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Electron paramagnetic resonance investigations of electrospun polyaniline fibers
Solid State Communications 124 (2002) 195–197 www.elsevier.com/locate/ssc
Electron paramagnetic resonance investigations of electrospun polyaniline fibers P.K. Kahola,*, N.J. Pintob a Department of Physics, Wichita State University, Wichita, KS 67260-0032, USA Department of Physics and Electronics, University of Puerto Rico, Humacao, PR 00791, USA
b
Received 2 July 2002; received in revised form 27 August 2002; accepted 2 September 2002 by J. Kuhl
Abstract A comparative study of the EPR magnetic susceptibility behavior of the camphorsulfonic acid-doped polyaniline (PANCSA) blends with polyethylene oxide (PEO) is reported. Compared to films, the fibers are found to exhibit increased Pauli susceptibility, more Lorentzian character in the EPR lineshape, and smaller EPR linewidth. These changes are interpreted as due to increased chain alignment in the fibers compared with the cast films. Electron localization effects in true nanofibers (diameter , 100 nm) of organic polymers can therefore be studied using the technique of electron paramagnetic resonance. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 72.80.Le; 75.40.Cx; 75.30.Cr; 76.30.Pk Keywords: A. Polymers; D. Electronic transport
1. Introduction Fabrication of nanofibers of conducting electronic polymers has recently been demonstrated [1 – 3]. Specifically, nanofibers of polyaniline doped fully with camphorsulfonic acid (PANCSA) have been obtained as a blend in polyethylene oxide (PEO—molecular weight 900,000) through the method of ‘electrospinning’ [2]. We will abbreviate PANCSA – PEO blends drawn from chloroform as (PANCSA)x(PEO)12x, where x refers to the amount of PANCSA in the blend by weight. On a (PANCSA)0.72(PEO)0.28 fiber with a diameter of 1320 nm, the room temperature dc conductivity was obtained as 33 S/cm [3], which is larger by a factor of 100 compared with the conductivity of a film of pure PANCSA cast from the same solvent chloroform [3]. This result implies that electrospinning in high electric fields perhaps leads to significant chain alignment in the polymer blend. In this paper, we make a comparative study of the EPR magnetic susceptibility behavior of the (PANCSA)0.72(PEO)0.28 nanofibers (to be * Corresponding author. Fax: þ 1-316-978-3350. E-mail address: [email protected] (P.K. Kahol).
abbreviated as PAPEOFIB) and cast films of PANCSA– PEO of the same composition as above (to be abbreviated as PAPEOFIL), with the underlying objective of exploring mesoscopic structural disorder and finding an interpretation for the observed behavior.
2. Experimental The details of the electrospinning technique for producing fibers have appeared in the literature in a number of papers [1,2]. The fibers were collected from the aluminum electrode and put into an EPR tube. The mass of the thus produced sample, PANCSA)0.72(PEO)0.28, for EPR measurements was 0.85 mg. Most of the fibers used in this study were found to have a diameter in the range 200– 600 nm. The EPR experiments were performed on a computer controlled Bruker EMX 6/1 spectrometer with necessary attachments (Oxford cryostat and ITC503 control) for studies down to helium temperatures. Low microwave power (0.2 mW) and small magnetic field modulation
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 4 9 2 - 1
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P.K. Kahol, N.J. Pinto / Solid State Communications 124 (2002) 195–197
Fig. 1. Temperature dependence of xT for polyaniline–PEO fibers (closed circles) and polyaniline–PEO films (open circles).
(0.3 G) were used to avoid saturation and lineshape distortion effects. Following EPR measurement, the derivative intensity data were twice integrated. The EPR magnetic susceptibility was obtained from the above data by dividing it with the double integral of the derivative intensity of a K3CrO8 standard, both in the solid and the liquid forms. The values so obtained were normalized to a two-ring repeat unit.
