2223
Langmuir 1992,8, 2223-2227
Stabilization and Skeletonization of Polyion-Complexed Langmuir-Blodgett Films by Two-Dimensional Imide Formation Burm-Jong Lee and Toyoki Kunitake'pt Molecular Architecture Project, JRDC, Kurume Research Park, Kurume 830, Japan Received March 3,1992 A polyion-complexed polymeric monolayer was prepared by spreading monoalkyl maleate copolymer 1 on the aqueous poly(ally1amine)subphase. Ita monolayer properties have been studied by the surface
pressure-area isotherm and fluorescence microscopy. The polyion-complexed Langmuir-Blodgett (LB) films on various substrates were characterized by FT-IR spectroscopy,X-ray photoelectron spectroscopy and ellipsometry. Two-dimensional cross-linking to form (XPS), scanning electron microscopy (SEM), a polymer network was achieved by imide formation through heat treatment under vacuum along with concurrent removal of alkyl tails. From quantitative XPS analyses, the degree of cross-linking and skeletonization was determined to proceed for up to 77% of the maleate unit under employed conditions. SEM observation of this film on a porous membrane filter showed that the four-layer film was sufficiently stable to cover the filter pore (0.1 pm). Immersion of this film in water and benzene did not cause any change in its appearance and in FT-IR spectra. Introduction Stabilization by polymerization and cross-linking is critical in order to improve the intrinsic fragility of monolayers and Langmuir-Blodgett (LB) films and to make their technological applications possible.'r2 Polymerization was applied to monomeric monolayers and LB films diene^,^^^ diacetylenes,+13 and amino acid of esters.14J5 Cross-linked monolayers have been obtained by polycondensation of octadecylureas16and amino acid esters.17J8 Bauer et al.19 carried out cross-linking of hydroxy moieties with epichlorohydrin in a lipoid monolayer. Tredgold et al. were among the first to prepare
* To whom correspondence should be addressed. t Permanent address: Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812,Japan. (1)Breton, M. J. Macromol. Sci., Reu. Macromol. Chem. 1981,C12, 61. (2)Baker, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adu. Polym. Sci. 1985, 69,1. (3)Beredjick, N.; Burlant, W. J. J.Polym. Sci., Part A-1 1970,8,2807. (4)Akimoto, A.; Dorn, K.; Gros, L.; Ringsdorf, H.; Schupp, H. Angew. Chem., Znt. Ed. Engl. 1981,20,90. ( 5 ) Puterman, M.; Fort, T., Jr.; Lando, J. B. J. Colloid Interface Sci. 1974,47, 705. (6)Miyashita, T.; Yoshida, H.; Murakata, T.; Matsuda, M. Polymer 1987,28,311. (7)Hupfer, B.; Ringsdorf, H.; Schupp, H. Makromol. Chem. 1981,182, 247. (8)Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid F i l m 1986, 133,39. (9)Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (IO)Sarkar, M.; Lando, J. B. Thin Solid Films 1983,99,119. (11)Albrecht, 0.;Johnston, D. S.; Villaverde, C.; Champman, D. Biochim. Biophys. Acta 1982,687, 165. (12)Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,77. (13)Kajzar, F.; Messier, J. Thin Solid Films 1983, 99,109. (14)Folds, T.; Gros, L.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1982,3,167. (15)Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1983, 99,87. (16)Rosilio, C.;Ruaudel-Teixier, A. J.Polym. Sci., Polym. Chem. Ed. 1975,13,2459. (17)Fukuda, K.; Shibasaki, Y.; Nakahara, H.; Endo, H. Thin Solid Films 1989,179, 103. (18)Hanabusa, K.; Yamasaki, J.; Koyama, T.; Shirai, H.; Hayakawa, T.; Kurose, A. J. Macromol. Sci., Chem. 1989,A26 (121,1571. (19)Bauer, S.; Heckman, K.; Six, L.; Strobl, C.; Bloecher, D.; Henkel, H.; Garbe, T.; Ring, K. Desalination 1983,46, 369.
