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Langmuir 1998, 14, 2134-2138
Monolayer Behavior of Poly(amic acid) Alkylamine Salts Containing the Dimethylsiloxane Structure and Their Langmuir-Blodgett Films Koji Hirano and Hiroyuki Fukuda* Nagoya Municipal Industrial Research Institute, Rokuban 3-chome, Atsuta-ku, Nagoya 456, Japan
Masa-aki Kakimoto Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received May 30, 1997. In Final Form: December 22, 1997 The behavior of monolayers of poly(amic acid) alkylamine salts containing different dimethylsiloxane chain lengths at the air-water interface was investigated. The limiting areas estimated from surface pressure-area isotherms were much larger than those calculated from both the alkyl chain number per repeating unit and the cross-sectional area of the alkyl chain. The direct observation of monolayers was carried out by Brewster angle microscopy (BAM). The BAM observation was affected by the length of the dimethylsiloxane chain in the poly(amic acid)s. The layered structure of the LB films of poly(amic acid) alkylamine salts with multichains was investigated by X-ray diffraction, Fourier transform infraredreflection absorption spectroscopy, and X-ray photoelectron spectroscopy. Consequently, it was found that the LB films consist of bilayer (Y-type) structures in which the alkyl chains are appreciably tilted from the normal direction and that the long dimethylsiloxane chain is concentrated in the outer surface of the LB films.
Introduction The Langmuir-Blodgett (LB) films have been paid much attention in many fields including microelectronics and sensing technology.1-3 It is anticipated that polymers with high performance are useful candidates for the materials of the LB films because a major factor for practical use is their durability. The LB films of polymers are fabricated by two routes. One method is the transfer of monomeric amphiphiles through the LB technique followed by polymerization on the substrate by irradiation with UV light.4 This method sometimes causes defects in LB films because of film shrinkage during the polymerization. On the other hand, another method is the construction of LB films built by monolayers of polymerized amphiphiles, which can avoid the structure change.5 Polyimide, which has excellent chemical, physical, and electrical properties, is widely used as a reliable insulating material and is known as a useful material for the orientation film of the liquid-crystalline cell. Therefore, many researchers have turned their attention toward polyimides, and their LB films have been investigated.6-12 (1) Kuhn, H. Thin Solid Films 1989, 178, 1. (2) Petty, M. C. Thin Solid Films 1992, 211, 417. (3) Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University: New York, 1996. (4) Naegele, D.; Lando, J. B.; Ringsdorf, H. Macromolecules 1977, 10, 1339. (5) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788. (6) Suzuki, M. Thin Solid Films 1989, 180, 253. (7) Baker, S.; Seki, A.; Seto, J. Thin Solid Films 1989, 180, 263. (8) Sotobayashi, H.; Schilling, T.; Tesche, B. Langmuir 1990, 6, 1246. (9) Akatsuka, T.; Tanaka, H.; Toyama, J.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Thin Solid Films 1992, 211, 458. (10) Tsukruk, V.; Mischenko, N.; Scheludko, E.; Krainov, I.; Tolmachev, A. Thin Solid Films 1992, 211, 620. (11) Schoch, K. F., Jr.; Su, W.-F. A.; Burke, M. G. Langmuir 1993, 9, 278.
We have developed the preparation of LB films for polyimide by means of a “precursor method”, consisting of the preparation of LB films for poly(amic acid) alkylamine salts, followed by chemical imidization with the elimination of alkylamine and water.13,14 The LB films of several polyimides with functional groups, including the porphyrin unit and azobenzene pendant group, were prepared, and their physical, photochemical, and electrochemical characteristics were examined.15,16 Also, we have reported that the behavior of monolayers of poly(amic acid) alkylamine salts is influenced by the number of alkyl chains of alkylamines, and the orientation of the resulting precursor LB films is dominated by the characteristics of the monolayers.17,18 Further, we have demonstrated that the collapse behavior of monolayers is not always predicted by their surface pressure-area (πA) isotherms on the basis of Brewster angle microscopy (BAM).19 On practical use of LB films for polyimides, it is important that polyimides should have excellent insulating properties, low dielectric constants, good thermal and oxidative stability, and good adhesive properties to the substrates such as glass, silicon wafer, and so on. To design the structure of the polyimides carrying the above(12) Uekita, M.; Awaji, H.; Murata, M.; Mizunuma, S. Thin Solid Films 1989, 180, 271. (13) Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986, 823. (14) Kakimoto, M.; Morikawa, A.; Nishikata, Y.; Suzuki, M.; Imai, Y. J. Colloid Interface Sci. 1988, 121, 599. (15) Nishikata, Y.; Morikawa, A.; Kakimoto, M.; Imai, Y.; Hirata, Y.; Nishiyama, K.; Fujihira, M. J. Chem. Soc., Chem. Commun. 1989, 1772. (16) Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1993, 9, 1086. (17) Hirano, K.; Sato, M.; Fukuda, H.; Kakimoto, M.; Imai, Y. Langmuir 1992, 8, 3040. (18) Hirano, K.; Nishi, Y.; Fukuda, H.; Kakimoto, M.; Imai, Y.; Araki, T.; Iriyama, K. Thin Solid Films 1994, 244, 696. (19) Hirano, K.; Fukuda, H. Langmuir 1995, 11, 4173.
