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Electron spin resonance in carbonized organic polymers—I
Carbon
1964, Vol. 2, pp. 227-237.
ELECTRON
Pergamon
Press Ltd.
SPIN RESONANCE ORGANIC C. JACKSON*
Northern
Coke Research
Printed in Great Britain
Laboratories,
IN CARBONIZED
POLYMERS-I
and W. F. K. WYNNE-JONES
School of Chemistry,
The University,
Newcastle
upon Tyne
1, England
(Received 10 February 1964)
Abstract-An investigation has been made into electron spin resonance (ESR) occurring in ranges of carbons prepared from eight organic polymeric materials. Free spin concentration, line width, line shape and “g’‘-factors have been measured. A correlation has been attempted of these measurements with chemical analysis, d.c. resistivity, X-ray diffraction and the chemical structure of the original
polymeric material. No correlation could be found between free spin concentration and the [C]/[Hj ratio. The temperature region where d.c. resistivity is decreasing rapidly is not related to that of rapid increase in line width. Those carbons which become conducting most readily are those for which the unpaired electrons are formed most readily. For cellulose carbons, there is reasonable agreement between number of free spins and number of layer planes. The stability and concentrations of free spins is discussed in the terms of HTT and chemical structure of the polymers. New phenomena have been observed in the variation of line width with HTT for five of the ranges of carbons examined by ESR. The discussion includes some considerations of the factors which affect line width and of the nature of the free spin in carbonaceous materials. Some difficulties limiting the interpretation of ESR measurements are outlined.
1. INTRODUCTION
of ESR is particularly opportune in view of the shortage of experiments on well characterized carbons the origins of which are well known, and the study is a.n attempt to contribute to the understanding of the origins of ESR in carbons. As the examined carbons have been prepared from pure organic materials, usually polymers, the chemical compositions of which are quite different, e.g. polydivinyl benzene and melamine formaldehyde, the effect of chemical composition upon the ESR has been investigated. The variations of the measurable quantities of electron spin susceptibility and line shape and width with heat treatment temperature (HTT) have been investigated. The various interactions which can effect the widths of ESR lines have been critically discussed because amorphous carbons are probably unique in that for a range of carbons prepared at different heat treatment temperatures, practically every type of interaction so far recognized has been invoked to explain the observed variations in line width and line shape. (1)
THE PHENOMENON of electron spin resonance (ESR) in carbonaceous materials is now clearly recognized though the understanding of the origins is far from complete. Recently SINGER(~) has critically reviewed almost the entire literature of this field of study and has stressed the extreme care needed in experimentation and interpretation of results. MARSH and WYNNJ+JONES@) have prepared eight series of carbons from pure organic materials and have examined their surface properties and chemical composition. These carbons are extremely well characterized and their method of preparation is described in detail. The examination of ESR in these carbons has been completed as part of a comprehensive study and selected results have already been used in discussions of chemical reactivity of carbons to mdecular oxygen.(s) This examination * Present address : School of Pharmacy, Sunderland Technical College, Sunderland, Co. Durham. 227
228
C. JACKSON
and W. F. K. WYNNE-JONES
2. EXPERIMENTAL
2.1 Carbcuzpreparation The carbons used in this study have been prepared from polydivinyl benzene, polyvinylidene chloride, ‘cellulose, polyfurfuryl alcohol, dibenzanthrone, polyvinyl cyanide, melamine formaldehyde and polyvinyl chloride, as described by MARSHand WYNNE-JONES.(~)The maximum HTT was maintained for 10 hr to establish the equilibrium concentration of free radicals.@) The carbonizations were all carried out in vacuum or nitrogen because of the known effects of carbonization in air.(l) In all preparations of samples for examination in the spectrometer the carbons were ground in air to less than 150 ,u. Weighed amounts of carbon (several milligrams) were then outgassed in weighed Pyrex tubes (O.D. of 5 mm) to 10-s cm Hg at 200°C and the tubes sealed. This procedure enabled the weight of the outgassed carbon to be accurately known. The main supply of the carbon was always stored in vacuum to avoid the slow chemisorption of oxygen which occurs at room temperature and which can alter the concentration of unpaired electrons in the carbon.(r) In these studies no dependence of free spin concentration on particle size down to sizes of 62 micron have been observed in outgassed carbons. All of the examined carbons contain negligible quantities of ash in so far as standard ash determinations show undetectable amounts. This is important because of the influence of mineral impurities upon line shape. (5) In addition, small amounts of carbons, produced from hexaiodobenzene, were examined. The hexaiodobenzene (m.p. 340°C) was prepared by RUPP’S method(s) and recrystallized from nitrobenzene. The red-brown, needle-like crystals were carbonized in nitrogen in a sample tube which, after evacuation, could be inserted directly into the spectrometer. Heat treatment up to 1000°C did not produce any detectable concentrations of unpaired electrons. 2.2 D.C. resistivity measurements Resistivity measurements are usually made with the specimen under compression to minimize interparticle effects. It was found that for all carbons examined in this study the resistance value decreased with increasing pressure up to the maximum pressure used of 1000 kg/cmz, but
showed no signs of having reached a saturation value. As these measurements were made to ascertain if the final broadening of the ESR absorption curves coincided with the sharp fall in d.c. resistivity it was considered that relative results would be adequate. All measurements were made at room temperature. The conductivity cell consisted of thick walled “Veridia” tubing, 6 cm in length and 0.25 cm in diameter. A specimen of l-2 mg of finely ground carbon (<150 p) was contained between two, highly polished, silver-steel electrodes which fitted closely inside the “Veridia” tubing, and terminated in brass contacts. Leads from the brass contacts were taken to a Wheatstone bridge and the resistance measured. The whole assembly could be accurately aligned and clamped, along with a compression gauge, between the jaws of a small vice and the powder compressed to the required value. 2.3 Crystallite size determinations X-ray, powder diffraction photographs have been taken of the cellulose carbons. The mean layer sizes of the turbostratic crystallites were obtained using Warren’s method as described by BLAYDEN,GIBSON and RILEY(~)which involves the measurement of the line width of the (lo)-band. These carbons contain both ordered and disordered material and an estimate of the ordered material was obtained by comparison of the absolute intensity of the (02)-band relative to graphite. 2.4 Examination of carbon by ESR The spectrometer was designed to operate at X-band, i.e. frequencies of the order of 9,000 MC/S and magnetic fields of 3200 G. Provision was made for both crystal video and phase sensitive detection. An Hsla cavity was made from standard waveguide with the distance between the iris plates of 3.95 cm, and O-281 in. diameter holes in the iris plates. A hole was drilled in the centre of the narrow side of the cavity, passing right through the cavity, to accept the Pyrex tubes containing the specimens. The specimen holder was designed so that the Pyrex tubes could always be placed accurately in the centre of the cavity. The holder was essentially
ELECTRON
SPIN
RESONANCE
IN CARBONIZED
