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Structural properties of conducting and semiconducting polymers
ELSEVIER
Physica B 234-236 (1997) 242-244
Structuralproperties of conducting and semiconducting polymers A. G a l a t a n u a' *, M.I. C h i p a r a a, M . D . C h i p a r a b, M. T o a c s e n a aNational Institute of Material Physics, Magurele, Bucharest, PO Box MG-6, R-76900 Romania blnstitute for Electrotechnical Researches, T. Vladimirescu Av., Bucharest, Romania
Abstract Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.
Keywords:Polymers; Composite materials; Semiconductors
I. Introduction The electrical conduction in macromolecular systems is an interesting research field, from both theoretical and experimental points of view. However, the polymers which exhibit conducting or semiconducting properties have either acceptable mechanical and electrical characteristics but low thermal stability (polyacetylenes) or a high thermooxidative stability, coupled with a high conductivity but poor mechanical features and low processability (polyanilines). To balance between mechanical properties, electrical characteristics and thermooxidative stability, we have focused our attention on polyaniline-based composites. Polyaniline is a polymer with an outstanding thermal stability, in air, which has electrical features ranging from an insulator to a metallic state, depending on the oxidation state and dopant (nature and concentration) [1].
2. Experimental methods and results Composites of polyethylene, containing various amounts ofpolyaniline (5%, 10%, 15%, 20% and 40% * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S092 1 - 4 5 2 6 ( 9 6 ) 0 0 9 2 5 - 8
in weight) have been investigated by electron spin resonance spectroscopy (ESR), using a JES-ME-3X spectrometer, operating in X band. The temperature dependence of resonance spectra in the range 170 K; 300 K has been studied using a JES-VT-3X, variable temperature accessory. To estimate accurately the g value, a nuclear magnetic resonance based gaussmeter and a frequency converter were used. The ESR spectrum of the composites polyethylene-polyaniline (PE-PANI) is a narrow single line, located closed to the free electron g value (g = 2.0023). Crude polyethylene presents no ESR signal at this position, whereas the pristine polyaniline has a resonance spectrum analogous to that recorded in PE-PANI. Accordingly, the resonance spectrum of PE-PAN1 is due solely to polyaniline. As may be noticed from Fig. 1, the peak to peak line width Hpp, ranges between 15 and 27 Gs, depending on the amount of polyaniline and on the temperature. The resonance line asymmetry is low, suggesting that the uncoupled electronic spins are not located in metallic islands, as has been suggested by some authors [2]. We have tentatively fitted the temperature dependence of Hpp, in the range 170-250K with an Arrhenius like equation, Hpp =H~p°) exp-EA/RT. The correlation coefficient is higher than 0.98, supporting an Arrhenius-like dependence of Hpp on
243
A. Galatanu et aL / Physica B 234-236 (1997) 242-244
It
27.5 -
.l
22.5-
•
v
!.io •" O o O
1.0 •
O
•l I " I 25.0 -
•
"1
•
O
',El ~ \
•
o o o
A •
[]
5% PANI - - -
O •
20% PANI - 40% PANI
0.9
O
,~ [] ~ ' < " o A X []
\~", ~'~q >
0.8-
~. 20.07-
• "~
•
"~., .. r~--
~': V
A
--
5% PANI
~7 •
...... .....
15% PANI 20% PANI
• 275
40% PANI , 300
0.7.
17.5"
.....'v
15.0-
O i 175
i 200
i 225
250
TEMPERATURE
"~. ~
0.6
160
( K )
i 180
i 200
i 220
~. '
ii 240
TEMPERATURE
Fig. 1. The temperaturedependenceof Hpp in PAPE composites.
A
2000,
70
60 1600.
~7 A
J
"o
.--t
V zk
Lk
m.
5o ~
2 4o
1200.
$
30
~ 4_.
