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Structure of PAN fibres and its relationship to resulting carbon fibre properties
FsbreScienceand Technology 13(1981) 147 151
Technical Note Structure of PAN Fibres and its Relationship to Resulting Carbon Fibre Properties
STRUCTURE OF P A N
Polyacrylonitrile fibre is of great importance both in the textile industry and also for making high quality carbon fibres. As far as the development of carbon fibres from PAN is concerned, the behaviour is largely dependent on its structure; hence it is most important to study the crystallographic structure of a particular PAN fibre before converting it into carbon fibres. Various authors have made attempts to study the structure of PAN fibres through X-ray diffraction analysis. However, none of them could conclusively establish a particular structure. Some authors 1-3 assign the orthorhombic structure (a = 10.2,~, b = 6.1 A,, c = 5.1 A) to PAN fibres while others 4 propose a hexagonal cell (a = 6 A., c = 5.1 A). Recently 5 the structure of PAN has been better described using a paracrystalline model, a structure which is well ordered laterally in two dimensions with little or no order longitudinally. 6"7 The present study was therefore undertaken to resolve the existing controversy by taking three different samples of PAN fibres, made under three entirely different spinning conditions. The three different PAN fibres studied here bear the trade names Beslon (Japan), Courtella (UK) and Toray (Japan) respectively. The wide angle X-ray diffraction patterns of these oriented PAN fibres were obtained under identical exposure and other experimental conditions and are shown in Fig. 1. Discrete equatorial diffraction maxima are clearly seen in all three types of fibre. The first diffraction maxima with d = 5.2A corresponds to (200) reflections of the orthorhombic structure or (100) of the pseudo-hexagonal cell as mentioned above. This intense reflection is however an indication of prominent lateral order in the fibre which does exist in all the three precursors. A more critical examination of the diffractograms reveals four off-equatorial peaks (diffraction maxima) in the case of the Beslon fibre (Fig. 1(a)) whereas only a diffuse ring is visible in the case of the Courtella fibre (Fig. l(b)). The Toray fibre, however, does not show any off-equatorial reflections at all (Fig. l(c)). These off-equatorial diffraction maxima at d = 3.61 A could be assigned 147 Fibre Science and Technology 0015-0568/81/0015-0147/$02.50
England, 1981 Printed in Great Britain
© Applied Science Publishers Ltd,
148
o.P.
BAHL, R. B. MATHUR, K. D. KUNDRA
(a)
(b) Fig. I.
X-ray diffraction patterns of various PAN precursors.
STRUCTURE OF PAN FIBRES
149
(c) Fig. 1--contd.
to the (201) reflection of the orthorhombic or the (101) of the pseudo-hexagonal cells. In the absence of other off-equatorial reflections it is difficult to conclusively establish an orthorhombic or hexagonal structure for PAN fibres. Lindenmeyer and Hoseman 5 on the other hand have established that these two cells can be derived from each other where the (200) planes of the hexagonal cell are the same as the (i 10) planes of the orthorhombic cell. Hence to say that PAN has a hexagonal structure 4 or a orthorhombic structure 2 is rather fallacious. In fact it can have either structure depending upon the spinning conditions, type of comonomers and the stretch ratio, etc. As discussed above, the present study has revealed that the Beslon fibre (Fig. 1(a)) unlike the Courtella fibre (Fig. l(b)) possesses a longitudinal order as well, corresponding to the off-equatorial diffraction maxima. In order to see if the offequatorial maxima and hence the longitudinal ordering, in the precursor has any effect on the ultimate carbon fibre properties, carbon fibres were made from all the precursors and their properties studied.
CARBON FIBRE PROPERTIES
The dependence of the mechanical properties of carbon fibres on the structure of the original P A N is well known, a'9 The better the orientation in the original PAN, the
150
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
better should be the mechanical properties in general, and Young's modulus in particular, of the resulting carbon fibres. Since the equatorial arc length is the measure of orientation of molecular chains, it follows from Fig. 1 that Beslon PAN has the smallest arc length and should give carbon fibres with maximum Young's modulus and that the Toray fibre which possesses the maximum arc length should give lower modulus carbon fibres. In our laboratory we have made carbon fibres under optimum conditions with these PAN fibres and their mechanical properties are summarised in Table 1. TABLE 1
No.
P A N fibre type
1. 2. 3.