a modified chuck. When used for crystal video detection, the lower hole in the cavity (designed to hold the 100 kc/s modulation loop) was sealed off with a tightly fitting brass insert. Under the best possible conditions the crystal video spectrometer was capable of detecting approximately 1 x 101s AH, free spins per gram (AHx is the absorptionline width in gauss at half power). It was, however, impossible to make accurate quantitative estimations of free radical concentrations as low as this. The concentrations of unpaired electrons in the amo~hous carbons were usually much higher than the limit of sensitivity of the apparatus, being of the order of 1 x 1019 to 2x 102s per gram. Since resolved hyperfine structure was not observed in the spectra associated with these amorphous carbons it was only possible to measure integrated intensity, width, shape and “g” factors of the absorption curves. In the calculation of spin concentration from integrated intensity it has been assumed that the free spin centres of the carbon are similar to those of stable free radicals with almost free spins, i.e. the number of unpaired spins is related to the static spin susceptibility by the usual Curie Law expression. In this way the measurements of spin concentration could be made by essentially comparative methods. These depend upon knowing the concentration of unpaired electrons in a known weight of the organic free radical diphenyl-picryl-hydrazyl (DPPH) which possesses one unpaired electron spin per molecule. The DPPH was used in such a way, as a sub-standard, that its absorption line appeared on the oscilloscope screen simultaneously with that of the carbon under Difficulties of differing dielectric examination. loss of various specimens and spurious fluctuations in power level were accordingly avoided. The fine capillary containing DPPH was placed in the position in the resonant cavity where the microwave magnetic field intensity was similar to that of the position occupied by the carbon specimen. The two superimposed absorption lines were displaced (22 G) by placing a piece of transformer steel outside the wave-guide to distort the magnetic field in the vicinity of the sub-standard. In the absence of any saturation effects the action of increasing the power into the cavity does not alter the area ratio of sub-standard to specimen. The reproducibility using this method was better than 5 per cent. The 3.5 mm photographs of the absorption curves were
ORGANIC
POLYMERS-I
229
projected on to plain paper, the curve traced, and the areas evaluated with a planimeter. The absorption curve widths, between points of maximum slope, were measured directly from the oscilloscope screen with the aid of a proton resonance meter. Oscilloscopic presentation of the first derivative of the absorption curve is made possible by the method of “double modulation”(s) in which simultaneous modulation of the resonance signal at 100 kc/s and 50 c/s is carried out. The method of TIKHOMIROVAand VEOVODSKII(Q) was used for line shape analysis. For the majority of carbons the line shape falls somewhere between the two extremes of Gaussian and Lorentzian. Line shape analysis involves taking certain measured parameters from the first derivative of the absorption curve, and plotting them in expressions which give rise to linear relationships if the curve is either Gaussian or Lorentzian. The measured parameters are J’ (the height of the derivative curve at any point) and y (the distance from the cross-over point to a particular value of 7’); this amounts to plotting log10 y/J’ against ya for a Gaussian curve and (y/~~~ against y2 for a Lorentzian curve. The g-factor is most simply determined by the accurate measurement of the magnetic field strength, and the frequency at which ESR occurs. Although magnetic field strength could be measured sufficiently accurately, provision was not available for the measurement of resonance frequency to the same degree. It was therefore necessary to measure the g-factors of carbons by comparison with ultramarine, whose g-factor is appreciably different to that of carbons. Theg-factor of ultramarine (gu) may be obtained by comparison from that of DPPH (gd). As &PHu=gafiHd then (g,-gd)/gd=(Hd-H~)/Hu. Substituting g~=2~0036&0~0002 and also for Hu and Hd (obtained by the method of double modulation) gave a value ofg,=2~0303f0~0004. Carbons were then mixed with ultramarine and the g-factors for carbons determined in the same way. 3. RESULTS .?.l
D.C. resistivity measurements
The results obtained are shown graphically in Fig. 1. The shape of the log10 resistivity against HTT plot is similar for all the series of carbons. The resistivity is initially high at low values of HTT
230
C. JACKSON
I
-7
I
I
I
600
700
830
I
LCD
500
and W. F. K. WYNNE-JONES
HEAT
TREATMENT
TEMPERATURE
1
I
irioo
900
I ‘C I
FIG. 1. Variation of d.c. resistivity with HTT for carbons prepared from organic polymers. - -A- - Polyfurfuryl alcohol - 0Polydivinylbenzene - -A - - Melamine formaldehyde - - 0- - Polyvinylidene chloride -@Dibenzanthrone -ACellulose
but decreases rapidly over a temperature range of 100 to 200°C and finally flattens off at the higher values of HTT. 3.2 Crystallite
size determination
The variation of the crystallite diameter L,, with percentage disordered material and HTT for cellu1.
TABLE
VARIATION IN THE CONCENTRATION
“C
400 500 550 600 700 800
TOTAL NUMBER OF AROMATIC LAYERS WITH Hl’T FOR CELLULOSE CARBONS
AND
FREE SPIN
Carbon content %
Total No. of layers per gram
Free spin concn.
A
Ordered material %
16.6 19.0 21.1 24.7 28.0 29.1
39.6 42.0 43.7 45,s 50.4 56.3
78.5 85.6 90.1 93.3 95.6 95.9
2.4 x 10s” 2.1 x 102s 1.9 x 102s 1.4 x 102s 1.2x1oss 1.2 x1020
0.8 1.4 2.0 l-3
LO
HTT
lose carbons is shown in Table 1. Also shown in the table are the corresponding values for the free spin concentrations as determined by ESR. Using the value of 5.24 A2 as the area of a single benzene ring and assuming a cylindrical shape for the crystallites, the number of carbon atoms in individual layer planes has been calculated. From the
dimension
per gram x x x x -
102s 102s toss 102s
ELECTRON
SPIN
RESONANCE
IN CARBONIZED
HEAT
TREATMENT
TEMPERATURE
ORGANIC
231
POLYMERS-I
1 ‘C 1
PIG. 2. The variation of free spin concentration
and line width with HTT for carbons prepared from organic polymers. -+ Polydiv~ylbe~ene - -a- - Cetlulose - -A - - Polyvinylidene chloride -APolyfurfuryl alcohol
1
200
LOO
600
800 HEAT
FIG.
3.
The variation of free spin concentration
1000 TREATMENT
200 TEMPERATURE
LOO
600
Bon
low
totI
and line width with HTT for carbons prepared polymers. -@-Polyvinylcyanide - -a- - Polyvinylchloride - -A - - Melamine formaldehyde -A-Dibenzanthrone
from organic
C. JACKSON and W. F. K. WYNNE-JONES
232 TBLE 2.
pRofzRTIES
ASSOCIATBD
WITH
CARBONS
POSSESSING
MAXIMUM
FIZSE
SPIN
CONCENTRATION
Carbon
prepared from
Heat treatment temp., “C.
Percentage carbon
430
72.1
80
22-5 x
560 565
950 92.9
35 44
18.6 x IO’” 20.7 x 101”
1.0 0.8
640
97-S
46
2@5 X1019
0.5
650
950
63
28.1 x IO’”
0.5
69.5
65.9
94
11.7 x 10’9
1.5
740
80.6
101
15.0 x 101”
0.8
Polyvinylidene chloride Polyvinyl chloride Cellulose Polydivinyl benzene Polyfurfuryl alcohol Melamine formaldehyde Polyvinyl cyanide
amounts of ordered material present in these cellu-
lose carbons values are obtained for the total number of aromatic condensed systems per gram of carbon (Table 1). 3.3 Examination of carbons by BSR The variation of free spin concentration with HTT for the eight ranges of carbons can be seen in Figs. 2 and 3 and Table 2. In all cases the integrated intensities are small at low HTT, gradually increase to maxima and finally decrease to small values when HTT approaches 1000°C. The break in the curve for the polyvinylidene chloride carbons will be discussed later. The remaining series show a maximum free spin concentration of the order of 2~ 1020 free spins per gram. These figures correspond approximately to one free spin per 200 carbon atoms. The HTT corresponding to the maximum free spin concentration varies considerably for the eight ranges. For carbons prepared from polyvinylidene chloride the temperature is 43O”C, whilst for melamine formaldehyde the temperature is 695°C. From the chemical analyses@) it is found that there is no correlation between the [C]/[H] ratio and free spin concentration@@ (Table 2). Also it appears that during the formation of free spin centres it is essentially the carbon content which is related to the number of free spin centres. Thus a dibenzanthrone carbon (HTT of 540°C) with a carbon
C/H
Free spin concn .
