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A Study of Langmuir and Langmuir-Blodgett Films of Polyaniline A. Dhanabalan,†,‡ R. B. Dabke,†,§ N. Prasanth Kumar,| S. S. Talwar,† S. Major,| R. Lal,⊥ and A. Q. Contractor*,† Departments of Chemistry, Physics, and Electrical Engineering, Indian Institute of Technology, Bombay 400 076, India Received November 12, 1996. In Final Form: May 20, 1997X
Formation of a stable monolayer of polyaniline (EB) at the air-water interface has been obtained, employing N-methylpyrrolidone (NMP) as processing solvent. Surface pressure-mean molecular area isotherm and compressibility of the polyaniline monolayer were studied under different subphase conditions such as the subphase temperature, the subphase pH, and the nature of counterions in the subphase. The mean molecular area found and compressibility results have been interpreted in terms of the rigidity of the polymer chains at the air-water interface. Further, polyaniline monolayers have been transferred onto different substrates and the multilayers were characterized by spectroscopic, electrical, and electrochemical techniques. The Langmuir-Blodgett multilayers were also deposited at different surface pressures. The transfer behavior and the electrochemical characteristics of these films indicate differences in their packing arrangement.
Introduction In the field of molecule-based electronic devices, conducting polymers are receiving increasing attention.1-4 Compared to other conducting polymers, polyaniline is of special interest owing to its higher environmental stability and ease of preparation. For many device applications, it is desirable to have the conducting polymer in a thin film structure, preferably with known thickness and molecular packing. The Langmuir-Blodgett (LB) technique has long been used for the preparation of ultrathin organic films, as it offers unique control over the film thickness and molecular orientation.5,6 The LB technique is mainly employed with amphiphilic molecules which possess distinct polar and nonpolar parts.7 Extensive work on the preparation of LB films of preformed amphiphilic polymers has been carried out by Tredgold and others.8,9 However, only a limited number of studies exist on the LB manipulation of conducting polymers. This is pri* Author for correspondence. E-mail: aliasgar@ ether.chem.iitb.ernet.in. † Department of Chemistry, Indian Institute of Technology. ‡ Department of Physics, Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, C.P. 369, CEP 13560-970, Sao Carlos, SP, Brazil. § Department of Chemistry, University of Mumbai, Mumbai 400 098, India. | Department of Physics, Indian Institute of Technology. ⊥ Department of Electrical Engineerings, Indian Institute of Technology. X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) Kaneto, K.; Kaneko, M.; Min, Y.; MacDiarmid, A. G. Synth. Met. 1995, 71, 2211. (2) Marsella, M. J.; Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 9832. (3) Gardnes, T. J.; Frisble, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (4) Rozsyai, L. W.; Wrighton, M. S. Chem. Mater. 1996, 8, 309. (5) Ulman, A. An Introduction to Ultrathin Organic Filmssfrom Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991; Part 2. (6) Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; Chapter 2. (7) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966; Chapter 4. (8) Tredgold, R. H. Thin Solid Films 1987, 152, 223. (9) Ringsdorf, H.; Schmidt, G.; Schneider, J. Thin Solid Films 1987, 152, 207.
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marily due to lack of processibility (solubility in organic solvents required for dispersing the molecules at the airwater interface) and amphiphilicity. Both these problems have often been overcome by derivatizing the polymer backbone with long alkyl chains. This approach has been successfully employed with polythiophenes10 and polyanilines.11,12 In another approach, the conducting polymer is codeposited along with amphiphilic molecules to form mixed LB films.13-15 Recently, LB films of polyaniline have been obtained, by exploiting the solubility of polyaniline in a N-methylpyrrolidone-chloroform mixture, both with or without acetic acid.14,16,17 It is to be noted that though polyaniline is not a typical amphiphilic molecule, a stable monolayer of polyaniline has been obtained and subsequently transferred as LB films. The influence of different subphase conditions on the π-A isotherms of polyaniline monolayer has, however, not been given much attention. We report herein the formation of a stable monolayer of polyaniline (emeraldine base (EB)), employing water miscible N-methylpyrrolidone (NMP), as processing solvent and discuss the implications of using NMP. π-A isotherms of polyaniline have been studied under different subphase conditions such as subphase temperature, subphase pH, and the presence of salt in the subphase. Further, the process of monolayer transfer under different deposition conditions has been studied, and the transferred multilayers have been characterized using spectroscopic and electrochemical techniques. (10) Watanabe, I.; Hong, K.; Rubner, M. F. Langmuir 1990, 6, 1164. (11) Ando, M.; Watanabe, Y.; Iyoda, T.; Honda, K.; Shimidzu, T. Thin Solid Films 1989, 179, 225. (12) Cheung, J. H.; Punkka, E.; Rikukawa, M.; Rosner, R. B.; Royappa, A. T.; Rubner, M. F. Thin Solid Films 1992, 210/211, 246. (13) Suwa, T.; Kakimoto, M.; Imai, Y.; Araki, T.; Iriyama, K. Mol. Cryst. Liq. Cryst. 1994, 255, 45. (14) Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 990. (15) Dhanabalan, A.; Dabke, R. B.; Datta, S. N.; Prasanth Kumar, N.; Major, S. S.; Talwar, S. S.; Contractor, A. Q. Thin Solid Films 1997, 295, 255. (16) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Phys. Chem. 1993, 97, 11580. (17) Agbor, N. E.; Petty, M. C.; Monkman, A. P.; Harris, M. Synth. Met. 1993, 55-57, 3789.
