J . Phys. Chem. 1990, 94, 3192-3196
3192
above 120 K, suggesting that a neutral radical rather than the ring-oDened distonic radical cation is observed in CF,CICFCI,. - . Acknowledgment. We are indebted to Drs. Martin Bakker and David Werst for technical assistance and for numerous useful discussions. Useful discussions with other members of the Radiation and Photochemistry Group and with Prof. Ffrancon
Williams of the University of Tennessee are acknowledged as well. This work was cerformed under the ausDices of the Office of Basic Energy Sciences, Division of Chemicai Science, US-DOE, under Contract No, w-3 -109-ENG-38. Registry No. I , 56324-44-8; T M C P , 41 27-47-3; Xe, 7440-63-3; CF,CICFCI,, 76-1 3-1; CH2CI,, 75-09-2.
Aqueous Reactivity of Polyacetylene: pH Dependence Anthony Guiseppi-Elie* AAI-ABTECH, P.O. Box 376, Yardley, Pennsylvania 19067
and Gary E. Wnek Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 (Received: July 11, 1989; In Final Form: November I , 1989)
The stability of iodine-doped polyacetylene in aqueous environments and the particular effect of variations in pH upon the stability of the intrinsically conducting polymer has been investigated in a variety of aqueous environments. Stability performance was established by simultaneously monitoring the four-probe electrical conductivity and the steady-state electrode rest potential during exposure of the metallically doped, [CHI,,l,4,20],, polymer over an approximate IO-day period. Initial polymer conductivities were in all cases between 100 and 300 S cm-’. The initially measured electrode potential was in all cases ca. 0.45 V vs SCE irrespective of the doping levels studied; y = 0.007, 0.168,0.210 or the pH of the test solution in the range 1-9. The rate and extent of degradation in these material properties was found to increase at the extremes of pH. Alkaline conditions were found to be generally more aggressive to the p-doped polymer compared to acidic conditions. The normally slow but ever present degradation of iodine-doped polyacetylene which occurs even under inert atmosphere conditions is accelerated in all aqueous environments leading to an often precipitous degradation of electrically based material properties.
Introduction The reactivity of doped and undoped plyacetylene in aqueous environments is relevant to its application as a fuel cell electrode,’ as a transducer-active material in chemical and biological sens o r ~ , *and , ~ in aqueous-based rechargeable storage b a t t e r i e ~ . ~ Equally important, the reactivity of doped and undoped p l y acetylene with water is relevant to a fundamental understanding of its environmental stability because moisture is an ubiquitous component of ambient. Considerable work has been reported on the stability and degradation of polyacetylene, but few have addressed its stability in aqueous environments. Polyacetylene is characterized both as an electrode and as an electroactive polymer. Implicit in this definition is the ability of polyacetylene to be an intrinsic source of charge carriers,j to act as a conduit for the movement of charge,6 and be redox active,’%* that is, be electrochemically oxidized or reduced. The redox properties of undoped trans-plyacetylene have been investigated by MacDiarmid et al.9 and the following summary has been provided: ( I ) Shirakawa, H.: Ikeda, S.; Aizawa, M.; Yoshitake, J.; Suzuki, S. Synth. Met. 1981, 4 , 43. (2) Guiseppi-Elie, A. Biosensor Applications of Polyacetylene. Paper presented before the NOBECCHE, Philadelphia, PA, April 1988. ( 3 ) Malmros, M. K.; Gulbrinski, J.; Gibbs Jr., W. B. Biosensors 1987/ 1988, 3, 7 1 . (4) Nigrey, P. J.; MacDiarmid, A. G.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1979. 594. (5) Maclnnes Jr.. D.; Druy, M. A.; Nigrey, P. J.; Nairns, D. P.; MacDiarmid, A. G.; Heeger, A. J . J . Chem. Sot., Chem. Commun. 1981, 317. ( 6 ) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger. A. J . J . Chem. Soc., Chem. Commun. 1977, 578. (7) Nigrey, P. J.; MacDiarmid, A. G.; Heeger, A. J. J . Chem. Soc.. Chem. Commun. 1979. 594. (8) Diaz, A. F.; Clarke, T. C. J . Electroanal. Chem. 1980, I I I , 1 1 5. (9) MacDiarmid, A. G.;Mammone, R. J.; Krawczyk, J . R.; Porter, S. J . Mol. Cryst. Liq. Cryst. 1984, 105, 89.
