LangmuirBlodgett Films of Novel Polyion Complexes of Conducting

Chapter 7

Langmuir-Blodgett Films of Novel Polyion Complexes of Conducting Polymers

Downloaded by STANFORD UNIV GREEN LIBR on October 13, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch007

A. T. Royappa and M . F. Rubner Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

A novel method of incorporating polyion complexes of conducting polymers into Langmuir-Blodgett films has been studied. The method involves forming polyion complexes of preformed conducting polymers with stearylamine. The polyion complexes form stable, easily transferable, expanded monolayers on the water surface. The resulting Langmuir-Blodgett films are highly ordered and Y-type in structure, with interdigitated stearylamine alkyl tails. The polymer chains are sandwiched in planes between stearylamine layers. The conductivity exhibited by these films is on the order of 10 S/cm. -2

Since the discovery of electrically conducting polymers more than a decade ago (1), a significant portion of the research on conducting polymers has been focused on those that are readily processable and environmentally stable, eg, the polythiophenes, polypyrroles and polyanilines. This paper deals with some novel methods of fabricating conducting Langmuir-Blodgett (LB) films of polythiophenes and polyanilines functionalized with ionizable side groups. The L B technique is aptly suited for the study and manipulation of conducting polymers because it allows for fine control of the molecular architecture of very thin films, yields a very high degree of anisotropic ordering, and holds out numerous possibilities for the construction of electronic devices and sensors (2). The most serious hurdle to processing any material by the L B method is a lack of surface activity on the part of the material. In this work, we present a novel approach to surmounting this hurdle for non-surface active conducting polymers, namely by forming surface active polyion complexes between the polymers and stearylamine, a commonly available surfactant. Although other workers have synthesized L B films containing polyion complexes of conducting polymer precursors (3), this research marks the first successful fabrication of L B films of a polyion complex of a preformed conjugated polymer. It should be noted at this point that this method is quite general, in that it is not restricted to conjugated polymers (3). The success of this method with the two disparate polymers mentioned above would seem to imply that any non-surface active polymer that can be functionalized with ionic groups can be reacted with a suitable surface-active agent to form surface active polyion complexes.

0097-6156/92/0493-0076$06.00/0 © 1992 American Chemical Society

In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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EXPERIMENTAL Poly(thiophene-3-acetic acid) (PTAA) was prepared as follows: commercially available ethyl thiophene-3-acetate (Lancaster Synthesis) was polymerized by the FeCl3 suspension method (4), and the resulting polymer was converted to P T A A by acid hydrolysis using aqueous HC1. The weight average molecular weight of the resulting polymer as determined by GPC (polystyrene equivalent calibration) was » 1700 g/mol, which corresponds to a degree of polymerization of ~ 12. This material should therefore be viewed more as an oligomer than a true high polymer. The polyion complex of P T A A and stearylamine was synthesized as follows: P T A A was dissolved in dimethylacetamide (DMAc), to which a solution of stearylamine in benzene was added dropwise, with stirring, to form the amine salt of P T A A . The structure of the complex is shown below: CH COCT H N - C H +

2

3

1 8

3 7

Sulfonated poly aniline (PAn) of number average molecular weight approximately 50,000 g/mol was obtained from Prof. A.J. Epstein of Ohio State University. The synthesis and properties of sulfonated polyaniline are detailed elsewhere (5). The polymer (PAn) provided to us was in the emeraldine salt form. The polyion complex of PAn was synthesized in the exact manner described above, resulting in the formation of the following amine salt of PAn: ^ N-C 3

H

1 8

H

"*H N-C H

3 7

3

18

37

H

The sulfonated polyaniline produces a dark green solution in D M A c , which turns to indigo-violet as the reaction with stearylamine takes place. This behavior is exactly identical to the reaction of the sulfonated polyaniline with NaOH in water. A l l L B results were obtained using a modified Lauda film balance. To study the behavior of these PTAA-stearylamine (PTAA-StNH2) salts on the L B trough, a set of solutions were made up, containing various mole ratios (1:1, 2:1 and 5:1) of thiophene structural units to stearylamine molecules. The only PAn-stearylamine (PAn-StNH2) complex examined had one stearylamine molecule per sulfonate group, ie, one stearylamine molecule for every other aniline subunit. A l l solutions contained 1.0 mg/ml of stearylamine, and the solvent was a 3:1 (v/v) mixture of benzene and D M A c , respectively. This mixture has been found to be a good solvent for a wide range of materials and a good spreading agent for monolayer formation (6). In order to make L B films these solutions were spread onto an ultra-high purity MiUi-Q water (>18 Mohmcm) subphase and compressed to form condensed monolayers, which in turn were used to build the multilayer thin films. The vertical dipping method was used (dipping speed: 10 mm/min.), and the films were held at a constant surface pressure of about 20 mN/m and a temperature of 20.0°C.


