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IR studies on preoxidized PAN fibres
Co&v!,
1976, Vol. 14, pp. 201-109.
Pergamon Press.
Printed in Great Britain
IR STUDIES ON PREOXIDIZED PAN FIBRES S. P. VARMA,B. B.
and N. K. SRIVASTAVA
LAL
Infrared Section, National Physical Laboratory, Hillside Road, New Delhi 110012,India (Receioed 9 Marck
1976)
Abstract-IR absorption spectra of preoxidized PAN fibres have been studied in the spectral region 2-15 ~1.Observed frequencies have been assigned to the different modes of vibration. The neighbouring frequencies appearing in the stretching mode region of the nitrile group has been interpreted in terms of Fermi-resonance and a basis for the correlation between their intensity ratio and the degree of oxidation has also been discussed. 1.INTRODUCTION
Molecular orientation and rearrangement play a very important role in understanding the mechanism involved among the oxidation process, degree of conjugation (or reaction) and contribution towards the strength and the modulus of carbon fibres. These relevant imformations may be obtained by various techniques, out of which IR technique has been proved to be very useful. Liang and Krimm [ l] have reported the IR spectra of polyacrylonitrile (PAN) in the region 70-32OOcm-’ and they have given tentative assignments to some of the observed frequencies. IR study of PAN fibres has also been made by Bailey and Clarke[2] and they have only indicated the approximate spectral regions relevant to some of the frequencies expected to be observed. On the basis of these observations the latter authors have also proposed a possible arrangement of the chemical groups for a partially oxidized PAN. However, this chemical structure does not correspond to the structure already suggested by Standage and Matkowsky[3] on the basis of multiple internal reflection IR spectroscopy. In this way, we observe that so far proposed chemical structures of preoxidized PAN fibres, as reviewed by Clarke and Bailey[4], are very inconsistent. The latter have also reported the IR spectra of the oxidized PAN fibres to understand the strength and structure but on the whole they are very ambiguous and no specific conclusion has been drawn. In the present study, the IR absorption spectra of preoxidized PAN fibres were recorded and an attempt has been made to assign the observed frequencies to different modes of vibration, to analyze the neighbouring frequencies appearing in the stretching mode region of the nitrile group and to have a close insight of their intensity ratios with respect to the degree of oxidation. 2.~mtu.
method of preparation of preoxidized PAN has been described elsewhere by Bahl and Manocha[S]. For our study, the KBr pellet technique has been employed and the fixed proportion of fibres, in each pellet, was taken for all the samples. The spectra were recorded on a Hilger and Watts H-800 spectrophotometer (2-15 1~)for several PAN fibre samples preoxidized in air at 200230°C. The oxidation time for these samples varied from 25 min to 4Ohr. Since the absorption spectra tend to be very broad and unresolved as the oxidation time is The
increased, spectra obtained from PAN preoxidized for 10hr are considered here so that an appreciable change in chemical structure is known with an assesseable distortion in the spectra. J.REsuL'TSAND DISCUSSION
observed absorption frequencies along with their assignments, and relative intensities are given in Table 1 for PAN fibres preoxidized in air for 1Ohr. In this case, the molecular symmetry has been assumed to belong to the C, point group. It is clear from Table 1 that the frequencies C=C, C=N, C=O, C-N, C-OH, O-H, N-H are observed in the IR spectra of preoxidized PAN along with the frequencies arising due to the original PAN fibres. These frequencies appear with appreciable intensities in the spectra. From this observation it can be easily inferred that the preoxidized PAN has not fully converted to a conjugated structure. Also, the presence of these IR frequencies indicate that the chemical structure of the preoxidized PAN fibres may be similar to the one suggested by Bailey and Clarke[2]. It is also observed that after oxidation the entire spectra have been broadened and overall intensity of absorption bands has increased. This broadening in spectra makes resolution of the specific bands difficult, and also suggests that the structure of PAN has changed to a complex structure due to oxidation. In the constituent chemical groups of the PAN fibres, there are some group frequencies which lie in the same spectral region, i.e. the overlapping of several frequencies due to different modes of vibration. This type of overlapping may cause broadening in the spectra. Also, after the preoxidation, oxygen comes in its chemical struture and the perturbation caused by the inter- and/or intra-molecular hydrogen bonding may result in to spectral broadening. The different environmental conditions of a specific mode of vibration in the long chain of the preoxidized PAN fibres may also be one of the reasons for this type of spectra. The neighbouring frequencies appearing in the stretching mode region of the nitrile group (GN), have been indicated in terms of resonance by Clarke and Bailey [4]. But the nature of the resonance and the interacting frequencies taking part in it have not been explained. In the present investigation, two frequencies in the stretching mode region of C=N have been observed at 2180 and 2240 cm-‘, and it is expected that the origin of these two The
