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Langmuir 1992,8,3168-3177
Novel Langmuir-Blodgett Films of Conducting Polymers. 1. Polyion Complexes and Their Multilayer Heterostructures A. T. Royappa and M. F. Rubner' Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 18,1992. In Final Form: October 8,1992
Novel Langmuir-Blodgett (LB) films incorporating conducting polymers have been fabricated from polyion complexesof acid-functionalizedpolythiophene and sulfonated polyaniline. The multilayer thin films of these new polyion complexeswere found to consist of ionicallybound layers of conjugatedpolymer chains isolated between insulating layers of amphiphilic stearylamine molecules. Using this approach, it was ale0possible to fabricatemultilayerheterostructuresin which monolayersof two differentconjugated polymerswere joined together to form an isolated conductingpolymer interface. The multilayer thin f i i s were rendered electrically conductive by doping with suitable oxidizing agents (polythiophenecase) or protonic acids (polyanilinecase). The level of conductivity achieved by doping was found to be sensitive to the composition and organization of the multilayer structure.
Introduction It is now well recognized that the junction formed between materials with different work functions can be exploited to fabricate a variety of technologically useful thin-film-based solid-state devices. A key element to the realization of such devices, however, is the development of the fabrication and processing techniques needed to manipulate the relevant materials into thin-film forms. Since the interaction that takes place between two materials forming a junction is essentially confined to the interface, it is important to utilize processing techniques that provide control over, among other things, the level and type of ordering at the interface and the thickness and supermolecular organization of the thin films used to fabricate them. Control over the supermolecular organization of thin filmsbecomes particularly important when one considers the fabrication of alternating layer superlattices and related thin-film heterostructures. The Langmuir-Blodgett (LB) technique currently represents the most versatile thin-film processing route available for the purpose of fabricating such structures from electroactive polymer materials. With this approach, it is possible to manipulate a wide variety of polymeric and monomeric materialsinto a unique collection of multilayer thin films.lV2 A particularlyintriguing applicationof the LB technique involves the fabrication of alternating layer thin-film heterostructures in which monolayer-thickfilms (10-40A thick) of electroactivecomponents are alternately stacked in an AB-type multilayer structure. In such an organization, the film is comprised of only interface-active material as the thickness of the individual layers can be designed to be as small as or smaller than the transition zone created at the junction between the layers. Through the use of semiconductive conjugated polymers as the electroactive components of the multilayer film, it is, in principle, possible to create a wide variety of dopantmodulated and band gap-modulated heterostructures with a diverse collection of electrical and optical properties. The possibility also exists to modulate further the properties of these thin films by externally stimulating chargetransfer interactions between the different layers. For (1) See for example: Proceedings of the Fifth International Conference on Langmuir-Blodgett Films, Paris, France. Thin Solid Films 1992,210, 211. (2) Rubner, M. F.; Skotheim, T. A. In Conjugated Polymers; Bredas, J. L., Silbey, R. J.,Eds.; Kluwer Academic Publishers: Boston, 1991; pp 363-403.
example, with the correct combination of electroactive layers, it may be possible to use lasers to induce ultrafast electrochromic switching and other such electrical and optical effects. This type of laser-activatedcharge transfer has already been observed when p-type electroactive polymers are interfaced to n-type semiconductor^.^ To realize the above heterostructures, it is fiit necessary to develop the chemistry and molecular level handling techniques needed to manipulate electroactive polymers into multilayer thin films. Toward this goal, we have developed a number of different approaches that can be utilized to fabricate LB films of conducting polymer^.^^ In this paper, we describe a new method of fabricating electrically conducting Langmuir-Blodgett films of conjugated polymers and show how this approach can be used to form alternating layer heterostructures in which monolayers of the electroactive polymers are joined together to form thin-film junctions that are isolated between insulating layers of amphiphilicmolecules. The basic approach is centered on the manipulation of surface-activepolyion complexes formed by ionically binding stearylamine molecules to the ionizable side groups of conjugated polyions. To date, we have successfullyused this approach to manipulate polyaniline- and polythiophene-basedpolyions. The LB manipulation of polyion complexes is well documented in the literature. For example, it has previously been shown that polyion complexesof conducting polymer precursors7can be readily manipulated into wellordered LB films. The research described in this paper, however, marks the first successfulfabrication of LB films containing polyion complexes of a preformed conjugated polymer. The advantage of this direct method over the precursor approach is that it obviates the need to convert thermally the precursor molecules into their electroactive (3) Chandraeekhar,P. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1992,33, 1206. (4) (a) Hong, K.; Rubner, M. F. Thin Solid Films 1988,160,187. (b) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Mater. 1990,2, 82. (c) Rosner, R. B.;Rubner, M. F. Mater. Res. SOC.Symp. Proc. 1990,173,363. (5) (a) Watanabe, I.; Hong, K.; Rubner, M. F. Langmuir 1990,61164. (b) Rikukawa,M.;Rubner,M. F. Synth.Met. 1992,47,203. (c) Wetanabe, I.; Cheung,J. H.;Rubner, M. F.J.Phys. Chem. 1990,94,8715. (d) P u k k a , E.; Rubner, M. F.; Hettinger, J. D.; Brooks, J. S.; Hannaha, S.T. Phys. Rev. B 1991,43,9076. (6) Rosner,R. B.;Rubner, M. F. J.Chem. Soc., Chem. Commun. 1991, 1449. (7) (a) Nishikata, Y.; Kakimoto, M. A.; Imai, Y. Thin Solid Films 1989, 179 191. (b) Era, M.; Kamiyama, K.; Yoshiura, K.; Momii, T.; Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Thin Solid Films 1989, 179, 1.
