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NMRD of polyaniline-based conducting polymers
Synthetic Metals 155 (2005) 681–683
NMRD of polyaniline-based conducting polymers夽 E. Murray a , D.F. Brougham b,∗ b
a School of Chemical Sciences, Dublin City University, Dublin 9, Ireland National Institute for Cellular Biotechnology, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
Received 28 June 2004; received in revised form 11 January 2005; accepted 30 June 2005 Available online 7 November 2005
Abstract Nuclear magnetic resonance dispersion (NMRD) is a technique for measuring NMR relaxation times, which is commonly used to study dynamics in condensed matter. In this paper, we present NMRD profiles, for polyanilines doped with trifluoromethanesulfonic acid (TFSA), recorded as a function of temperature and dopant concentration. This work demonstrates that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. The NMRD profile of the bulk polymer is sensitively dependent on the concentration of the dopant, while the response of the dopant is insensitive to concentration. Complete analysis of the NMRD profiles of these ICPs will give important information, including the rates of spin diffusion and of charge transport between dopant-rich domains. © 2005 Elsevier B.V. All rights reserved. Keywords: Nuclear magnetic resonance spectroscopy; Conduction; Dopant
1. Introduction The aim of this project is to use NMRD to study polymer dynamics and charge transport in the both the doped and undoped forms of the ICP polyaniline–TFSA. Nuclear magnetic resonance dispersion (NMRD) is a technique used to measure the longitudinal relaxation time (T1 ) as a function of NMR resonance frequency. NMRD can be used to study dynamics in condensed matter as T1 is determined by fluctuations in local fields. For normal polymers, these fluctuations usually arise due to molecular motions. So, the NMRD response of an undoped polymer is driven by chain motion and the NMRD profile (T1 versus frequency plot) should be a stretched-Lorentzian; the width of which is directly related to rate of molecular dynamics. In the case of intrinsically conducting polymers, ICP’s, charge carrier motion may also affect the polymer relaxation. Thus, it is our hypothesis that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. Through this work we hope to improve the
夽 Based on presentation at the International Conference on Synthetic Metals, Wollongong, Australia, June 28–July 2, 2004 (ICSM 2004). ∗ Corresponding author.
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.06.022
understanding of the charge transport mechanisms in this very important class of materials, specifically we are probing the interaction between the conducting (nano)domains and the bulk polymer. Polyaniline (PAni) was the first ICP to achieve worldwide commercial availability. It has achieved significant importance among conducting polymers due to its ease of processability, environmental stability and of course high conductivity. The ‘doped’ (protonated) form is green coloured (hence emeraldine) and has conductivity on a semiconductor level of the order of 10 S cm−1 (depending on dopant), many orders of magnitude higher than that of common polymers. The dopant used in this study, trifluoromethanesulfonic acid (TFSA) (Fig. 1), was chosen due to the fact its anion contains no protons to interfere with 1 H signal from the polymer backbone, while 19 F NMRD can be used to study the mobility of dopant. It is thought that the dopants are not homogeneously distributed through the polymer, but rather are concentrated in domains separated by regions of lower conductivity [1]. Thus, the charge transport mechanism is limited by the flow of charge between domains. The domains are relaxation sinks, linked at a molecular level to the entire polymer by the process of spin diffusion. Thus, the doped regions determine the NMRD response of the polymer.
E. Murray, D.F. Brougham / Synthetic Metals 155 (2005) 681–683
682
Fig. 1. Trifluoromethanesulfonic acid (TFSA).