207
S. P. V4uM.4 et al.
Table 1. IR absorption frequencies and their assignments for preoxidized PAN fibres Observed frequencies (cm-‘) 4425 3250 2890 2240 2180 1715 1656 1580 1460 1425 1330 1200 1150 1060 752
Species
s (br) s (br)
A’ a’
a’
a’ a’
ms ms S
a’
mw m
a' a’ a’
s (W ms ms ms
m m
a” a’ a’ a’ a”
m, medium; ms, medium strong; broad.
mw,
frequencies may be due to Fermi-resonance. The necessary requirements for Fermi-resonance are that the two frequencies must be very close in magnitude and both must belong to the same symmetry species. If it is accepted that these two frequencies in the nitrile group region result due to Fermi-resonance, the magnitude of the C =N stretching mode, which is one of the two interacting frequencies, must be the mean of the two observed frequencies. The symmetry of CSN stretching mode is of a’ type. The other component of the Fermi-resonance (probably an overtone or a combination band), therefore, should also belong to a’ symmetry species and should have nearly the same magnitude and intensity. In the present case, the mean of the two frequencies is 2210cm-’ which may be assigned to CSN stretching mode. As evident from Table 1, a strong absorption band due to C-N and/or C-OH ‘stretching mode appears at 1150cm-’ which belongs to a’ symmetry species, and another strong band due to C-CN stretching mode appears at 1060cm-’ which also belongs to a’ symmetry species. The combination band of these two will have a magnitude of -2210 cm-’ and would belong to a’ species. Thus, the 2210 cm-’ CZN stretching frequency and the above mentioned combination band at 2210cm-’ satisfy the requirements for Fermi-resonance, due to which these two frequencies may be equally shifted from their original position. Furthermore, a frequency at 4425 cm-’ has been observed in the spectrum which appears to be the first overtone of the C=N stretching frequency at 2210cm-‘. This confirms the presence of the latter fundamental frequency, derived from the observed Fermi-resonance components. However, the intensity of the overtone band at 4425cm-’ could not be justified and may be understood through the reasons related to the broadening of the spectrum. It is generally observed in the case of Fermi-resonance that the intensities tend to be shared equally between the
Assignment 2 x 2210= 4420 NH/ or CH stretching CH, stretching Fermi-resonance between CEN stretching and a combination of C-N/ or C-OH and C-CN stretching. C=O stretching C=C stretching C=N stretching NHz scissoring CH, scissoring CH in-plane-bending CH, wagging CH in-plane-bending C-N/ or C-OH stretching C-CN stretching CH2 rocking/ or CH outof-plane bending medium weak; s, strong (intensities). br,
two resonance components[6], but it is not so in the present case. This type of nature of intensities of the Fermi-resonance components have also been observed in few cases of aldehyde and benzoyl group frequencies [79]. However, there may be different reasons for this type of nature, especially in the present case. As oxidation proceeds, C=N groups reduce to C=N, and C-N groups. Consequently, concentrations of C=N and C-N groups increase simultaneously with a decrease in the concentration of CSN groups. Thus, the concentrations of C=N and/or C-N groups in preoxidized PAN fibres may also result this type of nature of intensities in the Fermiresonance. Also, by assigning these two neighbouring frequencies independently instead of interpreting them in terms of resonance, the nature of their intensities may be understood. It will be again based on the concentration of C E N group and its reduced constituent groups, i.e. C=N and/or C-N groups, present in preoxidized PAN fibres. However, with the spectra and structure available, to date, this type of attempt has not been possible. More extensive and systematic studies employing different analytical techniques may settle these types of ambiguities. Originally, PAN consists of =CH2, =CH and -C=N chemical groups in a non-conjugated form. When it is heat treated, there results partial conjugation and some new groups are added to it. The degree of conjugation, to some extent, depends upon the oxidation time and it contributes to the strength and the modulus of the PAN fibres. Moreover, the dependence between the degree of conjugation and strength as well as modulus is not yet well understood. However, the correlation between oxidation time and the degree of conjugation may be established by knowing the ratio of intensities of the two neighbouring frequencies observed in the CeN stretching region. An attempt in the case of “Courtelle” fibres has
209
IR studies on preoxidized PAN fibres
