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Field-Cycling NMR Relaxometry Study of Dynamic Processes in Conducting Polyaniline Eoin Murray,† Darren Carty,† Peter C. Innis,‡ Gordon G. Wallace,‡ and Dermot F. Brougham*,†,§ School of Chemical Sciences, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland, Intelligent Polymer Research Institute, Department of Chemistry, UniVersity of Wollongong, Northfields AVenue, Wollongong, NSW 2522, Australia, and National Institute for Cellular Biotechnology, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland ReceiVed: April 22, 2008
Fast-field cycling NMR relaxometry been applied to investigate dynamic processes in the conducting polymer, polyaniline. For a group of samples with different concentrations of the dopant trifluoromethanesulfonic acid, the 1H spin-lattice relaxation rates exhibit power law dependence on the Larmor frequency. The powers obtained are found to increase above a percolation threshold in dopant concentration and to show similar concentration and temperature dependence as is observed for the macroscopic polymer conductivity. These observations are discussed in terms of the accepted models for both the fast polaron dynamics and the slower, low-frequency, polymer dynamics. Introduction Polyaniline was the first intrinsically conducting polymer (ICP) to be commercially available worldwide. It has achieved importance among conducting polymers because of its ease of processability, environmental stability, and of course high conductivity.1 The polyaniline class of conducting polymer has been widely investigated for potential applications in fuel cells, rechargeable batteries, protection coatings, electrochromic displays, conducting composite materials, gas sensors, corrosion protection, electronics, biosensors, and optical devices.2-5 Polyaniline is a phenylene-based polymer having a secondary amine group flanked on either side by phenyl rings, which exhibits an insulator to conductor transition with increasing protonation due to doping with organic acids. The protonated form is the most common form of polyaniline, it is green colored (and hence is referred to as the emeraldine salt), and has conductivity in the semiconductor range, on the order of 10 Scm-1. This is many orders of magnitude higher than that of common polymers (104 Scm-1). On doping to the conducting emeraldine salt, the imine nitrogen atoms are protonated by organic acids (Figure 1). However, on the basis of optical and microtransport studies, it was proposed that partial protonation leads to phase segregation between the doped and undoped regions.6,7 The size of the conducting domains remains a subject of interest and controversy, as macroscopic charge transport is limited by the flow of carriers between domains. Microscopic charge transport in polyaniline occurs via electron transfer between localized states.8 The conduction mechanism of conducting polymers has been explained as the interchain hopping of polarons and bipolarons. When an electron is added to a pristine polymer, either by oxidation or by reduction due to a dopant, a characteristic bond deformation in the chain of about 20 monomer sites long is generated. The * To whom correspondence should be addressed. E-mail: dermot.brougham@ dcu.ie. † School of Chemical Sciences, Dublin City University. ‡ Department of Chemistry, University of Wollongong. § National Institute for Cellular Biotechnology, Dublin City University.
Figure 1. Schematic representation of the interconversion of the undoped polyaniline emeraldine base (upper) into fully doped conducting emeraldine salt (lower), drawn here in the delocalized polaron lattice form.
