Patterns produced in water-oil-block copolymer systems
Patterns produced in water-oil-block copolymer systems lead image
Amphiphilic block copolymers act like ordinary surfactants when their molecular weight is low but produce patterned domains at higher molecular weights. To investigate these patterns, Sugimura and Ohno carried out large-scale lattice Monte Carlo simulations of a ternary system consisting of water, oil and ABA triblock copolymers with hydrophilic A and hydrophobic B blocks.
The MC simulations were performed on a simple 51 by 51 by 51 cubic lattice with periodic boundary conditions in the x and y directions along with fixed boundaries in the z direction. The latter simulated a system confined between two parallel plates.
All lattice cells were filled with water, oil or polymer, with interaction energies imposed between particular cells. The Metropolis algorithm was used to generate new configurations from an arbitrary initial condition. Thermal equilibrium was achieved in about 500,000 MC steps.
The simulations allowed the investigators to generate a phase diagram showing the behavior of the system at different concentrations of copolymer, water, and oil. With no copolymer, water and oil are completely immiscible and form two separate phases, but when the copolymer concentration reaches 35%, phase separation into microdomain structures is observed.
A variety of patterns were observed including micelles, lamellae, a bicontinuous structure, and a layer-passage structure, where two water layers parallel to the wall have a vertical passage connecting them.
“We showed that the lattice Monte Carlo model can reproduce equilibrium patterns in this complex ternary system,” said author Natsuko Sugimura. “The formation of ordered patterns is mainly induced by confinement of the system by parallel walls.”
Future investigations will consider rheological properties such as viscosity and elasticity of these systems.
Source: “A Monte Carlo simulation of water+oil+ABA block copolymer ternary system I. Patterns in thermal equilibrium,” by Natsuko Sugimura and Kaoru Ohno. AIP Advances (2021) The article can be accessed at https://doi.org/10.1063/5.0034063 .
