Study on Arrangement of Diblock Copolymer Microdomains during Solvent Evaporation and in Hemi-spherically Confined System

Abstract

Block copolymer nanostructures are one of the cutting edge candidates to create long range ordered microstructures. AB block copolymers can be assembled into various nanoscale morphologies such as lamella, cylinder, sphere and gyroid depending on the A block composition f and the interaction strength between segments represented by the Flory-Huggins χ parameter. There have been intense efforts to use the block copolymer morphologies as scaffolds to position nanomaterials into ordered arrays for applications such as high density memory device, metamaterial, and photonic band gap materials. Solvent evaporation and annealing is one of the most common methods to create the block copolymer structure in thin films and enhance its alignment. However, the microdomains usually contain some uncontrolled defects and lack of long-range order due to entropic fluctuation and incomplete annealing. The optimum structural property required in the academic and industrial world can be outlined by the following three keywords: Large area, low defects, and short period. The detailed discussion for these three trendy keywords in this rapidly growing field is included in chapter I.

In chapter II, I introduce the main theoretical tool in this research, self-consistent field theory (SCFT), and its application. The most important part in SCFT method is the calculation of partition function which predicts statistical behavior of polymers by solving the modified diffusion equation with potential field w(r). The commonly used numerical methods to solve such equations are real space method, spectral method and pseudospectral method. In this study, I use the real space method to investigate the microstructure evolution of the diblock copolymer thin films in solvent. I also use the pseudospectral method to obtain the morphology of the block copolymers formed in the confined system with two controlled interfaces.

In the third chapter, I study the nanostructure evolution of diblock copolymer thin films in solvents. I use SCFT to study the equilibrium block copolymer phases and the quasi-equilibrium dynamic path of the morphology evolution during the slightly selective solvent vaporization process. During the evaporation process, I introduce a small noise field using random function to create the initial block copolymer morphologies. Then, the thickness of block copolymer film h is reduced as the global solvents volume fraction ψ decreases. During this process, I use the previous morphologies as an input for the morphology of the thinner film, which means I model the quasi-static morphology evolution of the thin film. As the solvent evaporates, the film thickness decreases and the number of defects also decrease. This method allows us to track down the detailed mechanism of the defect elimination. The onset of order-disorder transition and the observed morphology matches very well with the theoretical prediction.

Confinement of diblock copolymers under certain geometries can offer new methods to develop unique morphologies which have never been known before in bulk or thin film. In chapter IV, I study various block copolymer morphologies in hemispherical and ellipsoidal shape confinements and compare the results with experiments. In the experiment, PS-PMMA block copolymers are physically confined in hemispherical cavities prepared by anodic aluminum oxide template. The cavities have two controlled interfaces, one of them is the top surface of the cavity covered with preferential films and the other is the surface of cavity wall which interacts preferentially or randomly depending on the coating of the cavity wall. Our theoretical modeling uses SCFT which calculates the mean field density distribution of AB block copolymers in this confined geometry. The key parameters for the morphology determination are the size and shape of the container and the surface tension between components. For example, when the container wall is coated with PS polymers and neutral cover film is used, onion-shape lamellar phases with PS at the bottom is observed rather than the parallel lamellar phases. It is also found that preferential cover film promotes the alignment of domains. Our versatile method also allows us to model ellipsoid-shaped confinements, and other interesting morphologies are found depending on the eccentricity of the ellipsoid.