- Email: [email protected]
Synthesis of polyanilines with high electrical conductivity
S¥1iilTIHIITIIC M|TRLS ELSEVIER
Synthetic Metals 72 (1995) 135-140
Synthesis of polyanilines with high electrical conductivity Giulio Boara a, Massimo Sparpaglione b,, Enichem S.p./t., via F. Maritano 26, 20097 San Donato Milanese, Italy h Enichem S.p.A., via G. Fauser 4, 28100 Novara, Italy Received 2 June 1994; accepted 10 November 1994
Abstract
A procedure for the chemical synthesis of conductive polyanilines is presented. The effect of the relative concentration of reactants on the material properties is studied. The synthesis is realized with hydrochloric or phosphoric acids. It is shown that it is possible to perform the oxidative polymerization condensation at temperatures up to 99 °C and that its control can give rise to polyanilines with high conductivity. The optimum temperature depends on the acid used and is, for the cases considered, higher than the one used in the procedures reported so far. In particular, pellets of polyanilines synthesized with H3PO4 are produced with conductivities up to 58 S/cm. Keywords: Synthesis; Polyaniline; Conductivity
I. Introduction
Materials produced by the oxidative polymerization of aniline, hereinafter polyaniline, have received much attention in the last few years, mainly because of their electrical properties [1-9]. The bulk conductivity, ~r, of the emeraldine base of the polyaniline family can be varied from 10-10 up to 5-20 S/cm when, upon treatment with acids, protons are added to a fraction of the unprot0nated nitrogen sites [2-4]. Polyaniline is an interesting conducting polymer even if it has a conductivity up to three orders of magnitude lower than polyacetylene [10] because of its lower cost and higher environmental stability [11] which can make it preferable for practical applications. Polyanilines are also interesting compounds from the point of view of the fundamental aspects of the mechanism of the charge transport in conducting polymers [5-7]. The conductivity along the polyaniline backbone can be affected by the degree of oxidation of the polymer (the relative amount of amine and imine groups), the protonation acid [1,12] and the percentage of protonation [1,3,6]. The conductivity is also affected by the degree of water content [4,13,14], which presumably can affect the charge transport between chains, and also by the morphology and texture of the polymer * Corresponding author. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A, All rights reserved SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 3 3 7 - X
[15,16], its chain length [16] and degree of crystallization [1,17,18]. The insulating form of polyaniline was discovered more than 100 years ago and its synthesis is relatively simple [1,19]. Studies have been carried out to understand the relationship between the conditions of synthesis, both chemical and electrochemical, and the resulting material properties. For example, it was shown that if ammonium persulfate is used as oxidant and the molar ratio oxidant/aniline is below 1.15, the conductivity of the polyaniline produced is insensitive to the oxidant concentration; the lowest ratio considered was 0.25 [8]. Recently there have been several advances in the synthesis procedures: improvement of the solubility properties of the polyaniline by using suitable protonation acids [20,21] or improvement of the material thermal stability [22]. Nevertheless, it has been difficult to find a synthesis procedure which can act in a controlled way on the factors that are responsible for the electrical properties, not only because of the degree of complexity of the problem but also because there is still discussion on the mechanism of the aniline polymerization itself and on the charge transport phenomena in these materials. It is the purpose of the present work to present an improved method of synthesis which gives to the material a very high macroscopic bulk conductivity and is not limited to a single acid used either as reaction media
136
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
or protonation acid. During the development of the work certain criteria were followed which allowed the authors to define a set of measurements of the reaction conditions which can be related to the final property of the material; these will be discussed. Particular attention is devoted to HC1 because comparison with the literature is easier, HC1 being a widely used acid. The synthesis is described in Section 2; in Section 3 the results are presented and in Section 4 they are discussed.
2. Experimental
2.1. Materials All reagents were used as received: aniline (Merck, 9 8 % ) , H 3 P O 4 (Carlo Erba, 99%), (NH4)2S208 (Aldrich, 98%), HC1 in water solution (37 wt.%, Rudi Pont), NH4OH in water solution (33 wt.%, Merck).
2.2. Synthesis The reactions were performed in a reactor which allowed continuous stirring of the reaction media and also in situ measurement of the media pH and temperature. It was possible, when needed (see description of the procedure below), to immerse the reactor in a cooling bath. Two families of reactions are considered. They differ only on the acid used throughout the whole procedure: in one case HC1 and in the other H3PO4. The quantity of aniline used was 0.5 moles for all the samples here reported. The synthesis procedure is the same for all samples: first, the aniline salt is formed by the dropwise addition of aniline to 0.5 moles of the Bronsted acid in solution, under continuous stirring inside the reactor. The concentration of the acid in water is changed in the various procedures to find the optimum reaction condition. Then, for each mole of aniline, Z moles of the same acid are added, in the form of a concentrated solution in water (37 wt.% for HC1 and 70 wt.% for H 3 P O 4 ) . The amount of acid added at this step is given, for each sample in Tables 1 and 2, for HC1 and H3PO4, respectively. Up to now approximately 1 h has elapsed since the beginning. Next, 0.25 moles of (NH4)zSzOs, in water solution (concentration 45 wt.%), are added under vigorous stirring. As the polymerization reaction proceeds the temperature steadily increases and some polyaniline precipitates. When the temperature ceases to increase (hereinafter denoted Tr,,x) the reaction is quenched by immersing the reactor in a water-ice bath. The time elapsed after the addition of the persulfate to reach Tm,x mainly depends on the amount of water in the reactor, varying from 4-5 min for the most concentrated samples up to 18 rain for the highest
Table 1 Synthesis conditions and polyaniline characterizations using HCI " Sample no.