per gram -~-
1019
Line width (Gauss)
1.3
content of 94.9 per cent has a free spin concentration of l-9 x 1020 g-1, whilst a melamine formaidehyde carbon (HTT of 691°C) containing 65.9 per cent carbon has a free spin concentration of only 1.2~ lOsag1. However, beyond the temperature corresponding to the maximum free spin concentration the number of free spin centres no longer bears any relationship to the carbon content. The variations of line width with HTT for the eight ranges of carbons are also shown in Figs. 2 and 3. These curves fall into two main groups, the first containing carbons prepared from cellulose, polyvinyl cyanide and melamine formaldehyde with polyvinylidene chloride, dibenzanthrone, polydivinyl benzene, polyfurfuryl alcohol and polyvinyl chloride forming the second group. In the first group the line width has an initial value of 8 G at low HTT. The curves then narrow rapidly reaching minima in width corresponding to the maximum free spin concentrations, In the region of 1000°C the absorption curves are extremely broad and do not correspond in shape to either a Gaussian or a Lorentzian curve. The curves in the second group show the same initial narrowing of the absorption curve with increasing HTT, but this is followed by a broadening, followed by a second narrowing which precedes the final broadening. The curves from polyvinyIidene chloride and polyvinyl chloride have broadened beyond detection at values of HTT of 625 and 750°C respectively. This effect is quite real and occurs over a
ELECTRON SPIN RESONANCE IN CARBONIZED ORGANIC POLYMERS-I small range of temperature when the carbon is prepared either in nitrogen or in vacuum. With regard to line shape, for a 400°C melamine formaldehyde carbon and a 380°C cellulose carbon, the widths are four and seven gauss respectively. The widths predicted from dipolar broadening are four and ten gauss. Line shape analysis in both cases showed that the curves are Lorentzian in the centre and Gaussian in the wings. Line shape analysis of the 560°C cellulose carbon and the 700°C melamine formaldehyde carbon show that the curves are perfectly Lorentzian over the whole line shape. At HTT greater than 600°C for cellulose, and greater than 700°C for melamine formaldehyde, the line width increases rapidly and the line shape obeys neither a Lorentzian nor a Gaussian curve. The curves do not show any perfectly linear regions although the deviation from a Lorentzian shape is much smaller than from a Gaussian shape. The line shape of the 210°C polyvinylidene chloride carbon is Lorentzian in the centre and Gaussian in the wings as observed in the first group of carbons. At the position of the first minimum in line width at 43O”C, the line shape is Lorentzian further out from the centre of the line than the 210°C carbon but still followed a Gaussian relationship in the extreme wings (Fig. 4). With increasing HTT the line width increases and the shape becomes perfectly Lorentzian over the entire width (580°C polyvinylidene chloride carbon, Fig. 4). As HTT increases to 625°C the line broadens rapidly and is no longer perfectly Lorentzian but falls somewhere between the two shapes. At 650°C the second narrowing has occurred and the shape is once mere Lorentzian. This shape extends to the position of the second minimum in line width at an HTT of 835 “C. For the carbon prepared at 1000°C the line has broadened considerably and although no longer perfectly Lorentzian it resembles this shape more closely than Gaussian. The g-factors of carbonized organic materials always fall extremely close to the free spin value of 2+00t3. As a result the amount of information TABLE
Heat
tresitment
g-factor
3.
temp.
VARIATION
“C
OF R-FACTOR
233
which can be obtained from them is small. The range of cellulose carbons has been examined and the results (Table 3) show the g-factor to decrease with increasing HTT as has already been reported for other materials.trl) 4. DISCUSSION
The various mechanisms which can influence the shape and width of the absorption curves of amorphous carbons have been recently reviewed.(l) It is however stressed that any attempts at calculation of expected line widths at the present time can only be of a qualitative nature. The case of amorphous carbons in which the exact nature of the unpaired electrons is not unambiguously established is probably quite different from that of paramagnetic ions in a rigid crystal lattice for which the theories of line width and shape have been developed. It is also a feature of carbon work that strong narrowing mechanisms (exchange or delocalization) can completely outweigh a broadening process such as dipolar interaction for which a reasonable value may be predicted by theory. With the data available, it has not been possible to calculate the narrowing produced by exchange and delocalization effects, although it was possible to examine line shape and attempt some correlation with theoretical predictions. (12-15) It is against the classical work by Van Vleck and others that this discussion of line width and shape is made. The results for the first group of carbons (from cellulose, polyvinyl cyanide and melamine formaldehyde) prepared at low HTT show that the width could be explained on the basis of a static electron spin-spin dipolar interaction which with the low concentration of paramagnetic species could give lines which are approximately Lorentzian in shape. However, assuming the unpaired electrons to be associated with layer planes, then exchange of electrons between different regions of delocalization can give lines which are Lorentzian in the centre and Gaussian in the wings,(la) as was observed WITH
HTT FORCELLULOSE
CARBONS
380
415
490
520
565
615
2.0028
2.0028
2.0027
2.0026
2.0025
2ao24
234
C. JACKSON and W. F. K. WYNNE-JONES ,112
2 I
$
(Y/JI
LORENTZIAN
6 I
8 8
10 I
12 I
3w
Y2
200
l&l
loo~~fy/l’l
GAUSSIAN
FIG. 4. Line shape analysis for 430°C and 580°C pol~inyIidenc chloride carbons. continuous 430°C Polyvinylidene chloride dootted 580°C Polyvinylidene chloride experimentally. The possibility that unresolved hyperfine interaction is also present in these carbons cannot be ruled out, but the effect is probably masked by the stronger narrowing interactions. In this connexion it is interesting to note that SINGER and LEWIS(“) have produced well resolved proton hyperfine spectra from low temperature chars. It would be expected that as HTT increases the layer diameters and degree of perfection of the aromatic structures increase, and hence the de-
localization of an unpaired electron associated with such structures will also increase. Since the concentration of unpaired electrons is rapidly increasing in this region the unpaired electrons will be closer together and the exchange integral r should increase thus producing a greater narrowing and a more perfect Lorentzian curve. Loss of hydrogen which is shown to occur and delocalization of the unpaired electrons will also tend to reduce the effects of any hyperfine interaction. Quali~tively it would seem possible to explain the narrowing of
ELECTRON
SPIN RESONANCE IN CARBONIZED ORGANIC POLYMERS-I
the curve as HTT is increased. At the higher values of HTT (700°C for cellulose and 800°C for melamine formaldehyde) the curves broaden considerably. Again assuming that the unpaired electrons are associated with layer planes it would be expected that both delocalization and exchange narrowing should steadily increase with the growth of the layer planes, although there will be the counteracting effect of decreasing spin concentration to take into account. On a qualitative basis therefore the breakdown of exchange interaction would not be expected, SINGER et aZ.(lr) have shown that for sucrose carbons in the temperature range 600-900°C the spin-lattice relaxation time decreases from 10-7 to 10-s set, which can give values for the line width from 0.6 to 60 G. If such strong spin-lattice interactions also exist in the carbons examined, then this could lead to uncertainty broadening which would give a curve of approximately Lorentzian shape.(r*) The important question however is the reason for the more efficient relaxation processes as HTT increases. The variation of line width with HTT for the second group of carbons (polydivinyl benzene, polyvinylidene chloride, polyfurfuryl alcohol, dibenzanthrone and polyvinyl chloride) is the same as that described above with the exception of the broadening at intermediate HTT which is most pronounced in the polyvinylidene chloride series (Figs. 2 and 3). The observed line width at low HTT for all ranges is greater than that predicted by electron spin-spin dipolar broadening. Unresolved hyperfine splitting is probably therefore contributing to the observed widths in this region. However, an explanation of the early broadening and narrowing in the region of 625°C is required. No evidence of any narrowing of the line was obtained when the broad-line sample was examined at 77”K, indicating that spin-lattice interaction was not the principal broadening mechanism. The only other alternative would seem to be a breakdown in exchange interaction thus leaving a curve broadened by dipolar interaction and possibly unresolved hyperfine splitting. The value predicted for electron spinspin dipolar broadening is approximately 30 G, which when coupled with hyperfine interaction could explain the observed width. The breakdown in exchange interaction could be explained on the suggestion that bond strains
235
