Langmuir 1994,10, 557-562
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Two-Dimensional Polymer Networks of Maleic Acid Copolymers and Poly(allylamine) by the Langmuir-Blodgett Technique Burm-Jong Lee* Department of Chemistry, Inje University, Kimhae 621-749, Korea
Toyoki Kunitake Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan Received July 24,1993. I n Final Form: October 20,199P Two-dimensionally cross-linked Langmuir-Blodgett (LB) films of maleic acid copolymers and poly(allylamine) were prepared by employing a double-chain amine ( 1 ) as the monolayer template which was subsequently removed by thermal imide formation followed by extraction. Maleic acid copolymers used were poly(maleic acid-co-methyl vinyl ether) (PMAMVE), poly(maleic acid-co-styrene) (PMAS), and poly(maleic acid-co-ethylene) (PMAE). The polyion-complexed monolayers of three components, i.e., template amine 1, a maleic acid copolymer, and poly(allylamine), were formed at the air-water interface. Their monolayer properties have been studied by the surface pressure-area isotherm and fluorescence microscopy. The monolayers could be transferred on solid substrates. The polyion-complexed LB films were characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS),scanning electron microscopy (SEMI, and ellipsometry. Two-dimensional cross-linking to form a polymer network was achieved by imide formation through heat treatment under vacuum. SEM observation of a film with PMAMVE on a porous fluorocarbon membrane filter (pore diameter 0.1 pm) showed covering of the pores by two layers in the polyion complex state. Six LB layers were, however, required in order for the f i i not to be destroyed during the heat treatment. Extraction by chloroform followed by immersion in aqueous NaCl produced pin-hole defects in the film.
Introduction The formations of polyion-complexed monolayers at the air-water interface have been used as a distinctive process in the fabrication of Langmuir-Blodgett (LB) films.' By employing these polyion complexes, the preparation of LB films with special functionalities became possible. For example, we succeeded in forming a LB f i i of a fluorocarbon amphiphile by complexing with potassium poly(styrenesulfonate),2 which is otherwise intractable to transfer the monolayer onto a substrate. The LB films of azobenzene amphiphiles as polyion complexes were prepared by spreading monolayers on subphases containing anionic' or cationicS polymers. Ringsdorf et al. reported the domain formation of fatty acid monolayers on a poly(ethyleneiminel-containing~ubphase.~ Thin films of poly(imide)6*6and poly(ary1ene~inylene)~ were prepared by the LB method by attaching electrostaticallyhydrophobic chains to the precursor polymers which, by themselves, are incapable of monolayer formation. We have extended the polyion complexes to polymerpolymer systems of two kinds of polymers. We reported formations of salt bridges between a polymerized ammonium monolayer and an anionic polymer at the air-water interfacea and of a two-dimensional polymer network of 9
AbettactpubliahedinAduanceACSAbstracts, January15,1994.
(1) Shimomura, M.; Kunitake, T. Thin Solid Films 1988, 132, 243. (2) Hieanhi, N.;Kunitake, T. Chem. Lett. 1986, 106. (3) Umemura, J.; Hishiro, Y.; Kawai, T.; Takenaka, T.; Gotoh, Y.; Fujihsra, M. Thin Solid Films 1989,178, 281. (4) Chi,L. F.;Johnston, R. R.; R-orf, H. Langmuir 1991,7,2323. (6) S d , M.; Kakimoto, M.; Koniehi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986,396. (6) Baker,S.;Seki, A.; Seto, J. Thin Solid Films 1989,180, 263. (7) Era,M.;Kamiyama,K.; Yoshiura,K.; Momii,T.; Murata,H.;Tokito, S.;T~utsd,T.; Saito, 5.Thin Solid Films 1989, 179, 1.
(8) Kunitake, T.; Higashi, N.; Kunitake, M.; Fukwhige, Y. Macromolecules 1989,22, 486.