Aniline concentration (mol/l)
Z
Tm~ (°C)
o(S/cm)
0 17 37 61 62 63 64 69 71 74 14 27 67 72 73 68 66 70
0.11
8 0 0 0 0 0 0 0 0 0 1 1 1 1 1 2 4 4
25 55 45 99 74 68 60 56 54 50 50 60 61 58 56 62 66 61
12 36 15 20 29 32 34 38 28 16 22 45 32 36 27 25 0.2 1
6.14 3.8 2.75 2.16 1.77 1.51 1.31
2.16 1.77 1.51 2.16 2.75 2.16
( Z + 1) is the number of moles of acid used for each aniline mole in the synthesis procedure and Tm~xis the maximum temperature reached during the polymerization reaction.
Table 2 Synthesis conditions and polyaniline characterizations using H3PO4 ~ Sample no.
1 105 107 108 109 2 3 35 33 36 23 101 100 103 102 134
Aniline concentration (mol/l)
6.14 2.75 2.16 1.77
6.14 2.75 2.16 1.77 2.16
Z
Tm~x (°C)
o(S/cm)
0 0 0 0 0 i 2 2 2 2 3 4 4 4 4 5
30 85 65 63 60 30 30 32 60 75 55 60 55 52 50 56
2 10 6 5.5 4.5 5 22 22 20 24 41 22 36 45 34 58
( Z + I ) is the number of moles of acid used for each aniline mole in the synthesis procedure and T,,~ is the maximum temperature reached during the polymerization reaction.
diluted ones. About 1 h after quenching the stirring is stopped and the polyaniline is recovered by filtration, washing it several times with a 1 M solution of the acid under consideration. The resulting material is put in a crystallizer to dry in air for approximately 24 h and finally is washed with ether in a Soxhlet apparatus. Tables 1 and 2 give the aniline concentration, defined
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
as the moles of aniline over the total volume of water present in the reaction media, for the various samples. For comparison, sample no. 0 was synthesized according to the method described in the literature [3,8,9]. In this last case the aniline concentration was 0.11 mol/ l and a total of 9 moles of acid were included in the overall synthesis for each mole of aniline. 2.3. Characterization
The conductivity of polyaniline was measured on pellets, 7 mm in diameter and about 100-400/xm thick. The thickness was measured using a digital micrometer. The conductivity is measured by the four-probe method. A Keithley 127 voltmeter and a Keithley 224 programmable current source were used; typically a current of 1 mA was injected. The measurements were confirmed for the highest conducting samples by the van der Paw method (square geometry). A thermogravimetry analysis was made with a Netzsch 409 system; the weight loss below 150 °C was typically between 7 and 10%, and was attributed to water content. The molecular weight distribution of the unprotonated polyanilines was determined by gel permeation chromatography (GPC) using N-methylpyrrolidone or tetramethylurea as eluent. The GPC analysis was made with a Philips PU 4100 liquid chromatography with four TSK-GEL columns in series with molecular weights 10 4, 6× 104, 4× 105 and 4 x 10 6. The detector was a spectrophotometer measuring the absorbance at 590 nm. The polyaniline was unprotonated by washing it first with a solution of NH4OH (pH 11) and then with water.
137
developed after the addition of the persulfate mainly comes from the polymerization step. In general, the procedure gave very satisfactory results. Note, however, that the conductivity of samples nos. 70 and 66 is low compared to the other samples; this is due to a decomposition process that sets in during the oxidative polymerization. The amount of reaction byproducts is large if the HC1 concentration is too high and there is a large excess of acid with respect to the aniline present, as happens with the two previously mentioned samples. Furthermore, when the procedure described in Section 2 is carried out with 8 moles of extra acid (Z=8) and with aniline concentration equal to 2.16 mold, no polymer is obtained, only decomposition products. The amount of polyaniline recovered after filtration was lower for sample no. 0 than for the other samples synthesized with HCI reported here, with the exception of sample nos. 66 and 70. The yield was measured only for selected samples after being unprotonated and dried. In particular, for sample no. 14 the yield was approximately 57%, whereas for sample no. 0 it was about 37%. Consider now the measurements related to the molecular weight distribution. Fig. 1 shows the GPC analysis of sample no. 0 when the polyaniline is synthesized by the method described in the literature [3,8,9], whereas Fig. 2 shows the GPC analysis of sample no. 27; in both cases N-methylpyrrolidone was used as eluent. The qualitative characteristics of the distributions were confirmed by experiments made using tetramethylurea as eluent. A comparison between the two graphs clearly indicates that sample no. 0, prepared by the method described in the literature, has longer exit times and thus shorter polymer chain length.
3. Results
The study of the correlation of reaction product conductivity with the variation of reagent concentration focused essentially on two parameters: the aniline concentration and the concentration of acid in the reaction media. The definition of the quantity of reagents used, the conditions of the reaction medium, the maximum temperature reached in the oxidative polymerization reaction together with the results of the conductivity measurements performed on each sample are given in Tables 1 and 2 for HC1 and H3PO4, respectively. Since the temperature tends to increase during the formation of the aniline salt, unwanted high temperatures are avoided by slowly adding the aniline to the acid and also by adding extra acid after the salt is formed. After the addition of the persulfate the temperature remains approximately constant for 3 rain and then starts to rise, at the same time the reaction media begins to become greenish. This is an indication that the heat
e~ k.
e~
<
¢~,,i,,,,1,
5
,,,i,,,,i,,,,i,,,,i,,,,
10
15
Time
20
:;if
25
30
35
I
40
(min)
Fig. 1. Gel permeation chromatogram of sample no. 0 prepared with the method of Ref. [31.
