NMR observation of proton exchange in the conducting polymer polyaniline

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NMR observation of proton exchange in the conducting polymer polyaniline

M. Nechtschein, C. Santier

To cite this version:

M. Nechtschein, C. Santier. NMR observation of proton exchange in the conducting polymer polyani-

line. Journal de Physique, 1986, 47 (6), pp.935-937. �10.1051/jphys:01986004706093500�. �jpa-

00210289�

935

NMR OBSERVATION OF PROTON EXCHANGE IN THE CONDUCTING POLYMER POLYANILINE

M. NECHTSCHEIN+ and C. SANTIER

Centre d’Etudes Nucléaires de Grenoble, Département

de Recherche Fondamentale, Service de Physique

Groupe Dynamique ^de Spin ^et Propriétés Electroniques,

85 X, 38041 Grenoble Cedex, France

(Regu ^{Ze 4 mars} 1986, accept6 ^{Ze 7} avril 1986)

Résumé

Nous donnons des résultats de RMN qui ^montrent l’existence d’un echange rapide ^de protons ^{entre la} matrice rigide ^du polymère ^{et une} phase d’eau adsor- bée mobile.

Abstract

We report ^NMR experiments ^which supply ^direct

evidence for fast proton exchange ^{between the} polymer

skeleton of polyaniline and adsorbed water.

J. Physique ⁴⁷ (1986) ^{935-937 juin} 1986,

Classification

Physics ^Abstracts

77.60E - 66.90

Although polyaniline has been known as a conduc-

ting polymer ^{for a} long time 1,2, ^a renewed interest for this material has arisen recently, ⁱⁿ particular

related to its potential applications. ^{One of the}

most fascinating features of polyaniline is the very

high sensitivity ^{of its} transport properties ^{to the}

presence of mobile protons 3-6. As other conducting polymers polyaniline ^{needs to be} doped ^{-or oxidized-}

to present conductivity, ^{but in} addition the degree

of protonation ^{is also a} determinant parameter. ^A kind of metal-insulator transition has been reported

to occur as a function of the pH ^at which the poly-

aniline samples have been equilibrated. ^{This effect}

has been observed on conductivity, thermopower, and spin dynamics properties. Furthermore proton-exchange

processes ^are also probably involved in the high

redox activity of polyaniline 7. ^The proton-exchange

mechanism has been qualitatively ^{described as an}

acid-base reaction as follows :

In this letter we present ^a direct proof for the

proton-exchange mechanism in polyaniline and we re-

port preliminary ^NMR experiments which open the way to quantitative studies.

The samples have. been obtained by oxidation of aniline by amonium persulfate ^{in a} fluorhydric ^acid

medium. They were washed in acetonitril in a Soxhlet, equilibrated in a sulfuric acid solution of pH

0,

and dried under vacuum. They ^were introduced into an

NMR sample holder tube in a dry box. The NMR tube was

connected to a line which enabled us to monitor the water vapor pressure, pH 01 ^on the sample ^{while ob-} serving ^{the NMR} signal. We2used a pulsed NMR spectro- meter (Bruker SXP) ^{at the} frequency ^of 54MHz.

As shown in Fig. 1, ^the signal ^{of the} proton free induction decay ^{is the} superimposition ^{of two} components ^with diferent T2

+ER CNRS 216

a) ^{a short} T2 ^{signal, or S-signal,} (T2(S)= ^20vs),

which is partly masked in the dead time of the spectrometer ^{and that we attribute to the} protons of the rigid polymer skeleton,

b) ^a long T2 signal, ^or L-signal (T (L) 300vs), ^that corresponds ^to protons ^{in a} mobile phase.

We will call "solid" or S, ^and "liquid" ^or L, ^the types ^of protons corresponding ^to a) ^and b), respec-

tively. With the time scale used in Fig. 1 the L-si-

gnal appears ^{as a} piedestal ^{for the} S-signal. ^{As its} amplitude ^is reversibly governed by Par ^{0, we attri-}

bute the L-signal ^to mobile, ^water adsorbed at the

polymer ^surface.

