On the iodine-doping of polyaniline and poly-ortho-methylaniline

Synthetic Metals, 41-43 (1991) 739-744

ON THE IODINE-DOPING

7 39

OF POLYANILINE AND

POLY-ortho-METHYLANILINE

Lixiang W A N G , Xiabin JING and Fosong W A N G Polymer Physics Laboratory, C h a n g c h u n Institute of Applied Chemistry, Academia Sinica, C h a n g c h u n

130022, People's Republic of China

ABSTRACT The

reactions

of polyaniline

and

degrees with Iz were followed by

poly-o-methylaniline

FTIR and

of different

oxidation

electrical conductivity measurements.

The results showed that the reaction of c o m m o n

polyanilines with 12 was oxidation

in nature whereas that of the fully reduced ones was doping. The latter took place in two steps: oxidation of benzene-diamine units into quinone-diimine units (redox between I2 and the polymer chain) and formation of a conjugated system consisting of four aromatic rings (intramolecular chain redox).

INTRODUCTION Common

polyanlline,

benzene-diamine

composed

of nearly equal

amount

of quinone-diimine

units, can be doped to the metallic conducting regime w h e n

and it is

reacted with a protonic acid. This is referred as to "protonic acid doping" and has caused considerable attention [I-3]. By contrast, few research effort [4,5] has been devoted

to

oxidative

or

reductive

doping

for

polyanilines

although

they

are

well-known for polyacetylene and other conducting polymers. In this paper, w e

are to describe the reactions of c o m m o n

and

fully reduced

polyaniline and poly-o-methylaniline with iodine and discuss their mechanisms.

EXPERIMENTAL Common

polyaniline (C-PAn)

and

poly-o-methylaniline

(C-PMAn)

in base form

were synthesized according to the method described previously [6,7]. Fully reduced polyanlline (R-PAn) About

and poly-o-methylaniline

1.0 g of C - P A n

phenylhydazine

or C - P M A n

solution was

added

(R-PMAn)

were prepared as follows:

was dispersed in 250 ml ethanol and excess of into the dispersion.

The

mixture

was

stirred

" Project supported by the National Natural Science Foundation of China.

037%5779/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

740 magnetically overnight and centrifuged. vacuum-dried

a t room t e m p e r a t u r e

The r e a c t i o n s

of t h e

The p r o d u c t w a s

washed several times and

for more t h a n 24 h o u r s .

polyanilines

with iodine

p o l y m e r s w i t h i o d i n e s o l u t i o n in n - h e p t a n e

were carried out by t r e a t i n g

at proper concentration

atmosphere, filtrating and washing with n-heptane

the

under nitrogen

a n d a c e t o n e . The p o l y m e r s w e r e

r e a c t e d w i t h 1.0 M a q u e o u s HCI or HI s o l u t i o n to o b t a i n t h e i r s a l t or d o p e d forms. IR s p e c t r a was coated

w e r e r e c o r d e d on a DIGILAB F T S - 2 0 E

on a KBr p l a t e

n-heptane

and

contacted

with

FTIR s p e c t r o m e t e r . T h e s a m p l e

gaseous

HC1 or i o d i n e

solution

in

f o r d i f f e r e n t time.

RESULTS AND DISCUSSION 1. E l e c t r i c a l c o n d u c t i v i t y a n d d o p i n g l e v e l The highest electrical

conductivities

achieved

for v a r i o u s reaction

products are

l i s t e d in T a b l e 1.

TABLE 1 Electrical conductivities

(S/cm) of t h e r e a c t i o n p r o d u c t s of p o l y a n i l i n e s w i t h h ,

HI

a n d HCl Polymers

Reagents HI

h C-PAn R-PAn C-PMAn R-PMAn

3.9 2.8 5.4 4.6

It is s e e n t h a t : three

orders

respectively; HCl-reacted of

of (2)

x x x x

10 -4 10-* 10 -s 10 -3

HC1

4.2 x 10 -1 -3.6 x 10 -3 --

6.7 3.5 x 10 -5 1.3 x 10 -2 5.8 x 10 -7

(1) t h e r e a c t i o n p r o d u c t s of R - P A n a n d R-PMAn w i t h i o d i n e a r e

magnitude HI-

or

more

conductive

HCl-doped

C-PAn

than

those

and

C-PMAn

of

C-PAn are

and

C-PMAn,

conductors

while

R - P A n a n d R-PMAn a r e a t m o s t s e m i c o n d u c t o r s ; (3) t h e c o n d u c t i v i t i e s