electron plus the deformation pattern constitute a polaron. Polarons may diffuse freely along the polymer chain at the onchain diffusion rate, D|, so the limiting factor for bulk conductivity is thought to be the polaron interchain hopping rate, D|. Research into the effects of dopant content, morphology, and crystallinity on charge transport in these materials remains critically important, and a range of optical, electronic, and magnetic resonance techniques have been applied to study conducting polymers in general and polyaniline in particular.9,10 Fast field-cycling nuclear magnetic resonance is a technique used to measure the spin-lattice relaxation time (T1) as a function of the NMR resonance frequency. Briefly, the sample magnetism is polarized in a magnetic field with a flux density, Bpol, which is as high as possible. The relaxation process subsequently occurs in a second (usually lower) field, Brlx, which is maintained for a variable time, τ. The magnetism remaining after this interval is measured at a third, fixed field, Bacq, by applying a single RF pulse on-resonance to generate an FID. There is then an extended time delay at B ) 0 for the restoration of thermal equilibrium prior to the next field cycle. By varying the value of τ, the spin-lattice magnetization recovery curve at the chosen field, Brlx, can be obtained. By then repeating the measurement at different values of Brlx, the frequency depen-
10.1021/jp8034902 CCC: $40.75 2008 American Chemical Society Published on Web 10/17/2008
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TABLE 1: Bulk Physical Properties of the Amorphous Polyaniline-TFSA Samples y -1
conductivity (Scm )
0.23
0.27
0.35
0.36
0.37
0.38
0.39
0.40
0.41
0.49
0.05
0.03
1.12
1.91
2.51
4.19
4.35
4.87
5.48
11.46
dence can be obtained. Critical to the method is rapid fieldswitching, compared to the relevant T1 value, so the sample is not exposed to intermediate values of the magnetic field during the time τ. The technique is used to study dynamics in condensed matter, as T1 is determined by random fluctuations in local fields,11 that is, by the spectral density J(ω) at the resonance frequency ω ) γBrlx, where γ is the 1H gyromagnetic ratio. The accessible frequency range of 10 kHz to about 20 MHz provides sensitivity to dynamic processes with correlation times, τc, ranging from ∼10-4 to ∼10-9 s. For complex dynamic systems, this approach has advantages over conventional fixed-field NMR relaxometry, in that the number and shape of the spectral density contributions are readily apparent from the relaxation profile, which can be fitted using the appropriate spectral density function. Thus, whereas a single random dynamic process can be described using a Lorentzian function (the BPP approach),12 more complex processes may require multiple Lorentzians or other spectral density functions. For normal polymers, J(ω) is determined by chain or side group dynamics. In conducting polymers, the highfrequency 1H relaxation (>10 MHz) is dominated by fast spin carrier (polaron) dynamics along the polymer chains. The reduced dimensionality for the propagation of the fluctuation, imposed by the quasi-1D character of the chain, results in a power-law spectral density. As polarons are also charge carriers in conducting polyaniline, the transport properties in conducting polymers may thus be studied. This high-frequency relaxation has been described in detail. 10 In this article, we present field-dependent 1H NMR relaxation data recorded for polyaniline doped with trifluoromethanesulfonic acid (polyaniline-TFSA) to different extents; both below and above the measured charge percolation threshold. We find that, for samples doped in excess of this threshold, the 1H relaxation profiles in the accessible frequency range, < 20 MHz, are sensitive to the dopant content. Surprisingly, the dependence of the low-frequency profile on dopant content reflects the previously reported10 effect of the high frequency modes, that is, of the on-chain and off-chain polaron motions that determine the conductivity, on the high frequency NMR response. Experimental Methods Sample Preparation. Polyaniline emeraldine base (MW ∼65 000) was obtained from Aldrich chemicals. In a typical preparation, this fine blue-colored powder was dispersed in aqueous solutions of trifluoromethanesulfonic acid (TFSA) in an inert atmosphere under vigorous stirring for 24 h at ambient temperature. The product was filtered, washed with the acid solution, and then with water and dried.13 The doping procedure was repeated using a number of acid strengths to obtain a range of polymer samples doped to different extents. Scanning electron microscopy analysis on a wide range of these samples indicated the complete absence of any crystallites on the micrometer scale. For this reason, we shall refer to these samples as amorphous polyaniline-TFSA. This is not intended to suggest the absence of local order of the polymer chains that would be expected for such highly conducting materials. Polyaniline-TFSA was also synthesized using a variation of published methods.14 In a typical preparation, 0.02 mol aniline was dispersed in 50 mL chloroform and to this was slowly added
