Langmuir 1999, 15, 3273-3278
3273
Thin Films of a New Polar Substituted Polypyrrole Wolfgang M. Sigmund,† Werner A. Goedel,‡ Rosa Souto-Maior,§ Alexandro C. Teno´rio,| and Celso P. de Melo*,| Max-Planck-Institut fu¨ r Metallforschung, Pulvermetallurgisches Laboratorium, 70569 Stuttgart, Germany, Max-Planck-Institut fu¨ r Polymerforschung, 55128 Mainz, Germany, Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, 50732-901 Recife PE, Brazil, and Departamento de Fı´sica, Universidade Federal de Pernambuco, 50670-901 Recife PE, Brazil Received April 23, 1998. In Final Form: January 7, 1999 We have investigated the properties of thin films of a substituted polypyrrole synthesized via the chemical oxidation of a chloroform solution of 5-acetamido-4,5,6,7-tetrahydro-2H-benzo[c]pyrrole. Repeated compressions of Langmuir films prepared by spreading a dilute chloroform/dimethyl sulfoxide solution of the polymer at an air-water interface have established their long-term stability as floating monolayers. Hysteresis experiments and surface potential measurements have indicated that while the stiff condensed layer changes its structure upon the first compression, the size and distribution of floating domains remain invariant under subsequent cycles of compression and decompression. An infrared spectroscopic investigation was carried out for KBr mixed pellets of the material and for Langmuir-Blodgett (LB) films obtained by depositing six monolayers of the polymer on ZnSe crystals. The presence of a broad bipolaron band both in the transmission and in the attenuated total reflection (ATR) spectra indicates that in its pristine form the polymer was already slightly doped, a hypothesis confirmed by preliminary two-probe measurements. To better understand the nature of the organization of the deposited films we performed a linear dichroism analysis of the polarized ATR spectra and found evidence that the polymeric chains are transferred in a helical manner along the surface of the substrate.
1. Introduction Conducting polymers have been subject of great interest in the recent literature both from a fundamental point of view,1 in the continuous investigation of the basic processes of charge transfer and polarization response in these materials, and from an applied science perspective.2 Special attention in the latter case has been given to the possible development of new sensors and optoelectronic devices that exploit the unusual properties of these materials.3 Among the several types of conducting polymers under present investigation, polyaniline4 and polypyrrole5 derivatives stand out for being able to combine a large environmental stability to high electrical conductivity. Polymers of these families of compounds are commonly synthesized via the oxidative coupling of the respective monomers. The polymerization of an unsubstituted pyrrole usually leads to insoluble cross-linked polymeric salts in which the positive charge of the polymer chain is neutralized by incorporated negative counterions. These salts can be reduced to the neutral form, and the neutral polymer can be soluble if it has been synthesized from monomers * To whom correspondence should be addressed. Phone: 1-5581-2718450. Fax: 1-55-81-2710359. E-mail:
[email protected]. † Max-Planck-Institut fu ¨ r Metallforschung. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. § Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco. | Departamento de Fı´sica, Universidade Federal de Pernambuco. (1) Skotheim, T. A.; Elsenbalmer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel-Dekker: New York, 1997. (2) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. (3) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley-Interscience: New York, 1991. (4) McDiarmid, A. G.; Yang, L. S.; Huang, W. S.; Humphrey, D. B. Synth. Met. 1987, 18, 393. (5) Sakkopoulos, S.; Vitoratos, E.; Dalas, E. Synth. Met. 1998, 92, 63.
