Studies On Hierarchical Microstructres And The Properties Of Various Copolymer Systems

Hierarchical microstructures have received a lot of attentions in the studies of polymeric materials and their applications. Self-assembly is an efficient way to construct these hierarchical structures, especially for preparing the multifunctional materials, biomemictic materials, and electric equipments. With breakthroughs in chemical synthesis techniques, scientists have been capable of preparing a variety of polymer molecules, and carried out a series of studies on their self-assembled hierarchical structures. Based on these works, we employed self-consistent field theory (SCFT) and other theories, such as density functional theory (DFT), to study the hierarchical nanostructures self-assembled from different systems of block polymers in melts. In the presented dissertation, we found many new hierarchical microstructures, such as organic/inorganic hierarchical microstructures that are self-assembled from block copolymer tethered nanoparticles, and hierarchical liquid crystalline microstructures from rod-coil multiblock copolymer self-assembly. Meanwhile, we also studied the mechanical properties of such hierarchical structures.(1) Organic/inorganic hybrid hierarchical microstructures self-assembled form block copolymer tethered nanoparticlesConnecting coil AB diblock copolymer tethers to the P nanoparticles with covalent bond provides an alternative opportunity for creating a new class of macromolecules, which can produce novel complex self-assembled microstructures. These microstructures are distinct from those self-assembled from conventional AB diblock copolymers. Based on the different chemical compatibility of nanoparticles with flexible chains, two cases were studied:one is where the particles are chemically neutral to both A and B blocks, and the other is where the particles are unfavorable to neither of the two blocks. For neutral particles, the P particles are localized in B block domains, and the size of particles can influence the phase behavior. For unfavorable particles, the ABP molecules microphase separate to form distinct ordered structures. Hierarchical structures, such as cylinders with cylinders at the interfaces and lamellae with cylinders at the interfaces, were observed. These resulting hierarchical structures are mainly determined by two parameters:A block volume fraction and particle size. On the basis of the calculation results, phase diagrams were constructed. (2) Hierarchical liquid crystalline microstructures predicted from rod-coil multiblock copolymer self-assemblyRigid rod-like molecules can self-organize into well-defined microstructures while retaining the mobility on molecular level like fluid. It is the phase behavior of liquid crystal. When the liquid crystalline blocks are covalently connected to the coil blocks, the liquid crystalline polymers can be prepared. By designing molecular architectures (e.g., molecular shape or topology), linear Cx(RC)nRy multiblock copolymers are able to self-assemble into hierarchical liquid crystalline microstructures. In this system, due to the long rod-like Ry end block (x<<y), such Cx(RC)nRy multiblock copolymers can self-assemble into hierarchical isotropic-in-smectic and smectic- in-smectic phases at moderator interaction and fewer repeating unit n=1. It was discovered that the hierarchical smectic structures exhibit not only double periodicity in overall structure but also double orientational orders of rods. Additionally, the isotropic-in-amorphous structures can be tailed by tuning the relative length of coil blocks (x>> y). In such microstructure, there is only single type of liquid crystalline phase in the hierarchical structures.(3) Macroscopic mechanical response to hierarchical microstructuresThe linear flexible A(BC)n multiblock copolymers can self-assemble into a series of hierarchical structure-in-structures, such as lamellae-in-lamella. The mechanical properties of materials with such hierarchical microstructures can be characterized by calculating the extension and shear moduli as well as the Young's modulus. A remarkable improvement of elastic modulus in the process of morphology transformation from lamellar to lamellae-in-lamellar structure was discovered. From the physical analysis of each different blocks, it was found that internal energy and conformational entropy of BC blocks play a predominate role in improving the elastic moduli in the hierarchical lamellae-in-lamellar structures. Moreover, the mechanical response of hierarchical lamella is also shown to be dependent on the number of small-length-scale structures. Our findings are in agreement with the recent experimental observations and also predict some phenomenons that have not been obtained in the experiments. Therefore, it yields guidelines for designing hierarchical nanostructured materials with improved properties.