Orienting thin films of lamellar block copolymer: the combined effect of mobile ions and electric field
We study thin films of A/B diblock copolymer in a lamellar phase confined between two parallel plates (electrodes) that impose a constant voltage across the film. The weak-segregation limit is explored via a Ginzburg-Landau-like free-energy expansion. We focus on the relative stability of parallel and perpendicular orientations of the lamellar phase, and how they are affected by variation of four experimental controllable parameters: addition of free ions, the difference in ionic solubilities between the A and B blocks, the dielectric contrast between the A/B blocks, and the preferential interaction energy of the plates with the blocks. It is found that, in general, the addition of ions lowers the critical voltage needed to reorient the lamellae from being parallel to the plates, to being perpendicular to them. The largest reduction in critical voltage is obtained when the ions are preferentially solubilized in the block that is not preferred by the plates. This reduction is even further enhanced when the dielectric constant of this block has the higher value. These predictions are all subject to experimental verification.