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
138
100. 90
G" 8o4
~
70" 60"
e. 50" 40
3
<
~
Aniline
F i g . 3. M a x i m u m merization
temperature
reaction,
~
The experimental ~,llIFIf
5
10
15
Time
20
IIJl
25
IIf~
30
-
reached
during
the
oxidative poly-
Tin,x, a s a f u n c t i o n o f a n i l i n e c o n c e n t r a t i o n
the synthesis with HCI molar concentration ,,ill,
6
C o n c e n t r a t i o n (tool/I)
points are indicated
for
e q u a l to t h e a n i l i n e o n e .
by squares.
I~ff]
35
40
(min) 50-
F i g . 2. G e l p e r m e a t i o n with the method
chromatogram
presented
of sample
n o . 27 p r e p a r e d
in t h i s p a p e r . 40-
4. Discussion and conclusions
This work demonstrates than an increase in the reaction temperature does not compromise the overall reaction result provided that, at a certain moment, the oxidative polymerization reaction is quenched by suddenly immersing the reactor in an ice bath. If the reaction is allowed to continue after Tmax is reached a decomposition process occurs, undesired byproducts appear and the polyaniline conductivity significantly drops or is completely destroyed. Since the reaction heat is mainly developed during polymerization, we believe that the decomposition process is due to residual oxidant which, at high temperatures, attacks the already formed polymer, probably hydrolysing it [1,12,23]. The procedures described in the literature [3,8,9] avoid this effect by performing the oxidative polymerization with external cooling and with low aniline concentration, around 0.11 mol/l. In this way Tm,x does not exceed 25 °C because of the relatively large amount of reaction media to be heated and the contact with the cooling bath. In our procedure the variation of the aniline concentration is a simple and efficient way of T,,,x control without modifying the basic reaction evolution (provided that there is a minimum amount of water to ensure solubility). Fig. 3 shows the variation of Tma~with the aniline concentration when the acid is HC1 with molar ratio HC1/aniline equal to unity. A straight line gives an excellent fitting, as expected, because the heat developed in the reaction is the same for all experiments (the amount of aniline used for all the experiments is the same and there are no variations of the molar ratio aniline/persulfate) and only the amount of water that is heated up varies. Similar straight line fittings can
Z-=l z-4
•
30-
20-
10"
0 20
-
, 30
'
, 40
•
, 50
•
, 60
•
, 70
•
, 80
•
, 90
•
100
Tmax {°C) F i g . 4. C o n d u c t i v i t y
as a f u n c t i o n o f Tm.x f o r t h e s a m p l e s
with HCI and various values of acid concentration,
prepared
distinguished
by
the values of Z.
be obtained for each of the two acids and/or for different values of acid concentration. The slopes of the fitted straight lines depend on the particular conditions, indicating that there is a different reaction enthalpy and/ or different reaction medium heat capacity. We have shown that there is a signal (Tr, a,) in the procedure indicating when to quench the reaction and a way to predict and hence to control it. This internal control allows an easier study of the optimization conditions. Fig. 4 shows the measured conductivities of the various samples prepared with HCI as a function of Tm~. The curves for Z = 0 and Z = 1 have a maximum indicating that the optimum Tm~ is between 50 and 60 °C. The exact position depends on the kind of acid and on its concentration. The reaction using H3PO4 apparently has a different behaviour. At low acid concentrations ( Z = 0 and 2), there is no maximum in the curve of conductivity against Tma, (see Fig. 5), but for higher concentration (Z= 4) the conductivities are higher and the curve presents a maximum.
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140 50"
40"
7~2 Z=4
-----C}---
30"
=
20
..-.--.O
10
• 0
, 30
.
, 40
•
, 50
.
,
•
60
Tma x
, 70
.
, 80
.
, 90
• 100
(°C)
Fig. 5. Conductivity as a function of Tmax for the samples prepared with H3PO4 and various values of acid concentration, distinguished by the values of Z.
From these results we conclude that the acid concentration has an important effect on the oxidative polymerization. In the first place it is known that the presence of the acid is needed in the polymerization step [1] and also the acid protonates the polymer, so it is no surprise that there is a beneficial effect on the conductivity of the resulting polymer when the acid concentration is increased, but a highly acidic reaction medium causes decomposition processes, in the presence of the oxidant, and these are more severe at higher temperatures. The presence of two competing mechanisms gives a maximum in the curves of Fig. 4 and a maximum in Fig. 5 for Z = 4. We argue that, since H3PO4 is a weaker acid than HC1, the effects on the decomposition process occur at higher H3PO4 concentrations. Thus for Z = 0 and 2 there is a monotonic increase in conductivity with temperature, which is the consequence of a more efficient oxidative polymerization without the presence of a strong competing decomposition. Under these circumstances a way to increase conductivity is just to realize the reaction with a higher aniline concentration. Following this reasoning, sample no. 36 was synthesized which has a conductivity of 24 S/cm. However, the conductivity cannot be increased much further by concentrating the reactants because a minimum amount of water is needed to ensure solubility. To achieve higher conductivities a higher acid/aniline molar ratio is required with the consequent need of polymerization temperature control. Following this idea sample nos. 23, 103 and 134 were synthesized. To our knowledge, sample no. 134 reaches the maximum polyaniline conductivity reported so far for a pellet: 58 S/cm. There are reports of polyaniline films which, when stretched, can give up to 500 S/cm in the stretching direction but the conductivity of the polyaniline unstretched films reported in the same article are 'only' about 35 S/cm [18]. Moreover, in Ref. [16] films are reported to have a maximum directional conductivity of 350 S/cm, whereas pellets of the same material have