800 PANi ( % )
Fig. 2. The dependence of the activation energy and pre-exponential factor on the weight of polyaniline in polyethylene. temperature. The estimated activation energies, EA, varies between 950 and 1950 MJ/kmol, depending on the amount of polyaniline introduced in the system. The dependence of EA and Hp~p °) on the polyaniline content is given in Fig. 2. A minimum value of EA has been noticed for 20% of polyaniline introduced in polyethylene. As the temperature dependence of Hop may be related to the dimensionality of the conduction process, we have also tested a polynomial-like temperature dependence, Hpp Ao + A 1 T + A z T 2. The correlation coefficients for such a dependence are ranging from 0.95 to 0.975. The term A1 is ranging from 0.18 to 0.64 K whereas A2 is ranging from -0.001 to -0.00032 K 2. The weak contribution of A2 indicates a three-dimensional effect superimposed on =
•
~
o i 260
• •
i 280
1 300
( K )
Fig. 3. The temperaturedependenceof the resonance line double integral. the usual one-dimensional conductivity of polyaniline [1-3]. This behaviour may be related to the random distribution of polyaniline particles within polyethylene. The double integral of the resonance spectrum S has been estimated using the relation S = KIH2p/A, where K is the line shape factor, I is the resonance line amplitude and A the gain of the spectrometer. The temperature dependence of S (which is proportional to the static susceptibility of the sample) neither obeys an Arrhenius-like law nor a Curie -Weiss equation. The presence of a maximum in the temperature dependence of S may indicate a triplet state, specified by a total spin S = 1 (Fig. 3). However, we have failed to record any signal at 9 = 4.0. It is important to notice that such a signal should be within the experimental errors as the transition associated to the 9 = 4 line has a weak intensity (about 1/104 of the g = 2 line intensity).
3. Conclusions
Blends of polyaniline with various polymers (polyethyleneterephtalate-PET [3], polymethylmethacrylate-PMMA [4], ...) have been studied. We have investigated blends of polyaniline with polyethylene. In the case of PE-PANI composites, although the conductivity increases as the concentration of polyaniline is increased, the transition towards a metallic con-
244
A. Galatanu et al./ Physica B 234-236 (1997) 242-244
duction has not been noticed. These data agree with ESR data which consists of narrow single line, of low asymmetry, typical for defects localised in the energy gap. As happens with the blend with PMMA, the temperature dependence of spin susceptibility did not obey a Curie-Weiss dependence [5], suggesting a triplet state or severe exchange interactions. The peak-to-peak line width increases as the sample temperature is increased, followed by a weak line narrowing in the high-temperature range, due to the motional contributions.
References [1] F. Lux, Polymer 34 (1994) 2915. [2] Z. Wang, A. Ray, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B 43 (1991) 4373. [3] R. Pelster, G.Nimtz and B. Wessling, Phys. Rev. 49 (1994) 12718. [4] C.O. Yoon, M. Reghu, D. Moses, A.J. Heeger and Y. Cao, Synthetic Metals 63 (1994) 47. [5] N.S. SarieiRei, A.J. Heeger and Y.Cao, Phys. Rev. B 49 (1994) 5988.
Physica B 234-236 (1997) 242-244
Structuralproperties of conducting and semiconducting polymers A. G a l a t a n u a' *, M.I. C h i p a r a a, M . D . C h i p a r a b, M. T o a c s e n a aNational Institute of Material Physics, Magurele, Bucharest, PO Box MG-6, R-76900 Romania blnstitute for Electrotechnical Researches, T. Vladimirescu Av., Bucharest, Romania
Abstract Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.
Keywords:Polymers; Composite materials; Semiconductors
I. Introduction The electrical conduction in macromolecular systems is an interesting research field, from both theoretical and experimental points of view. However, the polymers which exhibit conducting or semiconducting properties have either acceptable mechanical and electrical characteristics but low thermal stability (polyacetylenes) or a high thermooxidative stability, coupled with a high conductivity but poor mechanical features and low processability (polyanilines). To balance between mechanical properties, electrical characteristics and thermooxidative stability, we have focused our attention on polyaniline-based composites. Polyaniline is a polymer with an outstanding thermal stability, in air, which has electrical features ranging from an insulator to a metallic state, depending on the oxidation state and dopant (nature and concentration) [1].
2. Experimental methods and results Composites of polyethylene, containing various amounts ofpolyaniline (5%, 10%, 15%, 20% and 40% * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S092 1 - 4 5 2 6 ( 9 6 ) 0 0 9 2 5 - 8
in weight) have been investigated by electron spin resonance spectroscopy (ESR), using a JES-ME-3X spectrometer, operating in X band. The temperature dependence of resonance spectra in the range 170 K; 300 K has been studied using a JES-VT-3X, variable temperature accessory. To estimate accurately the g value, a nuclear magnetic resonance based gaussmeter and a frequency converter were used. The ESR spectrum of the composites polyethylene-polyaniline (PE-PANI) is a narrow single line, located closed to the free electron g value (g = 2.0023). Crude polyethylene presents no ESR signal at this position, whereas the pristine polyaniline has a resonance spectrum analogous to that recorded in PE-PANI. Accordingly, the resonance spectrum of PE-PAN1 is due solely to polyaniline. As may be noticed from Fig. 1, the peak to peak line width Hpp, ranges between 15 and 27 Gs, depending on the amount of polyaniline and on the temperature. The resonance line asymmetry is low, suggesting that the uncoupled electronic spins are not located in metallic islands, as has been suggested by some authors [2]. We have tentatively fitted the temperature dependence of Hpp, in the range 170-250K with an Arrhenius like equation, Hpp =H~p°) exp-EA/RT. The correlation coefficient is higher than 0.98, supporting an Arrhenius-like dependence of Hpp on
243
A. Galatanu et aL / Physica B 234-236 (1997) 242-244
It
27.5 -
.l
22.5-
•
v
!.io •" O o O
1.0 •
O
•l I " I 25.0 -
•
"1
•
O
',El ~ \
•
o o o
A •
[]
5% PANI - - -
O •
20% PANI - 40% PANI
0.9
O
,~ [] ~ ' < " o A X []
\~", ~'~q >
0.8-
~. 20.07-
• "~
•
"~., .. r~--
~': V
A
--
5% PANI
~7 •
...... .....