been made[4] to achieve this type of correlation but its basis has not been explained. The stretching region of the nitrile group is the only region in the present case, throughout the entire spectrum which remains unaffected by the broadening. Also, the absorption peak positions are unshifted in the magnitude suggesting that the corresponding absorption cross-section has not changed. Thus, it implies that the changes in the intensity of these two frequencies (2240 and 2180 cm-‘) are proportional to the corresponding changes in the concentration of the groups involved, due to different length of oxidation. Obviously, here the involved groups are the nitrile group and its reduced constituent groups. Present IR studies on the preoxidized PAN fibres have revealed the dependence of the degree of conjugation with the oxidation time. A kinetic of the chemical reaction along with the changes in its chemical structure may be better understood from in situ IR measurements on PAN fibres preoxidized for different duration of time and in different environments. Further work on the absorption
studies at low temperature, with polarized light in the near and far IR regions is in progress and wiI1 be presented elsewhere. Acknowledgements-We are thankful to Prof. A. R. Verma and Dr. V. G. Bhide for their constant encouragement, and Dr. S. S. Chari for providing preoxidized PAN fibres along with his many helpful suggestions and inspirations. REFERENCES
1. C. Y. Liang and S. Krimm, J. Poly. Sci. 31, 513 (1958). 2. J. E. Bailey and A. 3. Clarke, Nature 234, 529 (1971). 3. A. E. Standage and R. Matkowsky, Nature 224, 689 (1969). 4. A. J. Clarke and J. E. Bailey, Nature 243, 146 (1973). 5. 0. P. Bahl and L. M. Manocha, Carbon 12,417 (1974). 6. F. A. Cotton, Chemical Applications of Group Theory, Second edn. Wiley-Interscience, New York (1971). 7. B. G. Viladkar, Proc. Symp. on Raman and Infrared Spectroscopy (University of Kerala, Trivendrum, India), p. 123(1964). 8. B. B. Lal, Spectral studies in organic molecules, Ph.D. Thesis, Banaras Hindu Univerity, Varanasi, India (1971). 9. B. B. Lal, K. Singh and I. S. Singh, Current Sci. 42,627 (1973).
1976, Vol. 14, pp. 201-109.
Pergamon Press.
Printed in Great Britain
IR STUDIES ON PREOXIDIZED PAN FIBRES S. P. VARMA,B. B.
and N. K. SRIVASTAVA
LAL
Infrared Section, National Physical Laboratory, Hillside Road, New Delhi 110012,India (Receioed 9 Marck
1976)
Abstract-IR absorption spectra of preoxidized PAN fibres have been studied in the spectral region 2-15 ~1.Observed frequencies have been assigned to the different modes of vibration. The neighbouring frequencies appearing in the stretching mode region of the nitrile group has been interpreted in terms of Fermi-resonance and a basis for the correlation between their intensity ratio and the degree of oxidation has also been discussed. 1.INTRODUCTION
Molecular orientation and rearrangement play a very important role in understanding the mechanism involved among the oxidation process, degree of conjugation (or reaction) and contribution towards the strength and the modulus of carbon fibres. These relevant imformations may be obtained by various techniques, out of which IR technique has been proved to be very useful. Liang and Krimm [ l] have reported the IR spectra of polyacrylonitrile (PAN) in the region 70-32OOcm-’ and they have given tentative assignments to some of the observed frequencies. IR study of PAN fibres has also been made by Bailey and Clarke[2] and they have only indicated the approximate spectral regions relevant to some of the frequencies expected to be observed. On the basis of these observations the latter authors have also proposed a possible arrangement of the chemical groups for a partially oxidized PAN. However, this chemical structure does not correspond to the structure already suggested by Standage and Matkowsky[3] on the basis of multiple internal reflection IR spectroscopy. In this way, we observe that so far proposed chemical structures of preoxidized PAN fibres, as reviewed by Clarke and Bailey[4], are very inconsistent. The latter have also reported the IR spectra of the oxidized PAN fibres to understand the strength and structure but on the whole they are very ambiguous and no specific conclusion has been drawn. In the present study, the IR absorption spectra of preoxidized PAN fibres were recorded and an attempt has been made to assign the observed frequencies to different modes of vibration, to analyze the neighbouring frequencies appearing in the stretching mode region of the nitrile group and to have a close insight of their intensity ratios with respect to the degree of oxidation. 2.~mtu.
