Pyrolysis of polyacrylonitrile prepared by electrochemical initiation and bromine doping of pyrolysis derivatives

Synthetic Metals, 16 (1986) 173 - 188

173

PYROLYSIS OF POLYACRYLONITRILE PREPARED BY ELECTROCHEMICAL INITIATION AND BROMINE DOPING OF PYROLYSIS DERIVATIVES G. MENGOLI, C. PAGURA, R. SALMASO, R. TOMAT and S. ZECCHIN* Consiglio Nazionale delle Ricerche, Istituto di Polarografia ed Elettrochimica Preparativa, Corso Stati Uniti 4, 35020 Padua (Italy)

(Received February 21, 1986; accepted April 30, 1986)

Abstract A comparison is made between the pyrolysis of polyacrylonitrile (PAN) prepared by anionic electro-initiation (E) and that of a commercial (chemically-prepared) polymer (C). The physical and chemical properties of pyrolysis derivatives at t ~> 250 °C are very close to each other, whereas different characteristics are observed at t ~ 250 °C in that CN group polymerization occurs much more easily for E than for C. A significant consequence is seen when the compounds are bromine loaded, only the samples cyclized through imine moieties achieve semiconduction (p ~ 10 s ~ cm). E.s.r. and other analytical data suggest a possible explanation for this.

Introduction In order to achieve conducting or semiconducting organic materials, an alternative pathway to synthesis is afforded by pyrolysis. Indeed, it has long been known that in the thermal conversion of some traditional polymers such as polyacrylonitrile to graphite-like fibres, controlled pyrolysis may lead to quasi-linear chain polyconjugated systems [1, 2]. With regard to polyacrylonitrile (PAN), it is mainly suggested that a ladder polymer is produced through two heating stages. First, a singleconjugated structure of imine moieties is formed by polymerization of pending CN groups. This stage is reported to occur at 200 - 300 °C, also in the presence of air [1 - 3]. The double-conjugated ladder structure would be achieved by heating in vacuum or under N 2 at temperatures > 3 0 0 °C [3].

~n

t:200-300°C* ~

n

t>300°C'

~n

*Author to whom correspondence should be addressed. 0379-6779]86]$3.50

© Elsevier Sequoia/Printed in The Netherlands

174 However, despite a number of investigations on the subject in the past 25 years, many points (other than the real structure of pyrolysis derivatives) are still controversial. Thus, with reference to the most recent papers, Teoh et al. [4] found that conductivities of PAN films far above those previously reported may be achieved after pyrolysis in vacuum over a narrow temperature range ( 3 9 0 435 °C). Such a dramatic conductivity increase for products pyrolysed at 427 °C was confirmed by Chung et al. [5]: through a detailed optical investigation, these authors, however, suggest that such material is an inhomogeneous mixture of different chemical species, comprising graphitized islands. A uniformphase polymer conversely appears at 200 - 250 °C which, in spite of a predictably low conductivity ( ~ 1 0 -1° ~ - 1 cm-1 [6, 7]), exhibits the relatively sharp absorption edge characteristic of a ~-~* transition in an organic semiconductor [ 5]. Lastly, Leroy et al. [8], in an i.r. study of the pyrolysed derivatives of PAN filmed on nickel by electropolymerization, suggest that thermal transformations in vacuum are: at 200 °C, cyclization; from 300 °C to 600 °C, crosslinking, followed by dehydrogenation and denitrogenation. The presence of air during pyrolysis causes degradation from 200 °C upwards. (This easy degradation, which is rather unusual with other PAN samples, might be related to the morphology and the low molecular weight of the investigated product.) The present work deals with the pyrolysis of PAN synthesized by electro-initiation, compared with a commercial polymer. Indeed, it has been reported elsewhere [ 9 - 11] that a polymer obtained by electrochemical initiation already contains cyclized moieties, and this fact might therefore influence the subsequent pyrolytic stages. This work also focuses on the possibility of Br2-doping of pyrolysis derivatives obtained at between 200 and 250 °C.