0743-7463/94/2410-0557$04.50/0
LB films of a polyamine-polycarboxylate salt.a Higashi et al. reported chain-length recognition of poly(acry1ic acid)s by the monolayer of a polymer-based amphiphile.lO In this paper, we describe two-dimensional polymer networks of maleic acid copolymers and poly(ally1amine) (PAA),which were prepared from monolayers consisting of three polyion-complexing components. Double-chain tertiary amine 1 was used as the template for enhancing the monolayer stability and deposition efficiency,and was eventually removed from the LB film. In this system, the maleic acid copolymer forms polyion salts with both the template amine 1 and subphase polymer PAA.
Experimental Section Materials. Thetemplate amine l,l,l-dioctadecyl-E[2-(N~dimethy1amino)ethyllglyceryl triether, was prepared as follows. 1,3-DioctadecylglyceryldietherlZ(2ClsOH)(2.0 g, 3.4 X 1Vmol) was dissolved in dried tetrahydrofuran, and 0.16 g (3.8 X 10-8 mol) of sodium hydride (60%in oil dispersion)was added. After
the solution became clear, p-toluenesulfonyl chloride (3.2 g, 1.7 x mol) dissolved in tetrahydrofuran was added dropwise to the solution. After 24 hat 60 O C under nitrogen flow,the reaction solutionwas cooled in an ice bath, and excessNaH was destroyed with ethanol. After condensingthe solutionby rotary evaporator, the solution was poured into 1.5 L of ethanol and stirred for 2 h. After cooling the solution, the precipitates of the tosylate (2CleOTs)were collected by filtration, and recrystallized twice from ethanol: white powder; 1H NMR (CDCb, ppm) 6 0.8-1.1 (t,6H),1.2-1.6 (m,64H),2.5 (8, 3H), 3.2-3.7 (m, 9H). 7.3 (d,2H), 7.9 (d, 2H); IR (KBr) 2916,2848, 1597,1467, 1360,1176, 1127, (9) Ueno, T.; Kunitake, T. Chem. Lett. 1990, 1927. (10) Higashi,N.; Matsumoto,T.; Nojima, T.;Niwa, M. Polym. R e p r . Jpn. 1991, 40 (3), 968.
(11) Okahata,Y.; Tanamachi, S.; Nagai, M.; Kunitake, T. J. Colloid Interface Sci. 1981,82,401. (12) Kimizuke, N.; Kunitake, T. J. Am. Chem. SOC.1989,111,3758.
0 1994 American Chemical Society
558 Langmuir, Vol. 10, No. 2, 1994
Lee and Kunitake 80
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1101, 951, 882, 820, 781, 718, 668 cm-l. Anal. Calcd for C d s O a S : C,73.54;H,11.54;5,4.27. Found C,73.59;H,11.27; S, 4.05. This tosylate (2.0 g, 2.7 X 10-8 mol) was dissolved in 50 mL (44.3 g, 0.5 mol) of (2-dimethy1amino)ethanolcontaining 1.5 g (2.7 X 10-2 mol) of potassium hydroxide as the dissolved state. This reaction was continued for 48 h at 100 "C. After cooling to room temperature, the solution was poured dropwise into 1.5 L of ice-cooled water and stirred for 2 h. The precipitates were collected by filtration and dissolved in diethyl ether. The ether solution was washed with water until neutrd pH. The ether solvent was exchanged with 1 N HC1 ethanol solution. After evaporation of ethanol, the 1-HC1salt was recrystallized twice from hexane. The free amine 1 was obtained through a strong anion exchange resin, Amberlyst A-27. After exchange of the counterionof the resin to hydroxide,the tetrahydrofuran solution of 1.HC1 was eluted. After evaporation of the fractions, the residue was recrystallizedfrom acetone to yield 1: white powder; mp 38-40 OC; 1H NMR (CDCla, ppm) b 0.8-1.1 (t, 6H), 1.2-1.6 (m, 68H), 2.1-2.7 (m, 8H), 3.4-3.9 (m, 11H);IR (KBr) 2916,2848, 1466,1376,1119,719cm-l. Anal. Calcd for CaHmNOs: C, 77.29; H, 13.42; N, 2.10. Found C, 77.31; H, 13.27; N, 1.97. The maleic acid copolymers used were those of polymer kits of Scientific Polymer Products, Inc. Poly(mdeic acid-co-styrene) and poly(maleic acid-co-ethylene)were prepared by hydrolysis of the corresponding maleic anhydride copolymers. Poly(allylamine)hydrochloride (M,= 1.0 % l(r)was purchased from Nitto Boseki Co. and treated with a strong anion exchange resin (Amberlite IRA-402) to obtain free poly(ally1amine). The concentrationof amino