can destroy the similarity of centres necessary for exchange to occur, and hence it is possible that the broad line obtained at 625°C reflects a highly strained structure. This may explain why the effect is most commonly observed for polymers particularly if there is extensive cross-linkage. It can also be suggested, as another mechanism, that the breakdown in the exchange interaction may be due to a transition between two distinct types of free spin centre. At low HTT the unpaired electrons could be localized to individual carbon atoms associated with conjugated aliphatic systems. As the number of unpaired electrons increases, exchange narrowing occurs, probably by the overlapping rr-orbitals of conjugated systems. Above 43O”C, condensed ring systems begin to form and exchange narrowing breaks down because the similarity of the centres necessary for the process is no longer present. With increasing HTT the condensed ring systems capable of stabilizing an unpaired electron are formed in increasing number and it is exchange between these that produces the narrowing of the observed line. A comparison of susceptibilities as calculated from ESR and static susceptibility measurements for these polyvinylidene chloride carbons has recently been completed by BLAYDENet aZ.(ss) and illustrates the complexity of interpretation. 4.2 D.C. resistivity On comparison of the resistivity/HTT curves (Fig. 1) with those of free spin concentration against HTT (Figs. 2 and 3) it is seen that there is no correlation between the temperature region where d.c. resistivity is decreasing rapidly and the temperature region where the line width is increasing rapidly. The most interesting feature of the results is that the carbons which become conducting most readily are those for which the unpaired electrons are formed most readily. If the carbons are placed in the order of the readiness with which they become conducting, the following sequence is obtained: polyvinylidene chloride, dibenzanthrone, polydivinyl benzene, polyfurfuryl cellulose, alcohol and melamine formaldehyde. This is exactly the same order as is obtained by placing the carbons in order of increasing HTT corresponding to the maximum values of free spin concentrations. In general therefore, it would seem that the formation of unpaired electrons is
236
C. JACKSON
and W. F. K. WYNNE-JONES
accompanied by increasing conductivity. MROZOWSKI@L)has regarded carbons prepared at temperatures in the region 500-800°C as intrinsic semiconductors possessing an energy band completely filled with electrons at absolute zero separated from an empty conduction band by an energy gap E. As hydrogen is evolved during carbonization it is considered that the a-electrons of the peripheral carbon atoms are left unpaired. A n-electron from the n-band forms a spin-pair creating a hole in the filled band. Consequently holes, or positive charge carriers account for the large fall in d.c. resistivity. The above band model may be too simple because of the non-existence of a continuous periodic lattice in these carbons. On the other hand it may be possible to describe the changes in resistivity in terms of conductivity of polynuclear hydrocarbons. ELEY and PARFITT(~~)have observed a high conductivity for the free radical diphenyl-picrylhydrazyl. It seems probable, when considering conductivity of polynuclear hydrocarbons that there exists an energy gap associated with the transfer of electrons from one layer to the next. The transfer will depend upon the ionization potential of the layers and COULSON et uZ.(33) have shown that ionization potentials decrease with increasing molecular size for condensed polynuclear hydrocarbons. On this basis alone, it would be expected that the d.c. resistivity should decrease with increasing HTT as the average layer diameter increases, as has been found in this study. 4.3 Nature of the free spin The nature of the free spin has proved extremely difficult to determine and there has been much argument as to whether or not the source of the free spin is a n- or a o-electron. There is probably no unequivocal answer to the problem, and there is no doubt that ESR is an involved phenomenon. The fact that “carbon” can exist in so very many different structural forms incorporating many elements in a variety of chemical structures, has not added to the understanding of the subject. However, by correlating some of the results of this study a qualitative analysis of the nature of the free spin is possible. An attractive feature of the idea that unpaired electrors (n-electrons) are associated with layer planes is that the maximum concentration of unpaired electrons is limited. Thus, the material
giving the largest concentration of unpaired electrons would on pyrolysis give the largest number of layer planes of just sufficient size to stabilize an unpaired electron. The evidence for the agreement between the total number of layer planes and the total number of free spin centres (Table 1) certainly requires further examination but it is interesting to note that examination of the carbon prepared from hexaiodobenzene at 600°C showed no ESR. This carbon was similar to those obtained from hexachlorobenzene and hexaiodobenzene by GIBSON et uZ.@J) which were found to be entirely amorphous showing no coherent scattering of X-rays. MOHUN(25)found similar X-ray behaviour from carbons prepared by the decomposition of metal carbides and again no ESR spectrum was observed. On the other hand, Mrozowski in his quest for anomalous chars(Ls,ss) has carbonized CsSs and a rubber with a high sulphur content (CHs-CHs-S-S-CHs). The variations found in intensity, line width and g-factor were quite different from the generally similar variations found in other carbonized materials. In these sulphur containing materials the nature of the free spin could be quite different. Unfortunately, as has been realized by ERGUNet 01,(27) it is extremely difficult, if not impossible, to ascertain quantitatively the types of linkage and the structure which exist in carbons of low HTT. Until such an analysis is possible, then any discussion of the origin and nature of ESR must be rather hypothetical. With the above limitation realized, it is possible to examine semiquantitatively the effect of variation in structure of the original polymer upon the ESR of the subsequent carbons. Assuming the hypothesis of unpaired 7c-electrons, it would seem that anything which hinders the formation of condensed aromatic ring systems will also hinder the formation of unpaired electrons. Thus it would be expected that dibenzanthrone which already has nine condensed rings would readily form unpaired electrons and in fact the maximum is found to occur at a temperature as low as 540°C. Furfuryl alcohol contains cyclic oxygen which must be removed from the furanose ring before extensive condensation can occur and this has the effect of raising the HTT for maximum free spin concentration to 650°C. Divinyl benzene might be expected to give condensed, hexagonal, ring structures but WINSLOW et al.@@ have shown that the initial
ELECTROS
SPIN
RESONANCE
IN CARBONIZED
polymer is highly cross-linked and that it is this cross-linking which hinders the aromatization process. The ability of polyvinylidene chloride to form unpaired electrons at extremely low values of HTT is interesting. The only prcduct of pyrolysis is hydrogen chloride and the formation of ring structures in which the carbon is in a different state of hybridization must be preferable to the highly unsaturated chain structures. (29) The two, nitrogen-containing ranges of carbons prepared from polyvinyl cyanide and melamine formaldehyde are rather different from the remaining series. The maximum free spin concentrations are only about one half of those associated with the other carbons. Also the free spins do not persist to those values of HTT which produce carbons of chemical composition similar to the other ranges. Apparently some process is removing unpaired electrons more rapidly than they are produced. This situation of substitutional nitrogen could be analogous to that of phosphorus or arsenic in germanium in which an excess electron is produced from each impurity atom. This in a sense would be the reverse of the removal of hydrogen from edges and could result in the removal of positive holes. This might be a possible explanation of why nitrogen-containing carbons contain a smaller number of unpaired electrons than others. The problem of unpaired electrons in carbons would therefore appear to be the highly theoretical one associated with the stability of electrons in carbonaceous structures which are considered to be made up of small units of relatively well-orientated crystallites contained within the “amorphous carbon”. These layers are probably distorted with folds, bends and structural defects and will have bonded to them such atoms as hydrogen, oxygen, nitrogen, chlorine or sulphur, depending upon the origin of the carbon. -4cknowledgementsThis study forms part of the fundamental research programme of the British Coke Research Association and we are grateful to the Council of the Association for permission to publish this paper. One of us C.J., also acknowledges financial assistance from the B.C.R.A. We are grateful to Drs H. Harker and H. Marsh, members of the Northern Coke Research Laboratory, for valuable discussion.
REFERENCES 1. SINGER L. S., Proceedings of the Fifth Carbon Conference, Vol. 2, p. 37. Pergamon Press, Oxford (1963).
6. 7.
8. 9. 10.
11.
12. 13. 14. 15. 16. 17. 18. 19. 20.
21. 22. 23.
24. 2.5. 26.
27.
28. 29.