The L-signal ^can be removed by pumping. ^After

one hour pumping (10-‘’ ^{torr at} the pump level) ^its amplitude ^{is reduced to a few} percents of that of the

S-signal. ^For PHQ ^{close to} saturating vapor pressure the L-signal amplitude ^is about half that of the

S-signal, ^which corresponds ^{to about one} H20 ^adsorbed

molecule per aniline ^monomer. However, ^{accurate esti-}

mate of the amount of adsorbed water is difficult due to the fact that the starting ^{of the} signal ^{is lost}

in the dead time. Detailed studies of the kinetics of water adsorbtion and desorbtion are in progress and will be published subsequently.

We have measured the proton spin-lattice ^relaxa-

tion time (TI) for the S- and for the L-signals. ^It

turns our that they ^are equal (within 10%). ^This equality ^{is not} fortuitous. For different samples prepared in somewhat different conditions, ^{and also}

for a given sample ^{measured a different} frequencies,

the value of T1 ^can vary. We have found values in the range 8 to 40ms. But in all cases we have obtained the same T1 ^{value for the two} signals. ^This equality

in T1 shows that there is a single spin temperature for the protons, either in the solid, ^or in the li-

quid phase. Hence, ^a strong coupling should exist between S- and L-protons. ^This situation is quite similar to that described by.Goldman ^{and Shen} (GS) ⁱⁿ

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01986004706093500

936

Fig. ^{1 - Proton} free induction decay signal ⁱⁿ polyaniline exposed ^{to different water}

vapor pressure. The NMR frequency ^{was 54}

MHz.

LaF38. ^In this material two types ^of fluorine ^are present : ^one type ^is mobile and the other is quasi-

motionless. Furthermore there exists ^a strong ^cross- relaxation between the two fluorine types. ^{In the} general ^{case two} mechanisms contribute to the cross- relaxation : spin flip-flop ^{due to} spin-spin dipole

interactions and exchange of nuclei between the two

phases. These mechanisms also contribute to the line- width of the L-protons. Taking ^{into account motional} narrowing ^one may write :

where DLL, ^and DLS, ^{are the} spin-spin dipole ^interac-

tions of protons ^{in the} L-phase, ^or between L- and

S-phases, respectively; _TM is the correlation time for the motion in the L phase; _Tex ^{is the} proton exchange ^time between L- and S-phases; Tcr ^{is the}

cross-relaxation time; NL is the number of L-protons, and NA the number of those which can be exchanged.

GS have proposed ^a pulse sequence ^to determine the cross-relaxation time. It consists in three x/2 pulses. The first two pulses ^{are used to} prepare the spin system ^{is such a} way that the magnetization ^of

the S-spins is zero, while the magnetization ^{of the} L-spins ^{is not zero.} The third pulse, which is send after a variable delay T, enables ^{us to} follow the rate at which the equality ^{of the} spin magnetizations

is restored.

In Fig. ^{2 we see the} amplitude ^{of the} L-signal

as a function of T. In the first 2ms the L-spin ^ma- gnetization ^{decreases because} part of it is transfer- red to the S-spins, ^via cross-relaxation. Then, ^the

magnetization increases ^{due to} spin-lattice ^relaxa-

tion (T1( ) = Tt

^25ms). Without the spin-lattice

relaxation effect the L-magnetization would decrease from Mp(L) ^{o to} M (L) = ¹ NL _{NL+NS 0} ^{m (L)}

Determining MiL) ¹

by extrapolation, ^{as shown in} Fig. ^{2 we obtain a}

value for the ratio of S- to L-spins : NS/NL=2.3.

With the assumption ^{that the} exchangeable proton spends half of the time in each phase ^we estimate the

hydratation ^{rate to} about 0.8 H20 molecule per ani- line ring.

From the data of Fig. 2 in the first 2ms, ^we

deduce the cross-relaxation time : Tcr= ^{0.7ms. At the}

present stage ^{it has not been} established which one

of the two contributions to the cross-relaxation

Fig. ^{2 -} Amplitude ^{of the} long T2 proton signal (measured ^{at time} TL) ^{as a function of the}

variable delay i after the two pulse pre- paration of the Goldman-Shen sequence.