appropriately substituted at the β-position of the pyrrole ring. Different n-alkyl substituents have been utilized for that purpose, although usually the resulting poly(nalkylpyrroles) remain soluble only in the neutral state.6 Although thin films of pyrrole derivatives in this oxidized neutral form can be shown to be electrically conducting, it is well-known that in conjugated systems cross-linking should lead to the reduction of the average π-electron delocalization length and therefore to a less efficient charge transport and a diminished nonlinear optical response.7 Some research groups have successfully spread polypyrroles at an air/water interface and built up multiple layers by LB methods.8,9 Highly oriented thin films of these functionalized polymers exhibit interesting anisotropic optical and electrical properties and, indeed, due to their more organized nature such structures are expected to be useful systems in the study of charge transport mechanisms in low-dimensional conductors.10 However, branching points caused by side reactions during the polymerization process will eventually result in films with a less well-defined two-dimensional nature. Since any distortion of the regular conducting structure associated to the conjugated backbone contributes to a lower conductivity of the material, it is desirable to reduce the possibility of branching of the polymeric chain. Recently, a new substituted pyrrole monomer, 5-acetamido-4,5,6,7-tetrahydro-2H-benzo[c]pyrrole (from now on referred to as AcBPY; see Figure 1), has been (6) Ru¨he, J.; Ezquerra, T. A.; Wegner, G. Makromol. Chem. Rapid Commun. 1989, 5, 1. (7) Anderson, T.; Roth, S. Braz. J. Phys. 1994, 24, 746. (8) Rahman, A. K. M.; Samuelson, L.; Minehan, D.; Clough, S.; Tripathy, S.; Inagaki, T.; Yang, X. Q.; Skotheim, T. A.; Okamoto, Y. Synth. Met. 1989, 28, C237. (9) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Mater. 1990, 2, 82. (10) Sarkar, D.; Paul, A.; Misra, T. N. Thin Solid Films 1993, 227, 105.
10.1021/la980470v CCC: $18.00 © 1999 American Chemical Society Published on Web 04/03/1999
3274 Langmuir, Vol. 15, No. 9, 1999
Figure 1. 5-Acetamido-4,5,6,7-tetrahydro-2H-benzo[c]pyrrole (AcBPY).
synthesized.11 This monomer has two promising characteristics: (i) Since both β positions are substituted, one should expect a lower degree of cross-linking upon polymerization. (ii) The presence of the polar acetamide side group should enhance its solubility in polar solvents. In fact, the polar substituted polypyrrole (from now on referred to as pAcBPY) obtained from this monomer is soluble even while in the oxidized state. Therefore, this polymer offers a good possibility of being processed into the electrically conducting state and the corresponding samples can be expected to exhibit a higher conductivity than those of unsubstituted polypyrroles. In this article we report an investigation of the characteristics of Langmuir films of these polymers formed on an air-water interface and present the results of a preliminary linear dichroism infrared analysis of the molecular orientation of the polymeric chains deposited via a Langmuir-Blodgett transfer procedure on ZnSe crystals. This can be considered as a first study of the structure and properties of LB films of these β-substituted polypyrroles. We expect that further work on these kinds of compounds could in fact corroborate their potential as anisotropic thin films for use in optoelectronic devices. 2. Materials and Methods
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Figure 2. Isotherm corresponding to the first compression of a floating film of pAcBPY (on a water subphase at a temperature of 23 °C) spread from a diluted solution. to measure the surface pressure. Surface potential measurements were done using the vibrating-plate method. Typical values of the barrier speeds used to compress the monolayer were of the order of 30 mm/min. Isotherms of surface pressure versus mean molecular area/molecule were measured at 23 °C. For spreading a (gastight) Hamilton syringe was used. The amount of solvent spread was 1000 µL with a concentration of 66 µg/mL. A molecular weight per repeat unit of 146.2 g/mol was adopted for all calculations. 