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MACROMOLECULAR ASSEMBLIES IN POLYMERIC SYSTEMS


For X-ray diffraction studies, UV-visible spectra, profilometry and conductivity measurements, the film substrates used were transparent hydrophobicized glass slides. X-ray patterns were measured on a Rigaku Rotaflex R U 300 automated X-ray diffractometer. Four-point van der Pauw conductivity measurements were made in a sealed glass flask containing the sample in dopant vapor, using a Keithley DC voltmeter and programmable current source. UV-visible spectra of film samples were recorded with an Oriel 250 mm multichannel spectrometer. A Sloan Dektak Π was used for profilometry. For infrared spectra of the L B films, IR-transparent zinc selenide plates were used as substrates in die 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°. A l l reflectance spectra were measured at an incident angle of 8°. FT-IR spectra were recorded using a Nicolet 51 OP spectrometer with a liq. N2 cooled M C T detector. Platinum coated glass slides were also used for ellipsometry, done on a Gaertner model L I 17. RESULTS Pressure-area isotherms on pure water at 20.0°C were recorded for the various polythiophene and polyaniline salts mentioned above, as well as for stearylamine. In every case, the limiting area per stearylamine molecule in the salt form was greater than that of pure stearylamine, as expected (see Table I, below). The abcissa was calibrated to the concentration of stearylamine in the spreading solution. A l l the complexes are best described as expanded monolayers on the water surface. It is of some interest to note that the limiting area per molecule is the same for the 2:1 polythiophene complex and the polyaniline complex. This is consistent with the fact that half of all the aniline rings have a stearylamine molecule attached at a sulfonic acid site, analogous to the 2:1 polythiophene complex, in which half the thiophene rings have a stearylamine molecule attached at the carboxylic acid side group. A l l the polyion complexes were found to be exceptionally stable (up to 12 hrs. in most cases) to film collapse on the water surface when held at a surface pressure of about 20 mN/m. This is in contrast to pure stearylamine which is unstable, possibly due to its partial solubility in water. The 2:1 and 5:1 polythiophene salts actually expanded in area by up to 5% over time when held at constant pressure, which we surmise is due to the redistribution of the stearylamine molecules along the polymer backbone by ion hopping. A l l of these salts transferred easily onto the substrates listed above, the L B films formed being Y-type in structure (equal transfer on both up and down strokes). Table I. Limiting area per molecule for various surface active materials Surface Active Material

Limiting Area per Molecule ( A )

Stearylamine l:lPTAA-StNH 2:1 P T A A - S t N H 5:1 P T A A - S t N H

18 46 28 24 28

2

PAn-StNH

2

2

2

2


7. ROYAPPA & RUBNER

L—B Films ofNovd Polyion Complexes

79


The transfer ratios did not vary significantly from layer to layer, indicating even, reproducible transfer throughout the deposition process. Figure 1, below, shows the UV-visible absorbance of L B films of these complexes as a function of the number of layers transferred.

Ο Polyaniline complex

+ 1:1 Polythiophene complex

0

20

40

60

#

80

100

120

140

layers

Figure 1. Absorbance as a function of #layers in L B films of P A n S t N H (at 600 nm) and 1:1 P T A A - S t N H (at 425 nm). 2

2

The L B films used in these experiments have an equal number of layers on each side of the substrate. The high degree of linearity in the above plots further supports the evenness of deposition in these L B films, even up to 120 layers. Polythiophene Multilayers The L B films of the 1:1 PTAA-StNH2 complex were clear yellow and free of gross structural defects such as streaks and spots, as viewed by the naked eye. However, the films of the 2:1 and 5:1 PTAA-StNH2 complexes showed some streakiness and cloudiness, the 5:1 being more disordered than the 2:1. UV-visible spectroscopy of the PTAA-StNH2 films showed the canonical π-π* transition of conjugated systems at around 420 nm, which shows that the polythiophene molecules were incorporated into the L B film. Infrared spectra of L B films of the salts showed that the free acid C=0 peak (1700 c m ) and the free acid O H peak (3100 cm" ) diminished as the mole ratio decreased from 5:1 to 1:1, concomitant with an increase in the amine salt C=0 peak (1575 cm"*), as would be expected. The other significant feature of the infrared spectra was the presence of very strong C-H peaks (2850 and 2920 cm" ) corresponding to the long alkyl chains of the stearylamine. -1