Experimental

(a) Preparation o f the polymer Polymerization was carried o u t at --15 °C under N2 in a two-compartment cell. 80 ml of either the pure m o n o m e r (vacuum-distilled before each run) or 20 - 25% AN solution in dried DMF were placed in the cathode compartment (mercury pool); the background electrolytes were TBAP or NaB(C6Hs) 4 in 0.1 M and 0.03 M concentrations respectively. In the other compartment, filled with a similar electrolyte, the anode was an Ag sheet. Electrolyses were amperostatic, using constant currents in the range 1 - 5 mA for 8 h. At the end of the run, the catholyte was added to 500 ml of CH3OH , thus recovering PAN as a yellow-orange precipitate. The crude product was purified by further dissolution and precipitation and then dried under vacuum at 60 - 70 ° overnight. The yields of these preparations ranged

175

from ~ 3 0 to ~ 1 5 0 m g / c o u l o m b , higher yields being obtained from A N DMF solutions for the lowest current densities. The molecular weight M was determined both viscosimetrically by [12]: [7] = 3.92 X 10 --4 × M °'Ts and by vapour p r e s s u r e o s m o m e t r y , using a Knauer osmometer. Typical molecular weights were Mv = 10 200 and/ ~ n 7200. The commercial PAN (M.W. = 150 000 from Polyscience) gave /l~v 100 000. =

(b) Heat t r e a t m e n t Weighed samples ( 0 . 8 - 1 . 0 g) were placed in an oven directly at the chosen t e m p e r a t u r e and left isothermally, under either air or N2, for p r o g r a m m e d times. T G / D T A curves were obtained by a Netzsch simultaneous thermoanalyser, model STA 429. PAN (0.05 g) was heated from r o o m t em perat ure to 350 - 400 °C at a rate of 2 °C/min. (c) R e a c t i o n with Br 2 0.250 g o f the pyr ol ys ed sample were made to interact with 4.2 g Br 2 dissolved in CC14 in a sealed flask at r o o m t em perat ure for times varying from 20 to 100 h. In some instances 1.30 g Br2 dissolved in 50 ml H20 containing KBr (2.9 g) and HBr to pH = 2 were used as the brominating system. The recovered p r o d u c t was carefully washed (in CC14 or H 2 0 / H B r pH = 2) till the c o n c e n t r a t i o n of Br2 in the filtrate, as determined colorimetrically, was below 10 3 M; the p r o d u c t was then kept under dynamic vacuum till no Br2 could be d e t e c t e d in the condensation traps and the sample had acquired a constant weight. Bromination was also carried out by exposure of samples to Br2 vapour for 20 h; Br 2 excess was then eliminated by keeping the derivate u n d e r dynamic vacuum as above. The 12 doping of pyrolysed PAN was a t t e m p t e d by means of similar procedures, b u t the experiments were unsuccessful (i.e., no iodine was taken up). (d) Electrical c o n d u c t i v i t y Conductivity was determined on pressed discs of the material: as pyrolysed PAN has very p o o r cohesion [6], it had to be mixed with 20% polyvinylalcohol in order to provide discs suitable for conductivity measurements. Conversely, pyr ol ys ed PAN after bromine exposure could easily be w o rk ed as such. (20% Polyvinylalcohol c o n t e n t decreases the conductivity in the b r o min ated samples by ~ 5 0 - 70% at most.) Electrical resistance was measured at r o o m t e m p e r a t u r e by a Kulicke and Soffa four-point pr obe head (linear array}, equipped with a 220 Keithley programmable current source and a 195 Keithley digital multimeter. Only

176

resistances below 101°- 1011 ~ cm could be determined. Once the VII ratio had been measured, the conductivity was obtained by [13]

p = V/I× 2~ × 0.1 (e) E.s.r. procedure E.s.r. measurements were performed on a Bruker ER 100 D X-band spectrometer equipped with a variable temperature unit. The position of the sample holder, a quartz tube with ~ = 0.3 cm, containing a weighed a m o u n t of sample (5 - 10 mg), was adjusted inside the cavity until m a x i m u m signal intensity was achieved. Modulation amplitude settings were optimized to avoid false broadening. The power of the exciting wave was low enough to rule out saturation, and in fact the signal was always linear with the square root of the power and had a line width independent of it. Spin density (Ns/g) was measured by standardization with VOSO4 × 5H:O crystals, by [14]:

h Z~-/pp2 signal intensity - - GM where h is the peak height, modulation amplitude.