2.4. Langmuir-Blodgett Films. For the Langmuir-Blodgett transfer of the floating thin films, (25 × 20 × 2) mm3 ZnSe crystals with 45° faces (Harrick Scientific, Ossining, NY) were used as substrate on vertical depositions. Those crystals have a high refractive index (2.42) and an appropriate transmission window for ATR studies, and before use they were submitted to a preliminary thorough cleansing with DMSO and a subsequent hydrophilic treatment12 using deionized water, toluene, and isopropylic acid. In each case six monolayers were transferred to the substrate at a working pressure of 20 mN/m (which corresponds to the inflection point of the isotherm right before collapse) and at a dipping speed of 2.0 mm/min. All depositions were done on the upward stroke of the substrate, leading to a Z-type structure. The forward (backward) speed of the barrier was set to 1.1 (1.0) mm/min. All transfer ratios (defined as the relation between the area swept by the barrier and the emerged area of the substrate) were of the order of 0.90 ( 0.05, a very good value if one considers the trapezoidal shape of the crystal (resulting from the 45° orientation of its superior and inferior faces). 2.5. Spectroscopic Investigation. For the initial spectroscopic characterization, pAcBPY:KBr pellets with thickness equal to 1.5 mm were prepared in the concentration 1:1000 in mass. For the Fourier transform infrared (FTIR) transmission spectra of these pellets 50 scans were collected on a BOMEM MB-100 (Quebec, Canada) spectrophotometer using a deuterated triglycine sulfate (DTGS) detector with cosine apodization and a resolution of 4 cm-1. Using a minimum of 2048 scans, attenuated total reflection (ATR) spectra were obtained for the material deposited on the ZnSe crystal, which was placed in a homemade accessory. For the linear dichroism studies, a rotating polarizer composed of wide aluminum strips on a germanium substrate (Graseby Specac Ltd., Kent, U.K.) was appropriately positioned before the sample.
2.1. Chemicals. The monomer, 5-acetamido-4,5,6,7-tetrahydro-2H-benzo[c]pyrrole (AcBPY) was synthesized using a procedure described elsewhere.11 All chemicals used were pro analysis (pa) grade: dimethyl sulfoxide (DMSO), from Merck (Rio de Janeiro, Brazil), chloroform (stabilized with ethanol), from Quı´mica Moderna (Rio de Janeiro, Brazil), and anhydrous FeCl3, from BDH (Poole, England). Ultrapure water with resistivity higher than 16 MΩ/cm, obtained after passage through a NanoPure500 Barnstead system (Boston, MA), was used as a subphase. All glassware was soaked with chromo-sulfuric acid and rinsed thoroughly with purified water before use. The spreading solution was a mixture of chloroform and DMSO (70/ 30 v/v). Although chloroform was the main agent for the spreading on water, we have found that complete dissolution of the polymer was only attained if DMSO was added prior to chloroform. 2.2. Polymerization of the Compound. A CHCl3 solution of AcBPY (5.32 g, 0.031 mol) was added to a CHCl3 solution of anhydrous FeCl3 (4.866 mg, 0.03 mol). The yellowish solution quickly turned into a black colored suspension, which was then stirred for 2 h. The polymer, a black solid with a dark blue hue, was collected by filtration of the reaction mixture and washed thoroughly, first with water and then methanol. The polymer was dried under vacuum and stored in a desiccator under nitrogen. 2.3. Monolayer Characterization. The monolayers were studied using a LB-5000 Langmuir-Blodgett system (KSV Instruments, Helsinki, Finland) consisting of two PTFE Langmuir troughs, each one equipped with a surface balance and a moving nylon barrier. The Wilhelmy plate method was applied
3.1. Spreading and Isotherm. pAcBPY was dispersed into the air/water interface by the careful spread of the chloroform/DMSO solution. The corresponding compression isotherm is shown in Figure 2. The floating monolayer exhibits a surface pressure onset point at a surface area per repeat unit of 13.0 ( 0.2 Å2. The monolayer collapse region was at an area of 7.0 ( 0.2 Å2 per repeat unit with an applied surface pressure of 25 mN/m. In general terms the isotherm resembles those observed for other substi-
(11) Goedel, A. W.; Ho¨ltz, G.; Wegner, G.; Rosemund, J.; Zotti, G. Polymer 1993, 34, 4341.
1.
