Phase behavior of disk-coil molecules : from bulk thermodynamics to blends with block copolymers

Abstract

In this thesis, we explore the phase behavior of discotic molecules in various circumstances. We first study the thermodynamics of disk-coil molecules. The system shows rich phase behavior as a function of the relative attractive strength of coils ([lambda]), the stacking interaction strength of disks ([mu]), the number of coarse-grained monomers of the coil (Nc), and the reduced temperature (T*). At high T*, a disordered phase is dominant. At intermediate T*, lamellar, perforated lamellar, and cylinder phases appear as y and Nc are increased. At low T*, disks crystallize into ordered lamellar, ordered perforated lamellar, and ordered cylinder phases. We find that the confinement imposed on the disks by the attached coils strongly contributes to the ordered stacking of the disks. In particular, the ordered cylinder phase contains highly ordered disks stacked in parallel due to the cylindrical confinement of the coils that restricts the system to a single degree of freedom associated with the director vector of the disks. Our results are important for understanding the self-assembly of supramolecular structures of disk-coil molecules that are ubiquitous in nature, such as chlorophyll molecules. Having established the importance of confinement on the phase behavior of discotic molecules, we next study blends of discotic molecules and block copolymers (BCPs) using self-consistent field theoretic simulations. In particular we explore systems containing a single sphere, rod, or discotic molecule confined within a BCP defect and systems containing multiple discotic molecules confined within BCP cylinders. In the former case, the sphere, rod, and discotic molecules are all trapped in the defect center where the cylinders of the surrounding BCPs make a junction. The director vector of the rod molecule aligns with the axial direction of one of the cylinders, while the director vector of the discotic molecule aligns perpendicular to the axes of all the cylinders. This preferential orientation is induced by the minimized stretching energy of the BCPs for these configurations. For the system with multiple discotic molecules confined within the BCP cylinders, all director vectors are aligned with the axial direction of the cylinder when the density of disks is high to minimize both the stretching energy of the BCPs and the polymer-mediated potential between the disks. These results provide design principles for next generation optoelectronic devices based on blends of discotic molecules and BCPs.