3. Results and discussion The room temperature conductivity of CSA doped polyaniline – PEO films in chloroform (PAPEOFIL) was measured in our laboratory to be 1 £ 1023 S/cm. As expected, this value is smaller than the value of , 0.1 S/cm obtained on a spun film of pure polyaniline doped with CSA [3]. We, however, have no explanation for
Fig. 2. EPR lineshape analysis at room temperature for polyaniline– PEO fibers (closed circles) and polyaniline–PEO films (open circles).
the values of room temperature conductivity for the polyaniline– PEO cast films, reported in Ref. [2], that are at least two orders of magnitude larger than our values including the one at x ¼ 0.72. On the other hand, we measured the room temperature conductivity of CSA doped polyaniline– PEO fibers (PAPEOFIB) to be in the range (1– 2) £ 1021 S/cm, which is much larger than the corresponding measured value for PAPEOFIL ( ¼ 1 £ 1023 S/cm). A similar value for the room temperature conductivity was reported in Ref. [2] for the fiber at x ¼ 0.72. To understand the observed difference between the room temperature conductivities of PAPEOFIL and PAPEOFIB, we report below our EPR investigations of these samples. The results for the EPR magnetic susceptibility for the two samples, namely PAPEOFIB (closed circles) and PAPEOFIL (open circles), are shown in Fig. 1 in the form of a xT versus T plot. The solid lines are least-squares fit of the data, from room temperature to the onset of nonlinear behavior, to the equation xT ¼ C þ xP T; where C ¼ ðN m2B =kB that is related to the number of spins per tworings (n ) as n ¼ 8C/3. The values of n obtained from such fits are: 0.024 for PAPEOFIB and 0.035 for PAPEOFIL. While the error in the absolute values of measured x is close to 5%, it is only of the order of 2% in the relative x values. An increased value of xP ¼ 49 £ 1026 emu/(mole tworings) for PAPEOFIB, compared with a value of xP ¼ 41 £ 1026 emu/(mole two-rings) for PAPEOFIL, is indicative of the presence of better structural order in PAPEOFIB compared with PAPEOFIL. Note that the number of Curie spins is 1.2 and 1.8 per 100 rings for PAPEOFIB and PAPEOFIL, respectively, and the origin of these spins is most likely in the amorphous regions. Since it is well known that electron states are increasingly localized with increasing disorder, we show the EPR line shapes for PAPEOFIB (solid circles) and PAPEOFIL (open circles) at room temperature in Fig. 2. The plot shows the reciprocal of the normalized EPR intensity as a function of the square of the static magnetic field (relative to the center of the EPR line) divided by the half-power linewidth [4]. While the continuous line curve in Fig. 2 corresponds to Lorentzian lineshape, the broken line curve represents lineshape due to one-dimensional motion [4]. Comparison between the lineshapes of PAPEOFIL and PAPEOFIB shows that the lineshape is slightly more Lorentzian-like in the case of polyaniline – PEO fibers compared to polyaniline – PEO films. Since the lineshape changes from one-dimensional type to three-dimensional type as the inter-chain diffusion rate becomes comparable with the intra-chain diffusion rate (or the EPR linewidth), the results shown in Fig. 2 indicate the existence of increased threedimensional electron delocalization in the case of fibers. We would like to remark that the EPR lineshape analysis in the present case is not unique. Since we were not able to fit the EPR lineshape with a simple sum of Lorentzian and Gaussian components, we decided to present the results in the framework of the model by Hennessy et al. [5]. This
P.K. Kahol, N.J. Pinto / Solid State Communications 124 (2002) 195–197
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Increased Pauli susceptibility, narrower EPR line, and increased Lorentzian character in the case of (PANCSA) 0.72(PEO)0.28 fibers compared with (PANCSA)0.72(PEO)0.28 films suggest increased chain alignment in the ordered regions of the polymer. While the Pauli susceptibility has increased somewhat, two orders of magnitude increase in conductivity is therefore speculated to be primarily due to increased structural order in the disordered regions that surround the ordered regions. We would like to stress that what our EPR data really suggest is increased effective inter-chain diffusion rate and hence increased three-dimensional delocalization. Fig. 3. Dependence of the EPR peak-to-peak linewidth on temperature for polyaniline–PEO fibers (open circles) and polyaniline–PEO films (closed circles).