a solution of 0.005 mol ammonium persulphate (APS) in 50 mL of aqueous TFSA solution to form an interface. A blue color formed at the interface within about 5 min. This typically spread within 10 min to fill the whole of the upper aqueous layer. Within 24 h, the upper layer turned to a green/black color, indicating the presence of doped polyaniline. The upper phase was then carefully removed, and the byproducts were dialysed overnight against deionized water, through 12 000-15 000 MW cutoff dialysis tubing. The dialysed sample was filtered, water washed, and dried. Scanning electron microscopy analysis on a wide range of these samples indicated the presence of crystallites of width ∼100 nm and length into the micrometer range. For this reason, we shall refer to these samples as nanocrystalline polyaniline-TFSA. Conductivity. Conductivity measurements were performed using the four-point probe method. Polyaniline samples were pressed into pellets of ∼1 cm diameter. For most of the samples, the current was varied from -20 to +20 mA and voltages were measured in the -50 to + 50 mV range. Field-Cycling NMR Relaxometry. 1H spin-lattice relaxation data was recorded in the range from 0.01-20 MHz on a Stelar FFC2000 operating at a 1H detection frequency, Bacq, of 9.25 MHz. Standard field cycling experiments were applied.11 A field slew rate of 20 MHz/ms was used in all cases with a switching time of 3 ms to allow the electromagnet to settle. A digitization rate of 1 MHz was used, and the dead time of the spectrometer was about 24 µs. The FID was sampled in the time range of 25-540 µs after the 90° pulse, which was of 7 µs duration. The relaxation rates, R1, were determined from the magnetization recovery curves by least-squares fitting. The R1 values obtained were not sensitive to time window applied to the FID. In a typical experiment, ∼0.2 g of polyaniline-TFSA powder was used. The sample temperature was controlled using a thermostatted flow of dried air that ensured temperature precision to within 1 K over the full temperature range of 253-343 K. Temperatures were calibrated externally using a Cu-Al thermocouple in a 10 mm NMR tube. Dopant Content. The dopant concentration, or y value, is the number of dopant molecules per monomer unit along the polymer backbone. The theoretical maximum is y ) 0.5, which corresponds to one dopant molecule for every two aromatic rings, that is, for each amine and imine pair of the emeraldine base, as shown in Figure 1. The imine nitrogen cannot normally be protonated as this would disrupt the conjugation along the chain. The dopant concentration was determined by CHNF elemental microanalysis and expressed as the y value. Results Dopant Content and Conductivity. The conductivity was found to be dependent on the y value. For samples with y e 0.35, the conductivity is low, above this value an increase in dopant content results in an increase in conductivity (Table 1). This indicates the presence of a percolation threshold, yperc, arising from the onset of charge carrier motion between conducting regions. An increase in dopant concentration should result in either an increase in the number of conducting domains and/or their size. Thus, increasing dopant content results in shorter paths through the nonconducting regions and increased conductivity.
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Murray et al. different dopant content. Representative data is shown in Figure 3 as a semilog plot, for the purposes of clarity. In all cases, it is possible to fit the NMR profiles with a power law spectral density function, once the quadrupolar dip region is excluded. The main observation, from Figure 3, is that only samples with dopant content in excess of the percolation threshold exhibit significant temperature dependence of the spin-lattice relaxation across the entire frequency range. For these samples, R1 decreases with temperature, which indicates that the dynamic process is in the fast motion limit at all frequencies, that is, ω < τc-1. Discussion
Figure 2. Representative 1H NMR profiles, recorded at 298 K, for amorphous polyaniline-TFSA samples with y ) 0.49 (9), 0.39 (•), and 0.27 (2). The solid lines are fits obtained using the power law spectral density function, eq 1. The open symbols are data points in the range where 14N quadrupole dips influence the 1H relaxation, these points were omitted from the fits. 1H NMR Relaxation. 1H NMR relaxation profiles were recorded for the polyaniline-TFSA samples over a wide range of y values (Figure 2). The 1H magnetization recovery curves were monoexponential for all the samples at all temperatures and magnetic fields. In the range 0.3 - 3.0 MHz, quadrupole dips15 are observed where the low-field 14N nuclear quadrupole and 1H resonance frequencies match. As quadrupolar nuclei relax very rapidly, they provide an efficient relaxation sink for the 1H magnetization. It is possible to fit all of the NMR profiles reported in this article to a power law spectral density function, eq 1, once the data in this range is excluded. This confirms that the quadrupolar dips are always the dominant relaxation mechanism in this frequency range for polyaniline-TFSA.
