Phase behavior of diblock copolymers under an external electric field.

摘要

Block copolymers have been shown to exhibit amazing variety of structural phases, which can be tailored for different technological uses. In this thesis we especially focus on the phase behavior of diblock copolymers in the presence of an external electric field. Since electrostatic energies corresponding to different structural phases vary, it is possible to manipulate these microscopic structures by means of the electric field. For the bulk system of copolymer melts, certain spatial symmetries will be broken by the field inevitably for phases characterized by more than one axis of symmetry. The increasing electric field will bring about the instability of these phases over phases characterized by only one axis of symmetry or the disorder one, and eventually lead to a phase transition. We study this transition as a function of the temperature and the polymer architecture within our SCFT framework by incorporating the electrostatic contribution and solving the Maxwell's equation exactly. The results agree with the experimental observations and earlier theoretical studies in some limiting cases.; For copolymer thin films, surface interactions and the electrostatic interaction are competing with each other to determine the alignment of microscopic structures. We specifically apply SCFT calculations to a diblock copolymer film in the cylindrical phase, which is relevant to many applications, bounded by two electrode plates and investigate structures aligned either parallel or perpendicular to the plates. It is shown that for weak surface fields, with an increasing electric field the phase of cylinders parallel to the plates make a direct transition to the phase of cylinders perpendicular to the plates throughout the sample. However, for stronger surface fields, there emerges an intermediate state, in which cylinders are aligned with the electric field only in the interior of the film but terminate near the plates. Instead there is a boundary layer of hexagonal symmetry at the plates composed mostly of monomer favored by the surface interactions. The phase diagrams as a function of the film thickness and the electric field strength are plotted for both strong and weak surface fields.