Chapter 11 Flory Interaction Parameters for Illinois Number 6 Coal Extracts and Benzene 1
Downloaded by MONASH UNIV on November 5, 2012 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch011
Leonor Lopez-Froedge, James E. Ball, Min-Wei Chen , and Thomas K. Green Center for Coal Science, Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101
Flory interaction parameters (χ) for the untreated and O-alkylated pyridine-extracts of an Illinois No. 6 coal and benzene were determined using an established vapor pressure technique. The extract and O-methylated extracts sorb benzene slowly and behave like glassy materials. A hole-filling/dissolution model for sorption of vapors by glassy polymers was applied to the data in order to separate adsorption from true swelling. The interaction parameters are determined to be 1.5 and 1.2 for the extract and O-methylated extract. O-butylated and O-octylated extracts sorb more benzene and at a faster rate. The presence of the larger alkyl groups lowers the average solubility parameter of the extract, which results in increased swelling by benzene.
Coals are considered macromolecular solids.(l) Although they are not polymers in the sense that they possess a repeating unit, they do possess several fundamental properties typical of synthetic crosslinked polymers.Q) One of these properties is the ability of coals to swell in organic solvents without dissolving. In recent years, there has been a rapid growth in the number of publications that deal with the solvent swelling of coals. Much of this effort has been directed toward the application of modern polymer and network theories to coals, with the purpose of better understanding their network structures. One of the most fundamental parameters of a network structure is M , the number average molecular weight between crosslinks. Consequently, several research groups have attempted to estimate M 's for coal from solvent swelling data and the Flory-Rehner equation.(2z2) The equation incorporates both the Flory-Huggins theory of polymer solutions and the Gaussian elastic c
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Current address: East China University of Chemical Technology, Shanghai, People's Republi of China 0097-6156/91/0461-0137$06.50A) © 1991 American Chemical Society
In Coal Science II; Schobert, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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network theory. An important parameter embodied in the Flory-Huggins theory is the interaction parameter, χ. χ is a thermodynamic parameter describing the free energy of the polymer-solvent interaction. A reliable evaluation of χ is essential to an estimation of M for crosslinked networks using the Flory-Rehner equation. Larsen gi â l have developed a method that relies on regular solution theory to determine χ parameters for nonpolar solvent-coal systems.(6.7) However, few other methods have been developed so that comparisons can be made. It is the purpose of this research to develop such a method. One of the most common techniques for determining χ parameters for polymer-solvent systems is the vapor pressure method.(lO) In this approach, the uncrosslinked polymer is exposed to solvent vapor of known pressure, p. The polymer absorbs solvent until equilibrium is established, χ is related to ρ and v , the volume fraction of polymer at equilibrium, by the Flory-Huggins equation:(H)
Downloaded by MONASH UNIV on November 5, 2012 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch011
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2
X = In ί(ρ/Ρο)/(1·ν )] - V2(l-l/x) 2
(1)
where p is the saturation pressure of the solvent vapor and x is the degree of polymerization or the number of segments in the polymer chain. Measurement of ρ as a function of v can be used with Equation 1 to obtain values for χ over a wide range of concentrations. According to Flory-Huggins theory, in the limit of χ ·+ «, the critical χ parameter is 0.5.(11) Below this value the polymer and solvent will be miscible in all proportions. Above this value, the solvent will not dissolve the polymer, but will act only as a swelling solvent. Thus, the pure solvent may not dissolve the polymer even though it is not crosslinked. If χ is not