group in aqueoussolutionwas determined by colloid titration with standard poly(viny1 sulfate) solution using Toluidine Blue 0 as the indicator. Pressure-Area ( P A ) Isotherm. A computer-controlledf i i balance systemFSD-50 (San-esuKeisoku)was used for measuring surface pressure as a function of molecular area (trough size 150 X 600 "2). Isotherms were taken at a compression rate of 0.4 mm/s, and the temperature of the aqueous subphase was maintained at 30.0 f 0.1 "C. A tetrahydrofuran/dimethyl sulfoxide(6/4, by volume) mixture was employedas the spreading solvent. Monolayers were spread on pure water or on aqueous poly(ally1amine) (2 X 1W mol/-NHz) and incubated for 10 and 30 min, respectively, before starting the compression. LB Deposition of Monolayer onto Solid Substrates. The deposition of monolayer was performed in the vertical mode by using a computer-controlled film balance (FSD-BO) and lifter (FSD-51) system (San-esu Keisoku). The transfer onto solid substrates was carried out at a surface pressure of 30.0 & 0.2 mN/m and a deposition rate of 4 mm/min. The temperature of the subphase was kept at 30 & 0.1 OC. The employed substrates were fluorocarbonmembrane filters (FP-010,Sumitomo Electric Co.) for SEM observation and XPS measurement, CaF2 plates (GL Sciences)for FT-IR measurement, and Si wafers (E&MCo.) for ellipsometry.
Area (nm*/molecule) Figure 1. Surface pressurearea isotherms of 1 fixed electrostatically with maleic acid copolymers: (a) on pwe water; (b) on aqueous PAA at 30 OC; (A) PMAE; (B) PMAMVE; (C) PMAS. The concentration of the amino group of PAA is 2 X lo-' M.
Measurements. FT-IR measurements were carried out on a Nicolet 710 FT-IR spectrometer by the transmission method. In order to minimize the influence of water vapor and COSon the spectra, the system was purged with dry air for 1-2 h before measurement. XPS spectra were obtained on an ESCA 5300 X-ray photoelectron spectrometer (Perkin-Elmer), which was operated at 12 kV and 25 mA (300W)with a Mg Ka X-ray source and at less than 7.0 X 10-8 Torr. The sample stage was cooled to below -100 OC by an internal thermal condudor connected to an external cryogenic Dewar. The survey spectrum in a range of O-lOOO eV was measured with a pass energy of 89.45 eV with a sampling step of 1.0 eV/step. The data aquisition time was 1 min. In the narrow-regionmeasurement (394-414eV for the Nh peak and 525-545 eV for the Oh peak), the pass energy of 8.94 eV was used with a sampling step of 0.1 eV/step and a scan time of 1-2 min. The take-off angle was fiied at 45O. Data analysis was carried out with an Apollo Domain 3500 computer. Scanning electron microscopy (SEM; Hitachi 5-900) was used to observe the surface morphology of the FP-010 membrane filter (pore diameter 0.1 pm). An acceleration voltageof 2 kV was employed. The sample was sputtered with Pt-Pd before observation. The film thickness was determined by an ellipsometer (Gaertner L-115B). Before the measurement ofthe thickness of thesample film on a Si wafer, the optical parameters of the blank Si wafer cleaned were measured,and the values were input into the analysis program for data correction. Data analysis was automatically carried out with a Hewlett-Packard 9OOO computer. The monolayer structure on water was monitored with a fluorwence microscope (Zeiss, Axiophot) equipped with a SIT TV camera (HamamatauModel C-2400). Fluorescein-PE(Molecular Probea, Inc.) was used as a fluorescence probe. A tetrahydrofuran/ dimethyl sulfoxide solution of 1 and a maleic acid copolymer containing the probe (0.5 mol 7% maleic acid unit) was spread on aqueousPAA(2x 1Wmol)(troughsize13X7cm2). Themovable barrier was controlled by a FSD-20 system (Sa-eeu Keisoku). A 150-W Hg lamp with a filter (Zeiss, BP 546/12) was used for excitation, and the light emitted from the monolayer was passed through a filter (Zeiss, L P 590).