Z~/pp the

recorded width, G the gain and M the

Results

(a) Thermal analysis Figure 1 shows the thermal response obtained either in air or under N2 from PAN samples prepared by electro-initiated polymerization. Thermogravimetric traces indicate that a slight weight loss takes place from ~ 2 0 0 °C, but the p h e n o m e n o n increases sharply near 250 °C: above this temperature, degradation is faster in N2 than in air. At 250 °C DTA analysis shows an exothermic peak, the area of which is independent of the presence of 02 in the atmosphere. This exotherm is generally ascribed to --CN polymerization {cyclization), although simultaneous evolution of NHa and HCN were tested for at this temperature [15]. Figure 2 shows the data obtained under the same experimental conditions from the commercial polymer. No weight change could be detected up to ~ 2 5 0 °C; above this temperature not only does a sharp weight loss occur, but DTA analysis indicates a vigorous exotherm, dramatically enhanced by air.

(b ) Pyrolysis at 250 °C Following these indications, E (electrochemical) and C (chemical) PAN samples were isothermally heated at 250 °C either in air or under N 2. The duration of treatment was fixed at 2.5 h, at which time condensation appeared extensive w i t h o u t intolerable polymer loss.

177

°ct

500

llOmg

400

l

lmV

300

200

100

60

120

180

P

min.

Fig. 1. T G a n d D T A c u r v e s f o r P A N p r e p a r e d b y e l e c t r o - i n i t i a t i o n in N 2 ( (. . . .

) a n d in air

).

°C

I

50G

\ 400

I

\

lOmg 1mY

\\x mV \

300

200

100

0

60

120

180

min.

Fig. 2. T G a n d D T A c u r v e s f o r c o m m e r c i a l P A N in N 2 (

) a n d in air ( . . . .

).

178 TABLE 1 Weight loss, e l e m e n t a l analysis a n d resistivity o f e l e c t r o c h e m i c a l (E) a n d c h e m i c a l (C) P A N s a m p l e s a f t e r pyrolysis at 2 5 0 °C Original P A N

Weight loss % C H N Total p(~cm)

E

C

--

-

65.7 5.9 25.2 96.8 >101°

67.2 5.6 26.4 99.2 >101°

Eair

EN 2

Cair

CN2

Cair]N 2 a

13.6 64.1 4.7 23.2 92.0 >101°

23.4 66.3 5.2 24.4 95.9 >101°

21.6 61.0 2.3 20.9 84.2 >101°

11.7 66.4 5.0 24.4 95.8 >101°

32.4 65.4 1.9 20.7 88.0 15.5×10 s

a S a m p l e s f u r t h e r p y r o l y s e d at 4 2 0 °C u n d e r N2 for 1 h o u r .

Table 1 reports the elemental analyses of these products. Some indications of Figs. 1 and 2 are thus substantiated: the composition of E is only slighted affected when air is present during heating; indeed, 02 seems to stabilize the material, since the weight loss is lower; the behaviour of C is opposite, in that weight loss is much higher in air, in which pyrolysis also causes significant oxygen uptake. All the derivatives exhibit significant spin activity, and the electric resistance of pressed discs is in any case high (p > 101° - 1011 ~2 cm). Only when the derivatives are submitted to further heating (above 400 °C under N2) does the resistivity drop to ~ 106 ~ cm. The last column of Table 1 gives details of this treatment. Figure 3 shows the i.r. pattern of an E sample before (a) and after (b) heating at 250 °C in N2. With reference to the typical 2240 cm -1 CN band, this is seen to fade in favour of new ones testing CN polymerization, such as those at 2200 cm -1, due to terminal ~ C = N H stretching, and 1600 - 1380 cm -1 (broad) assigned t o ~ ( C=Ng~ [8]. The original intensities of the bands at 2 9 5 0 - 2 8 8 0 cm -1, due to aliphatic C--H (a) appear to be decreased (b) more than expected from elemental analysis data (Table 1). The broad absorption from 1600 to 1400 cm -1 may indicate, although not unequivocally, c a r b o n - c a r b o n unsaturation. The i.r. spectrum of E pyrolysed in air is the same as Fig. 3(b). Only C heated in air gives a different spectrum, as shown by Fig. 3(c): aliphatic vibrations are no longer present, in agreement with elemental analysis, showing a higher hydrogen loss in this case. The higher absorption of sample C above 2000 cm -1 may be related with its higher 02 uptake during pyrolysis in air, whereby h y d r o x y l and carboxyl functions have entered the polymer.

