Theoretical Study On The Formation Of CaTiO₃-Type Structure Self-Assembled By Block Copolymer

Block copolymer self-assembly provides a robust platform for the fabrication of ordered nanostructures with the feature size of 5-100 nm,which have potential applications in a wide range of fields,and thus it has attracted continuously increasing interest to exploring novel structures.At present,CaTiO₃ structure is a research hotspot,expecially for its special application in the fields of solar cells and light-emitting devices.However,it has not be obtained by self-assembly of diblock copolymers.Therefore,the research objective of this study is to design suitable block copolymer systems to obtain a stable CaTiO₃ structure and analyze its formation mechanism from both thermodynamic and kinetic perspectives.Up to date,the main strategies for enriching the self-assembly of block copolymer in bulk include:（1）changing the topological structure of the molecular chains or increasing the complexity of the blocks,and（2）blending the simple block copolymer.For this new structure,we first use the self-consistent field theory（SCFT）method combined with the above two strategies to design various block copolymer systems to explore the stability of the CaTiO₃ structure,then based on the calculated phase diagram including the stable CaTiO₃ structure,we use the string method to calculate the free energy path of different competing structures evolving to this target structure to understand their kinetic evolution process.The main content includes:1.Firstly,the B₁AB₂CB₃ linear block copolymer is designed.By adjusting the length of the bridging B₂ block and the relative length of the two end blocks B₁ and B₃ blocks to regulatethe coordination number and asymmetry of the A/C binary crystal structure,and be expected to obtain the target CaTiO₃ structure.However,there is little trend of the stability for CaTiO₃ structure formed in this system.Subsequently,the ABC triblock copolymer system was designed as a branched architecture.By altering the number of the branchs（n）in the ABC_n block copolymer to regulate the conformational asymmetry of the C block.Nonetheless,it was also found that the CaTiO₃ phase could not been stabilized in this system.Then the ABC triblock copolymer blending with ABC_n branched copolymer is studied,and it was found that the CaTiO₃ structure become stable underthe certainly blending conditions.Finally,after several parameter adjustments,a set of parameters thatbeneficialto the stability of the target structure was summarized,and further simplify the previous blend system to an ABC/AB blending system.By systematically exploring the effect of the volume fractions of different blocks,the chain lengthsand concentration of the two polymers on the stability of the CaTiO₃ structure,several two-dimensional phase diagrams regarding these parameters were constructed.,that thestable regionsof the CaTiO₃ structure is indentified and its thermodynamic mechanism is unveiled.2.According to the obtained phase diagram of the ABC/AB blending system constructed using SCFT method,the kinetic evolution path to the target CaTiO₃ structure is studied by string method.We main consider two initial structures to the target structure,the one is structureformed in the pure ABC system,the other one is Na Cl structure which is the most competitive structure to the target structure as the stable region adjacent to the one of the CaTiO₃.It was found that the structureundergoes a process of transform from C cylinder into C sphere on the four faces,and the nucleation growth of C sphere on the upper and lower faces,then evolves into the CaTiO₃ structure.In contrast,the Na Cl structure undergoes a process of disappearance of A sphere on edges and nucleation and growth of A sphere on vertices before transforming into the CaTiO₃ structure.The study of the kinetic evolution process of these structures can deepen the understanding of the self-assembly mechanism of the target structure.To unveil the self-assembly mechanisms of targeted CaTiO₃ structure from the aspects of both thermodynamics and kinetics,the SCFT is used to determine the stability of the targeted structures by calculating the free energy in various block copolymer systems,and the string method is applied to identify the minimal free-energy path of the targeted structures.Our study will be helpful to understand the self-assembly of block copolymers,and will provide an efficient route to precisely fabricate special structures similar to CaTiO₃.