Beslon Courtella Toray
Carbon fibre properties at 1000 oC Tensile strength Young's modulus lb / in 2 Ib / in z 300 × 103 300 × 103 140 × 103
25 × 106 29 × 106 13 × 106
Surprisingly the Young's modulus is maximum in the case of Courtella fibres and not in the case of Beslon fibres. As discussed above, the Beslon precursor possesses lateral ordering together with the not so perfect longitudinal ordering (diffused offequatorial maxima). In the process of pyrolysis, this longitudinal ordering hinders the process of molecular chain orientation during the preoxidation stage and this is the most probable reason why Beslon-based carbon fibres give a lower Young's modulus than the Courtella fibres. In the case of the Toray PAN fibre the diffraction pattern is almost a complete ring, showing therefore, that the initial orientation or the Young's modulus of the fibre is very low, which effects the mechanical properties of the fibre which are also comparatively very low as can be seen in Table 1. We can therefore conclude that: (i)
(ii)
It is possible to have PAN fibres possessing either orthorhombic or hexagonal structures, depending on the spinning conditions and the composition. For making good quality carbon fibres one should look for the PAN precursor having only lateral ordering.
ACKNOWLEDGEMENTS
The authors are grateful to Dr A. R. Verma, Director, National Physical Laboratory, for his interest in and permission to publish this work. Sincere thanks are due to Dr G. C. Jain and Dr S. S. Chari for the encouragement and interest in the work.
STRUCTURE OF PAN FIBRES
15 [
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
R. STEFANI, M. CHEVERTON, J. TENIER and C. EYRAND, Comp. Rend., 248 (1959) p. 2006. R. STEFANI, M. CHEVERTON, T. TERRIERand C. EYRAND, Comp. Rend., 251 (1960) p. 2174. G. HUIRICHSENand H. ORTH, Kolloid-Z., 247 (1971) p. 844. G. W. URBARCZYK, Przeglad Wlokienizy, 15 (1961) p. 216. P. H. LINDENMEYERand R. HOSEMANN, J. Appl. Phys., 34 (1963) p. 42. C. R. BOrIN, J. R. SCHAEEGENand W. O. STATTON, J. Polymer Sci., 55 (1961) p. 531. L. G. WALLNERand K. RIGGERT, J. Polymer Sci., B1 0963) p. l l l . O. P. BAHLand R. B. MATHUR, Fib. Sci. Tech., 12 (1979) pp. 31-9. O. P. BAI-IL,R. B. MATHURand K. D. KUNDRA, Fib. Sci. Tech., 13 (1980) pp. 155 62. O . P. BAHL, R. B. MATHUR, K . D . KUNDRA,
Carbon Technology Unit, National Physical Laboratory, Hillside Road, N e w Delhi 110012 (India)
Technical Note Structure of PAN Fibres and its Relationship to Resulting Carbon Fibre Properties
STRUCTURE OF P A N
Polyacrylonitrile fibre is of great importance both in the textile industry and also for making high quality carbon fibres. As far as the development of carbon fibres from PAN is concerned, the behaviour is largely dependent on its structure; hence it is most important to study the crystallographic structure of a particular PAN fibre before converting it into carbon fibres. Various authors have made attempts to study the structure of PAN fibres through X-ray diffraction analysis. However, none of them could conclusively establish a particular structure. Some authors 1-3 assign the orthorhombic structure (a = 10.2,~, b = 6.1 A,, c = 5.1 A) to PAN fibres while others 4 propose a hexagonal cell (a = 6 A., c = 5.1 A). Recently 5 the structure of PAN has been better described using a paracrystalline model, a structure which is well ordered laterally in two dimensions with little or no order longitudinally. 6"7 The present study was therefore undertaken to resolve the existing controversy by taking three different samples of PAN fibres, made under three entirely different spinning conditions. The three different PAN fibres studied here bear the trade names Beslon (Japan), Courtella (UK) and Toray (Japan) respectively. The wide angle X-ray diffraction patterns of these oriented PAN fibres were obtained under identical exposure and other experimental conditions and are shown in Fig. 1. Discrete equatorial diffraction maxima are clearly seen in all three types of fibre. The first diffraction maxima with d = 5.2A corresponds to (200) reflections of the orthorhombic structure or (100) of the pseudo-hexagonal cell as mentioned above. This intense reflection is however an indication of prominent lateral order in the fibre which does exist in all the three precursors. A more critical examination of the diffractograms reveals four off-equatorial peaks (diffraction maxima) in the case of the Beslon fibre (Fig. 1(a)) whereas only a diffuse ring is visible in the case of the Courtella fibre (Fig. l(b)). The Toray fibre, however, does not show any off-equatorial reflections at all (Fig. l(c)). These off-equatorial diffraction maxima at d = 3.61 A could be assigned 147 Fibre Science and Technology 0015-0568/81/0015-0147/$02.50
England, 1981 Printed in Great Britain
© Applied Science Publishers Ltd,
148
o.P.
BAHL, R. B. MATHUR, K. D. KUNDRA
(a)
(b) Fig. I.