S0743-7463(97)00570-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/18/1998
Poly(amic acid) Alkylamine Salts Containing Dimethylsiloxane
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Figure 1. Synthesis and chemical structures.
mentioned properties, we employed diamines having poly(dimethylsiloxane) and bis(phenyl)fluorene residues. In this paper, we describe the behavior of monolayers of poly(amic acid) alkylamine salts containing the dimethylsiloxane structure by means of BAM as well as the characterization of their LB films. Predominantly, it will be discussed that the length of the dimethylsiloxane chains of the poly(amic acid)s and the number of alkyl chains of the alkylamines affect the characteristics of the monolayers at the air-water interface and the orientation of the LB films. Experimental Section Along with the synthesis scheme, the structures of the compounds studied in this experiment are identified in Figure 1 by roman numerals. o-Hexadecanoyldimethylethanolamine (I.1), o,o′-dihexadecanoylmethyldiethanolamine (I.2), and o,o′,o′′trihexadecanoyltriethanolamine (I.3) were synthesized according to the previously reported method.17 The solutions (25 wt %) of the poly(amic acid)s containing the dimethylsiloxane structures (II, m ) 2; III, m ) 9) were prepared by adding aromatic diamine to the mixture of the diamine having the dimethylsiloxane structure and acid dianhydride in N-methyl-2-pyrrolidone (NMP)-diethylene glycol dimethyl ether (DGDE) (1:1 (v/v)). Poly(amic acid) without dimethylsiloxane (IV) was synthesized according to the previously reported method.13 The solutions of II and III were prepared at a concentration of 1 mM in a mixture of NMP, DGDE, and benzene (1:1:2 (v/v)), and those of I and IV were done at the same concentration in a mixture of N,N-dimethylacetamide and benzene (1:1 (v/v)), respectively. The solutions of poly(amic acid) alkylamine salts were prepared by mixing the solutions of poly(amic acid) and alkylamine, just before casting on pure water (Milli-Q SP grade). The π-A isotherms of the monolayers were measured at the compression rate of 60 mm2/s with a computer-controlled film balance (San-esu Keisoku, FSD-20). The subphase was kept at 20 °C. The observation of monolayers at the air-water interface was carried out under compression by a Brewster angle micro-
scope (Nippon Laser and Electronics Laboratories, NLEMM633).19 The microscope system was mounted on the movingwall-type film balance (Nippon Laser and Electronics Laboratories, NL-LB150MW). The LB films were prepared by transferring the monolayers onto glass slides or Au-coated ones in the vertical mode (San-esu Keisoku, FSD-21). A dipping speed of 10 mm/min was applied for all depositions. The surface pressure used for the depositions was from 20 to 35 mN/m for II.1, II.2, III.1, and III.2 and from 20 to 30 mN/m for II.3 and III.3, where poly(amic acid) alkylamine salts formed stable monolayers. The deposition was started from the upward stroke. All depositions were completed after the upward stroke at an arbitrary odd number of monolayers. The cleaning procedure of the substrates was as follows. The glass slides were immersed over 1 week in H2SO4/H2O2 (1:1) at room temperature. Then the glass slides were repeatedly washed for 20 min in a sonic bath and extensively rinsed with pure water, and the Au-coated glass slides were immersed for 2 h in acetone and then thoroughly rinsed with pure water, prior to use. The elemental analysis of the LB films was carried out by X-ray photoelectron spectroscopy (XPS) (JEOL, JPS-90 SXV, X-ray source Mg KR 40 kV, 100 mA). The measurement of X-ray diffraction was carried out by a Geigerflex Rad-C system (Rigakudenki) with a Cu KR X-ray source (40 kV, 20 mA). Fourier transform infrared-reflection absorption (FTIR-RA) spectra of the LB films (9 layers) deposited on Au-evaporated glass slides were measured by a FTIR spectrometer (JASCO, FT/IR-8000) with a reflection attachment (incident angle, 80°).