0022-3654/90/2094-3 192$02.50/0
(CH+q+‘)x + (ax)e--
(CH+q),
Eo vs Li+ ( 1 M)/Li = 3.1; Eo vs H+ ( 1 M)/f/,H2 = 0.0
-
(CH-99, + (ax)e(CH-9), E” vs Li+ ( 1 M)/Li = 1.8; Eo vs H+ (1 M)/f/,H2 = -1.3 These reduction potentials define the valence and conduction band edges, respectively, of this narrow band gap (1.3-1.4 eV), polymeric, semiconductor. This implies, as illustrated in Figure I , that all redox-active species that have reduction potentials more positive than the oxidation potential of (CH),, Le., greater than 0.0 V vs NHE, will spontaneously react with polyacetylene and result in charge-transfer oxidation or p-doping. Similarly, all redox-active species that have reduction potentials less than the reduction potential of plyacetylene, Le., more negative than -1.3 V vs NHE, will spontaneously react with polyacetylene resulting in charge-transfer reduction or n-doping. The term “doping” is used with the understanding that this process, while it produces a result similar to the doping of inorganic semiconductors, that is, a change in electrical properties, is indeed a redox process involving charge transfer. Since reduced or ndoped polyacetylene is extremely reactive with water,I0 this will not be discussed here. Oxidized or p-type polyacetylene is considerably less reactive, can be handled in air, and has been demonstrated to be stabilized by immersion in aqueous chloride solutions.”.l2 We have been engaged in a study of the aqueous environmental stability or reactivity of polyacetylene for some time now11-13 and in developing ways to chemically functionalize and (IO) Chiang, C. K.; Gau, S. C.; Fincher Jr., C. R.; Park, Y. W.; MacDiarmid, A. G.; Heeger, A. J. Appl. Phys. Left. 1978, 33(1), 18. ( I I ) Guiseppi-Elie, A.; Wnek, G. E. Stabilization of Conductive Polymers in Aqueous Environments. U.S.Patent 4,499,007, 1985. (12) Guiseppi-Elie. A.; Wnek, G. E. J . Chem. Soc.,Chem. Commun. 1983. 63. ( 1 3 ) Guiseppi-Elie, A.; Wnek,
G.E. J . Phys., Colloq. 1983, C3, C3-193.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3193
Aqueous Reactivity of Polyacetylene (CW,
ELECTROLYTE
TABLE I: Constitution and Composition of pH Buffered Solutions
PH solution constitution 1 .O potassium chloride/hydrochloric acid (0.05 M) 3.0 5.0 7.0 9.0
potassium biphthalate/hydrochloric acid (0.05 M) potassium biphthalate/sodium hydroxide (0.05 M) monobasic potassium phosphate/sodium hydroxide (0.05 M) boric acid/potassium chloride/sodium hydroxide (0.10 M)
Figure 1. Energy diagram of undoped trans-polyacetylene.
derivatize the surface for enhanced aqueous wettabilityI4-l6 and in order to confer specificity to the transducer-active responses of these chemically sensitive f i h ” J Our interest in this subject is motivated by consideration of potential aqueous-based chemical sensor and biosensor applications of polyacetylene. In this paper we report on the stability of predominantly cis-polyacetylene that has been doped to metallic levels using iodine and is exposed to various aqueous solutions of varying pH.
Background The reaction of (CH), with molecular iodine leads to spontaneous charge transfer and is accompanied by the loss of electrons from the highest occupied bonding orbitals of the polymer, Le., oxidation. (CH),
-
(CH+Y),
+ (xy)e-
(1)
There is concommitant acceptance of the charge by the iodine molecule leading to its reduction to iodide and the formation of various iodide species.