model deals with spin motion in a homogeneous spin system. Although polyaniline is basically an inhomogeneous system, the presence of relatively fewer number of Curie spins affords analysis in terms of intra-chain and inter-chain diffusion rates for the entire system. Krinichny et al. [6] have recently reported an elegant EPR lineshape analysis of H2SO4-doped polyaniline in terms of Lorentzian and Gaussian components. The g values of PAPEOFIL and PAPEOFIB are 2.0032, and they stay more or less temperature independent down to helium temperatures. These values also indicate electron localization over a few carbon atoms of the rings, since the g-value of an electron near a nitrogen – hydrogen bond is 2.0054 and that of an electron near a carbon –hydrogen bond is 2.0031. The observed value is closer to 2.0034, which is the mean of the g-values of six carbons and one nitrogen. Fig. 3 shows a plot of the EPR peak-to-peak linewidth as a function of temperature for PAPEOFIL (closed circles) and PAPEOFIB (open circles). At all temperatures down to about 5 K, decrease in temperature leads to an increase in the linewidth, which is due to increased electron localization that leads to reduced motional narrowing.
4. Conclusions An EPR study on fibers of polyaniline blends has been shown to reflect mesoscopic structural/electronic changes induced by electrospinning. It is therefore suggested that the EPR technique can be applied to investigate electron localization effects in true nanofibers (diameter ,100 nm) of organic polymers.
References [1] D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216–223. [2] I.D. Norris, M.M. Shakar, F.K. Ko, A.G. MacDiarmid, Synth. Met. 114 (2000) 2. [3] A.G. MacDiarmid, W.E. Jones, I.D. Norris, J. Gao, A.T. Johnson, N.J. Pinto, J. Hone, B. Han, F.K. Ho, H. Okuzaki, M. Llaguno, Synth. Met. 119 (2001) 27 –30. [4] N.J. Pinto, P.K. Kahol, B.J. McCormick, N.S. Dalal, H. Wan, Phys. Rev. B 49 (1994) 13983. [5] M.J. Hennessy, C.D. McElwee, P.M. Richards, Phys. Rev. B 7 (1973) 930. [6] V.I. Krinichnyi, H.-K. Roth, G. Hinrichsen, F. Lux, K. Lueders, Phys. Rev. B 65 (2002) 155205.
Electron paramagnetic resonance investigations of electrospun polyaniline fibers P.K. Kahola,*, N.J. Pintob a Department of Physics, Wichita State University, Wichita, KS 67260-0032, USA Department of Physics and Electronics, University of Puerto Rico, Humacao, PR 00791, USA
b
Received 2 July 2002; received in revised form 27 August 2002; accepted 2 September 2002 by J. Kuhl
Abstract A comparative study of the EPR magnetic susceptibility behavior of the camphorsulfonic acid-doped polyaniline (PANCSA) blends with polyethylene oxide (PEO) is reported. Compared to films, the fibers are found to exhibit increased Pauli susceptibility, more Lorentzian character in the EPR lineshape, and smaller EPR linewidth. These changes are interpreted as due to increased chain alignment in the fibers compared with the cast films. Electron localization effects in true nanofibers (diameter , 100 nm) of organic polymers can therefore be studied using the technique of electron paramagnetic resonance. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 72.80.Le; 75.40.Cx; 75.30.Cr; 76.30.Pk Keywords: A. Polymers; D. Electronic transport
1. Introduction Fabrication of nanofibers of conducting electronic polymers has recently been demonstrated [1 – 3]. Specifically, nanofibers of polyaniline doped fully with camphorsulfonic acid (PANCSA) have been obtained as a blend in polyethylene oxide (PEO—molecular weight 900,000) through the method of ‘electrospinning’ [2]. We will abbreviate PANCSA – PEO blends drawn from chloroform as (PANCSA)x(PEO)12x, where x refers to the amount of PANCSA in the blend by weight. On a (PANCSA)0.72(PEO)0.28 fiber with a diameter of 1320 nm, the room temperature dc conductivity was obtained as 33 S/cm [3], which is larger by a factor of 100 compared with the conductivity of a film of pure PANCSA cast from the same solvent chloroform [3]. This result implies that electrospinning in high electric fields perhaps leads to significant chain alignment in the polymer blend. In this paper, we make a comparative study of the EPR magnetic susceptibility behavior of the (PANCSA)0.72(PEO)0.28 nanofibers (to be * Corresponding author. Fax: þ 1-316-978-3350. E-mail address: [email protected] (P.K. Kahol).
abbreviated as PAPEOFIB) and cast films of PANCSA– PEO of the same composition as above (to be abbreviated as PAPEOFIL), with the underlying objective of exploring mesoscopic structural disorder and finding an interpretation for the observed behavior.