X-ray diffraction patterns of various PAN precursors.
STRUCTURE OF PAN FIBRES
149
(c) Fig. 1--contd.
to the (201) reflection of the orthorhombic or the (101) of the pseudo-hexagonal cells. In the absence of other off-equatorial reflections it is difficult to conclusively establish an orthorhombic or hexagonal structure for PAN fibres. Lindenmeyer and Hoseman 5 on the other hand have established that these two cells can be derived from each other where the (200) planes of the hexagonal cell are the same as the (i 10) planes of the orthorhombic cell. Hence to say that PAN has a hexagonal structure 4 or a orthorhombic structure 2 is rather fallacious. In fact it can have either structure depending upon the spinning conditions, type of comonomers and the stretch ratio, etc. As discussed above, the present study has revealed that the Beslon fibre (Fig. 1(a)) unlike the Courtella fibre (Fig. l(b)) possesses a longitudinal order as well, corresponding to the off-equatorial diffraction maxima. In order to see if the offequatorial maxima and hence the longitudinal ordering, in the precursor has any effect on the ultimate carbon fibre properties, carbon fibres were made from all the precursors and their properties studied.
CARBON FIBRE PROPERTIES
The dependence of the mechanical properties of carbon fibres on the structure of the original P A N is well known, a'9 The better the orientation in the original PAN, the
150
O. P. BAHL, R. B. MATHUR, K. D. KUNDRA
better should be the mechanical properties in general, and Young's modulus in particular, of the resulting carbon fibres. Since the equatorial arc length is the measure of orientation of molecular chains, it follows from Fig. 1 that Beslon PAN has the smallest arc length and should give carbon fibres with maximum Young's modulus and that the Toray fibre which possesses the maximum arc length should give lower modulus carbon fibres. In our laboratory we have made carbon fibres under optimum conditions with these PAN fibres and their mechanical properties are summarised in Table 1. TABLE 1
No.
P A N fibre type
1. 2. 3.
Beslon Courtella Toray
Carbon fibre properties at 1000 oC Tensile strength Young's modulus lb / in 2 Ib / in z 300 × 103 300 × 103 140 × 103
25 × 106 29 × 106 13 × 106
Surprisingly the Young's modulus is maximum in the case of Courtella fibres and not in the case of Beslon fibres. As discussed above, the Beslon precursor possesses lateral ordering together with the not so perfect longitudinal ordering (diffused offequatorial maxima). In the process of pyrolysis, this longitudinal ordering hinders the process of molecular chain orientation during the preoxidation stage and this is the most probable reason why Beslon-based carbon fibres give a lower Young's modulus than the Courtella fibres. In the case of the Toray PAN fibre the diffraction pattern is almost a complete ring, showing therefore, that the initial orientation or the Young's modulus of the fibre is very low, which effects the mechanical properties of the fibre which are also comparatively very low as can be seen in Table 1. We can therefore conclude that: (i)
(ii)
It is possible to have PAN fibres possessing either orthorhombic or hexagonal structures, depending on the spinning conditions and the composition. For making good quality carbon fibres one should look for the PAN precursor having only lateral ordering.
ACKNOWLEDGEMENTS
The authors are grateful to Dr A. R. Verma, Director, National Physical Laboratory, for his interest in and permission to publish this work. Sincere thanks are due to Dr G. C. Jain and Dr S. S. Chari for the encouragement and interest in the work.
STRUCTURE OF PAN FIBRES
15 [
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
R. STEFANI, M. CHEVERTON, J. TENIER and C. EYRAND, Comp. Rend., 248 (1959) p. 2006. R. STEFANI, M. CHEVERTON, T. TERRIERand C. EYRAND, Comp. Rend., 251 (1960) p. 2174. G. HUIRICHSENand H. ORTH, Kolloid-Z., 247 (1971) p. 844. G. W. URBARCZYK, Przeglad Wlokienizy, 15 (1961) p. 216. P. H. LINDENMEYERand R. HOSEMANN, J. Appl. Phys., 34 (1963) p. 42. C. R. BOrIN, J. R. SCHAEEGENand W. O. STATTON, J. Polymer Sci., 55 (1961) p. 531. L. G. WALLNERand K. RIGGERT, J. Polymer Sci., B1 0963) p. l l l . O. P. BAHLand R. B. MATHUR, Fib. Sci. Tech., 12 (1979) pp. 31-9. O. P. BAI-IL,R. B. MATHURand K. D. KUNDRA, Fib. Sci. Tech., 13 (1980) pp. 155 62. O . P. BAHL, R. B. MATHUR, K . D . KUNDRA,
Carbon Technology Unit, National Physical Laboratory, Hillside Road, N e w Delhi 110012 (India)