iodine-reacted

R-PAn

and

R-PMAn

are

surprisingly

identlcal

H I - d o p e d C - P A n a n d C-PMAn, r e s p e c t i v e l y . T h e s e d a t a s u g g e s t t h a t

with

those

of

t h e r e a c t i o n of

p o l y a n i l i n e s w i t h i o d i n e d e p e n d s on t h e i r o x i d a t i o n d e g r e e a n d t h e r e a c t i o n of t h e r e d u c e d p o l y a n i l i n e s w i t h i o d i n e is e q u i v a l e n t

to H I - or H C l - d o p i n g o f t h e common

p o l y a n i l i n e s a s f a r a s c o n d u c t i v i t y is c o n c e r n e d a n d t h e r e f o r e is d o p i n g in n a t u r e . When R - P A n or R-PMAn is r e a c t e d of

the

polymer

changes

disappears

gradually

Therefore

it

is

I t is f o u n d

that

i o d i n e / R - P A n is l e s s t h a n

to

w i t h i o d i n e s o l u t i o n in n - h e p t a n e ,

light-brown

or r e m a i n s

possible

reaction by examining the starch.

from

to

brown, depending

determine

the

color and detecting the

filtrate

0.75.

green on

amount the

and the of

that

of

amount iodine

free iodine

the

the

of i o d i n e

consumed in t h e

used. in

filtrate

is c o m p l e t e l y c o l o r l e s s w h e n w e i g h t

Beyond t h i s ratio,

f r e e i o d i n e is d e t e c t e d .

color

solution

the with

ratio

of

In t h i s

741

way, the

highest molar doping level,

shown previously

I/CsH4NH, is d e t e r m i n e d

to be a b o u t

0.54.

As

16], t h e d o p i n g l e v e l o f common p o l y a n i l l n e s is a p p r o x i m a t e l y 0.5.

T h e s e d a t a a r e in a g r e e m e n t w i t h t h e c o n d u c t i v i t y

v a l u e s in T a b l e

1.

2. FTIR s p e c t r a In o u r p r e v i o u s w o r k [8,9], we r e p o r t e d polyanilines be

doping

during their

in n a t u r e .

reactions

Fig.

t h e i n f r a r e d s p e c t r a l c h a n g e s of common

w i t h HC1. T h e s e

1 shows the

IR s p e c t r a

reactions

have

been

s h o w n to

of C-PMAn r e c o r d e d

during

its

r e a c t i o n w i t h i o d i n e . Some o f t h e s p e c t r a l c h a n g e s in Fig. 1 a r e a n a l o g o u s to w h a t happen

in

HCl-doping

of C-PMAn

[8]. F o r

example,

the

baseline

over

4000-1800

cm -~ is r a i s e d s l o w l y ; a n e w p e a k a p p e a r s a t 3200 c m - h t h e two p e a k s a t 1110 cm -~ g r o w s i m u l t a n e o u s l y the

1110

cm -~ p e a k .

On

and the

the

other

hand,

substantial

c

I

I

f

I

differences

are

observed

°

B

4000

1160 a n d

1160 cm-* p e a k b e c o m e s m u c h s t r o n g e r t h a n

~

~

I

.71

2000

1-00

c

I

i

400

,

,

i

400

i

i

, i

2000

WAVENUMBER

12'00

400

WAVENUMBER

Fig. 1. IR s p e c t r a o f C-PMAn d u r i n g i t s r e a c t i o n w i t h h: A - 0, B 3 min.

1, C - 2, D -

Fig. 2. IR s p e c t r a o f R-PMAn d u r i n g i t s r e a c t i o n w i t h h: A - 0, B 5, E - 7 rain.