ORGANIC
POLYMERS-l
237
MARSHH. and WYNNE-JONES W. F. K., Carbon 1,269 (1964). HARKERH., JACKSONC. and WYNNE-JONES W. F. K., Proc. Roy. Sot. 262A, 328 (1961). PASTOR R. C., WIEL J. A., BROWN T. H. and TURKEVICHJ., Adwances in Catalysis 9, 107 (1957). SINGER L. S. and WAGONER G., Proceedings of the Fifth Carbon Conference, Vol. 2, p. 65. Pergamon Press, Oxford (1963). RUPP E., Chem. Ber. 29, 1625 (1896). BLAYDEN H. E., GIBSON J. and RILEY H. L., The Ultra-fine Structure of Coal and Cokes, p. 176. B.C.U.R.A., London (1944). INGRAMD. I. E.. ‘Free Radicals’ as Studied bv ESR. p. 79. But&wo&s, London (1958). d ’ TIKHOMIROVAN. N. and VEOVODSKIIV. V., Optics and Spectroscopy 7, 486 (1959). UEBER~FELDJ. and ERB E., Proceedings of the Third Carbon Conference, p. 103. Pergamon Press, Oxford (1959). SINGER L. S., SPRY W. J. and SMITH W. H., Proceedings of the Third Carbon Conference, p. 121. Pergamon Piess, Oxford (1959). VAN VLECK T. H.. Phvs. Rev. 74. 1168 (1948). ANDERSONP: W.‘and*WEIss P. fi., Rev: MO;. Phys. 25, 269 (1953). PRYCE M. H. L. and STEVENSK. W. H., Proc. Phys. Sot. A63, 36 (1950). K1~~~~C.andA~~~~~~sE.,Phys.Ra.90,238(1953). VAN VLECK J. H., Nuooo Cimento Suppl. Series IO 6, 993 (1956). SINGER L. S. and LEWIS I. C., Carbon 1, 400 (1964). PAKE G. E., Am. J. Phys. 18, 478 (1950). MROZOWSKIS. and WOBSCHALLD., r. C&z. PhJjs. 57, 915 (1960). BLAVDENH. E. and WE~TCOTTD. T., Proceedings of the Fifth Carbon Conference, Vol. 2, p. 97. Pergamon Press, Oxford (1963). MROZOWSKIS., Phys. Rev. 85, 6009 (1952); 86, 1056 (1952). ELEY D. D. and PARFITTG. D.. Trans. Faruduy Sot. 51, 1529 (1955). COUISON C. A.. SCHAADL. 1. and BURNELLE L.. Proceedings of the Third Carbbn Conference, p. 27: Pergamon Press, Oxford (1959). GIBSON J., HOLAHANM. and RILEY H. L., J. Chem. Sot. 456 (1946). MOHUN W. A., Proceedings of the Fourth Carbon Conference, p. 443. Pergamon Press, Oxford (1960). MROZOWSKI S., Proceedings of the Fifth Carbon Conference, Vol. 2, p. 79. Pergamon Press, Oxford (1963). MENTSER M., O’DONNELL H. J. and ERGUN S., Proceedings of the Fifth Carbon Conference, Vol. 2. p. 493. Fergimon Piess, Oxford (1963). WINSLOW F. H.. BAKER W. 0.. PAPE N. R. and MATREW( W., Jl Polymer Sci. lb, 101 (1955). WINSLOW F. H., BAKER W. 0. and YAGER W. A., Proceedingsof the First and Second Carbon Conferences, p. 93. University of Buffalo (1956).
1964, Vol. 2, pp. 227-237.
ELECTRON
Pergamon
Press Ltd.
SPIN RESONANCE ORGANIC C. JACKSON*
Northern
Coke Research
Printed in Great Britain
Laboratories,
IN CARBONIZED
POLYMERS-I
and W. F. K. WYNNE-JONES
School of Chemistry,
The University,
Newcastle
upon Tyne
1, England
(Received 10 February 1964)
Abstract-An investigation has been made into electron spin resonance (ESR) occurring in ranges of carbons prepared from eight organic polymeric materials. Free spin concentration, line width, line shape and “g’‘-factors have been measured. A correlation has been attempted of these measurements with chemical analysis, d.c. resistivity, X-ray diffraction and the chemical structure of the original
polymeric material. No correlation could be found between free spin concentration and the [C]/[Hj ratio. The temperature region where d.c. resistivity is decreasing rapidly is not related to that of rapid increase in line width. Those carbons which become conducting most readily are those for which the unpaired electrons are formed most readily. For cellulose carbons, there is reasonable agreement between number of free spins and number of layer planes. The stability and concentrations of free spins is discussed in the terms of HTT and chemical structure of the polymers. New phenomena have been observed in the variation of line width with HTT for five of the ranges of carbons examined by ESR. The discussion includes some considerations of the factors which affect line width and of the nature of the free spin in carbonaceous materials. Some difficulties limiting the interpretation of ESR measurements are outlined.
1. INTRODUCTION
of ESR is particularly opportune in view of the shortage of experiments on well characterized carbons the origins of which are well known, and the study is a.n attempt to contribute to the understanding of the origins of ESR in carbons. As the examined carbons have been prepared from pure organic materials, usually polymers, the chemical compositions of which are quite different, e.g. polydivinyl benzene and melamine formaldehyde, the effect of chemical composition upon the ESR has been investigated. The variations of the measurable quantities of electron spin susceptibility and line shape and width with heat treatment temperature (HTT) have been investigated. The various interactions which can effect the widths of ESR lines have been critically discussed because amorphous carbons are probably unique in that for a range of carbons prepared at different heat treatment temperatures, practically every type of interaction so far recognized has been invoked to explain the observed variations in line width and line shape. (1)
THE PHENOMENON of electron spin resonance (ESR) in carbonaceous materials is now clearly recognized though the understanding of the origins is far from complete. Recently SINGER(~) has critically reviewed almost the entire literature of this field of study and has stressed the extreme care needed in experimentation and interpretation of results. MARSH and WYNNJ+JONES@) have prepared eight series of carbons from pure organic materials and have examined their surface properties and chemical composition. These carbons are extremely well characterized and their method of preparation is described in detail. The examination of ESR in these carbons has been completed as part of a comprehensive study and selected results have already been used in discussions of chemical reactivity of carbons to mdecular oxygen.(s) This examination * Present address : School of Pharmacy, Sunderland Technical College, Sunderland, Co. Durham. 227
228
C. JACKSON
and W. F. K. WYNNE-JONES
2. EXPERIMENTAL
2.1 Carbcuzpreparation The carbons used in this study have been prepared from polydivinyl benzene, polyvinylidene chloride, ‘cellulose, polyfurfuryl alcohol, dibenzanthrone, polyvinyl cyanide, melamine formaldehyde and polyvinyl chloride, as described by MARSHand WYNNE-JONES.(~)The maximum HTT was maintained for 10 hr to establish the equilibrium concentration of free radicals.@) The carbonizations were all carried out in vacuum or nitrogen because of the known effects of carbonization in air.(l) In all preparations of samples for examination in the spectrometer the carbons were ground in air to less than 150 ,u. Weighed amounts of carbon (several milligrams) were then outgassed in weighed Pyrex tubes (O.D. of 5 mm) to 10-s cm Hg at 200°C and the tubes sealed. This procedure enabled the weight of the outgassed carbon to be accurately known. The main supply of the carbon was always stored in vacuum to avoid the slow chemisorption of oxygen which occurs at room temperature and which can alter the concentration of unpaired electrons in the carbon.(r) In these studies no dependence of free spin concentration on particle size down to sizes of 62 micron have been observed in outgassed carbons. All of the examined carbons contain negligible quantities of ash in so far as standard ash determinations show undetectable amounts. This is important because of the influence of mineral impurities upon line shape. (5) In addition, small amounts of carbons, produced from hexaiodobenzene, were examined. The hexaiodobenzene (m.p. 340°C) was prepared by RUPP’S method(s) and recrystallized from nitrobenzene. The red-brown, needle-like crystals were carbonized in nitrogen in a sample tube which, after evacuation, could be inserted directly into the spectrometer. Heat treatment up to 1000°C did not produce any detectable concentrations of unpaired electrons. 2.2 D.C. resistivity measurements Resistivity measurements are usually made with the specimen under compression to minimize interparticle effects. It was found that for all carbons examined in this study the resistance value decreased with increasing pressure up to the maximum pressure used of 1000 kg/cmz, but