Results and Discussion Monolayers at the Air-Water Interface. Figure 2 shows the π-A isotherms and BAM images of the monolayers of poly(amic acid) II with a short dimethylsiloxane chain and its alkylamine salts (II.1, II.2, and II.3). II does not form a stable monolayer at the airwater interface, but its alkylamine salts form stable monolayers. In Figure 2, only the expanded phase is shown in the π-A isotherm for the monolayer of II.1, which
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Hirano et al.
Figure 2. Surface pressure-area isotherms and BAM images of monolayers for II (a), II.1 (b), II.2 (c), and II.3 (d). (1) II.1, 42 mN/m; (2) II.2, 52 mN/m; (3) II.2, 54 mN/m; (4) II.3, 32 mN/m; (5) II.3, 38 mN/m.
Figure 3. Surface pressure-area isotherms and BAM images of monolayers for III (a), III.1 (b), III.2 (c), and III.3 (d). (1) III.1, 43 mN/m; (2) III.2, 51 mN/m; (3) III.3, 32 mN/m; (4) III.3, 37 mN/m.
is coincident with the previously reported result of the poly(amic acid) alkylamine salt for IV.1.19 Although the π-A isotherms for II.2 and II.3 with multichains exhibited the inflection points, the distinct phase transition behaviors were not observed because of the slow increase of their surface pressures. The limiting areas for II.1, II.2, and II.3 obtained from the π-A isotherms are 1.9, 2.4, and 2.1 nm2/unit, respectively. The direct observation of monolayers at the air-water interface was carried out by BAM. BAM is one of the useful methods to observe the morphology of monolayers.19-21 The distinct destruction of monolayers could not be observed in the case of II.1 (Figure 2 (1)). The BAM image for II.3 shows the destruction of monolayers perpendicular to the direction of compression at about 32 mN/m, which is below the socalled collapse pressure estimated from the π-A isotherm (Figure 2 (4)). Further compression led to the continuous destruction of the monolayer (Figure 2 (5)). The monolayer for II.2 was destroyed at about 52 mN/m (Figure 2 (2)). In the cases of II.2 and II.3, the increase of surface pressures was observed even after the destruction of monolayers by further compression (Figure 2 (3, 5)). These behaviors of monolayers for II.1, II.2, and II.3 were similar to those for IV.1, IV.2, and IV.3 reported previously.19 These results suggest that the surface pressure of the monolayers for poly(amic acid) alkylamine salts with multichains continuously increases because of their rigidity, although monolayers have already been destroyed. Figure 3 shows the π-A isotherms and BAM images of the monolayers of poly(amic acid) III with a long dimethylsiloxane chain and its alkylamine salts (III.1, III.2, and III.3). III does not form a monolayer at the airwater interface. The limiting areas for III.1, III.2, and III.3 are 1.9, 2.3, and 2.1 nm2/unit, respectively, which are consistent with the results for II.1, II.2, and II.3 with a short dimethylsiloxane chain. The limiting areas are far larger than the areas estimated from the number of alkyl chains per repeating unit of the poly(amic acid)s. It is thought that the limiting areas are affected by the unit area of the poly(amic acid)s. However, it is interesting that the limiting areas are dependent only on the number of alkyl chains but independent of the length of the dimethylsiloxane chain. BAM observation shows that the compression behaviors of the monolayers of III.1 and III.3 are similar to those of II.1 and II.3 (Figure 3 1, 3, 4)). On the contrary, the
BAM images for II.2 and III.2 gave different results even though the same alkylamines with double chains were employed. The distinct destruction could not be observed for the BAM image of the monolayer for III.2 (Figure 3 (2)), but the destruction of the monolayer for II.2 could be clearly recognized at about 52 mN/m, corresponding to the so-called collapse pressure. This can be explained by the difference in the lengths of the dimethylsiloxane chain in the poly(amic acid)s. As poly(dimethylsiloxane) is inherently soft and flexible, the residue of poly(dimethylsiloxane) may act as the damper. The monolayer for IV.2 without the dimethylsiloxane unit is, in fact, destroyed at a surface pressure below the so-called collapse pressure.19 The residue of poly(dimethylsiloxane) may hinder the BAM observation of destruction of monolayers, since the distinct destruction in the monolayer for III.2 cannot be recognized even beyond the so-called collapse pressure. Characterization of the LB Films. The monolayers of II.2, II.3, III.2, and III.3 with multichains were deposited at each upward and downward