The Effect of Orientation in Double-Gyroid Thin Films*

The effects of thin-film confinement on lamellar and cylindrical phases in block polymers are well understood, but this understanding cannot be easily extended to the double-gyroid (DG) phase due to its three-dimensionally bicontinuous domains. For instance, DG thin films almost always form with the (422) lattice plane located at the top surface of the film, but the reasons for this preferred termination plane are unclear. To better understand the effect of thin-film confinement on the DG phase, we use self-consistent field theory to compute the equilibrium morphology and energetics of DG thin films confined between non-preferential walls. By comparing films with various termination planes, we observed a common feature: the DG morphology is distorted relative to the bulk in a manner that results in a wetting angle of around 90° between the A/B interface and the wall. The amount of distortion required to accommodate this preferred angle varies depending on the termination plane; we show that the (422) lattice plane requires relatively little distortion compared to other alternatives, thus explaining its stability. We also provide an explanation for this preferred wetting angle and discuss its implications for the phase behavior of DG thin films.