Viscoelastic behaviour of a heat treated isotropic pitch

1406

Letters to the Editor 1608

,

1604

' I ' Charging , ~ l m m m l

_

Zone H

"~ 1 6 0 0 ~

1596

~

1592

~=~_~

1588

1584

~ 0

Discharging I 1

~

I 2

3

Voltage / (V) Fig. 3. The voltage dependence for the high frequency Raman modes of the PPP-700 electrode obtained by fits to a Lorentzian line shape. wave number shows no change with decreasing voltage up to 1.0 V, and in the other (Zone II, 1.0 V ~ 0.3 V) corresponds to downshift with decreasing voltage for charging. For discharging, the change in peak position takes place almost reversibly with some residual effect. This result suggests that in Zones I and II the Li ions have different insertion behaviors. The change in zone II could be due to a charge transfer effect like in GIC occurring in the PPP-700 electrode, which would modify the electron density and electrical conductivity. Finally because of the increase of the conductivity, there is a lowering of the Raman peak frequency shown in Fig. 3. It is noteworthy also that even in the present very disordered structure of PPP-700, charge transfer can take place to store Li ions between the defective carbon layers, and this charge transfer is phenomenologically consistent with the Li storage model proposed earlier [I]. On the other hand, Li storage behavior in Zone ! might be largely different from that in Zone II. Since it is still difficult to clarify

the Li storage mechanism, it could be suggested that Li in Zone I is firstly stored at preferred sites, and charge transfer affecting the peak wave number does not occur. I n - s i t u Raman spectroscopy is used to investigate the electrochemical Li insertion into disordered PPP-700, obtained by the Kovacic method after heat-treatment at 700°C. The Raman spectrum provides insight into the mechanism of lithium insertion into the PPP-700 electrode. The intensity changes of the Raman bands observed in PPP-700 are correlated with the electrical conductivity associated with lithium insertion and release. The large Li storage capacity of the PPP-700 electrode could be largely due to the formation of very small flat pre-graphitic clusters coexisting with polymer-like (quinoid-like) segments. Acknowledgement - This research was partially supported by the MinisU'y of Education, Science, Sports and Culture, Grant-Aid for Scientific Research on Priority Areas (Carbon Alloys), No.09243105, 1997. The M1T authors acknowledge support from NSF grant, DMR 95-10093. REFERENCES

1. Sato. K., Noguchi. M., Demachi. A., Oki. N., and Endo. M., Science, 1994, 264,555. 2. Noguchi. M., Miyashita. K., and Endo. M., Tanso, 1992, 155, 315. 3. Matthews. M.J., Dresselhaus. M.S., Endo. M., Sasabe. Y., Takahashi. T., and Takeuchi. K., J. Mater. Res., 1996, 11, 3099. 4. Mabuchi. A., Tokumitsu. K., Fujimoto. H., and Kasuh. T., J. Electrochent Soc., 1995, 142, 1041. 5. Fong. R., Sacken. U., and Dahn. J. R., J. Electrochem. Soc., 1990, 137, 2009. 6. Odziemkowski. M., Krell. M., and Irish. D. E., J. Electrochem. Soc., 1992, 139, 3052. 7. Matthews. M.J., Bi. X.X., Dresselhaus. M.S., Endo. M., and Takahashi. T., Appl. Phys. Lett., 1996, 68, 1078. 8. Inaba. M., Yoshids. H., and OgumL Z., J. Electrochem. Soc., 1996, 143, 2572. 9. Dresselhans. M.S. and Dresselhaus. G., Adv. Phys., 1990, 30, 139. 10. Endo, M., Kim. C., Hiraoka. T., Karaki. T., Nishimura. K, Matthews. M.J., Brown. S.D.M., and Dresselhans. M.S., J. Mater. Res., 1998, 13, (unpublished).

Viscoelastic behaviour of a heat treated isotropic pitch J. DAJI, B. RAt,to and M. TORPINI University of Leeds Carbon Centre, Department of Materials, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, UK. (Received 22 January 1998; accepted in revised form 10 June 1998)

Key words - A. Petroleum pitch, C. rheometry, D. viscoelasticity

The chemical and physical properties of p i t c h precursors, especially their rheological behaviour, are critical in their processing to carbon materials. In the case of mesophase pitch, being a discotic nematic liquid crystal, its rheological behaviour is known to be much more complex than that of isotropic pitches, and this has 1Current address: Department of Chemical Engineering, Loughborough University, UK

been demonstrated by recent work [1-3]. The unique aspect of mesophase pitch is the ability of the liquid crystalline molecules to be oriented during flow, and it is possible to design the system such that most of the texture generated can be maintained in the solid state. Hence, the flow properties (i.e. rheology) may be directly related to the structure and properties of the final carbon product, The flow characteristics of apparently single

