Neutron scattering from pressurized polyolefin blends near the limits of metastability

Small-angle neutron scattering experiments were conducted on a series of of f-critical binary polymethylbutylene/polyethylbutylene (PMB/PEB) blends ove r a wide range of blend compositions, component molecular masses, temperatu res, and pressures. The blends become more immiscible with either decreasin g temperature or increasing pressure. A simple extension of the Flory-Huggi ns theory that accounts for finite volume changes of mixing (DeltaV) is pre sented. Our extension demonstrates the validity of the usual mean-field the ory of scattering from polymer mixtures based on the random phase approxima tion (RPA) at elevated pressures. We use this framework to analyze the temp erature and pressure dependence of the small-angle neutron scattering profi les obtained from binary PMB/PEB blends. We propose that the volume change of mixing is a linear response to the repulsive interactions between monome rs. We demonstrate that off-critical PMB/PEB blends can be undercooled or s uperpressurized deep into the metastable two-phase region (e.g., up to 50 d egreesC undercooling) without detectable signs of phase separation. The chi parameters and the statistical segment lengths obtained by fitting the dat a obtained in the metastable region are within experimental error of those determined from stable, single-phase PMB/PEB blends well-removed from a pha se boundary. This indicates that the concentration fluctuations in the meta stable region of the phase diagram have a mean-field character similar to t hose in stable, single-phase blends that have been extensively characterize d by the RPA-based theory.