0 1992 American Chemical Society
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Novel LangmuipBlodgett Films of Conducting Polymers
forms. Such thermal treatments inevitably alter and destroy the unique molecular organization created during the fabrication process, thereby making it difficult to build the kinde of complex heterostructures possible with the direct manipulation method. It should also be noted that the polyion method in general is quite versatile as it can be used with any non-surface-active polymer that can be functionaliied with ionic groups. Ae alluded to earlier,the LBmanipulationof conjugated polyions provides a unique opportunity to fabricate heterostructures containing isolated thin-film junctions of conjugated polymers. These new complex molecular organizations can be readily built by alternate deposition of two different homopolymer polyion complexes. In this work, this was accomplished with acid-functionalized polythiophene molecules and sulfonated polyaniline molecules. The heterostructure formed from these materials may be viewed as a collection of regularly spaced, isolated interfaces formed between monolayers of two conjugated polymers in molecular contact with each other. The monolayer interfaces are separated from each other by intervening layere of stearylamine. These new heterostructure f i e provide an ideal model systemfor studying the interfacial behavior of conducting polymers without interference from the bulk. The .overarching conclusion drawn from this work is that the polyion complex method-in conjunction with the LB technique-is a singularly potent means of fabricatinghighly ordered t h i n - f i systemswith complex molecular architectures. In addition, this approach can be readily applied to a wide range of materials, thereby opening the door to the fabrication of completely new molecular organizations of electroactive polymers.
Experimental Section Synthesis of Poly(thiophene-3-acetic acid) (PTAA). A 40-g sample of commerciallyavailable ethyl thiophene-3-acetate (Lancaeter Synthesis) was polymerized by the FeClS suspension methods to form poly(ethy1thiophene-3-acetate). This method was chosen instead of the direct polymerization of thiophene3-acetic acid because the carboxylic acid group interferes with the FeC13 polymerization. The reaction was carried out under nitrogen in 2 L of chloroform, with a 4:l mole ratio of FeC13 to thiophene monomer. The reaction mixture was heated at 50 OC for 2 h, followed by stirring overnight at room temperature. A mechanical stirrer was used since magnetic stirrers were not powerful enough to stir the rather viscous mixture. The resulting black precipitate was dissolved in 2 L of tetrahydrofuran (THF). It was found to be completely soluble, indicating that this poly(thiophene ester) was not cross-linked. As expected, the IR spectrum of this polymer showed a strong ester C=O stretch at 1730 cm-l (identical to the monomer C=O stretch), thiophene ring vibrations in the fingerprint region from 600 to 1700 cm-l, and no 0-H peaks in the 3OOO-cm-l region. The THF solution was rotovapped down to 200 mL and poured into 1L of a 10 g/L solution of NaOH in water. A thick brown precipitate of Fe(0H)s formed at once, while the polymer was converted to the water-soluble sodium salt of PTAA. The Fe(OHh was filtered out by vacuum filtration, and the filtrate was acidified with 25 mL of concentrated HCl to precipitate the PTAA (see Figure l a for the structure of this polymer). The PTAA was washed with 2.6 L of pure water and dried under vacuum for 2 days to yield 19g of a brown powder; the yield was 58% of the theoretical yield (32.9 g). PTAA is soluble in THF, dimethylacetamide (DMAc), and concentrated aqueous NaOH, and insoluble in chloroform and water. The solutions of PTAA were orange in color. The FTIR spectrum of PTAA showed the presence of a strong free carboxylic acid C=O stretch at 1700 cm-1, thiophene ring vibrations in the same region as for the polythiophene ester, and (8) Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoehino, K. Chem. Express 1986,1,636.
3
C:&WJH:
PTAA
Stearylamine
Polyion Complex
b
SPAn-StNH2 Polyion Complex
Figure 1.. (a) Synthesis of the PTAA-StNH2 polyion complex. (b) Synthesis of the SPAn-StNH2 polyion complex. a broad 0-H peak at 3100 cm-1. Cast films of this polymer were reddish in color,and the W-vie spectrum of this material revealed the classical T-T* transition peak at ca. 470 nm. The numberaveragemolecular weight of the PTAA obtained by gel permeation chromatography (polystyrene calibration) was 1100g/molwith a polydispersity of -1.4, which implies an average degree of polymerization of -8 thiophene repeat units. This material should therefore be viewed more as an oligomer than as a true high polymer. References to ‘polythiophene” in the text will mean PTAA, not pure unsubstituted polythiophene, which was not used in any of these experiments. Sulfonated Polyaniline (SPAn). The sulfonated polyaniline used in these experiments was synthesized usingthe procedures of Epstein and co-workers. The synthesis and properties of this material have been described elsewhere.9 The polymer is a freeflowing green powder, which is essentially insoluble in water, partially soluble in DMAc, and readily soluble in dilute aqueous base such as aqueous ammonia or NaOH. Note that, at this point, the sulfonated polyaniline backbone is in its (self-doped) emeraldine salt form, i.e., the conducting state, and that only half of the phenyl rings are sulfonated. Although the structure shown in Figure l b depicts sulfonate groups on alternating rings, sulfonated rings may also be adjacent to each other in the chain. The synthetic procedure used for this polymer is believed to produce materials with a molecular weight of about 50 OOO g/mol.1° Synthesis of the Poly(thiophene-3-acetic acid)-Stearylamine Polyion Complexes (PTAA-StNHa). A 5.2-mg sample of PTAA was dissolved in 2.5 mL of DMAc and addedwithstirring to 10.0 mg of stearylamine (CI~HUNH~, StNH2) dissolved in 7.5 mL of benzene to form the PTAA-StNH2 polyion complex. The reaction involved is a straightforward acid-base reaction between the carboxylic acid groups of the PTAA and the amine group of stearylamine. The resulting polyion complex is simply an amine salt of PTAA with stearylamine, as shown in Figure la. In this complex, the mole ratio of thiophene repeat units to stearylamine molecules is 1:1; i.e., one stearylamine molecule is attached to every thiophene ring in the polymer chain. PTAAStNHzpolyion complexeswith 2 1and 5 1moleratios of thiophene
-
(9) (a) Yue, J.; Epstein, A. J. J. Am. Chem. SOC.1990,112,2800. (b) Yue, J.; Epstein, A. J.; MacDiarmid, A. G. Mol. Cryst. Liq. Cryst. ISSO, 189,265. (10) Yue, J. Private communication.
