Polymers for Fibers and Elastomers - American Chemical Society

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M o l e c u l a r Weight D e t e r m i n a t i o n a n d D i s t r i b u t i o n in Pitches 1

S. H. CHEN, W. C. STEVENS, and R. J. DIEFENDORF Downloaded by UNIV OF NEW HAMPSHIRE on March 5, 2015 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch014

Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181 Vapor pressure osmometry (VPO) and gel permeation chromatography (GPC), using 1.2.4 -trichlorobenzene (TCB) solvent, have been applied to measure molecular weight and molecular weight distribution of petroleum pitches. A high osmometry temperature is required to prevent molecular association/phase separation of high molecular weight species. VPO appears to be satisfactory for molecular weight determination with most TCB soluble fractions of petroleum pitches up to and including toluene insoluble fraction. Different pitches have different GPC calibration curves and each sample should be used to define its own molecular weight/elution volume relationship. The higher molecular weight species show a higher UV absorption coefficient. This discrete absorption coefficient makes analysis difficult. Petroleum pitches and coal tar pitches are becoming increasingly important as raw materials for the production of low cost, high performance carbon fibers. This development has been primarily due to the ability of the pitch to form an oriented liquid crystalline mesophase. Since the 1st observation of mesophase spheres in pitch material by Brooks & Taylor (1), a number of studies on the kinetics of mesophase formation have been made by several different groups. A free radical polymerization mechanism was proposed by Singer & Lewis (2). However, studies of heat-treatment of heavy oil, by Riggs & Diefendorf (3) indicates that mesophase formation is preceded not necessarily by extensive polymerization reactions which builds up the molecular weight and aromaticity to satisfy the average structural requirements for mesophase formation. Rather, volatilization increases molecular weight and alkyl cracking reactions increase the degree of aromatic carbon, although decreasing molecular weight. Mesophase can be thermotropic or lyotropic in nature. For thermotropic systems, the size of the molecules, or the molecular weight 1

Current address: American Cyanamid, Stamford, CT 06904. 0097-6156/84/0260-0235$06.00/0 © 1984 American Chemical Society

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

Downloaded by UNIV OF NEW HAMPSHIRE on March 5, 2015 | http://pubs.acs.org Publication Date: August 29, 1984 | doi: 10.1021/bk-1984-0260.ch014

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(MW), as well as shape i s important for mesophase formation. For lyotropic systems, mesophase formation i s dependent on s u f f i c i e n t s o l u b i l i t y of a thermotrope i n an i s o t r o p i c solvent to form a mesophase, or a l t e r n a t i v e l y enough p l a s t i c i z a t i o n of a thermotrope by a p l a s t i c i z e r to allow flow. Mesophase pitches appear to be of the second type and are controlled by the molecular weight/structure d i s t r i b u t i o n . Obviously, molecular weight i s an important parameter for mesophase formation i n pitches. However, early studies indicated that mesophase was insoluble i n a l l common solvents, and molecular weight could not be determined. Even for the more soluble asphaltenes various techniques produced values of molecular weight varying over a range of vLOOO to greater than 50,000 (4). Further, a study of asphaltene molecular weight by Speight (5), using the vapor pressure osmometry (VPO) technique, shows that the MWs were dependent on the nature of the solvent and the VPO temperature. A higher VPO temperature or higher d i e l e c t r i c constant of the solvent reduced the observed MWs. The molecular association, which i s responsible for the controversy regarding the determination of MW, can be removed by using a better solvent and increasing the VPO temperature. This work was performed to determine i f "absolute molecular weight of solvent fractionated mesophase pitches could be determined by VPO. The molecular weight d i s t r i b u t i o n (MWD) which i s important for describing mesophase formation i n pitches was determined by gel permeation chromatography (GPC). While the solvent/solute/gel i n t e r actions depend on more than just the size of the solute, smaller molecules generally are retained longer than larger molecules. The success of the technique depends on good solvent and columns, the elution volume/molecular weight c a l i b r a t i o n and the detector response/concentration c a l i b r a t i o n . Good solvent and high system temperature prevent the molecular association. Evidence of associated molecules i s the observation of a high molecular weight peak at 30°C. In our studies, t h i s peak could be removed by increasing the system temperature to 135°C. The c a l i b r a t i o n of MW vs elution volume for l i n e a r f l e x i b l e chains and r i g i d rod polymeric materials often uses standard samples of polystyrene. The pitch molecules are s i g n i f i c a n t l y d i f f e r e n t from the PS polymer and large discrepancies between measured p i t c h MWs and those implied from PS standards are inevitable. Aromatic compounds appear a t t r a c t i v e as standards but are limited to 600 MW, and are s i g n i f i c a n t l y d i f f e r e n t from the structure of pitches. Synthetic model compound pitches provide good standards, as the i n d i v i d u a l polymer peaks are well resolved by GPC (6). Retention volumes for molecular weights up to 1000, and sometimes higher, can be determined. However, d i f f e r e n t pitches have d i f f e r e n t c a l i b r a t i o n curves (7), (Figure 1). Hence, each p i t c h sample should be used to define i t s own MW/elution volume r e l a t i o n . 11

