A method of thermal analysis of polymers by measurement of electrical conductivity

A Method of Thermal Analysis of Polymers by Measurement of Electrical Conductivity" M. I. POPE Conventional methods of thermal analysis involve measuring either changes in mass or enthalpy of a substance, which is heated at a constant rate of rise of temperature in a controlled atmosphere. Work on a range of organic polymeric materials has now shown that, if the rate of change in electrical conductivity with temperature is plotted against temperature, then the curve obtained is characteristic of that particular substance. With the aid of in[ormation obtainable by a number of other techniques, it .has proved possible to ascribe each of the peaks in the various curves to physical or chemical changes occurring in the heated polymer. The utility o[ this technique is illustrated by its application to a medium rank coal, both be[ore and after chemical treatment, and to a sample of unplasticized polyvinyl chloride.

THE idea that measurement of changes in electrical conductivity during heating could be used to characterize complex organic compounds was first suggested by the results of research into the low temperature carbonization of coals1. It had long been known 2 that, at some stage during the carbonization process, coals changed from electrical insulators to relatively good conductors, with properties approaching those of graphite. The object of the research project was originally to investigate the possibility of partially de-volatilizing coals on heating a finely ground powder in a fluidized bed, by means of passing electrical current through the coal. Results soon indicated that this process was impracticable due to the extremely low initial conductivity of the coal; aggravated by the fact that much of the room temperature conductivity was due to adsorbed watera, which is rapidly lost at temperatures above about 150°C. However, carbonization studies of a large number of bituminous coals, differing quite widely in rank, indicated that the rise in electrical conductivity with increase in temperature of carbonization was remarkably similar in each case. Measurements were made on compacted samples in an atmosphere of oxygen-free nitrogen, maintaining a constant rate of rise of temperature of 3 deg. C/min by use of an apparatus described elsewhere 1. A typical graph of conductivity against temperature is shown in F i g u r e 1 and refers to Markham Black Shale, a coal of rank number 401. The differences between the behaviour of the individual coals only become apparent where the rate of change in electrical conductivity is plotted against temperature [Figure 2(a)]. The resulting curve shows essentially three characteristic peaks, although the peak occurring in the region of 600 ° to 800°C frequently splits into two distinct maxima. Each of these *Based on a lecture given before the Thermal Analysis Group symix~sium h e l d at S a l f o r d o n 21 t o 22 A p r i l 1966.

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2--The rate of change of conductivity (in arbitrary units) plotted against temperature of carbonization for Markham Black Shale which was: (a) untreated; (b) methylated; (c) dehydrogenated; and ((t) brominated

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A METHOD OF THERMAL ANALYSIS OF POLYMERS peaks corresponds to a definite stage in the carbonization process and the reasons for their occurrence are discussed below. If we consider a hypothetical, intrinsic, organic .semi-conductor, which does not decompose on heating, then the rate of increase in conductivity with temperature will fall progressively as the temperature is increased, in accordance with the equation o-=A exp ( - E / 2 k T ) where o- is the electrical conductivity at absolute temperature T, k is Boltzmann's constant, E is the energy of activation for semi-conduction and A is a constant. The peaks observed in Figure 2 (a) would therefore be expected to be due to one, or more, of the following factors. (1) Decomposition of the solid may result in a new mechanism of conduction becoming possible. (2) Evolution of a decomposition product having a significantly higher conductivity than that of the residue. (3) Sintering, or partial melting, bringing about an improvement in electrical contact between the particles of the solid. Factors (1) to (3) would all cause the rate of increase in conductivity to be greater than that expected for our hypothetical semi-conductor. In addition, a fourth factor needs to be considered. (4) Evolution of non-conducting decomposition products may (a) disrupt the grain structure of the solid, causing a fall in the area of inter-particle contacts, or (b) lead to the formation of a nonconducting film over the surface of the particles. Factor (4) would cause the rate of increase in conductivity to be less than expected. Although conductivity results are not sufficient in themselves to give a clear picture of the different decomposition reactions which occur during the carbonization of coals, it has proved possible4, with the aid of information obtained by a number of other techniques, to put forward a reasonable interpretation of the mechanism of the carbonization process. It appeared that up to 150°C the loss of physically adsorbed water predominated and little change in the coal structure occurred; then above 150°C and below about 350°C, a small amount of alkyl aromatic material was given o ~ and was presumably due to the evolution of molecules trapped within the coal structure during the coalification process. Primary carbonization commenced at between 400 ° and 500°C, and involved the fission of the carbon--carbon bonds between aliphatic bridges which join together the aromatic clusters in the coal. A certain amount of the aromatic material is thereby liberated to form tar, while the hydrogen from the broken aliphatic bonds disproportionates between the residue and the tar, the latter being richer in hydrogen. Where a methylene group is activated by replacement of one or both hydrogen atoms, e.g. by an aromatic group, bond fission would be expected to occur at a lower temperature than with the unsubstituted group. The bulk of the non-aromatic material remaining above 500°C appears to be hydrogen and methyl groups at the 51

