Characterization of Micro-Domain Structure of Solvent-Swollen Coal by Proton Spin Diffusion and Small Angle Neutron Scattering

The macromolecular structure of a high volatile bituminous coal (the Blind Canyon) is characterized by osmotic dilation experiments using deuterated solvents (pyridine or benzene/ pyridine mixed solvent). Nano scale structural changes were quantitatively revealed using proton spin diffusion experiments and qualitatively verified with small-angle neutron scattering (SANS) measurements. The SANS data reveal systematic changes in the coherent scattering behavior of osmotically dilated coal particles in a binary solvent system of benzene + pyridine. These results show that volumetric strain derived from osmotic stresses results in the development of phase-separated domains at nanoscales. Quantitatively, the effect of solvent-to-coal mass ratio (S/C) on the proton relaxation characteristics was examined. It is shown that the fractional amount of mobile hydrogen yielding the exponential decay, ∫MH, increased to a maximum of 0.5 with increasing solvent concentration. Above S/C = 2.24, ∫MH remained nearly constant, indicating that solvent impenetrable regions exist in the swollen coal even at S/C ratios as high as 4.72. Although there exist at least two distinct structural regions in the swollen coals based on the transverse relaxation characteristics, longitudinal relaxation experiments are fit best by a single component revealing that spin diffusion is rapid in the swollen coals. The dynamics of spin diffusion were determined using a partially modified Goldman-Shen pulse sequence and analyzed by simple geometric models of two-phase systems. The average distance between the adjacent solvent impenetrable domains, d_i, was estimated on the basis of the diffusive path length for each spatial dimension by employing simple one-, two-, and three-dimensional models. Estimates of d_i derived from the SANS data lie between those derived for the one- and two-dimensional diffusion models suggesting that the actual structure may be roughly laminar, but with sufficient curvature or other irregularity to yield diffusion behavior lying between the two simple models considered. The results of this study clearly reveal that volumetrically measured osmotic strain is not affine down to the macromolecular level.