3040
Langmuir 1992,8, 3040-3042
Well-ordered Langmuir-Blodgett Films from Polyamic Acid tert-Amine Salts Bearing Multichainsl Koji Hirano, Makoto Sato, and Hiroyuki Fukuda' Nagoya Municipal Industrial Research Institute, Atsuta-ku, Nagoya 456,Japan
Masa-aki Kakimoto' and Yoshio Imai Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Received March 27, 1992. I n Final Form: July 23, 1992 tert-Amines bearing multichains such as methyldioctadecylamine (2a), 0,O'-dihexadecanoyl-Nmethylethanolamine (2b), and 0,O"O"-trihexadecanoyltriethanolamine (2c) were prepared in order to match the repeating unit area of polyamic acid (l),prepared from pyromellitic anhydride and bis(4aminophenyl) ether, with the s u m of the cross section of two molecules of tert-amine. Monolayers of polyamic acid tert-amine salts (3b and 3c) at the air-water interface showed phase transitions at about 20 mN/m, which reflects the behavior of tert-amines. Monolayers of 3b and 3c were transferred by Y-type deposition, whereas transfer of 3a was Z-type. X-ray diffraction patterns of Langmuir-Blodgett films of 3b and 3c showed sharp peaks with d-spacing of 5.06 and 5.36 nm, respectively. Introduction
It is well-known that wholly aromatic polyimides are useful engineering plastics with excellent chemical, physical, and electrical properties and widely used as reliable insulating materials in electronics. We have reported the preparation of ultrathin films of polyimides using the LangmuhBlodgett (LB)technique? spreading a polyamic acid alkylamine salt as an amphiphilic precursor at the air-water interface, followed by (i) deposition of the precursor monolayer on a suitable substrate and (ii) chemical imidization by means of acetic anhydridepyridine. It is of particular importance for the LB technique that the occupied area of the repeating unit of polyamic acid should fit with that of the hydrophobic alkyl chains of two molecules of tert-amine. In a previous paper? we estimated the unit area of polyamic acid from pyromellitic anhydride and bis(Caminopheny1) ether to be 1.28 nm2 using the HGS molecular model. This value seemed to be too large for single-chain tert-amines to align vertically at air-water interface4 because the cross section of alkyl chains is ca. 0.2 nm2. In order to overcome this mismatching, it should be essential tQ use the multichain tertamine instead of the single-chain tert-amine. We would like to describe herein the first example of well-ordered LB films of polyamic acid salts by using multichain tert-amines on the basis of the above concept. Experimental Section Materials. Unless stated otherwise, aJl reagenta and chemicals were obtained from Wako Pure Chemical Industries, Ltd., and
used without further purification. Polyamic acid ( 1)6and 0,O'(1) Supported by the Nagoya Municipal Government (Grant 1990-02) and Naito Foundation (Grant 1991-08) for H.F. (2) Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai,Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986, 823. (3) Kakimoto, M.; Morikawa, A.; Nishikata, Y.; Suzuki, M.; Imai, Y. J. Colloid Interface Sci. 1988,121, 599. (4) Tsukruk, V.;Mischenko, N.; Scheludko, E.; Tolmachev, A. Makromol. Chem., Macromol. Symp. 1991,46, 277. (5) Nishikata, Y.; Morikawa,A.; Takiguchi, Y.; Kanamoto, A.; Suzuki, M.; Kakimoto, M.; Imai, Y. Nippon Kagaku Kaishi 1987, 2174.