179 100

-

(a)

50

o

lOO

o

I00"

so~~_(c) o 4000

I 3000

I 2000

T 1500

I 1000

I 500

WAVENUMBER (cm ') Fig. 3. Infrared spectra for: (a) original PAN prepared by electro-initiation; (b), (a) after 2.5 h at 250 °C under N2; (c) commercial PAN after 2.5 h at 250 °C in air.

(c ) Bromination The samples of Table 1 were interacted with Br2 under various conditions. The consequent composition changes and electrical properties are listed in Table 2. TABLE 2 Weight gain, elemental analysis and resistivity of E and C PAN samples after pyrolysis at 250 °C (Table 1 ) a n d interaction with Br 2 Brominating systenl

Br 2 in CCI a

Sample

Eai r

Cair

CN2

Weight gain%

78.0 80.4 68.0

C H N Br Total

34.5 2.7 12.3 36.7 86.2

32.4 3.0 11.7 34.9 82.0

34.4 2.3 12.2 34.0 82.9

4.5

4.4

1.9

p x 10 - s ( ~ cm)

EN2

Br 2 gas

Br2/Br- in H20

Calf/N2

EN2

Eai r

81.2

13.6

98.3 124.0

88.2

33.8 3.3 12.1 37.6 86.8

56.3 1.8 17.8 7.5 83.4

27.8 3.0 9.9 43.9 84.6

30.2 2.9 10.9 44.3 88.3

36.9 3.2 12.9 36.0 89.0

4.4

3.7

2.5

5.0

0.75

CNz

180

The first row of Table 2 shows that any sample, with the exception of that pyrolysed at 420 °C, gains from 70 to 120% of its original weight. Bromine in fact enters the elemental composition in percentages ranging between 35 and 45%. Comparing the elemental analyses of Tables 1 and 2, it follows that C/H ratios do n o t substantially vary after bromination, but moisture absorption may well have occurred (analyses fail to reach 100% much more frequently than those of the original pyrolysis derivatives). The i.r. spectra appear to be unmodified below 200 cm-1; conversely, there is an intensity drop of the aliphatic CH band. The electronic spectra of a typical E sample before and after bromination are shown in Fig. 4. The relevant samples were obtained by evaporating a PAN solution onto a glass substrate, with subsequent pyrolysis and exposure to Br2. The effects of bromination are not very great; however, a bathochromic shift can be noted. The easier ~ ~ ~* transition observed for the bromine complex might well indicate a lower band gap for this material with respect to the original pyrolysis derivative. The most significant fact is shown by the last row of Table 2, which shows that reasonable semiconduction is acquired. Resistivity in fact drops b y more than five orders of magnitude. The c o m p o u n d pyrolysed at 420 °C, which has a low bromine content, shows a conductivity increase that may be

1.0

o I= ¢1 Ii J~

am MI O Z a,i 0 m

(,)\

I 350

I 450

\ (b)

[

I

550

650

Fig. 4. E l e c t r o n i c s p e c t r a o f p y r o l y s e d P A N p r e p a r e d b y e l e c t r o - i n i t i a t i o n b e f o r e (a) a n d a f t e r (b) e x p o s u r e to Br 2.