139
20 S/cm. It is well known [11] that polymer films have a higher conductivity than the pellets because a stretched film is usually oriented. Notice also that the two competing mechanisms leave, at least with our synthesis procedure, narrower optimization ranges for increasing acid concentration. In Fig. 4 the Z = 0 curve is broader than the Z = 1 one. The GPC measurements indicate that the polymer obtained through our method has a higher molecular weight if compared with the one obtained following the literature procedure. Also, this is an indication of a more efficient polymerization reaction or, we should say, the polymerization competes in a more favourable way against decomposition. In Ref. [24] the kinetics of aniline polymerization in a monolayer is studied and it is concluded that higher temperatures favour a high molecular weight polymer. The extension of this conclusion to solution polymerization can be made if the reaction conditions are such that the decomposition process is not present, otherwise the reaction procedure has to be optimized in order to get a high molecular weight polymer. We believe that controlled conditions during the oxidative polymerization and the quick lowering of the reaction media temperature when the polymerization reaction is happening favour a higher conductivity because the polymer backbone has a higher structural regularity, i.e. the counterions are distributed along the backbone in a more regular way, increasing the charge carrier mobility. This is supported from proton-spin lattice nuclear magnetic resonance relaxation time measurements [25]. Another reason for the high conductivity values achieved is the relative high molecular weight of our samples. In conclusion, it was shown that the oxidative polymerization of aniline can be successfully realized at temperatures well above room temperature with an optimum between 50 and 60 °C, provided that the reaction is quenched once this temperature is achieved. Moreover, the molar ratio acid/aniline is a relevant reaction parameter. A large ratio can increase the material conductivity and can also enhance decomposition processes. The optimum ratio has to be found in each case. The polyanilines synthesized with the proposed procedure have a quasi monomodal molecular weight distribution; high molecular weights are favoured and give, in a reproducible way, polyanilines the bulk conductivities of which are above 40 S/cm.
Acknowledgements We thank Dr K.W. Jen from Enichem for starting our interest in the present subject and Mr G.M. Lawlor from Trinity College, Dublin, for his help with some of the synthetic work. We also thank Professor G.
140
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
Ottaviani and Dr Michelini from the University of Modena (Italy) for their kind collaboration with some of the conductivity measurements. This work has been carried out with the partial support of the European Economic Community through Contracts RACE 1020 and RACE 2012.
References [1] E.M. Geni~s, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139 and refs. therein; F. Lux, Polymer, 35 (1994) 2915. [2] R. de Surville, M. Jozefowicz, L. Yu, J. Perichon and R. Buret, Electrochim. Acta, 13 (1968) 1451; M. Jozefowicz, J. Perichon, L Yu and R. Buvet, UK Patent No. 1216569 (1970). [3] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N. Somasiri and A.J. Epstein, in L. Alcacer (ed.), Conducting Polymers, Reidel, Dordrecht, 1987, p. 105. [4] M. Angelopoulous, A. Ray, A.G. MacDiarmid and A.J. Epstein, Synth. Met., 21 (1987) 21. [5] H.Y. Choi and E.J. Mele, Phys. Rev. Lett., 59 (1987) 2188. [6] T. Hjertberg, M. Sandberg, O. Wennerstr6m and I. Lagerstedt, Synth. Met., 21 (1987) 31. [7] R.R. Chance, D.S. Boudreaux, J.F. Wolf, L.W. Shacklette, R. Silbey, B. Th6mans and J.L. Br6das, Synth. Met., 15 (1986) 105. [8] S.P. Armes and J.F. Miller, Synth. Met., 22 (1988) 385. [9] Y. Wei, K.F. Hsueh and G. Jang, Macromolecules, 27 (1994) 518.
[10] J.L. Br6das, in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 2, Marcel Dekker, New York, 1986. [11] J.R. Ellis in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986. [12] Y. Cao, A. Andreatta, A.J. Heeger and P. Smith, Polymer, 30 (1989) 2305 and Refs. therein. [13] O.N. Timofeeva, B.Z. Lubentsov, Ye.Z. Sudakova, D.N. Chernyshov and M.L. Khidekel, Synth. Met., 40 (1991) 111. [14] W.W. Focke, G.E. Wneek and Y. Wei, J. Phys. Chem., 91 (1987) 5813. [15] H.S.O. Chan, P.K.H. Ho, K.L. Tan and B.T.G. Tan, Synth Met., 35 (1990) 333. [16] A.P. Monkman and P. Adams, Synth. Met., 40 (1991) 87. [17] J.E. Fischer, Q. Zhu, X. Tang, E.M. Scherr, A.G. MacDiarmid and V.B. Cajipe, Macromolecules, 27 (1994) 5094. [18] M. Wan, M. Li, J. Li and Z. Liu, J. Appl. Polym. Sci., 53 (1994) 131. [19] A.G. Green and A.E. Woodhead, J. Chem. Soc., Trans., 97 (1910) 2388. [20] A. Profi, J. Laska, J.-E. 0sterholm and P. Smith, Polymer, 34 (1993) 4235. [21] S.A. Chen and G.W. Hwang, J. Am. Chem. Soe., 116 (1994) 7939. [22] H.S.O. Chan, S.C. Ng and P.K.H. Ho, Macromolecules, 27 (1994) 2159. [23] N. Gospodinova, P. Mokreva and L. Terlemezyan, Polymer, 35 (1994) 3102. [24] R. Bodalia, J. Manzanares, H. Reiss and R. Duran, Macromolecules, 27 (1994) 2002. [25] L. Pavesi, S. Aldrovandi, M. Corti, A. Rigamonti, G. Boara and M. Sparpaglione, Proc. Ampdre Congress, Athens, Greece, 1992; L. Pavesi, S. Aldrovandi and M. Corti, J. Phys.: Condens. Matter, 5 (1993) B25.
