Phase behavior of a binary fluid mixture of quadrupolar molecules

Miscibility and exchange chemical potential of ring polymers in symmetric ring-ring blends

Generally, differences of polymer topologies may affect polymer miscibility even with the same repeated units. In this study, the topological effect of ring polymers on miscibility was investigated by comparing symmetric ring-ring and linear-linear polymer blends. To elucidate the topological effect of ring polymers on mixing free energy, the exchange chemical potential of binary blends was numerically evaluated as a function of composition ϕ by performing semi-grand canonical Monte Carlo and molecular dynamics simulations of a bead-spring model. For ring-ring blends, an effective miscibility parameter was evaluated by comparing the exchange chemical potential with that of the Flory-Huggins model for linear-linear polymer blends. It was confirmed that in the mixed states satisfying χN > 0, ring-ring blends are more miscible and stable than the linear-linear blends with the same molecular weight. Furthermore, we investigated finite molecular weight dependence on the miscibility parameter, which reflected the statistical probability of interchain interactions in the blends. The simulation results revealed that the molecular weight dependence on the miscibility parameter was smaller in ring-ring blends. The effect of the ring polymers on miscibility was verified to be consistent with the change in the interchain radial distribution function. In ring-ring blends, it was indicated that the topology affected miscibility by reducing the effect of the direct interaction between the components of the blends.

Speeding up the study of diluted dipolar systems

We study the regimes of a diluted dipolar system through Monte Carlo numerical simulations in the NVT ensemble. To accelerate the dynamics, several approximations and speed-up algorithms are proposed and tested. In particular, it turns out that "cluster move Monte Carlo" algorithm speeds-up to two decades faster than the traditional Monte Carlo, depending on temperature and density. We find simple-fluid, chain-fluid, ring-fluid, gel, and antiparallel columnar regimes, which are studied and characterized through positional, orientational, and thermodynamical observables.

Time-Resolved Structured Illumination Microscopy for Phase Separation Dynamics of Water and 2-Butoxyethanol Mixtures: Interpretation of "Early Stage" Involving Micelle-Like Structures

Phase separation dynamics of a water/2-butoxyethanol (2BE) mixture was studied with newly developed time-resolved structured illumination microscopy (SIM). Interestingly, an employed hydrophobic fluorescent probe for SIM showed spectral shifts up to 500 ns after a laser-induced temperature jump, which suggests 2BE micellar-like aggregates become more hydrophobic at the initial stage of phase separation. This hydrophobic environment in 2BE aggregates, probably due to the ejection of water molecules, continued up to at least 10 μs. Time-resolved SIM and previously reported light scattering data clearly showed that the size of a periodic structure remained constant (ca. 300 nm) from 3 to 10 μs, and then the growth of periodic structures having the self-similarity started. We think that the former and the latter processes correspond to "early stage" (concentration growth) and "late stage" (size growth), respectively, in phase separation dynamics. Here we suggest that, in the early stage, the entity to bear 2BE phase be water-poor 2BE aggregates, and the number density of these aggregates would simply increase in time.