Pergamon

Carbon, Vol. 32, No. 6, pp. 1133~1136, 1994 Copyright 0 1994 Elsevier Science Ltd

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0008-6223(94)00055-7

IR STUDIES OF PAN FIBRES THERMALLY STABILIZED AT ELEVATED TEMPERATURES

J. MITTAL, 0. P. BAHL,* R. B. MATHUR, and N. K. SANDLE’ Carbon Technology Unit, National Physical Laboratory,

Dr. K.S. Krishnan Marg, New Delhi-l 10012, India ‘Chemistry Department, IIT, Delhi, India

(Received 19 July 1993; accepted in revised form 13 April 1994)

Abstract-It was proposed earlier that the additional exothermic peak observed at 350°C in differential scanning calorimetry of PAN fibre is associated with certain intermolecular cross linking, as well as to the aromatization of structure, which results in a better stabilized structure. It has been further confirmed in the present study through infrared spectroscopy that these reactions take place in the fibre and occur only in the presence of oxygen.

Key Words-PAN fibre, stabilization, infrared spectra, aromatization, intermolecular crosslinking.

1. INTRODUCTION

Low-temperature thermal stabilization of PAN fibres has been an interesting area of studies over the last three decades[l-61, since it influences the resulting carbon-fibre properties to a large extent. Several re- action mechanisms have been proposed[7-131 to un- derstand the reaction kinetics during this step. The authors in their recent study[14] had proposed a new approach to thermal stabilization of PAN in which stabilization temperature was extended up to 400°C. The studies were in fact initiated after critically ana- lyzing the DSC curve of PAN fibres, which shows the existence of a second exothermic peak, shown in Fig. 1. Occurrence of the second maxima was pro- posed by the authors[ 141 to be possibly due to addi- tional cross-linking reactions. The authors have tried to confirm these reactions after studying the structure of these PAN fibres through the WAXD technique[ 151.

In the present paper we have studied IR spectra of these samples, which provide further evidence of our hypothesis, that is, promotion of intermolecular cross-linking reactions when PAN fibres are stabilized at elevated temperatures (i.e., 300-400°C) proposed earlier.

2. EXPERIMENTAL

Courtelle PAN precursor of 1.1 denier a single tow that contains 12,000 monofilaments, was used for the present studies. Oxidation experiments have been per- formed on a bench scale using the set-up described ear- lier[ 161. PAN fibres were heated to 250°C (sample A) at a heating rate of 1 “Urnin. These fibre samples were subjected to higher oxidation temperatures, 300”, 350”, and 400” (samples B to D, respectively) isothermally for 1 minute only in each case. The fibre tows regis- tered an elongation of 10% up to 250°C heat treat- ment, whereas no shrinkage/elongation was observed

*Author with whom correspondence should be addressed.

during the isothermal heating of the samples above

300°C. IR spectra of the samples was recorded on a Nicolet DX FTIR using the KBr pallet method, cov- ering the range 400-4000 cm-‘.

3. RESULTS AND DISCUSSION

IR spectra of PAN fibers have been studied and an- alyzed by several workers earlier[l7-211. A typical

spectrum of Courtelle PAN fibre (curve P, Fig. 2) shows two characteristic peaks, one at 2240 cm-’ due to -C=N dangling bond and another at 2940 crn~.-’ due to -CH2 groups. It also shows two additional peaks at 1730 cm-’ and 1170 cm-’ due to the presence of comonomers like methyl acrylate/methylmethacry- late. These groups can be either C=O or C-0[20].

When courtelle PAN fibers are heated above 180°C in air, peaks at 2940 cm-’ and 2240 cm-’ start de-

Maxima I

l”“l”“l”“l 100 200 300 400

Temperature “C -

Fig. 1. DSC curves of PAN fibres (a) in the presence of air and (b) in the presence of nitrogen. Heating rate S”C/min.

CAR 32:6-E 1133

1134 J. MITTAL et al.

/

/ :: 5 E P

i

E /

GIN $, c”2 I I I I I I

1 3zoo 2400 1800 1500 1100 8% 650 400

Wove Numbers (cm?)

Fig. 2. IR spectra of PAN fibres stabilized at different tem- peratures: (P) original precursor, (A) 25O”C, (B) 300°C

(C) 35O”C, (D) 4OO”C, (E) 400°C in N,.

creasing[20] (curve A, of Fig. 2) as a result of initia- tion of the cyclisation reaction[21]. Simultaneously, new peaks appear around 1590 cm-’ and 800 cm-‘, and a shoulder at 1655 cm-’ start to appear. While

the shoulder is due to /3-diketone group (conjugated ketonic groups), the corresponding peak at 1590 cm-’ is due to C=C and C=N groups and 800 cm-’ is due to =C-H groups. It has been reported earlier[20] that the peak at 800 cm-’ is absent when PAN is heated in the nitrogen atmosphere. The decrease in the intensity of CH2 groups and increase in the intensity of =C-H groups with treatment temperature in the present study suggests that =C-H is formed only in the presence of oxygen-causing aromatization of the structure.