15% PANI 20% PANI
• 275
40% PANI , 300
0.7.
17.5"
.....'v
15.0-
O i 175
i 200
i 225
250
TEMPERATURE
"~. ~
0.6
160
( K )
i 180
i 200
i 220
~. '
ii 240
TEMPERATURE
Fig. 1. The temperaturedependenceof Hpp in PAPE composites.
A
2000,
70
60 1600.
~7 A
J
"o
.--t
V zk
Lk
m.
5o ~
2 4o
1200.
$
30
~ 4_.
800 PANi ( % )
Fig. 2. The dependence of the activation energy and pre-exponential factor on the weight of polyaniline in polyethylene. temperature. The estimated activation energies, EA, varies between 950 and 1950 MJ/kmol, depending on the amount of polyaniline introduced in the system. The dependence of EA and Hp~p °) on the polyaniline content is given in Fig. 2. A minimum value of EA has been noticed for 20% of polyaniline introduced in polyethylene. As the temperature dependence of Hop may be related to the dimensionality of the conduction process, we have also tested a polynomial-like temperature dependence, Hpp Ao + A 1 T + A z T 2. The correlation coefficients for such a dependence are ranging from 0.95 to 0.975. The term A1 is ranging from 0.18 to 0.64 K whereas A2 is ranging from -0.001 to -0.00032 K 2. The weak contribution of A2 indicates a three-dimensional effect superimposed on =
•
~
o i 260
• •
i 280
1 300
( K )
Fig. 3. The temperaturedependenceof the resonance line double integral. the usual one-dimensional conductivity of polyaniline [1-3]. This behaviour may be related to the random distribution of polyaniline particles within polyethylene. The double integral of the resonance spectrum S has been estimated using the relation S = KIH2p/A, where K is the line shape factor, I is the resonance line amplitude and A the gain of the spectrometer. The temperature dependence of S (which is proportional to the static susceptibility of the sample) neither obeys an Arrhenius-like law nor a Curie -Weiss equation. The presence of a maximum in the temperature dependence of S may indicate a triplet state, specified by a total spin S = 1 (Fig. 3). However, we have failed to record any signal at 9 = 4.0. It is important to notice that such a signal should be within the experimental errors as the transition associated to the 9 = 4 line has a weak intensity (about 1/104 of the g = 2 line intensity).
3. Conclusions
Blends of polyaniline with various polymers (polyethyleneterephtalate-PET [3], polymethylmethacrylate-PMMA [4], ...) have been studied. We have investigated blends of polyaniline with polyethylene. In the case of PE-PANI composites, although the conductivity increases as the concentration of polyaniline is increased, the transition towards a metallic con-
244
A. Galatanu et al./ Physica B 234-236 (1997) 242-244
duction has not been noticed. These data agree with ESR data which consists of narrow single line, of low asymmetry, typical for defects localised in the energy gap. As happens with the blend with PMMA, the temperature dependence of spin susceptibility did not obey a Curie-Weiss dependence [5], suggesting a triplet state or severe exchange interactions. The peak-to-peak line width increases as the sample temperature is increased, followed by a weak line narrowing in the high-temperature range, due to the motional contributions.
References [1] F. Lux, Polymer 34 (1994) 2915. [2] Z. Wang, A. Ray, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B 43 (1991) 4373. [3] R. Pelster, G.Nimtz and B. Wessling, Phys. Rev. 49 (1994) 12718. [4] C.O. Yoon, M. Reghu, D. Moses, A.J. Heeger and Y. Cao, Synthetic Metals 63 (1994) 47. [5] N.S. SarieiRei, A.J. Heeger and Y.Cao, Phys. Rev. B 49 (1994) 5988.