J. Phys. Chem. B 2005, 109, 22725-22729
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Optically Active Polymer Carbon Nanotube Composite Marc in het Panhuis,*,† Raquel Sainz,‡ Peter C. Innis,§ Leon A. P. Kane-Maguire,§ Ana M. Benito,‡ M. Teresa Martı´nez,‡ Simon E. Moulton,§ Gordon G. Wallace,§ and Wolfgang K. Maser‡ Department of Chemistry, UniVersity of Hull, Hull HU6 7RX, United Kingdom, Instituto de Carboquimica (CSIC), C/Miguel Luesma Casta´ n 4, E-50018, Zaragoza, Spain, and ARC Centre for ElectroactiVe Materials, Intelligent Polymer Research Institute, UniVersity of Wollongong, Wollongong NSW 2522, Australia ReceiVed: June 7, 2005; In Final Form: September 21, 2005
A completely soluble optically active polyaniline-multiwalled carbon nanotube composite was investigated by spectroscopic and microscopic techniques. It was found that the polymer’s optical activity was retained in the presence of carbon nanotubes. Solutions were found to be easily processable into thin films, which exhibited dendritic structures only in the presence of nanotubes.
1. Introduction Electroactive and inherently conducting polymers such as polypyrrole, polythiophene, and polyaniline are organic semiconductors.1 An important feature of these polymers is that their oxidation/reduction processes are reversible and can be initiated at moderate potentials. The doped oxidized forms exhibit good electrical conductivity, whereas the reduced forms have very low conductivity. It is this dynamic character of these polymers which has intrigued many researchers. For example, electroactive polymers, so-called “intelligent materials”, have been actively investigated for applications such as electrochromic devices, rechargeable batteries, and chemical and biological sensors.2 In addition, it has been shown that polyaniline can be synthesized as optically active (chiral) emeraldine salt, by doping with an optically active dopant.3-6 It is well-known that a high proportion of chemicals important to the pharmaceutical and agricultural industries, such as pesticides and drugs, are chiral. Standard synthetic routes generally yield racemic mixtures, which exhibit remarkably different biological effects. Often only one form of a drug or pesticide has the desired effect. Therefore effective methods for producing enantiomerically pure forms are being actively pursued.7-9 It is envisaged that chiral molecules such as polyaniline could aid in the development of two of such methods: as conducting polymer membranes in chiral separation and as novel chiral electrodes for asymmetric electrosynthesis. Carbon nanotubes (CNTs) are under active investigation due to their phenomenal physical (mechanical, electrical, and thermal conductivity) properties.10 Unfortunately CNTs are not easily processed due to their hydrophobicity and therefore lack of solubility in many solvents. However, the surface carbon atoms in nanotubes present an excellent platform for chemical functionalization, and have been utilized to address the processability issue. Polymer-carbon nanotube composites can be produced using noncovalent (side-wall) functionalization of nanotubes with polymers11-15 (ex-situ solution mixing) or by in-situ * To whom correspondence should be addressed. E-mail: M.Panhuis@ hull.ac.uk. † University of Hull. ‡ Instituto de Carboquimica (CSIC). § University of Wollongong.
Figure 1. Photographs of polyaniline-MWNT composite solutions (A) and as thin films (B) in emeraldine base (blue) and emeraldine salt (green) form.
polymerization in the presence of carbon nanotubes.16-21 Additional methods for composite production are based on electrochemical growth of composite films on aligned arrays of nanotubes,22 epoxies,23 and spinning from microextruded blends of multiwalled nanotubes (MWNTs) and polyamides.24 The combination of polymers and carbon nanotubes into composite materials enhances their functionality, generally not achievable for each of the components in their own right. Current applications for polymer carbon nanotube composites include antistatic packaging, whereas using CNTs as reinforcement materials could yield exceptionally strong and lightweight materials.25
10.1021/jp053025z CCC: $30.25 © 2005 American Chemical Society Published on Web 11/10/2005
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Figure 2. UV-visible spectra (normalized) of polyaniline in emeraldine salt (ES) form and polyaniline-MWNT composite (MWNT/ES) in emeraldine salt form in NMP.
Figure 3. Circular dichroism spectra (normalized) of polyaniline in emeraldine salt (ES) form, and polyaniline-MWNT composite (MWNT/ES) in emeraldine salt form in NMP.