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3170 Langmuir, Vol. 8, No. 12,1992 repeat units to stearylamine were synthesized in an exactly analogous manner: the concentration of stearylamine in all solutions of PTAA-StNH2 was held constant at 1.0 mg/mL, while the PTAA concentration was 0.52,1.04, and 2.60 mg/mL for the 1:1, 2:1, and 5:l complexes, respectively. Likewise, the solvent in every case consisted of a 3:l (v/v) mixture of benzene and DMAc. In the 2:l and 5:l complexes, a stearylamine molecule is attached, on average, to every other and every fifth thiophene ring in the chain. These solutions appeared orange in color. The solutions were stored in the refrigerator, and were stable for several months up to a year; Le., there was no visible precipitation of polymer over this time period. The FTIR spectrum of a cast f i iof the 1:l complex showed a shift in the C = O stretch from 1700 cm-1 in the free carboxylic acid to 1575 cm-l in the amine salt, and a dramatic increase in the intensity of the C-H peaks at 2850 and 2920 cm-l (compared to PTAA) due to the large number of stearylamine - C H r groups present in the complex. This confirmed the formation of the amine salt in the polyion complexes. In addition, IR spectra showed that the free acid C 4 peak (1700 cm-l) and the free acid O-H peak (3100 cm-l) diminished as the mole ratio decreased from 5:l to 1:1,concomitant with an increase in the amine salt C=O peak (1575 cm-l), as would be expected. Synthesis of the Sulfonated Polyaniline-Stearylamine Polyion Complex (SPAn-StNH2). A 9.7-mg sample of SPAn was dissolved in 2.5 mL of DMAc with stirring, to give a dark green solution. A 10.0-mg sample of stearylamine dissolved in 7.5 mL of benzene was added to this solution with stirring. The color of the solution immediately started turning blue-purple upon addition of the stearylamine, as an indication of the reaction between the sulfonic acid groups and the stearylamine getting underway. The mixture was placed in an ultrasonic bath to assist in disintegrating the larger fragments of undissolved SPAn. Stirring was continued overnight until most of the SPAn went into solution. The final color of the solution was indigo. These solutions,contrary to those of the PTAA-StNH2 complexes,were not stable for longer than a couple of months. Precipitation was noticeable after several weeks, and this phenomenon was exacerbated at lowtemperatures, such as in a refrigerator. These solutions were therefore stored at room temperature. The reaction between SPAn and stearylamine is identical to the reaction between SPAn and other bases such as NaOH and aqueous ammonia described in ref 9, and is shown in Figure lb. It is important to note that since only half the phenyl rings in SPAn are sulfonated, only those rings have stearylamine molecules attached to them. General Procedures. Ultrapure Milli-Q water (resistivity 218 MO cm) was used as the LB subphase for all experiments; the subphase temperature was always maintained at 20 OC. All isotherms were run at a compression rate of -60 cm2/min. Hydrophobic glass slides were made by exposing precleaned vapor for microscope slides to 1,1,1,3,3,3-hexamethyldisilazane 36 h. The spreading solvent for all LB solutions was the 3:l (v/v) mixture of benzene and DMAc mentioned above. This mixture has been found to be a good solvent for a wide range of materials and a superlative spreading agent for monolayer formation of polyions.ll All spreading solutions were filtered through 0.5-gm solvent-resistant filters prior to use. A computer-controlled Lauda film balance, model FW2, was used in all the experiments except for the fabrication of the polyion heterostructures, which were made using a Nima trough, model TKB2410A, fitted with an alternating layer dipper mechanism. Transfer ratios were determined by dividing the area transferred by the substrate area. In the Nima trough, the substrate area could not be determined accurately because the entire dipper and slide holder were coated during dipping due to the nature of the alternating dipper mechanism. Therefore, the substrate area was set at an arbitrary constant value of 12.5cm2, the area of a glass slide. The transfer areas reported for the heterostructure are therefore relative, not absolute. Polythiophene and Sulfonated Polyaniline LB Films. After spreading the polyion solution on the water surface, the surface monolayer was compressed to the target pressure and (11) Nishikata, Y.;Katimoto, M. A.; Morikawa,A.; Imai, Y. Thin Solid
Films 1988, 160, 15.
