PHASE-BEHAVIOR AND MACROMOLECULAR STRUCTURE OF SWOLLEN COALS - A LOW-TEMPERATURE H-1 AND H-2 NMR-STUDY

H-1 and H-2 NMR line-shape studies have been used to investigate the low-temperature phase behavior of pyridine-d(5) and N-methylpyrrolidinone-d(9) (NMP) swollen-Pittsburgh No. 8 coal, Illinois No. 6 coal, and Zap lignite. Good solvents like pyridine and NMP greatly reduce secondary interactions within coals, transforming them from the glassy to the rubbery state at room temperature when the solvent-to-coal mass ratio (W-s/W-c) is over about 1. Using deuterated swelling solvents permits complementary studies of the coal structure (with H-1 NMR) and dynamics of the deuterated solvent molecules (with H-2 NMR). For solvent swollen coals with W-s/W-c similar to 2, the H-1 NMR line width is reduced by more than an order of magnitude, indicating the onset of a high level of molecular motion which averages the dipolar interactions among protons. The increase of line width with decreasing temperature is reversible, independent of solvent employed, and similar for all three coals examined (lignites to high-volatile type A bituminous coals). The line-width increase extending from similar to 280 to 170 K and centered at 210 K is attributed to a glass-to-rubber transition. The H-2 NMR studies of the deuterated solvent molecules suggests that they become immobile at significantly lower temperatures than that observed for the transition of the coals. Analyses of the NMR data suggest isolation of the individual components of the coal as a consequence of the swelling process.