Conductive polymer composites and copolymers of pyrrole and N-vinylcarbazole

Synthetic Metals, 40 (1991) 239-246

239

Conductive polymer composites and copolymers of pyrrole and N-vinylcarbazole Uwe

G e i s s l e r a, M a n f r e d

L. H a l l e n s l e b e n a'* and L e v e n t T o p p a r e ~''~'*

aInstitute of Macromolecular Chemistry, University of Hannover, D-3000 Hannove~" (F.R. C.) t'Department of Science, Middle East Technical University, 05531 Ankara (Turkey)

(Received April 4, 1990; accepted November 29, 1990)

Abstract The synthesis of composites and copolymers of pyrrole and N-vinylcarbazole is described. Both insulating and conducting polymers of N-vinylcarbazole are utilized in the electroinitiated polymerization of pyrrole. The influence of coating either of the conducting polymers prior to the other on the percolating threshold is discussed. The initial presence of both monomers in the reaction medium leads to a copolymer, whereas consecutive electrode coatings from the monomer solution yield inhomogeneous composites together with grafts. Characterizations are carried out via FT-IR, DSC, TGA and SEM analyses.

Introduction Since t h e initial r e p o r t on the s y n t h e s i s of c o n d u c t i n g p o l y ( p y r r o l e ) ( P P y ) v i a the e l e c t r o c h e m i c a l o x i d a t i o n of p y r r o l e [ 1 ], n u m e r o u s studies h a v e b e e n p u b l i s h e d c o n c e r n i n g the c o n d u c t i v e p o l y m e r s y n t h e s i s as well as c o n d u c t i v e c o p o l y m e r s [2]. E l e c t r o c h e m i c a l d o p i n g of poly(N-vinylcarb a z o l e ) (PNVC) w a s also s h o w n p r e v i o u s l y [3]. M u c h effort h a s b e e n s p e n t o n the s y n t h e s i s o f c o n d u c t i v e p o l y m e r b l e n d s [ 4 - 9 ] . The hon~ogeneity of the c o m p o s i t e s h a s b e e n a t t r i b u t e d to a p o s s i b l e H - b o n d i n g b e t w e e n the insulating h o s t p o l y m e r a n d c o n d u c t i n g p o l y m e r [9]. On the o t h e r hand, b l e n d i n g of t w o c o n d u c t i n g p o l y m e r s t h r o u g h c o n s e c u t i v e o x i d a t i o n f r o m their m o n o m e r s o l u t i o n s m a y lead to o n e - p h a s e s y s t e m s i n d e p e n d e n t of Hb o n d i n g c a p a c i t i e s of the m o n o m e r s . This idea m a y b e valid e v e n if one of the c o m p o u n d s initially is a n insulating p o l y m e r p r o v i d e d t h a t it c a n b e e l e c t r o c h e m i c a l l y d o p e d during t h e b l e n d i n g p r o c e s s . F o r t h a t p u r p o s e , the c h o i c e of N - v i n y l c a r b a z o l e a n d p y r r o l e is n o t r a n d o m since the o x i d a t i o n p o t e n t i a l s o f the m o n o m e r s are c o m p a r a b l e , H o w e v e r , s t r u c t u r a l differences b e t w e e n t h e m m a y l e a d to different diffusion r a t e s t h r o u g h the o t h e r film w h i c h in t u r n c a u s e a different p e r c o l a t i o n t h r e s h o l d in b l e n d s ( n a m e l y PNVC/ *Authors to whom the reprint requests should be sent.

0379-6779/91/$3.50

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240 PPy and PPy/PNVC). Yet another factor worth considering is the possible o c c u r r e n c e of a copolymerization reaction when both m o n o m e r s are initially in the reaction medium. Thus the present investigation deals with the differences between the products synthesized by making use of several types of electrochemical polymerizations of the two monomers.