181

mainly due to the fact that after bromination it could be sampled as a disc w i t h o u t any binder. (d) Pyrolysis under milder conditions E and C samples were also heated at 200 °C for 20 h. This experimental choice was derived from both Fig. 1 (weight loss started from ~ 2 0 0 °C) and suggestions in the literature, which indicate that exhaustive CN polymerization may be achieved after 15 h at 200 °C [5]. Table 3 reports the elemental analysis of samples pyrolysed in this way. As shown by the first line, E samples show greater weight losses than C samples, but the latter are again more sensitive to oxygen, which enters the polymer composition. However, a more significant difference between E and C is shown with respect to bromine uptake {Table 3). In fact, the bromine content easily reaches ~40% in E, versus ~20% in C. This has a direct consequence on resistivity: only E gains semiconducting properties. An examination of the i.r. patterns of E and C before and after exposure to Br2 (Fig. 5) may account for their different behaviours. After 20 h at 200 °C (a), E shows absorption very similar to that achieved at 250 °C, as CN groups appear to be largely converted to imino moieties. Subsequent bromination (Fig. 5(b)) brings about an additional drop of the C--H bands at ~ 2 9 0 0 cm -1. Instead, the VCN intensity of C at 2240 cm I after pyrolysis (d) is only slightly changed and subsequent exposure of the derivative to Br 2 has minor effects (e). This comparison was carried out for samples pyrolysed in air, in which thermal modifications appeared more significant. For temperatures between 200 and 250 °C, similar results were obtained. Thus, after 4 h at 220 °C in N/, E could be brominated to the semi-

TABLE 3 Weight variation, e l e m e n t a l analysis a n d resistivity o f E a n d C PAN samples a f t e r pyrolysis at 2 0 0 °C a n d i n t e r a c t i o n w i t h Br 2 Treatment

Pyrolysis

Sample

Eai r

Pyrolysis + Br 2 in CCI4 EN2

Cai r

CN2

Eai r

EN:

Cai r

CN2

Weight --8.1 variation % C 65.1 H 5.6 N 24.2 Br . . Total 94.9

--10.5

--2.8

--0.12

+88.4

+116.0

+30.8

+25.6

33.3 3.3 12.3 42.2 91.1

27.4 3.5 10.0 41.5 82.4

47.8 3.6 17.0 19.7 88.1

48.1 4.3 18.5 18.4 89.3

p (~ cm)

> 1010

> 1010

64.6 61.1 5.5 3.9 24.2 22.8 . . 94.3 87.8 > 1010

66.0 5.3 25.5 96.8 > 101°

2.5 × l 0 s

1.25 × 10 # > 1010

> 101°

182 ,vv

100-

(d)

(a)

50-

0 100"

0 100

(b)

T~s o - ~ ~ o 100

"

"~

o 4000

so0 100

(c)

3o'oo

2o'0o

(e)

(f)

4000

WAVENUMBER(cm-')

I 3000

I 2000

Fig. 5. Infrared spectra of PAN pyrolysed in air at 200 °C for 20 h: (a), electrochemical sample (E) after pyrolysis; (b), (a) after bromination; (c), (b) after debromination; (d), chemical sample (C) after pyrolysis; (e), (d) after bromination; (f), (e) after debromination.

conducting condition (p ~ 1 0 s ~2 cm). After the same treatment, C only reached 15% bromine c o n t e n t and turned out to be practically insulating. E was also submitted to heating in N: at 185 °C. After 20 h a reddish material was obtained, which in more than 100 h of exposure to bromine only t o o k up ~ 3% of halogen. On prolonging heating for 65 h, the colour darkened and spin activity rose; after this treatment, ~20% bromine was able to enter the material. Products obtained under milder pyrolysis are generally brominated more slowly than samples pyrolysed at 250 °C; the limiting bromine content is in fact achieved after 2 - 3 days of interaction with Br:.

(e) Dehalogenation A qualitative assay on the brominated samples by mass spectrometry at a moderate temperature (160 °C) indicated the presence of Br2 and HBr. In order to determine better the fate of the halogen inside the pyrolysis derivatives, the following procedure was adopted: 200 mg were treated with 500 ml of 0.1 M KOH for 20 h and the recovered product was carefully

183 TABLE 4 Weight loss and elemental analysis of E and C PAN samples after pyrolysis, bromination (Tables 2 - 3) and alkaline dehalogenation Pyrolysis temperature

250 °C

200 °C

200 °C

200 °C

200 °C

Sample

Eair a

Eair

EN2

Cair

CN 2

Weight loss % C H N Br S Total

48.5 (not det.) 52.2 (49.7) 3.5 (3.8) 18.4 (16.7) 5.5 (12.2) --(1.2) 79.6 (83.6)

62.4 52.3 4.8 18.7 5.8 . 81.6

74.0 48.0 4,7 16.5 3.8 . 73.0

28.0 52.4 4.0 19.3 3.1

32.0 61.1 4.8 23.4 2.5

78.8

91.8

.

