13
P i t c h - S o l v e n t Interactions a n d T h e i r Effects
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on Mesophase Formation 1
J. G. VENNER and R. J. DIEFENDORF Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181 High performance pitch carbon fiber can only be manufactured from mesophase forming pitches. Generally, commercially available pitches require an extensive heat-treatment to increase the molecular weight of the pitch by removing or condensing the low end species, prior to the mesophase formation. A selective solvent can also be used to remove the low molecular weight, mesophase-inhibiting species. The multidimensional solubility parameters of a solvent provide qualitative information as to the type of molecular interactions a solvent is capable of undergoing, and the suitability of a solvent for separating a mesophase-forming fraction from the pitch. The effects of varying the selective solvent and altering the pitch to solvent ratio are reported in this paper. High strength, high modulus, pitch based carbon fiber can be made from a precursor pitch which forms an anisotropic phase, or a mesophase. The aligned pitch molecules in the mesophase provide the fundamental structural elements of graphitizable carbons (1,_2). Unfortunately, commercially available pitches, such as Ashland A-240, are complex mixtures of mesophase forming species and mesophase disrupting species(3). A commercial pitch must be processed to remove the low molecular weight mesophase inhibiting species to provide a pitch fraction that will form a coalesced mesophase rapidly upon melting (Figure 1). Formerly, it was thought that a pitch could not form 100% coalesced mesophase without undergoing extensive heattreatments to increase molecular weight and aromaticity(4). The nonmesophase forming species were thought to react to form larger, planar molecular blocks more capable of alignment. Recently, however, Riggs and Diefendorf(5) patented the use of selective solvent as an alternative to extensive heat-treatments. A selective solvent can 'Current address: Exxon Enterprises Materials Division, Fountain Inn, SC 29644. 0097-6156/84/0260-0219S06.00/0 §> 1984 American Chemical Society
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
73
m Three-dimensional pseudo-phase diagram of a t y p i c a l p i t c h . Figure 1.
c/5
mr >
α
>
C/5
53 m73
73
C/5
*n Ο
m73
"0
Ο ζ
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
VENNER AND
13.
DIEFENDORF
Pitch-Solvent Interactions
221
dissolve many of the lower molecular weights, disordering species leaving the larger, more orientable species, insoluble. The i n s o l uble d i s t r i b u t i o n s are capable of forming a 100% coalesced structure rapidly upon melting provided enough of the low molecular weight species have been removed, and the insolubles have the proper structure to form a mesophase. The mesophase forming tendencies of a p i t c h f r a c t i o n depend l a r g e l y on the structure of the precursor p i t c h , and on the extraction c a p a b i l i t y of the solvent. Therefore, the selection of an extracting solvent and extraction conditions are c r i t i c a l . This research focused on answering three fundamental questions: 1. What solvents can be used to extract a f r a c t i o n from a commercial p i t c h (such as A-240) which i s capable of forming a coalesced mesophase r a p i d l y upon melting, and what c r i t e r i a w i l l predict their effectiveness? 2. What extraction conditions are c r i t i c a l ? 3. Can the extraction be modified, or t a i l o r e d to produce desirable fractions? Solvent
Selection
The regular d i s s o l u t i o n of a s o l i d i n a n o n - e l e c t r o l y t i c solvent has been described i n d e t a i l by Hildebrand(6) who showed that mole f r a c tion of s o l i d dissolved depends on the molecular weight of the s o l i d , (which a f f e c t s i t s enthalpy of fusion), i t s melting temperature, the system temperature, and the difference between the s o l b i l i t y parameter of the solvent and the solute. 4.575
F
•
L O g
1 x
=
AH m(Tm-T) 4.575TmT
ACD. Tm^T ACp log Tm 4.575 ( T )T7y87 T
2 ^ l
}
2
1
x^
= mole f r a c t i o n of s o l i d soluble i n solvent
(1)
AHm Tm ACp
= heat of fusion at the melting point = melting point of the s o l i d = the difference between the heat capacities of the s o l i d l i q u i d solute - molar volume of solute
and
V^ 6^,
6^ = s o l u b i l i t y parameters of solvent and
and $
= volume f r a c t i o n of the
solute
solvent
If the solute, the concentration, and the system temperature are kept constant, then differences i n s o l u b i l i t y depend s o l e l y on the difference between the s o l u b i l i t y parameter of the s o l i d and the solvent. Riggs showed the s o l u b i l i t y of a p i t c h i n a series of solvents with increasing s o l u b i l i t y parameters followed a b e l l shaped curve, with maximum s o l u b i l i t y occurring when the s o l u b i l i t y parameter of the solute and solvent were equal(5). In general, non-polar solvents with t o t a l s o l u b i l i t y parameters ranging from 8.0-9.5 were desirable for extracting mesophase forming fractions from pitches. In t h i s
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
P O L Y M E R S FOR F I B E R S A N D E L A S T O M E R S
222
work, A-240 p i t c h was dissolved i n 38 solvents, both dispersive and polar (Table I ) . A plot of weight percent dissolved versus solub i l i t y parameter shows that A-240 s o l u b i l i t y i n dispersive solvents does follow the t y p i c a l b e l l shaped curve, however, the t o t a l parameter s o l u b i l i t y f a i l s to describe A-240 s o l u b i l i t y i n polar solvents (Figure 2). Alcohols, aldehydes, a c e t o n i t r i l e , acetone, dioxane, and other non-dispersive solvents dissolve less than the predicted percentage of A-240. As Hildebrand's s o l u b i l i t y parameter was intended for use i n dispersive systems, t h i s i s not surprising. The inadequacy of using the t o t a l s o l u b i l i t y parameter to describe the behavior of polar and hydrogen bonded solvents i n polymer systems led Charles Hansen(7) to generalize Hildebrand's parameter. The t o t a l s o l u b i l i t y parameter was broken down into three components representing the three types of interactions possible between molecules: dispersive, polar, and hydrogen bonding.