© 1997 American Chemical Society
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Figure 2. π-A isotherm of polyaniline monolayer at subphase temperature of 20 °C and pH of 4.0. The arrows in the figure indicate the surface pressures at which the films were transferred. Figure 1. The experimental setup used for electrochemical studies of polyaniline LB films: RE, reference electrode (calomel electrode); CE, counter electrode (platinum wire); WE, working electrode (platinum plate); electrolyte, 0.5 M sulfuric acid.
Experimental Section Polyaniline (EB) was prepared chemically employing the method reported by Huang et al.18 NMP used was obtained from Sisco Research Labs, India, and the purity was 99.5%. Control experiments were carried out by spreading the solvent on the surface of water and subsequent compression to confirm the absence of any surface active impurities. Deionized and Millipore ultrafiltered water (resistivity ) 18.2 MΩ cm) was used as the subphase. The salts (KCl and K2SO4) used were AR grade. The spreading solution was obtained by dissolving the mixture of polyaniline and acetic acid (10:1 ratio) in NMP in an ultrasonicator for an hour. The undissolved particles were filteredoff prior to use. The percent weight fraction of undissolved particles is about 40%. We have considered the amount of undissolved particles while calculating the mean molecular area, and hence the molecular area reported in this study is based on the polymer portion dissolved in NMP. Freshly prepared spreading solution 0.08 mg/mL was used within 10 days. The subphase pH measured after the spreading was 4.0. Mean molecular area has been calculated on the basis of a benzenoid unit, a quinoid unit, and an imine unit (molecular formula, C24H18N4; formula weight, 362). Surface pressure-mean molecular area isotherm studies and multilayer LB deposition were carried out using a KSV-3000 instrument kept in a clean room of class 10000. Quartz and platinum plates were used as substrates. Quartz substrates were cleaned by treating with a hydrogen peroxide (30%) aqueous ammonia (1:1) mixture at 70 °C followed by ultrasonic cleaning with organic solvents and kept in deionized water prior to use. Platinum plates were degreased by treating with chromic acid and then cleaned electrochemically in galvanostatic mode by passing anodic and cathodic currents of 10 mA/cm2 for 20 min each in 0.5 M sulfuric acid. UV-vis spectra were obtained with a Shimadzu-160A spectrophotometer. Electrochemical measurements were done with an EG&G PARC 273 scanning potentiostat equipped with Linseis 1900-II XY recorder. The experimental setup used for electrochemical studies is shown schematically in Figure 1. The LB film deposited on a platinum plate was held horizontally, and a Teflon-coated silicone “O” ring (area 0.442 cm2) was placed on the film. A drop of 0.5 M sulfuric acid was released on the area defined by the “O”-ring and the platinum plate was connected to the working electrode lead. Platinum wire and saturated calomel electrode were used as counter and reference electrodes respectively. The potential was scanned between -200 and 800 mV at a scan rate of 50 mV/s. The in-plane dc conductivity of polyaniline LB films was measured at room temperature by the four probe technique using gold contacts. The thickness of the film was measured with a Fizzeau interferometer (Å scope) after coating it with a very thin layer of aluminum. The film thickness was found to be 35 ( 5 Å per layer; this value was used for calculating the conductivity of the film. (18) Huang, W. S.; Humphery, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2385.