held at that pressure for approximately 1h. The monolayers of these polyion complexes were held at a constant pressure of 20 mN/m; the downstroke and upstroke dipping speeds were 10 mm/min. The fiist two layers deposited on the glass slide were allowed to dry for 2 h before further deposition was continued. The pause at the top of each dipping sequence (substrate out of the water) was 245 min and 30 s at the bottom of each dip (substrate underwater). Thickness Measurements. Ellipsometric measurements of the thickness of the PTAA-StNH2 and SPAn-StNH2 polyion complex LB films were done on a Gaertner Model L117 ellipsometer. Platinum-coated glass slideswereused as substrates for these films. The refractive index of all the LB films was taken to be -1.5, since the dominant thickness component in every case is the long hydrocarbon tail group of the amphiphiles used. The ellipsometer was calibrated usinga cadmium stearate LB film of known thickness as a reference. For each polyion complex, three films were studied having 10,14, and 28 layers. In all cases including the cadmium stearate reference film, four or more independent thickness measurements were made for each sample. All profilometry was done using a Sloan Dektak I1 profilometer with a diamond stylus. The stylus weight was 117 mg, and all scans were performed at the lowest scan speed setting. The fibs whose thicknesses were to be measured were scored with a sharp steel blade in five different places about 1 mm apart prior to measurement. The average of the five different score depths was used as one measure of f i i thickness. Since steel is harder than most commonly used LB substrate materials other than glass, only films deposited on glass substrates were analyzed by profilometry. For Fourier transform infrared spectra of the LB films, IRtransparent zinc selenide plates were used as substrates in the transmission mode. In the reflectance mode, platinum-coated glass slides were used in conjunction with a Spectra Tech Model 500 specular reflectance attachment,capable of tilting specimens down to grazing angles of 5 O , measured from the substrate plane. All reflectance spectra were measured at an incident angle of 8 O . FTIR spectra were recorded using a computer-controlled Nicolet 510P spectrometer equipped with a liquid nitrogen-cooledMCT detector. All spectra were averaged over 256 scans, and polynomial baseline correction was performed as necessary to compensate for anomalous baseline curvature. The techniques of Umemura et al.12as modified by Watanabe et al.&were closely followed in performing transmission/reflection FTIR spectroscopy on these films. UV-Vis Spectroscopy. UV-vis spectra of film samples were recorded with a computer-controlled Oriel Model 77200 250-mm multichannel spectrometer in laboratory atmosphere at room temperature. Films analyzed by UV-vis spectroscopy had an equal number of layers on both sides of the glass substrate. Two different polarization experiments were carried out on LB fiis using the UV-vis spectrometer. The first of these dichroicstudies was to determine whether the polymer chains in the LB films were preferentially oriented in the dipping direction, and the second was to determine whether the chains lay preferentially in the plane of the LB substrate. In the first experiment, the electric field vector (polarization direction) of the incident radiation is perpendicular and parallel to the dipping direction. Therefore, one would expect the absorption intensities to be higher in the parallel p o l a r i i spectrum than in the perpendicular polarized spectrum if the polymer chains are oriented in the dipping direction. In the second experiment, the electric field vector is parallel to the substrate plane (P) and at 4 5 O to the substrate plane (SI.In this case, one would expect the absorption intensities to be higher in the P polarization direction than in the S polarization direction if the polymer chains lie preferentially in the plane of the substrate. In these polarization experiments, a separate reference spectrum was collected for each different orientation of the polarizer and the sample. This was done in order to account for the different absorption characteristics of the polarizer at Oo and 90° positions, and also the different 0 and 45' to the beam. absorbances of the reference substrate at ' (12) Umemura, J.;Kamata, T.; Kawai,T.;Takenaka,T.J.Phys. Chem. 1990, 94, 62.
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Novel LangmuipBlodgett Films of Conducting Polymers
Spin-Coating. The polymer solutions used for spin-coating contained approximately 5% ' polymer by weight, and all solutions were fiitered through 0.5-rm solventresistantfilters before using. The substrates for spin-coated films were clean glass slides. All slides were wetted with the solvent used before applying the polymer coating. About 6-8 drops of the polymer solution were dripped on to the glass slide which was then spun at 2000-6000 rpm for 10s to produce arange of thicknesses depending inversely on the spinning rate. X-ray Diffraction Patterns. All X-ray diffraction patterns of the LB f i i were recorded on a Rigaku Rotaflex RU 300 automated X-ray diffractometer with a rotating copper anode as the X-ray source. 28 scan rates were typically 2 deg/min with a step size of 0.02". The range of 28 in the scans taken was normally 1.5-6.0°, except when analyzing a sample with unknown X-ray diffraction characteristics, in which case the range was 1.5-16O. LB film bilayer repeat distances d were calculated using the Bragg equation. All LB films analyzed by X-ray diffraction had at least 20 monolayers deposited per side. Glass substrates were used for all X-ray measurements. Doping of Polythiophene-ContainingFilms. Several different reagents were examined for use as potential dopants for PTAA-StNHz LB films. The fiist of these was a 1 mg/mL solution of nitrosyl hexafluorophosphate (NOPF6)in acetonitrile. Although doping was very rapid (the yellow films turned green in seconds), dedoping was equally rapid in air. The LB films also had a tendency to dissolve in solution, which made this a very unsatisfactory dopant. The secondwas a 10mg/mL solution of FeCla in nitromethane. The films were doped in about 1min as evidenced by the change in film color, and the solution did not dissolve the LB films. However, dedoping of the sample also occurred in about 1 min in air. It was therefore decided to try vapor-phase doping, which has the advantage of shielding the sample from exposure to air. The two gas-phase dopants studied were 12 and antimony pentachloride, SbClb. Of these, only SbClb was capable of doping the sample, and therefore it was the dopant of choice in spite of the fact that SbC16-doped f i i s also dedoped rather quickly in air. All doping and conductivity results for the polythiophene-containingLB films are therefore based on gasphase SbCls-doped samples. The doping procedure was as follows: after all the electrical contacts had been made, the LB film was placed in a sealed three-neck round-bottom flask. A couple of boiling chips were dropped in the bottom of the flask, which was then backfilled with dry Nz. A few drops of a 1.0 M SbCla solution in dichloromethane (Aldrich) were introduced into the flask, and the flask was evacuated. The dichloromethaneevaporatedalmost instantly, leaving the higher-boiling SbCb to fill the flask slowly with vapor after the flask was disconnected from the vacuum. Spin-coated films of PTAA homopolymer (from a solution in 3:l benzene/DMAc) were doped using the same procedure detailed above for polythiophene LB films. Doping of Sulfonated Polyaniline-Containing Films. SPAn-StNHz LB filmswere doped by immersionin a 1M solution of aqueous HCl for 15min. The fiims were then dried in a stream of dry Nz for 2 h followed by drying under vacuum for approximately 10 min. Electrical contacts were made at this point, and the sample was placed, as before, in a sealed, dry Nz-filledthree-neck round-bottom flask containing boiling chips. The conductivity of the sample was measured under these dry conditions first. Following this measurement, a few drops of water were introduced into the flask and the flask was evacuated and sealed, causing it to be filled with water vapor. The conductivity of the sample was measured again in this saturated water vapor atmosphere. This was done in order to examine the effectsof moisture on sample conductivity in a controlled fashion, since it had been observed that it had a noticeable effect on the conductivity of these films. Sulfonated polyaniline was dissolved in 0.1 M aqueous NHdOH to form a solution of the ammonium saltof SPAn as described in ref 9. The spin-coated films made from the ammonium salt of SPAn spontaneously released the weak base NH3 to produce films of SPAn in the self-doped emeraldine salt form. These films required no further doping, and their conductivity was measured in air. As a check, however, these fiims were also
75 A B
- Stearylamine
- 2 I PTAA-SINHZ C - 5 I PTAA-StNH2 D - I I PTAA-StNH2
3
v,
15
0
t
0
7
21 35 0 Area per Molecule ( A 2 )
49
Figure 2. Pressure-area isothsrms of stearylamine and the 1:1, 2:1, and 5 1 PTAA-StNH2 polyion complexes on pure water at
20 "C.
