STABILITY OF IODINE-DOPED POLYACETYLENE IN AQUEOUS ENVIRONMENTS

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STABILITY OF IODINE-DOPED POLYACETYLENE IN AQUEOUS ENVIRONMENTS

A. Guiseppi-Elie, G. Wnek

To cite this version:

A. Guiseppi-Elie, G. Wnek. STABILITY OF IODINE-DOPED POLYACETYLENE IN AQUE- OUS ENVIRONMENTS. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-193-C3-197.

�10.1051/jphyscol:1983339�. �jpa-00222691�

JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-193

STABILITY OF IODINE-DOPED POLYACETYLENE IN AQUEOUS ENVIRONMENTS A. Guiseppi-Elie and G.E. Wnek

Department of Materials Science and "Engineering, Massachusetts Institute of Technology, Cambridge, MA. 02139, U.S.A.

Résumé : Le polyacétylène dopé à l'iode montre une stabilité exceptionnelle dans des environnements aqueux débarrassés d'air.

Il se dégrade en présence de nucléophile's aqueux avec un taux qui augmente quand l'activité du nucléophile augmente.

Abstract; Iodine-doped polyacetylene has been found to display exceptional stability in deaerated, aqueous environments and to degrade in the presence of aqueous nucleophiles with a rate which increases with increasing nucleophilicity.

I INTRODUCTION The potential utility of highly conductive polymers as electrode materials has been demonstrated by polypyrrole(1) and underscored by the fabrication of lightweight, rechargeable, storage batteries based on polyacetylene(2), (CH)", and poly(p-phenylene)(3). We have begun to explore the use of conductive poiymers as electrode materials in aque- ous media. Pater is an attractive electrolyte medium because of its ready availability, low cost, non-toxicity and because useful fuel in the form of H? can be obtained from its photoelectrolysis(4). Although it is doubtful that n-type (reduced) organic conductors will ever have a finite lifetime in water (due to notoriously facile protonation exhibited by carbanions), p-type (oxidized) polymers appear to be much less sensitive in aqueous media(5). In fact, polyacetylene may be rendered a p-type conductor by simple exposure to aqueous KI/I- solut- ions and by electrochemical oxidation in aqueous KI solutions (5,6).

This is contrary to our intuition concerning the susceptibility of carbenium ions to nucleophilic attack by water, as illustrated for p- type, trans-(CH) :

{1}

We report here our observations of exceptional stability of iodine- doped (CH) in aqueous environments and our preliminary study of the reactivityxof p-type doped (CH) with aqueous nucleophiles. From the latter work an environment for the enhanced stabilization of (C H I n)x

has been identified.

II EXPERIMENTAL Cis-polyacetylene films were prepared at -78°C using techniques similar to those developed by Shirakawa and coworkers(7,8). Elemental analyses of the as-synthesized, pristine polymers indicated CH contents of >99%. Iodine doping was achieved by exposing the pristine polymer to a vapor stream of iodine carried in deoxygenated and dried argon. The doped polymer was subsequently subjected to dynamic vacuum (io_5 torr) for 24 hours to remove adsorbed iodine. Elemental analyses of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983339

C3-194 JOURNAL DE PHYSIQUE

resulting doped polymers produced compositions in the range (CHI 0.18- 0.20)x and were in good agreement with simultaneously determined com- positions via weight uptake. The resulting room temperature four- robe conductivities were in the range 200-300 a-lcm-1. The conductivities of the doped films were followed as a function of exposure time under purified argon; in deaerated, deionized, distilled water; in laboratory air and in deaerated, aqueous solutions of analytical grade NaI, NaSCN, NaBr, NaCl and NaF made from water of similar high quality. For the immersion exposure experiments, samples were mounted in a dr~r box onto four-probe platinum electrodes using Flectrodag 502 and the leads were individually sealed with Yicrostoo Stopoff Lacquer. This latter step was done to insure that the electrical conductivity of the doped (CE)x was exclusively measured without ionic contributions from the electro- lyte solutions. The mounted films were quickly transferred (throughair) to 250 ml three-neck flasks containing the desired deaerated, aqueous solution. The solutions were blanketed with argon throughout the duration of exposure (approximately nine days).

