The macromolecular nature of bituminous coal

Abstract

Considerable insight into the macromolecular state of coal has been obtained by examining the optical anisotropy of untreated solvent-swollen, and chemically derivatized thin sections of coal. From the effect of pressure on the optical anisotropy, and from the rate and degree of recovery after release of pressure, it was found possible to determine whether the coal is in a plastic or rubbery state, whether a rubbery state is cross-linked and how mobile the macromolecular chain segments are. The experimental technique utilized for this work was transmission optical microscopy in polarized light of uncontaminated thin sections of vitrinite from a bituminous coal. The study included in-situ microscopic examination of swollen coal immersed in pyridine, THF, toluene and several other solvents. Some samples were O-methylated to assess the impact of hydrogen bonding. New results and conclusions derived from this study include: (1) the vitrinite of raw bituminous coal is a plastic macromolecular substance; (2) coal swollen in pyridine (and some other ‘specific’ solvents) is a cross-linked rubber and its macromolecular chain segments have substantial mobility; (3) when pyridine-extracted coal dries, it reverts to a plastic; (4) the large discrepancies previously found between values of M c (molecular weight between crosslinks) measured by solvent-swelling and by stress-strain techniques is caused by differences in secondary interactions; (5) various solvents can, by their effect on secondary interactions, create appreciably different macromolecular structures in the coal; (6) different solvents, depending on their effect on secondary interactions in the coal, can be expected to extract chemically different molecules from a coal - rates of extraction and the ability of solvents to extract larger molecules should also differ; (7) O-methylated coal is a plastic, and thus, in addition to hydrogen bonding, other secondary interactions are of great importance; (8) it is likely that in their dry condition, solvent-treated coal and O-alkylated coal, as well as untreated coal, are in glassy states; (9) pyridine by itself appears to relax substantially all secondary interactions which are weakened by O-methylation, only permanent bonds are not relaxed; (10) previous measurements of M c can now be reassessed in view of these results.