R1(ν) ) 1 ⁄ T1(ν) ) aν-b + c
(1)
Where a is a measure of the strength of coupling of the spins to the lattice, b is the exponent or power. The parameter c is a field-independent offset, which accounts for the contribution to the 1H relaxation of the higher frequency contributions to the spectral density, probably due to polaron motion, as discussed below. The observation of the same power law dependence in both fitted ranges is strong evidence that this spectral density contribution is constant across the measured range, as would be expected for dynamic processes orders of magnitude faster than those studied here. The values of c we obtained were low, 30 ( 16 s-1, and no dependence of c on the dopant content was observed. Typical fits are shown in Figure 2. Using the field-cycling relaxometer we found the signal intensity to be; (i) very low for undoped polyaniline, and; (ii) to increase with increasing dopant content, that is for the more highly doped amorphous samples, and also for the nanocrystalline samples. We interpret this as arising from an increased contribution to the signal from the doped regions, which clearly have longer T2 at the measurement frequency of 9.25 MHz. We could not study this issue further due to the unsuitability of the field-cycling technique for T2 measurements. Despite the changes in signal intensity, there was no indication of systematic deviation of the data from eq 1, or of significant improvement in the fitting errors, for any of the samples across the entire y range. NMR relaxation profiles were also recorded, at a number of temperatures, for amorphous polyaniline-TFSA samples of
Polymer Dynamics and Conductivity. In a previous 19F NMR relaxation study on polyaniline-TFSA,16 we observed that the dynamic process that drives the spin-lattice relaxation of the dopant molecules is fast, τc ∼ 37 × 10-9 s, and is insensitive to the dopant content. This demonstrates that the anion is a bystander in the conduction process and hence the y-dependent low-field dispersion observed in the 1H profiles is not due to a slow charge transfer or other dynamical process associated with polymer anion interactions. Mizoguchi et al.10 reported the field dependence of both the ESR and high-frequency 1H NMR relaxation (6-340 MHz) of polyaniline samples doped with HCl to different extents. The 1H profiles recorded were fitted with a power law model, yielding ω-0.5 dependence of the relaxation rate as expected for 1D on-chain polaron diffusion, down to a cutoff frequency (ωc ∼ 107-108 rads-1), which corresponds to the onset of interchain polaron hopping. Their analysis yielded the interchain (D|) and intrachain (D⊥) spin diffusion coefficients. It was observed that D| was independent of dopant concentration, therefore neighboring chains do not affect the on-chain diffusion. The D⊥ values were found to decrease rapidly below a percolation threshold in dopant concentration. The drop in D⊥ was consistent with a decrease in σDC, indicating that conductivity is limited by interchain polaron hopping. It is important to note that the frequency window of the field-cycling experiments on polyaniline-TFSA presented here, lies below the cross-over frequency identified for polyaniline-HCl. The low-field dispersion we observe for polyaniline-TFSA arises from a slower dynamic process, approximately on the 10-4 to 10-7 s time scale. It is very surprising, therefore, that both the power law relaxation exponents and the σDC values show similar dependence on dopant content (Figure 4). For low dopant content samples, y < 0.35, the conductivity is low and ∼ω-0.5 dependence of the relaxation rate is observed over the temperature range of 253-343 K. As we have noted, this exponent is consistent with the spectral density for a 1D dynamic process. However, it is also commonly observed for entangled polymers, which are in the fast motion (high mode number) limit,17 for instance in the case of polybutadiene, Mw 65 500, at temperatures of 253 K and below. For polyaniline-TFSA, above a threshold in dopant concentration, yperc ∼0.35, charge carrier percolation begins and the conductivity increases (Figure 4). In this range, the parallel increase in the relaxation exponent shows that the dopant mole fraction determines the 1H spectral density at low frequency. Changes in the relaxation exponent have been commonly observed as a function of frequency and/or temperature for a range of polymers. 11 These transitions are usually sharp and arise when dynamics on different timescales begin to influence the spectral density. The gradual change in exponent as y is increased from ∼0.35 to ∼0.50, for polymer samples of the
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Figure 3. 1H NMR relaxation profiles recorded at (2) 253, (9) 298, and (b) 343 K, for amorphous polyaniline-TFSA samples with (a) y ) 0.35 < yperc, and (b) y ) 0.41 > yperc.