stroke (Y type) on the glass slides. On the other hand, the deposition mode of II.1 and III.1 with a single chain was Z type, where the monolayer is transferred only upon withdrawal. These deposition behaviors are consistent with those of poly(amic acid) (IV) alkylamine salts without the dimethylsiloxane structure.17 The layer structure of the LB films was investigated by X-ray diffraction. The LB films of II.1 and III.1 with a single chain showed no X-ray diffraction peaks, but the LB films for II.2, II.3, III.2, and III.3 with multichains showed the X-ray diffraction peaks, as shown in Figure 4. However, the intensities and widths of the X-ray diffraction peaks were weaker and broader than those of the LB films for IV.2 and IV.3.17 The d spacings of the LB films for II.2, II.3, III.2, and III.3 (25 mN/m) estimated from the diffraction angle (2θ) were 4.1, 4.9, 4.3, and 4.6 nm, respectively. These are smaller than those of the corresponding LB films for IV.2 and IV.3. The sum of the cross-sectional area of the alkyl chains per repeating unit is smaller than the unit area for II and III, even when the alkylamine I.3 with triple chains is employed. Therefore, the decrease of d spacings is due to the tilt of the alkyl chains of the amine. The d spacings for II.2 and III.2 are smaller than those for II.3 and III.3. The d spacings are larger than the thickness of the monolayer calculated from the molecular models of the poly(amic acid) alkylamine salts. The d spacings verify that the LB films for II.2, II.3, III.2, and III.3 are bilayer (Y-type) structures, as expected from the deposition procedure. The influence of
(20) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (21) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936.
Poly(amic acid) Alkylamine Salts Containing Dimethylsiloxane
Figure 4. X-ray diffraction patterns of LB films (25 mN/m) for II.2 (a), II.3 (b), III.2 (c), and III.3 (d).
Figure 5. FTIR-RA spectra of LB films for II.1 (25 mN/m) (a), II.2 (25 mN/m) (b), II.2 (30 mN/m) (c), and II.3 (25 mN/m) (d).
Figure 6. FTIR-RA spectra of LB films for III.1 (25 mN/m) (a), III.2 (25 mN/m) (b), III.2 (30 mN/m) (c), and III.3 (25 mN/m) (d).
the deposition pressure on the d spacings of the LB films (II.2, III.2) was investigated to find that the d spacings hardly depended on the deposition pressures from 20 to 35 mN/m. The orientation of alkyl chains in the LB films was examined by FTIR-RA spectroscopy. FTIR spectra of LB films for II.1, II.2, and II.3 (25 mN/m), II.2 (30 mN/m), III.1, III.2, and III.3 (25 mN/m), and III.2 (30 mN/m) are shown in Figure 5 and Figure 6, respectively. Two bands at 2853 and 2922-2924 cm-1 are assigned to the sym-
Langmuir, Vol. 14, No. 8, 1998 2137
metric and asymmetric CH2 stretching vibrations of the alkyl chains, and two bands at 2874 and 2961-2963 cm-1 are due to the symmetric and asymmetric CH3 stretching vibrations. The transition dipole moments of the CH2 stretching vibrations are perpendicular to the long axis of the alkyl chain, whereas the symmetric and asymmetric CH3 stretching vibrations have components along the chain axis.22 The LB films (25 mN/m) for II.2 and III.2 hardly exhibited two bands due to the CH3 stretching vibrations, but those for II.3 and III.3 showed two CH3 stretching bands. The intensities of the CH2 stretching vibrations of II.3 (25 mN/m) and III.3 (25 mN/m) were almost identical to those of II.2 (25 mN/m) and III.2 (25 mN/m), despite the increase of the number of alkyl chains per repeating unit of poly(amic acid). The CH2 stretching vibrations of the LB films (25 mN/m) for II.1 and III.1 exhibited a weaker intensity compared with those for II.2, II.3, III.2, and III.3. However, the intensity of the bands of CH3 and CH2 became stronger with the increase of deposition pressure, because the concentration of alkyl chains increased with compression. These results imply that the alkyl chains in the LB films (25 mN/m) for II.3 and III.3 are less tilted from the normal direction and the alkyl chains (25 mN/m) for II.1, II.2, III.1, and III.2 are appreciably tilted, which are in good agreement with the X-ray diffraction results. In the cases of II.2 and III.2, the relative intensities of the bands of the CH3 stretching vibrations against the bands of the CH2 stretching vibrations increased with increasing deposition pressure. This result suggests that the alkyl chains in the LB films for II.2 and III.2 rise toward the normal direction of the substrate with compression. The bands of the asymmetric CH2 stretching vibration for II.3 and III.3 were shifted to