Mesophase Formation in Polynuclear Aromatic Compounds

12 Mesophase Formation in Polynuclear Aromatic Compounds Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch012

A Route to Low Cost Carbon Fibers R. J. DIEFENDORF Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181 P i t c h precursor carbon f i b e r s have the potential of providing good mechanical properties at low cost. The high preferred o r i e n t a t i o n of graphite basal planes that is necessary for high modulus is obtained by forming a d i s c o t i c l i q u i d c r y s t a l p i t c h . Requirements for l i q u i d c r y s t a l l i n i t y are discussed for model compounds. These considerations are generalized and expanded to describe l i q u i d c r y s t a l formation i n pitches. The element carbon i n the form of graphite has the highest s p e c i f i c s t i f f n e s s (stiffness/kg) and highest t h e o r e t i c a l , t e n s i l e strength. While graphite i s s t i f f and strong i n the basal plane networks, i t i s compliant and weak normal to the planes. Furthermore, the layer planes can e a s i l y shear over each other i n perfect graphite. A problem to be solved i n producing a high performance carbon f i b e r i s how to a l i g n the graphite basal planes roughly p a r a l l e l to the f i b e r axis, and have a high defect structure such that basal plane shear i s d i f f i c u l t . Since graphite can not be dissolved and only melts at very high temperature under pressure, the f i b e r must be made from a precursor which can be converted subsequently to carbon. Desired c h a r a c t e r i s t i c s of the precursor are that i t : 1) develops an oriented structure which i s carried through to the carbon f i b e r , 2) has a high y i e l d of carbon, 3) i s cheap, and 4) i s easy to handle through the various processing steps such as spinning and carbonization. Pitch materials appear to be able to meet a l l four of these c r i t e r i a . Carbon Y i e l d The carbon y i e l d from pitches can be understood by studying the y i e l d s of the generic components of p i t c h using the method of corbett (1,2). The amounts of saturates, napthene-aromatics, polar aromatics & asphalentenes can be determined for d i f f e r e n t pitches for various 0097-6156/ 84/0260-0209S06.00/0 © 1984 A m e r i c a n C h e m i c a l Society

In Polymers for Fibers and Elastomers; Arthur, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

P O L Y M E R S FOR FIBERS A N D E L A S T O M E R S

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 24, 2015 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch012

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heat treatment temperatures and times, Figure 1. Saturates and napthene-aromatics are observed to evaporate during heat treatment leaving the less v o l a t i l e polar aromatics and asphaltenes, Figure 2. The carbon y i e l d at 800°C i s found to be l i n e a r l y related to the amount of polar aromatics and asphaltenes i n the precursor p i t c h , Figure 3. Although y i e l d depends upon more than molecular weight, the carbonization of model polynuclear aromatic compounds indicates that a minimum molecular weight of 400 i s needed to prevent v o l a t i l i z a t i o n before molecular weight increasing, condensation reactions occur. Figure 4. From these r e s u l t s an asphaltene p i t c h would give a high y i e l d and would be low cost. Orientation Liquid c r y s t a l l i n i t y i s determined by the length to diameter r a t i o , L/d, for rod-like molecules, and diameter to thickness, D/h, for d i s c - l i k e molecules. The r e s u l t s of calculations predict l i q u i d c r y s t a l l i n i t y i n rod-like molecules with L/d r a t i o s greater than 6.3 and i n d i s c - l i k e molecules with D/h r a t i o s greater than 3.5 (4,5) . Liquid c r y s t a l l i n i t y i s observed i n the rod-like para polyphenylene series when the L/d r a t i o approaches or exceeds 6.3, Figure 5. While both quinque- and s e x i - paraphenyl have s u f f i c i e n t l y high L/d r a t i o s to form r o d - l i k e l i q u i d systems, no d i s c - l i k e polynuclear aromatic compounds are available with a high enough D/h r a t i o to form a l i q u i d c r y s t a l . Ovalene, which has ten fused rings, i s close (D/h=3.0-3.2), but l i q u i d c r y s t a l l i n i t y has not been observed for i t or for the binary system of ovalene/coronene, Figures 5. Ovalene has a melting point of 472°C. Presumably, fused ring systems with higher D/h would have higher melting points, and thermal decomposition would occur before the crystalline/mesophase t r a n s i t i o n temperature i s reached. Brooks and Taylor (6) showed that heat treatment of coal tar and petroleum p i t c h forms a l i q u i d c r y s t a l (mesophase) with d i s c o t i c symmetry. Generally, mesophase formation was assumed to occur by polymerization and condensation of smaller p i t c h molecules to form large planar aromatics. I t i s true that the molecular weight increases upon heat treatment, but our studies on carbon y i e l d indicates that v o l a t i l i z a t i o n might be more important than polymerization and condensation. I f so, the o r i g i n a l i s o t r o p i c p i t c h already may contain the mesogenic species. Mesophase can be observed i n pitches which have a higher molecular weight than ovalene yet can have a softening point well below the melting point of ovalene. Several factors cause t h i s . F i r s t l y , a wide range of species with d i f f e r i n g molecular weights and structures are present i n a p i t c h . Melting point depression w i l l occur, and mesophase regions may be uncovered. For example, i n a binary eutectic system, two nonmesogenic compounds often w i l l "uncover" a l i q u i d c r y s t a l l i n e region near the eutectic composition where the melting point depression i s greatest, Figure 6. Secondly, the wide range of species makes c r y s t a l l i z a t i o n sluggish, so that the systen can be supercooled and mesophase may form before c r y s t a l l i z a t i o n occurs or the glass t r a n s i t i o n temperature i s reached. Thirdly, mesophase pitches often appear to consist of f l e x i b l y linked polynuclear aromatics with three to s i x rings with a number average molecular weight of 1000 or more. By analogy with calculated e f f e c t s of adding f l e x i b l e l i n k s to r i g i d - r o d molecules, the softening point i s more


