Density-Induced Phase Separation in Poly(ethylene-<i>co</i>-1-butene)−Dimethyl Ether Solutions

Abstract

Dynamic light scattering measurements are reported for poly(ethylene- co -20.2 mol % 1-butene) (PEB 10 ) in dimethyl ether (DME) at 110−170 °C and pressures to 2500 bar. The cloud-point curve for the PEB 10 −DME system exhibits both low- and high-pressure upper consolute solution temperature (UCST) branches. The polymer infinite dilution translational diffusion coefficient, D 0, increases with increasing temperature and decreasing pressure as expected. The observed variation of D 0 is inversely proportional to the solvent viscosity, indicating that the polymer coil hydrodynamic size is independent of temperature and pressure. The dynamic second virial coefficient, k D, which represents a balance between thermodynamic interactions and hydrodynamic forces, displays values that lie within the bounds expected for ϑ and good solvent conditions. While the low-pressure UCST is classical in that the polymer−solvent interactions become unfavorable upon approach to this phase boundary, the high-pressure UCST branch exhibits anomalous behavior wherein polymer−solvent interactions improve as this phase boundary is approached. Such behavior suggests that the phase separation is entropic in origin and is driven by unfavorable mixing effects.