Studies On Microphase Separation Of Copolymer Mixture

Manufacture of nanostructral materials controllably and tunably has been widely applicated in the fields of new energy materials, biomedical materials, and special materials. Mixing is an easy and efficient way to obtain nanostructures. It can not only overcome the diffculty of multi-component copolymeric synthesis, but control the self-assembly of these mixtures easily as well. Taking the advantage of controlling and tuning, the study of copolymer mixtures has become a research topic in macromolecular science. In this dissertation, we studied the self-assembly behavior of copolymer mixtures by using self-consistent field theory (SCFT). It includes the self-assembly of diblock copolymer mixtures in dilute solution, the self-assembly of supramolecular mixtures in dilute solution and in bluk, and the self-assembly of graft copolymers in concentrated solution. The morphologies of the microstructures can be controlled by tuning many parameters, such as the mixture ratio of polymers. Furthermore, it becomes more stable by introducing supramolecular non-covalent bonds into the mixtures, and a series of hierarchically ordered microstructures were observed. Besides, in concentrated solution, graft copolymers can self-assemble into various ordered structures, and yield a physical gel with local crosslinked structure or gobal crosslined structure. The dissertation includes four parts, which are presented as follows:(1) Self-assembly behavior of diblock copolymer mixtures in dilute solutionIn selective solvent, the AB/AC amphiphilic diblock copolymer mixtures can self-assemble into a series of hybrid aggregates, such as. micelles, rings, quasi-vesicles, and vesicles. The AB and AC copolymers have a common block A but different blocks B and C. Two cases were studied:one is that the A block is hydrophobic, and the other is that the A block is hydrophilic. The morphologies of the cooperatively self-assembling aggregates were found to be dependent on the AB/AC mixture ratio and the interaction between the B and C blocks. For the AB/AC copolymers with different hydrophobic and hydrophilic lengths, chain segregation was found in the formed hybrid aggregates. Based on the obtained calculation results, phase diagrams as functions of the mixture ratio and interaction between B and C blocks were constructed. It symterically studied the self-assembly of AB/AC copolymer mixtures, and agreed with the experimental results well. (2) Self-assembly of supramolecular mixtures with reversible bonds in selective solventPolymer mixtures in dilute solution can become more stbale by the introduction of the macromolecular bonds. The amphiphilic AB diblock copolymer and hydrophobic C homopolymer mixtures with supramolecular reversible bonding interactions can also self-assemble into a variety of micelles, such as core-shell-corona micelles, segment wormlike micelles, etc. The supramolecular reversible bond was formed between the free end of hydrophobic B block and one end of hydrophobic C homopolymer. Therefore, the mixtures consist of unbonded AB diblock copolymers, unbonded C homopolymers, and supramolecular ABC triblock copolymers. The constituents of the mixturess are dependent on the supramolecular bonding strength and the initial mixture ratio, which is essential to the micellar morphologies. With the reversible bonding strength increasing, the volume fraction of supramolecular ABC triblock copolymers increases and transition from core-shell-corona micelles to wormlike micelles occurs. Besides, the morphological transition of theses micelles is observed with the volume fraction of homopolylmers changed.(3) Phase behavior of supramolecular mixtures with reversible bonds in bulkIn bulk, it is easier to macrophase separate than to microphase separate for AB diblock copolymer/C homopolymer mixtures. The introduction of the reversible bond into the mixing system, which is formed between the B free end of AB diblock copolymers and one end of C homopolymers, could effectively suppress the macrophase separation and encourage the self-assembly of molecules into hierarchical nanostrucutres. Therefore, the mixture consists of AB diblock copolymers, C homopolymers, and supramolecular ABC terpolymers. The constitutients of the mixtures are dependent on the reversible bonding strength and the mixture ratio, which is essential to nanostructures. In these hierarchical nanostructures, the C homopolymers exhibit a swollen effect on the C substructures. The coordination number of A substructures around each C substructures decreases as the bonding strength increases. To understand the detailed structures of the hierarchical nanostructures, one-dimensional density profiles were plotted. The results were finally compared with the existing experimental findings, and an agreement was shown. The obtained results provided an insight into the role of the supramolecular interactions on the hierarchical nanostructures.(4) Phase behavior of graft copolymers in concentrated solutionIn addition to linear copolymers, amphiphilic graft copolymers can also self-assemble into ordered microstructures in selective solvent. This system can be considered as a mixture of graft copolymer and small molecular solvent. The effects of polymer concentration and molecular architecture, such as the number of graft points and the position of the first point, on the phase behavior were investigated. Phase diagrams were mapped out by comparing the free energy of each ordered structure. Besides, the density distributions and domain spacing were calculated to understand the filling of solvents and packing of graft copolymers in the ordered structures. Since the backbone can take the bridging conformation, the graft copolymers can take either a local crosslinked structure or a global crosslinked structure in the concentrated solution, which can yield a vital material of physical reversible gels. The understanding of the microstructures of graft copolymers in concentrated solution provides useful information for designing high-performance gels.