12.

saturates

saturates


Mesophase Formation in Polynuclear Aromatics

DIEFENDORF

naphthene aromatics

naphthene aromatics

saturates

naphthene aromatics

naphthene aromatics

polar aro.

211

naphthene aromatics polar aro.

polar aromatics

asphaltene

asphaltene

polar aro. polar grornatics

asphaltene

asphaltene

Carbon Black Oil R

Carbon Black Oil B

Dau Bottom

Figure 1 .

Ashland 240

CTP

Ashland

240

260

The saturate, napthene-aromatic, polar aromatic, and asphaltene content of valious pitches and heavy o i l s .

CARBON BLACK OIL B 4 2 5 «fc

ABSOLUTE

saturates

FRACTIONATION

naphthene aromatics

naphthenes I—naphthenes———* M—napiimeiieo Jj--polar a r o m a t i c s — — polars — polar aromatics asphaltenes

asphaltenes

asphaltenes

asphaltenes

asphaltenes mesophase

- mesophase-

AS RECEIVED Figure 2.

38

MIN

68MIN

117MIN

256MIN

The generic fractions for a c a t a l y t i c cracker o i l upon heat treatment at 425C.



212

POLYMERS

F O R FIBERS A N D

ELASTOMERS

100

200

Molecular Figure 4.

400

600

800

Weight

The carbon y i e l d of polynucleat aromatic compounds upon heating to 800C.



12.