method of preparation of preoxidized PAN has been described elsewhere by Bahl and Manocha[S]. For our study, the KBr pellet technique has been employed and the fixed proportion of fibres, in each pellet, was taken for all the samples. The spectra were recorded on a Hilger and Watts H-800 spectrophotometer (2-15 1~)for several PAN fibre samples preoxidized in air at 200230°C. The oxidation time for these samples varied from 25 min to 4Ohr. Since the absorption spectra tend to be very broad and unresolved as the oxidation time is The
increased, spectra obtained from PAN preoxidized for 10hr are considered here so that an appreciable change in chemical structure is known with an assesseable distortion in the spectra. J.REsuL'TSAND DISCUSSION
observed absorption frequencies along with their assignments, and relative intensities are given in Table 1 for PAN fibres preoxidized in air for 1Ohr. In this case, the molecular symmetry has been assumed to belong to the C, point group. It is clear from Table 1 that the frequencies C=C, C=N, C=O, C-N, C-OH, O-H, N-H are observed in the IR spectra of preoxidized PAN along with the frequencies arising due to the original PAN fibres. These frequencies appear with appreciable intensities in the spectra. From this observation it can be easily inferred that the preoxidized PAN has not fully converted to a conjugated structure. Also, the presence of these IR frequencies indicate that the chemical structure of the preoxidized PAN fibres may be similar to the one suggested by Bailey and Clarke[2]. It is also observed that after oxidation the entire spectra have been broadened and overall intensity of absorption bands has increased. This broadening in spectra makes resolution of the specific bands difficult, and also suggests that the structure of PAN has changed to a complex structure due to oxidation. In the constituent chemical groups of the PAN fibres, there are some group frequencies which lie in the same spectral region, i.e. the overlapping of several frequencies due to different modes of vibration. This type of overlapping may cause broadening in the spectra. Also, after the preoxidation, oxygen comes in its chemical struture and the perturbation caused by the inter- and/or intra-molecular hydrogen bonding may result in to spectral broadening. The different environmental conditions of a specific mode of vibration in the long chain of the preoxidized PAN fibres may also be one of the reasons for this type of spectra. The neighbouring frequencies appearing in the stretching mode region of the nitrile group (GN), have been indicated in terms of resonance by Clarke and Bailey [4]. But the nature of the resonance and the interacting frequencies taking part in it have not been explained. In the present investigation, two frequencies in the stretching mode region of C=N have been observed at 2180 and 2240 cm-‘, and it is expected that the origin of these two The
207
S. P. V4uM.4 et al.
Table 1. IR absorption frequencies and their assignments for preoxidized PAN fibres Observed frequencies (cm-‘) 4425 3250 2890 2240 2180 1715 1656 1580 1460 1425 1330 1200 1150 1060 752
Species
s (br) s (br)
A’ a’
a’
a’ a’
ms ms S
a’
mw m
a' a’ a’
s (W ms ms ms
m m
a” a’ a’ a’ a”
m, medium; ms, medium strong; broad.
mw,
frequencies may be due to Fermi-resonance. The necessary requirements for Fermi-resonance are that the two frequencies must be very close in magnitude and both must belong to the same symmetry species. If it is accepted that these two frequencies in the nitrile group region result due to Fermi-resonance, the magnitude of the C =N stretching mode, which is one of the two interacting frequencies, must be the mean of the two observed frequencies. The symmetry of CSN stretching mode is of a’ type. The other component of the Fermi-resonance (probably an overtone or a combination band), therefore, should also belong to a’ symmetry species and should have nearly the same magnitude and intensity. In the present case, the mean of the two frequencies is 2210cm-’ which may be assigned to CSN stretching mode. As evident from Table 1, a strong absorption band due to C-N and/or C-OH ‘stretching mode appears at 1150cm-’ which belongs to a’ symmetry species, and another strong band due to C-CN stretching mode appears at 1060cm-’ which also belongs to a’ symmetry species. The combination band of these two will have a magnitude of -2210 cm-’ and would belong to a’ species. Thus, the 2210 cm-’ CZN stretching frequency and the above mentioned combination band at 2210cm-’ satisfy the requirements for Fermi-resonance, due to which these two frequencies may be equally shifted from their original position. Furthermore, a frequency at 4425 cm-’ has been observed in the spectrum which appears to be the first overtone of the C=N stretching frequency at 2210cm-‘. This confirms the presence of the latter fundamental frequency, derived from the observed Fermi-resonance components. However, the intensity of the overtone band at 4425cm-’ could not be justified and may be understood through the reasons related to the broadening of the spectrum. It is generally observed in the case of Fermi-resonance that the intensities tend to be shared equally between the