All four of the above reaction products have been identified17and are believed to occur in dynamic equilibrium with adsorbed molecular iodine. The stability or degradation of the resulting conducting polymer can be described as arising from three broadly defined possible time course of reactions. These arise from ( I ) reactivity of the dopant counteranion with the p-doped polymer itself, (2) reactivity of the dopant counteranion with constituents of the environment, and (3) reactivity of the p-doped polymer backbone with constituents of the environment. The fact that stability of p-doped polyacetylene is so vastly varied given doping by widely differing dopant^^^,^* suggests that the issue of stability is generally dominated by reactions involving the counteranions rather than by reactions of the p-doped polymer backbone itself.
Experimental Section Materials. Polyacetylene was synthesized as the free-standing cis-polymer film by the general procedure of Shirakawa et al.’9*20 Samples were typically 20% trans isomer2I and were handled in a Vacuum Atmospheres drybox. The polyacetylene films were iodine doped to a general metallic composition of [CHI0,1B-0,20]x (14) Guiseppi-Elie, A.; Wnek, G. E. J . Phys., Colloq. 1983, C3, C3-154. (15) Guiseppi-Elk, A.; Wnek, G . E. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2601. (16) Guiseppi-Elie, A.; Wnek, G . E.; Wesson, S. P. Langmuir 1986, 2, 508. (17) Rolland, M.; Lefrant, S.;Aldissi, M.; Bernier, P.; Rzepka, E.; Schue, F. J . Electron. Mater. 1981, 10(4), 619. (18) Druy,M. A.; Rubner, M. F.; Walsh, S.P. Synth. Met. 1986, 13, 207. (19) Ito, T.; Shirakawa, H.; Ikeda, S.J . Polym. Sci., Polym. Chem. Ed. 1974, 12, 1 1. (20) Chein, J. C. W. Polyacetylene: Chemistry, Physics and Materials Science: Academic Press: Orlando, FL, 1984; Chapter 2. (21) Ito, T.;Shirakawa, H.; Ikeda, S.J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 1943.
u Figure 2. Aqueous exposure conductivity cell.
in a vapor stream and subjected to overnight dynamic vacuum at Torr. Upon doping to these levels the cis polymer component isomerizes completely to the trans polymer.20 The pH sensitivity of iodine-doped polyacetylene was examined under three different conditions: in solutions of unbuffered perchloric acid (HC104) at pH 1.0, 3.0, and 5.0; test solutions of unbuffered, pH-adjusted 0.50 M NaCl at pH 1 .O, 3.0, 5.0,6.9, and 9.0; and, finally, in pH-buffered solutions prepared by using the reagent compositions shown in Table 1. In all cases the test solution volume was made large (250 mL) relative to the volume of film used (0.005-mL geometric volume) to avoid any possible film buffering effects. Electrical Conductiuity. Conductivity measurements were made according to the standard four-probe technique22for both aqueous and ambient measurements. Electrodag 502 paste (Acheson Colloids) was used to both define the contact area and provide Ohmic contact to the film. Test samples were strips 0.5 cm wide X 2.0 cm long X 100 pm thick with electrode separations of 0.5 cm. Each electrode contact and the entire platinum lead were completely coated in Microstop Stopoff Lacquer (Pyramid Plastics Inc., AR). This ensured that the electronic conductivity of the film was measured, exclusive of ionic contributions arising from electrolysis at the bare platinum electrodes. Molecular oxygen is a well-known dopant for p~lyacetylene~~ and has a longer term deleterious effect on its material proper tie^.,^ To eliminate the influence of dissolved oxygen, the solutions were deaerated using a 15-min purge with prepurified argon and a positive pressure of argon was maintained within the test vessel over the time course of each experiment. Mounted electrodes were rapidly transferred from the drybox to the deaerated test solution held in a three-neck, round-bottom flask as shown in Figure 2. Initial conductivities were typically 100-300 S cm-I. Electrode Potential Measurements. Electrode potentials were taken as the steady-state, rest, or open-circuit potential measured vs a remote saturated calomel electrode (SCE). The SCE was connected to the cell via a Luggin capillary placed within 1 mm of the polymer’s surface. Rest potential measurements were made using a > IOI4 ohm input impedance Keithley 616 Electrometer. To obviate the possibility of spurious readings resulting from ionic polarization effects, the electrode potentials were measured before the application of the impressed voltage needed for conductivity measurements. (22) Wider, H. H. Laboratory Notes on Electrical and Galvanomagnetic Measurements; Materials Science Monographs 2; Elsevier: Amsterdam, 1979. (23) Haleem, M. A,; Billaud, D. Polymer 1982, 23, 1400. (24) Pochan, J. M.; Pochan, D. F.; Rommelmann, H.; Gibson, H. W. Macromolecules 1981, 14. 1 IO.
