Phase behavior of diblock copolymer in thin films and membranes
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Abstract
Thin films of block copolymers have attracted intensive research interest because these materials can self-assemble into a variety of well-ordered nanostructures. These systems have been employed in numerous applications in nanotechnology, including photonic crystals, magnetic storage media, etc.. However, one of the problems with self-assembled block copolymers is the lack of long-range order due to the presence of topological defects.
In this thesis, we perform a theoretical study to investigate the morphologies in thin diblock copolymer films using self-consistent field theory (SCFT).
We focus on the stability and orientation of the cylindrical phase formed by AB diblock copolymers when confined within substrates of different geometries.
Firstly, we apply the SCFT method to study AB diblock copolymers confined between two planar surfaces. We determine minima in the free energy landscape and the optimum thickness of multilayer cylindrical films to analyze the stability of films. For the thin film systems investigated in this work, we show that the global equilibrium state of these films is the one where a monolayer film coexists with the islands of thicker multilayer films. Additionally, the coupling mechanism between the bottom and upper layers is investigated as well.
We show that the first aligned layer can help to order the upper layers, and then to propagate order in thicker thin films.
In general, when the surface fields attract the majority block sufficiently strongly, the cylindrical phase formed by the diblock copolymers will orientparallel to the film plane (C_{||}). However, their orientation can switch to perpendicular (C_{⊥}) or new morphologies can be found under certain conditions. Our SCFT results indicate that there is an orientational phase transition, C_{||} -> C_{C⊥}, when the film thickness is reduced below the natural size of the monolayer of parallel cylinders. Furthermore, we also find the formation of the perforated lamellae (PL) and lamella (L) phases in the region in which the thickness is about half the optimal thickness of a single layer of parallel cylinders. When the thickness is very small, the films become unstable and dewet from the substrate.
We also study the diblock copolymers confined in curved substrate (coaxial cylindrical surfaces). We consider two types of model systems: free-standing membranes and curved supported thin films. We calculate the bending constant of the free-standing membranes. The results imply that the local orientation of patterns has a strong coupling with the geometry of confinement. Namely, the block copolymer cylinders tend to align along the direction of curvature at
high curvatures. Moreover, at low curvatures, there is a transition C_{⊥} -> C_{||} in supported films, which is absent in the free-standing membranes.
Therefore, the mean curvature not only acts as a guiding field to produce well ordered patterns but also generates defects at specific regions in space. The stability of the thin films against curvature-induced dewetting is also analyzed.