1

1

A comparison of the FT-IR spectra of 1:1 P T A A - S t N H in reflection and transmission modes showed significant dichroism, indicating a high level of molecular orientation in the L B films. Measurements of the tilt angle (relative to the substrate normal) of the hydrocarbon tails of stearylamine in the L B films were accomplished by analyzing the differences between the transmission and reflectance FT-IR spectra, according to the methods described in (7) and (8). The tilt angle of these tails in the 1:1 P T A A - S t N H complex was found to be 10°±5°. 2

2


80



X-ray diffraction scans revealed signs of layer ordering in the L B films. As judged by the number of peaks appearing in the X-ray scans, the degree of order varied as follows: 1:1 > 2:1 > 5:1, confirming the visual observation mentioned above. For the 1:1 salt, the 2q peaks observed were at 2.6° and 5.2°. Assuming that these two peaks correspond to n=l and n=2 Bragg reflections, the rf-spacing for the 1:1 PTAA-StNH2 complex was found to be ca. 28À. This spacing corresponds to the bilayer repeat distance, and is consistent with measurements made on the same system by ellipsometry and profilometry. This means that each layer contributes approximately 14A to the film thickness. Preliminary doping studies of cast films of the PTAA-StNH2 salts showed that they can be successfully doped either in solution (using NOPF6 in acetonitrile or FeCl3 in nitromethane) or in vapor (using gaseous SbCl5). The films turned greenish upon doping, and the UV-visible spectra of the doped films showed the growth of the bipolaron band at around 750 nm accompanied by a marked decrease in the intensity of the π-π* peak. The maximum conductivity achieved for these L B films is about 10"^ S/cm for the 2:1 salt doped with SbCl5. The conductivity of these doped samples decayed rapidly when exposed to air, dropping to less than 10' S/cm in about 10 minutes. 5

Polyaniline Multilayers The L B films made from the PAn-StNH2 complex were clear violet and also of very high optical quality. The UV-visible spectra were identical to the spectrum of the sodium salt of sulfonated polyaniline, which confirms that we do indeed form a polyion complex, and that the polyaniline is transferred onto the L B substrate. The infrared spectra of L B films of this salt showed N - H peaks (3300 cm- ) and the strong stearylamine C-H peaks (2850 and 2920 cm' ) observed before. The tilt angle of the hydrocarbon tails in the PAn-StNH2 complex was found by the same method as above tobe4°±5°. 1

1

The X-ray diffraction patterns were remarkably similar to those of the PTAA-StNH2 salts; the 2q peak positions are 2.6° and 5.4°. Assuming that these peaks are the n=l and n=2 Bragg reflections as before, the ^-spacing for this system comes out to be about 32Â. Again, this value of the bilayer thickness was confirmed using ellipsometry and profilometry, which gave consistent results with X-ray diffraction. This similarity between the two polyion complexes leads us to believe that the structure in their respective L B films is dictated by the stearylamine alkyl chains attached to the polymer backbone. The L B films of PAn-StNH2 could be successfully doped in H Q solution. The violet films turned green upon immersion for 15 min., and the UV-visible spectrum of the doped film showed an increase in the polaron peak intensity at 800 nm and above, and a concomitant decrease in the H O M O - L U M O excitonic peak at 600 nm (5). The conductivity could then be measured after placing the samples in a stream of dry nitrogen and/or dynamic vacuum to remove the excess HC1 solution adhering to the samples. The conductivity of these L B films was found to be 5 x l 0 ' S/cm (measured in a moist environment), which is equal to the conductivity reported for the parent material (5). The conductivity is very sensitive to moisture, dropping to less than 10" S/cm when the sample is dried, and rising back to the above mentioned levels when it is remoistened, eg, by suspending the sample in a flask filled with water vapor. 2

5

Dichroic behavior observed while recording UV-visible spectra of L B films oriented at 45° (see Figure 2a below) to the polarized incident beam indicated that the polymer chains lie in planes parallel to the substrate. In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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L-JB Films of Novel Polyion Complexes