3. Results and Discussion
(12) Bardwell, J. A.; Dignan, M. J. J. Colloid Interface Sci. 1987, 6,
Thin Films of a Polar Substituted Polypyrrole
Figure 3. Hysteresis and corresponding surface potential changes for a pAcBPY film on a water surface at 23 °C. 13
tuted pyrroles. However, this polymeric thin layer is very stiff and when compressed to a small surface area it causes the swiveling of the Wilhelmy plate at small angles to the side: hence, for more accurate measurements in the collapse region a trough with two opposite moving barriers might be necessary.14 The cross-sectional area of the repeat unit extrapolated to zero lateral pressure is somewhat smaller than the typical value (of the order of 25 Å2) expected for pyrroles.9,15 In principle, we can think of three possible explanations for this result. First, one could have incomplete spreading; i.e., due to formation of bundles of the stiff molecules a thin film composed of different coalesced domains (but not a pure monolayer) could result. In our case, this would happen if upon spreading the DMSO dissolves into the subphase faster than the chloroform could disperse the pAcBPY chains. Second, the chains could be attached to the surface not as a flat layer but, for instance, in a loopike structure. Structures of this kind have been associated by some authors16 to polymer monolayers whose crosssectional areas are smaller than 10 Å2. Finally, a third possible explanation is that the molecular weight per repeat unit used in the calculation might be too low, due to the neglecting of counterions in the oxidized form of the polymer. However, as we will discuss laterssee section 4sthe existence of a marked linear-infrared dichroism in the LB films of the polymer can be taken as an evidence of an organized nature for the Langmuir films. This fact is therefore a strong argument against the presence of disorganized structures in the air-water floating monolayer. A slight concentration effect on the spreading solution was observed; i.e., the higher the concentration of the solution used, the smaller would be the measured value of the mean molecular area per repeat unit. (The experiments reported here correspond to the polymer concentration (66 µg/mL) which gave the largest mean molecular area per repeat unit.) This observation is consistent with the hypothesis that a more complex thin structure is formed on the floating monolayer. 3.2. Hysteresis and Surface Potential. Further characterization of the thin layer has been done by applying surface potential measurements and performing hysteresis experiments. The corresponding results are shown in Figure 3, for an experiment done after completion of the first compression-decompression cycle. Note that (13) Sigmund, W. M.; Bailey, T. S.; Hara, M.; Sasabe, H.; Knoll, W.; Duran, R. S. Langmuir 1995, 11, 3153. (14) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (15) Iyoda, T.; Ando, M.; Kaneko, T.; Ohtani, A.; Shimidzu, T.; Honda, K. Tetrahedron Lett. 1986, 27, 5633. (16) Gaines, G. L., Jr., Langmuir 1991, 7, 834.
Langmuir, Vol. 15, No. 9, 1999 3275
Figure 4. Isobaric creep experiment for a thin layer of pAcBPY on a water surface at 23 °C. The applied surface pressure was 20 mN/m.
the onset of the compression isotherm is shifted to 12.0 Å2. It was also observed that, after a delay time of 6 min at the fixed pressure of 25mN/m, a relaxation of the thin layer seems to occur: the decompression isotherm follows a different route, and for each value of the applied pressure, a smaller mean molecular area is found. However, if a waiting time of at least 10 min with no lateral pressure applied is observed, we find that the isotherm of the first compression is reproduced. As seen in Figure 3, during the compression step the surface potential changes are of the order of 400 mV, a value comparable to that observed for several polar compounds.17 The fact that the onset of the surface potential variation occurs earlier than that of the surface pressure indicates that the orientation of the dipoles begins before a condensed layer stage is reached. 3.3. Stability of the Thin Layer. The stability of the monolayer on the trough can be investigated by compression of the thin layer to a certain surface pressure (typically about 20 mN/m) and then allowing the barrier to move freely to keep the surface pressure constant over a long period of time. In these isobaric creep experiments, changes in the surface area associated with the eventual relaxation of the floating monolayer can be directly observed. Although no variation of surface area should occur for a completely stable thin layer, some decline is observed for most substances. On the other hand, strong deviations from the original value of the surface area can be associated to either the collapse of the thin layer or to its dissolution into the subphase (although a third possibilitysthe creeping of the monolayer under the barriersalso exists). The results obtained for the pAcBPY thin film are depicted in Figure 4 and show that a very stable monolayer was formed with only a minor decrease in the surface area per repeat unit. It should be pointed out that the insolubility of pAcBPY in water had been proven before, when the polymer was thoroughly washed with water. 3.4. Transmission Spectrum. As it could be expected, the transmission spectrum (obtained in a normal incidence configuration) of the pAcBPY:KBr pellets (Figure 5) has several features in common with that of polypyrrole,18 e.g., (i) a broad band in the region above 1800 cm-1, characteristic of the presence of bipolaron conformational defects,9,19 (ii) a peak at 1430 cm-1, assigned to C-N stretching,18 and (iii) peaks in the region of 650-760 cm-1, attributed to out-of-plane pyrrole ring deformation.20 On the other hand, in the fingerprint region below 2000 cm-1 one can also identify exclusive pAcBPY bands such as (i) (17) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (18) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892. (19) Hong, K.; Rubner, M. Thin Solid Films 1989, 179, 215. (20) Lopez Navarrete, J. T.; Tian, B.; Zerbi, G. Synth. Met. 1990, 38, 299.
