Observing Nucleation Close to the Binodal by Perturbing Metastable Polymer Blends

Abstract

The initial stages of liquid−liquid phase separation in an off-critical binary polymer blend were studied by time-resolved small-angle neutron scattering (SANS). Phase separation is triggered by quenching homogeneous blends from the single-phase to the two-phase region of the phase diagram. Our objective was to determine the size of the critical nucleus, Rc, over a wide range of quench depths, κ. We present results obtained from two-step quenches. In the first step, the blend is subjected to a deep quench, κ‘, for a brief period of time. This is followed by a reduction in quench depth to κ. One can view the first quench as a perturbation. We demonstrate that under certain conditions the only effect of the perturbation is to hasten nucleation kinetics at κ. This allows the determination of Rc at low quench depths near the binodal, where nucleation barriers preclude the formation of viable nuclei after single-step quenches on experimental time scales. Our experiments at large κ provide no evidence for a cha...