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Langmuir 1997, 13, 4882-4886
Composite Langmuir Monolayers and Langmuir-Blodgett Films from 16-Mer Polyaniline A. Dhanabalan,† A. Riul, Jr.,† L. H. C. Mattoso,‡ and O. N. Oliveira, Jr.*,† Instituto de Fı´sica de Sa˜ o Carlos, USP, CP 369, 13560-970, Sa˜ o Carlos, SP, Brazil, and CNPDIA/EMBRAPA, CP 741, 13560-970, Sa˜ o Carlos, SP, Brazil Received March 31, 1997. In Final Form: June 16, 1997X
The formation of Langmuir monolayers and Langmuir-Blodgett films is reported for the first time for a polyaniline oligomer, 16-mer, that was mixed with cadmium stearate. The processibility of this oligomeric material was greatly improved as compared to the parent polyaniline, which allowed one to use common solvents like chloroform as the spreading solvent. Analogously to composite polyaniline monolayers, the stability and the transferability are found to depend on the 16-mer content in the composite film, with poor stability and low transfer ratios above 60% of 16-mer. Deposited Langmuir-Blodgett films were found to be undoped (or only weakly doped) since they were obtained from monolayers spread onto aqueous subphases (pH ) 6.0). Uniform Langmuir-Blodgett (LB) deposition was confirmed by optical microscopy and UV-vis spectra whose intensity increased linearly with the number of layers deposited and also with the 16-mer content. Fourier transform infrared (FTIR) results revealed that cadmium stearate was transferred together with the 16-mer. Upon doping in HCl vapor, the electrical conductivity of the deposited LB films was seen to increase by 1 order of magnitude.
Introduction Oligomers of conducting polymers have received great attention of late due to their exciting properties, in particular, their improved electrical characteristics. MacDiarmid et al.,1 for instance, reported the preparation of the tetramer, octamer, and 16-mer of polyaniline, starting from the dimer. Polyaniline 16-mer could be obtained in the conjugated, half-oxidized emeraldine state, which after doping reached high electrical conductivity. The mechanical properties of these oligomers, however, were relatively poor,1 probably owing to the shorter per chain conjugation length. By the same token, on the other hand, short-chain materials can be packed more efficiently. Additional advantages of oligomers would be in their ease of crystallization compared with their polymer counterparts, which is likely to enhance the inter-molecular component for electrical conductivity. Indeed, increases in the crystallinity and ordering have been pursued in conducting polymers as a way of minimizing defects and enhancing conductivity. In this context, the LangmuirBlodgett (LB) technique has long been recognized as a powerful method since it offers a means to organize the molecules in a desired way, with the resulting LB films often possessing a high degree of packing order at the molecular level.2,3 After some slow progress because of poor processibility and nonamphiphilicity associated with parent conducting polymers, the LB method has been extensively applied to conducting polymers using a variety of processing approaches.4-14 LB films have also been * To whom correspondence should be addressed. Telephone: +55 16 271 5365. Fax: +55 16 271 3616. E-mail: CHU@ IFQSC.SC.USP.BR. † Instituto de Fı´sica de Sa ˜ o Carlos. ‡ CNPDIA/EMBRAPA. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Zhang, W.; Feng, J.; MacDiarmid, A. G.; Epstein, A. J. Abstract presented at the International Conference on Science and Technology of Synthetic Metals, Snowbird, UT, 1996. (2) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Ulman, A. An Introduction to Ultrathin Organic Filmssfrom Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (4) Robitaille, L.; Bergeron, J. Y.; Da´prano, G.; Leclerc, M.; Callender, C. L. Thin Solid Films 1989, 244, 728. (5) Cheung, J. H.; Punkka, E.; Rikukawa, M.; Rosner, R. B.; Royappa, A. T.; Rubner, M. F. Thin Solid Films 1992, 210/211, 246.