Guiseppi-Elie and Wnek
3194 The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
-
a
---
Aspon
-----
Water
-----
pH30
pH10
W5 0
..................
2
0
4
6
10
8
12
14
(Thousands) TIME (mid
0.80
w
-
0.60
sg -E>
4
0.40 Lr-.
------ -_------____-__
s
0.60}
0.20"\\,
'
0.00
0.00
1 0
2
4
8
6
1
0
1
2
4
(Thousands) TIME (mid
Figure 5. (a) Normalized four-probe electrical conductivity vs exposure time of (CHI,), (y = 0.18-0.20) in deaerated, buffered solutions of pH 1.0, 3.0, 5.0, 7.0, and 9.0. (b) Electrode potential vs SCE vs exposure time of (CHI,), (y = 0.18-0.20) in deaerated, buffered solutions of pH 1.0, 3.0, 5.0, 7.0, and 9.0.
P=
lo-' 0
2
4
6
10
8
12
14
(Thousands)
0.00' 0
.
.
.
.
.
2
4
5
7
9
.
.
'
1 1 1 2 1 4
TIME (mid
_____
--_pH50
b
0.00
'
0
pW9
---
pH1 0
Figure 6. pH dependence of the measured electrode potential, E,, vs SCE for iodine-doped polyacetylene, (CHIJXt for compositions y = 0.007, 0.168, 0.210.
W3 0
pH90
I
2
4
6
8
10
12
14
(Thousands) TIME (mid
Figure 4. (a) Normalized four-probe electrical conductivity vs exposure time of (CHI,), (y = 0.18-0.20) in deaerated 0.50 M NaCl of pH 1.0, 6.9, and 9.0. (b) Electrode potential vs SCE vs exposure time of (CHI,), (y = 0.18-0.20) in deaerated 0.50 M NaCl of pH 1.0, 6.9, and 9.0.
Results and Discussion Figure 3a shows the conductivity change versus the time of exposure to perchloric acid for iodine-doped polyacetylene. The accompanying changes in electrode potential over the same period of approximately 9 days is shown in Figure 3b. Plotted along with the conductivity changes are previously reportedI3 changes in
similarly measured, normalized conductivity made in deaerated, deionized, distilled water and under dry, prepurified argon. Figure 4 shows similar results in pH-adjusted 0.5 M saline solution, and in Figure 5 are shown the normalized conductivity and electrode potentials measured in buffered solutions of pH 1.0, 3.0, 5.0, 7.0, and 9.0. Generally similar trends are observed in buffered solutions as in unbuffered perchloric acid and unbuffered saline solutions. There is a clear parallel in the two material properties of electrical conductivity and electrode potential with both trends toward reduction in these properties with time. The rates of degradation are considerably greater than that which is considered normal for iodine-doped polyacetylene in deaerated, deionized, distilled water and more so over that which occurs spontaneously, and is the result of poor counteranion stability, in an inert atmosphere. Figure 6 shows the measured steady-state electrode potential vs SCE of iodine-doped polyacetylene films, (CHly)x, for the compositions y = 0.007,0.168, and 0.210. Measured in deaerated, unbuffered solutions of various pH, each potential measurement was made on separate samples after 15 s of immersion. The electrode potentials at all three compositions are seen to be
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3195
Aqueous Reactivity of Polyacetylene
-3
. -2 u!