mSULTS AND DISCUSSION

Figure 1 shows the stability nerformance of iodine-doped polyacetylene as indicated by the variation in the normalized conductivity (oe/oeo) followed as a function of exposure time in purified argon; deaerated, deionized, distilled water and laboratory air. Iodine-doped (CEIx is observed to degrade even under highly controlled, inert conditions

(argon), the conductivity falling to 27% of its initial value over the approximate 9 day period. This decay in conductivity is presumed to result from slow iodination of the (CE)Y backbone via: ^'

Reaction {2) leads to an interruptionof chain conjugation and annhia- lation of charge carriers. For this reaction the following general, chemical rate expression may be written,

where K (specific rate) is a constant and n is the reaction order with respect to the conductivity variable. Equation ( 3 ) is valid if the conductivity is proportional to the amount of chemical entity which is resoonsible for conduction, i.e. the char9e transfer complex, charged soliton or carbenium ion. Taking loglO of equation I31

should produce a straight line of slove n. Figure 2 is such a plot of d'Ge'o%) o

loglO ⁽ e/oeo ) and is shown to not be a straiqht line. This indicates that the conductivitv decay does not fit the general, chemical rate expression and the assumptions inherent therein.

This is at variance with the oseudo-first order kinetics observed by Pochan - et al(9) - for this reaction. Thus conductivity is therefore not simply pro?ortional to the abundance of carbenium ions. This is as expectedandis consistent with the fact that the conductivity is also proportional to the charge mobility as can be seen in the general conductivity expression,

where n is the number of carriers, the carrier mobility and e the charge on the carrier. This confirms what is often suggested and

intuitively understood about doped (CH) - that changes in conductivity resulting from reaction of the active csrbenium ions generally also result in changes in the carrier mobility. The latter results from interruptionof the backbone conjugation.

,,,

.J, p L o r s OF THE WRWLILED C~NDUCTIVITI (0. fa-...) IIEXVOSURC TIHE (MIN.) FOR IODINE M P E D POLYACETILENE, (CH11,20)X , UNDER PURIFIED ARGON, DEAERATED, DEIOIIIZED D I S T I L L E D WATER AND I N LABORATORY AIR.

FIG. 2. PLOT OF THE G E N E R I L I W RATE EXPRESSION FOR THE DECAY I N CONDUCTIVITY OF IODINE DOPED POLYACETIUNE (CH10,20)r

UNDE* ARGON.

Figure 3 shows data obtained for the degradation of iodine-doned (CH)x under argon plotted as lou (-d loq ('e/5ea) vs - log (Oe/oeo )

.

produces a straioht line of s$gpe n=2.9 and intercept -3.29 with a correlation coefficient of 0.992. This suggests that the decay kinetics can be emnerically represented by an expression of the form

Such a logarithmic dependence of the conductivity on the copulation of carriers has also been observed by Xanicki et al(10) in their study of the iodine doping kinetics and was interpreted to suwport a three- dimensional variable range hopping conduction mechanism.

F I G . 3. P U T OF THE MODIFIED RATE E W R E S S l O l FOR THE DECAY I N c O # W C T l V l T Y OF IODINE WPm W L Y A C m W E (uIO.l~)r

UNDER ARWN.

FIG. 4, PLOTS OF THE W R M L I I E D CONDUCTIVITY (%/%I II EXPOSURE T I M (HI*) FOR IODINE-mVED POLYACETYLEWE, (CHlo,ll.o,la)x, I N DEAERArED. AWEOUS IODIDE SOLUTIONS OF CONLIYTRATIOWS 9.1% - 12.3" CMPARED U I I I I BElllVIOR WDLR PURIFIED ARBOM AND DEAERITED, DEIOIIIZED, D I S T I L L E D HATER.

There avoears in Fiqure I little difference in the perforntance of highly doped in deaerated, deionized, distilled water and labor- atory air. This would suggest that the influence of oxygen in acceler- ating degradation is secondary to that of moisture over the nine dav period of observation.