M. I. P O P E

periphery of aromatic clusters. The loss of these methyl groups would be expected to take place at a temperature higher than that required to split methylene bridges, from bond energy considerations. For example, the strength of the methylene bond in CtH~CHr---CH~C6H~ is quotedB as being 47 kcal, as compared with 87 kcal for CtH~---CI-I~. Secondary carbonization involves the loss of this periphery material, mainly as methane and hydrogen, thus leaving the aromatic groups free to grow into sheets similar to, but much smaller than, those in graphite. The mean layer diameter of these sheets appears to have reached a steady value by about 800°C, and little further structural change seems to take place below 1 000°C. In accordance with this picture, the free radical concentration in carbonized coals reaches a maximum7,8 value between 400 ° and 600°C, due to the bond fission associated with primary and the beginning of secondary carbonization. When aliphatic groups adjacent, or attached, to an aromatic cluster :are split off, electron traps are formed; electrons from the aromatic ~" band can now fall into trapping o- orbitals9 where bond fission has occurred, leaving conducting holes in the ,r band. The onset of primary carbonization (at about 400°C) is therefore accompanied by a rapid increase in electrical conductivity. The free radical concentration begins to fall off sharply7 above 600°C, as a result of coalescence of the aromatic clusters to form small graphite-like sheets1°; however, the conductivity continues to increase steeply up to about 800°C due to intrinsic semi-conduction in the aromatic sheets, since the energy of activation for semi-conduction has been shown11 to be approximately inversely proportional to the area of the aromatic sheet. It follows that any alteration in the structure of a coal brought about by chemical treatment, which affects the mechanism of the carbonization process, should lead to corresponding changes in the electrical conductivity during carbonization. Measurements have therefore been made1~on samples of Markham Black Shale (hereafter referred to as MBS) which were (a) untreated, (b) approximately 50 per cent methylated using alkaline dimethyl sulphate, (c) dehydrogenated by heating with sulphur at 190°C and (d) brominated by treating acetylated MBS with ~r-bromosuccinimide. This particular coal was chosen because a great deal of information concerning its structure and properties was already available through the work of Dicker, Gaines et al. 13,". The three main peaks observed with the untreated sample of MBS are illustrated in Figure 2(a) and occur in the ranges of temperature (t3 150 ° to 350°C, (iz3 300 ° to 500°C and (iii) 500 ° to 800°C. Each peak, or group of peaks, is associated with the corresponding stage involved in the carbonization process. Peak (0 has been ascribedTM to the disruption of the grain structure of the coal when trapped volatile material escapes. Peak (if) results from the primary carbonization process, and peak (iit3 from secondary carbonization. Methylation of the hydroxyl groups in MBS appears to leave the mechanism of the carbonization process virtually unchanged [cf. Figure 2(a) and 2(b)]. The peak (0 at 250°C is somewhat reduced in size, due probably to 52

