Studies On Hierarchically Ordered Microstructure Self-Assembled From Copolymer Systems

Hierarchical structures are assemblies of molecules or their aggregates that are intertwined with other phases, which in turn are similarly organized at increasing length levels. Study of hierarchical structures is central to the field of self-assembly. The self-assembly of hierarchical structures can provide excellent candidates of materials designed for multifunctional applications and bring us one-step closer to achieve natural assembly. In this dissertation, we directed towards the study of the structures with hierarchy at nanoscale, i.e., hierarchical microstructure, by using self-consistent field theory and other theoretical methods. The dissertation was organized into two main sections, including hierarchical microstructure in bulk and hierarchical microstructure in dilute solution. In bulk, by solving diffusion equations with fourth-order backward differentiation formula, we successfully discovered a series of microstructures that existed at higher interaction strength. The calculations can not only reproduce the experimental results, but also predict various new hierarchical microstructures. In selective solvents, we proposed a concept of multcore micelles and further confirmed that the linear terpolymers are capable of forming multicore micelles. Furthermore, we discovered a variety of hierarchcal vesicles with hierarchical self-assembly process in graft copolymer solutions, and various hybrid aggregates in the solution of nanoparticle tethered block copolymers.1. Hierarchical Microstructures Self-Assembled from Copolymers in Bulk1) Hierarchical Microstructures Self-Assembled from A(BC)n Multiblock CopolymerA(BC)n multiblock copolymer are capable of self-assembling into various hierarchical microstructures such as lamellae-in-lamella. In the hierarchical microstructures, the large-length-scale structures were formed by the separation between A blocks and (BC)n blocks, while the small-length-scale structures were obtained by the separation between B and C blocks. The hierarchical microstructures can be tuned by changing the A block length. As A block length increases, the A domain can be changed from sphere to cylinder, then to lamella, and finally to matrix, while the small-length-scale structures still remain lamella. In addition, the interaction strength between each blocks have an influence on the hierarchical structures. When the interaction strength between A and B blocks is smaller than that between A and C blocks, the small structures and the large structures in hierarchical microstructures are packed in parallel; in reverse, the hierarchical structure become more complex, where the small structure are arranged perpendicular to the large structures as the interaction strength between B and C blocks is significantly high.2) Hierarchical Microstructures Self-Assembled from Coil-Comb Block CopolymersIn addition to ternary copolymers, binary coil-comb block copolymers are able to self-assemble into hierarchical microstructures including parallel and perpendicular lamellae-in-lamella. The large-length-scale structures are formed by the sparation between coil blocks and comb blocks, whereas the small-length-scale structures are obtained by the self-assembly within the comb blocks. The variation of the coil block length can not only change the large-length-scale structure, but also the small-length-scale structures. Furthermore, the change of interaction strength can lead to a change of the hierarchical microstructures.3) Microphase Saparation in Multigraft Copolymer MeltsBecause of the nature of coil-comb block copolymer, the phase behavior of multigraft copolymers was also investigated. The study of random phase approximation reveals that the spinodals are independent of the junction number and junction position as the junction number and junction position are large enough. In this sense, the graft copolymers perform as the coil-comb block copolymers. Using the self-consistent field theory, phase diagrams of the conventional structures were also mapped out.2. Hierarchical Microstructrues Self-assembled from Copolymers in Dilute Solution1) Self-Assembly of Linear ABC Terpolymers in B-Selective SolventsIn selective solvents, linear ABC terpolymer combining a solvophilic midblock and two mutually imcompatible solvophobic endblocks are capable of self-assembling into hierarchical microstructures like superhelix. The characteristic of these hierarchical microstructures is different from the multicompartment micelles, and therefore a concept of multicore micelles was proposed. The length and solubility of the blocks shows a pronounced effect on the morphology of multicore micelles. The theoretical calculations not only reproduced the experimental results, but also provided a deep insight into the experimental phenomena.2) Self-Assembly of Graft Copolymers in Backbone-Selective SolventsIn addition to ternary copolymers, binary graft copolymer can also self-assemble into hierarchical microstructures in a backbone-selective solvent. The example of hierarchical microstructures includes multilamellar vesicles and large-compound vesicles. The self-consistent field calculations revealed that the hierarchical vesicles are thermodymic stable. The formation of hierarchical vesicles depends on the solubility of blocks as well as the interaction strength between backbone and graft arms. The dissipative particle dynamics simulations demonstrated that the self-assembly process of the hierarchical vesicle is hierarchical, depending on the formation of multiple intermediate structures.3) Self-Assembly of Nanoparticle Tethered Block Copolymers in Dilute SolutionIn addition to flexible copolymers, nanoparticle tethered block copolymers are capable of self-assembling into hierarchical microstructures in selective solvents. The self-assembly of nanoparticle tethered block copolymers involve not only the microphase separation between different blocks, but also the close packing of nanoparticles. Similar to flexible copolymers, the self-assembled microstructures are dependent of the block lengths as well as the incompatibility between polymeric blocks and nanoparticles.The study is helpful for expanding the morphological window of copolymers and understanding the self-assembly mechanism of hierarchical microstructures. The results provided a theoretical support for the experiments and further guide the future experiments. The work is anticipated to facilitate the application of hierarchical materials, such as the design of photovoltaic devices, the development of materials with enhanced mechanical response, and the production of hierarchical organic/inorganic hybrid nanocomposites.