"doped" with HCl and their conductivities measured according to the procedure listed above for the SPAn-StNH2 LB f i i . Heterostructure LB Films. In the Nima trough used to fabricate the heterostructure LB films, each of the two separate chambers was set up with a monolayer of 1:lPTAA-StNHz and SPAnStNH2, respectively. Then, with the aid of the alternating dipper mechanism, monolayers of 1:lPTAA-StNHZ and SPAnStNHz were alternately deposited onto the substrate, forming a heterostructure c0mprise.d of head-to-head layers of the two conjugated polymers. The heterostructure films were doped in an identical manner to the SPAn-StNH2 films, Le., by immersion in 1M HC1. Thus, only the sulfonated polyaniline layers in the heterostructure were doped, while the polythiophene layers remained undoped. Conductivity Measurements. The standard four-point Van der Pauw method was used to measure the resistivity of all samples. Electrical contacts were made to the four points of the sample, usually the four corners, using gold wire and silver paint or graphite paste (Electrodag). All conductivity measurementa reported represent the in-plane conductivity of the samples. Although the multilayer structures described in this paper should give rise to highly anisotropicconductivities, difficultiesassociated with the deposition of metal electrodes onto the surfaces of these very thin and delicate f i i s made it not possible to measure their transverse conductivities.
Rssults and Discussion LB Fabricationand Structureof thePolythiophene Polyion Complexes. The pressurearea isotherms of the various PTAA-StNH2 polyion complexes on pure water at 20 O C are shown in Figure 2. The isotherm of pure stearylamine is also included for reference. The abcissa was calibrated solely to the concentration of stearylamine in the spreading solution; the limiting area per molecule was obtained by dividing the area extrapolated from the linear part of the curve by the number of stearylamine molecules applied onto the surface. This makes it possible to evaluate directly the influence of the ionically bound polymer chains on the molecular packing of the stearylamine molecules. As expected, in every case the limiting area per molecule of stearylamine in the complexed form is greater than that of pure stearylamine. This is because the effective area occupied by each stearylamine molecule in the polyion complex has an additional contribution from the repeat units of the attached polymer. The limiting areas per molecule (A2)for stearylamine and the various polyion complexes are stearylamine,l&1:l PTAA-StNHz, 46;2 1 PTAA-StNH2,28; 5:l PTAA-StNH2,24. Thus, as the number of thiophene repeat units relative to stearylamine molecules decreases, the area per molecule steadily becomes greater than that of pure stearylamine. It is to be expected that the ability of the stearylamine molecules to achieve their tightly packed monolayer organization
Royappa and Rubner
3172 Langmuir, Vol. 8, No. 12, 1992 will be most inhibited in the more sterically crowded 1:l
complex. It is not possible with simple pressure-area isotherm data alone to ascertain the configuration of the conjugated chains attached to the stearylamine molecules (Le., are they attached as loops or extended chains). Given the rigid nature of the backbone, however, we assume that they are attached as fully extended or near fully extended Chaina. All of the polythiophene complexes were found to be very stable (several hours to days) to film collapse on the water surface when held at a surface pressure of 20 mN/ m. This is in stark contrast to pure stearylamine which is unstable, due to its partial solubilityin water.13 However, the 2:l and 5:l salts actually expand in area over time when held at constant pressure. This phenomenon is probably due to the redistribution of stsarylamine molecules along the polymer backbones by ion hopping. In these complexes, stearylamine molecules on adjacent thiopheneringeexperienceelectrostaticrepulsion between their positively charged ammonium head groups. It would therefore be more energetically favorable for such molecules to move apart from each other by attaching themeelves to carboxylic acid groups on thiophene rings further apart. The 1:l PTAA-StNH2 complex did not expand when held at constant pressure, but remained at ita equilibrium area. This is in keeping with the above speculation since there are no free carboxylic acid groups for the stearylamine molecules to transfer to in the 1:l complex because by definition, every ring in this complex already has a stearylamine molecule attached to it. Monolayers of the polythiophene-containiigpolyion complexes were found to transfer onto hydrophobicglass slides, platinum-coated glass slides,and zinc selenideplates with ease. Transfer was approximately equal on the up and down strokes, indicating a Y-typeLB film structure. The transfer ratios were typically close to unity after the first dip, and did not vary significantly from dip to dip. As produced, the polythiophene-containing multilayer films are yellow in color. The filmsof the 1:l complex are clear and of excellent optical quality (devoid of defecta to the naked eye), indicating a high level of order. However, the films of the 2 1 and 5 1 complexes appear translucent with a few streaks and spots,such defects being particularly pronounced in films of the 5 1 complex. This is one indication of a decreased level of order in the 2 1 and 5 1 complexes. The W-vis spectra (not shown) of the multilayer LB films of 1:1, 2 1 , and 5 1 PTAA-StNH2 display the canonical r-r* transition of the conjugated polythiophene system at ca. 425 nm, regardless of the composition of the polyion complex. The presence of this peak is evidence that the PTAA-StNH2 complex does not dissociate on the water surface or dissolve into the subphase but that the polythiophene molecules are transferred on to the substrate. In order to evaluate the reproducibility of transfer, the UV-vis spectrum of an LB f h of 1:l PTAAStNHz was recorded at several different stages during its fabrication. The absorbance of this film at 425 nm was found to be a linear function of the number of layers deposited, with a correlation coefficient of 0.999. Such a high degreeof linearity indicatesnear-ideal reproducibility of transfer in this system. UV-vis dichroic studies performed on LB films of the 1:l complex reveal that the spectra generated with light polarized parallel and perpendicular to the dipping direction are identical. That is to say that there are no ~~
(13)Tajima, K.;Takahashi, M.;Kobayashi, K. Thin Solid Films 1989,
178, 381.
