Effect of mobile ions on the electric field needed to orient charged diblock copolymer thin films

Abstract

We examine the behavior of lamellar phases of charged/neutral diblock copolymer thin films containing mobile ions in the presence of an external electric field. We employ self-consistent field theory and focus on the aligning effect of the electric field on the lamellae. Of particular interest are the effects of the mobile ions on the critical field, the value required to reorient the lamellae from the parallel configuration favored by the surface interaction to the perpendicular orientation favored by the field. We find that the critical field depends strongly on whether the neutral or charged species is favored by the substrates. In the case in which the neutral species is favored, the addition of charges decreases the critical electric field significantly. The effect is greater when the mobile ions are confined to the charged lamellae. In contrast, when the charged species is favored by the substrate, the addition of mobile ions stabilizes the parallel configuration and thus results in an increase in the critical electric field. The presence of ions in the system introduces a new mixed phase in addition to those reported previously.

Orienting Thin Films of Lamellar Block Copolymer: The Combined Effect of Mobile Ions and Electric Field

“…Our qualitative explanation is simple, although it does not retain the more complex coupling between the various parameters. In addition, it is in agreement with previous theoretical works [36][37][38]. As the charge separation for the L A is about the film thickness L minus half a periodicity, while this separation spans the entire film thickness, L, for the L ⊥ , it leads to a difference in polarization between the two phases; roughly, an effective charge multiplied by d 0 /2.…”

“…More recently, Dehghan et al 36 revisited the effect of added ions in a BCP film. They considered the intermediate-segregation regime, χN = 20, and utilized SCFT in which the polymer segment profiles and electric potential are statistical variables calculated self-consistently.…”

“…Motivated by these experiments, several theoretical studies − focused on the orientation transition of BCP thin films subject to a constant voltage imposed by a voltage power source. The parameters used to model the film (besides χ, N , and f ) are the surface energies of the A and B blocks with the substrate, the film thickness L , and the corresponding dielectric constants ε A and ε B of the two blocks.…”

“…The parameters used to model the film (besides χ, N , and f ) are the surface energies of the A and B blocks with the substrate, the film thickness L , and the corresponding dielectric constants ε A and ε B of the two blocks. The theoretical methods vary from a simple capacitor model of parallel and series stacking of alternating A and B dielectric layers, applicable in the strong-segregation limit of large χ N , , to Ginzburg–Landau (GL) expansions for the free energy − in the weak-segregation limit, close to the order–disorder temperature (ODT), χ c N , and to self-consistent field theories (SCFT). − …”

Instability and reorientation of block copolymer microstructure by imposed electric fields

“…There are various extensions of this work which could address other phenomenon related to electric field alignment. For example, the alignment of lamellar microstructures with electric fields may be enhanced in the presence of mobile ions [17,40], or by addition of nanoparticles [41]. Morphological transitions, such as those between spherical, cylindrical and gyroid BCP phases, may also be initiated by applying external fields [7,28].…”