2. Experimental The details of the electrospinning technique for producing fibers have appeared in the literature in a number of papers [1,2]. The fibers were collected from the aluminum electrode and put into an EPR tube. The mass of the thus produced sample, PANCSA)0.72(PEO)0.28, for EPR measurements was 0.85 mg. Most of the fibers used in this study were found to have a diameter in the range 200– 600 nm. The EPR experiments were performed on a computer controlled Bruker EMX 6/1 spectrometer with necessary attachments (Oxford cryostat and ITC503 control) for studies down to helium temperatures. Low microwave power (0.2 mW) and small magnetic field modulation
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 4 9 2 - 1
196
P.K. Kahol, N.J. Pinto / Solid State Communications 124 (2002) 195–197
Fig. 1. Temperature dependence of xT for polyaniline–PEO fibers (closed circles) and polyaniline–PEO films (open circles).
(0.3 G) were used to avoid saturation and lineshape distortion effects. Following EPR measurement, the derivative intensity data were twice integrated. The EPR magnetic susceptibility was obtained from the above data by dividing it with the double integral of the derivative intensity of a K3CrO8 standard, both in the solid and the liquid forms. The values so obtained were normalized to a two-ring repeat unit.
3. Results and discussion The room temperature conductivity of CSA doped polyaniline – PEO films in chloroform (PAPEOFIL) was measured in our laboratory to be 1 £ 1023 S/cm. As expected, this value is smaller than the value of , 0.1 S/cm obtained on a spun film of pure polyaniline doped with CSA [3]. We, however, have no explanation for
Fig. 2. EPR lineshape analysis at room temperature for polyaniline– PEO fibers (closed circles) and polyaniline–PEO films (open circles).
the values of room temperature conductivity for the polyaniline– PEO cast films, reported in Ref. [2], that are at least two orders of magnitude larger than our values including the one at x ¼ 0.72. On the other hand, we measured the room temperature conductivity of CSA doped polyaniline– PEO fibers (PAPEOFIB) to be in the range (1– 2) £ 1021 S/cm, which is much larger than the corresponding measured value for PAPEOFIL ( ¼ 1 £ 1023 S/cm). A similar value for the room temperature conductivity was reported in Ref. [2] for the fiber at x ¼ 0.72. To understand the observed difference between the room temperature conductivities of PAPEOFIL and PAPEOFIB, we report below our EPR investigations of these samples. The results for the EPR magnetic susceptibility for the two samples, namely PAPEOFIB (closed circles) and PAPEOFIL (open circles), are shown in Fig. 1 in the form of a xT versus T plot. The solid lines are least-squares fit of the data, from room temperature to the onset of nonlinear behavior, to the equation xT ¼ C þ xP T; where C ¼ ðN m2B =kB that is related to the number of spins per tworings (n ) as n ¼ 8C/3. The values of n obtained from such fits are: 0.024 for PAPEOFIB and 0.035 for PAPEOFIL. While the error in the absolute values of measured x is close to 5%, it is only of the order of 2% in the relative x values. An increased value of xP ¼ 49 £ 1026 emu/(mole tworings) for PAPEOFIB, compared with a value of xP ¼ 41 £ 1026 emu/(mole two-rings) for PAPEOFIL, is indicative of the presence of better structural order in PAPEOFIB compared with PAPEOFIL. Note that the number of Curie spins is 1.2 and 1.8 per 100 rings for PAPEOFIB and PAPEOFIL, respectively, and the origin of these spins is most likely in the amorphous regions. Since it is well known that electron states are increasingly localized with increasing disorder, we show the EPR line shapes for PAPEOFIB (solid circles) and PAPEOFIL (open circles) at room temperature in Fig. 2. The plot shows the reciprocal of the normalized EPR intensity as a function of the square of the static magnetic field (relative to the center of the EPR line) divided by the half-power