Orienting thin films of lamellar block copolymer: the combined effect of mobile ions and electric field

  Bin Zheng  ,  Xingkun Man  ,  Zhong-Can Ou-Yang  ,  M. Schick  ,  David Andelman  
Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Tel Aviv, Israel
CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China

We study thin films of A/B diblock copolymer in a lamellar phase confined between two parallel plates (electro- des) 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 the following four experimental controllable parameters: addition of free ions, the difference in ionic solubilities between the A and B blocks, their dielectric contrast, 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. A large 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 test.