.

aIn brackets: data for dehalogenation with 0.1 N H2SO4 for 20 hours

washed with H20 and vacuum-dried overnight (see Table 4 for elemental analysis data). As a consequence of the alkaline treatment, E samples underwent a higher weight loss. This was expected, owing to their higher bromine content, although some release of oligomers may also have occurred [16]. However, the elemental analyses of E and C emphasize that most of the halogen has been eliminated from the polymer. From the i.r. pattern of a debrominated sample, (Fig. 5(c)), it appears that the intensities of Vcn and VCN bands are in some way restored with respect to (b), while the band at 2200 cm -1 is definitely lost: the reactive ~ C = N H groups were probably either oxidized by bromine or hydrolysed. It is significant that the material from alkaline dehalogenation can again be bromine-loaded, regaining its semiconductivity (typically the resistivity of an E sample, pyrolysed at 250 °C in air, goes from 4.5 × l 0 s fl cm after bromination to 2 × l 0 s ~2 cm after subsequent dehalogenation and rebromination, without any significant change in composition). It must be noted that the dehalogenated products are also completely incapable of taking up iodine. Table 4 shows that a sample could be significantly debrominated by treatment with dilute (0.1 N) H2SO4. Consequently, C--Br bond cleavage by KOH seems unlikely as a possible dehalogenation pathway. (f) E.s.r. results As mentioned above, pyrolysis derivatives of both E and C samples display significant spin activity, as evidenced by e.s.r, measurements. The e.s.r, signal appears as a single line, as reported in the literature [17,181. This Section reports the dependence of signal characteristics on the history (E or C, pyrolysis conditions, bromination, etc.) of each sample; Table 5 gives a summary of the changes. E derivatives exhibit somewhat lower activity than C ones. The highest pyrolysis temperature leads to highest spin concentration.

Sample

C

C

E

E

C

C

C

E

E

E

Pyrolysis c o n d i t i o n s

250 °C in N2

250 °C in air

250 °C in N 2

250 °C in air

420 °C in air/N2

200 °C in N2

200 °C in air

200 °C in N2

200 °C in air

185 °C in N 2 (65 h)

E.s.r. data

TABLE 5

E

E

E

C

C

C

E

E

C

C

CBr

EBr

) EBr

) EBr

)

Br ~ )

Br 2

Br 2

Br 2

CBr

) CBr

) EBr

) EBr

> CBr

> CBr

Br2)

Br 2

Br 2

Br 2

Br 2

Br 2

Reactions

KOH

KOH

KOH

KOH

KOH

~E

) E

:* C

>C

) E

Br2 ) EBr

(Ns/g) and AHpp (gauss)

.-).

2 × 10 -.`8

--~

_->

-->

2 X 1019 4 1.5 x 1016 --> 2 x 1018 5 10.5 2 X 1017 --> 2 × 1018 5 6.5

6.5 X 1017 1.5 × 1016 7 6 4 × 1017 4 × 1016 9 5

1018 8

8 × 1017 --> < 1016 --> 8 X 1017 10 ---5 8.5

2.5 X 1019 3.5 1.5 X 1018 9.5 1.5 × 10 is 5.5

5.5 5.5 1018 --> 5 × 1016 8 7 2.5 X 1018 6 X 1017 2.5 X 1018 6.5 × 1017 5 "-> 6 "+ 5 -> 6

1019

1.5 x 1018 _-). 2 × 1017 9 4.5

Spin c o n c e n t r a t i o n

185

3.5"

>, c e) c

2.5-

2.0-

O

1'0

2'0

3'0

Attenuation

(dB)