Experimental Materials N-Vinylcarbazole (NVC), pyrrole (Py) and tetrabutylammonium tetrafluoroborate (TBAFB) were supplied by Aldrich Chemical Co. and used after c o m m o n purification techniques; HPLC-grade acetonitrile (Aldrich Chemical Co.) was used as obtained. Polymerization of NVC into its insulating polymer was achieved both electrochemically and by the use of boron trifluoride etherate via a cationic mechanism. The details of the constant-potential electrolysis system were given elsewhere [10, 11 ]. Platinum foil (2.5 cm 2) working electrodes were used at + 1.2 V versus an Ag/Ag + (0.01 M) reference electrode.

Synthesis of composites and copolymers The following scheme summarizes the electrochemical reactions that were carried out by the same technique as described above. All m o n o m e r concentrations were 0.1 mol 1-1. NVC

Py

(A)

Pt

, Pt(PNVC)

(B)

Pt ey) Pt(PPy)

-)

Nvc,

Py, NV¢

(C)

Pt

(D)

Pt (iJ) NVC)

(E)

Pt(PNVC*)

(i) Py

ey

,

Py, PNVC*

(F)

Pt

)

Pt stands for the platinum working electrode and PNVC* for the insulating poly(N-vinylcarbazole) as p r e p a r e d by electroinitiation and via boron trifluoride etherate. In reactions (A) and (B) several runs for each m o n o m e r were carried out in order to obtain different compositions of composites and thus a conductivity profile. Reaction (C) represents electrolysis where both m o n o m e r s were initially present in the reaction system. In reaction (D) the m o n o m e r NVC was introduced into the system while the electrolysis of pyrrole was already proceeding. For reactions (A) to (D) the medium was an acetonitrile-TBAFB solvent-electrolyte couple. Addition of 50 t~l of water prevents the formation of nonconducting PNVC* [12] which normally precipitates in acetonitrile during polymerization in the absence of water. In Scheme (E)

241

the electrode is coated with PNVC* simply by dip-coating from 1% polymer solution. Scheme (F) indicates the reaction of pyrrole and the insulating PNVC in dichloromethane.

Physical methods The conductivity m e a s ur e m ent s were carried out with a Keithley 600B e l e c t ro meter c o n n e c t e d to a Signatone four-probe head with osmium tips. The composition analysis of schemes (A) and (B) were done by weighing the electrodes using a high-sensitivity Sartorius 5503 MP6 electronic microbalance. Infrared spectra were taken on a Nicolet 60 SX Y'F-IR spectrometer. Scanning electron micrographs were obtained by a JEOL Superprobe 733 electron microscope. Differential scanning calorimetry was conduct ed on a Perkin-Elmer DSC-2C instrument and gravimetric analyses were carried out with a Du Pont 951 thermal analysis system.

Results

and discussion

The conductivity curves owing to the polymerizations of pyrrole on a PNVC-coated (conducting polymer) electrode (A) and N-vinylcarbazole on a PPy-coated electrode (B) are given in Figs. l(a) and Co), respectively. The conductivity value of 0.5 S cm-1 can be achieved with a small content of pyrrole in the composite (c. 20%) in the polymerization of pyrrole (A). In the latter case it takes c. 80% pyrrole content in the resulting films to reach the same degree of conductivity. This behaviour can be attributed to the diffusibility of the m o n o m e r through the polymer of the other monomer. Table 1 summarizes the conductivity of the other samples at the end of c. 30 min of electrolysis for schemes other than (A) and (B). The DSC scans of the two composites show no glass transition temperatures, a characteristic of conducting polymers. In the case of reactions (E) and (F) there is also no glass transition t em perat ure observed for any of the reaction products which indicates the complete transformation of

i

"7

0-

_

Q ~ i b

'E

E

•

-]-

%

~'o (a)

~

6'o

Weight % Poly(pyrrole )

8'o

loo

4 '

(b)

2~

'

&

'

ob

'

8b

'

t~

Weight % Poly(pyrrole)

Fig. 1. Relationship between conductivity and poly(pyrrole) content (wt.%): (a), PPy/PNVC, scheme (A); (b), PNVC/PPy, scheme (B).