Letters to the Editor

1407

Table 1 Typical properties for pitches CTP, A240 and Aerocarb 75. "~"

C'IP

A240

Aerocarb 75

Carbon Yield (%)

58

50

75

*Helium Densit~ (~,/cm3) Softening Point, Ts (°C) Pdn~ & Ball Method *Glass Transition Temperature, Tg (°C)

1.34

1.24

1.26

116

119

234

50

56

165

Qninoline Insolubles, QI (%) Toluene Iasolubles, TI (%)

5.7

0

0

5

0 -1 -4

-3

-2

-1

0

1

2

3

4

5

27.6 8 26 * Data measuredby the authors,and not the manufacturer.

Log reduced ~equency, at, co(Hz) Fig. 1. Schematic representation of storage modulus, G' and creep compliance, J, behaviour for A240 which is Newtonian at the the low reduced frequencies, and a Mesophase pitch (MP-A) derived from it which is nonNewtonian and viscoelastic at corresponding frequencies. phase isotropic pitch, such as coal tar and petroleum pitch, at processing temperatures are often considered to be Newtonian. However, two phase pitch systems and 100% optically anisotropic mesophase pitches show marked deviations from Newtonian behaviour, with evidence of a more complex viscoelastic behaviour [3] of the latter. The elasticity of mesophase pitch was tentatively attributed to the time dependent relaxation process of the perturbed liquid crystalline "domains". Under comparable conditions, Ashland A240, a typical isotropic pitch of low softening point, showed no such elasticity and was Newtonian in character. These points are illustrated schematically in Figure 1. The G' Master curve (where, G' is the shear storage (or elastic) modulus) of a 100% optically anisotropic mesophase pitch (produced from A240), was found to be from 30 to 300 times higher than the G' Master curve of A240 in the region of low reduced frequency (10 -1 to 10-4 Hz, respectively). This region is equivalent to behaviour at measurement temperatures that are above the softening point (but below the decomposition temperature) and low shear rates. The storage modulus, G', represents the elastic component of the complex shear modulus, G*, in dynamic oscillatory rheometry. This very much larger elasticity at corresponding low reduced frequencies (ar,O~ where, aT is the shift factor as determined from the time-temperature superposition principle [7]) for the mesophase pitch also results in a significant change in the slope of the G' Master curve as compared with that of A240. Also, as illustrated in Figure 1, the mesophase pitch in the region of 240 to 320°C, showed clear evidence of elastic recovery in constant stress creep experiments, an effect absent in A240. In this letter we present results, first shown in preliminary form in 1996 [4], which show that such elastic effects are not unique to the mesophase materials, but are also exhibited by an optically isotropic heat treated petroleum pitch, Aerocarb 75. Aerocarb 75 has a relatively large average molecular weight compared to many other isotropic pitches and the pitch, A240, from which it is derived. The lower molecular weight species have been removed in its preparation. Table 1 gives -

typical characteristic features. The larger average molecular weight results in higher softening point temperatures, T s, higher glass transition temperatures, Tg, and higher carbon yields than A240. The Carrimed CSL500 controlled stress rheometer was used to perform controlled stress oscillatory rheometry (CSOR) and constant stress creep measurements on the petroleum derived pitch, Aerocarb 75 f r s = 234°c), at temperatures from 190°C to 270°C in 10°C intervals. This range encompasses the conventional softening temperature. To compare results, the same rheological (CSOR and creep) analysis was carried out on two other isotropic pitch samples of lower softening point, at temperatures from 90°C to 160°C in 10°C in intervals. These were the parent petroleum pitch, A240 (T s = 119°C) and a coal tar pitch, CTP (T s = 116.5°C) chosen because of its similar Ts, T~ and carbon yield to A240, but toluene insoluble (TI) content similar to Aerocarb 75. Samples were first ground to 1251xm and pressed into pellets of 40mm diameter and 1.05mm thickness. All measurements were performed within the linear viscoelastic region (LVER), where the bounds of the LVER were first determined by a stress sweep at each measurement temperature (using fresh samples for each measurement). The detailed procedure has been outlined elsewhere [6]. CSOR frequency sweep measurements were made using fresh samples at each measurement temperature over a frequency range of 0.05 - 5Hz, where the dynamic viscoelastic parameters; the shear storage (elastic) modulus, G', shear loss (viscous) modulus, G " , dynamic viscosity, rl', and the phase angle, & were measured. Due to the equivalent effect of frequency and temperature (the timetemperature superposition principle [8]), and provided that no chemical changes have occurred during the test, these dynamic viscoelastic parameters, G', G " , 1"1'and 8 data can be shifted along the log frequency axis to form a Master curve at a reduced (or reference) temperature. This was chosen here to be the softening temperature, T s. The shift of the data between each measurement temperature is known as the shift factor, aT. The Master curves for Aerocarb 75 are shown in Figure 2 and are reduced to the softening temperature of T s = 234°C. It can be seen in Figure 2 that at a reduced frequency of between approximately 1 and 10 2 Hz the G' Master curve levels out approaching the G " Master curve in magnitude, and in the same region the 8 Master curve falls from about 1.5 radians (90 °) to 1.2 radians (69°), indicating an increase in elasticity in this region. Non-Newtonian behaviour is demonstrated by the