dihexadecanoyl-N-methylethanolamine(2b)6were synthesized by the reported methods. Dioctadecylaminewas purchased from Fluka Chemie AG and purified by recrystallization (3 times) from ethanol. Methyldioctadecylamine (2a). A mixture of dioctadecylamine (2.3 g, 4.6 mmol), ethanol (100 mL), and 97% formic acid (1.5 mL) was heated at 50 O C until the solid was dissolved completely. After the solution was cooled to 40 O C , 35 % aqueous formaldehyde (0.5 mL) was added and then the mixture was heated at 60 "C for 30 min. After the evolutionof carbon dioxide was completed, the mixture was refluxed for an additional 2 h and then neutralized with aqueous sodium hydroxide. The solvent was removed under reduced pressure and the organic layer was extracted with hexane (50 mL X 2) and dried over anhydraus magnesium sulfate. After hexanewas removed under reduced pressure, the residue was recrystallized from ethanolacetone (1:l (v/v))to give 2a (2.1 g, 86%), mp 39-41 O C . Calcd for C87H77N: C, 82.88; H, 14.51; N, 2.61. Found C, 82.95; H, 14.42; N, 2.49. O,Q,Q'-Trihexadecanoyltriethanolamine (2c). To a solution of triethanolamine (0.75 g, 5 mmol) in NJV-dimethylformamide (DMF) (25 mL) was added a solution of hexadecanoyl chloride (5.0g, 18.2 mmol) in DMF (25 mL) at 0 OC. The mixture was then warmed to room temperature and stirred overnight. After the solvent waa removed under reduced pressure, the reaidue was dissolved in chloroform (100 mL) and washed with 1 N aqueoussodium hydroxide (50 mL). The organiclayer was further washed with water (50 mL X 2) and dried over anhydroue magnesium sulfate. After chloroformwas removed under reduced pressure, the residue was recrystallized from hexane to yield 2c (4.0 g, 92%),mp 48.5-50.5 O C . Calcd for Ca$Il&Oe: C, 75.01; H, 12.27; N, 1.62. Found C, 75.14; H, 12.18; N, 1.53. Monolayers and LB Films. Monolayers were obtained by spreading benzendVJV-dimethylacetamide(1:l (v/v))solutions. The concentrations of solutions of 1 and 2a-2c were 1 0.01 mmoVL. The spreading solutions of 3a-30 were prepared by mixingthe solutions of 1and 2a-2c, just prior to the experiments. The surfacepressurearea ( S A )isothermswere measured under a continuouscompressionof monolayer (0.6 cm2/s)by a computercontrolled film balance (FSD-20,San-Esu Keisoku Co., Japan). The temperature of pure water (Milli-QSPgrade)in the subphase was kept constant at 20 i 0.2 "C. LB films were deposited by means of the computer-controlled film balance with a film deposition device (FSD-21, San-Eau Keisoku Co., Japan). The surface pressure during deposition
*
(6) Kippenberger,D.;Rosenquist, K.;Odberger,L.;Tundo,P.;Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 1129.
0743-746319212408-3040$03.00/0 0 1992 American Chemical Society
LB Film of Amine Salts
0
Langmuir, Vol. 8, No. 12, 1992 3041
1 .o
0.5
1
0
Area (nm2/molecule)
2
3
A r e a of r e p e a t unit (nm')
Figure 1. Surface pressure-area isotherms for tert-amines bearing multichains at 20 O C : 2a,- - -; 2b,- - -; 2c,
-.
Figure 2. Surface pressure-area isotherms for polyamic acid tert-amine salts at 20 O C : 3a,- - -; 3b,- - -; 3c, -.
Scheme I
1
1
-
[IO-( CH2 ) 2.
c151131c0c1
HO-(CH2),"-CH3
2)
HO-(CH2)2 HO.(CII~)~~N HO-(C112)2
I N NeOll
1) CISH31COC1
>-
2)
I N NaOll
2a
0 C113(C112)14COOCH
CII
Cl13(Cll,)l,CO~CH~Cll~~N~cH3
2b Cl13(CH2)14COOC112CI12 C H ~ ( C I I ~ ) ~ ~ C O O CI CI I I\ N CH3(CH2)14COOCH2CH2 2/
2c
process was kept constant at 25 mN/m. The dipping speed of the substrate was kept constant at 10 mm/min for both up- and downward strokes. Characterizations of LB fiis were performed by UV spectroscopy (Hitachi330),X-ray photoelectron spectroscopy(JEOL JPS-90 SXV) with a Mg Ka X-ray source (10 kV,15 mA), and X-raydiffractionanalysis(Rigakudenki,Rotaflex, R a d 4 System) with a Cu Ka X-ray source (40 kV, 100 mA).