S0743-7463(97)00330-2 CCC: $14.00
fabricated from mixed quinque-, sexi-, and septithiophenes.15 In the present work, we reportsfor the first time to our knowledgeson the study of composite Langmuir monolayers containing oligomeric 16-mer polyaniline and cadmium stearate. The choice of the mixed-film approach was essentially due to the high uniformity that may be achieved when a conducting polymer is transferred onto a solid substrate along with an amphiphilic material such as cadmium stearate.12 We felt that such a study may provide key information on the effect of the polymer chain length on the packing of polymer molecules in the LB film structure. Also, the main problems associated with the LB processing of the parent polyaniline can be alleviated. For instance, using 16-mer processed with a functionalized acid eliminates the need of employing water-miscible spreading solvents such as N-methylpyrrolidone (NMP) and filtration of undissolved high molecular weight polymers from the spreading solution. The 16-mer polyaniline is found to be more easily processible than the high molecular weight polyaniline obtained either by electrochemical or by chemical means. The surface pressure and surface potential results obtained with 16mer polyaniline composite monolayers have been compared with those obtained with chemically prepared polyaniline. These composite monolayers have also been transferred and characterized. (6) Zhou, H.; Stern, R.; Batich, C.; Duran, R. S. Makromol. Chem. Rapid Commun. 1990, 11, 409. (7) Ando, M.; Watanabe, Y.; Iyoda, T.; Honda, H.; Shimidzu, T. Thin Solid Films 1989, 179, 225. (8) Agbor, N. E.; Petty, M. C.; Monkman A. P.; Harris, H. Synth. Met. 1993, 55-57, 3789. (9) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Phys. Chem. 1993, 97, 11580. (10) Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 990. (11) Suwa, T.; Kakimoto, M.; Imai, Y.; Araki, I.; Iriyama, K. Mol. Cryst. Liq. Cryst. 1994, 255, 45. (12) Dhanabalan, A.; Dabke, R. B.; Datta, S. N.; Prasanth Kumar, N.; Major, S. S.; Talwar S. S.; Contractor, A. Q. Thin Solid Films, in press. (13) Riul, A., Jr.; Mattoso, L. H. C.; Telles, G. D.; Herrmann, P. S. P.; Colnago, L. A.; Parizotto, N. A.; Baranauskas, V.; Faria, R. M.; Oliveira, O. N., Jr. Thin Solid Films 1996, 284-285, 177. (14) Riul, A., Jr.; Mattoso, L. H. C.; Mello, S. V.; Telles, G. D.; Oliveira, O. N., Jr. Synth. Met. 1995, 71, 2067. (15) Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. Thin Solid Films 1988, 160, 87.
© 1997 American Chemical Society
Composite Langmuir Monolayers and LB Films
Langmuir, Vol. 13, No. 18, 1997 4883
Experimental Section Stearic acid (99+%) was purchased from Aldrich Chemical Co. and used without further purification. The 16-mer polyaniline was a kind gift from Prof. MacDiarmid of The University of Pennsylvania; it was prepared employing the procedure reported in ref 1. Chloroform (HPLC grade, Merck) was used as the spreading solvent, and the spreading solutions were obtained by dissolving different proportions of 16-mer polyaniline and stearic acid along with camphor sulfonic acid, CSA (Aldrich, 99%) and m-cresol (Merck, 99%) by ultrasonification for 15 min. The ratio (by weight) of 16-mer and CSA was about 1:1.3. The spreading solution was found to be homogeneous so that no filtration was required. Equal amounts of m-cresol were used in all mixtures in order to avoid possible effects from varying the amount of this water-soluble component. Ultrapure water supplied by a Milli-RO coupled to a Milli-Q system whose resistivity is 18.2 MΩ cm-1 was used for the preparation of the subphase solutions. These solutions also contained 4 × 10-4 M cadmium chloride, and for the pH to be kept at ca. 6.0, 5 × 10-5 M sodium bicarbonate was added to the subphase. All experiments were carried out at a temperature of 21 ( 1 °C. Langmuir monolayers were characterized by surface pressure-mean molecular area (Π-A) and surface potential-mean molecular area (∆V-A) measurements and transferred in the form of LB films using a KSV LB5000 trough mounted on an antivibration table in a class 10 000 clean room. The surface pressure and surface potential were measured with a Wilhelmy plate and a Kelvin probe, respectively, both provided by KSV. The composite monolayers were compressed with a barrier speed of 10 mm/ min; their stability was investigated by keeping the monolayer in the compressed state at the surface pressure used during transfer (29 mN/m) and monitoring the changes with time in the mean molecular area. The mean molecular area axis (abscissa) is calibrated to the concentration and molecular weight of stearic acid in the spreading solution, i.e., the number of stearic acid molecules spread onto the subphase during each experiment. This has been done to investigate the effect from the addition of various