monolayers from preformed polymersz0 and cross-linkable polymersz1in order to avoid alteration of conformation and spatial arrangement during polymerization which usually leads to defect formation due to contraction or dilatation. We have reported stabilization of monolayers and LB films by electrostatic interaction of ionic polymers with oppositely-charged amphiphiles22and by covalent cross-linking of ionically interacting p0lpers.23 On the other hand, attempts to synthesize ultimately thin films have led to the skeletonization technique. Thin films of polyimideZ4and poly(arylenevinylene)2s were prepared by using precursor polymers. Polycondensation of amino acid esters26 produced cross-linked films along with detachment of the esteric alcohol. Removal of the detached species was usually performed during reactions in by heat treatment,%and by immersion in a solvent.26 In this paper, we describe cross-linking of LB films by imide formation along with concurrent removal of alkyl tails that are required for monolayer formation. As a closely related system, we reported the imide formation in polyion-complexed LB films of poly(monomethy1maleThe ate-co-octadecyl vinyl ether) and p~ly(allylamine)?~ reacted LB films, however, retained long alkyl chains. In the present system, the pendant alkyl moiety was introduced as a long-chain ester of the maleic acid unit and was removed during imide formation with poly(ally1amine). Experimental Section Materials. The monomer bis[(octadecyloxy)methyll maleic acid monoester and its copolymer 1 were prepared as follows. 1,3-Dioctadecylglyceroln (2ClsOH) (2.0 g, 3.4 X 10-8 mol) wae (20)Tregold, R.H.; Winter, C. S J. Phys. D 1982,16,L55. (21)Jones, R.; Tredgold, R. H.; Davis, F.; Hodge, P. Thin Solid Film 1990,186,L61. (22)Shimomura, M.; Kunitake, T. Thin Solid F i l m 1985,132,243. (23)Ueno, T.; Kunitake, T. Chem. Lett. 1990,1927. (24)Kakimoto, M.; Suzuki, M.;Konishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986,823. (25)Nishikata,Y.;Kakimoto, M.; Imai,Y. J. Chem.Soc., Chem. Commum 1988. 1040. . _.__ ---(26)Fukuda, K.; Shibasaki, Y.; Nakahara, H.; Endo, H. Thin Solid F i l m 1989,179, 103. (27)Okahata, Y.; Tanamachi, S.; Nagai,M.; Kunitake, T. J. Colloid Interface Sci. 1981,82,401. I
0 1992 American Chemical Society
2224 Langmuir, Vol. 8, No. 9, 1992
Lee and Kunitake
dissolved in dried tetrahydrofuran, and 0.15 g (3.8 X 10-3 mol) of sodium hydride (60% in oil dispersion) was added. The ICH
I
o=c
I OH
-CI H m C H 2 - C HI E c=o
O(CHd&H,
I
OCH[CHz0(CH2),,CH3I2 1
tCHz-CHk
I
C‘H2 NHz PAA
solution was kept at 60 OC for 5 h and turned clear. Maleic mol) dissolved in tetrahydrofuran anhydride (1.67 g, 1.68 X was added dropwise to the solution. The reaction was continued for 20 h at 60 OC under nitrogen. The violet solution was concentrated and poured into 1 L of water and stirred for 6 h. Dark brown precipitates were filtered and treated with activated carbon in chloroform far 12 h. The clear fiitrate was evaporated, and the residual white solid diesolved in tetrahydrofuran was reprecipitated from methanol: yield 53%;mp 46-48 OC; IR(KBr, cm-l) 3430 (v+H, carboxylic acid), 2916 (VC-H, methylene), 2848 (VC-H, methylene), 1717 ( V M , ester and acid), 1641 (V-, ethylene), 1465 (SC-H, methylene), 1172 (YC-O, ether); NMFt (CDCb, ppm) S 1.0-2.1 (70 H, m), 3.6-4.2 (9 H, m), 6.9 (2 H, 8). Anal. Calcd for CaHmOe: C, 74.30; H, 11.89. Found C, 74.52; H, 12.14. The copolymerizationof the monoalkyl maleate monomer was carried out in an open flask under nitrogen at 60 “C for 24 h by using n-butylvinyl ether aa comonomer and solvent. 2,2’-Azobis(isobutyronitrile) (AIBN;1 mol % of the monomaleate monomer) was used as initiator. The polymer was precipitated from 1propanol and water and dried at 30 OC under vacuum. Disappearance of the vinyl group was c o n f i i e d by IR and N M R spectra. The weight-averagemolecular weight was 5.9 X l(r (Mw/ M. = 4.5) as determined by gel permeation chromatography (polystyrene calibration). The polymer composition was determined to be 50 % monomaleate unit by elemental analyeis. Anal. Calcd for C&uO, (as 50% monomaleate unit): C, 74.00; H, 11.91. Found: C, 74.05; H, 12.01. n-Butyl vinyl ether was purchased from Wako Pure Chemicals and used without further purification. Poly(ally1amine)hydrochloride (M,= 1.0 X l(r)was purchased from Nitto Boseki Co. and treated with a strong anion exchange resin (Amberlite IRA402) to obtain free poly(allylamine). The concentration of the amino group in aqueous solution waa determined by colloid titration with standard poly(viny1sulfate) solution28using Toluidine Blue 0 as indicator. Press-Awg ( P A ) Isotherm. A computer-controlledf i i balance system FSD-50 (San-esuKeieoku)was used for measuring surface pressure as a function of molecular area (trough size, 150 X 600 mm). Isotherms were taken at a compression rate of 0.4 “/e, and the temperature of the aqueous subphase was maintained at 30.0 f 0.1 OC. Benzene was employed as spreading solvent. Monolayers were spread on pure water or on aqueous poly(allylamine) (2 x 1o-L mol/-NHz) and incubated for 10 and 30 min, respectively, before starting the compression. LB Deposition of Monolayer onto Solid Substrates. The deposition of the monolayer was performed in the vertical mode by using a computer-controlled film balance (FSD-50)and lifter (FSD-51) system (San-esu Keisoku). The transfer onto solid substrates was carried out at a surface pressure of 15.0 f 0.2 mN/m and a deposition rate of 4 mm/min. The temperature of the subphase was kept at 30 f 0.1 OC. The employed substrates were fluorocarbon membrane filters (FP-010, Sumitomo Electric Co.) for scanning electron microscopy (SEM) observations and X-ray photoelectron (XPS) measurements, CaFz plates (GL Sciences)for FT-IRmeasurements, and Si wafers (E&M Co.) for ellipsometry. Measurements. FT-IR measurements were carried out on a Nicolet 710 FT-IRspectrometer by the transmission method. In order to minimize the influence of water vapor and COz on the spectra, the system was purged by dry air for 1-2 h before measurement. XPS spectra were obtained on an ESCA 5300 X-ray photoelectron spectrometer (Perkin-Elmer), which was X-ray source operated at 12 kV and 25 mA (300W) with a Mg KCY and at less than 7.0 X 1O-g Torr. The sample stage was cooled to below -100 OC by an internal thermal conductor connected to an external cryogenic Dewar. The survey spectrum in a range
0
0.1
0.3 0.4 Area(nm*/molecule)
0.2
0.5
0.6
Figure 1. Surface pressurearea isotherms of 1: (a) on pure water; (b) on aqueous PAA at 30 OC. The concentration of the amino group of PAA is 2 X 1o-L M. of 0-1OOO eV was measured with a pass energy of 89.45 eV with a sampling step of 1.0 eV/step. The data acquisition time was 1 min. In the narrow-region measurement (394-414 eV for the N1, peak and 525-545 eV for the 01, peak), the paes 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 takeoffangle was fiied at 45O. Data analysis was carried out with an ApolloDomain 3500 computer. Scanning electron microscopy (SEM Hitachi S-900)was used to observe the surface morphology of a FP-010 membrane fiiter (pore diameter, 0.1 pm). An accelerationvoltage of 2 kV was employed. The sample was sputtered with Pt-Pd before observation. The film thickness was determined by an ellipsometer (Gaertner L115B). Data analysis waa automatically carried out with a Hewlett-Packard 9OOO computer. The monolayer structure on water was monitored with a fluorescence microscope (Zeiee, Axiophot) equipped with a SIT TV camera (Hamamatau, model C-2400). Fluorescein-PE (Molecular Probes, Inc.) was used as a fluorescence probe. A benzene solution of the polymer containing the probe (0.5 mol % monomaleate unit) was spread on aqueous poly(allylamine) (PAA;2 X l(r M)(trough size 13 X 7 cm2). The movable barrier was controlled by an FSD-20 system (San-esu 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, LP 590).
Results and Discussion Monolayer Formation and Its Transfer on Solid Substrates. Benzene solutions of polymer 1 were spread on pure water or on aqueous PAA. T-A isothermsof Figure 1 show that polymer 1 forms stable monolayers. These monolayers give expanded phases. When compared with the isotherm on pure water, a more expanded area is revealed on aqueous PAA,though it is more condensed at the initial step of compression. 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 No noticeable expansion of the isotherm is observed with increasing incubation times from 30 min to 2 h. The structure change of the monolayer on aqueous PAA was more clearly noticed in observation by fluorescence microscopy during compression of the monolayer. A benzene solution of the polymer containing fluoresceinPE was spread over aqueous PAA. Figure 2 shows fluorescence micrographs of the monolayer taken from TV pictures. Any structure formation is not observed at 0.50 nm2/molecule, which shows the presence of a macroscopically homogeneous monolayer at the initial state of compression (Figure 2a). Parta b and c of Figures 2 show white and straight lines appearing at molecular areas of 0.30 and0.25 nm2,respectively. These areas correspond (28) Terayama, H. Kagakunokenkyu (in Japanese) 1949,4, 31. (29) Kimizuka, N.; Kunitake, T. J . Am. Chem. SOC.1989,111,3758.