Synthetic Metals 72 (1995) 135-140
Synthesis of polyanilines with high electrical conductivity Giulio Boara a, Massimo Sparpaglione b,, Enichem S.p./t., via F. Maritano 26, 20097 San Donato Milanese, Italy h Enichem S.p.A., via G. Fauser 4, 28100 Novara, Italy Received 2 June 1994; accepted 10 November 1994
Abstract
A procedure for the chemical synthesis of conductive polyanilines is presented. The effect of the relative concentration of reactants on the material properties is studied. The synthesis is realized with hydrochloric or phosphoric acids. It is shown that it is possible to perform the oxidative polymerization condensation at temperatures up to 99 °C and that its control can give rise to polyanilines with high conductivity. The optimum temperature depends on the acid used and is, for the cases considered, higher than the one used in the procedures reported so far. In particular, pellets of polyanilines synthesized with H3PO4 are produced with conductivities up to 58 S/cm. Keywords: Synthesis; Polyaniline; Conductivity
I. Introduction
Materials produced by the oxidative polymerization of aniline, hereinafter polyaniline, have received much attention in the last few years, mainly because of their electrical properties [1-9]. The bulk conductivity, ~r, of the emeraldine base of the polyaniline family can be varied from 10-10 up to 5-20 S/cm when, upon treatment with acids, protons are added to a fraction of the unprot0nated nitrogen sites [2-4]. Polyaniline is an interesting conducting polymer even if it has a conductivity up to three orders of magnitude lower than polyacetylene [10] because of its lower cost and higher environmental stability [11] which can make it preferable for practical applications. Polyanilines are also interesting compounds from the point of view of the fundamental aspects of the mechanism of the charge transport in conducting polymers [5-7]. The conductivity along the polyaniline backbone can be affected by the degree of oxidation of the polymer (the relative amount of amine and imine groups), the protonation acid [1,12] and the percentage of protonation [1,3,6]. The conductivity is also affected by the degree of water content [4,13,14], which presumably can affect the charge transport between chains, and also by the morphology and texture of the polymer * Corresponding author. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A, All rights reserved SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 3 3 7 - X
[15,16], its chain length [16] and degree of crystallization [1,17,18]. The insulating form of polyaniline was discovered more than 100 years ago and its synthesis is relatively simple [1,19]. Studies have been carried out to understand the relationship between the conditions of synthesis, both chemical and electrochemical, and the resulting material properties. For example, it was shown that if ammonium persulfate is used as oxidant and the molar ratio oxidant/aniline is below 1.15, the conductivity of the polyaniline produced is insensitive to the oxidant concentration; the lowest ratio considered was 0.25 [8]. Recently there have been several advances in the synthesis procedures: improvement of the solubility properties of the polyaniline by using suitable protonation acids [20,21] or improvement of the material thermal stability [22]. Nevertheless, it has been difficult to find a synthesis procedure which can act in a controlled way on the factors that are responsible for the electrical properties, not only because of the degree of complexity of the problem but also because there is still discussion on the mechanism of the aniline polymerization itself and on the charge transport phenomena in these materials. It is the purpose of the present work to present an improved method of synthesis which gives to the material a very high macroscopic bulk conductivity and is not limited to a single acid used either as reaction media
136
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
or protonation acid. During the development of the work certain criteria were followed which allowed the authors to define a set of measurements of the reaction conditions which can be related to the final property of the material; these will be discussed. Particular attention is devoted to HC1 because comparison with the literature is easier, HC1 being a widely used acid. The synthesis is described in Section 2; in Section 3 the results are presented and in Section 4 they are discussed.
2. Experimental
2.1. Materials All reagents were used as received: aniline (Merck, 9 8 % ) , H 3 P O 4 (Carlo Erba, 99%), (NH4)2S208 (Aldrich, 98%), HC1 in water solution (37 wt.%, Rudi Pont), NH4OH in water solution (33 wt.%, Merck).
2.2. Synthesis The reactions were performed in a reactor which allowed continuous stirring of the reaction media and also in situ measurement of the media pH and temperature. It was possible, when needed (see description of the procedure below), to immerse the reactor in a cooling bath. Two families of reactions are considered. They differ only on the acid used throughout the whole procedure: in one case HC1 and in the other H3PO4. The quantity of aniline used was 0.5 moles for all the samples here reported. The synthesis procedure is the same for all samples: first, the aniline salt is formed by the dropwise addition of aniline to 0.5 moles of the Bronsted acid in solution, under continuous stirring inside the reactor. The concentration of the acid in water is changed in the various procedures to find the optimum reaction condition. Then, for each mole of aniline, Z moles of the same acid are added, in the form of a concentrated solution in water (37 wt.% for HC1 and 70 wt.% for H 3 P O 4 ) . The amount of acid added at this step is given, for each sample in Tables 1 and 2, for HC1 and H3PO4, respectively. Up to now approximately 1 h has elapsed since the beginning. Next, 0.25 moles of (NH4)zSzOs, in water solution (concentration 45 wt.%), are added under vigorous stirring. As the polymerization reaction proceeds the temperature steadily increases and some polyaniline precipitates. When the temperature ceases to increase (hereinafter denoted Tr,,x) the reaction is quenched by immersing the reactor in a water-ice bath. The time elapsed after the addition of the persulfate to reach Tm,x mainly depends on the amount of water in the reactor, varying from 4-5 min for the most concentrated samples up to 18 rain for the highest
Table 1 Synthesis conditions and polyaniline characterizations using HCI " Sample no.