AE
Vm
=
AEd
Vm
+
AEJJ
Vm
+
AE
h
(2)
Vm
+ 6
(3)
Although generalizing the s o l u b i l i t y parameter into three dimensions i s not e n t i r e l y rigorous (as the geometric mean expression i s used to describe hydrogen bonding i n t e r a c t i o n s ) , the three dimensions provide an improved q u a l i t a t i v e understanding of solute-solvent interactions, and should be useful i n predicting s o l u b i l i t y of pitches. A solub i l i t y map can be created by p l o t t i n g the weight percentage of p i t c h dissolved as a function of dispersive and polar components of the s o l u b i l i t y parameter. Symbols or contours are used to highlight regions of reduced or enhanced s o l u b i l i t y . A s o l u b i l i t y plot i s shown i n Figure 3 where s o l u b i l i t y i s indicated by the shape of the symbol used to show l o c a t i o n on the p l o t . Strong dispersive solvents with moderate polar character are most able to dissolve A-240, while solvents with small dispersive parameters or large polar parameters, are generally incapable of d i s s o l v i n g an appreciable percentage of the p i t c h . In between these two regions exists an area of p a r t i a l s o l u b i l i t y . Solvents, such as benezene, dioxane, toluene, xylene, carbon tetrachloride, DMF, or DMA, dissolve between 75 and 96% of A-240. Because many of the lower molecular weight species can be removed by these solvents, their insoluble fractions should be ideal candidates for forming mesophase. The mesophase forming tendencies of the insoluble fractions were investigated by encapsulating samples under vacuum, then heat-soaking to 375°C for two hours. A l i s t of the mesophase forming fractions i s presented i n Table I I . As expected, solvents with s o l u b i l i t y parameters located at the outer edge of the enhanced s o l u b i l i t y region extract A-240 fractions capable of forming coalesced mesophase. These solvents range i n t o t a l s o l u b i l i t y parameter from 8.65 to 12.14 ( g l ) l / 2 , with dispersive parameters ranging from 8.7 to 9.3. Solvents, such as DMF and acetaldehyde, show appreciable polar parameter while DMF and dioxane are strongly hydrogen bonded. MeCl, c
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
13.
VENNER A N D DIEFENDORF
Pitch-Solvent Interactions
223
TABLE I E f f e c t o f S o l u b i l i t y Parameter on Weight % A2**0 Dissolved
, . Solvent e
1,2,1* Trichlorobenzene Bromobenzene Acetophenone Nitrobenzene Tetrahydrofuran Quinoline Pyridine Chlorobenzene Tetramethylurea Ethylene bromide Trichloroethylene Carbon d i s u l f i d e Ethylene d i c h l o r i d e Chloroform Methylene chloride Dimethyl acetamide Benzene Xylene Dioxane Toluene Dimethyl formamide Carbon t e t r a c h l o r i d e Decalin Diethyl amine Acetaldehyde Benzyl alcohol Nonane Decane Acetone Dimethyl sulfoxide Octane Nitromethane Hexane Pentanol Acetonitrile Propanol Ethylene g l y c o l Methanol
* v v • +4 Abbreviation
S o l u b i l i t y Parameter ( «l/cc)* c
TCB Bb A Nb Tf Q Py Cb Tmu Eb Tee Cd Ed Cf Mc DMA B X D T DMF Ct DI Da Aa Ba Nn Dn Act DMS On Nm Hn Pol Acn Ppl EG M
10.U5 10.60 10.60 10.62 9.52 10.75 10.61 9.57 10.60 11.70 9.20 10.00 9.80 9.2 9.93 11.10 9.15 8.80 10.00 8.91 12. ll* 8.65 8.80 7.97 9.86 11.6U 7.70 7.70 9.77 12.93 7.60 12.90 7.30 10.61 11.90 11.97 16.30 ll*.28
*Room temperature l g pitch/100 ml solvent.
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Wt.? A2**0 Dissolved* 99.8 99.8 99.8 99.7 99.7 99.6 99.3 98.9 98.9 98.6 98.0 97.5 97.3 96.7 96.0 95.0 88.8 88.6 87. k 86.8 86.1 82.8 66.1 63.6 1*9.8 1*8.0 1*1*.8 1*1*.7 1*0.5 36.6 36.3 31.8 31.1 22.7 19.5 16.0 7.1 2.5
224
POLYMERS FOR FIBERS AND ELASTOMERS
T
100
Tee
C
B
Tf .^CfT.-A Cf i M c
Py Tmu A Bb Nb x • Eb |D\EA •
•
\ \
\-
• Ct
ESTIMATED
DMF
80h
\ •
1
Da •DI
Q W
60h
5
O co to M Q O CM