linewidth [4]. While the continuous line curve in Fig. 2 corresponds to Lorentzian lineshape, the broken line curve represents lineshape due to one-dimensional motion [4]. Comparison between the lineshapes of PAPEOFIL and PAPEOFIB shows that the lineshape is slightly more Lorentzian-like in the case of polyaniline – PEO fibers compared to polyaniline – PEO films. Since the lineshape changes from one-dimensional type to three-dimensional type as the inter-chain diffusion rate becomes comparable with the intra-chain diffusion rate (or the EPR linewidth), the results shown in Fig. 2 indicate the existence of increased threedimensional electron delocalization in the case of fibers. We would like to remark that the EPR lineshape analysis in the present case is not unique. Since we were not able to fit the EPR lineshape with a simple sum of Lorentzian and Gaussian components, we decided to present the results in the framework of the model by Hennessy et al. [5]. This
P.K. Kahol, N.J. Pinto / Solid State Communications 124 (2002) 195–197
197
Increased Pauli susceptibility, narrower EPR line, and increased Lorentzian character in the case of (PANCSA) 0.72(PEO)0.28 fibers compared with (PANCSA)0.72(PEO)0.28 films suggest increased chain alignment in the ordered regions of the polymer. While the Pauli susceptibility has increased somewhat, two orders of magnitude increase in conductivity is therefore speculated to be primarily due to increased structural order in the disordered regions that surround the ordered regions. We would like to stress that what our EPR data really suggest is increased effective inter-chain diffusion rate and hence increased three-dimensional delocalization. Fig. 3. Dependence of the EPR peak-to-peak linewidth on temperature for polyaniline–PEO fibers (open circles) and polyaniline–PEO films (closed circles).
model deals with spin motion in a homogeneous spin system. Although polyaniline is basically an inhomogeneous system, the presence of relatively fewer number of Curie spins affords analysis in terms of intra-chain and inter-chain diffusion rates for the entire system. Krinichny et al. [6] have recently reported an elegant EPR lineshape analysis of H2SO4-doped polyaniline in terms of Lorentzian and Gaussian components. The g values of PAPEOFIL and PAPEOFIB are 2.0032, and they stay more or less temperature independent down to helium temperatures. These values also indicate electron localization over a few carbon atoms of the rings, since the g-value of an electron near a nitrogen – hydrogen bond is 2.0054 and that of an electron near a carbon –hydrogen bond is 2.0031. The observed value is closer to 2.0034, which is the mean of the g-values of six carbons and one nitrogen. Fig. 3 shows a plot of the EPR peak-to-peak linewidth as a function of temperature for PAPEOFIL (closed circles) and PAPEOFIB (open circles). At all temperatures down to about 5 K, decrease in temperature leads to an increase in the linewidth, which is due to increased electron localization that leads to reduced motional narrowing.
4. Conclusions An EPR study on fibers of polyaniline blends has been shown to reflect mesoscopic structural/electronic changes induced by electrospinning. It is therefore suggested that the EPR technique can be applied to investigate electron localization effects in true nanofibers (diameter ,100 nm) of organic polymers.
References [1] D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216–223. [2] I.D. Norris, M.M. Shakar, F.K. Ko, A.G. MacDiarmid, Synth. Met. 114 (2000) 2. [3] A.G. MacDiarmid, W.E. Jones, I.D. Norris, J. Gao, A.T. Johnson, N.J. Pinto, J. Hone, B. Han, F.K. Ho, H. Okuzaki, M. Llaguno, Synth. Met. 119 (2001) 27 –30. [4] N.J. Pinto, P.K. Kahol, B.J. McCormick, N.S. Dalal, H. Wan, Phys. Rev. B 49 (1994) 13983. [5] M.J. Hennessy, C.D. McElwee, P.M. Richards, Phys. Rev. B 7 (1973) 930. [6] V.I. Krinichnyi, H.-K. Roth, G. Hinrichsen, F. Lux, K. Lueders, Phys. Rev. B 65 (2002) 155205.