amounts of 16-mer polyaniline to the isotherm of pure cadmium stearate. For the purpose of comparison, the mean molecular areas were also obtained by considering the weighed molecular weight average of the 16-mer polyaniline monomer repeating unit (91) and stearic acid. Langmuir-Blodgett films were deposited on hydrophilic BK7 glass and calcium fluoride plates that were cleaned thoroughly before use. Deposition was carried out using the vertical dipping method with dipping speeds ranging from 1 to 3 mm/min. Each deposited layer was usually dried in air for 10 min prior to a subsequent dipping. UV-vis and transmission FTIR measurements were conducted using a Hitachi U200 spectrophotometer and a BOMEM Michelson series instrument (resolution ) 4 and 32 scans), respectively. Optical microscopy analysis was carried out with a Olympus BX 40 instrument. In-plane dc electrical conductivity measurements were carried out with the four-probe method using a Keithley-167 electrometer.
Results and Discussion Π-A Isotherms. The surface pressure-mean molecular area isotherm of cadmium stearate is characterized by a steep rise in surface pressure at ∼20 Å2, indicative of a condensed monolayer. The absence of a typical liquid region that is normally observed with pure stearic acid is correlated to the complexation of acid headgroups with cadmium ions.16 Isotherms of composite monolayers containing cadmium stearate and different weight percentages of 16-mer polyaniline are shown in Figure 1. In the present study, the weight percentage rather than the mole percentage of 16-mer polyaniline has been used for describing the effect of the addition of different amounts of 16-mer polyaniline. This choice was mainly because different values for the repeating unit of polyaniline have been used in the literature. Up to 60% 16-mer polyaniline, a condensed monolayer was obtained with a collapse (16) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.
Figure 1. Π-A isotherms of the composite monolayers containing cadmium stearate and different weight percentages of 16-mer polyaniline. The mean molecular area was calculated considering only the cadmium stearate molecules.
pressure in the range of 60-65 mN/m, which is close to that measured for pure cadmium stearate monolayers. In addition, very little or no hysteresis was observed during subsequent compression-expansion cycles analogously to the behavior of pure cadmium stearate. Reproducible isotherms excluded the possibility of any material dissolution into the subphase. Above 60% 16-mer, however, the composite monolayer exhibited an expanded isotherm with no clear solid region and considerably lower collapsing pressures which decreased with increasing 16-mer content. The limiting mean molecular area, obtained by extrapolating the steep region of the isotherm to zero surface pressure, increases with the increasing amount of 16-mer in the composite, as illustrated in Figure 2a. Also shown in Figure 2a is the change in the critical area for the onset of surface potential (as explained later) vs the mean molecular area. For the purpose of comparison, the mean molecular areas calculated on the basis of the weighed molecular weight average of the 16-mer polyaniline monomer repeating unit and stearic acid were plotted in Figure 2b against the weight percentage of 16-mer polyaniline in the composite. The mean molecular area thus obtained was found to decrease near linearly with increasing weight percentage of 16-mer polyaniline, obeying the simple additive rule of mixing. Such an observation is normally related to the formation of separate domains of the components without molecular level mixing, as has also been confirmed by the observation of clear diffraction peaks corresponding to cadmium stearate domains in the XRD pattern of the transferred LB films. Stability plots for the composite monolayers obtained as described in the Experimental Section are shown in Figure 3. It is noted that the mean molecular area (at 29 mN/m) varies for different amounts of 16-mer, as indicated in the figure together with the weight percentage of 16-mer. In order to compare the data, all stability plots are shown in the same picture, and hence, Figure 3 reveals only the relative change in the mean molecular area from the initial value in each case. Stable composite monolayers could be obtained up to 50% of the polymer content, above which a decrease in mean molecular area is observed on holding the monolayer at the deposition surface pressure. In the cases of 60% and 75% 16-mer, monolayers became stable after 1 h or so, but the monolayer with 90% 16-mer did not get stabilized even
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Figure 3. Time stability curves of composite monolayers containing cadmium stearate and different weight percentages of 16-mer polyaniline.