1, C - 3, D -

b e t w e e n t h e s e two c a s e s : (1) T h e b a n d s a t 1 6 0 0 / 1 5 8 0 cm -1 a r e g r e a t l y s t r e n g t h e n e d a n d s h i f t e d to 1580 cm-*, i t s i n t e n s i t y

b e c o m e s a s h i g h a s t h r e e t i m e s of t h e b a n d

at

the

relative

intensity

(2) A n e w p e a k

appears

at

1500

cm -* w h e r e a s

in

remains almost unchanged; associated peaks the

with

in t h e

whole

C-N

HCl-doping

stretching

in

region of 1300-1200

spectrum

at

last

with

the

neighborhood

cm -~ g r o w r a p i d l y its

maximum

at

of

these

two

bands

1370 cm -~ a n d it is u s u a l l y of and 1305

a

quinoid

become t h e cm-*;

(4)

ring;

(3)

strongest The

The in

spectral

742 changes caused

by h c a n n o t be e l i m i n a t e d

NHs. In p a r t i c u l a r , The

reaction

the

product

relative

intensity

is

conductive

not

by tre a ting

of 1 5 8 0 / 1 5 0 0 and

can

the

reaction product with

cm-* d o e s n o t d e c r e a s e ;

not

be

made

(5)

conductive

by

r e a c t i n g w i t h HCI e i t h e r . T h e a b o v e IR i n f o r m a t i o n a s well a s t h e p r e v i o u s r e s u l t s for C-PAn

19] s u g g e s t s

that

the

reaction

o f common p o l y a n i l i n e s

with

iodine

is

m a i n l y o x i d a t i o n a n d is n o t a d o p i n g r e a c t i o n in n a t u r e . Fig. 2 s h o w s IR s p e c t r a o f R-PMAn d u r i n g i t s r e a c t i o n w i t h i o d i n e . I t is n o t i c e d that more

the

main

and

spectral

more

disappears

and

broadened

changes

inclined; the

and

the

bands

at

strengthened

are:

the

band

at

1600/1580

greatly;

baseline 1620

the

over

1800

cm -~ c h a r a c t e r i s t i c valley

between

f i l l e d g r a d u a l l y a n d two b a n d s a p p e a r a t 1560 a n d 1305 a n d

4000-

cm -* c h a r a c t e r i s t i c of

1600

cm-* b e c o m e s

of

benzoid

quinoid and

ring

ring

1500

are

cm -* is

1540 cm -* a t l a s t ; t h e b a n d s a t

1240 cm -* grow u p a n d t h e r e a p p e a r s a n e w p e a k a t

1260 cm-*; t h e

1160

a n d 1110 cm -1 b a n d s i n c r e a s e g r e a t l y . In s h o r t , t h e f i n a l i n f r a r e d s p e c t r a look like those

of

HCl-doped

C-PMAn

[8],

indicating

that

the

h-doped

R-PMAn

and

H C l - d o p e d C-PMAn h a v e e s s e n t i a l l y t h e s a m e m o l e c u l a r c h a i n s t r u c t u r e .

0 liIt '

4000

'

'

'

I

I

2000

I

a

1200

,

400

,

4000

3. IR s p e c t r a

of R-PAn

during

I

I

I

2000

WAVENUMBER Fig.

i

I

I

1200

400

WAVENUMBER its

reaction

with

I~: A -

O, B -

1, C -

2, D -

3, E - 5 rain. Fig. 4. IR s p e c t r a of t h e 12-doped R - P A n d u r i n g i t s d e d o p i n g b y NHs: A - 0, B C - 3, D - 5, min.

1,

743 This conclusion is also valid for the case of R-PAn.

As shown in Fig. 3, its IR

spectral changes during the reaction with iodine are in agreement R-PMAn.

Its final spectra look like those of the HCl-doped C - P A n

Fig. 4 shows IR spectra of the reaction product of R - P A n subsequently

treated with

ammonia.

with those of

[9].

with iodine w h e n it is

It is noticed that the baseline over 4000-

1800 cm -i becomes horizontal and the band near 1160 c m -i is decreased gradually. Similar changes were observed w h e n HCl-doped C - P A n

was dedoped by a m m o n i a [9].

Sure enough, Spectrum 4D looks like the IR spectrum of C - P A n

or that of dedoped

P A n [9]. But it differs from Spectrum 3 A at least in that the relative intensity of 1587 vs. 1500 cm-* for 4D is m u c h higher than that of 1600 vs. 1500 cm -I for 3A. That is to say, after a doping-dedoping 0.5. Because

circle, it gains an oxidation degree near

the determined highest h-doping

level of R - P A n

(I/C6H4NH)

is 0.54,

the iodine element is believed to exist in the form of I- instead of 13-. 3. Reaction schemes The

above

experimental

iodine depends

on

results show

their oxidation

that

degree.

the

The

reaction

reduced

PAn

of polyanilines is oxidized

with to an

intermediate oxidation degree and is converted into a structure found in HCl-doped PAn.