2. Experimental 2.1. Sample synthesis Polyaniline emeraldine base (MW ∼ 65,000) was obtained from Aldrich chemicals. This fine blue powder was dispersed in a 0.7 M solution of trifluoromethanesulfonic acid in an inert atmosphere under vigorous stirring for 24 h at ambient temperature. The product was filtered, washed with the acid solution and with water and dried [2]. The synthesis was repeated using a 0.35 M solution of trifluoromethanesulfonic acid in order to obtain a sample of lower dopant concentration. Characterisation of the materials by fluorine determination and conductivity measurements is ongoing. 2.2. NMRD measurements Solid-state NMRD T1 data in the range from 10 kHz to 20 MHz were obtained using a Stelar Spinmaster-FFC 2000 Fast-Field Cycling NMR Relaxometer [3]. 3. Results and discussion 3.1. 1 H NMRD 3.1.1. Low dopant concentration At low dopant concentrations the conductivity is low and the observed NMRD response is typical for a solid polymer (Fig. 2); the curve is a stretched-Lorentzian (the width of which is directly related to rate of molecular dynamics). The response is weakly
Fig. 3. NMRD profile of high dopant concentration PAni–TFSA.
temperature dependent in the accessible temperature range; its shape is indicative of correlated chain motions. 3.1.2. High dopant concentration At moderate to high dopant concentrations, the NMRD response clearly demonstrates that polymer chain dynamics alone do not drive relaxation. The widths of the Lorentzians (Fig. 3) are observed to decrease with temperature, hence they cannot be simply related to a motional process; for which the rate will normally increase with temperature. Furthermore there is a change in shape of the profile between +24 and −20 ◦ C. Similar behaviour was observed for a polyaniline sample doped with phosphohexafluoric acid (HPF6 ). Clearly, a second relaxation mechanism is operating, we believe this is due to the conducting domains. The data clearly demonstrate that the degree of doping determines the NMRD response of the polymer. It is probable that the NMR properties of the bulk material are determined by coupling to the localised sinks by spin diffusion. 3.2.
14 N
quadrupolar dips
Whereas one might expect to see a gradual, smooth increase in T1 with field strength, with doped ICP samples this increase is often superimposed by three pronounced ‘dips’ where the T1 value drops by 10% or even more over a small range of field strengths. These are known as ‘quadrupole dips’ [4] and are found at well-defined field strengths (0.74 MHz, 1.56 MHz and 2.30 MHz) where the Nitrogen-14 nuclear quadrupole resonance (NQR) and the proton NMR frequencies are equal (Fig. 4). 3.3.
Fig. 2. NMRD profile of low dopant concentration PAni–TFSA.
19 F
NMRD
In order to probe dopant mobility, we measured the 19 F NMRD profile (Fig. 5). We discovered that the profile is independent of dopant concentration and could be fitted using the well-known Cole–Cole distribution [5,6] for spectral densities
E. Murray, D.F. Brougham / Synthetic Metals 155 (2005) 681–683
683
tumbling is on the expected timescale for molecular tumbling in a moderately restricted environment at this temperature. The δ value is indicative of a significant degree of correlation of the dopant reorientational motion. The fact that both curves can be fitted with the same parameters demonstrates that the anions are in very similar environments irrespective of the dopant level. There are two possible explanations for this; either the dopant is homogeneously distributed through the material and is relatively dilute, i.e. the anion environment is determined by the polymer backbone and partial de-doping does not affect the anion motional characteristics. Alternatively dopants may reside in domains of comparable size and dopant density, in which case the decrease in conductivity on de-doping is due to a decrease in the number of domains, with a resulting increase in the interdomain separation. This is consistent with the observations of domains from EFM microscopy [7], XRD and EPR [1] of other workers. Future work will involve measuring the temperature dependence of τ c , which will allow the determination of the barrier height of the to motion. We are also developing a model to fit the profiles of the polymer backbone.
Fig. 4. 1 H NMRD of PAni–TFSA at 24 ◦ C showing 14 N quadrupolar dips.
4. Conclusions 1 H NMRD can be used to probe the magnetic coupling of the polymer backbone of ICPs to dopant-rich domains, which act as NMR relaxation sinks. The coupling processes are interesting as they are related to the mechanisms of conduction in these materials. 19 F NMRD data suggest that conductivity is limited by the extent of inter-domain communication.
Acknowledgments Fig. 5. 19 F NMRD profile, at 24 ◦ C, of both high and of low dopant concentration PAni–TFSA fitted with the Cole–Cole distribution model.