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Figure 5. FTIR absorbance spectrum of a pAcBPY:KBr pellet (concentration 1:1000 in mass). See text for the discussion of the characteristic peaks.
Sigmund et al.
the material. These alkyl groups were probably the result of a partial breakdown of the cyclohexane ring (one should note that the vibration at 1020 cm-1 associated to the out-of-plane deformation of the cyclohexane ring remains present). The above assignment is corroborated by the fact that when a heatgun was used to expedite the evaporation of DMSO from drops of the solution deposited on the ZnSe crystal, the intensities of the 2848 and 2919 cm-1 bands were substantially increased in the corresponding ATR spectrum at the expenses of that of the 1020 cm-1 peak. 3.6. Linear Dichroism Investigation. Using the rotating polarizer, the transmission spectra of the LB films could be obtained for radiation polarized along different directions.25 In particular, for the 1040 cm-1 band (which corresponds to the out-of-plane vibration of the cyclohexane ring), the transmission dichroic ratio DT ) A|/A⊥ relating the absorbances along two arbitrary orthogonal directions26 was found to have a value of 1.0. Therefore the projections of this vibration mode can be considered as isotropically distributed along the plane of the substrate, in an indication that on the average the cyclohexane ring has no preferential spatial direction. In ATR configuration the dichroic ratio
DATR )
Ey2 Ex2 + Ez2
(1)
for an isotropic film (where x is the dipping direction and z is the normal to the substrate) gives a direct relationship between the intensities of the components of the electric field.27 As a consequence of the Fresnel equations,28 which express the boundary conditions satisfied by the electric field at the interface between two media, one can write27 Figure 6. ATR spectrum of six monolayers of pAcBPY deposited on a ZnSe crystal. Please see text for the discussion of the characteristic peaks.
