We examine the alignment of thin film diblock copolymers subject to a perpendicular electric field. Two regimes are considered separately: weak segregation and strong segregation. For weakly segregated blocks and below a critical value of the field, Ec, surface interactions stabilize stacking of lamellae in a direction parallel to the surfaces. Above the critical field, a first-order phase transition occurs when lamellae in a direction perpendicular to the confining surfaces (and parallel to the field) become stable. The film morphology is then a superposition of parallel and perpendicular lamellae. In contrast to Helfrich-Hurault instability for smectic liquid crystals, the mode that gets critical first has the natural lamellar periodicity. In addition, undulations of adjacent inter-material dividing surfaces are out-of-phase with each other. For diblock copolymers in the strong segregation regime, we find two critical fields E1 and E2 > E1. As the field is increased from zero above E1, the region in the middle of the film develops an orientation perpendicular to the walls, while the surface regions still have parallel lamellae. When the field is increased above E2 the perpendicular alignment spans the whole film. In another range of parameters, the transition from parallel to perpendicular orientation is direct. a perfect alignment of the lamellae, several techniques have been used. In the bulk, mechanical shear proved to be a successful technique. Alignment by application of an electric field [7][8][9][10] is also possible, but for macroscopic samples it requires high voltage difference between the two bounding electrodes. Nevertheless, this technique is especially suitable for thin films, because the thickness involved makes the required large fields (typically 10 − 30 V/µm) accessible.We consider in this paper thin films of lamellar diblock copolymers, under the influence of a perpendicular electric field. Initially, the lamellae are parallel to the confining surfaces, because of preferential short-range interactions with the surfaces. In section II we consider diblock copolymers in the weak segregation regime. We show in section III that electric field applied perpendicular to the surfaces can cause the melt to transform from a parallel to a perpendicular orientation through a first-order phase transition. The critical field E c for this transition is caused by a competition between the electric field and surface interactions. In section IV we investigate the thin-film alignment for diblocks in the strong segregation regime. In this regime, the surface correlations are finite, and thus the range of parallel ordering induced by the surfaces is finite as well. We give the transitions between parallel, perpendicular and mixed lamellae in terms of the system parameters, using a phenomenological model. In both weak and strong segregation regimes, large distortions are present in the copolymer film, and these could be observed in experiments. II. WEAKLY SEGREGATED LAMELLAEThe copolymer order parameter φ(r) = φ A ...

We argue that the presence of dissociated ions in block copolymers under electric fields can induce strong morphological changes and even lead to phase transitions. We investigate, in particular, diblock copolymers in the body centered cubic (bcc) phase. In pure dielectric materials (no free charges), a dielectric breakdown is expected to occur for large enough electric fields, preempting any structural phase transition. On the other hand, dissociated ions are predicted to induce a phase transition to a hexagonal array of cylinders, at fields of about 10 V/microm or even lower. The strength of this mechanism can be tuned by controlling the amount of free ions present.

The structure and thermodynamic state of a system changes under the influence of external electric fields. Neutral systems are characterized by their dielectric constant ε, while charged ones also by their charge distribution. In this Colloquium several phenomena occurring in soft-matter systems in spatially uniform and nonuniform fields are surveyed and the role of the conductivity σ and the linear or nonlinear dependency of ε on composition are identified. Uniform electric fields are responsible for elongation of droplets, for destabilization of interfaces between two liquids, and for mixing effects in liquid mixtures. Electric fields, when acting on phases with mesoscopic order, also give rise to block copolymer orientation, to destabilization of polymer-polymer interfaces, and to order-order phase transitions. The role of linear and nonlinear dependences of ε on composition will be elucidated in these systems. In addition to the dielectric anisotropy, existence of a finite conductivity leads to appearance of large stresses when these systems are subject to external fields and usually to a reduction in the voltages required for the instabilities or phase transitions to occur. Finally, phase transitions which occur in nonuniform fields are described and emphasis on the importance of ε and σ is given.

Phase separation in liquid mixtures is mainly controlled by temperature and pressure, but can also be influenced by gravitational, magnetic or electric fields. However, the weak coupling between such fields and concentration fluctuations limits this effect to extreme conditions. For example, mixing induced by uniform electric fields is detectable only at temperatures that are within a few hundredths of degree or less of the phase transition temperature of the system being studied. Here we predict and demonstrate that electric fields can control the phase separation behaviour of mixtures of simple liquids under more practical conditions, provided that the fields are non-uniform. By applying a voltage of 100 V across unevenly spaced electrodes about 50 micro m apart, we can reversibly induce the demixing of paraffin and silicone oil at 1 K above the phase transition temperature of the mixture; when the field gradients are turned off, the mixture becomes homogeneous again. This direct control over phase separation behaviour depends on field intensity, with the electrode geometry determining the length-scale of the effect. We expect that this phenomenon will find a number of nanotechnological applications, particularly as it benefits from field gradients near small conducting objects.