(Eqs. (1) and (2)). 1 1 = Cdip 6 (aJω + bJ2ω ) T1 r 2 Jcc (ω, τc , δ) = sin ω
δπ 2
(1)
(ωτc )δ × 1 + (ωτc )2δ + {2 cos(δπ/2)}(ωτc )δ
EM gratefully acknowledges the financial support of the Irish Research Council for Science, Engineering and Technology Embark scholarship funded by the National Development Plan. DB acknowledges Enterprise Ireland Research Innovation Fund and the Higher Education Authority for the Republic of Ireland for support in equipment purchase. The authors would like to thank Gordon Wallace and Peter Innis of the Intelligent Polymer Research Institute, University of Wollongong for their helpful comments and insights. References
(2)
where Cdip is dipole–dipole constant, Jcc the spectral density, ω the angular frequency (rad/s), τ c the correlation time and δ is the degree of correlated motion and distribution of correlation times. Using the Cole–Cole distribution model to fit the 19 F data of PAni–TFSA, we found a correlation time, τ c , of 3.69(8) × 10−8 s and a value of 0.386(8) for δ. A τ c of 37 ns for TFSA anion
[1] J. Too, S.M. Long, Phys. Rev. B: Solid State 57 (1998) 9567. [2] P.K. Kahol, K.K. Satheesh Kumarb, S. Geetha, D.C. Trivedi, Synth. Met. 139 (2003) 191. [3] E. Anoardo, G. Galli, G. Ferrante, Appl. Magn. Reson. 20 (2001) 365. [4] E. Anoardo, D.J. Puisol, Phys. Rev. Lett. 76 (1996) 3983. [5] P.A. Beckmann, Phys. Rep. 171 (3) (1988) 85. [6] K.S. Cole, R.H. Cole, J. Chem. Phys. 19 (1951) 1484. [7] J.N. Barisci, R. Stella, G.M. Spinks, G.G. Wallace, Synth. Met. 124 (2001) 407.
NMRD of polyaniline-based conducting polymers夽 E. Murray a , D.F. Brougham b,∗ b
a School of Chemical Sciences, Dublin City University, Dublin 9, Ireland National Institute for Cellular Biotechnology, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
Received 28 June 2004; received in revised form 11 January 2005; accepted 30 June 2005 Available online 7 November 2005
Abstract Nuclear magnetic resonance dispersion (NMRD) is a technique for measuring NMR relaxation times, which is commonly used to study dynamics in condensed matter. In this paper, we present NMRD profiles, for polyanilines doped with trifluoromethanesulfonic acid (TFSA), recorded as a function of temperature and dopant concentration. This work demonstrates that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. The NMRD profile of the bulk polymer is sensitively dependent on the concentration of the dopant, while the response of the dopant is insensitive to concentration. Complete analysis of the NMRD profiles of these ICPs will give important information, including the rates of spin diffusion and of charge transport between dopant-rich domains. © 2005 Elsevier B.V. All rights reserved. Keywords: Nuclear magnetic resonance spectroscopy; Conduction; Dopant
1. Introduction The aim of this project is to use NMRD to study polymer dynamics and charge transport in the both the doped and undoped forms of the ICP polyaniline–TFSA. Nuclear magnetic resonance dispersion (NMRD) is a technique used to measure the longitudinal relaxation time (T1 ) as a function of NMR resonance frequency. NMRD can be used to study dynamics in condensed matter as T1 is determined by fluctuations in local fields. For normal polymers, these fluctuations usually arise due to molecular motions. So, the NMRD response of an undoped polymer is driven by chain motion and the NMRD profile (T1 versus frequency plot) should be a stretched-Lorentzian; the width of which is directly related to rate of molecular dynamics. In the case of intrinsically conducting polymers, ICP’s, charge carrier motion may also affect the polymer relaxation. Thus, it is our hypothesis that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. Through this work we hope to improve the
夽 Based on presentation at the International Conference on Synthetic Metals, Wollongong, Australia, June 28–July 2, 2004 (ICSM 2004). ∗ Corresponding author.