242 TABLE 1 Conductivity of resultant free-standing films Sample

Conductivity (S c m - ' )

(C) (D) (E)

i 0 - ' ~,b 4 ~, 10-3 b

(F)

10-' .,b

aElectrode side. bSolution side. cSeveral different values are available because of the nonuniformity of the films.

120

I

1.2

100t 594 53oc IT)

0.S

S0 ~

"0.6

,~

(0.4836 mg)

~ /

0.4. > (O.2317 rag)

21

-O.O o

260

460

Temperature

(°C) s~o

86o

• - 0 . 2

1ooo

Fig. 2. TGA curve of a composite synthesized through reactions (A) and (B) at a heating rate

of 10 °C min-'. PNVC* into c o n d u c t i v e p o l y m e r . T h e r m a l g r a v i m e t r i c a n a l y s e s yield a onep h a s e s y s t e m f o r b o t h c o m p o s i t e s (A) a n d (B) (Fig. 2). T h e IR s p e c t r a o f c o m p o s i t e s (A), (B), (E) a n d (F) yield the c h a r a c t e r i s t i c s of b o t h c o n d u c t i v e p o l y m e r s d o p e d with B F 4 - a n i o n s (Figs. 3 a n d 4). It w a s p r e v i o u s l y r e p o r t e d [ 13, 14] t h a t c o n d u c t i v e PNVC c o n t a i n s e x t r a b a n d s s u c h as 8 0 0 a n d 8 8 0 c m -1 ( a n d 1 1 0 0 c m -1 b e c a u s e o f the d o p a n t ) as a p r o o f o f l i n k a g e s in t h e 3- a n d / o r 6 - p o s i t i o n of t h e c a r b a z o l e ring. Yet a n o t h e r t w o b a n d s , 9 1 0 c m -~ ( b r o a d ) a n d 9 6 5 - 9 7 0 c m -1, a p p e a r in all s a m p l e s in the IR s p e c t r a o f s a m p l e s (A), (B) a n d (C). T h e first p e a k ( 9 6 5 - 9 7 0 c m -~) is a t t r i b u t e d to the linkage b e t w e e n the t w o c o n d u c t i v e p o l y m e r s p o s s i b l y t h r o u g h t h e i r a r o m a t i c sites (3- a n d / o r 6-

243

(a) (b)

!r /j

(b)

isis

i610

i~iO

i~iO

iSiS

sis

181o

isoo

i@9o

WAVENUMBER

liso

WAVENUMBER

@70

~SO

SSO

Fig. 3. Fr-IR spectra of composites derived from (a), scheme A; (b), scheme (B). Fig, 4. Fr-IR spectra of products derived from (a), scheme (E); (b), scheme (F).

isiO

isso

iqso

i~70

16SO

910

7so

WAVENUMBER Fig. 5. I~-IR spectra of (a), copolymer (C); Co), copolymer (D).

position of PNVC and with 2- and/or 5-position of PPy); thus the end product (schemes (A), (B)) contains grafts to a certain extent. This band is also emphasized in reactions (C) and (D) possibly yielding a copolymer (Fig. 5). A control experiment under the same conditions utilizing carbazole and pyrrole confirms the presence of this band but lacks the 910 cm-1 broad band. This observation rules out the assignment of the 9 6 5 - 9 7 0 cm-1 band to vinyl group linkage between PNVC and PPy (which is also absent in schemes (E) and (F)). On the contrary, the possibility of having a sort of 10- and/or l l - p o s i t i o n of N-vinylcarbazole with a 2- and/or 5-position of pyrrole linkage may explain this 910 cm -1 broad band which exists in (A),

