alkyl chains (Figure 3c). The monolayer was locally destroyed at about 47 mN/m (Figure 3d) during the steep rise of surface pressure, though the n-A isotherm still showed a condensed phase. The collapse of the monolayer was wholly extended a t about 55 mN/m, similar to 3c, when the
Notes
4176 Langmuir, Vol. 11, No. 10,1995
It should be noted that the occupied areas at the collapse pressures for 3b and 3c were about 0.8 and 1.3 nm2, respectively, whose values were almost same as the sum of their cross section of tertiary amines per unit of polyamic acid salts. These facts suggest that the collapse behavior 0 hl 0 of polyamic acid salts is directly dependent upon the employed tertiary amines. Therefore, it can be assumed that the structure of the tertiary amine is the most important factor determining the morphology of the monolayer for the polyamic acid tertiary amine salt. - 200nm - 200nm However, the reason why the direction of the monolayer Figure 4. BAM micrographs of monolayers for 2b and 2c: (a) collapse is influenced by the number of alkyl chains of the 2b (55 mN/m); (b) 2c (50 mN/m). tertiary amine is not clear at present. In conclusion, we found that the morphology of the surface pressure changed from the steep rise region to the monolayer for polyamic acid tertiary amine salt was gentle rise one (Figure 3e). It was revealed from the BAM affected by the alkyl chains of the tertiary amine. The observation as well as the n-A isotherms, that the increase of the number of alkyl chains per the unit of monolayer of 3b had the intermediate characteristics polyamic acid led to the formation of a rigid monolayer between 3a and 3c. with compression. It became apparent that the monolayer It has been represented by means of electron microscopy for a polyamic acid tertiary amine salt with multichains and electron diffraction analysis that the aggregation broke at a surface pressure below the so-called collapse structure of the fatty acid monolayer could be designated systematically on the basis of the subphase t e m p e r a t ~ r e . ~ ~ pressure. The monolayers for polyamic acid tertiary amine salts with multichains collapsed perpendicular to the However, in the case of polyamic acid tertiary amine salts, direction of the compression whereas that for a polyamic 3, the morphologies of the monolayersfor 3 were different acid tertiary amine salt with a single chain was destroyed according to the tertiary amines used. The aggregation parallel to the compression. We believe that the charstructure of the monolayers for tertiary amines, 2a-c, acteristics of the monolayers for ion-paired amphiphilic was examined with the Brewster angle microscope in order polymers such as polyamic acid saltscould be easily altered to investigate how the structure of tertiary amines by changing the pair, compared with the amphiphilic contributed to the morphology of monolayers for salts, 3. polymers from covalent bonds. Although the monolayers for 2b and 2c were destroyed perpendicular to the direction of the compression at surface pressures of 55 and 50 mN/m, respectively, their BAM Acknowledgment. We are grateful to Dr. Masa-aki micrographs were somewhat different from those of 3b Kakimoto of Tokyo Institute of Technology for his stimuand 3c, as shown in Figure 4.22 These collapse pressures lative and helpful discussions. We also thank Dr. Katsumi were naturally coincident with the reported results. l9 On Yoneda and Mr. Nobuo Ohga (Nippon Laser and Electhe other hand, a monolayer for 2a could not be observed tronics Lab.) for their technical assistance and helpful because of the low contrast of the BAM micrograph of 2a. discussions. This research was supported in part by the Naito Foundation (Grant 94-07).
s
1
(23)Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992,8, 1563.
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