Figure 2. Plots of the weight percentage of 16-mer polyaniline (a, top) in the composite monolayer vs limiting mean molecular area (4) and critical mean molecular area for the onset of surface potential isotherms (O) and (b, bottom) plot of weight percentage of 16-mer polyaniline in the composite monolayer vs the limiting mean molecular area calculated based on the weighed average molecular weight of 16-mer polyaniline and stearic acid.
after 2 h, and blue crystallites could be seen by the naked eye during this period. Hence, the 60% and 75% composite monolayers were transferred only after stabilization had been achieved, and no attempt was made to transfer the 90% composite monolayer. The observed decrease in collapse pressure and the loss of stability may indicate that the monolayer characteristics of the composites are mostly being dictated by the oligomeric 16-mer above 60%. The poor stability may be related to the squeezing out of some 16-mer molecules from the cadmium stearate monolayer when it is held at higher surface pressures. In subsidiary experiments, monolayers of pure 16-mer were spread on an acidic subphase. Analogously to what had been reported for high molecular weight polyaniline,13 relatively stable monolayers can only be obtained on acidic subphases. On pure water, aggregates are formed which can be seen by the naked eye as the monolayer is compressed. The mean molecular area obtained for 16mer polyaniline is about 5-6 Å, which is comparatively
Figure 4. ∆V-A isotherms of composite monolayers containing cadmium stearate and different weight percentages of 16-mer polyaniline.
smaller than that observed with high molecular weight polyaniline. The transfer onto a solid substrate, however, is not straightforward, and the transfer ratios are usually in the 0.7-0.9 range. Moreover, the resulting LB multilayers are considerably less homogeneoussas viewed from electron microscopy or atomic force microscopysthan the composite LB films containing either PAni/cadmium stearate or 16-mer/cadmium stearate. ∆V-A Isotherms. Figure 4 shows surface potential isotherms for mixed 16-mer/cadmium stearate monolayers containing various weight percentages of 16-mer. The monolayer surface potentials can be related to the group dipole moments of the monolayer-forming molecules using the Demchak-Fort approach,17,18 but this only applies to simple compounds. If the monolayers are ionized, an additional contribution appears from the electric doublelayer.19 In the case of polymeric materials, one cannot identify the dipoles (and their orientation) which will give (17) Oliveira, O. N., Jr.; Taylor, D. M.; Lewis, T. J.; Salvagno, S.; Stirling, C. J. M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1009. (18) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508. (19) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. Chem. Phys. Lett. 1989, 161, 147.