showed no signs of having reached a saturation value. As these measurements were made to ascertain if the final broadening of the ESR absorption curves coincided with the sharp fall in d.c. resistivity it was considered that relative results would be adequate. All measurements were made at room temperature. The conductivity cell consisted of thick walled “Veridia” tubing, 6 cm in length and 0.25 cm in diameter. A specimen of l-2 mg of finely ground carbon (<150 p) was contained between two, highly polished, silver-steel electrodes which fitted closely inside the “Veridia” tubing, and terminated in brass contacts. Leads from the brass contacts were taken to a Wheatstone bridge and the resistance measured. The whole assembly could be accurately aligned and clamped, along with a compression gauge, between the jaws of a small vice and the powder compressed to the required value. 2.3 Crystallite size determinations X-ray, powder diffraction photographs have been taken of the cellulose carbons. The mean layer sizes of the turbostratic crystallites were obtained using Warren’s method as described by BLAYDEN,GIBSON and RILEY(~)which involves the measurement of the line width of the (lo)-band. These carbons contain both ordered and disordered material and an estimate of the ordered material was obtained by comparison of the absolute intensity of the (02)-band relative to graphite. 2.4 Examination of carbon by ESR The spectrometer was designed to operate at X-band, i.e. frequencies of the order of 9,000 MC/S and magnetic fields of 3200 G. Provision was made for both crystal video and phase sensitive detection. An Hsla cavity was made from standard waveguide with the distance between the iris plates of 3.95 cm, and O-281 in. diameter holes in the iris plates. A hole was drilled in the centre of the narrow side of the cavity, passing right through the cavity, to accept the Pyrex tubes containing the specimens. The specimen holder was designed so that the Pyrex tubes could always be placed accurately in the centre of the cavity. The holder was essentially
ELECTRON
SPIN
RESONANCE
IN CARBONIZED
a modified chuck. When used for crystal video detection, the lower hole in the cavity (designed to hold the 100 kc/s modulation loop) was sealed off with a tightly fitting brass insert. Under the best possible conditions the crystal video spectrometer was capable of detecting approximately 1 x 101s AH, free spins per gram (AHx is the absorptionline width in gauss at half power). It was, however, impossible to make accurate quantitative estimations of free radical concentrations as low as this. The concentrations of unpaired electrons in the amo~hous carbons were usually much higher than the limit of sensitivity of the apparatus, being of the order of 1 x 1019 to 2x 102s per gram. Since resolved hyperfine structure was not observed in the spectra associated with these amorphous carbons it was only possible to measure integrated intensity, width, shape and “g” factors of the absorption curves. In the calculation of spin concentration from integrated intensity it has been assumed that the free spin centres of the carbon are similar to those of stable free radicals with almost free spins, i.e. the number of unpaired spins is related to the static spin susceptibility by the usual Curie Law expression. In this way the measurements of spin concentration could be made by essentially comparative methods. These depend upon knowing the concentration of unpaired electrons in a known weight of the organic free radical diphenyl-picryl-hydrazyl (DPPH) which possesses one unpaired electron spin per molecule. The DPPH was used in such a way, as a sub-standard, that its absorption line appeared on the oscilloscope screen simultaneously with that of the carbon under Difficulties of differing dielectric examination. loss of various specimens and spurious fluctuations in power level were accordingly avoided. The fine capillary containing DPPH was placed in the position in the resonant cavity where the microwave magnetic field intensity was similar to that of the position occupied by the carbon specimen. The two superimposed absorption lines were displaced (22 G) by placing a piece of transformer steel outside the wave-guide to distort the magnetic field in the vicinity of the sub-standard. In the absence of any saturation effects the action of increasing the power into the cavity does not alter the area ratio of sub-standard to specimen. The reproducibility using this method was better than 5 per cent. The 3.5 mm photographs of the absorption curves were
ORGANIC
POLYMERS-I
229
projected on to plain paper, the curve traced, and the areas evaluated with a planimeter. The absorption curve widths, between points of maximum slope, were measured directly from the oscilloscope screen with the aid of a proton resonance meter. Oscilloscopic presentation of the first derivative of the absorption curve is made possible by the method of “double modulation”(s) in which simultaneous modulation of the resonance signal at 100 kc/s and 50 c/s is carried out. The method of TIKHOMIROVAand VEOVODSKII(Q) was used for line shape analysis. For the majority of carbons the line shape falls somewhere between the two extremes of Gaussian and Lorentzian. Line shape analysis involves taking certain measured parameters from the first derivative of the absorption curve, and plotting them in expressions which give rise to linear relationships if the curve is either Gaussian or Lorentzian. The measured parameters are J’ (the height of the derivative curve at any point) and y (the distance from the cross-over point to a particular value of 7’); this amounts to plotting log10 y/J’ against ya for a Gaussian curve and (y/~~~ against y2 for a Lorentzian curve. The g-factor is most simply determined by the accurate measurement of the magnetic field strength, and the frequency at which ESR occurs. Although magnetic field strength could be measured sufficiently accurately, provision was not available for the measurement of resonance frequency to the same degree. It was therefore necessary to measure the g-factors of carbons by comparison with ultramarine, whose g-factor is appreciably different to that of carbons. Theg-factor of ultramarine (gu) may be obtained by comparison from that of DPPH (gd). As &PHu=gafiHd then (g,-gd)/gd=(Hd-H~)/Hu. Substituting g~=2~0036&0~0002 and also for Hu and Hd (obtained by the method of double modulation) gave a value ofg,=2~0303f0~0004. Carbons were then mixed with ultramarine and the g-factors for carbons determined in the same way. 3. RESULTS .?.l
D.C. resistivity measurements
The results obtained are shown graphically in Fig. 1. The shape of the log10 resistivity against HTT plot is similar for all the series of carbons. The resistivity is initially high at low values of HTT
230
C. JACKSON
I
-7
I
I
I
600
700
830
I
LCD
500
and W. F. K. WYNNE-JONES
HEAT
TREATMENT
TEMPERATURE
1
I
irioo
900
I ‘C I
FIG. 1. Variation of d.c. resistivity with HTT for carbons prepared from organic polymers. - -A- - Polyfurfuryl alcohol - 0Polydivinylbenzene - -A - - Melamine formaldehyde - - 0- - Polyvinylidene chloride -@Dibenzanthrone -ACellulose
but decreases rapidly over a temperature range of 100 to 200°C and finally flattens off at the higher values of HTT. 3.2 Crystallite
size determination
The variation of the crystallite diameter L,, with percentage disordered material and HTT for cellu1.
TABLE
VARIATION IN THE CONCENTRATION
“C
400 500 550 600 700 800
TOTAL NUMBER OF AROMATIC LAYERS WITH Hl’T FOR CELLULOSE CARBONS
AND
FREE SPIN
Carbon content %
Total No. of layers per gram
Free spin concn.
A
Ordered material %
16.6 19.0 21.1 24.7 28.0 29.1
39.6 42.0 43.7 45,s 50.4 56.3
78.5 85.6 90.1 93.3 95.6 95.9
2.4 x 10s” 2.1 x 102s 1.9 x 102s 1.4 x 102s 1.2x1oss 1.2 x1020
0.8 1.4 2.0 l-3
LO
HTT
lose carbons is shown in Table 1. Also shown in the table are the corresponding values for the free spin concentrations as determined by ESR. Using the value of 5.24 A2 as the area of a single benzene ring and assuming a cylindrical shape for the crystallites, the number of carbon atoms in individual layer planes has been calculated. From the
dimension
per gram x x x x -
102s 102s toss 102s
ELECTRON
SPIN
RESONANCE
IN CARBONIZED
HEAT
TREATMENT
TEMPERATURE
ORGANIC
231
POLYMERS-I
1 ‘C 1
PIG. 2. The variation of free spin concentration
and line width with HTT for carbons prepared from organic polymers. -+ Polydiv~ylbe~ene - -a- - Cetlulose - -A - - Polyvinylidene chloride -APolyfurfuryl alcohol
1
200
LOO
600
800 HEAT
FIG.
3.
The variation of free spin concentration
1000 TREATMENT
200 TEMPERATURE
LOO
600
Bon
low
totI
and line width with HTT for carbons prepared polymers. -@-Polyvinylcyanide - -a- - Polyvinylchloride - -A - - Melamine formaldehyde -A-Dibenzanthrone
from organic
C. JACKSON and W. F. K. WYNNE-JONES
232 TBLE 2.
pRofzRTIES
ASSOCIATBD
WITH
CARBONS
POSSESSING
MAXIMUM
FIZSE
SPIN
CONCENTRATION
Carbon
prepared from
Heat treatment temp., “C.