Ey )
2 cos Φ
x1 - n312
(2a)
the NCO stretching of the acetamide group21 at 1640 cm-1 and (ii) the out-of-plane vibration of the cyclohexane ring22 at 1020 cm-1. One should note that in this spectrum the typical pyrrole peak at 3400 cm-1 (associated with the NH stretching23) cannot be identified, probably because it is immersed in the broad bipolaron band. 3.5. ATR Spectrum. Despite the high viscosity of the floating pAcBPY Langmuir film, we have been able to obtain successful transfers to the ZnSe crystal using the Langmuir-Blodgett vertical method. In Figure 6 we present the ATR spectrum obtained of the corresponding LB films. All characteristic features observed in the transmission experiment can once again be observed. One should note, however, that the broad bipolaron band (which extends to the visible region) is less intense here than before. Besides the difference in the organizational structure of the material when in the form of compressed pellets or when disposed in two-dimensional LB films, the high pressure used in the preparation of the pellets should reduce the distances among individual fibers and favor the formation of defects. A prominent feature of this spectrum is the presence of symmetric and asymmetric CH2 stretching vibrations24 (at 2848 and 2919 cm-1, respectively), revealing the presence of alkyl groups in
where Φ is the incidence angle of the radiation, n31 ) n3/n1 is the relative refractive index between the atmosphere and the ATR crystal, and in a similar manner n32 ) n3/n2 is the relative refractive index between the atmosphere and the sample. Therefore, since the refractive index of ZnSe is known, the determination of the dichroic ratio providessthrough use of eqs 1 and 2sa direct estimate of the refractive index of the LB sample. An isotropic film of pAcBPY was then prepared by the transfer of six monolayers at a lateral pressure above the collapse value, and the dichroic ratio of the cyclohexane deformation vibration was used (accordingly to eq 1) to determine the refractive index of the sample. The resulting value (n2 ) 1.7) is consistent with the fact that polymeric
(21) Fraser, R. D. B. J. Chem. Phys. 1953, 25, 1511. (22) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1975. (23) Jego, C.; Agricole, B.; Vicentini, F.; Barrouillet, J.; Mauzuc, M. M.; Mingotaud, C. J. Phys. Chem. 1994, 98, 13408. (24) Bubeck, C.; Holtkamp, D. Adv. Mater. 1991, 3, 32.
(25) Ahn, D. J.; Franses, E. I. Thin Solid Films 1994, 244, 971. (26) Takenaka, T.; Nogami, K.; Gotoh, H.; Gotoh, R. J. Colloid Interface Sci. 1971, 35, 395. (27) Haller, G. L.; Rice, R. W. J. Phys. Chem. 1970, 74, 4368. (28) Jackson, J. D. Classical Electrodynamics, 2nd ed.; John Wiley & Sons: New York, 1975.
Ex )
Ez )
2 cos Φxsin2Φ - n312
x1 - n312x(1 + n312) sin2 Φ - n312 2 cos Φ(sin Φ)n322
x1 - n312x(1 + n312) sin2 Φ - n312
(2b)
(2c)
Thin Films of a Polar Substituted Polypyrrole
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thin films (estimated thickness ∼500 Å) was determined to be of the order of 10-5 S/cm, a value larger than those of LB films of pure undoped alkylpyrroles.9,18 4. Conclusions
Figure 7. Comparison of the S- and P-polarized ATR spectra for six monolayers of pAcBPY deposited on a ZnSe crystal. Note that the dichroic ratio varies with frequency. Table 1. Estimated Average Orientation Angles for the Most Important Vibrations Identified on the Polarized ATR Spectra for Six Monolayers of PAcBPY Deposited on a ZnSe Crystal band (cm-1)
assgt
dichroic ratio
orientation angle (deg)
1640 1540 1035
CdO CdCstr (C6H12)torsion
0.78 0.80 1.16
35.6 42.7 61.6
films usually form more dense optical media than fatty acids and oligomers.29 On the other hand, a strong anisotropy can be identified in the polarized ATR spectra (Figure 7) of organized LB films obtained when six monolayers were deposited on the ZnSe crystal at a lateral pressure below that corresponding to the collapse of the Langmuir film. If a simple uniaxial model of orientation of the transition moments of interest is assumed,30 the ATR dichroic ratio can be written as31
DATR )
Ey2 sin2 〈γ0〉 Ex2 sin2〈γ0〉 + 2Ez2 cos2 〈γ0〉
(3)
where 〈γ0〉 is the average angle of the transition moment considered relative to the normal to the substrate surface. As can be seen in Figure 7, the dichroic ratio varies with the frequency and therefore the estimated average orientation angle32 is different for each of the relevant vibrations, as shown in Table 1. Since the orientation angle for the torsion vibration of the cyclohexane is relatively close to the “magic value”31 of 54.7°, one can note that even in the better organized samples this saturated ring is randomly oriented in space. (The interested reader will find a detailed discussion of the use of FTIR methods in orientational studies in refs 25, 29, and 30.) 3.7. Conductivity Measurement. A preliminary investigation of the room-temperature dc conductivity of the deposited films was performed using the two-probe technique. For this, using conducting epoxy (Circuit Works-Chemtronics, Kennesaw, GA) two contacts 8.0 mm apart were fixed on top of a 20 monolayers LB film. When a current of 14 nA was injected, the conductivity of these (29) Umemura, J.; Kamata, T.; Kuwai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (30) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific: New York, 1987. (31) Ahn, D. J.; Franses, E. I. J. Phys. Chem. 1992, 96, 9952. (32) Watanabe, I.; Cheung, J. H.; Rubner, M. F. J. Phys. Chem. 1990, 94, 8715.
