Synthesis And Microstructure Control Of Amphiphilic Block Copolymers

Conjugated polymer, which broke the traditional concept of nonconductive polymers, have been emerging as kinds of special functional polymers in the field of scientific research for the last forty years and have promising applications in organic semiconductor devices such as organic light emitting diodes, organic field-effect transistors and solar cells. How to improve the photoelectric properties is a core research project of the conjugated polymers. It’s well known that the photovoltaic performance of conjugated polymers is determined not only by its chemical composition, but also by its microstructures on thin films. Controlling and optimizing the. nanostructures helps to the dissociation and diffusion of excitons, high mobility of charge carrier, thus enhances the performance of semiconductor devices. One of these effective ways to control film morphology is introducing conjugated polymer into block copolymer system, and control the microstructure by microphase separation and self-assembly behavior of the block copolymer. Hence, both domestic and overseas scientists focus on the preparation of new conjugated block copolymers and the regulation of their microstructures in order to obtain the best photoelectric performance.This thesis mainly focus on the P3ATs systems and study the synthesis and microstructure control of amphiphilic rod-coil block copolymers, and further compare the results with the self-assembly behaviors of amphiphilic coil-coil block copolymers which helps us to understand and master the control methods and influencing factors. Currently, our work and the results are as follows:First, a series of amphiphilic rod-coil block copolymers poly(3-hexylthiphene)-b-poly(ethylene oxide)(P3HT-b-PEO) were synthesized via modified Grignard metathesis (GRIM) polymerization and Click Chemistry with a fixed PEO block length and gradually increased P3HT lengths.2-Bromo-5-iodo-3-hexylthiphenes were used as monomers here which have a good selectivity of activation by the Grignard reagent and hence ensure the high stereoregularity and narrow polydispersity indice (PDI). Characterizations of thermal, crystalline, optical properties and solvent-induced structure formation in thin films as well as vapor annealing treatments were systematically investigated. A series of morphologies including two-phase separated nanostructure, nanofibrils and their mixed morphologies could be obtained. It revealed that there is a subtle balance between microphase separation of the copolymer and the π-π stacking fibrillar aggregation of the conjugated P3HT and such balance can be controlled by changing the solvents with different selectivity in solution and the length of P3HT block. Besides that, fractal structures were obtained by annealing the P3HT-b-PEO films, which were spin-coated from its THF solution, in carbon disulfide for a period of time.Second, in order to delve into the control methods of the microstructures in block copolymer systems, we prepared uniform ring-shaped micelles from polyisoprene-block-poly(2-vinylpyridine)(PI-b-P2VP) diblock copolymer in THF/ethanol solvent mixture on the basis of Chang’s work. Besides that, we systematically investigated the influencing factors on the ring-shaped morphology including copolymer concentration, the ratio of THF to ethanol, solvent selectivity, solvent evaporation rate, solvent dropping sequence and the dropping rates of selective solvent. It was found that the solvent ratio, solvent selectivity and solvent evaporation rate have significant influences on the formation of ring morphology while copolymer concentration, dropping sequence and the dropping rates of selective solvent affect the uniformity of the rings.In order to analyze the structure of the ring morphology, chloroauric acid (HAuCl4) was selectively loaded into P2VP phase by complexation and detected by AFM. Combining these results with cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), static light scattering (SLS) and the theoretical research, a possible formation mechanism of rings was proposed:with the addition of ethanol which is a poor solvent for PI, PI-b-P2VP self-assembled into micelle structure with the shell of P2VP and the core of PI.Based on a series of research mentioned above, we acquired a deeper understanding of the influencing factors on the PI-b-P2VP system, which helps to achieve effective control on the self-assembly morphology. Compared with the morphology research of rod-coil P3HT-b-PEO system, it helps to deepen our concepts of microphase separation and self-assembly and.lay a solid foundation for the future research work.