Percentage carbon
430
72.1
80
22-5 x
560 565
950 92.9
35 44
18.6 x IO’” 20.7 x 101”
1.0 0.8
640
97-S
46
2@5 X1019
0.5
650
950
63
28.1 x IO’”
0.5
69.5
65.9
94
11.7 x 10’9
1.5
740
80.6
101
15.0 x 101”
0.8
Polyvinylidene chloride Polyvinyl chloride Cellulose Polydivinyl benzene Polyfurfuryl alcohol Melamine formaldehyde Polyvinyl cyanide
amounts of ordered material present in these cellu-
lose carbons values are obtained for the total number of aromatic condensed systems per gram of carbon (Table 1). 3.3 Examination of carbons by BSR The variation of free spin concentration with HTT for the eight ranges of carbons can be seen in Figs. 2 and 3 and Table 2. In all cases the integrated intensities are small at low HTT, gradually increase to maxima and finally decrease to small values when HTT approaches 1000°C. The break in the curve for the polyvinylidene chloride carbons will be discussed later. The remaining series show a maximum free spin concentration of the order of 2~ 1020 free spins per gram. These figures correspond approximately to one free spin per 200 carbon atoms. The HTT corresponding to the maximum free spin concentration varies considerably for the eight ranges. For carbons prepared from polyvinylidene chloride the temperature is 43O”C, whilst for melamine formaldehyde the temperature is 695°C. From the chemical analyses@) it is found that there is no correlation between the [C]/[H] ratio and free spin concentration@@ (Table 2). Also it appears that during the formation of free spin centres it is essentially the carbon content which is related to the number of free spin centres. Thus a dibenzanthrone carbon (HTT of 540°C) with a carbon
C/H
Free spin concn .
per gram -~-
1019
Line width (Gauss)
1.3
content of 94.9 per cent has a free spin concentration of l-9 x 1020 g-1, whilst a melamine formaidehyde carbon (HTT of 691°C) containing 65.9 per cent carbon has a free spin concentration of only 1.2~ lOsag1. However, beyond the temperature corresponding to the maximum free spin concentration the number of free spin centres no longer bears any relationship to the carbon content. The variations of line width with HTT for the eight ranges of carbons are also shown in Figs. 2 and 3. These curves fall into two main groups, the first containing carbons prepared from cellulose, polyvinyl cyanide and melamine formaldehyde with polyvinylidene chloride, dibenzanthrone, polydivinyl benzene, polyfurfuryl alcohol and polyvinyl chloride forming the second group. In the first group the line width has an initial value of 8 G at low HTT. The curves then narrow rapidly reaching minima in width corresponding to the maximum free spin concentrations, In the region of 1000°C the absorption curves are extremely broad and do not correspond in shape to either a Gaussian or a Lorentzian curve. The curves in the second group show the same initial narrowing of the absorption curve with increasing HTT, but this is followed by a broadening, followed by a second narrowing which precedes the final broadening. The curves from polyvinyIidene chloride and polyvinyl chloride have broadened beyond detection at values of HTT of 625 and 750°C respectively. This effect is quite real and occurs over a
ELECTRON SPIN RESONANCE IN CARBONIZED ORGANIC POLYMERS-I small range of temperature when the carbon is prepared either in nitrogen or in vacuum. With regard to line shape, for a 400°C melamine formaldehyde carbon and a 380°C cellulose carbon, the widths are four and seven gauss respectively. The widths predicted from dipolar broadening are four and ten gauss. Line shape analysis in both cases showed that the curves are Lorentzian in the centre and Gaussian in the wings. Line shape analysis of the 560°C cellulose carbon and the 700°C melamine formaldehyde carbon show that the curves are perfectly Lorentzian over the whole line shape. At HTT greater than 600°C for cellulose, and greater than 700°C for melamine formaldehyde, the line width increases rapidly and the line shape obeys neither a Lorentzian nor a Gaussian curve. The curves do not show any perfectly linear regions although the deviation from a Lorentzian shape is much smaller than from a Gaussian shape. The line shape of the 210°C polyvinylidene chloride carbon is Lorentzian in the centre and Gaussian in the wings as observed in the first group of carbons. At the position of the first minimum in line width at 43O”C, the line shape is Lorentzian further out from the centre of the line than the 210°C carbon but still followed a Gaussian relationship in the extreme wings (Fig. 4). With increasing HTT the line width increases and the shape becomes perfectly Lorentzian over the entire width (580°C polyvinylidene chloride carbon, Fig. 4). As HTT increases to 625°C the line broadens rapidly and is no longer perfectly Lorentzian but falls somewhere between the two shapes. At 650°C the second narrowing has occurred and the shape is once mere Lorentzian. This shape extends to the position of the second minimum in line width at an HTT of 835 “C. For the carbon prepared at 1000°C the line has broadened considerably and although no longer perfectly Lorentzian it resembles this shape more closely than Gaussian. The g-factors of carbonized organic materials always fall extremely close to the free spin value of 2+00t3. As a result the amount of information TABLE
Heat
tresitment
g-factor
3.
temp.
VARIATION
“C
OF R-FACTOR
233
which can be obtained from them is small. The range of cellulose carbons has been examined and the results (Table 3) show the g-factor to decrease with increasing HTT as has already been reported for other materials.trl) 4. DISCUSSION
The various mechanisms which can influence the shape and width of the absorption curves of amorphous carbons have been recently reviewed.(l) It is however stressed that any attempts at calculation of expected line widths at the present time can only be of a qualitative nature. The case of amorphous carbons in which the exact nature of the unpaired electrons is not unambiguously established is probably quite different from that of paramagnetic ions in a rigid crystal lattice for which the theories of line width and shape have been developed. It is also a feature of carbon work that strong narrowing mechanisms (exchange or delocalization) can completely outweigh a broadening process such as dipolar interaction for which a reasonable value may be predicted by theory. With the data available, it has not been possible to calculate the narrowing produced by exchange and delocalization effects, although it was possible to examine line shape and attempt some correlation with theoretical predictions. (12-15) It is against the classical work by Van Vleck and others that this discussion of line width and shape is made. The results for the first group of carbons (from cellulose, polyvinyl cyanide and melamine formaldehyde) prepared at low HTT show that the width could be explained on the basis of a static electron spin-spin dipolar interaction which with the low concentration of paramagnetic species could give lines which are approximately Lorentzian in shape. However, assuming the unpaired electrons to be associated with layer planes, then exchange of electrons between different regions of delocalization can give lines which are Lorentzian in the centre and Gaussian in the wings,(la) as was observed WITH
HTT FORCELLULOSE
CARBONS
380
415
490
520
565
615
2.0028
2.0028
2.0027
2.0026
2.0025
2ao24
234
C. JACKSON and W. F. K. WYNNE-JONES ,112
2 I
$
(Y/JI
LORENTZIAN
6 I
8 8
10 I
12 I
3w
Y2
200
l&l
loo~~fy/l’l
GAUSSIAN
FIG. 4. Line shape analysis for 430°C and 580°C pol~inyIidenc chloride carbons. continuous 430°C Polyvinylidene chloride dootted 580°C Polyvinylidene chloride experimentally. The possibility that unresolved hyperfine interaction is also present in these carbons cannot be ruled out, but the effect is probably masked by the stronger narrowing interactions. In this connexion it is interesting to note that SINGER and LEWIS(“) have produced well resolved proton hyperfine spectra from low temperature chars. It would be expected that as HTT increases the layer diameters and degree of perfection of the aromatic structures increase, and hence the de-
localization of an unpaired electron associated with such structures will also increase. Since the concentration of unpaired electrons is rapidly increasing in this region the unpaired electrons will be closer together and the exchange integral r should increase thus producing a greater narrowing and a more perfect Lorentzian curve. Loss of hydrogen which is shown to occur and delocalization of the unpaired electrons will also tend to reduce the effects of any hyperfine interaction. Quali~tively it would seem possible to explain the narrowing of
ELECTRON
SPIN RESONANCE IN CARBONIZED ORGANIC POLYMERS-I
the curve as HTT is increased. At the higher values of HTT (700°C for cellulose and 800°C for melamine formaldehyde) the curves broaden considerably. Again assuming that the unpaired electrons are associated with layer planes it would be expected that both delocalization and exchange narrowing should steadily increase with the growth of the layer planes, although there will be the counteracting effect of decreasing spin concentration to take into account. On a qualitative basis therefore the breakdown of exchange interaction would not be expected, SINGER et aZ.(lr) have shown that for sucrose carbons in the temperature range 600-900°C the spin-lattice relaxation time decreases from 10-7 to 10-s set, which can give values for the line width from 0.6 to 60 G. If such strong spin-lattice interactions also exist in the carbons examined, then this could lead to uncertainty broadening which would give a curve of approximately Lorentzian shape.(r*) The important question however is the reason for the more efficient relaxation processes as HTT increases. The variation of line width with HTT for the second group of carbons (polydivinyl benzene, polyvinylidene chloride, polyfurfuryl alcohol, dibenzanthrone and polyvinyl chloride) is the same as that described above with the exception of the broadening at intermediate HTT which is most pronounced in the polyvinylidene chloride series (Figs. 2 and 3). The observed line width at low HTT for all ranges is greater than that predicted by electron spin-spin dipolar broadening. Unresolved hyperfine splitting is probably therefore contributing to the observed widths in this region. However, an explanation of the early broadening and narrowing in the region of 625°C is required. No evidence of any narrowing of the line was obtained when the broad-line sample was examined at 77”K, indicating that spin-lattice interaction was not the principal broadening mechanism. The only other alternative would seem to be a breakdown in exchange interaction thus leaving a curve broadened by dipolar interaction and possibly unresolved hyperfine splitting. The value predicted for electron spinspin dipolar broadening is approximately 30 G, which when coupled with hyperfine interaction could explain the observed width. The breakdown in exchange interaction could be explained on the suggestion that bond strains
