SANS study of polymer-supercritical fluid solutions: Transitions from liquid to supercritical fluid solvent quality

Phase behavior and small-angle neutron scattering (SANS) measurements are r eported for poly(ethylene-co-l-butene) (PEB) dissolved in pentane, pentane plus ethane, and supercritical ethane to pressures of 2000 bar and temperat ures of 130 and 150 degrees C. The solution microstructure and solvent qual ity are probed using the high-concentration isotopic labeling technique at the mixture critical concentration to determine the variation of intra- and intermolecular correlations on approach to the phase boundary with isother mal changes in pressure. The SANS results for the three solutions show that the intramolecular radius of gyration remains close to the unperturbed val ue over the entire pressure range investigated. However, the intermolecular correlation length changes dramatically as the phase boundary is approache d and is approximately 3 times larger in ethane as compared to pentane even at 1000 bar from the phase boundary. The intermolecular correlation length data suggest that ethane is a poorer quality solvent than pentane at condi tions close to the phase boundary even though both solvents have similar ma ss densities.