1408

Letters to the Editor 6-

"~

7

"~

5

•~ .._.5

"F. ,.-,4

I

-

~3

~,2

f

0

,.d

6

-1 -4

I

~

i

I

I

i

I

-3

-2

-1

0

1

2

3

r 4

0 -1

5

-4

Log reducedfi'equency,aT,CO(Hz)

-3

-2

-1

0

1

2

3

4

Log ReducedFrequency, aT,o) (Hz)

Fig. 2. Master curves for Aerocarb 75 reduced to a temperature of 234°C.

Fig. 4. Master curves for CTP reduced to a temperature of 120°C.

increase in dynamic viscosity, ~ ' , in this region. It is important to note here that it is the quantitative difference between G' and G " which governs the extent of the elasticity, which is then also reflected by a decrease in & This can be contrasted with Figure 3, which shows the corresponding Master curves for A240. For A240 it can be seen that G' does not level out, but continuously diverges from G " , and becomes very small in comparison as it approaches correspondingly low reduced frequencies. In this region 8 is constant at approximately 1.5 radians (or 90°), thus A240 displays Newtonian behaviour. The behaviour displayed is that of a glassy system at high reduced frequencies, with G' decreasing rapidly until simple viscous (Newtonian) character is displayed at the lower reduced frequencies. This type of behaviour is comparable to that of a low molecular weight uncrosslinked polymer [6]. The G' behaviour displayed by Aerocarb 75 is quite similar to that of the 100% optically anisotropic mosophase pitch observed previously [3], and shown schematically in Figure 1. However, the Master curves of CTP (Figure 4) are very similar to those of A240, although the G' and ~ data show some scatter and do not exactly obey the time - temperature superposition shifts. It is

possible that the scatter is caused by one of two things. The reason for this are not precisely known at the present time. Possible explanations will be discussed in a later publication when data from a wide range of isotropic pitches will be reported. Figures 5 and 6 show the creep response of Aerocarb 75 at two extremes of measurement temperature, 190°C and 250°C, respectively. These creep compliance curves show that Aerocarb 75 undergoes relaxation after a constant stress (creep) retardation. In the case of the creep curve at 190°C (Figure 5) the instantaneous elastic response and .recovery is more pronounced due to the measurement being made nearer to the glassy region (Tg of Aerocarb 75, measured by dynamic mechanical theimal analysis (DMTA), was around 165°C). Also, the time dependence of the retardation part of the creep curve is non-linear. These results demonstrate that Aerocarb 75 does not behave as a simple viscous material, but is viscoelastic at these temperatures. This type of creep behaviour is not displayed by A240 and CTP. It demonstrates that once the initial structure of Aerocarb 75 is disturbed, some form of elastic recovery occurs. Elasticity in other pitches of high softening point has also been reported by the authors [5] and also by Menendez et al. [7] for air blown pitches. This suggests the phenomenon is quite widespread in such materials. Flexible chain polymers show viscoelastic

6 #O~O~OO

"~. _ 5

O

1.75 x 10 "T

Relaxation

Retardation

~ 4

~o

J (mZ/N)

3

0.88 _ x 10"~

E ~0

0 -1

>e -4

-3

-2

I

-1

0

1

2

3

Log reduced fi'equency, aT,~ (Hz) Fig. 3. Master curves for A240 reduced to a temperature of 120°C.

150

t

I

I

300

450

600

Time (s) Fig. 5. Creep curve showing creep compliance, J, behaviour at 190°C for Aerocarb 75 showing viscoelastic behaviour.