81

V


Detector

Sample

Polarizer

Source

Figure 2a. Experimental set-up for UV-visible dichroic studies, showing the sample oriented at 45° to the plane of the polarized incident beam. Figure 2b shows the UV-visible spectra of PAn-StNH2 for 0° and 90° polarizations (electric field vector in the plane and partially out of the plane, respectively). The dichroic effect was more pronounced for the polyaniline complex than for the 1:1 polythiophene complex, probably due to the much greater chain length of the former. However, there is no indication of any preferred orientation of the chains within these planes (eg, in the dipping direction). DISCUSSION To account for a) the anomalously small monolayer thicknesses observed for both systems studied and b) the dichroic behavior indicating polymer chains sandwiched in planes between stearylamine layers, we propose that the hydrocarbon tails of the stearylamine in our L B films are interdigitated, with the polymer chains ionically bound to the stearylamine molecules. Since the length of a fully extended stearylamine molecule is about 25Â, mere tail tilting of 4°-10° is not sufficient to account for this low thickness value. Since we employ a surface pressure during film deposition that is very much lower than the film collapse pressure, interdigitation is not an unreasonable proposition, because the stearylamine tails would be loosely packed at the pressure used for dipping. Figure 3 is a schematic depiction of the proposed structure for the L B films of these complexes. 0.3 -,

u.u -j

400

,

500

,

,

,

600

700

800

Wavelength (nm) Figure 2b. Dichroism in the UV-visible spectra of PAn-StNH2 for 0° (top) and 90° (bottom) polarizations. In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

82


Stearylamine


Polymer

Figure 3. Proposed structure for L B films of polyion complexes showing interdigitation of stearylamine tails.

We have built C P K atomic scale models of the proposed structure, and we find that the thicknesses per monolayer reported above are consistent with the structure described in Figure 3. CONCLUSIONS A novel method of incorporating non-surface active conducting polymers into L B films is described. The conducting polymers used in this study (poly(thiophene acetic acid) and sulfonated polyaniline) have been functionalized with ionizable side groups and subsequently treated with a commonly available surfactant (stearylamine) to render them surface active. They can then be easily manipulated into very good quality L B films. The resulting L B films are Y-type in structure with interdigitated alkyl tails and polymer chains that are sandwiched in planes parallel to the L B substrate. These films can be doped using conventional techniques to conductivities on the order of 10 S/cm. -2

LITERATURE CITED 1. Shirakawa, H.; Louis, E.J.; MacDiarmid, A.G.; Chiang, C.R.; Heeger, A.J. J. Chem.Soc.,Chem. Commun. 1977, p.578. 2. See, for example: Punkka, E.; Rubner, M. F.; Hettinger, J. D.; Brooks, J. S.; Hannahs, S. T.; Phys. Rev. Β 1991, 43, p.9076. Rosner, R. B.; Rubner, M. F.; Mat. Res. Soc. Symp. Proc. 1990, p.363. Hong, K.; Rosner, R. B.; Rubner, M. F.; Chemistry of Materials 1990, 2, p.82. Watanabe, I.; Hong, K.; Rubner, M. F.; Langmuir 1990, 6, p.1164. 3. See, for example: Nishikata, Y.; Kakimoto, M-A.; Imai, Y. Thin Solid Films 1989, 179, p.191. Era, M.; Kamiyama, K.; Yoshiura, K.; Momii, T.; Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Thin Solid Films, 1989, 179, p.1. 4. Sugimoto, R.; Takeda, S.; Gu, H.B.; Yoshino, K.; Chemistry Express 1986, 1, p.635. 5. Yue, J.; Epstein, A. J.; Macdiarmid, A. G.; Mol. Cryst. Liq. Cryst. 1990, 189, p.255. Yue, J.; Epstein, A.J.; J. Am. Chem. Soc. 1990, 112, p.2800. 6. Nishikata, Y.; Kakimoto, M-A.; Morikawa, Α.; Imai, Y.; Thin Solid Films 1988, 160, p.15. 7. Umemura, J.;Kamata, T.;Kawai, T.;Takenaka, T.;J. Phys. Chem. 1990, 94, p.62. 8. Watanabe, I.; Cheung, J. H.; Rubner, M. F.; J. Phys. Chem. 1990, 94, p.8715. RECEIVED September 24, 1991


LangmuirBlodgett Films of Novel Polyion Complexes of Conducting

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