Composite Langmuir Monolayers and LB Films
rise to the surface potential. Therefore, no quantitative analysis can be made of the measured surface potentials for the mixed monolayers discussed here. There are, nevertheless, features in the surface potential isotherms which may provide useful information. The most important one is the sharp increase in potential when a critical area is reached. Such an increase has been attributed to the lowering of the effective dielectric constant at the monolayer/water interface20 and appears to be a general feature for all types of monolayer. It is related to the coming together of microdomains known to exist even for very large areas per molecule,21 and as a consequence, non-zero surface potentials are obtained when aggregates are formed at large areas. As shown in Figure 4, upon increasing the 16-mer content, the surface potential for large areas gradually increased, with the exception of the 90% 16-mer, which displays an odd behavior as compared to the other monolayers. The observation of a relatively high surface potential even at larger molecular areas for monolayers containing a higher amount of 16-mer polyaniline may possibly be related to the formation of larger domains of 16-mer. In addition, the onset for the curve at the critical area became less sharp. The shift in critical area, also illustrated in Figure 2a, is not surprising, as the mean molecular area also increases with the 16-mer content. Critical areas are generally twice that of the limiting mean molecular area for condensed monolayers. The sharp increase in area when the 16-mer content was increased from 75% to 90% is nonetheless striking, which again demonstrates that this monolayer has a distinct behavior that was also reflected in its poor stability. It should be stressed that for poorly stable composite monolayers, the compression process is carried out under a nonequilibrium condition. The measured areas cannot, therefore, be considered as a “true” representation of the molecular area for the composite monolayer containing 90% 16-mer polyaniline. Transfer and Characterization of Composite Monolayers. The composite monolayers were also transferred onto solid substrates as multilayer LB films. It is of interest to see whether the observed difference in the monolayer characteristics from the various 16-mer contents is being reflected in the transfer process. In order to make the comparison straightforward, all monolayers were transferred at 29 mN/m. Stable monolayers containing up to 50% 16-mer could easily be transferred as type Y (transfer during both up- and downstrokes) with a near unity transfer ratio (TR) up to 25 layers. However, with the 60% and 75% composites, though transfer ratios for the upstrokes were close to unity, those for the downstrokes were poor (TR ) 0.6-0.4), indicating a Y-type transfer with some Z-type character. This behavior is similar to that already observed with composite LB films of cadmium stearate and high molecular weight polyaniline.22 Transferred LB films were characterized by UV-vis, FTIR, optical, and preliminary electrical conductivity measurements. Figure 5 shows the typical absorption spectra of a 50% composite LB film. The as-deposited film exhibits a strong absorption at ∼590 nm (curve a), and the spectrum is similar to that observed with composite LB film from polyaniline (EB) and cadmium stearate.22 One thus concludes that the 16-mers in the composite LB film are present in the emeraldine base (20) Oliveira, O. N., Jr.; Taylor, D. M.; Morgan, H. Thin Solid Films 1992, 210, 76. (21) Pen˜acorada, F.; Reiche, J.; Katholy, S.; Brehmer, L.; RodriguezMe´ndez, M. L. Langmuir 1995, 11, 4025. (22) Dhanabalan, A.; Riul, A., Jr.; Oliveira, O. N., Jr. Supramolec. Sci., submitted.
Langmuir, Vol. 13, No. 18, 1997 4885
Figure 5. UV-vis absorption spectrum of a 13-layer composite LB film containing cadmium stearate (50%) and 16-mer polyaniline (50%). (a) As-deposited and (b) after treatment with HCl vapor.
Figure 6. Plots of the number of strokes vs absorbance (at 590 nm) of composite LB films containing cadmium stearate and three weight percentages of 16-mer polyaniline: (0) 25%; (O) 50%; (4) 75%.
form. Similar spectra were seen for all LB films, regardless of their polymer content. Interestingly, the spreading solution containing 16-mer was green due to doping by CSA, and therefore, the UV-vis results indicate that dedoping had occurred during the LB processing. It is possible that CSA molecules might have got dissolved into the subphase (pH ) 6.0); such dedoping has also been observed during the LB processing of functionalized aciddoped polyaniline.23 Nevertheless, these composite LB films could again be doped upon treatment with HCl vapor. The spectral characteristics of the doped films (curve b) are similar to those of doped polyaniline LB films. Uniform transfer of 16-mer in the composite LB films was confirmed by measuring the UV-vis absorbance of films with distinct numbers of up- and downstrokes and also with different amounts of polymer. As shown in Figure 6, for a given composite, the absorbance at 590 nm is found to increase almost linearly with increasing number of strokes. Note that the absorbance for a given number of layers is less (23) Punkka, E.; Laakso, K.; Stubb, H.; Levon, K.; Zheng, W.-Y. Thin Solid Films 1994, 243, 515.
