Dissipative particle dynamics simulation of diblock co-polymer microphase separation

The Dissipative Particle Dynamics (DPD) simulation method describes the statistical mechanics of soft spheres, including hydrodynamic interactions. Each sphere represents many atoms. When they are connected by harmonic springs, the simulation model gives a mesoscopic representation of polymers. The connection between this model and the Flory-Huggins theory is established, and the method is applied to meso-phase formation of diblock co-polymer melts, by introducing two types of soft spheres, A and B. When the melt is cooled down A-blocks and B-blocks tend to form separate phases, but because of the chemical connectivity macroscopic phase separation is prevented. instead, the melts forms a micro-phase separated structure. We find lamellar, perforated lamellar, hexagonal and micellar phases as the polymer becomes more asymmetric, in line with experiments and mean-field theory. The strength of the DPD method lies in its capability to predict the dynamical pathway along which the melt finds its equilibrium structure after a temperature quench. The stable structure emerges via a non-trivial pathway, where a series of metastable phases can be formed before equilibrium is reached. It is concluded that hydrodynamic interactions are very important for the evolution of the mesophases.