Aniline concentration (mol/l)
Z
Tm~ (°C)
o(S/cm)
0 17 37 61 62 63 64 69 71 74 14 27 67 72 73 68 66 70
0.11
8 0 0 0 0 0 0 0 0 0 1 1 1 1 1 2 4 4
25 55 45 99 74 68 60 56 54 50 50 60 61 58 56 62 66 61
12 36 15 20 29 32 34 38 28 16 22 45 32 36 27 25 0.2 1
6.14 3.8 2.75 2.16 1.77 1.51 1.31
2.16 1.77 1.51 2.16 2.75 2.16
( Z + 1) is the number of moles of acid used for each aniline mole in the synthesis procedure and Tm~xis the maximum temperature reached during the polymerization reaction.
Table 2 Synthesis conditions and polyaniline characterizations using H3PO4 ~ Sample no.
1 105 107 108 109 2 3 35 33 36 23 101 100 103 102 134
Aniline concentration (mol/l)
6.14 2.75 2.16 1.77
6.14 2.75 2.16 1.77 2.16
Z
Tm~x (°C)
o(S/cm)
0 0 0 0 0 i 2 2 2 2 3 4 4 4 4 5
30 85 65 63 60 30 30 32 60 75 55 60 55 52 50 56
2 10 6 5.5 4.5 5 22 22 20 24 41 22 36 45 34 58
( Z + I ) is the number of moles of acid used for each aniline mole in the synthesis procedure and T,,~ is the maximum temperature reached during the polymerization reaction.
diluted ones. About 1 h after quenching the stirring is stopped and the polyaniline is recovered by filtration, washing it several times with a 1 M solution of the acid under consideration. The resulting material is put in a crystallizer to dry in air for approximately 24 h and finally is washed with ether in a Soxhlet apparatus. Tables 1 and 2 give the aniline concentration, defined
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
as the moles of aniline over the total volume of water present in the reaction media, for the various samples. For comparison, sample no. 0 was synthesized according to the method described in the literature [3,8,9]. In this last case the aniline concentration was 0.11 mol/ l and a total of 9 moles of acid were included in the overall synthesis for each mole of aniline. 2.3. Characterization
The conductivity of polyaniline was measured on pellets, 7 mm in diameter and about 100-400/xm thick. The thickness was measured using a digital micrometer. The conductivity is measured by the four-probe method. A Keithley 127 voltmeter and a Keithley 224 programmable current source were used; typically a current of 1 mA was injected. The measurements were confirmed for the highest conducting samples by the van der Paw method (square geometry). A thermogravimetry analysis was made with a Netzsch 409 system; the weight loss below 150 °C was typically between 7 and 10%, and was attributed to water content. The molecular weight distribution of the unprotonated polyanilines was determined by gel permeation chromatography (GPC) using N-methylpyrrolidone or tetramethylurea as eluent. The GPC analysis was made with a Philips PU 4100 liquid chromatography with four TSK-GEL columns in series with molecular weights 10 4, 6× 104, 4× 105 and 4 x 10 6. The detector was a spectrophotometer measuring the absorbance at 590 nm. The polyaniline was unprotonated by washing it first with a solution of NH4OH (pH 11) and then with water.
137
developed after the addition of the persulfate mainly comes from the polymerization step. In general, the procedure gave very satisfactory results. Note, however, that the conductivity of samples nos. 70 and 66 is low compared to the other samples; this is due to a decomposition process that sets in during the oxidative polymerization. The amount of reaction byproducts is large if the HC1 concentration is too high and there is a large excess of acid with respect to the aniline present, as happens with the two previously mentioned samples. Furthermore, when the procedure described in Section 2 is carried out with 8 moles of extra acid (Z=8) and with aniline concentration equal to 2.16 mold, no polymer is obtained, only decomposition products. The amount of polyaniline recovered after filtration was lower for sample no. 0 than for the other samples synthesized with HCI reported here, with the exception of sample nos. 66 and 70. The yield was measured only for selected samples after being unprotonated and dried. In particular, for sample no. 14 the yield was approximately 57%, whereas for sample no. 0 it was about 37%. Consider now the measurements related to the molecular weight distribution. Fig. 1 shows the GPC analysis of sample no. 0 when the polyaniline is synthesized by the method described in the literature [3,8,9], whereas Fig. 2 shows the GPC analysis of sample no. 27; in both cases N-methylpyrrolidone was used as eluent. The qualitative characteristics of the distributions were confirmed by experiments made using tetramethylurea as eluent. A comparison between the two graphs clearly indicates that sample no. 0, prepared by the method described in the literature, has longer exit times and thus shorter polymer chain length.
3. Results
The study of the correlation of reaction product conductivity with the variation of reagent concentration focused essentially on two parameters: the aniline concentration and the concentration of acid in the reaction media. The definition of the quantity of reagents used, the conditions of the reaction medium, the maximum temperature reached in the oxidative polymerization reaction together with the results of the conductivity measurements performed on each sample are given in Tables 1 and 2 for HC1 and H3PO4, respectively. Since the temperature tends to increase during the formation of the aniline salt, unwanted high temperatures are avoided by slowly adding the aniline to the acid and also by adding extra acid after the salt is formed. After the addition of the persulfate the temperature remains approximately constant for 3 rain and then starts to rise, at the same time the reaction media begins to become greenish. This is an indication that the heat
e~ k.
e~
<
¢~,,i,,,,1,
5
,,,i,,,,i,,,,i,,,,i,,,,
10
15
Time
20
:;if
25
30
35
I
40
(min)
Fig. 1. Gel permeation chromatogram of sample no. 0 prepared with the method of Ref. [31.