400
500
600
700
000
Wavelength (nm )
Figure 3. UV-vis spectra of LB films of (a) 1:l PTAA-StNH2 in the P (top) and S (bottom) polarization modes and (b)SPAnStNHz in the P (top) and S (bottom) polarization modes.
signs of preferred orientation of the polymer chains in the dipping duection. However, there are significant differences in the spectra taken with light polarized in the S and P directions. For example, the absorbance of the conjugated polymer chaine in the P polarization mode, parallel to the LB substrate, was found to be higher than for the corresponding S polarization mode. These spectra are shown in Figure 3a. It can therefore be stated that the polymer chaina in LB films of 1:l PTAA-StNH2 exhibit a tendency to be preferentially oriented in the plane of the substrate. This result suggests that the PTAA chaine lie in planes that are sandwiched between layers of the attached stearylamine molecules. The X-ray diffraction pattern of an LB film of 1:l PTAA-StNH2 is shown in Figure 4 a The presence of multiple Bragg reflection peaks in this pattern indicates the existence of a layerlike structure. Assuming that the two peaks in this diffraction pattern correspond to the n = 1 and n = 2 Bragg reflections, the average d spacing calculated for multilayer films of the 1:l PTAA-StNH2 complex is about 33 A. This spacing corresponds to the bilayer repeat distance, which means that each monolayer contributes approximately 17 A to the film thickness. Proflometry experimentaperformed on a &monolayer LB film of 1:l PTAA-StNH2 yields a total film thickness of approximately 870 A, which translates to an average bilayer d spacingof 29 A, or an averagemonolayer thicknese of about 15A. This technique works well with these films, as they are rigid enough to produce thickness profiles with sharp, well-defined edges. Ellipsometric measurements yielded the followingresults for the totalthickness of three different 1:l PTAA-StNHz LB films on platinum-coated glass substrates: 141,297, and 383 A for lo-,14-, and 28monolayer films,respectively. Condensing these data into a single number produces an average bilayer d spacing of 33 A which Corresponds to a monolayer thickness of 16 A, on average. Thus, the thicknesses contributed per monolayer to the 1:l PTAA-StNH2 LB films obtained from X-ray diffraction, profilometry, and ellipsometry-three fundamentallydisparate experimental methods-are mutually consistent, within experimental error.
Langmuir, Vol. 8, No. 12,1992 3173 0.0200p
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Figure 4. X-ray diffraction patternof LB filmsof (a) 1:l PTAAStNHz and (b) SPh-StNH2. A bilayer d spacing of about 33 A is rather small, particularly considering that a single fully extended stearylamine molecule is about 25 A long and that there are two layers of stearylamine molecules per bilayer in additionto the PTAA chains. To achievethis anomalously low thickness, the stearylaminetails must either tilt away from the substrate normal or interdigitate. A simple calculation reveals that the stearylamine tails must tilt approximately 55O from the substrate normal to achieve a Y-typestructure with these d spacings,which is unusually large for such systems. Interdigitationof the stearylamine tailstherefore appearsto be the most plausibleexplanation for the observed bilayer thickness. It should be noted at this juncture that the transfer process is conducted well below the collapsepressure of the film on the water surface, while the monolayer is in a loose, expanded state (not a tightly packed condensed state). Thus, it is entirely possible that during transfer, the tail groups of the stearylamine molecules on the water surface can interdigitate with those in the LB films. As will become apparent shortly,it is also possibleto rule out the existence of moleculeswith large tilt anglesvia reflection/abaorption
FTIR.
The transmission FTIR spectra of LB films of 1:1,21, and 5 1 PTAA-StNH2 are presented in Figure 5. These spectra display the same features present in the spectra of solution cast films, including a shift in the location of the C 4 peak to 1575 cm-', proportionality of the free acid C 4 absorbance to the mole fraction of free acid in the complex, and the intense C-H peaks at 2850 and 2920 cm-l due to the hydrocarbon tails of stearylamine. This last piece of information, together with the presence of the PTAA n-r* transition in W-vis spectra of these f i i (seeFigure 3a),provides completeevidencethat these polyion complexes do not dissociate on the water surface but are transferred intact (i.e., incorporationof both PTAA and stearylamine)onto the LB substrate. As an additiod test of the reproducibilityof transfer, a series of different thickness LB f i i of 1:l PTAA-StNH2 on zinc selenide substrates was examined by transmiasion FTIR spectroscopy. The absorbance of the asymmetric (2920cm-l) and symmetric (2860 cm-I) C-H stretches for these various films was found to be a linear function of the number of
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Figure 6. FTIR spectra of LB films of the 1:l (bottom), 2 1 (middle),and 5 1 (top) PTAA-StNH2 complexes. 0.0150 00130 0.0050 W C V
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Figure 6. Transmission (a) and reflection (b)FTIR spectra of an LB film of 1:l PTAA-StNH2.