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.06.022
understanding of the charge transport mechanisms in this very important class of materials, specifically we are probing the interaction between the conducting (nano)domains and the bulk polymer. Polyaniline (PAni) was the first ICP to achieve worldwide commercial availability. It has achieved significant importance among conducting polymers due to its ease of processability, environmental stability and of course high conductivity. The ‘doped’ (protonated) form is green coloured (hence emeraldine) and has conductivity on a semiconductor level of the order of 10 S cm−1 (depending on dopant), many orders of magnitude higher than that of common polymers. The dopant used in this study, trifluoromethanesulfonic acid (TFSA) (Fig. 1), was chosen due to the fact its anion contains no protons to interfere with 1 H signal from the polymer backbone, while 19 F NMRD can be used to study the mobility of dopant. It is thought that the dopants are not homogeneously distributed through the polymer, but rather are concentrated in domains separated by regions of lower conductivity [1]. Thus, the charge transport mechanism is limited by the flow of charge between domains. The domains are relaxation sinks, linked at a molecular level to the entire polymer by the process of spin diffusion. Thus, the doped regions determine the NMRD response of the polymer.
E. Murray, D.F. Brougham / Synthetic Metals 155 (2005) 681–683
682
Fig. 1. Trifluoromethanesulfonic acid (TFSA).
2. Experimental 2.1. Sample synthesis Polyaniline emeraldine base (MW ∼ 65,000) was obtained from Aldrich chemicals. This fine blue powder was dispersed in a 0.7 M solution of trifluoromethanesulfonic acid in an inert atmosphere under vigorous stirring for 24 h at ambient temperature. The product was filtered, washed with the acid solution and with water and dried [2]. The synthesis was repeated using a 0.35 M solution of trifluoromethanesulfonic acid in order to obtain a sample of lower dopant concentration. Characterisation of the materials by fluorine determination and conductivity measurements is ongoing. 2.2. NMRD measurements Solid-state NMRD T1 data in the range from 10 kHz to 20 MHz were obtained using a Stelar Spinmaster-FFC 2000 Fast-Field Cycling NMR Relaxometer [3]. 3. Results and discussion 3.1. 1 H NMRD 3.1.1. Low dopant concentration At low dopant concentrations the conductivity is low and the observed NMRD response is typical for a solid polymer (Fig. 2); the curve is a stretched-Lorentzian (the width of which is directly related to rate of molecular dynamics). The response is weakly
Fig. 3. NMRD profile of high dopant concentration PAni–TFSA.
temperature dependent in the accessible temperature range; its shape is indicative of correlated chain motions. 3.1.2. High dopant concentration At moderate to high dopant concentrations, the NMRD response clearly demonstrates that polymer chain dynamics alone do not drive relaxation. The widths of the Lorentzians (Fig. 3) are observed to decrease with temperature, hence they cannot be simply related to a motional process; for which the rate will normally increase with temperature. Furthermore there is a change in shape of the profile between +24 and −20 ◦ C. Similar behaviour was observed for a polyaniline sample doped with phosphohexafluoric acid (HPF6 ). Clearly, a second relaxation mechanism is operating, we believe this is due to the conducting domains. The data clearly demonstrate that the degree of doping determines the NMRD response of the polymer. It is probable that the NMR properties of the bulk material are determined by coupling to the localised sinks by spin diffusion. 3.2.
14 N
quadrupolar dips
Whereas one might expect to see a gradual, smooth increase in T1 with field strength, with doped ICP samples this increase is often superimposed by three pronounced ‘dips’ where the T1 value drops by 10% or even more over a small range of field strengths. These are known as ‘quadrupole dips’ [4] and are found at well-defined field strengths (0.74 MHz, 1.56 MHz and 2.30 MHz) where the Nitrogen-14 nuclear quadrupole resonance (NQR) and the proton NMR frequencies are equal (Fig. 4). 3.3.