Figure 4. Conductivity (O) and power law exponent (9) of amorphous polyaniline-TFSA as a function of dopant mole fraction, recorded at 298 K. The solid lines are guides for the eye and the dashed line marks the suggested onset of percolation. Data for nanocrystalline polyaniline-TFSA (exponents only) are included for comparison (b).
same Mw at a single temperature, is very unusual. The simplest interpretation is that there is a gradual change in the slow chain dynamics, arising from an increase in the dimensionality of the faster charge carrier motions. At very high dopant concentrations, y ∼0.5, the exponent becomes less strongly dependent on dopant content, as the polymer is almost fully doped with one dopant molecule per monomer unit (i.e., for each pair of aromatic rings) and the percolation network is complete. However, the conductivity continues to rise, probably due to the formation of bipolarons and polaron pairs. These contribute to conductivity, but as they have zero spin, do not directly affect the 1H relaxation. Polymer Morphology. The observation of monoexponential spin relaxation at all frequencies and temperatures, for all of the samples studied, indicates that all detected 1H nuclei, that is, those residing in the doped regions, are at a common spin temperature due to fast spin diffusion.18 The poor field homogeneity of the field-cycling relaxometer prevented estimation of the percent crystallinity of the samples on the basis of
the FID. However, as mentioned, scanning electron microscopy confirmed that none of the amorphous polyaniline-TFSA samples had any micron-scale crystallinity. High field 1H and 2H NMR spin-counting experiments on conducting polyaniline have been reported, 19 which demonstrated that signal can be lost due to RF reflectance and by dephasing due to the presence of unpaired electrons. However, we have found that extensive grinding of amorphous polyaniline-TFSA samples, with y both above and below yperc, does not alter their conductivity, the 1H signal intensity, or the relaxation exponents. This confirms that the observed changes in these properties are not due to changes in the grain size, or from limited penetration of the RF field. Included in Figure 4 are spin relaxation exponents for samples of nanocrystalline polyaniline-TFSA emeraldine base. SEM studies have confirmed that these materials are composed of micron sized grains. It was not possible to obtain reliable conductivity data for these samples, perhaps because the grain size was comparable to the tip size of the probe. However, we observed very similar dependence of the exponents on dopant
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Murray et al.
content, to that obtained for amorphous polyaniline-TFSA. Taken as a whole, these observations are strong evidence that the polymers are homogeneously doped, that the observed magnetism arises from the doped (long T2) regions, and that the dependence of the NMR relaxation exponents on dopant content does not arise from changes in the macroscopic crystallinity. Physical Interpretation of the Spin Relaxations Exponents. Spectral density functions for anomalous diffusion due to a random walk on a fractal network, of dimension d, can be derived, for example for the case of polaron motion.9,10 The power law exponent, b, is given by:
b)-
( 2d - 1)
(2)
1D motion therefore gives rise to ω-0.5 power law dependence. For amorphous polyaniline-TFSA at room temperature we find; for y ) 0.49, b ) 0.69, while below the threshold for y ) 0.23, b ) 0.44. Application of eq 2 to our data suggests a dimension for the slow polymer motion of approximately unity (∼1.1) below yperc, decreasing to sub-1D (∼ 0.6) above yperc, which is the opposite of the expected behavior. Clearly, this model is not directly applicable to the low-frequency spectral density in amorphous polyaniline-TFSA, which suggests that the exponent is not interpretable in terms of the dimension for the propagation of a dynamic process. Korb and Bryant20,21 developed a model for chain fluctuations localized, or trapped, in multiple sites along the polymer backbone, which also gives rise to a power law spectral density function for spin relaxation. This approach has been successfully applied to interpret relaxation profiles of homopolypeptides and proteins. The power law exponent, b, that emerges from this treatment is,