In this paper, we report the combination of the unusual properties of (chiral) polymers and (conducting) carbon nanotubes into optically active conducting composites (using in-situ polymerization). 2. Experimental Details An in-situ approach for the polymerization of aniline was adopted. Multiwalled carbon nanotubes were produced by arc discharge (see refs 18 and 19 for additional details). A carbon nanotube containing composite was fabricated by in-situ polymerization of aniline in the presence of MWNTs as previously described.18,19 Elemental analysis indicated that the resulting composite contains about 50% (by weight) MWNT-containing material. Polyaniline in emeraldine base form was synthesized under similar conditions without carbon nanotubes. Optically active polyanile was prepared by doping a (blue) solution of emeraldine base (EB) in N-methyl-2-pyrrolidinone (NMP, Acros) with (S)-(+)-10-camphorsulfonic acid (HCSA, Aldrich), resulting in a (green) emeraldine salt (ES) solution (see Figure 1A). All solutions were prepared by dissolving 1.5 mg of EB in 3 mL of NMP and doped through addition of 70 mg of HCSA (0.1 M). A similar procedure was followed for doping the MWNT-containing composite. A 150 µL aliquot of each solution was freshly diluted in 2850 µL of NMP. UV-visible spectra and circular dichroism (CD)
spectra were recorded immediately after dilution using a Shimadsu UV-1601 spectrophotometer and a Jobin-Yvon Dichrograph 6. All spectra were normalized to the highest wavelength UV-vis band. Transmission electron microscopy (TEM) images were obtained on a JEOL 2011 TEM. Diluted composite solutions were evaporated onto Cu 300 mesh grids. A Deerac Fluidics Equator single-tip liquid handling system (Equator) was used to deposit droplets (100-400 nL) onto a glass substrate from a reservoir containing (freshly prepared) polymer-nanotube dispersions (see ref 26 for details). This allowed for noncontact dynamic deposition of droplets with great accuracy in the droplet volume. The droplets were allowed to dry in air. The resulting composite thin films were blue (indicating EB) or green (indicating ES); see Figure 1B. Optical micrographs of the thin film samples were obtained using a Olympus BX-51 optical microscope fitted with a DP50 digital camera. 3. Results and Discussion The properties and characteristics of the nanotube-polyaniline composites, fabricated by in-situ polymerization, were reported previously.18,19 It was reported that the composite in emeraldine base form (MWNT/EB) is completely dispersable in NMP, resulting in a blue and stable solution. However,
Optically Active Polymer Carbon Nanotube Composite conversion into emeraldine salt composite (MWNT/ES) through addition of HCl immediately led to complete flocculation (destabilization) of most nanotube material. In contrast, upon addition of HCSA (0.1 M) to our MWNT/ EB sample the color changed rapidly from blue to green, characteristic of emeraldine salt. The resulting (green) MWNT/ ES composite solution is stable; no floating particles and no particulate matter could be observed, even after several days. To the best of our knowledge this is the first time a soluble composite has been achieved in emeraldine salt form. Figure 2 shows the UV-visible spectra of (freshly converted/diluted solutions of) ES and MWNT/ES in NMP. The UV-vis spectrum (Figure 2) of ES showed the three characteristic absorption bands at 345, 415, and 810 nm associated with π-π*, polaron-π*, and π-polaron band transitions of polyaniline in the emeraldine salt form.6 The chains of polyaniline in emeraldine salt form are generally thought to exist in two distinctive conformations: “compact coil”, indicating tightly coiled chains exhibiting a localized polaron (UV-vis) absorption band at ca. 800 nm, and “expanded coil”, indicating an expansion of the coiled chains exhibiting an intense, broad absorption in the near infrared (which replaces the highwavelength polaron band). Thus the spectral features are consistent with a “compact coil” conformation. The following changes can be observed when comparing the spectrum of the MWNT/ES composite with ES: the 345 nm band has blueshifted, broadened, and increased in intensity, while a new, very strong feature appeared at 295 nm. Both polaron bands are least affected, although the 415 nm showed a small increase in intensity. Most significant is the new feature at 295 nm, which was previously observed for MWNT/EB and assigned as a π-π* transition centered on the quinoid unit.18 The UV-vis spectrum for MWNT/ES is dominated by polymer features; this is similar to our previous observations on various other polymer-nanotube composites.13-15,18 In particular, it was noted that a shift in the π-π* and/or a new (emerging) feature could be explained by absorption of polymer onto the nanotube surface. It has been proposed that this process is facilitated by polymer backbone exposure,12 resulting in efficient absorption onto the nanotube surfaces. Computer simulation studies indicated that backbone exposure affects the polymer conformation. The new feature in the MWNT/ES spectrum (in Figure 2) suggests polymer backbone exposure upon interaction with nanotubes. Both aforementioned solutions are optically active (see Figure 3). The ES solution is optically active with bands at 408 nm, 455 nm, and the first component of a bisignate band at 748 nm. These observations are consistent with the polymer adopting a compact coil conformation. The CD spectrum for MWNT/ ES showed that the polymerization of PANI in the presence of nanotubes and doping does not inhibit the polymer’s ability to become optically active. The presence of nanotubes results in a red shift for the lower wavelength bands to 424 and 458 nm, with no observable shift for the high wavelength band. The spectrum indicates that the polymer conformation is similar to compact coil. Thus the following picture emerges: in-situ polymerization (in the presence of MWNT) affects the polymer stacking behavior (as shown by shifts in π-π* bands), but does not result in a significant alteration of the overall compact coil conformation (as shown by π-polaron band transition). Electron microscopy is an excellent technique for investigating the wetting between polymers and nanotubes, as both can be readily identified in the images. Figure 4 shows TEM images
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Figure 4. Transmission electron microscopy (TEM) images of polyaniline-MWNT composite in emeraldine salt form.