244 (B) a n d (C) r e a c t i o n p r o d u c t s . It s e e m s t h a t s e v e r a l c o m p e t i n g r e a c t i o n s occur simultaneously. SEM m i c r o g r a p h s r e v e a l the d i f f e r e n c e s b e t w e e n f r e e - s t a n d i n g c o n d u c t i n g films. In all c a s e s the e l e c t r o d e sides are s m o o t h e r t h a n the solution sides indicating t h a t the c o n d u c t i n g p o l y m e r starts to f o r m on active c e n t r e s on the e l e c t r o d e f r o m w h i c h the p o l y m e r diffuses t h r o u g h the h o s t p o l y m e r s p r e a d i n g in all directions. T h e d i f f e r e n c e s in t h e p r o d u c t s o f r e a c t i o n s (A) a n d (B) are also clear f r o m SEM studies with the s a m e magnification. T h e e l e c t r o d e sides of the (A) a n d (B) p r o d u c t s are b o t h s m o o t h , b u t (A) s h o w s s o m e d i m p l e s with a d i a m e t e r o f 1 /zm o n t h e s u r f a c e . E v e n the solution sides exhibit different p i c t u r e s (Fig. 6). The s o l u t i o n side f r o m c o m p o s i t e (A) builds up f r o m g l o b u l a r p r o j e c t i o n s with a d i a m e t e r of 0 . 5 - 1 /zm a n d a r o u g h surface. In c o m p o s i t e (B) t h e s e p r o j e c t i o n s are b i g g e r b y a f a c t o r of five a n d it a p p e a r s

(a)

(b)

(c) .......... ~ .

(d) (¢) (f) Fig. 6. SEM micrographs of the electrode sides of the samples: (a), sample (A), magnification ×4000; (b), sample (B), ×4000; (c), sample (C), × 1000; (d), sample (D), × 1000; (e), sample (E), ×200; (f), sample (F), × 1000. (All micrographs reduced 45% in reproduction.)

245 that these projections build up from spherical particles with a diameter of under 0.1 t~m. In reaction (C) the surface shows rough spherical projections with a diameter of 2 t~m; these particles build a rugged solution side. The solution side from (D) is also rugged, but there are two kinds of particles; the first has a diameter of 2 tLm and a smooth surface, the second has a diameter of 0 . 1 - 0 . 5 t~m and a rough surface. This indicates that there are two different products in this composite. These products might be the h o m o p o l y m e r (PPy) and a copol ym e r (PPy/PNVC), which is understandable because N-vinylcarbazole is added to pyrrole electrolysis after a period of time. Therefore, a PPy layer is formed before copolymerization occurs. In principle, both the insulating polymer PNVC* and pyrrole are involved in the electrochemical processes (E) and (F). During the synthesis of the composite two reactions are competing, namely, the transformation of the insulating polymer to a conducting one and the polymerization of pyrrole into its conducting film not mentioning the cross-coupling of the active species. When the electrode is coated with the PNVC* prior to pyrrole polymerization (E), it yields micrographs different from the rest of the schemes. The electrode side shows a smooth but shrunk surface; this can be attributed to a swollen polymer (here PNVC*) which has been dried. The solution side has an island structure; this behaviour belongs also to the insulating PNVC*, which swells in acetonitrile, but the degree of swelling is very p o o r so that PPy primarily grows through the thinner regions of the film. After PPy has grown through PNVC*, further polymerization takes place on PPy because it has the higher conductivity. Therefore, the polymerization of pyrrole into PNVC* by this technique leads to inhomogeneous films. The solution side of (F) differs from all other structures. It is built up from spherical particles with a diameter of 2 - 5 t~m, but there is also another structure which looks like a network laid over the globular projections. It seems that is possible to synthesize several products showing different microstructures from the two different m o n o m e r s pyrrole and N-vinylcarbazole via different electrochemical reactions. In addition, these experiments show that a sufficient h o m ogenei t y of the films can also be achieved by blending/ grafting a conducting with a nonconducting polymer which has an electroactive group.

Acknowledgements L. Toppare thanks the Alexander von Humboldt Foundation for a scholarship which made it possible for him to visit the University of Hannover. We also acknowledge SEM micrographs by U. Anemtiller and H. Krause of the Institut fiir Werkstoffkunde, University of Hannover, and TGA and VI'IR m eas u r emen ts by the Deutsches Institut fiir Kautschuktechnologie e.V., Hannover. Financial support by Deutsche Forschungsgemeinschaft is gratefully acknowledged.

246

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