DIEFENDORF


213

depressed than the mesophase/isotropic t r a n s i t i o n temperature. Fourthly, short a l k y l side arms on the polynuclear aromatic cores may enhance mesophase formation s i m i l a r l y to the effects of a l k y l t a i l s on rod-like l i q u i d c r y s t a l s . F i n a l l y , mesophase pitches have many of the c h a r a c t e r i s t i c s of lyotropic l i q u i d c r y s t a l systems. In t h i s case, low molecular weight nonmesogenic species act as solvents or p l a s t i c i z e r s for the high molecular weight mesogenic species. Without these low molecular weight p l a s t i c i z e r s , the high molecular weight species would decompose before a sample could be heated hot enough to form f l u i d mesophase. These f i v e factors which enhance formation of mesophase are hard to study independently. Chemical reactions are occuring at the temperatures required to study p i t c h mesophase such that the system c h a r a c t e r i s t i c s are continuously changing. V i s c o s i t y i s also f r e quently high, and o p t i c a l microstructures can be misleading as e v i dence for mesophase formation. Quenched and slow cooled specimens may appear i s o t r o p i c and 100% mesophase respectively.) Pitches are not very ideal solutions. Hence, mesophase formation i s altered by s o l u b i l i t y considerations, which considerably clouds interpretation of experimental data. F i n a l l y , the complexity of a pitch makes the a n a l y t i c a l characterization very d i f f i c u l t (10,11). Simple averages such as number average molecular weight are just i n s u f f i c i e n t , and even these values may be hard to measure properly (12). These problems make for ambiguities i n the interpretation of experimental data. A gross s i m p l i f i c a t i o n i s to c l a s s i f y p i t c h molecules into two general groups. One group consists of mesogens with r e a l mesophase/ i s o t r o p i c t r a n s i t i o n temperatures (T&), Figure 7. Generally, the higher molecular weight, more aromatic species would belong to t h i s group. Mesogenic species where the mesophase/isotropic transition temperature i s just above the softening point would be e a s i l y d i s rupted by small additions of non-mesogenic species. Larger mesogenic species would have higher mesophase/isotropic t r a n s i t i o n temperatures, and would require larger amounts of non-mesogenic species to destroy the mesophase. The second group consists of nonmesogens which have v i r t u a l t r a n s i t i o n temperatures (Ty), e.g., the mesophase/isotropic t r a n s i t i o n temperature i s below the softening point and only an i s o t r o p i c l i q u i d i s observed. Nonmesogens which have v i t r u a l transi t i o n s temperatures close to the softening point w i l l not be very disruptive to a mesophase, while a nonmesogen with a low v i r t u a l t r a n s i t i o n temperature would be very d i s r u p t i v e . Also, an i s o t r o p i c pitch may contain an appreciable quantity of mesogens. The only requirement i s that the t r a n s i t i o n temperature (range) be v i r t u a l , e.g., below the softening point. S i m i l a r l y , a mesophase p i t c h may contain appreciable quantities of nonmesogenic species. In a two phased, i s o t r o p i c & mesophase pitch, both phases w i l l contain mesogeni c & nonmesogenic species but with d i f f e r e n t r e l a t i v e concentrations. The proof that an i s o t r o p i c p i t c h can contain an appreciable mesogenic component can be obtained by removing a s u f f i c i e n t amount of the nonmesogenic species. V o l a t i l i z a t i o n has been successfully used to reduce the lower molecular weight nonmesogenic species. However, appreciable chemical reaction occurs at the temperatures required to remove the less v o l a t i l e nonmesogens. Hence, the chemical reactions may be the cause for mesophase formation rather than v o l a t i l i z a t i o n . Solvent fractionation of pitches circumvents t h i s problem as the


POLYMERS FOR FIBERS AND ELASTOMERS


214

Figure 5.

The chemical structures of quinque parapolyphenylene and ovalene.

ISOTROPIC

LIQUID

NONMESOGEN A

Figure 6.

NONMESOGEN B

A schematic binary phase diagram of two nonmesogens which show mesophase formation near the eutectic composition.

Molecular Weight

Figure 7.

A schematic i l l u s t r a t i n g that an i s o t r o p i c p i t c h can be considered to consist of mesogens and nonmesogens.



12.

