A Monte Carlo simulation of water + oil + ABA block copolymer ternary system. I. Patterns in thermal equilibrium

Abstract

We carried out a large-scale lattice Monte Carlo simulation of a ternary system comprising water/oil/ABA triblock copolymers with hydrophilic A and hydrophobic B blocks to investigate the dependence of the pattern on the volume fraction of the components. The attractive interaction energies due to hydrogen bonding between the hydrophilic A block and water and between waters were defined as the relative values to the van der Waals bonding between water and oil. We created a phase diagram at thermal equilibrium. At low polymer concentrations, the water and oil were perfectly immiscible. As the polymer concentration increased, the pattern changed from one characterized by macro-phase separation to one characterized by micro-domain structures. At intermediate polymer concentrations, the pattern changed from water-in-oil micellar to lamellar to layer–passage structures as the water concentration increased. A bicontinuous pattern emerged when the concentrations of the polymer and water were the greatest. At low water concentrations, micelles formed at various copolymer concentrations. These pattern changes were approximately and qualitatively similar to those of pre-existing experimental phase diagrams of water/oil/ABA block copolymers. In our model, the lamellar structure comprised a lipid bilayer, and the area where ordered structures such as lamella appeared was limited to the narrow area of the phase diagram. The formation of ordered structures was due to the confinement of the system by the parallel walls and depended on the choice of a suitable ratio of the three components and the appropriate polymer length to produce ordered patterns commensurate with the distance between the walls.