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
138
100. 90
G" 8o4
~
70" 60"
e. 50" 40
3
<
~
Aniline
F i g . 3. M a x i m u m merization
temperature
reaction,
~
The experimental ~,llIFIf
5
10
15
Time
20
IIJl
25
IIf~
30
-
reached
during
the
oxidative poly-
Tin,x, a s a f u n c t i o n o f a n i l i n e c o n c e n t r a t i o n
the synthesis with HCI molar concentration ,,ill,
6
C o n c e n t r a t i o n (tool/I)
points are indicated
for
e q u a l to t h e a n i l i n e o n e .
by squares.
I~ff]
35
40
(min) 50-
F i g . 2. G e l p e r m e a t i o n with the method
chromatogram
presented
of sample
n o . 27 p r e p a r e d
in t h i s p a p e r . 40-
4. Discussion and conclusions
This work demonstrates than an increase in the reaction temperature does not compromise the overall reaction result provided that, at a certain moment, the oxidative polymerization reaction is quenched by suddenly immersing the reactor in an ice bath. If the reaction is allowed to continue after Tmax is reached a decomposition process occurs, undesired byproducts appear and the polyaniline conductivity significantly drops or is completely destroyed. Since the reaction heat is mainly developed during polymerization, we believe that the decomposition process is due to residual oxidant which, at high temperatures, attacks the already formed polymer, probably hydrolysing it [1,12,23]. The procedures described in the literature [3,8,9] avoid this effect by performing the oxidative polymerization with external cooling and with low aniline concentration, around 0.11 mol/l. In this way Tm,x does not exceed 25 °C because of the relatively large amount of reaction media to be heated and the contact with the cooling bath. In our procedure the variation of the aniline concentration is a simple and efficient way of T,,,x control without modifying the basic reaction evolution (provided that there is a minimum amount of water to ensure solubility). Fig. 3 shows the variation of Tma~with the aniline concentration when the acid is HC1 with molar ratio HC1/aniline equal to unity. A straight line gives an excellent fitting, as expected, because the heat developed in the reaction is the same for all experiments (the amount of aniline used for all the experiments is the same and there are no variations of the molar ratio aniline/persulfate) and only the amount of water that is heated up varies. Similar straight line fittings can
Z-=l z-4
•
30-
20-
10"
0 20
-
, 30
'
, 40
•
, 50
•
, 60
•
, 70
•
, 80
•
, 90
•
100
Tmax {°C) F i g . 4. C o n d u c t i v i t y
as a f u n c t i o n o f Tm.x f o r t h e s a m p l e s
with HCI and various values of acid concentration,
prepared
distinguished
by
the values of Z.
be obtained for each of the two acids and/or for different values of acid concentration. The slopes of the fitted straight lines depend on the particular conditions, indicating that there is a different reaction enthalpy and/ or different reaction medium heat capacity. We have shown that there is a signal (Tr, a,) in the procedure indicating when to quench the reaction and a way to predict and hence to control it. This internal control allows an easier study of the optimization conditions. Fig. 4 shows the measured conductivities of the various samples prepared with HCI as a function of Tm~. The curves for Z = 0 and Z = 1 have a maximum indicating that the optimum Tm~ is between 50 and 60 °C. The exact position depends on the kind of acid and on its concentration. The reaction using H3PO4 apparently has a different behaviour. At low acid concentrations ( Z = 0 and 2), there is no maximum in the curve of conductivity against Tma, (see Fig. 5), but for higher concentration (Z= 4) the conductivities are higher and the curve presents a maximum.
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140 50"
40"
7~2 Z=4
-----C}---
30"
=
20
..-.--.O
10
• 0
, 30
.
, 40
•
, 50
.
,
•
60
Tma x
, 70
.
, 80
.
, 90
• 100
(°C)
Fig. 5. Conductivity as a function of Tmax for the samples prepared with H3PO4 and various values of acid concentration, distinguished by the values of Z.
From these results we conclude that the acid concentration has an important effect on the oxidative polymerization. In the first place it is known that the presence of the acid is needed in the polymerization step [1] and also the acid protonates the polymer, so it is no surprise that there is a beneficial effect on the conductivity of the resulting polymer when the acid concentration is increased, but a highly acidic reaction medium causes decomposition processes, in the presence of the oxidant, and these are more severe at higher temperatures. The presence of two competing mechanisms gives a maximum in the curves of Fig. 4 and a maximum in Fig. 5 for Z = 4. We argue that, since H3PO4 is a weaker acid than HC1, the effects on the decomposition process occur at higher H3PO4 concentrations. Thus for Z = 0 and 2 there is a monotonic increase in conductivity with temperature, which is the consequence of a more efficient oxidative polymerization without the presence of a strong competing decomposition. Under these circumstances a way to increase conductivity is just to realize the reaction with a higher aniline concentration. Following this reasoning, sample no. 36 was synthesized which has a conductivity of 24 S/cm. However, the conductivity cannot be increased much further by concentrating the reactants because a minimum amount of water is needed to ensure solubility. To achieve higher conductivities a higher acid/aniline molar ratio is required with the consequent need of polymerization temperature control. Following this idea sample nos. 23, 103 and 134 were synthesized. To our knowledge, sample no. 134 reaches the maximum polyaniline conductivity reported so far for a pellet: 58 S/cm. There are reports of polyaniline films which, when stretched, can give up to 500 S/cm in the stretching direction but the conductivity of the polyaniline unstretched films reported in the same article are 'only' about 35 S/cm [18]. Moreover, in Ref. [16] films are reported to have a maximum directional conductivity of 350 S/cm, whereas pellets of the same material have
139
20 S/cm. It is well known [11] that polymer films have a higher conductivity than the pellets because a stretched film is usually oriented. Notice also that the two competing mechanisms leave, at least with our synthesis procedure, narrower optimization ranges for increasing acid concentration. In Fig. 4 the Z = 0 curve is broader than the Z = 1 one. The GPC measurements indicate that the polymer obtained through our method has a higher molecular weight if compared with the one obtained following the literature procedure. Also, this is an indication of a more efficient polymerization reaction or, we should say, the polymerization competes in a more favourable way against decomposition. In Ref. [24] the kinetics of aniline polymerization in a monolayer is studied and it is concluded that higher temperatures favour a high molecular weight polymer. The extension of this conclusion to solution polymerization can be made if the reaction conditions are such that the decomposition process is not present, otherwise the reaction procedure has to be optimized in order to get a high molecular weight polymer. We believe that controlled conditions during the oxidative polymerization and the quick lowering of the reaction media temperature when the polymerization reaction is happening favour a higher conductivity because the polymer backbone has a higher structural regularity, i.e. the counterions are distributed along the backbone in a more regular way, increasing the charge carrier mobility. This is supported from proton-spin lattice nuclear magnetic resonance relaxation time measurements [25]. Another reason for the high conductivity values achieved is the relative high molecular weight of our samples. In conclusion, it was shown that the oxidative polymerization of aniline can be successfully realized at temperatures well above room temperature with an optimum between 50 and 60 °C, provided that the reaction is quenched once this temperature is achieved. Moreover, the molar ratio acid/aniline is a relevant reaction parameter. A large ratio can increase the material conductivity and can also enhance decomposition processes. The optimum ratio has to be found in each case. The polyanilines synthesized with the proposed procedure have a quasi monomodal molecular weight distribution; high molecular weights are favoured and give, in a reproducible way, polyanilines the bulk conductivities of which are above 40 S/cm.