A METHOD OF THERMAL ANALYSIS OF POLYMERS the leaching out of some of the adsorbed organic material from the coal structure during methylation and the subsequent washing. Dehydrogenation by heating with sulphur has a marked influence on carbonization, as can be seen from Figure 2(c). Peak (1) at 250°C is again slightly reduced in size, due probably to the pre-heating of the MBS at 1900C during dehydrogenation. The primary carbonization peak (i0 occurring in the untreated material at 4500C, is now split into two separate peaks at ca. 400 ° and 500°C. Evidence, discussed elsewhere 1~, suggests that the 400* peak is due to loss of hydroxyl groups present in the original coal and also to the evolution of sulphur compounds, resulting from the rupture of bonds formed between the added sulphur and aromatic material in the coal. The 500°C peak is in the region where it is known that the rupture of methylene bridges occurs and is presumably due to fission of those aliphatic bridges which were not converted to aromatic material during the dehydrogenation process. The secondary carbonization peak (iit) is also much changed in the dehydrogenated sample [Figure 2(c)], the rate of increase in conductivity becoming very great at about 600°C. This is not unexpected since the rate of coalescence of aromatic groups is likely to be governed by the rate at which edge groups, mainly CH3-- and hydrogen, are split off. The rapid onset of secondary carbonization thus suggests that dehydrogenation by sulphur has resulted in removal of a proportion of the hydrogen attached directly to the aromatic ring systems, a conclusion also reached by Dicker et al. 14. Bromination of MBS considerably alters peaks (i) and (iO but leaves the secondary carbonization peak (iiz~ little changed [Figure 2(d)]. The marked increase in the size of the 200* peak was shown by TGA to be associated with the splitting-off of hydrogen bromide, leaving a residue containing a much increased proportion of carbon-carbon double bonds. The primary carbonization peak appears now to have been displaced towards a lower temperature, relative to untreated MBS; the position now corresponds closely with that of the lower primary carbonization peak of the sulphur dehydrogenated sample. The higher (500"C) peak occurring during primary carbonization of the sulphur dehydrogenated sample is either absent or has merged with the secondary carbonization peak. Absence of this peak would suggest that all the aliphatic bridges in the coal had been converted into aromatic ring systems; such an occurrence is most improbable and should have led to a very large increase in electrical conductivity, which was not observed. It thus appears that the peak must have moved to a higher temperature region, resulting in the apparent broadening of [Figure 2(d)] the secondary carbonization peak. Such a movement would be explained by the conversion of the single carbon-carbon bonds in the methylene bridges to double bonds. In polyatomic molecules, the bond energies of ---C--C-- and --C-----C-- are 83 kcal and 146 kcal respectivelye, indicating that the latter bonds are thermally more stable and would therefore be expected to break at a higher temperature. The success achieved by using rising temperature conductivity measure53

M . I . POPE

ments to study the decomposition of coals (and coal models4), suggested that this technique might prove to be of general use in identifying and following the thermal degradation of polymers. Wartield is has already shown that resistivity measurements can be used to study the thermal stability of polymers, in the temperature range 25 ° to 400°C. The procedure used by Wariield has been restricted to crosslinked polymers and differs markedly from the present work; blocks of polymer weighing about 350g were heated at a range of constant temperatures for 16 h prior to resistivity measurements being made. Although this procedure was termed electrothermal analysis (ETA), it shows little similarity to such techniques as DTA and TGA, both in the amount of sample consumed and in the time required. On the other hand, the technique discussed here is comparable with D T A in both respects and leads to a graph of similar form, suggesting that it might be useful in the routine analysis of polymers. To illustrate the application of this technique to polymers, polyvinyl chloride has been chosen, because of its relatively simple chemical structure. Figure 3 illustrates the changes in conductivity of a compact prepared from PVC powder as a function of temperature, while the rate of change in conductivity is plotted against temperature in Figure 4. From Figure 3 it can be seen at once that the thermal decomposition involves two distinct stages : stage I occurs in the region of 250°C and is accompanied by the evolution 1

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conductivity of unplasticized polyvinyl chloride plotted against temperature

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A M E T H O D O F T H E R M A L ANALYSIS O F P O L Y M E R S

of hydrogen chloride while stage II occurs at 400 ° to 500°C. The curve shows good reproducibility up to 250°C and above 550°C, but between these limits a certain amount of scatter occurs, due to the disruption of the grain structure of the compact as large quantities of gas are evolved. Differential thermal analysis likewise indicates a two-stage reaction18,in the same range of temperatures. Kipling and McEnaney 17 have shown by thcrmogravimetric analysis that stage I involves a 42 per cent loss in weight while the loss for stage II is 17.8 per cent. U p to 250°C, the weight loss is due almost entirely to evolution of hydrogen chloride, without any appreciable loss of carbon or hydrogen18; the mechanism of the reaction is (-~CH~C1--),