2 -1
P O
PH
Figure 7. Poubaix diagram showing the conduction and valence band edges of trans-polyacetylene compared with the E,, of water over the pH range 0-14.
clustered in a narrow band about a fixed potential which is ca. 0.45 V vs SCE and maintains this value over a major portion of the pH range, between pH 1 and 9. There is a notable dip in potential at values below pH 1. However, there is a very clearly defined and dramatic fall in electrode potential at values of pH greater than 9, the magnitude of the fall being larger the higher the pH. In addition, the previously noted band of variation tightens very sharply to yield a near Nernstian (54.0 mV per pH unit) straight-line fall in the electrode potential between pH 9 and 14. In the absence of electrochemical reactions, the polyacetylene/electrolyte interface accumulates charge and develops25 (1) a space charge layer within the semiconductor, (2) trapped surface states and charges associated with adsorbed ions or ionized surface groups in the surface of the semiconductor, and (3) a compensating space charge layer in the electrolyte. For few surface trapping states and few adsorbed ionized species, (such as occurs at the surface of a hydrophobic, polymeric, semiconductor like polyacetylene16) the potential difference between the interior of the semiconductor and the bulk of the electrolyte approximates the space charge potential drop within the semiis ca. conductor.26 The carrier concentration in [CHIo~184).20]x 3 X 1021per cm3 and is comparable with that of the electrolytes (1019cm-3 at 0.01 M) with which it is in contact. For this reason, highly doped polyacetylene is not expected to contribute a significant space charge layer to the interfacial potential. The net effect of polymer/electrolyte contact is to drive the equilibration of the Fermi energies of the semiconductor and the redox electrolyte. The measurement of changes in the electrode potential of polyacetylene vs an adequate reference electrode therefore establishes changes in the Fermi energy of the "doped semiconductor" and is instructive as to changes in carrier (hole) population, p, via the equation2' Ef = Evb - kT In ( p / N v )
(5)
where Ef is the Fermi energy, Evbis the energy of the valence band edge, k is Boltzmann's constant, T is absolute temperature, p is the density of carriers in the bulk semiconductor, and N, is the density of states at the valence band edge. The applicability of eq 5 and the Fermi level concept to a polymeric semiconductor may be criticized28on the grounds that there are no free electrons associated with the redox couples in either of the two phases of this system. However, there are also justifications for using this concept in the study of organic/electrolyte systems.29 In the Poubaix-like diagram of Figure 7 is shown the established conduction and valence band edges of undoped, trans-polyacetylene plotted along with the pH dependence of the redox potential of the constituents of water. The diagram uses the solid-state (25) Gerischer, H. In Physical Chemistry: An Advanced Treatise; Eyring, H.; Henderson, D.; Jost, W., Eds.; Academic Press: New York, 1970; Vol. 9A, Chapter 5, p 496. (26) Gerischer, H. J . Electroanal. Chem. 1975, 68, 263. (27) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980; p 401. (28) Bockris, J. 0. M.; Khan, S. U. M. Appl. Phys. Lett. 1983,42, 124. (29) Gratzel, M. Heterogeneous Photochemical Electron Transfer; CRC Press: New York, 1988; Chapter 1, p 21.