b)3-
2ds - ds df
(3)
where the fractal dimension, df, describes the spatial distribution of protons, whereas ds describes the dimensionality of propagation of the disturbance through the polymer network. The exponents we obtain for polyaniline-TFSA, of between 0.44 and 0.69, are within the range described by Korb and Bryant. This model suggests that for polyaniline-TFSA either: (i) The dimension of the disturbance propagation, ds, decreases with dopant content, which is similar to the inference provided by the random walk model. One would expect the reverse, as the dimensionality of the fast polaron motions increases above yperc, due to the onset of interchain hopping; or (ii): The dimension of the spatial 1H distribution, df, increases with dopant content. We believe that this is a more reasonable interpretation, as the increase in conductivity is associated with an increase in local polymer order. A key feature of this model is that the relaxation rate, R1, is directly proportional to temperature. However, for highly conducting polyaniline-TFSA we observe that R1 decreases with increasing temperature (part b of Figure 3). Deviations from the expected linear dependence of R1 and also from the power law spectral density have been observed for the polypeptide polyalanine.21 These were interpreted as arising from side-chain motions altering the spectral density, as was confirmed by the absence of such deviations for polyglycine, which has no side chains. Thus, polyaniline-TFSA appears to be an anomalous case, in that power law relaxation behavior is observed but linear dependence of R1 on temperature is not.
Figure 5. Temperature dependence of the exponent for polyaniline-TFSA for samples above the percolaton threshold; y ) 0.40 (O) and 0.41(9). The solid lines are straight line fits. The highlighted region corresponds, within the estimated fitting error, to quasi 1D behavior (using eq 2), i.e. where b ) 0.5 ( 0.03.
Temperature Dependence of the 1H Spin Relaxation Exponents. Further insight into the relaxation exponent can be gained from its temperature dependence. The change in the nature of the spin dynamics at yperc is underlined by the fact that below the percolation threshold the exponent obtained is temperature independent, as seen in part a of Figure 3, whereas above yperc the exponent is strongly temperature dependent, as seen in part b of Figure 3 and in Figure 5. It should also be noted that the field-independent offset, c, (eq 1) was found not to change with temperature for a given sample. It has been shown that for conducting polyaniline both the conductivity and the polaron interchain hopping rate, D⊥, are thermally activated for temperatures above 150 K.22 For samples doped in the range from yperc to y ) 0.5, where the conducting network is partially complete, increasing temperature is expected to increase its dimensionality. Thus, for conducting polyanilineTFSA, the observed temperature dependence of the low frequency 1H relaxation reflects both the temperature- and the dopant-induced changes in the fast polaron dynamics. We suggest that on raising the temperature, chains in disordered regions23 become aligned, which increases both the off-chain polaron diffusion and the dimensionality of the spatial 1H distribution. Similar changes in the relaxation behavior, to that shown in Figure 5, have been observed at low temperature for bovine serum albumin,24,25 with the relaxation exponents increasing with temperature up to 200 K, above which they become temperature independent. These observations were interpreted as arising due to multiple trapping of the chain disturbance with a low barrier, which limits the dimensionality of the disturbance propagation at low temperature. However, we do not observe a plateau in the temperature dependence of the relaxation exponents up to 343 K. This sets a lower limit for the average barrier to the polymer motions driving 1H relaxation in polyaniline-TFSA of ∼ 2.9 kJmol-1, as compared to the value of ∼5.0 kJmol-1, which can be inferred for bovine serum albumin. In conclusion, for polyaniline-TFSA we have observed an increase in the relaxation exponent with increasing dopant content and with increasing temperature, both of which are correlated with increased rapid polaron motion through the network. The most plausible interpretation is that the increases