> (---CH=CH--), + nHC1

The resulting straight-chain polyene at once undergoes a certain amount of cyclization, with the proportion of aromatic material increasing with temperature. Stage II occurring at about 400°C, is accompanied by the evolution of simple gaseous hydrocarbons and a rapid fall in the H / C ratiC". The presence of polycyclic aromatic regions in PVC chars heated above 400°C has been demonstrated by Winslow et al?° and by Kipling~°. Above 500°C, the predominant decomposition reaction observed by Gilbert and Kipling was the evolution of hydrogen. In accordance with this picture of the decomposition process, Wynne-Jones et aI. have shown that the specific surface21 of the residue passes through a maximum in the region 500 ° to 550°C and that the free spin concentration~ passes through a maximum value at 560°C. The mechanism of carbonization of PVC is thus far closer to that of a coal than might have been expected; this is reflected in the general similarity between Figures 2(a) and 4. The peaks of Figure 4 can therefore be explained as follows. (1) Loss of HC1 in the region 200 ° to 250°C initially leads to a rapid increase in conductivity as the polyene is formed; but the sudden evolution of gas disrupts the grain structure of the compacted powder to such an extent that the conductivity shows a subsequent fall. (2) Primary carbonization occurs in the region 400 ° to 500°C with the evolution of tar and volatile decomposition products, some of which must be good conductors of electricity. (3) The double peak at 600 ° to 700°C appears to result from the evolution of hydrogen and coalescence of the aromatic residue during secondary carbonization, as has been observed with medium and low rank coals. CONCLUSION

Measurements of electrical conductivity during carbonization have now been carried out on widely differing polymeric, organic compounds. The results indicate that this technique could prove useful in studying both the chemical structure and the mechanism of thermal decomposition of such materials. Since a plot of the rate of change of electrical conductivity with temperature, against temperature, gives a curve which is characteristic of a particular compound, the technique provides a new method of thermal 55

M . I . POPE

analysis. T h e technique described shows m a n y points of general similarity with such well-established m e t h o d s of t h e r m a l analysis as D T A a n d T G A . It therefore seems r e a s o n a b l e to suggest that this technique be referred to as 'Electro T h e r m a l A n a l y s i s ' , even though Warfield has used the term in a rather different sense.

Department of Chemistry, College o f Technology, Portsmouth, Hants (Received June 1966) REFERENCES 1POPE, M. I. and GREGG,S. J. Brit. J. appl. Phys. 1959, 10, 507 2 St~qDOg, J. Proceedings of the Conference on the Ultrafine Structure of Coals and Cokes, p 342. British Coal Utilisation Research Association: London, 1944 a GREGG,S. J. and POPE, M. I. Fuel, Lond. 1960, 39, 301 4PONE, M. I. Proceedings of the Second Conference on Industrial Carbon and Graphite, p 474. Society of Chemical Industry : London, 1965 HOLDEN, M. W. and ROBB,J. C. Fuel, Lond. 1960, 39, 39 6Co'rrRELL, T. L. The Strengths of Chemical Bonds, 2nd edition. Butterworths: London, 1958 7 AOSTEN, D. E. G., INGRAM,D. J. E. and TAPLEY, J. G. Trans. Faraday Soc. 1958, 54, 400 8 SMIDT,J. and VAN KREVELEN, D. W. Fuel, Lond. 1959, 38, 355 9 MROZOWSKI, S. Symposium on the Properties of Carbon, p 1. Durham (1956) 10 DIAMOND, R. and HIRSCH, P. B. Proceedings of the Conference on Industrial Carbon and Graphite, p 197. Society of Chemical Industry : London, 1958 11SCHUYER,J. and VAN KREVELEN, D. W. Fuel, Lond. 1955, 34, 213 lg POPE, M. I. 6th International Conference on Coal Science, MUnster (1965) 13 DICKER, P. H., GAINES, A. F. a n d STANLEY, L. 1. appl. Chem. 1963, 13, 455 14 DICKER, P. H., FLAGG, M. K., GAINES, A. F. and MARTIN, T. G..L appl. Chem.

1963, 13, 444 1~WARFIELD,R. W. Nature, Lond. 1961, 189, 1002. See also Testing of Polymers, edited by J. V. SCHMITZ,Vol. I, p 292. Interscience : New York, 1965 16YotmG, R. N. 1st International Conference on Thermal Analysis, p 70. Aberdeen (1965) 1~ KIPLING, J. J. and McEN~aqEY, B. Fuel, Lond. 1964, 43, 367 16 GILBERT, J. B. and KIPLING,J. J. Fuel, Lond. 1962, 41, 493 19 WINSLOW, F. H., BAKER, W. O. and YAGER, W. A. 2nd Conference on Carbon, p 93.

Buffalo (1956) 2o KIPLING, J. J. and SHOOTER, P. V. Proceedings of the Second Conference on

Industrial Carbon and Graphite, p 15. Society of Chemical Industry: London. 1965 21 MARSH, H. and WCN~4ONES, W. F. K. Carbon, 1964, 1, 269

JACKSON,C. and WY~a~m-JONES,W. F. K. Carbon, 1964, 2, 227

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