of polyaniline-MWNT composites in emeraldine salt form. Nanotubes are readily identified as thin straight lines, with the hollow core of the nanotube visible within, and appear wellgraphitized as can be expected from the arc discharge method. It is evident from the images that the nanotubes are nicely covered by polymer. This is indicative of excellent wetting as the nanotubes support a large amount of polymer. Excellent polymer wetting of the nanotubes’ surface is essential for the solubility/dispersability of composite materials. Hence doping of a soluble composite in EB form (MWNT/EB) does not affect the solubility of the composite material. A Deerac Fluidics Equator single-tip liquid handling system was used to deposit droplets (100 nL) from a reservoir containing freshly prepared solutions (either in EB or ES form) onto a specific position on a substrate and allowed to dry. Optical microscopy was used to investigate the morphology of the thin films. Figure 5A shows the typical morphology of a thin film (either polymer or composite) in emeraldine base form.
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Figure 5. Optical micrograph images of thin films of polyaniline-MWNT composites: (A) composite in emeraldine base form (MWNT/EB), (B) composite in emeraldine salt form (MWNT/ES), and (C, D) dendritic structures in MWNT/ES at different magnification. Thin films were prepared by pinpoint deposition of 100 nL droplets of composite solutions. Blue light is a microscope artifact.
In contrast, thin films prepared from freshly prepared solutions in emeraldine salt form have completely different appearance (see Figure 5B). Two distinctive morphologies can be observed in the MWNT/ES composite thin films: fibrous assemblies and dendritic structures (see Figure 5C,D). Dendritic structures were not observed in ES thin films and appear to be mediated by the presence of carbon nanotubes (in the composite material). The fibrous structures (also observed in ES thin films) can span along almost the entire diameter of the thin film, with diameters up to 15 µm. Thus it is suggested that the fibrous structures are related to “free” polyaniline, while the dendritic structure is related to polyaniline/MWNT. This is supported by our previous observation of destabilization of all material upon addition of HCl to MWNT/EB.18 After 24 h (green) dendritic structures could be identified at the bottom of the beaker. We did not further analyze these structures, as they are difficult to collect, but this could provide additional evidence. 4. Conclusions In conclusion, this work has shown the fabrication of a soluble and optically active polymer-nanotube composite. Circular dichroism showed that the chiroptical properties of polyaniline are retained in the presence of carbon nanotubes. Spectroscopy and microscopy analysis has shown excellent wetting between polymers and nanotubes. Polymer and composite solutions, in emeraldine salt form, could be easily processed into thin films. Optical microscopy analysis has shown significant difference between the morphology of composite films in emeraldine base and salt forms. In particular, it is suggested that both fibrous and dendritic structures are indicative of emeraldine salt, but only dendritic structures are mediated by the presence of nanotubes. Thus, combining the unusual properties of (chiral)
polymers and (conducting) carbon nanotubes into optically active conducting composites could aid the development of multifunctional materials, with applications ranging from production of chemicals in enantiomerically pure form to electrical sensors and lightweight, strong conducting materials. Acknowledgment. M.i.h.P. gratefully acknowledges financial support from The Royal Society for an International Exchange Grant (U.K. to Australia). The Group of the Instituto de Carboquı´mica gratefully acknowledges financial support from the Spanish Ministry of Education and Science (MEC) under Project NANOSIN (MAT 2002-04540-C05-04). The continued support of the Australian Research Council is gratefully acknowledged. Mrs. J. Halder of the University of Hull Microscopy Facility is thanked for electron microscopy images. References and Notes (1) Wallace, G. G.; Innis, P. C. J. Nanosci. Nanotechnol. 2002, 2, 441451. (2) Wallace, G. G.; Spinks, G. M.; Kane-Maguire, L. A. P.; Teasdale, P. R. ConductiVe ElectroactiVe PolymerssIntelligent Materials Systems; CRC Press: Boca Raton, FL, 2003. (3) Majidi, M. R.; Kane-Maguire, L. A. P.; Wallace, G. G. Polymer 1994, 36, 3597-3599. (4) Egan, V.; Bernstein, R.; Hohmann, L.; Tran, T.; Kaner, R. B. Chem. Commun. 2001, 2001, 800-801. (5) Havinga, E. E.; Bouman, M. M.; Meijer, E. W.; Pomp, E. W.; Simenon, M. M. J. Synth. Met. 1994, 66, 93-97. (6) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995, 7, 443-445. (7) Lakshmi, B. B.; Martin, C. R. Nature 1997, 388, 758-760. (8) Guo, H.; Knobler, C. M.; Kaner, R. B. Synth. Met. 1999, 101, 4447. (9) Afonso, C. A. M.; Crespo, J. G. Angew. Chem., Int. Ed. 2004, 43, 5293-5295. (10) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792.
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