A new electrically conducting polypyrrole, pAcBPY, was prepared and characterized by infrared transmission spectroscopy. Since both β positions in the monomer were substituted, one could expect a lower degree of crosslinking and therefore a better organization of the polymeric chains. Upon being spread at an air-water interface, the corresponding thin film formed has shown a high stability. As usual for polymers spread on surfaces, we have found a small cross-sectional area per repeat unit at collapse. This small value can be understood as resulting from the formation of a more complex thin film structure at the interface (rather than a simple floating monolayer). Although comparatively stiff, the film shows a slow relaxation of domains, as evidenced both by the reproducible character of the hysteresis curves and by surface potential measurements. An initial analysis of the FTIR spectrum of pAcBPY: KBr pellets has allowed us to establish the similarities and differences between this material and regular pyrrole derivatives. An important feature is the presence of a broad absorption band above 1800 cm-1, characteristic of bipolarons, which has led us to conclude that the pristine polymer is already in a (at least partially) doped form, probably as a consequence of the usage of FeCl3 in the polymerization process. After using the LangmuirBlodgett deposition technique to transfer six Langmuir layers of these polymeric chains to ZnSe crystals, we have performed an attenuated total reflectance infrared study of the solid films. The fact that the bipolaron band is relatively less intense in the FTIR spectra of the LB film than in that of the pellet can be attributed to the pressure used in the preparation of the latter, which should reduce the average interchain distance and favor the formation of defects. The room-temperature dc conductivity of these LB films was estimated to be in the range of 10-5 S/cm, a value which compares favorably to the maximum of 4 × 10-3 S/cm determined for the half-oxidized free-standing electrochemically prepared films of a much thicker (50 µm) nature.8 A linear dichroism analysis of the ATR spectra of these films obtained using polarized radiation has permitted us to conclude that the average orientation angles are distinct for different groups present in the unit cell of the polymer. The observation of a definite linear dichroism in the transferred films then suggests that no disorganized structures are present in the floating monolayer. Rather, we advance the hypothesis that the hydrophilic interactions of both the NH and NCO groups lead to a deformation of the unit cell33 (which therefore should be under mechanical stress). This type of anchorage of the unit cell precludes the assignment of the so-called horizontal and vertical locking models34 for the orientation of the deposited polymeric chains. As a matter of fact, these results are consistent with the helical model,25 in which the polymeric chains are on the average disposed in a parallel manner to a preferential direction9 (for instance, the longitudinal axis of the substrate used in the transfer process). Additional theoretical studies (such as a molecular mechanics investigation of the possible degree of deformation of the unit cell) and experimental work (e.g., (33) Teno´rio, A. C.; de Melo, C. P. Work in progress. (34) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 2183.
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on the dichroic ratio of films deposited along different directions on the substrate surface) would be required before a good estimate of this preferential angle could be advanced. Acknowledgment. The authors greatly acknowledge financial support of the Office of Naval Research and the Brazilian Agencies FINEP, CNPq-RHAE, and FACEPE.
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We thank Mr. C. G. dos Santos for technical assistance in different aspects of this work and Mr. F. L. dos Santos for the conductivity measurements. We thank especially Prof. R. Duran (University of Florida, Gainesville, FL) for the incentive for this work and the kind permission for using the surface potential probe. LA980470V