Langmuir, Vol. 10, No. 2, 1994 559
Two-Dimensional Polymer Networks
Results and Discussion Monolayer Formation and Its Transfer on Solid Substrates. The monolayers were formed by spreading a tetrahydrofuran/diemthyl sulfoxide (6/4, by volume) solution of equimolar amounts of 1 and a maleic acid copolymer (with respect to repeat units) on pure water or on aqueous PAA. The T-A isotherms of Figure 1show that amine 1 fixed electrostatically with maleic acid copolymers forms stable monolayers. The monolayers of 1-PMAMVE and 1-PMAE give condensed phases, while that of 1-PMAS shows an expanded phase. The bulky phenyl group of PMAS is supposed to be an origin of the expanded phase. When compared with the isotherm on pure water, a more increased molecular area at a given pressure is revealed on aqueous PAA. This change between the two isotherms is attributed to the formation of a polyion complex at the interface and the consequent change of the monolayer organization, as already described for other systems.1$9J2 The structure change of the monolayer on aqueous PAA was noticed in the observation by fluorescencemicroscopy during compression of the monolayer. All the monolayers show island structures upon spreading but become homogeneous by compression, as shown representatively in Figure 2a,b. Figure 2c shows white lines appearing at a molecular area of 0.30 nm2. This area corresponds to the state far beyond the collapse point of a ca. 0.50 nm2in the isotherm. Therefore, the white lines are attributed to overlayers of collapsed monolayers.l3 Monolayer transfer was possible onto various substrates such as a fluorocarbon membrane filter, a CaF2 plate, and a Si wafer. In the cases of 1-PMAMVE-PAA and 1-PMAE-PAA systems, Y-type deposition was found at a surface pressure of 30 mN/m and a transfer rate of 4 mm/min, although the deposition started from an upward stroke in the case of the CaF2 plate and Si wafer. In the case of the 1-PMAS-PAA monolayer, the Y-type deposition was changed to Z-type deposition from the second dipping run on the fluorocarbon membrane filter, while 2-type deposition was shown in the substrates of the CaF2 plate and Si wafer from the start. The reason for this different deposition behavior is not clear. The only result we have is that the same deposition behavior was found in the system of the corresponding itaconic acid copolymer containinga bulky hydrophobicgroup.14 The transfer ratio was close to unity (1.0-1.1) on all the substrates. These monolayers were stable enough to maintain a constant area for 3 h when kept at a surface pressure of 30 mN/m, where the monolayers were transferred. Polyion-Complexed LB Film. The formation and composition of a polyion complex in the resulting LB films
Scheme 1. Imide Formation from a Polyion Complex Film through Heat Treatment
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could be confirmed by means of FT-IR and XPS measurements. Parts a and b of Figure 3 show the FT-IR spectra of the film of the 1-PMAMVE system prepared from pure water and aqueous PAA, respectively. The spectrashow that the peak (1721cm-l) from the carboxylic acid group in the film prepared from the pure water subphase is remarkably reduced in the film prepared from the aqueous PAA subphase, while the peak (1565 cm-l) from carboxylate salt is increased. This change together with the appearance of the NH bending (1650 cm-l) peak in Figure 3b shows that polymer PMAMVE forms salts with both template amine 1 and subphase polymer PAA (see Scheme 1). Carbonyl peaks similar to those of this film were obtained in FT-IR spectra from 1-PMAS-PAA and 1-PMAE-PAA films, and