PHYSICAL STRUCTURAL CHARACTERIZATION OF BITUMINOUS COALS - STRESS-STRAIN ANALYSIS IN THE PYRIDINE-DILATED STATE

Measurement of the viscoelastic properties of bituminous coals in the pyridine-dilated state reveals that the characteristic behavior of the coals is similar to that of rubbery materials. This substantiates similar observations and conclusions made by Brenner. Quantification of the viscoelastic parameters indicate that the modulus of coal and its solidlike behavior results from either a cross-linked network or extensive entanglement coupling of noncovalently bound macromolecular strands. Viscous compliance observed after long experimental times is consistent with the physical structure of coal being an entangled network; however, a cross-linked network capable of steady-state bond rupture and re-formation would also explain the presence of viscous compliance at long times. In the case of an entangled physical structure the cross-link density in the coals studied would be very low and could conceivably even be zero. The number-averaged molecular weights between entanglement junctions or cross-links are calculated using the statistical theory of rubber elasticity and considering two extreme cases of viscoelastic behavior (one case including osmotic compressibility and the other assumes zero compressibility). The values span the range of previously reported values obtained through solvent-swelling studies assuming cross-linked networks.