Langmuir, Vol. 8, No. 9, 1992 2225
Polyion-Complexed Langmuil-Blodgett Films
Figure 2. Fluorescence micrograph of monolaver 1 on queous PAA: (a) at 0.50 nm2/molecule; (b) at 0.30 nm2/molecule; (c) at 0.25 nm2/molecule. Fluorescein-PE was used as the' fluorescence probe (0.5 mol 76 maleate unit).
.-
c
h
C
3
2
.-E
c
e v
8 m
e v)
n
e
3
1800
1600 1400 1200 Wavenumber(cm")
1000
Figure 3. Transmission FT-IR spectra of LB f i i s (15X 2layers) on a CaF2 plate: (a) as-deposited film; (b) heat-treated at 180 "C for 10 h in a vacuum; (c) immersed in benzene for 24 h.
to the state far beyond the collapse point of ca. 0.43 nm2 in the isotherm. Therefore, the white lines are attributed to overlayers of collapsed monolayers. Since the straight linesare parallel,the surfacepressure must operate evenly over the monolayer, indicating satisfactory homogeneity of the monolayer.30 The polyion-complexed monolayer can be transferred onto various solid substrates such as a porous fluorocarbon membrane filter, slideglass, CaF2plate, Si wafer, and poly(propylene) membrane filter. In all cases, the Y-type deposition was found at a surface pressure of 15 mN/m and a transfer rate of 4 mm/min; however, no deposition was observed at the first downward stroke in the case of the slide glass, CaF2 plate, and Si wafer. The transfer ratio was 1.2 to 0.9 on a CaF2 plate for 11layer depositions. On the other hand, the monolayer on pure water shows X-type deposition. When a surface pressure of 15mN/m was applied to the monolayer for 1 h, its area gradually decreased by about 10%. This is in contrast to the behavior of the polyion-complexed monolayer, which becomes so stable that a spontaneous area decrease is not at all observed during the transfer period of up to 4 h. Polyion Complex Film. The formation and compo(30) Yanagi,M.;Tamamura, H.; Kurihara, K.; Kunitake, T. Langmuir 1991, 7,167.
68.0
w.0
4x0
45.0
m.0 4M.o ~imiwt[ICRGY. ev
404.0
61.0
m.0
399.0
398.0
Figure 4. XPS spectra of the N1,region of a 16-layer LB film on a fluorocarbon membrane filter (SumitomoElectric, FP-010): (a) as-deposited film; (b) after heat treatment at 180 "C for 10 h in a vacuum. The charging shift was not corrected.
sition of a polyion complex could be confirmed by means of FT-IR and XPS measurements. Figure 3a shows two characteristic carbonyl peaks at 1733and 1575cm-', which are attributed to ester and carboxylate salt, respectively. The appearance of a shoulder peak at around 1650 cm-' is due to the N-H bending mode, showing incorporation of PAA into the film. An elemental analysis of this film was performed by XPS measurement. The N1, peak observed by XPS (Figure4a) is made of two components which are attributed to NH3+ (binding energy 405.9 eV) and NH2 (402.2 eV).23 The NH3+ peak is 13.6% of the total nitrogen area. The content of the N atom in the film is estimated to be 2.4 per maleate unit by taking into account the atomic sensitivity factor and the relative area of the N1, and 01, peaks. Therefore, the content of the ammonium nitrogen becomes 33% of the maleate unit. This is small,compared with the value of 90 ?6 in the case of a copolymer of monomethyl maleate and octadecylvinyl ether.= The bulky ester group next to the carboxyl group may diminish the ion pair formation by steric hindrance. SEM photographs of Figure 5a,c show the polyion complex films deposited onto porous fluorocarbon membranes (FP-010). We can readily observe that the original
2226 Langmuir, VoZ.8, No. 9,1992
Lee and Kunitake ?'
e
lr) r_
i
I
:
,
.
.
t
*
*
I
e
.
1:
1.0 pm
4.