This process

can be considered to take place in two steps: oxidation and

protonic doping. The protons involved in the doping originate step, i.e., the N - H

from the oxidation

groups in the original molecular chain are converted into +N-H.

Based on the "four ring B Q derivatives" model proposed for HCl-doped polyanilines [10, 11], the above reaction can be depicted as follows:

I 12

B II

The

common

PAn

B'

or PMAn.

Q,

which

g

Bt

is composed

of nearly equal

amounts

of

quinone-dilmlne and benzene-diamine units, is also oxidized by h. But the reaction product is not a conductor and increase

in

the

amount

of the

can not be doped quinone-diimine

by HCI either. as

evidenced

by

Based on the the

relative

intensification of 1580 vs. 1500 c m -I band, following reaction scheme is proposed:

744

where p r o d u c t F is a s a l t . T h e - N = a t o m s s e e m to be p r o t o n a t e d . But it does n e t m e a n p r o t o n i c doping, b e c a u s e t h e c o n d i t i o n for i n t r a - m o l e c u l a r c h a r g e t r a n s f e r is not

fulfilled,

i.e.,

there

are

no

benzene-diamine

units

as

its

nearest

neighbors

[11l. It s h o u l d be p o i n t e d o u t t h a t a l t h o u g h r e d u c e d PAn or PMAn c a n be c o n v e r t e d by h to s e m i - o x i d i z e d s t a t e

a n d common PAn or PMAn c a n be c o n v e r t e d by h to

f u l l y - o x i d i z e d s t a t e , t h e r e d u c e d PAn or PMAn c a n n o t be c o n v e r t e d by 12 d i r e c t l y to f u l l y - o x i d i z e d s t a t e . T h e k e y p o i n t h e r e is t h a t t h e common PAn or PMAn is in t h e b a s e form a n d t h e s e m i - o x i d i z e d R - P A n or R-PMAn is in t h e s a l t

or doped

form. T h e l a t t e r is more s t a b l e for f u r t h e r o x i d a t i o n . In s u m m a r y , s e m i - o x i d i z e d p o l y a n i l i n e s c a n reduced almost

polyanilines the

T h e r e f o r e it reductant.

can

be

same molecular

doped chain

c a n be d e d u c e d t h a t

This

has

been

by

an

be doped by a p r o t o n i c a c i d

oxidant.

structure

and

These

thus

two

similar

reactions

in

our

laboratory.

lead

conducting

fully-oxidized polyanllines can

observed

Related

and to

ability.

be doped by a results

will

be

published elsewhere.

REFERENCES 1

A.G. MacDiarmid, J.C. Chiang, A.F. Richter and A.J. Epstein, Synth. Met., 18 (1987) 285.

2

F. Wudl, R.O. Angus Jr., F.L. Lu, P.M. Allemand, D.J. Vachon, M. Novak, Z.X. Liu and A.J. Heeger, J. Am. Chem. Soc., 109 (1987) 8677.

3

A.J. Epstein, J.M. Ginder, F. Zuo, R.W. Blgelow, H.S. Woo, D.B. Tanner, A.F. Richter, W.S. Huang and A.G. MacDiarmid, Synth. Met., 18 (1987) 803.

4

Y. Cao, S. Li, Z. Xue and D. Guo, Synth. Met., 16 (1986) 805.

5

L.A. Ray, G.E. Asturias, D.L. Kershur, A.F. Richter, A.G. MacDiarmid and A.J. Epstein, Synth. Met., 29 (1989) E141.

6

F. Wang, J. Tang, X. Jing, S. Ni and B. Wang, Acta PolymericR Sinica, _5 (1987) 384.

7

L. Wang, ×. Jing, S. NI a n d F. Wang. S c e i n c e in C h i n a ( S e r i e s B), 3 2 (1989)

8

1422. L. Wang, X. J i n g a n d F. Wang, A c t a Chimica Slnica, Eng. Ed., ! (1989) 53.

9

J. T a n g , X. Jlng, B. Wang a n d F. Wang, S y n t h . Met., 2 4 (1988) 231.

10 X. Jing, J. T a n g , Y. Wang, L. Lel, B. Wang a n d F. Wang, Sceince in C h i n a (Series B), 3 3 (1990) 787. 11 L. Wang, ×. J i n g a n d F. Wang, S y n t h . Met., 2 9 (1989) E363.