Results and Discussion Langmuir Films. Preparation of polyaniline LB films, in which a mixture of NMP-CHCl3 was used as the spreading solvent, has been reported earlier.14,16,17 In comparison, pure NMP has been used as the spreading solvent in the present study. With the NMP-CHCl3 mixture, only a dilute spreading solution (concentration 0.01 mg/mL) of polyaniline could be obtained. This poses a problem as a large quantity of solution is required to be spread on the water surface in order to obtain a reasonable monolayer area which can be used for multilayer LB deposition. With NMP as the solvent, a more concentrated spreading solution (concentration 0.08 mg/mL) of polyaniline could be obtained.15 As NMP is water miscible, one might expect that the polymer may either dissolve into the subphase along with NMP or precipitate on the water surface. However, we obtained stable monolayers at the air-water interface, as is evident from the isotherm described below. Figure 2 shows the surface pressure-mean molecular area (π-A) isotherm of polyaniline monolayer at a subphase temperature of 20 °C and pH of 4.0. The isotherm shows considerable curvature indicating expanded monolayer behavior. The monolayer was found to collapse at about 30 mN/m. In the region of high surface pressure (20 < π < 30 mN/m), the isotherm has a nearly constant slope and this part of the isotherm has been extrapolated to zero surface pressure to calculate the limiting mean molecular area. Keeping the monolayer in the compressed state for 2 h or so did not cause any significant change in mean molecular area, suggesting the formation of a stable monolayer. The limiting mean molecular area observed in our studies was found to be smaller (∼8 Å2) than the reported limiting mean molecular areas of 12-1614,16 and 27 Å2 17 for polyaniline monolayer obtained with NMP-CHCl3 as the processing solvent. The variation in the values of mean molecular area observed by different workers may be understood as follows: The monolayers of nonamphiphilic polymeric material (polyaniline in the present case) are probably always folded. The extent of folding may depend upon the solvent system used for dissolution and spreading (due to variation in the molecular weight fraction of polymer dissolved in the solvent and variation in the miscibility of solvent with subphase). Different methods of polymerization used to obtain polymer samples, which in turn determine the molecular weight distribution and the structure of polymer chains, may also be the reason for the observed variation in the values of mean molecular areas. The relatively smaller mean molecular area observed in our study may be indicative of the formation of folded layers on the water surface rather than a “true” monolayer. It is of interest to study the surface pressure-mean molecular area isotherms under different subphase con-
LB Films of Polyaniline
Figure 3. π-A isotherms of polyaniline monolayer at two different subphase temperatures at a subphase pH 4.0: a, 25 °C; b, 10 °C. Dotted lines are plots of compressibility versus mean molecular area at two different subphase temperatures.
Figure 4. (a) π-A isotherms of polyaniline monolayer at two different subphase pH values at a constant subphase temperature 25 °C: a, 0.25 M K2SO4, pH 5.8; b, 0.25 M K2SO4 + 0.07 M H2SO4, pH 0.9. Dotted lines are plots of compressibility versus mean molecular area at two different subphase pH. (b) π-A isotherms of polyaniline monolayer at two different subphase pH values at a constant subphase temperature 25 °C: a, 0.25 M KCl, pH 4.8; b, 0.25 M KCl + 0.06 M HCl, pH 0.7. Dotted lines are plots of compressibility versus mean molecular area at two different subphase pH.
