Self-Assembly Of Diblock Copolymer Thin Films Confined In The Saw-Toothed Substrate

Block copolymer, a special polymeride obtained from two or more polymer chain segments of distinct properties via covalent bond, can self-assembly to produce high ordered patterns of nano-scale and rich forms, due to some diversity of the monomer types, the topological structures, and the block compositions. Thus, they attract wide attentions from large numbers of researchers in chemistry, physics, and materials. In recent years, it has gradually been a hot topic that manufacturing some specific substrates with chemical or physical graphics to induce the self-assembly behavior of the block copolymers (BCPs).In this dissertation, applying the real-space self-consistent mean field theory to the linear diblock copolymer thin films, we will set the Gaussian chain model as the starting point to investigate the directed self-assembly (DSA) of it as well as explore the influences of the inherent characteristics of the saw-toothed substrate on the order-to-order transitions (OOTs) between the given morphologies. By constructing 2-dimensional morphological phase diagram between wide-range lateral periodicity of the substrate and film thickness of the diblock copolymers, we not only demonstrated the fact that it is going to form the structures with the microdomains perpendicular to the substrate as the film thickness is thin enough, but also observed some novel hierarchical structures when the film thickness is thick enough. For instance, it will eventually be the bilayer cylindrical structures with the microdomain orientation in the upper layer orthogonal to the microdomain orientation at the bottom layer, of course, which could be a stable morphology over a very wide range of the film thickness. Then, we also analyzed the order-to-order transitions in details between specific morphologies under the condition of certain fixed lateral periodicity of the substrate, elucidating the importance of the conformational entropic free energy of the polymer chains further. We took the effect of the energetic preferences of the substrate at the bottom surface for the minority component segment into consideration so that we can obtain a much more comprehensive understanding over the impact of the substrate confinement on the OOTs. When the interaction between the substrate and the block copolymers is strongly repulsive, we found the transition morphology between the single-layer cylinder microdomain whose orientation is parallel to the y-axis direction of the substrate and the bilayer cylinder microdomains whose orientations are both also parallel to the y-axis direction of the substrate easily tends to be a combination structure with the sphere microdomain at the bottom layer and the cylinder microdomain in the upper layer whose orientation is parallel to the x-axis direction of the substrate. However, once this interaction turns strongly attractive, the transition morphology between them would like to be the bilayer cylinder microdomains, of which both the upper layer structure and the bottom layer structure are cylinders whose orientations are parallel to the x-axis direction of the substrate instead. Besides that, we have discussed the effect of the characteristic topology (or the tilted angles) of the saw-toothed substrate on the OOTs as well. The results suggest that the greater the angle is, the more easily the combination structure of sphere and cylinder takes shape. By comparing with the lateral periodicity of the substrate, we also found the critical value of the initial film thickness equals to 1.10 Rg0. It is obvious that we can efficiently tailor the DS A and the OOTs behavior of the BCPs by the saw-toothed substrate.