Field-Cycling NMR Relaxometry in the exponent arise from an increase in the fractal dimension that describes the distribution of 1H nuclei in space. Hence, we tentatively ascribe the changes in the low frequency dynamics to increased local ordering of the polymer chains. We are currently extending this study to include other dopant molecules and polymers such as polypyrrole and polythiophene. This work will establish whether the influence of fast polaron dynamics on the low-frequency 1H spin-lattice relaxation is a common feature in this important class of material. Acknowledgment. E.M. gratefully acknowledges the financial support of the Irish Research Council for Science, Engineering and Technology Embark scholarship funded by the National Development Plan. D.B. and D.C. would like to thank Enterprise Ireland for support through a basic research grant, SC/2002/336, and the Higher Education Authority of the Republic of Ireland for supporting the purchase of the relaxometer. References and Notes (1) (a) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; Macdiarmid, A. G. Phys. ReV. Lett. 1977, 39 (17), 1098–1101. (b) Mathew, R.; Mattes, B. R.; Espe, M. P. Synth. Met. 2002, 131 (1-3), 141–147. (2) Bleier, H.; Finter, J.; Hilti, B.; Hofherr, W.; Mayer, C. W.; Minder, E.; Hediger, H.; Ansermet, J. P. Synth. Met. 1993, 57 (1), 3605–3610. (3) Heeger, A. J. Synth. Met. 1993, 57 (1), 3471–3482. (4) Echigo, Y.; Asami, K.; Takahashi, H.; Inoue, K.; Kabata, T.; Kimura, O.; Ohsawa, T. Synth. Met. 1993, 57 (1), 3611–3616. (5) Paddeu, S.; Ram, M. K.; Carrara, S.; Nicolini, C. Nanotechnology 1998, 9 (3), 228–236. (6) Javadi, H. H. S.; Cromack, K. R.; Macdiarmid, A. G.; Epstein, A. J. Phys. ReV. B 1989, 39 (6), 3579–3584.
J. Phys. Chem. C, Vol. 112, No. 45, 2008 17693 (7) Stafstrom, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; Macdiarmid, A. G. Phys. ReV. Lett. 1987, 59 (13), 1464–1467. (8) Sariciftci, N. S.; Kolbert, A. C.; Cao, Y.; Heeger, A. J.; Pines, A. Synth. Met. 1995, 69 (1-3), 243–244. (9) Nechtstein, M. Electron Spin Dynamics. In Handbook of Conducting Polymers; Reynolds, J. R., Ed.; Dekker: New York, 1998. (10) Mizoguchi, K.; Nechtschein, M.; Travers, J. P.; Menardo, C. Phys. ReV. Lett. 1989, 63 (1), 66–69. (11) Kimmich, R.; Anoardo, E. Prog. Nucl. Magn. Reson. 2004, 33, 257–320. (12) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. ReV. 1948, 73, 679–712. (13) Kahol, P. K.; Kumar, K. K. S.; Geetha, S.; Trivedi, D. C. Synth. Met. 2003, 139 (2), 191–200. (14) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17 (13), 1679. (15) Anoardo, E.; Pusiol, D. J. Phys. ReV. Lett. 1996, 76 (21), 3983– 3986. (16) Murray, E.; Brougham, D. F. Synth. Met. 2005, 155 (3), 681–683. (17) Kimmich, R.; Anoardo, E. J. Chem. Phys. 1998, 108 (5), 2173– 2177. (18) McBrierty, V. J.; Packer, K. J. In Nuclear Magnetic Resonance in Solid Polymers; Cambridge University Press: New York, 1979. (19) Goddard, Y. A.; Vold, R. L.; Hoatson, G. L. Macromolecules 2003, 36, 1162–1169. (20) Korb, J. P.; Bryant, R. G. J. Chem. Phys. 2001, 115 (23), 10964– 10974. (21) Goddard, Y.; Korb, J. P.; Bryant, R. J. Chem. Phys. 2007, 126, 175105. (22) Mizoguchi, K.; Kume, K. Solid State Commun. 1994, 89 (12), 971– 975. (23) Goddard, Y. A.; Vold, R. L.; Cross, J.; Espe, M. P.; Hoatson, G. L. J. Chem. Phys. 2005, 122 (5), 054901. (24) Kimmich, R.; Winter, F. J. Phys. Chem. 1988, 92 (23), 6808–6814. (25) Nusser, W.; Kimmich, R. J. Phys. Chem. 1990, 94 (15), 5637–5639.
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