Fig. 2. NMRD profile of low dopant concentration PAni–TFSA.
19 F
NMRD
In order to probe dopant mobility, we measured the 19 F NMRD profile (Fig. 5). We discovered that the profile is independent of dopant concentration and could be fitted using the well-known Cole–Cole distribution [5,6] for spectral densities
E. Murray, D.F. Brougham / Synthetic Metals 155 (2005) 681–683
683
tumbling is on the expected timescale for molecular tumbling in a moderately restricted environment at this temperature. The δ value is indicative of a significant degree of correlation of the dopant reorientational motion. The fact that both curves can be fitted with the same parameters demonstrates that the anions are in very similar environments irrespective of the dopant level. There are two possible explanations for this; either the dopant is homogeneously distributed through the material and is relatively dilute, i.e. the anion environment is determined by the polymer backbone and partial de-doping does not affect the anion motional characteristics. Alternatively dopants may reside in domains of comparable size and dopant density, in which case the decrease in conductivity on de-doping is due to a decrease in the number of domains, with a resulting increase in the interdomain separation. This is consistent with the observations of domains from EFM microscopy [7], XRD and EPR [1] of other workers. Future work will involve measuring the temperature dependence of τ c , which will allow the determination of the barrier height of the to motion. We are also developing a model to fit the profiles of the polymer backbone.
Fig. 4. 1 H NMRD of PAni–TFSA at 24 ◦ C showing 14 N quadrupolar dips.
4. Conclusions 1 H NMRD can be used to probe the magnetic coupling of the polymer backbone of ICPs to dopant-rich domains, which act as NMR relaxation sinks. The coupling processes are interesting as they are related to the mechanisms of conduction in these materials. 19 F NMRD data suggest that conductivity is limited by the extent of inter-domain communication.
Acknowledgments Fig. 5. 19 F NMRD profile, at 24 ◦ C, of both high and of low dopant concentration PAni–TFSA fitted with the Cole–Cole distribution model.
(Eqs. (1) and (2)). 1 1 = Cdip 6 (aJω + bJ2ω ) T1 r 2 Jcc (ω, τc , δ) = sin ω
δπ 2
(1)
(ωτc )δ × 1 + (ωτc )2δ + {2 cos(δπ/2)}(ωτc )δ
EM gratefully acknowledges the financial support of the Irish Research Council for Science, Engineering and Technology Embark scholarship funded by the National Development Plan. DB acknowledges Enterprise Ireland Research Innovation Fund and the Higher Education Authority for the Republic of Ireland for support in equipment purchase. The authors would like to thank Gordon Wallace and Peter Innis of the Intelligent Polymer Research Institute, University of Wollongong for their helpful comments and insights. References
(2)
where Cdip is dipole–dipole constant, Jcc the spectral density, ω the angular frequency (rad/s), τ c the correlation time and δ is the degree of correlated motion and distribution of correlation times. Using the Cole–Cole distribution model to fit the 19 F data of PAni–TFSA, we found a correlation time, τ c , of 3.69(8) × 10−8 s and a value of 0.386(8) for δ. A τ c of 37 ns for TFSA anion
[1] J. Too, S.M. Long, Phys. Rev. B: Solid State 57 (1998) 9567. [2] P.K. Kahol, K.K. Satheesh Kumarb, S. Geetha, D.C. Trivedi, Synth. Met. 139 (2003) 191. [3] E. Anoardo, G. Galli, G. Ferrante, Appl. Magn. Reson. 20 (2001) 365. [4] E. Anoardo, D.J. Puisol, Phys. Rev. Lett. 76 (1996) 3983. [5] P.A. Beckmann, Phys. Rep. 171 (3) (1988) 85. [6] K.S. Cole, R.H. Cole, J. Chem. Phys. 19 (1951) 1484. [7] J.N. Barisci, R. Stella, G.M. Spinks, G.G. Wallace, Synth. Met. 124 (2001) 407.