The morphology and phase behavior of diblock polymer double-gyroid thin films

The double-gyroid mesophase (DG) has been experimentally observed in diblock polymer thin films, primarily with the (211) plane oriented parallel with the substrate. However, the thermodynamics of these thin films are poorly understood, largely due to a lack of computational research seeking to understand their phase behavior and morphology. Using self-consistent field theory, we modelled DG thin films and studied the resulting 3D composition profiles to identify the phenomena that drive the preference for certain orientations of DG in a thin film, and to predict the conditions at which DG could be the globally stable mesophase. Results show that, in the absence of preferential wetting by either block at the upper or lower interface of the film, the favorability of any given orientation of DG depends on the extent to which the polymers that are in contact with the substrate are able to lie flat with minimal excess chain stretching (which occurs when the A/B interface is normal to the substrate at contact). We predict the stable DG orientation as a function of film thickness, block fraction, and preferential wetting in the DG-forming region of phase space, and identify the factors driving their stability by comparing the relative chain stretching energies of each orientation.