235
can destroy the similarity of centres necessary for exchange to occur, and hence it is possible that the broad line obtained at 625°C reflects a highly strained structure. This may explain why the effect is most commonly observed for polymers particularly if there is extensive cross-linkage. It can also be suggested, as another mechanism, that the breakdown in the exchange interaction may be due to a transition between two distinct types of free spin centre. At low HTT the unpaired electrons could be localized to individual carbon atoms associated with conjugated aliphatic systems. As the number of unpaired electrons increases, exchange narrowing occurs, probably by the overlapping rr-orbitals of conjugated systems. Above 43O”C, condensed ring systems begin to form and exchange narrowing breaks down because the similarity of the centres necessary for the process is no longer present. With increasing HTT the condensed ring systems capable of stabilizing an unpaired electron are formed in increasing number and it is exchange between these that produces the narrowing of the observed line. A comparison of susceptibilities as calculated from ESR and static susceptibility measurements for these polyvinylidene chloride carbons has recently been completed by BLAYDENet aZ.(ss) and illustrates the complexity of interpretation. 4.2 D.C. resistivity On comparison of the resistivity/HTT curves (Fig. 1) with those of free spin concentration against HTT (Figs. 2 and 3) it is seen that there is no correlation between the temperature region where d.c. resistivity is decreasing rapidly and the temperature region where the line width is increasing rapidly. The most interesting feature of the results is that the carbons which become conducting most readily are those for which the unpaired electrons are formed most readily. If the carbons are placed in the order of the readiness with which they become conducting, the following sequence is obtained: polyvinylidene chloride, dibenzanthrone, polydivinyl benzene, polyfurfuryl cellulose, alcohol and melamine formaldehyde. This is exactly the same order as is obtained by placing the carbons in order of increasing HTT corresponding to the maximum values of free spin concentrations. In general therefore, it would seem that the formation of unpaired electrons is
236
C. JACKSON
and W. F. K. WYNNE-JONES
accompanied by increasing conductivity. MROZOWSKI@L)has regarded carbons prepared at temperatures in the region 500-800°C as intrinsic semiconductors possessing an energy band completely filled with electrons at absolute zero separated from an empty conduction band by an energy gap E. As hydrogen is evolved during carbonization it is considered that the a-electrons of the peripheral carbon atoms are left unpaired. A n-electron from the n-band forms a spin-pair creating a hole in the filled band. Consequently holes, or positive charge carriers account for the large fall in d.c. resistivity. The above band model may be too simple because of the non-existence of a continuous periodic lattice in these carbons. On the other hand it may be possible to describe the changes in resistivity in terms of conductivity of polynuclear hydrocarbons. ELEY and PARFITT(~~)have observed a high conductivity for the free radical diphenyl-picrylhydrazyl. It seems probable, when considering conductivity of polynuclear hydrocarbons that there exists an energy gap associated with the transfer of electrons from one layer to the next. The transfer will depend upon the ionization potential of the layers and COULSON et uZ.(33) have shown that ionization potentials decrease with increasing molecular size for condensed polynuclear hydrocarbons. On this basis alone, it would be expected that the d.c. resistivity should decrease with increasing HTT as the average layer diameter increases, as has been found in this study. 4.3 Nature of the free spin The nature of the free spin has proved extremely difficult to determine and there has been much argument as to whether or not the source of the free spin is a n- or a o-electron. There is probably no unequivocal answer to the problem, and there is no doubt that ESR is an involved phenomenon. The fact that “carbon” can exist in so very many different structural forms incorporating many elements in a variety of chemical structures, has not added to the understanding of the subject. However, by correlating some of the results of this study a qualitative analysis of the nature of the free spin is possible. An attractive feature of the idea that unpaired electrors (n-electrons) are associated with layer planes is that the maximum concentration of unpaired electrons is limited. Thus, the material
giving the largest concentration of unpaired electrons would on pyrolysis give the largest number of layer planes of just sufficient size to stabilize an unpaired electron. The evidence for the agreement between the total number of layer planes and the total number of free spin centres (Table 1) certainly requires further examination but it is interesting to note that examination of the carbon prepared from hexaiodobenzene at 600°C showed no ESR. This carbon was similar to those obtained from hexachlorobenzene and hexaiodobenzene by GIBSON et uZ.@J) which were found to be entirely amorphous showing no coherent scattering of X-rays. MOHUN(25)found similar X-ray behaviour from carbons prepared by the decomposition of metal carbides and again no ESR spectrum was observed. On the other hand, Mrozowski in his quest for anomalous chars(Ls,ss) has carbonized CsSs and a rubber with a high sulphur content (CHs-CHs-S-S-CHs). The variations found in intensity, line width and g-factor were quite different from the generally similar variations found in other carbonized materials. In these sulphur containing materials the nature of the free spin could be quite different. Unfortunately, as has been realized by ERGUNet 01,(27) it is extremely difficult, if not impossible, to ascertain quantitatively the types of linkage and the structure which exist in carbons of low HTT. Until such an analysis is possible, then any discussion of the origin and nature of ESR must be rather hypothetical. With the above limitation realized, it is possible to examine semiquantitatively the effect of variation in structure of the original polymer upon the ESR of the subsequent carbons. Assuming the hypothesis of unpaired 7c-electrons, it would seem that anything which hinders the formation of condensed aromatic ring systems will also hinder the formation of unpaired electrons. Thus it would be expected that dibenzanthrone which already has nine condensed rings would readily form unpaired electrons and in fact the maximum is found to occur at a temperature as low as 540°C. Furfuryl alcohol contains cyclic oxygen which must be removed from the furanose ring before extensive condensation can occur and this has the effect of raising the HTT for maximum free spin concentration to 650°C. Divinyl benzene might be expected to give condensed, hexagonal, ring structures but WINSLOW et al.@@ have shown that the initial
ELECTROS
SPIN
RESONANCE
IN CARBONIZED
polymer is highly cross-linked and that it is this cross-linking which hinders the aromatization process. The ability of polyvinylidene chloride to form unpaired electrons at extremely low values of HTT is interesting. The only prcduct of pyrolysis is hydrogen chloride and the formation of ring structures in which the carbon is in a different state of hybridization must be preferable to the highly unsaturated chain structures. (29) The two, nitrogen-containing ranges of carbons prepared from polyvinyl cyanide and melamine formaldehyde are rather different from the remaining series. The maximum free spin concentrations are only about one half of those associated with the other carbons. Also the free spins do not persist to those values of HTT which produce carbons of chemical composition similar to the other ranges. Apparently some process is removing unpaired electrons more rapidly than they are produced. This situation of substitutional nitrogen could be analogous to that of phosphorus or arsenic in germanium in which an excess electron is produced from each impurity atom. This in a sense would be the reverse of the removal of hydrogen from edges and could result in the removal of positive holes. This might be a possible explanation of why nitrogen-containing carbons contain a smaller number of unpaired electrons than others. The problem of unpaired electrons in carbons would therefore appear to be the highly theoretical one associated with the stability of electrons in carbonaceous structures which are considered to be made up of small units of relatively well-orientated crystallites contained within the “amorphous carbon”. These layers are probably distorted with folds, bends and structural defects and will have bonded to them such atoms as hydrogen, oxygen, nitrogen, chlorine or sulphur, depending upon the origin of the carbon. -4cknowledgementsThis study forms part of the fundamental research programme of the British Coke Research Association and we are grateful to the Council of the Association for permission to publish this paper. One of us C.J., also acknowledges financial assistance from the B.C.R.A. We are grateful to Drs H. Harker and H. Marsh, members of the Northern Coke Research Laboratory, for valuable discussion.
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