convention and represents the thermodynamically calculated reduction potentials for the hydrogen evolution and oxygen reduction reactions expressed as Eo vs pH. The diagram assumes that the conduction band edge and the valence band edge of trans-polyacetylene are themselves invariant with pH. One expects that charge-transfer equilibration across the film/electrolyte interface, as a function of E,,, will move the interfacial band edges accordingly. However, the absence of charged surface states and the existence of fully satisfied valence at the surface of transpolyacetylene makes this unlikely. Over most of the pH range, the hydrogen evolution reaction sits well within the 1.35-eV band gap, and therefore there exists no possibility for spontaneous charge-transfer reaction between water and trans-polyacetylene which will lead to hydrogen evolution. However, at low pH, the reduction potential for hydrogen evolution falls below the conduction band edge and accordingly establishes the possibility for spontaneous charge transfer leading to oxidation of the polymer and reduction of hydrogen ions. This feature could in principle adequately explain the observed protonic acid doping of polyacetylene;30however, kinetic limitations to hydrogen evolution at a valence-saturated, organic surface precludes this from being the only possible explanation. It is also instructive to note from this diagram that molecular oxygen, by sitting always more positive than the valence band edge over the full pH range, is energetically capable of spontaneous oxidation of trans-polyacetylene. Molecular oxygen is therefore expected to be a charge-transfer dopant for polyacetylene under all conditions. Under ambient conditions and in the presence of adsorbed, pH-neutral water vapor, the reduction product of oxygen doping is expected to be the hydroxyl anion which also serves as the only available counteranion. The hydroxyl anion is known to be an aggressive nucleophile for p-doped polya~etylene.~~.'~ It is likely that, under these conditions, charge-transfer reduction products, such as OH-, react further leading to a mixture of products containing carbonyl fun~tionalities~' and to an accompanying loss of conductivity. Recently it has been reported that by "activating" the undoped polyacetylene, that is, by using a brief immersion in a solution such as 7 M HBF, or by electrochemical doping followed by undoping in Li+BFL, there results an improved stability in the presence of oxygen. This observation supports the foregoing and previously reportedl1J2view as it presents an example of the availability of a less reactive counteranion, BF,-, serving to extend film life in the presence of an otherwise degradative oxidant as oxygen. When it is iodine doped, polyacetylene produces energy states in the gap. The decay in the initial high electrical conductivity of iodine-doped polyacetylene under argon atmosphere has been adequately.explained as arising from secondary reactions involving the (If, Is-) counteranions with these midgap states on the polymer backbone.32 When polyacetylene is immersed in deaerated, deionized, water, the increase in the rate and extent of degradation likely arises from the parallel contribution of reactive hydroxyl anions similarly reacting with these nonbonding, midgap states. The measured electrode potential of iodine-doped polyacetylene is found in all cases to originate ca. 0.45 V vs SCE (0.69 V vs N H E or 5.19 V on the vacuum scale). This value, E,,,, is evident irrespective of the doping level achieved and irrespective of the pH value of the electrolyte solution into which the film is immersed. These electrode potentials are steady and stable and show no systematic variation with the initial dopant composition over the range y = 0.007, 0.168, and 0.210. Equally important, no systematic variation in electrode potential with pH is observed over the range 0-9. This can be clearly noted in the initial potentials measured in all three pH test environments and is particularly evident in Figure 6. This value of potential, E, = 0.45 vs SCE or 0.69 vs NHE, corresponds exactly with that reported by MacDiarmid et al.9 for the reaction (30) Frommer, J. E.; Chance, R. R. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1986; Vol. 5, p 462. (31) Gibson, H. W.; Pochan, J. M. Macromolecules 1982, 15, 242. (32) Guiseppi-Elie, A. Sc.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1983.
3196
Guiseppi-Elie and Wnek
The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990
(CH+o,l+u ),
+ (ax)e-
-
(CH''.'),