DIEFENDORF


215

separation can be carried out at ambient temperature. The wide range of a v a i l a b l e solvents allows a t a i l o r i n g of the separation process (13). This technology i s a c r i t i c a l factor i n producing low cost carbon f i b e r s as i t allows a 100% mesophase p i t c h to be made which can be spun at temperatures where chemical reactions occur slowly. (The 100% mesophase structure produces high modulus i n the carbon f i b e r . ) E a r l ier p i t c h precursor carbon f i b e r s were made from pitches containing only 55% mesophase to allow a suitable, lower spinning temperature. Solvent separations depend on solvent/solute i n t e r a c t i o n s , and the mesogen/nonmesogen separation obtainable with solvents may not be very good as solvent/solute interactions are not controlled by mesogenicity. However, the toluene insoluble f r a c t i o n of a commercial p i t c h , Ashland A240, d i r e c t l y forms mesophase upon heating. The gel permeation chromatogram for the whole p i t c h and the toluene soluble portion shows, by difference, that the toluene insoluble molecular, weight d i s t r i b u t i o n i s quite broad, Figure 8. Although the toluene soluble portion of the p i t c h s t i l l contains an appreciable f r a c t i o n of mesogens, they do not appear to be separable with common solvents without including too high an amount of nonmesogens. A f i n a l proof that mesophase formation i s mainly due to the physical separation of nonmesogenic species rather than chemical reactions was performed by adding 25% of Ashland A240 p i t c h back to the toluene insolubles of A240 mesophase p i t c h . Almost a l l the mesophase was destroyed upon heating even for t h i s r e l a t i v e l y minor addition of v i r g i n p i t c h (14). Grouping p i t c h molecules into mesogenic and nonmesogenic f r a c t i o n s allows construction of a "pseudo" binary phase diagram, Figure 9. Transition temperatures become broadened to t r a n s i t i o n temperature ranges because of the d i s t r i b u t i o n s . Conventional heat treatment of i s o t r o p i c pitches would increase the temperature to the heat treatment temperature with l i t t l e change i n composition. As the nonmesogens evaporate, the composition moves towards the mesophase region. I f the temperature i s s u f f i c i e n t l y high and the time s u f f i c i e n t l y long, the composition w i l l change u n t i l mesophase i s formed. By contrast, s o l vent f r a c t i o n a t i o n removes the nonmesogens and the composition w i l l be in the mesophase region. Heat treatment need only be severe enough to cause softening and s u f f i c i e n t domain growth to produce microscopically observable mesophase. In general, the mesophase/isotropic t r a n s i t i o n i s not observed i n mesophase pitches because of molecular weight increases at higher temperatures and coking. There are many types of natural and synthetic pitches. Some consist of large polynuclear aromatic cores with a large number of r e l a t i v e l y long a l k y l side arms which s o l u b i l i z e and soften the species. Others with similar softening points consist of r e l a t i v e l y small polynuclear cores with no or at most a few methyl groups attached. The structures for a solvent fractionated, c a t a l y t i c cracker mesophase p i t c h are i l l u s t r a t e d i n Figure 10, (15). The lowest molecular weight species are nonmesogenic but p l a s t i c i z i n g . The highest molecular species won't soften by themselves, but s t a b i l i z e the mesophase structure. The most probable molecule has a molecular weight of about 1000. Molecular structures are constructed by combining r e s u l t s from IR, UV/VIS, NMR, VPO, GPC, HPLC and elemental analysis (16). These structures are only based on the average a n a l y t i c a l r e s u l t s . Even for averages, a wide range of structures, only one of which i s i l l u s t r a t e d , can be drawn. Since molecules can deviate from these average r e s u l t s , a much wider range of structures w i l l exist with


216

POLYMERS FOR

FIBERS A N D

ELASTOMERS


A 240

Elution

Figure 8.

Time

—•

The gel chromatogram for Ashland A240 p i t c h and toluene soluble (TS) f r a c t i o n .

the

LU QC D QC UJ CL LU

SOLID

Mesogen

Figure 9.

Nonmesogen

A pseudo binary phase diagram which explains mesophase formation i n a p i t c h . MESOPHASE

MW=230

Figure 10.

MW= 950

PITCH

MW *

1400

Typical average chemical structures for the toluene insoluble f r a c t i o n of a c a t a l y t i c cracker pitch.


12.

DIEFENDORF


217

the only requirement being that the d i s t r i b u t i o n must give the average analytical results.