Acknowledgements We thank Dr K.W. Jen from Enichem for starting our interest in the present subject and Mr G.M. Lawlor from Trinity College, Dublin, for his help with some of the synthetic work. We also thank Professor G.
140
G. Boara, M. Sparpaglione / Synthetic Metals 72 (1995) 135-140
Ottaviani and Dr Michelini from the University of Modena (Italy) for their kind collaboration with some of the conductivity measurements. This work has been carried out with the partial support of the European Economic Community through Contracts RACE 1020 and RACE 2012.
References [1] E.M. Geni~s, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139 and refs. therein; F. Lux, Polymer, 35 (1994) 2915. [2] R. de Surville, M. Jozefowicz, L. Yu, J. Perichon and R. Buret, Electrochim. Acta, 13 (1968) 1451; M. Jozefowicz, J. Perichon, L Yu and R. Buvet, UK Patent No. 1216569 (1970). [3] A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N. Somasiri and A.J. Epstein, in L. Alcacer (ed.), Conducting Polymers, Reidel, Dordrecht, 1987, p. 105. [4] M. Angelopoulous, A. Ray, A.G. MacDiarmid and A.J. Epstein, Synth. Met., 21 (1987) 21. [5] H.Y. Choi and E.J. Mele, Phys. Rev. Lett., 59 (1987) 2188. [6] T. Hjertberg, M. Sandberg, O. Wennerstr6m and I. Lagerstedt, Synth. Met., 21 (1987) 31. [7] R.R. Chance, D.S. Boudreaux, J.F. Wolf, L.W. Shacklette, R. Silbey, B. Th6mans and J.L. Br6das, Synth. Met., 15 (1986) 105. [8] S.P. Armes and J.F. Miller, Synth. Met., 22 (1988) 385. [9] Y. Wei, K.F. Hsueh and G. Jang, Macromolecules, 27 (1994) 518.
[10] J.L. Br6das, in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 2, Marcel Dekker, New York, 1986. [11] J.R. Ellis in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986. [12] Y. Cao, A. Andreatta, A.J. Heeger and P. Smith, Polymer, 30 (1989) 2305 and Refs. therein. [13] O.N. Timofeeva, B.Z. Lubentsov, Ye.Z. Sudakova, D.N. Chernyshov and M.L. Khidekel, Synth. Met., 40 (1991) 111. [14] W.W. Focke, G.E. Wneek and Y. Wei, J. Phys. Chem., 91 (1987) 5813. [15] H.S.O. Chan, P.K.H. Ho, K.L. Tan and B.T.G. Tan, Synth Met., 35 (1990) 333. [16] A.P. Monkman and P. Adams, Synth. Met., 40 (1991) 87. [17] J.E. Fischer, Q. Zhu, X. Tang, E.M. Scherr, A.G. MacDiarmid and V.B. Cajipe, Macromolecules, 27 (1994) 5094. [18] M. Wan, M. Li, J. Li and Z. Liu, J. Appl. Polym. Sci., 53 (1994) 131. [19] A.G. Green and A.E. Woodhead, J. Chem. Soc., Trans., 97 (1910) 2388. [20] A. Profi, J. Laska, J.-E. 0sterholm and P. Smith, Polymer, 34 (1993) 4235. [21] S.A. Chen and G.W. Hwang, J. Am. Chem. Soe., 116 (1994) 7939. [22] H.S.O. Chan, S.C. Ng and P.K.H. Ho, Macromolecules, 27 (1994) 2159. [23] N. Gospodinova, P. Mokreva and L. Terlemezyan, Polymer, 35 (1994) 3102. [24] R. Bodalia, J. Manzanares, H. Reiss and R. Duran, Macromolecules, 27 (1994) 2002. [25] L. Pavesi, S. Aldrovandi, M. Corti, A. Rigamonti, G. Boara and M. Sparpaglione, Proc. Ampdre Congress, Athens, Greece, 1992; L. Pavesi, S. Aldrovandi and M. Corti, J. Phys.: Condens. Matter, 5 (1993) B25.