those from the I-PMAEPAA system are shown in Figure 4. The composition of a polyion complex was estimated by XPS measurements. The N1, XPS spectra of the LB film of the 1-PMAS-PAS system are revealed in Figure 5. The N1, peak (Figure 5a) of the as-deposited film is made of two components which are attributed to ammonium N (binding energy 405.0 eV) and amino N (401.8 eV). The ammonium N peak is 33.2 % of the total nitrogen area. The incorporation content of PAA in the film is estimated by taking into account the atomic sensitivity factor and the relative area of the N1, and 01, peaks. The results including 1-PMAMVE-PAA and 1-PMAE-PAA systems are summarized in Table 1. This XPS result reveals that ca. 2 equiv (amino group) of subphase polymer PAA are incorporated into the as-deposited film with respect to the repeat unit of maleic acid copolymers. SEM observation of the 1-PMAMVE-PAA film on porous fluorocarbon membrane filters (FP-010) showed covering of the pores, which are seen as long and somewhat dark appearances in Figure 6, by two layers in the polyion complex state. The covering ability was different accord-
Figure 2. Fluorescence micrographs of monolayers of 1-PMAMVE on aqueous PAA: (a) at 0.7 nm2/molecule; (b) at 0.5 nm2/molecule; (c) at 0.3 nm2/molecule. Fluorescein-PE was used as the fluorescence probe (0.5 mol % maleic acid unit).
560 Langmuir, Vol. 10, No. 2, 1994
Lee and Kunitake
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Wavenumber(cm") Figure 3. TransmissionFT-IRspectra of LB films (13 X 2 layers) of 1-PMAMVE on a CaFz plate: (a) as-deposited film from the pure water subphase; (b) as-deposited film from the aqueous PAA subphase; (c) heat-treated film of (b) at 150 "C for 10 h in a vacuum; (d) film of (c) placed in aqueous NaCl and in CHC13 for 1 day, respectively.
ing to the employed polymers. In the cases of 1-PMASPAA and 1-PMAE-PAA films, hole defects, which are seen as dark spots as indicated in Figure 6d, were seen even in four layers. Although we did not further pursue the systematic investigation about the relation between polymer structure and covering ability, the flexibility of the polymer backbone is supposed to play a role for enhancing the covering ability from the present result. The thickness of the deposited layer was determined by ellipsometry. It was proportional to the number of deposited layers. The unit layer thickness as estimated from the slope is shown in Table 2. It ranged from 42.2 to 44.9 A in the state of polyion complex. Assuming that the alkyl tails are perpendicular to the layer plane and the ion pairs are tight as shown in Scheme 1,the CPK model building suggests that the estimated layer thickness is ca. 39 A. This somewhat larger measured value is supposed to be owed to incorporation of water molecules as the hydrated form around the ionic groups. Imide Cross-LinkedFilm. The polymer network with an imide bridge was achieved by heat treatment of the polyion-complexed film at 150 "C in a vacuum for 10 h. The formation of imide bonds (Scheme 1) could be confirmed by FT-IR spectra. New peaks at 1764and 1703 cm-l as shown in Figure 3c, which is of the heat-treated 1-PMAMVE-PAA film, are assigned to asymmetric and symmetric stretching modes, respectively, of the imide group. This conversion to imide was also confirmed from FT-IR spectra of 1-PMAS-PAA and 1-PMAE-PAA (Figure 4) films. The XPS spectrum given in Figure 5b also supports the formation of the imide group, since a (13) Lee, B.-J.;Kunitake, T. Langmuir 1992,8, 2223. (14) Lee, B.-J.; Kunitake, T. An unpublished result.
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