JOURNAL DE PHYSIQUE

Figures 4 through 8 shows the stability performance of iodine-doped (CH)x on exposure to deaerated, aqueous solutions of the nucleophiles

-

I , SCN-, ~ r - , ~ 1 - and F- (arranged in order of decreasing nucleophil- icity. Each nucleoyhile was studied at concentrations of 0.1Y, 0.5P4, 1.011 and at saturation. In general the extent of degradation increases with increased nucleonhilicity of the test anion though the effect of concentration is somewhat variable.

.Ar

10%

.

s . . -.

Go _10:

. .

lo!

. . .... .. .

FIG. 5. PLOTS OF THE WORIUL12FD CONOUCr(vlTY (s I%) EXPnsURE T1lK ('lH' FIG. 6. PLOTS OF THE IIOPVALIZED C O R W C T l V l l l ( C Y. EXPOSURE 11- ( U l N ) VOR FOR l O D l n E - M P E o PoLYACETILENE~ (CII10,19.0.20)x, I N DEAERATEDI A(IUEOUs

IHIOCIIHATE s n u m ~ n m w c o m e n ~ ~ ~ r ~ o n s %lh\ - ^?7.2n CWP~UED wnu m r

IODINE-WPED POLYACEWLENE, (C!IIo,,,.,.20), I N DEAERATED. IaUEOUS B R M I D E BEHAVIOR UNDER PURIFIED ARGOH AND DEAERATED, D E l O t l l l E D l D I S T I L L E D *hTEn.

SOLUrlCNS OF CONCEHTRATIMS 9 . h - I!.% CWWARED U l T N THE BEWVIOR UNDER PURIFIED ARC4N AND DEAERATED, DEIONIZED, D I S T I L L E D WATER,

-

_{' .}

_{.. .. . . . . .}

' 1

_. . _. . ...

nil-

=. . .

'.

.. . - .

5 ^{* - . ,}

.

;'

.

Z ^{* 0 . 1}

A 0 1 ⁰

ldi F& / M

TIME /mln %lo3 TIME /mln ,,los

rlc, 7, PLOTS OF 1 1 1 ~ RDRMLIIED C e n W C T I v I T Y (a. 1%) VI E ~ Q a S ~ 'llY

OIIHS.) FOR IODINE-MPED POLIACETYLENEI ( C l l l o . ~ a - o . z o ) x 'n ^{FIG. 8. PLOTS} OF THE WOR)Y\LIZED CONDUC~~VIIY VI EXPOSURE TI= (WIN) FOR OEAER&~ED, AouEOUE C l L O R l D E SOLUTIONS OF C M C E H l R * T l O N s '.IR - IODINE-MPED POLIlCETlLENE, (C~II0.1(I-o,20)L, I N DEAERATED, AWfOUS F L U D I I D E

15.m ~ ~ M P A R E " r l r t r nc s E i t k v 1 a n wnEn P U R I F I E D * S O N AND mAERATED*

s o ~ u r ~ o u s OF C a n c t n r n A r l w s 0.llk-1.h CMPARED WITN 11% BEWVIOR U D E s PURIFIED ARbON AND DIIERATED. DEIONIZED1 D I S T I L L E D U T E R .

D E l n H l l E O , D l r r l l l E n n n r E R .

"he case for chloride ion is ancmalous and is shown in Figure 7 to enhance stability over that observed under argon at concentrations of 1.OM and greater(l1). This anomaly possibly results from the formation of the interhalide, 12C1 , which acts to decrease the concentration of free iodine and create a more stable counteranion for the charge carriers.

The conductivity of iodine-doped polyacetylene is found to degrade even under inert conditions. The degradation kinetics is not treatable by the classical, general chemical rate expression but is found to obey a logarithmic dependence on carrier population. Eighly p-doyed

(CE) is found to be quite stable in aqueous environments with accel- eratgd decay kinetics in the presence of added nucleophiles. In general the decay increases with increased nucleo~hilicitv of the added nucleophile.

T h i s w o r k w a s surlported by t h e ?-$IT C e n t e r f o r ! r a t e r i a l s Science and E n g i n e e r i n g (PJSF-YRL C o r e F u n d DFR 7 8 - 2 4 1 8 5 )

.

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