-dipole interactions, ^or proton exchange- ^is dominant,

but the reality of proton exchange ^{has been} proved ^as follows.

In order to visualize the proton-exchange pheno-

menon we have performed ^the experiment ^{as shown in} Fig. ^{3. The} amplitude ^{of the} L-signal is measured as a function of time. The FID amplitude ^{is taken} during

the aperture ^{of a} gate (1s), 45ps after the end of the rf pulse (point tL ^{in Fig. 1). At time} tAthe sample ^is pumped off. About 90% of the adsorbed water is removed by pumping ^{for one} hour. At time tB ^a

pressure of 9.2 ^torr of deuterated water is suddenly

introduced. We observe that the rp oton L-signal ^in-

creases.

Fig. ^{3 -} Amplitude ^{of the} proton ^FID signal ^measu-

red at time TL(see ^{Fig. 1) as a function}

of time.

At time tA : ^start pumping.

At time t ^: pressure of D20 (p = ^9.2

torr) suddenly introduced.

937

This is evidence that the following processes take place :

a) D20 adsorbtion at the polymer surface. From the gaseous phase, D20 ^{molecules are} condensed into the adsorbed, liquid phase, ^L.

b) Exchange ^between proton in the solid phase ^and deuteriums in the L-phase.

Hence, owing ^to the presence of D20, protons ^are transferred from the solid to the liquid phase ^and

contribute to the L-signal. ^{The rate at} which the

L-signal increases after introduction of D20 ^{is the}

same as after introduction of H20, which shows that the kinetics of the L-signal amplitude ^is governed by

gas diffusion in the polymer matrix and not by proton exchange.

The L-signal ^{reaches a} maximum in about half an

hour, ^{and then} decreases, ^{with a time constant} of N 3 hours. We attribute the decay ^to isotopic exchange

between the liquid ^and the gaseous phase. ^Protons being exchanged ^with deuteriums are transferred into the gaseous phase ^{and are} lost for the L-signal.

We have benefitted by discussions with G. Bidan,

F. Devreux, E. Genies, ^B. Lamotte, ^H. Lecavellier and J.P. Travers.

REFERENCES

1 R. de Surville, ^M. Jozefowicz, ^L.T. Yu, ^J.

Périchon and R. Buvet, Electrochimica Acta, 13,

(1968) ¹⁴⁵¹

2 M. Doriomedoff, ^F. Hautière-Cristofini, ^{R. de} Surville, ^M. Jozefowicz, ^{L.T. Yu et R.} Buvet, J.

Chim. Phys., 68 ^{(1971) 1055.}

3 J.P. Travers, ^J. Chroboczek, ^F. Devreux, F.

Genoud, ^M. Nechtschein, A.Syed, E. Genies and C.

Tsintavis, ^Mol. Cryst. Liq. Cryst., 121 ⁽¹⁹⁸⁵⁾

195

4 A.G. MacDiarmid, ^J.C. Chiang, ^W.S. Huang, ^B.D.

Humphrey and N.L.D. Sumasiri, ^Mol. Cryst. Liq.

Cryst., 125 ^{(1985) 309}

A.G. MacDiarmid, ^J.C. Chiang, ^M. Halpern, ^W.S.

Huang, ^S.L. Mu, ^N.L.D. Somasiri, ^W. Wu, ^S.I.

Yaniger, ^Mol. Cryst. Liq. Cryst., ¹²¹ (1985) 173

5 T. Kobayashi, ^H. Yoneyama ^{and N.} Tamura, J.

Electroanal. Chem., 161 ^{(1984) 419;} 177 ⁽¹⁹⁸⁴⁾

293

6 W.R. Salaneck, ^I. Lundström, ^W.S. Huang ^{and A.G.}

Mac Diarmid, ^{to be} published

J.C. Chiang ^{and A.G.} MacDiarmid, Synthetic Metals, 13 ^{(1986) 193}

7 E. Genies, ^A Syed ^{and C.} Tsintavis, ^Mol. Cryst.

Liq. Cryst., ¹²¹ (1985) ¹⁸¹

8 M. Goldman and L. Shen, Phys. ^Rev. B, 144 ⁽¹⁹⁶⁶⁾

321