low wavenumber, compared with those for II.1, II.2, III.1, and III.2. The wavenumber of the absorption for the asymmetric CH2 stretching band of a crystalline polymethylene chain is lower than that for the liquid state.23 Therefore, the orientation of alkyl chains in the LB films increases with increasing the number of alkyl chains per repeating unit of poly(amic acid). Other characteristic absorption bands were observed at 1745, 1650, 1250, and 1000 cm-1. These are assignable to the CdO stretching vibration of the ester group, the CdO stretching of the amide, the CH3 bending vibration of SisCH3, and the SisO stretching vibration, respectively. The intensity of the SisO stretching vibration and the CH3 bending vibration of Si-CH3 for alkylamine salts of II are weaker than those of III, which reflects the difference of the length of the dimethylsiloxane chain. The elemental analysis of the surface of the LB films (49 layers) for II.2 and III.2 was measured by XPS. The obtained results are as follows. Anal. Calcd for ((C115, N4, O16, S1)0.7 (C100, N4, O17, S1, Si2)0.3)n, (II.2); C, 83.45; N, 3.02; O, 12.31; S, 0.76; Si, 0.45. Found: C, 80.08; N, 3.11; O, 15.36; S, 0.73; Si, 0.68. Anal. Calcd for ((C115, N4, O16, S1)0.7 (C114, N4, O24, S1, Si9)0.3)n, (III.2); C, 81.46; N, 2.84; O, 13.07; S, 0.71; Si, 1.92. Found: C, 72.42; N, 2.76; O, 19.17; S, 0.87; Si, 4.74. The analytical values for II.2 with a short dimethylsiloxane chain are in appreciable agreement with the calculated ones. On the other hand, the found values for III.2 with a long dimethylsiloxane chain, especially Si and O from the siloxane structure, are far different from the calculated ones. It is well-known that the residues of poly(22) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (23) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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(dimethylsiloxane) in block copolymers24-26 and graft copolymers27-30 accumulate at the air-side surface because of the lower surface energy of poly(dimethylsiloxane) (surface energy γ ) 21mN/m). To estimate the extent of the surface concentration of poly(dimethylsiloxane), the Si/S atomic ratio for III.2 was calculated. Although the overall atomic concentration ratio, Si/S, is 2.7 from its chemical formula, the calculated ratio from XPS analysis is 5.45. This value indicates that the surface concentration of poly(dimethylsiloxane) is about 2 times higher than the bulk poly(dimethylsiloxane) concentration. It is reported that the sampling depth of XPS is about 2-10 nm depending on the density of the organic material.26-29 Therefore, it is reasonable to assume that the poly(dimethylsiloxane) residues in LB film for III.2 are gathered in the outer surface of the LB film by the flip(24) Chen, X.; Gardella, J. A., Jr.; Kumler, P. L. Macromolecules 1992, 25, 6621. (25) Chen, X.; Gardella, J. A., Jr.; Kumler, P. L. Macromolecules 1993, 26, 3778. (26) Chen, X.; Lee, H. F.; Gardella, J. A., Jr. Macromolecules 1993, 26, 4601. (27) Kawakami, Y.; Aoki, T.; Yamashita, Y.; Hirose, M.; Ishitani, A. Macromolecules 1985, 18, 580. (28) Smith, S. D.; DeSimone, J. M.; Huang, H.; York, G.; Dwight, D. W.; Wilkes, G. L.; McGrath, J. E. Macromolecules 1992, 25, 2575. (29) Tezuka, Y.; Nobe, S.; Shiomi, T. Macromolecules 1995, 28, 8251. (30) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. J. Polym. Sci., Polym. Phys. Ed. 1988, 26, 1781.
Hirano et al.
flop30,31 of the poly(dimethylsiloxane) part while maintaining the layered structure. Conclusion The formation of monolayers by ion pairing of alkylamines containing single, double, and triple chains with poly(amic acid) derivatives containing a dimethylsiloxane structure was demonstrated. The morphology change of the surface of the monolayers under compression was observed by BAM. The destruction of the monolayers was promoted by the increase of the number of alkyl chains per repeating unit and relaxed by the introduction of a dimethylsiloxane chain. The LB films obtained from poly(amic acid) alkylamine salts with multichains consisted of bilayer (Y type) structures. The degree of vertical orientation of alkyl chains in the LB films increased with the increase of the number of alkyl chains per repeating unit. The dimethylsiloxane chain was gathered in the outer surface of the LB films, when the long dimethylsiloxane chain was employed. Acknowledgment. We thank Dr. Y. Nishi of Nagoya Municipal Industrial Research Institute for the XPS measurement. LA970570L (31) Tezuka, Y.; Ono, T.; Imai, K. J. Colloid Interface Sci. 1990, 136, 408.