Processability Mesophase p i t c h can be melt spun into f i b e r s . Major requirements are that the p i t c h i s : 1) thermally stable at spinning temperatures, 2) free from p a r t i c u l a t e s , and 3) rheologically acceptable. Solvent fractionated mesophase pitches can be made which meet these three requirements. As spun strength of p i t c h f i b e r s i s low but i n the same range as some other commercially produced t e x t i l e f i b e r s . Subsequent processing to carbon f i b e r i s similar to that used for polyacrylonitrileprecursor carbon f i b e r . A more complete description of this process and f i b e r properties are presented i n the paper of Riggs & Diefendorf i n t h i s volume (17). Conclusions Calculation and model compounds show that a length to diameter r a t i o of 6.3 i s s u f f i c i e n t to form a r i g i d rod l i q u i d c r y s t a l . Similarly, c a l c u l a t i o n indicates that a diameter to thickness r a t i o of at least 3.5 i s required to form a d i s c o t i c l i q u i d c r y s t a l . Mesophase formation i n pitches i s enhanced by the broad molecular weight d i s t r i b u tions, f l e x i b l e l i n k s between polynuclear aromatic elements and a l k y l side groups. Generically, pitches can be considered to consist of two species: mesogenes and nonmesogens. Solvent fractionation can be used to p a r t i a l l y separate the two types of species. F i n a l l y , a 100% mesophase p i t c h can be produced which can be melt spun and processed into a carbon f i b e r with high mechanical properties. Acknowledgment The author i s indebted to h i s students, p a r t i c u l a r l y : S.H. Chen, D.M. Riggs, W.C. Stevens, and J . Venner. Special thanks should be given to S.H. Chen for the artwork and D. Ruddy for typing the manuscript.

References 1. 2. 3. 4. 5.

Riggs, D. M. ; Diefendorf, R. J. "Factors Controlling the Thermal Stability and Liquid Crystal Forming Tendencies of Carbonaceous Materials", Preprint, CARBON '80, Baden-Baden, 1980, pp. 330-333. Corbett, L. W. "Relationship Between Composition and Physical Properties of Asphalt" presented at the Assoc. of Asphalt Paving Technologists, Kansas City, Missouri, Feb. 1970. Riggs, D. M.; Diefendorf, R. J. "Thermal Stability of Aromatic Compounds", Extended Abstracts, 14th Biennial Conf. on Carbon, Pennsylvania State Univ., June 1979, pp. 350-351. Flory, P. J.; Ronca, G. "Theory of Rod-like Particles - Part II: Thermotropic Systems with Orientation-Dependent Interactions", Mol. Cryst. Liq. Cryst., 1979, Vol. 54, pp. 311-330. Alben, R., Phy. Rev. Lett. 24, p. 1041, 1970.


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6.

Brooks, J. D.; Taylors, G. H. "Chemistry and Physics of Carbon", P. L . Walker, Jr. and P. A. Thrower, Eds., Marcel Dekker, New York, 1968, Vol. 4, pp. 243-268. 7. Smith, G. W. "Phase Behavior of Some Condensed Polycyclic Aromatics", Mol. Cryst. Liq. Cryst., Vol. 64, Letts., pp. 15-17. 8. Chen, S. H.; Diefendorf, R. J. "Mesophase Formation in Polynuclear Aromatic Compounds", Extended Abstracts, 16th Biennial Conf. on Carbon, U.C. San Diego, July, 1983, pp. 28. 9. Riggs, D. M.; Diefendorf, R. J., This volume. 10. Greinke; O'Conner, L. H. "Determination of Molecular Weight Distributions of Polymerized Petroleum Pitch by Gel Permeation Chromatography with Quinoline Eluent", Anal. Chem., 52, 1980, pp. 1877-81. 11. Chen, S. H., Stevens, W. C. and Diefendorf, R. J. "Molecular Weight and Molecular Weight Distributions in Pitches" International Symposium on Carbon, Toyohashi, Japan, 1982, pp. 59-72. 12. Chen, S. H.; Diefendorf, R. J. "Molecular Weight Determination of Pitches", Extended Abstracts, 16th Biennial Conf. on Carbon, U. C. San Diego, July, 1983, p. 433. 13. Venner, J.; Diefendorf, R. J., This volume. 14. Riggs, D. M.; Diefendorf, R. J. "A Phase Diagram for Pitches", Preprint, CARBON '80, Baden-Baden, 1980, pp. 326-329. 15. Chen, S.H.; Diefendorf, R. J. Unpublished data. 16. Chen, S. H.; Diefendorf, R. J., This volume. 17. Riggs, D. M.; Diefendorf, R. J., This volume. RECEIVED January 17, 1984


Mesophase Formation in Polynuclear Aromatic Compounds

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