Experimental Since both VPO and GPC r e l y on the complete solution of the p i t c h sample, s o l u b i l i t y study was performed. Results on a wide range of solvents showed 1.2.4- trichlorobenzene (TCB) to be the best compromize for p i t c h materials and was chosen for both VPO and GPC. Several solvents such as dimethylformamide (DMF), produce low insolubles.


14.

C H E N ET AL.

Molecular Weight Distribution

in Pitches

237


These solvents apparently have strong associations with the p i t c h molecules or disperse the p i t c h as proton NMR does not generate any aromatic s i g n a l . A Corona/Wescan Model 232A VPO was used for measuring the number average molecular weight of the samples. The molecular weight d i s tributions of p i t c h samples were determined by a Waters 150 C GPC. An UV detector was used i n this study. The 305 my wavelength cut o f f for the TCB solvent prevents the use of a more desirable lower detector wavelength. ^ / V i s spectra of model compounds and pitches were used to select the compromise of 365 my detector wavelength. However, small aromatics such as napthalene have no absorption above 365 my and hence are not detected. Results and Discussions The following equation can be derived for the vapor pressure osmometry determination of MW from the v i r i a l theorem and the Clausius-Clapeyron equation: ^9 9

Mi

9

v9 q

q

AV = K' C J L + K'a (J^y C + K" $ (=V C + (1) w M2 V-L w vi w where AV i s voltage output of the VPO K' i s a c a l i b r a t i o n constant i s weight concentration of solute, gro/c.c. Mj & M2 are molar mass of solvent and solute a &_B are the second and t h i r d v i r i a l c o e f f i c i e n t s and V\ ct V2 are the p a r t i a l s p e c i f i c volume of solvent and solute Neglecting the higher terms, Equation 1 i s modified to plot of AV/c vs CV w

^ C w

K' 2* + K* a M2

(II)

2

V l

C w

(2)

The plot of voltage output of the VPO per unit concentration vs concentration was used to determine the MW. The intercept determines the MW and the slope a. Alpha i n turn, provides information on the interaction between solute and solvent. The sign and magnitude of the slope determine i f good s o l u b i l i t y i s obtained for a given temperature. VPO i s a r e l a t i v e method for MW determination and a c a l i b r a t i o n i s required. Although there i s some controversy i n the l i t e r a t u r e regarding c a l i b r a t i o n of VPO, the c a l i b r a t i o n with a material of 200 MW appears to be v a l i d up to 100,000 for the apparatus used (8). To c l a r i f y this point, several model compounds were used as c a l i brants. Table I. Although three compounds were pure unsubstituted polynuclear aromatics and the other two contained oxygen, the c a l i bration constants agreed within a few percent. The c a l i b r a t i o n data for coronene are shown i n Figure 2. The negative slope i s i n d i c a t i v e of a poor solution at the given temperatures. The reason i s that the s o l u b i l i t y parameter for coronene, estimated from the Riggs (9) plot i s about 12.5, while that for TCB i s only 10.4. Therefore, coronene i s sparingly soluble i n TCB and even though d i l u t e solutions are used i n VPO studies, there i s a


POLYMERS FOR FIBERS AND ELASTOMERS


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MW

10

14

18

Elution Volume, ml

Figure 1.

GPC c a l i b r a t i o n curves, using VPO, shows that d i f f e r e n t pitches have d i f f e r e n t c a l i b r a t i o n curves.

Calibration of VPO Coronene MW = 300 o 122 °C • 132°C A^ = 38-1.1 7 c . c

AV

o—o

2

r = 0.997 K=11,410

Polymers for Fibers and Elastomers - American Chemical Society

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