ditions. π-A isotherms of polyaniline monolayer obtained at two different subphase temperatures (subphase pH ) 4.0) are shown in Figure 3. Also shown are corresponding compressibility plots against the mean molecular area. It is observed that on lowering the subphase temperature from 25 to 10 °C, the isotherm shifts toward higher mean molecular areas. The overall compressibility of the monolayer is smaller at the lower temperature, which is consistent with the observed increase in mean molecular areas. These results suggest that the polymer chains tend to occupy a larger area at a lower subphase temperature. The influence of subphase pH on the monolayer characteristics at a constant temperature has been studied by adding H2SO4 and HCl. To avoid effects arising from changes in ionic strength, the effect of subphase pH has been assessed by carrying out experiments in the presence of an excess of the corresponding potassium salt (K2SO4 or KCl) in the subphase. The results are summarized in Figure 4, which shows π-A isotherms and the corresponding compressibility plots at a subphase temperature
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of 25 °C. The presence and nature of the counterion have a significant effect on the monolayer characteristics as is evident from a comparison of corresponding isotherms and compressibility curves in Figures 3, 4a, and 4b. Figure 4a also shows the effect of subphase pH on the polyaniline monolayer spread on 0.25 M K2SO4 subphase. On lowering the subphase pH by adding sulfuric acid, the isotherm shifts toward higher mean molecular areas and a decrease in overall compressibility is observed. Figure 4b shows the results of similar experiments with 0.25 M KCl solution as subphase. On lowering the subphase pH by adding hydrochloric acid, the isotherm shifts again toward higher mean molecular area, and the overall compressibility is found to decrease, though the magnitude of the change is smaller compared to that observed with sulfate counterion. The above set of experiments were also performed at a lower subphase temperature of 11 °C (not shown in figures). For the purpose of comparison, the mean molecular areas at 25 mN/m under different subphase conditions are presented in Table 1. The results can be summarized as follows: The change in mean molecular area with subphase pH at a constant temperature, (∆A/ A)T, is larger at the higher temperature. It may be mentioned that at the lower temperature, the compressibility curves (not shown) were also found to be practically unaffected by a change of subphase pH. Further, the change in mean molecular area with subphase temperature at a constant subphase pH, (∆A/A)pH, is larger at the higher subphase pH. In general, the above behavior is consistent with the observation that the overall compressibility is higher at the higher temperature and pH. The changes observed in the nature of isotherms for different subphase conditions may be understood on the basis of change in the rigidity of the polymer chains at the air-water interface. At a given subphase pH, the increase of subphase temperature decreases the rigidity of polymer chains as evident from higher overall compressibility. As a result the polymer chains are packed closely, leading to a lower mean molecular area. Conversely, the polymer chains are comparatively more rigid at lower temperature, thereby imposing restriction on close packing which results in increase in mean molecular area. The change of slope of curves in the region of smaller mean molecular areas indicates the onset of collapse. It may be noted from Figure 3 that at a lower subphase temperature, the onset of collapse appears at a comparatively larger mean molecular area, indicating that the rigid monolayer tends to collapse earlier. Similar collapse behavior was observed for monolayers on subphase containing K2SO4 and KCl. Further, at a given temperature, the observed increase in mean molecular area as well as lowering of overall compressibility on decreasing the subphase pH indicates an increase in the rigidity of polymer chain. It is known that on lowering the bath pH, the conductivity of polyaniline increases. In the conducting state, the polymer adopts a rigid conformation in which there is an extended π-electron delocalization. A similar correlation has been proposed to explain the increase in conductivity due to “secondary doping” of polyaniline.19,20 However, as indicated earlier, a significant change in mean molecular area and compressibility with decrease of subphase pH was not observed at lower subphase temperature. It is possible that at lower subphase temperature, a monolayer which is already relatively rigid is less prone to undergo further changes on lowering subphase pH. Solubility of polymer chains, i.e., the extent of distribution of polymer (19) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. (20) Avlyanov, J. K.; Min, Y.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 72, 65.
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Table 1. Variation of Mean Molecular Area (mMa) at 25 mN/m under Different Subphase Conditions no.
subphase
subphase temp, °C
subphase pH
mMa at 25 mN/m (Å2)
1 2 3 4 5 6 7 8 9 10
water water 0.25 M K2SO4 0.25 M K2SO4 0.25 M K2SO4 0.25 M K2SO4 0.25 M KCl 0.25 M KCl 0.25 M KCl 0.25 M KCl
25 10.5 25 25 12 12 25 25 11 11
4.0 4.0 5.8 0.9a 5.8 0.9a 4.8 0.7b 4.8 0.7b
5.25 6.07 5.19 5.67 5.78 5.93 3.53 3.72 4.15 4.12
a
(∆A/A)pH (%)
(∆A/A)T (%)
15.6 (1 w 2) 9.2 (3 w 4) 11.4 (3 w 5) 4.6 (4 w 6)
2.6 (5 w 6) 5.4 (7 w 8)
17.6 (7 w 9) 10.8 (8 w 10)
0.0 (9 w 10)
b
pH was reduced by adding H2SO4. pH was reduced by adding HCl.