Assignment 2 x 2210= 4420 NH/ or CH stretching CH, stretching Fermi-resonance between CEN stretching and a combination of C-N/ or C-OH and C-CN stretching. C=O stretching C=C stretching C=N stretching NHz scissoring CH, scissoring CH in-plane-bending CH, wagging CH in-plane-bending C-N/ or C-OH stretching C-CN stretching CH2 rocking/ or CH outof-plane bending medium weak; s, strong (intensities). br,
two resonance components[6], but it is not so in the present case. This type of nature of intensities of the Fermi-resonance components have also been observed in few cases of aldehyde and benzoyl group frequencies [79]. However, there may be different reasons for this type of nature, especially in the present case. As oxidation proceeds, C=N groups reduce to C=N, and C-N groups. Consequently, concentrations of C=N and C-N groups increase simultaneously with a decrease in the concentration of CSN groups. Thus, the concentrations of C=N and/or C-N groups in preoxidized PAN fibres may also result this type of nature of intensities in the Fermiresonance. Also, by assigning these two neighbouring frequencies independently instead of interpreting them in terms of resonance, the nature of their intensities may be understood. It will be again based on the concentration of C E N group and its reduced constituent groups, i.e. C=N and/or C-N groups, present in preoxidized PAN fibres. However, with the spectra and structure available, to date, this type of attempt has not been possible. More extensive and systematic studies employing different analytical techniques may settle these types of ambiguities. Originally, PAN consists of =CH2, =CH and -C=N chemical groups in a non-conjugated form. When it is heat treated, there results partial conjugation and some new groups are added to it. The degree of conjugation, to some extent, depends upon the oxidation time and it contributes to the strength and the modulus of the PAN fibres. Moreover, the dependence between the degree of conjugation and strength as well as modulus is not yet well understood. However, the correlation between oxidation time and the degree of conjugation may be established by knowing the ratio of intensities of the two neighbouring frequencies observed in the CeN stretching region. An attempt in the case of “Courtelle” fibres has
209
IR studies on preoxidized PAN fibres
been made[4] to achieve this type of correlation but its basis has not been explained. The stretching region of the nitrile group is the only region in the present case, throughout the entire spectrum which remains unaffected by the broadening. Also, the absorption peak positions are unshifted in the magnitude suggesting that the corresponding absorption cross-section has not changed. Thus, it implies that the changes in the intensity of these two frequencies (2240 and 2180 cm-‘) are proportional to the corresponding changes in the concentration of the groups involved, due to different length of oxidation. Obviously, here the involved groups are the nitrile group and its reduced constituent groups. Present IR studies on the preoxidized PAN fibres have revealed the dependence of the degree of conjugation with the oxidation time. A kinetic of the chemical reaction along with the changes in its chemical structure may be better understood from in situ IR measurements on PAN fibres preoxidized for different duration of time and in different environments. Further work on the absorption
studies at low temperature, with polarized light in the near and far IR regions is in progress and wiI1 be presented elsewhere. Acknowledgements-We are thankful to Prof. A. R. Verma and Dr. V. G. Bhide for their constant encouragement, and Dr. S. S. Chari for providing preoxidized PAN fibres along with his many helpful suggestions and inspirations. REFERENCES
1. C. Y. Liang and S. Krimm, J. Poly. Sci. 31, 513 (1958). 2. J. E. Bailey and A. 3. Clarke, Nature 234, 529 (1971). 3. A. E. Standage and R. Matkowsky, Nature 224, 689 (1969). 4. A. J. Clarke and J. E. Bailey, Nature 243, 146 (1973). 5. 0. P. Bahl and L. M. Manocha, Carbon 12,417 (1974). 6. F. A. Cotton, Chemical Applications of Group Theory, Second edn. Wiley-Interscience, New York (1971). 7. B. G. Viladkar, Proc. Symp. on Raman and Infrared Spectroscopy (University of Kerala, Trivendrum, India), p. 123(1964). 8. B. B. Lal, Spectral studies in organic molecules, Ph.D. Thesis, Banaras Hindu Univerity, Varanasi, India (1971). 9. B. B. Lal, K. Singh and I. S. Singh, Current Sci. 42,627 (1973).