Manipulating the kinetics and mechanism of phase separation in dynamically asymmetric LCST blends by nanoparticles.

The addition of nanoparticles in dynamically asymmetric LCST blends is used to induce preferred phase-separating morphology by tuning the dynamic asymmetry, and to control the kinetics of phase separation by slowing down (or even arresting) the domain growth. For this purpose, we used hydrophobic and hydrophilic fumed silica, which self-assemble during phase separation into the bulk of the slow (PS-rich) and fast (PVME-rich) dynamic phases, respectively. Both types of nanoparticles slow down considerably nucleation and growth (NG), spinodal decomposition (SD), and viscoelastic phase separation (VPS) at volume fractions as low as 0.5%. Remarkably, beyond a critical volume fraction of hydrophobic nanosilica thermodynamically controlled phase separation mechanisms (NG and SD) change to the VPS mechanism due to enhanced dynamic asymmetry. However, in the presence of hydrophilic nanosilica dynamic asymmetry decreases and beyond a critical particle volume fraction a transition from the VPS to the SD mechanism is observed. Phase separation is arrested at 2% nanoparticle loading, and VPS percolating networks as well as co-continuous SD structures are completely stabilized by hydrophobic silica or hydrophilic silica, respectively. Electron microscopy images confirm that double percolated structures are induced in the presence of 2 vol% of either hydrophobic or hydrophilic nanoparticles.