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Figure 7. Plot of absorbance per layer of composite LB films for various percentages of 16-mer polyaniline vs weight percentage of 16-mer polyaniline.
Figure 8. FTIR transmission spectrum of a 21-layer composite LB film containing cadmium stearate (40%) and 16-mer polyaniline (60%).
for the 75% than for the 50% composite LB film. This could well be due to the poor transfer in the LB deposition of the 75% composite monolayer. Indeed, as shown in Figure 7, the average absorption per layer is found to increase linearly up to 50-60 wt % oligomer present in the composite, but a decrease is noted for the 75% 16-mer. The FTIR transmission spectrum of an as-deposited 60% composite LB film (21 layers) deposited on a calcium fluoride substrate is shown in Figure 8. The presence of an absorption peak at 1548 cm-1 corresponding to the carboxylate group and the absence of a peak at 1700 cm-1
Dhanabalan et al.
indicate that the film contained mainly stearate salt. Strong absorption peaks at 2917 and 2849 cm-1 corresponding to antisymmetric and symmetric vibrations of the CH2 group were also seen. The strong peaks at 1587, 1503, 1320, and 1159 cm-1 clearly indicate the transfer of 16-mer along with cadmium stearate. The absence of strong absorption in the 4000-2000-cm-1 region reveals that 16-mers are in the undoped state, supporting the UV-vis spectral results. The film uniformity was inferred through optical microscopy analysis. No distinct structure could be identified, which indicates excellent uniformity of these composite films to the resolution of an optical microscope. The dc electrical conductivity measured for the asdeposited 50% composite LB film (21 layers) is about 10-5 S cm-1 which is close to that observed with a composite LB film containing high molecular weight polyaniline (processed with CSA) and cadmium stearate.22 This conductivity value appears not to be related to the CSA molecules remaining in the film, as no trace of CSA has been observed in the FTIR spectra of the deposited films. Furthermore, similar values have also been reported for undoped polyaniline LB films prepared with NMP as the processing and spreading solvent.12 When the film was submitted to HCl vapor treatment, the conductivity increased to 10-4 S cm-1. The small increase in conductivity is comparable to the 2-3 order magnitude increase observed with composite LB films of high molecular weight polyaniline (processed with CSA) and cadmium stearate. The reason for such small increases in conductivity seems to lie in the use of functionalized acid like CSA in the LB processing of polyaniline. A detailed structural and electrical characterization of the composite LB films is under progress which may confirm this hypothesis. Conclusions In the present study, it has been demonstrated that a stable and easily transferable composite monolayer of cadmium stearate and oligomeric polyaniline (16-mer) could be obtained. Monolayer stability is found to depend on the oligomer content of the composite. Both stability and transferability are found to decrease above 60% oligomer content in the film. UV-vis as well as FTIR results for deposited LB films clearly indicate the transfer of undoped 16-mer along with cadmium stearate. Uniform transfer of oligomers is demonstrated by the linear increase of absorbance with increasing film thickness. In general, the monolayer behavior and transfer characteristics of the oligomeric composite are similar to those observed with composite monolayers containing high molecular weight polyaniline. Acknowledgment. The financial support for this work is provided by FAPESP and CNPq. We are grateful to Miss. S. V. Mello for helpful discussions. Also, the supply of 16-mer by Dr. A. G. MacDiarmid and J. Feng is gratefully acknowledged. LA9703306