Figure 5. A plot of cumulative transfer parameter versus the number of strokes on platinum substrate. Films were transferred at 25 mN/m and at subphase pH 4.0 and temperature 20 °C.
chains in the adjacent bulk phase (water) under different subphase conditions, might also contribute to the observed changes, especially with respect to change of temperature. Langmuir-Blodgett Films. The monolayer has been transferred onto different substrates as LB multilayers by the vertical dipping method. Multilayer LB depositions were carried out at a subphase pH of 4.0 (with pure water as subphase) and temperature of 20 °C after holding the monolayer at a constant surface pressure of 25 mN/m. Irrespective of the substrates, during the initial six to eight strokes, the deposition occurred during both the lifting and the dipping modes (Y-type), while in subsequent strokes, the deposition occurred only during the lifting mode (Z-type). With quartz substrates, a near unity transfer ratio was noticed up to 51 layers. On gold/glass and tin oxide/glass substrates the transfer ratio was found to be in the range of 0.7-0.8. The transfer on platinum substrates was characterized by transfer ratios in the range of 0.8-1.1 up to 25 layers or so, above which the transfer ratio was found to decrease with increasing number of strokes. In order to compare the transfer characteristics, we have used a parameter which we call the cumulative transfer parameter (CTP), which is obtained as the sum of transfer ratios for a given number of strokes. The cumulative transfer parameter for a perfect Y-type deposition should be equal to the total number of both up and down strokes (n), while for a perfect Z-type deposition, it should be nearly half the total number of strokes (n + 1)/2. This parameter has been used as a measure of the quantity of material transferred during a given number of strokes. A plot of CTP versus the number of strokes on platinum substrate is shown in Figure 5. The observed nonlinear dependence of CTP on the number
Figure 6. A plot of cumulative transfer parameter for 17stroke films versus the surface pressure at which the films were transferred: substrate, platinum; subphase pH, 4.0; temperature, 20 °C. Table 2. Transfer Characteristics at Different Surface Pressure along with Mean Molecular Area Values target surface pressure (mN/m)
mean molecular area (Å2)
transfer ratio (TR) (TR for 1st layer)
8 13 18 25 30
7.56 6.60 5.95 5.22 4.66
(1.5) 0.9-0.3 (1.5) 1.1-0.6 (1.3) 1.0-0.7 (1.8) 1.1-0.8 (2.4) 2.2-1.6
of strokes is indicative of the initial Y-type deposition followed by poor Z-type transfer above 25 strokes or so. Polyaniline monolayers were also transferred onto platinum substrates at constant surface pressures of 8, 13, 18, 25, and 30 mN/m as indicated by arrows in Figure 2. Transfer characteristics at different surface pressures along with the mean molecular area values are given in Table 2. For the transfers at the surface pressures 13, 18, and 25 mN/m (in the near-linear region of the isotherm, Figure 2), transfer ratios were mostly in the range of 1.10.7. Transfer at 30 mN/m (close to collapse pressure) was characterized by high transfer ratio (2.2-1.6), while only a poor transfer ratio (0.3-0.9) was observed during the transfer at 8 mN/m. These features are clearly reflected in Figure 6, in which the CTP for 17 strokes is plotted against the constant surface pressure at which the films were transferred. As shown in the figure, the CTP values observed at the surface pressures 13, 18, and 25 mN/m are in the range of 8.5-10.0, indicating good Z-type transfer (expected value for a perfect Z-type transfer being 9). Further, the CTP values of 5.5 and 21 observed at 8 and 30 mN/m, respectively, are indicative of poor transfer. The UV-vis absorption spectrum of a 21-stroke LB film of polyaniline transferred at surface pressure of 25 mN/m (subphase pH 4.0 and temperature 20 °C) on quartz is shown in Figure 7a. The characteristic strong absorption
LB Films of Polyaniline
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Figure 7. (a) Absorption spectrum of polyaniline LB film (21 strokes) on quartz. Film was transferred at 25 mN/m: subphase pH, 4.0; temperature, 20 °C. (b) A plot of absorbance at 590 nm and the number of strokes.
