A finite volume based implementation of the binary Cahn–Hilliard equation was implemented using an open source library, OpenFOAM. This was used to investigate the development of droplet and co-continuous binary polymer microstructures. It was shown that the initial concentrations of each phase define the final form of the resultant microstructure, either droplet, transition or co-continuous. Furthermore, the mechanical deformation response of the representative microstructures were investigated under both uniaxial and triaxial loading conditions. The elastic response of these microstructures were then compared to a classic representative microstructure based on a face centred cubic arrangement of spheres with similar volume fractions of each phase. It was found that the numerically predicted composite Young’s modulus closely followed the upper Hashin–Shtrikman bound for both co-continuous and classical structures, while significant deviations from analytical composite theory were noted for the calculated values of Poisson’s ratio. The yield behaviour of the composite microstructures was also found to vary between the co-continuous microstructures and the representative microstructure, with a more gradual onset of plastic deformation noted for the co-continuous structures. The modelling approach presented allows for the future investigation of binary composite systems with tuneable material properties.