Mesoscopic Simulation Of The Self-assembly Of Diblock Copolymers In Selective Solvents

Block copolymers are a type of macromolecules composed of a series of different repeating units linked together. Because of the incompatibility between different structural units, the blocks repel each other. On the other hand, the repeating units are connected by chemical bonds, and microphase separation can occur. Block copolymers, in bulk and in selective solvents, can self assemble into a number of ordered morphologies, such as lamellar, hexagonally packed cylinders, spherical, and even more complicated structures such as vesicles, complex micelles, ring micelles, multicompartment micelles and so on. In addition to the flexible block copolymers, a series of rod-coil and rod block copolymers are also synthesized. Studies show that copolymers with rigid blocks have distinct self-assembly behavior, which can be used in optics, microelectronics, membrane engineering and etc.Computer simulation has been widely used to study chemical problems. It not only supports experiments in explaining the phenomena and clarifying microscopic mechanism, but can also guide experiments. Dissipative particle dynamics method is a kind of coarse-grained simulation method. It contains fluid interaction, conservation of momentum, and employs soft potential to enable large integration time step, and has been widely used in the study of self-assembly of block copolymers. In this thesis, we apply dissipative particle dynamics to examine the self-assembly behavior of a diblock copolymer (with a certain degree of rigidity) in a selective solvent. We computed the phase diagram as a function of the concentration and the interaction parameters between the particles, and examined their effect on the location and regional size of each ordered phase. In addition, the evolution of ordered phase is very important for studying the microscopic mechanism and guiding for experiments, therefore, we investigated the evolution process of lamellar, cylinders and spherical phase. Results show that there is a variety of evolution process for cylinders and spherical phase, and the effect of interaction parameter on the evolution of ordered structures. Radial distribution function is an important tool to characterize the degree of spat ial order, but has been rarely used in analyzing dissipative particle dynamics results. I n this thesis, we use the radial distribution function to analyze the relative position of t he particles in different ordered phases, getting a quantitative understanding of the mi croscopic information of the ordered morphologies. In addition, we also study the effe ct of the interaction parameters on the distance between ordered morphologies.