at ∼590 nm and the typical absorption pattern of polyaniline14 in the 280-360 nm region reveal that transfer of polyaniline has taken place. The linear relationship observed between absorbance at 590 nm and the number of strokes indicates a uniform layer by layer transfer of polyaniline (Figure 7b). It is interesting to note that for a given number of strokes, the absorbance values obtained either at 360 nm or at 590 nm in the present study are almost three times those reported by earlier workers.16,17 Though the absorption measured under different experimental setups may vary slightly, the observed higher absorption seems to be in agreement with the lower mean molecular area observed in the present study in comparison to earlier reports. Hence, it is likely that with NMP as processing solvent, one is obtaining a stable and more close packed monolayer which gets transferred uniformly onto different substrates. Thicknesses of the LB films were measured using the Å-scope and the thickness per layer was calculated assuming Z-type deposition. The thickness per layer was found to be 35 ( 5 Å from several measurements with different numbers of layers transferred. In-plane dc conductivity of polyaniline LB films transferred at a surface pressure of 25 mN/m in both the as-deposited and doped states were obtained by a four-probe technique using the measured thickness per layer. In-plane dc conductivity for the undoped polyaniline LB film is ∼10-4 S cm-1, which is 4 orders of magnitude higher than that reported for as deposited polyaniline LB films.17 Further, on treating polyaniline LB film with HCl vapor (0.1 M HCl) for 5 min, the conductivity increased to a value of ∼0.1 S
Figure 8. Cyclic voltammograms of polyaniline LB films (17 strokes) transferred on platinum substrate at various surface pressures: electrolyte, 0.5 M H2SO4; scan rate, 50 mV/s. The cumulative transfer parameter corresponding to the transfer is indicated alongside in each case.
cm-1, which is comparable to that reported for doped polyaniline LB films.17 Cyclic voltammograms of polyaniline LB films (17 strokes) transferred on a platinum plate at a subphase pH of 4.0 and temperature of 20 °C and at various surface pressures are shown in Figure 8. In general, the CVs exhibit redox features characteristic of polyaniline; the currents at the anodic and cathodic limits are significantly higher than those observed for electrochemically deposited or chemically cast films. This shape of cyclic voltammograms has consistently been observed for all LB films transferred onto platinum or gold substrates. The films transferred onto tin oxide have features similar to electrochemically deposited films. It may be recalled that as shown in Figure 6, the films transferred at different surface pressures exhibited different transfer characteristics. It is interesting to see whether the observed transfer characteristics have any impact on the redox charge of the polyaniline LB films obtained at different surface pressures. While there was no significant change in the qualitative features of the cyclic voltammograms obtained for these films, there were considerable differences in the redox charge capacity of films transferred at various surface pressures. The total redox charge (Qredox) calculated by the integration of I-V response can be quantitatively related to the influx and efflux of anions through
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qredox calculated for the film transferred near the collapse pressure was found to be minimum. The higher qredox associated with the films transferred at 8 mN/m indicates that the film is more porous in comparison with the films deposited at higher surface pressures. This facilitates the easy movement of anions through the film during electrochemical scanning. There was no significant change in qredox associated with the films transferred at 13, 18, and 25 mN/m, indicating that these three films have a similar packing density. Further, the smaller qredox associated with the film transferred near the collapse pressure indicates a more close packed film structure.
Figure 9. A plot of redox charge per unit area divided by the cumulative transfer parameter versus the surface pressure at which the films were transferred: subphase pH, 4.0; temperature, 20 °C.
the polyaniline film during electrochemical scanning.21 Figure 9 shows a plot of the redox charge per unit area divided by the cumulative transfer parameter ((Qredox/ area)/CTP ) qredox) versus the surface pressure used for the transfer. It was noted that the value of qredox for the film transferred at 8 mN/m was higher than that observed for the films transferred at higher surface pressures. In addition, there was no significant change in qredox for the films transferred at 13, 18, and 25 mN/m, which are located in the condensed part of the π-A curve. The value of (21) Orata, D.; Buttry, D. A. J. Am. Chem. Soc. 1987, 109, 3574.
Conclusions In the present study, it has been demonstrated that a stable and close packed monolayer of polyaniline can be obtained at the air-water interface using N-methylpyrrolidone as processing solvent. The study of the effect of various subphase conditions reveals an increase in mean molecular area and decrease of compressibility on lowering of subphase temperature and subphase pH, indicating an increase in rigidity of monolayer under these conditions. Polyaniline monolayers were transferred as LB multilayers onto different substrates and UV-vis absorption studies revealed a linear increase of absorption with increasing number of layers, indicating a uniform transfer. Multilayer depositions carried out at different surface pressures exhibited different transfer characteristics. Electrochemical studies of these films revealed that a higher redox charge is associated with the film transferred at lower surface pressure in comparison with the films transferred at higher surface pressures. This indicates the formation of films with loosely packed polymer chains at lower surface pressures which facilitates easy movement of anions through the film during electrochemical scanning. A detailed study of electrical and electrochemical characteristics of polyaniline LB film is being reported separately. Acknowledgment. We are grateful to Department of Science and Technology, Government of India, for financial support of this work. LA961093U
