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 s olvents (pyridine or benzene/ pyridine mixed solvent). Nano scale structura l changes were quantitatively revealed using proton spin diffusion experime nts and qualitatively verified with small-angle neutron scattering (SANS) m easurements. The SANS data reveal. systematic changes in the coherent scatt ering behavior of osmotically dilated coal particles in a binary solvent sy stem of benzene + pyridine. These results show that volumetric strain deriv ed from osmotic stresses results in the development of phase-separated doma ins at nanoscales. Quantitatively, the effect of solvent-to-coal mass ratio (S/C) on the proton relaxation characteristics was examined. It is shown t hat the fractional amount of mobile hydrogen yielding the exponential decay , f(MH), increased to a maximum of 0.5 with increasing solvent concentratio n. Above S/C = 2.24, f(MH) remained nearly constant, indicating that solven t impenetrable regions exist in the swollen coal even at SIC 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, longi tudinal relaxation experiments are fit best by a single component revealing that spin diffusion is rapid in the swollen coals. The dynamics of spin di ffusion were determined using a partially modified Goldman-Shen pulse seque nce and analyzed by simple geometric models of two-phase systems. The avera ge distance between the adjacent solvent impenetrable domains, di, was esti mated on the basis of the diffusive path length for each spatial dimension by employing simple one-, two-, and three-dimensional models. Estimates of di derived from the SANS data lie between those derived for the one- and tw o-dimensional diffusion models suggesting that the actual structure may be roughly laminar, but with sufficient curvature or other irregularity to yie ld diffusion behavior lying between the two simple models considered. The r esults of this study clearly reveal that volumetrically measured osmotic st rain is not affine down to the macromolecular level.