which we believe is a characteristic open-circuit potential which defines the metallically doped regime of polyacetylene. Above pH 9, however, the instantaneously measured electrode potential is appreciably lower (more negative) than 0.45 V vs SCE and is lower the higher the pH is, irrespective of the starting polymer composition. There is a near Nernstian fall of ca. 54 mV per decade in electrode potential at pH values above 9. We note that the equilibrium redox potential of the O2 reduction reaction is at pH 9 calculated to be 0.70 V vs NHE or 0.46V vs This value corresponds exactly with the steady-state electrode potential of metallically doped (CH), when measured in the range pH 1-9. Above pH 9 the Eo of the oxygen reduction reaction is more negative than the E , of (CH), and these lines should cross on the Poubaix-like diagram of Figure 7. Under these conditions there is observed a corresponding fall in the measured electrode potential of polyacetylene and this line almost exactly tracks the Eo of the oxygen reduction reaction. This fall in instantaneously measured electrode potential is mirrored in a rapid and precipitous decay in the time-dependent electrode potential and is also noted in the time-dependent changes in electrical conductivity. So rapid is the decay under these extremes of alkaline conditions that long term stability tests such as are reported here in aqueous perchloric acid, pH-adjusted saline solution, and pH-buffered solutions, were not possible at pHs greater than 9. The fall in electrode potential likely results from a fall in carrier population leading to a change in the population of states in the gap. This change is invariably called compensation and is due in this case we believe to direct chemical reaction via nucleophilic attack by hydroxyls at carbocation sites on the polymer backbone rather than by any possible charge-transfer reactions. Our earlier report of preliminary work using a series of nucleophilies of varying nucleophilicity established the view that the polyacetylene cation behaves not unlike a molecular cation in its reactivity with aqueous nucleophiles. Changes in electrical conductivity are also related to changes in carrier population34though the scalar approximation a = npp
(7)
where a is the conductivity in S cm-I, n is the population of carriers per cm3,p is the charge on the carrier, and is the free carrier mobility cm*/(V s). If we assume that the mobility change, dp, over the period of measurement is negligible, then changes in conductivity, du/dt, are reflective only of changes in carrier population. We therefore are provided, along with the electrode potential, two independent measurements of carrier population in the doped polymer. The time-dependent change in electrical conductivity was noted to follow the same general pattern under all test conditions studied. There was first a slight increase in the electrical conductivity in all samples which occurs over the first approximately 5 min (33) Shrier, L. L. Corrosion, 2nd ed.; Newnes-Butterworths: London, 1976; Vol. 1, Chapter 1.4, p I:%. (34) Duke, C . B.; Gibson, H. W. In Kirk-Othmer: Encyclopedia of Chemical Technology, 3rd ed.; Wiley: New York, 1982; Vol. 18, p 755.
TABLE 11: Rate of Decrease of Electrical Conductivity during Exposure to Various Test Environments A parameter"
HC104 0.5M N a C l buffered
argon
water
1.6
3.1
pH 1 p H 3 p H 5 p H 7 p H 9
8.6 17.2
12.4 8.9 8.0
8.5 8.6 4.9
3.1 8.3 7.7
6.6 17.1
"All A values a r e negative and a r e XIO-'; log ut/uO= -A?.
following immersion in the aqueous environment. Such short-term increases in conductivity are on the order of 1% and have been observed beforeI7J5 in other test environments. This initial phase, not represented in these figures, likely results from brief exposure to molecular oxygen as the sample is introduced into its test flask. This initial phase is followed by a period of rapid decline in conductivity. This then is followed by a third period of leveling off to give a quasiliniear, log ut/go = -At, decline in conductivity, where A is a rate parameter. Finally, these is in some instances an apparent approach to saturation of the conductivity level. The values of the rate parameter, A , for the linear region of the log at/uo = -At plot are presented in Table I1 for the various test environments used. It can be seen from this table that the most aggressive environments yielding the largest A values are at the extremes of pH. Both acidic and alkaline conditions are extremely deleterious to iodine-doped polyacetylene. A comparison of the relative magnitudes of the rate parameter in acid and in alkaline environments confirms that alkaline conditions are generally more aggressive to the material properties of iodine-doped polyacetylene than are acidic conditions.
Summary Implicit in this discussion has been the notion of alternate reaction pathways for dopant species. In the study of the stability of doped polyacetylene we are concerned with secondary, postdoping reactions which lead to loss of electrical properties. While polyacetylene is adequately defined as an electrode and an electroactive material, polyacetylene is also a chemically reactive material36capable of other types of reactions in addition to charge transfer. The possibility of these other types of reactions serving as alternate pathways depend in part on where these reactants sit on the energetics diagram. Secondary reactions involving hydroxyl anion and dopant counteranion reactions with midgap states are seen as the most important cause of loss of electrical properties in aqueous environments. Acknowledgment. Part of this work was done under support of the Center for Materials Science and Engineering at MIT under NSF-MRL Core Fund DMR 78-24185. Registry No. NaCI, 7647-14-5; HC104, 7601-90-3; I,, 7553-56-2; trans-polyacetylene, 25768-71-2. (35) Inoue, T.; Osterholm, J. E.; Yasuda, H. K.; Levenson, L. L. Appl. Phys. Letr. 1980, 36, 101. (36) Wnek, G. E.; Whitney, D. H. Substituted Acetylenic Polymers and Conductive Materials Formed Therefrom. U.S. Patent 4,672,093, 1987.