monolayers deposited (correlation coefficients -0.994), again confirming the reproducibility of the deposition process for these polyion complexes. A comparison of the FTIR spectra of 1:l PTAA-StNH2 LB films in reflection and transmission modes shows significantdichroism, indicating a high level of molecular orientation, as seen in Figure 6. Measurements of the tilt angle (relativeto the substrate normal) of the hydrocarbon tails of stearylamine in the LB films were accomplished by analyzingthe differentabsorbancesof the C-H stretches in the transmission and reflectance FTIR spectra, according to the method of Umemura et al.12 The tilt angle of these tails in the 1:l PTAA-StNH2 LB films was calculated to be loo f 8O. This is clearly not sufficient tilting to account for the low bilayer d spacingfound earlier (recallthat a tilt angle of ca. 55O was required). This result
Royappa and Rubner
3174 Langmuir, Vol. 8, No. 12, 1992
mj:: Pol?mer
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Figure 7. Simplified structure of the polyion complexLB films
Figure 8. UV-visspectra of as-preparedSPAn (a)and its sodium salt (b).
therefore supports the existence of interdigitation of the stearylamine tails in these LB films. The FTIR spectra of LB films of 2:l and 5 1 PTAA-StNH2 showed considerably less dichroism than those of the 1:l complex, consistent with the lower level of molecular order present in films fabricated from these complexes. On the basis of the above information, we propose that the molecular organization of the multilayer films of the 1:l PTAA-StNH2 polyion complex consists of randomly oriented PTAA molecules (within the plane) lying as extended chains parallel to the substrate, and sandwiched between layers of stearylamine molecules whose long hydrocarbon tails are interdigitated. This structure is depicted in an idealized (and simplified) representation in Figure 7. Clearly, the multilayer organization is not as perfect as that indicated in this figure although it does reflect the important basic features of this system (ionically bound chains isolated in the head group region of the multilayer structure). All of the PTAA-StNH2 LB films were rendered electrically conductive when doped with SbCl5. As judged by the small size of the bipolaron peak at 750 nm in the UV-vis spectrum of the doped film (not shown), rapid dedoping occurs when the sample is placed in air (Le., within 5-10 min). This rapid dedoping is further exacerbated by the reactivity of the doped polymer to moisture in the atmosphere. Conductivity measurements were therefore made in controlled environments as soon as possible after doping. The conductivities of the polythiophene-based LB films (S/cm) are 1:l PTAA-StNH2, 1 X 10-4; 2:l PTAA-StNH2, 4 X 10-3; and 5:l PTAAStNH2, 2 X 10”’. It is significant to note that the conductivity of LB films of the 2:l complex is highest. One would expect that LB films of the 5:l complex would have the highest conductivity since it has the highest mole fraction of conducting species. However, these results reflect the interplay between the mole fraction of polythiophene and the level of order inherent in the LB structure of the various complexes-two major factors governing the conductivity of the film. Thus, the 5:l complex, even though it has the highest mole fraction of polythiophene, does not exhibit the highest conductivity because LB films of this complex also have the lowest level of order. Similarly, despite the high degree of order in LB films of the 1:l complex,their conductivities are not the highest because these films contain the least amount of polythiophene. LB films of the 2:l complex therefore represent an optimal balance between order and polythiophene mole fraction. By contrast, cast films of pure PTAA exhibit a conductivity of 4 X Slcm when doped with SbClb. This shows that these LB films can reach
conductivities approachingthat of the native conducting polymer. This is interesting considering that the LB films are comprised of a high volume fraction of stearylamine, which is an insulator. LB Fabrication and Structure of the Sulfonated Polyaniline Polyion Complexes. The indigo-colored monolayers of SPAn-StNH2 are visible to the naked eye on the water surface, because the sulfonated polyaniline chains in the complex are very strong chromophores. The pressurearea isotherm of the SPAn-StNH2 polyion complex on pure water looks very similar in shape to the pressure-areaisotherm of the 2 1 PTAA-StNHz monolayer shown in Figure 2. The limiting area per molecule was calculated to be 28 A2 (on the basis of only the concentration of stearylamine molecules in the spreading solution). It is worth noting that the two polyion complexes which are most similar to each other physically-21 PTAA-StNH2 and SPAn-StNH2, both of which carry one stearylamine molecule on every other polymer repeat unit-exhibit about the same limiting area per molecule. Since the repeat units of the polymers comprising these polyion complexes are quite similar in size (a thiophene ring and a phenyl ring, respectively), this result is not too surprising. The SPAn-StNH2 complex is also exceptionally stable for several hours on the water surface when held at a surface pressure of 20 mN/m. The area merely held constant, however; it did not increase, as in the case of the 2:l and 5 1 polythiophene complexes. This is as expected, since there can be no ion hopping to other phenyl rings on the chain. As in the case of the polythiophene complexes, the sulfonated polyaniline complex also transfers with ease onto hydrophobic glass, platinum-coated glass, and zinc selenide substrates. Agan, transfer was found to be approximatelyequal on both the up and down strokes during dipping, implying a Y-type LB structure. The transfer ratios vary little from dip to dip, indicating reproducibility of the deposition process. As with the polythiophene complexes, the transfer ratios were found to be close to unity after the first dip. Multilayer LB f i i s of the SPAn-StNH2 polyion complex are of excellent optical quality, appearing clear and indigo-colored to the naked eye. UV-vis spectroscopy confirms the presence of sulfonated polyaniline in the LB films of this complex (seeFigure 3b). These spectra display a broad excitonic peak at ca. 625 nm which is characteristic of the emeraldine base form of sulfonated polyaniline. The spectra of as-prepared SPAn (the conducting form) and its sodium salt are shown in Figure 8. Note the striking similarity between the spectrum of the sodium salt of SPAn (formed by dissolving SPAn in aqueous NaOH) and the spectrum of the LB fiis of SPAn-StNH2 (Figure 3b). This confirms the formation of a polyion complex, and that the reaction of SPAn with stearylamine is identical to its reaction with NaOH: both are simple acid-base
showing ionically bound monolayers of conjugated polymer sandwiched between stearylamine spacer groups with interdigitated hydrocarbon tails.