. I
Figure 5. Scanning electron micrographs of LB films deposited on fluorocarbon membrane filters (FP-010): (a) as-deposited film of two layers; (b) two layers after heat treatment; (c) as-deposited film of four layers; (d) four layers after heat treatment. The samples were sputtered with Pt-Pd and observed at 2 kV. Typical defects are indicated with arrows.
pores, which are seen as longish and with a somewhat dark appearance (Figure 5a), of the substrate membrane are covered by a thin surface film. Defects with sizes of 1 W 2 0 0nm are seen as dark spots. Although those pinhole defects are observed in the two-layer film (Figure5a),good covering of the pores is found in the four-layerfilm (Figure
5c). Satisfactory coverage of the pore by a four-layer film indicates that this LB film has a good self-supporting ability. The thickness of the deposited layer was determined by ellipsometry as shown in Figure 6. It was proportional to the number of deposited layers. The unit layer thickness
Langmuir, Vol. 8, No. 9, 1992 2227
Polyion- Complexed Langmuil-Blodgett F i l m
400 Polybn Canpiexed Film
L
OO
5
10
15
20
Number of layers Figure 6. Thickness of LB films on Si wafers determined by ellipsometry.
Scheme I. Imide Formation from a Polyion Complex Film through Heat Treatment
I I oA.o.
o=c,)-o
NH,+
22
A
2-
as estimated from the slope is 25.4 A in the case of the polyion complex film. Assuming that the alkyl tails are perpendicular to the layer plane as shown in Scheme I, the Corey-Pauling-Koltun (CPK) model building suggests that the estimated layer thickness is 32 A. This smaller measured value means that the alkyl tail of polymer 1 is tilted against the layer plane and the PAA is incorporated with uncoiled form in the LB film. Imide Cross-LinkedFilm. The polyion complex film was subsequently subjected to heat treatment at 180 "C in a vacuum for 10 h. The formation of imide bonds (Scheme I) can be confiied by FT-IRspectra. New peaks at 1767 and 1703 cm-l as shown in Figure 3b are assigned to asymmetric and symmetric stretching modes, respectively, of the imide group. Concurrently, peaks of carboxylate (1575 cm-') and amino groups (1650 cm-') are weakened. This reaction results in removal of esteric alcohol tails, as evidenced by decreased intensity of the peaks due to methylene (1466 cm-l) and ether (1121 cm-') moieties. An X P S spectrum given in Figure 4b also supports the formation of the imide group, since a third peak appears
at 403.0 eV in the N1, region.23 Diminished areas of the NH3+and NH2 peaks are consistent with this supposition. We can estimate from the change of relative area of each component of the N1, peaks and the 01,peak that the 77 % of the maleate unit is converted to the imide group. An IR shoulder peak at 1733cm-l (Figure 3b) also indicates the existence of unreacted esters. Parts b and d of Figure 5 are SEM photographs of the LB film after heat treatment. By comparing with parts a and c, we can see that the formation of the imide bond, i.e., cross-linkingand skeletonization, does not give further defects to the covered film. This means that only four monomolecular layers of the polymer network are enough to cover the pores of the membrane filter. Immersion of this f i i in water or in benzene for 1day did not cause any change in its appearance. Ita FT-IR spectrum after immersion (Figure 3c) does not give any indication of structure changes. The unit thickness measured by ellipsometry decreased from 25.4 to 14.0 A by heat treatment, as shown in Figure 6. This reduction of thickness of 11.4 A is caused by removal of esteric alcohol tails in the formation of the imide bond. In the case of the methyl monomaleate and octadecyl vinyl ether copolymer system, the thickness decrease is only 4.8 A,23 as the imide cross-linked system still contains the alkyl tails.
Comparison with Other LB Films The preparation of cross-linked and skeletonized LB films composed of two oppositely charged polymers was accomplished,and the film displayed good covering of the porous membrane filters only in four layers. Of examples of skeletonized LB films, Kakimoto et al. reported multilayer films of an aromatic polyimide with a unit thickness of 4 A.24 On the other hand, Ogata et al. prepared ultrathin films of aromatic polymers by cyclodehydrogenation of aromatic p ~ l y a n i l s . The ~ ~ latter polybenzimidazole film showed a unit thickness of 20 A. In our system of a nonaromatic cross-linked polymer, the unit thickness is 14 A. It appears that the cross-linked polymer possesses a tightly knit structure. The pore-coveringability of LB films has been employed as an index of film quality such as self-supporting ability and lack of defects.23 In a recent communication,Tredgold et al.21reported covering of 200-nm pores by six layers of a cross-linkable polymer with 80% probability. In our system, the coverage of pores (FP-010)by four layers was 98% complete, as estimated by SEM observation. The advantage of the present system is clear. (31)Ueda, T.;Yokoyama, S.;Watanabe, M.;Sanui, K.;Ogata, N.J. Polym. Sci., Polym. Chem. Ed. 1990,28, 3221.