5'0

Fig. 6. E.s.r. signal intensity for commercial PAN (in arbitrary units) as a function of microwave power at room temperature: (©) after pyrolysis at 250 °C in N2; (e) after pyrolysis at 250 °C in N 2 and subsequent bromination. However, the most significant point relates to bromination: after taking up Br2, there is a general drop in spin concentration by one to two orders o f magnitude. The width of the signal, AHpp, remains practically unchanged for PAN derivatives pyrolysed in air, whereas it is reduced to 50% for samples p y r ol ys ed under N> Undoping by KOH restores and possibly increases the pristine spin activity. Further bromination o f the u n d o p e d sample causes a new dr op in the signal. The intensities o f e.s.r, spectra, recorded in the t e m p e r a t u r e range 120 - 300 K, on bot h simply pyrolysed and brominated samples, were f o u n d to follow Curie's law. However, different behaviour was observed with respect to saturation: the signal of pyrolysis derivatives is easily saturated {Fig. 6), indicating weak spin-lattice interaction, while the corresponding b r o m i n a t e d samples are less prone to saturation.

Discussion The data of this work indicate that the synthesis rout e followed in preparing the p o l y m e r does influence the characteristics of the pyrolysed products. At relatively low temperatures, pyrolysis causes two main processes, which are related but n o t strictly interdependent, i.e., f o r m a t i o n of a certain a m o u n t o f c a r b o n - c a r b o n conjugation (which seems to be favoured by

186 oxygen) and the polymerization of CN groups. These processes may be independently tested, b y the increase in spin activity due to increased C--C conjugation, and the decrease of infrared absorption at 2240 cm -1, respec-tively. In fact, it has been shown elsewhere [19] that spin activity due to polyene conjugation is several orders of magnitude higher than that of the short (5 - 10 units) polyimine sequences (<~1016 spins/g) [19, 20]. We found that C samples pyrolysed at 200 °C exhibit fair spin activity and a relatively small decrease of the VCN band. These derivatives are expected to bear a reduced n u m b e r of poly(cyanoacetyl)ene sequences (hydrogen content is high), but also to have undergone only partial cyclization. Conversely, E samples are much more prone to CN polymerization which, at 200 °C, is extensive and prevails over the formation of C--C double bonds, the latter being similar to that of C samples. This fact is not surprising, as the original polymer already bears some cyclized moieties [ 9 - 11]: either the consequent sterical orientation of the polymer or some catalysis caused by imine moieties causes CN polymerization at more moderate temperatures. The most striking consequences of the different situations of C and E are seen with respect to bromination. After interacting with Br2 the spin activity of each sample drops. This is precisely what happens for I2
187

For instance, during the several stages undergone by a typical sample, the intensity of the aliphatic Vcn bands drops (Figs. 3(a) and 5(a) - (c)), whereas the C:H ratio (Tables 1, 3, 4) exhibits minor changes. Part of this excess hydrogen may be accounted for by either intramolecular migrations (e.g., to form ~ C = N H groups) or protonations (to form %I~ / Br- moieties), I H but the derivatives are also expected to seize acertain a m o u n t of moisture. The structural effects of H20 and 02 capture and the consequences on conductivity have n o t yet been elucidated. Some more points for discussion concern debromination by alkaline aqueous solution. Besides the fact that some bromine reacted irreversibly, a puzzling result was the full restoration of the spin signal. A tentative explanation may be given as follows: the oxidation potential of the conjugated sequences may be considered intermediate between that of the Br:/Br- couple, an effective oxidizing agent (E 0 = 1.08 V v e r s u s NHE), and that of the I2/I couple, which is ineffective (E0 = 0.62 V). However, in alkaline medium, the redox potential is no longer that of Br2/Br-, as the bromine in the polymer is converted into other systems such as BrO3-, thus acquiring a fairly low potential (E = 0.6 + 0.5 V a t p H = 12 + 13) [24]. Under these conditions, reduction of positive charges to uncharged spin-bearing sites is possible.

Acknowledgement The authors are indebted to Dr. Sergio Sitran of I C T R - C N R for thermal analysis measurements.

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