Novel LangmuipBlodgett Film of Conducting Polymers reactions. Again, in order to evaluate the reproducibility of transfer, the UV-vis spectrum of an LB film of SPAnStNH2 was taken at several different stages during its production. The absorbance of the film at 625 nm was found to be a linear function of the number of layers deposited (correlation coefficient 0.999), indicating nearideal reproducibility of transfer in this system. W-vis dichroicstudies performed on LB filmsof SPAnStNH2 indicate no orientation of polymer chains in the dipping direction, since spectra taken with light polarized in directions parallel and perpendicular to the direction of dipping are identical. However, there are strong signs that the SPAn polymer chains are preferentially oriented in the plane of the LB substrate. The spectra of an LB film taken with light polarized in the S and P directions are shown in Figure 3b. It can be seen from this figure that the absorbance in the P polarization mode (parallel to the LB substrate) is significantly stronger than in the corresponding S polarization mode. This observed dichroism is strong evidence that the SPAn polymer chains are preferentially oriented in the plane of the LB substrate which is consistent with the claim that the polymer chains in these conjugated polyion complexes lie in planes sandwiched between stearylamine layers, as depicted in Figure 7. Also note that the dichroism observed in the SPAn-StNH2 LB films is more pronounced than in the 1:l PTAA-StNHz LB films, probably because the SPAn chains are significantly longer than their (oligomeric) PTAA counterpts and therefore more susceptible to orientation during the deposition process. The X-ray diffraction pattern for an LB film of SPAnStNH2 is shown in Figure 4b. As can be seen this pattern is very similar to that obtained from LB films of the 1:l PTAA-StNH2 complex. The peak intensities of the sulfonated polyaniline-containing f i i with similar thicknesses, however, are greater, indicating a more developed level of layer ordering. The average bilayer d spacing for these films is about 34 A, 17 A per monolayer, as it was for LB films of the 1:lPTAA-StNH2 complex. Thus, the multilayer structures of the LB films of the different polyion complexes are remarkably similar. Profilometry measurements performed on a 60-monolayer LB film of SPAn-StNHzyielded a total film thickness of approximately 955 A, which tranalates to an average bilayer d spacing of 32 A, or an averagemonolayer thickness of 16 A. Ellipsometric measurements,on the other hand, yielded the following results for the total thickness of three different SPAn-StNH2 LB films on platinum-coated glass substrates: 134, 199, and 584 A for lo-, 14-, and 28monolayer films, respectively. This works out to an average bilayer d spacing of 32 A which corresponds to an average monolayer thickness of about 16 A. Once again, as for the polythiophene LB films, the thickness data obtained from X-ray diffraction, profilometry, and ellipsometry and mutually consistent within experimental error. The similarity in thickness per layer of LB films of both 1:l PTAA-StNHz and SPAn-StNH2 is a clear indication that the primary controlling factor in determining film structure is the organization of the stearylamine molecules. The transmission FTIR spectrum of an LB film of SPAn-StNH2 in Figure 9 shows an N-H peak at 3300 cm-l, C-H peaks at 2850 and 2920 cm-l due to stearylamine, and several weaker peaks in the fingerprint region from 600 to 1700 cm-l due to sulfonated polyaniline. This information, in conjunction with the presence of the strong sulfonated polyaniline excitonic peak in the UV-vis spectra of LB films of SPAn-StNHz, serves to prove that this
Langmuir, Vol. 8, No.12, 1992 3175
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Wovelength ( n m )
Figure 10. UV-vis spectra of an LB film of SPAn-StNH2 in the as-prepared (a) and HC1-doped (b) state.
polyion complex does not dissociate on the water surface either. Rather, it is transferred whole on to the LB substrates during deposition. Analysis of the transmission and reflection FTIR spectra (also in Figure 9) yields a tilt angle of 4O f 8 O for the stearylamine hydrocarbon tails, which is quite similar to the tilt angle obtained for the polythiophene films. This constitutes yet another indication of the fundamental structural similarity between LB films of these two systems. The stronger absorbance of the polyaniline ring vibrations in the transmission spectrum as compared with the reflection spectrum also indicates that the polymer chains are preferentially oriented in the plane of the film. A more detailed analysis of this region of the spectrum is currently underway. Given the above information, it can be concluded that the structure of LB films of SPAn-StNHz is also generally described by Figure 7. LB films of SPAn-StNH2 doped by immersion in 1M HC1 turned green, indicating the conversion of the sulfonated polyaniline chains from the nonconducting emeraldine base form to the conducting salt form. The LB films of SPAn-StNH2 are quite robust, inasmuch as they do not delaminate or otherwise disintegrate in the dopant solution. The UV-vis spectra of an LB film of SPAn-StNH2 in the pristine and doped form are shown in Figure 10. Note the absence of the broad exciton peak at 625 nm in the doped-state spectrum, and the similarity of this spectrum to that of pure, electrically conductive SPAn in Figure 8. The conductivity of the SPAn-StNH2 LB films was extremelysensitive to moisture. This moisture sensitivity has been documented and studied for other polyaniline
Royappa and Rubner
3176 Langmuir, Vol. 8,No. 12,1992 systems,14and is generally attributed to the mobility of previously fixed waters of hydration upon exposure of polyaniline samples to moisture. Sulfonated polyaniline is known to contain one water of hydration for every sulfonated group, or every other phenyl ring in the chaineg In order to study the effectsof moisture on the conductivity of the LB films in a controlled manner, the conductivity was measured after drying the filmsand after placing them in a saturated water vapor atmosphere as detailed in the Experimental Section. The conductivity of the doped SPAn-StNH2 LB films was unmeasurable (