It is seen from the curve A of Fig. 2 that the in-

tensity of peaks at 2940 cm-’ and 2240 cm-’ gets decreased considerably in the samples heated up to 250°C only. However, these peaks completely disap- peared after heating PAN fibre for just one minute be- yond 300°C (curves B, C, and D of Fig. 2). It is shown earlier[22] that these peaks do not completely disap- pear even after prolonged isothermal heating at low temperatures (<25O”C) (Fig. 3). Disappearance of the peak at 2240 cm-i at 300°C shows that remaining ni- trile groups are lost in the form of HCN beyond 250°C (Fig. 4), as proposed earlier[l4]. It was proposed ear- lier that the additional aromatization in structure takes place while heating in presence of oxygen beyond

3 4 5 8 1012.5

Wavelength

Fig. 3. Infrared spectra of polyacrylonitrile fibers (copoly- mer) heated in the presence of oxygen at 215°C for differ- ent times: (a) 0 min; (b) 25 min; (c) 200 min; (d) 400 min.

‘CP >,H’ \ CH/cH2\ , CH

b rT: : I

:A-7 -2HCN ,c?, CL

I I I - CH’ \ ; II l I

LJ_J L!_A cN

/Cn\c”Fn, I

dN

Fig. 4. Intermolecular cross-linking reaction involving free nitrile groups during stabilization of PAN between 300-400°C.

3OO”C, as indicated by the decrease in the percentage of hydrogen and disappearance of -CH2 and -CH groups shown in Fig. 5. It is now confirmed through IR by the disappearance of the peaks at 2940 cm-i (due to -CH2) and 1155 cm-’ (due to -CH) be- yond 250°C (Fig. 2).

The shoulder at 1655 cm-‘, however, behaves in a quite different manner. The shoulder gradually increases up to a temperature of 250°C (curve A of Fig. 2), beyond which it registers a gradual fall in in- tensity, as shown in curves B-D, Fig. 2. The decrease in the intensity of the shoulder is due to disappearance of ketonic groups as well as the oxygen-containing

functional groups in the fibre structure. This can be explained by some intermolecular cross linking, as shown in Fig. 6[22], taking place when the fibre is ox- idized to higher temperatures (300-400°C).

j$T& +o*_ &+2H20 lu

Fig. 5. Aromatization reaction in stabilized fibre in the pres- ence of oxygen.

IR studies of PAN fibres

// N \/ N, C C C

2\CB N\C, \C~N\C%@N\C/N\CPN\C,

I I I CH, ,CH\

C ci

,CH

A2

\,i’,C,clH\

I /H, ,C H

I 1 I

C \ C AC”, 2”, C C ,CH,

0 H b -4&O II H

II I I

9 H2

! 7

\cH/c\ 1% CH CH

,“\c”,“‘,H~

/ c \cH/C\CH/C\cH/C\cH/ I I I I 1

Y I I I ::

/c~N/c\N/c\NAN~c\

/c\N/c-==+N/c\N/c\N / \

Fig. 6. Intermolecular cross-linking reaction in PAN during the thermal stabilization step between 300-400°C.

1135

Further, the peak at 800 cm-’ (=C-H), which is very strong in the 250°C-treated sample, starts decreasing with subsequent high-temperature stabiliza- tion. This also suggests the occurrences of intermolec- ular cross-linking reactions by elimination of hydrogen in the form of H,O or HCN discussed earlier[22].

In Fig. 2, significant changes are observed in the peak around 600 cm-‘. However, this peak has not been attributed to any chemical change occurring in the PAN during oxidation, up to about 250°C. We

believe that this peak is due to vibrational modes of skeleton PAN due to various modes of CN, C-CN as reported earlier[l7]. Interestingly, in the present stud-

ies where the oxidation is extended up to 4OO”C, this peak gradually disappears. This may be due to inhi- bition of such free skeleton modes because of inter-

4000 3mo 2400 ,800 I%0 1100 850 650 400

Wave Numbers (cm?

Fig. 7. IR spectra of stabilized PAN fibre (B) carbonized to different temperatures.

molecular cross links hypothesized by us[14], when PAN is heated to 300°C and beyond. In order to fur-

ther confirm that these reactions are taking place only in air oxidation, the sample oxidized at 250°C was fur- ther heat-treated to 400” in N2 atmosphere. As shown in curve E of Fig. 2, the intensity of the peak around 600 cm-’ is the same as that for sample oxidized at

250°C. This, therefore, shows that there is no change in the basic skeleton of PAN if heated in the presence

of N, at 400°C. Figure 7 shows the IR spectra of 3OO”Coxidized

samples heated in the presence of Nz up to different temperatures. A general picture that emerges from these spectra is the decrease in the intensity of the peak

at 800 cm-’ and the shoulder at 2100 cm-‘. This sug- gests that the main reactions are consumption of =C-H and /3-diketonic groups during intermolecu-

lar crosslinkings. It is well understood that during early stages of carbonization (around 400°C), the mo- lecular chains in the oxidized PAN fibres start cross linking through dehydrogenation and condensation re- actions, resulting in the coalescence of the cyclized structure. Because a similar trend is observed in the IR spectra of the fibres oxidized in the presence of air between 300-400°C (Fig. 2), it can be concluded that the development of structure within this temperature range can also be due to intermolecular cross-linking

reactions.

4. CONCLUSIONS

It is now confirmed through IR that the reactions in the range 300-400°C are aromatizations (Fig. 5) and intermolecular crosslinkings (Figs. 4 and 6), and occur only in the presence of oxygen, as proposed earlier [ 141.

Acknowledgement-The authors wish to thank E. S. Raj- gopal, Director, NPL, for his constant encouragement dur- ing the course of this work and permission to publish the results. One of us (J. Mittal) is thankful to CSIR for its award of a Research Associateship.

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