Letters to the Editor Retardation

1.20-

Relaxation

J (m2.~l

0.60-

0

I

150

I

I

I

300 450 600 Time(s) Fig. 6. Creep curve showing creep compliance, J, behaviour at 260°C for Aerocarb 75 showing viscoelastic behaviour. behaviour due to the structure being governed by an entanglement network which accounts for such rheological behaviour as shear thinning, normal stresses, elastic recoil, fading memory and long relaxation times. However, for an isotropic pitch of relatively low molecular weight composed of rigid disc-like molecular units an entanglement network does not seem possible. In mesophase pitch we know that the discotic nematic liquid crystalline molecules align in the direction of shear, as shown by the melt spinning process of carbon fibres. This approximate alignment of molecules is retained through to fibre drawdown and solidification. However, a complicated orientation and relaxation mechanism exists from the extrusion, spinning, drawdown and through to solidification as reported by Fathollabi and White [2]. Fleurot et al. [1] have shown in a constant shear experiment, where they measured the stress response to a sudden imposed strain after varying times, that once the mesophase pitch attained steady flow the domain structure broke down into a finer structure. The existence of a spectrum of relaxation times, giving rise to the rheological characteristics observed, clearly is not due to any structure observable on the optical scale of sizes. It is possible that a sub-micron micellar structure exists, the perturbation of which provides the origin of the effect. MiceUar models of pitch have been around for a long time [9,10], in which the pitch is viewed as a colloidal system and have been described by Riggs and Diefendorf [11]. It was suggested that the aromatic moieties are solubilised by a sheath of polyaromatics and napthenics. If this outer sheath is eliminated by solvent fractionation, then the remnant material can show mesogenie character at elevated temperatures when the molecules are mobile. The preparation method for Aerocarb 75 involving the evaporation of the small "solubilising" molecules, will also modify severely any micellar structure and probably creates a pitch in which there is much greater molecular

1409

interaction between the aromatic cores. Whilst the precise origins of the elasticity observed here are not known at the present time, it seems likely that its explanation will lie in an understanding of the volume fraction, size distribution and degree of coalescence of any micellar units, the extent to which these are disrupted by solvent or thermal treatment and by the response of this structure to shear. Further work is underway to elucidate these mechanisms and to establish the extent to which this behaviour is widespread within pitches of high Tg. Heat treated pitches are increasingly being used in the manufacture of carbon products, for example, precursors for carbon composite matrices. They are used because of their higher carbon yield, which results in a pitch with a high glass transition temperature, Tg. There is also a new "breed" of heat treated coal tar pitches called Carbores, from which the low molecular weight species along w i t h the majority of the carcinogenic species are removed. These pitches have been shown to produce carbon products of improved properties [12]. Thus, if these pitches, which are also optically isotropic, show unusual rheologieal character, this would need to be taken into account during processing. Therefore, a detailed rheological investigation of pitch materials of different character is needed to achieve an understanding of their behaviour to obtain correlation between pitches and to model their behaviour. ~

N

~

1. Fleurot, O and Edie, D.D., in Ext. Abst. "Carbon 96", Univ. of Newcastle Upon Tyne, 1996, p. 425. 2. Fathollahi, B. and White, J.L., J. Rheology, 1994, 38, 1591. 3. Turpin, M., Cheung, T. and Rand, B., Carbon, 1996, 34, 265. 4. Daji, J., Rand B. and Turpin, M., in Ext. Abst. "Carbon 96", Univ. of Newcastle Upon Tyne, 1996, p. 140. 5. Daji J. and Rand, B., in Ext. Abst. 23rd. Biennial Conference on Carbon, Penn State Univ., Vol. II, 1997, p. 184. 6. Turpin, M., Cheung, T. and Rand, B., Carbon, 1994, 32, 225. 7. Menendez, R., Figueiras, A., Bermejo, J., Fieurot, O. and Edie, D.D., in Ext. Abst. 23rd. Biennial Conference on Carbon, Penn State Univ., Vol. II, 1997, p. 204. 8. Ferry, J.D., Viscoelastic Properties of Polymers, (3rd edn.) Wiley, New York, 1970, pp. 292-307. 9. Rand, B., in Strong Fibres, ed. W. Watt and B. V. Perov, Elsevier, Amsterdam, 1985, pp. 514-515. 10. Rand, B., Fuel, 1987, 66, 1491. 11. Riggs D.M. and Diefendorf, R.J., in Ext. Abst. 14th Biennial Conference on Carbon, Penn State Univ. 1979, p. 407. 12. Boenigk, W., Niehoff A. and Wildforster, R., Light Metals, 1992, 581.