Results and Discussion Preparations of polyamic acid (11, tert-amines bearing multichain (2a-2c), and polyamic acid tert-amine salts (3a-3c) were carried out according to Scheme I. The F-A isotherms for 2a-2c are shown in Figure 1. The curves of 2b and 2c clearly indicate the condensedto-expanded phase transition at ca. 20 mN/m, whereas that of 2a shows only the expanded phase. The limiting areas (Ao) estimated by extrapolating the steepest region of the condensed phases to zero pressure are 0.41 and 0.65 nm2/moleculefor 2b and 2c, respectively. These areas are in good agreement with the sum of cross section of alkyl chains in amine, taking into account the assumption that A0 of alkyl chain is ca. 0.2 nm2. As shown in Figure 2, the phase-transition behavior is observed in F-A isotherms for 3b and 3c, while the monolayer Of 3a exhibits only the expanded phase. These results suggest that the
20 40 N u m b e r of l a y e r s
60
Figure 3. Plots of UV absorbance at 260 nm against number of layers of polyamic acid tert-amine salts: 3b (deposited at 25 mN/m), 0;3c (deposited at 25 mN/m), 0; 3c (deposited at 10 mN/m), 0. behavior of monolayers of polyamic acid tert-amine salts are directly influenced by the nature of tert-amines. Estimated AOS' for 3b (1.10nm2) and 3c (1.40 nm2) are somewhat larger than the calculated areas from two molecules of amine per the repeating unit but fairly cloee to the previously estimated area of monomericunit of 1.3,4 The depositions of 3a-3c onto a quartz plate were carried out under the constant surface pressure at 25 mN/m. This value is just above the phase transitions of 3b and 3c and is chosen so as to guarantee the transfers of the condensed phases and to ensure the stability of monolayers. The obtained LB film from 3a had the 2-type structure, which was the same as the case of the polyamic acid tert-amine salts having a single chain. On the other hand, monolayers of 3b and 3c were transferred in Y-type deposition. Linear relationships between the intensity of absorbance and number of deposited layers were obtained as shown in Figure 3. The difference of slope of these lines can be explained by the occupied areas, i.e., LB film deposited under smaller occupied areas should show higher intensity of absorbance compared with that deposited under larger occupied area at the same surface pressure. X-ray photoelectron spectroscopy is one of the useful tools for surface analysis of LB films.' The results of elementary analysis of the surface of LB films, 3a-3c, are coincident with the calculated valuetx8 These results indicate that the monolayer at the air-water interface is ~~
~~~~
~
(7)Ishitani, A.; Ishida, H.; Nakayama, Y.; Saaanuma, Y. Nippon Kagaku Kaiahi 1990, 1096, and references are therein. Schreck, M.; Schmeiseer,D.;Gopel, W.;Schier,H.;Habermeier,H.U.;Roth,S.;Duleg, L. Thin Solid Films 1989, 175, 95. (8) LB film 38, Calcd for CSHI&LO,: C, 1.00; N, 0.04; 0, 0.07. Found: C, 1.00; N, 0.04; 0.0.09. LB f h 3b, Calcd for C&&~O~s: C, 1.00, N, 0.04; 0,0.16. Found C, 1.00, N, 0.04; 0,0.17. LB f h &, CalcdforC&2YN,O1D: C, 1.00;N,0.03;0,0.15;FoundC, 1.00;N,0.03; 0, 0.13.
Hirano et al.
3042 Langmuir, VoI. 8, No. 12, 1992 15
41 a v1
h . 2 Y R
I
1
3 2 8
20
I o
5
7
5
7
1 deg
I
o t 1
lo 3
2 8 I deg Figure 4. X-ray diffraction patterns of LB f i b s of polyamic acid tert-amine salts: (a) 3b (32 layers); (b) 3c (32layers).
repeatedly transferred onto a substrate with the salt structure between polyamic acid and tert-amine maintained. To examine the orientation of multichain tert-amine in LB film, X-ray diffraction measurement (0.9O C 28 C 8.0°) was carried out. As can be seen in Figure 4 , 4 orders of diffraction of multilayered structure are observed for LB film of 3c whereas only the first-order peak is detected for
that of 3b. Higher orders of diffraction and larger intensity of the fiist-order peak strongly suggest that LB film of 3c is more packed and ordered than that of 3b. On the other hand, no peak was detected in the X-ray diffraction patterns for LB f i i of 3a? These resulta indicate that the fine structure of LB films is duplicated from the orientation of the monolayer at the air-water interface, because F-A isotherms for 3b and 3c show the presence of the condensed phase above the surface pressure of 20 mN/m. The d-spacings calculated from the diffraction angles for 3b and 3c are 5.06 and 5.36 nm, respectively, the values of which are consistent with the width of the expected bilayer structure from the Y-type deposition. AB for the LB film Of 3a, the disordered amorphousmonolayer structure might be deposited. It can be anticipated that these fine-layered LB films from polyamic acid tert-amine salta bearing multichains are valuable precursors for wellordered ultrathin films of polyimides. In fact, these LB films are completely converted into polyimides by chemical imidization. Studies that are currently in progress are aimed at examining the influence of structure of tert-amine for F-A isotherms and the mode of deposition and the characterization of LB films before and after imidization.
Acknowledgment. We are grateful to Dr. Yasuo Nishi (Nagoya Municipal Industrial Research Institute) for technical assistance and helpful discussions. Registry No. 2a,4088-22-6; 2b,88703-86-0;2c, 88931-91-3; NH((CHZ)I,CH&, 112-99-2;MeN((CHz)zOH)Z, 105-69-9; CHs(CH2)lrCOCI, 112-67-4;N((CH&OH)s, 102-71-6. @)Very recently Tsukruk et al. have reported that d-epacing of polyamic acid salt from 1 and Nfl-dimethyloctadecylamine is 1.8 nm and the alkyl chains pack on the tilt similar to smectic C type liquid crystals (ref 4).
