Dynamic Monte Carlo Simulation On The Formation Of Diblock Copolymer Vesicle And The Directed Loading Of Nanoparticles In Vesicle

Recently, various efforts have been made to synthesize inorganic materials with controlled size or morphology, because of their potential applications in various fields such as catalysis, medicine, dye, and cosmetic. A large amount of experiments have found that block copolymers exert strong effect on the nucleation, the growth of crystal, and subsequently affect the morphology of crystal. However, the mechanism of the block copolymer controlled or directed crystal growth is still not clear. Computer simulations can provide fundamental insight into crystallization process directed by polymer and knowledge on important parameters governing the fabrication of inorganic particles with complex structures. So far, however, rare such computer simulation efforts have been reported.In this thesis, the formation mechanism of diblock copolymer vesicle was investigated using bond-fluctuation dynamic Monte Carlo method based on simple cubic lattice. The influence of the chain concentration, chain structure and the solvent property on the vesicle size was discussed. The solvent property was found to affect the vesicle formation process. At last, the directed loading of nanoparticles and mixed nanoparticles in vesicle was studied in detail.Chapter 1 introduced the experimental reports and computer simulation results on the self-assembly of block copolymers in solution.Chapter 2 introduced the principle of Monte Carlo method and its application in polymer science. The model used in this thesis was also elucidated.In chapter 3, the formation mechanism of amphiphilic diblock copolymer vecisle was investigated in detail. AmBn represents the amphiphilic diblock copolymer chain with hydrophobic A segments and hydrophilic B segments. Pairwise nearest-neighbor (NN) interactions and next nearest-neighbor (NNN) interactions are considered among chain segments A, B and solvent segment S. The amphiphilic property of A3B1 chain is represented by the attractionεAA = -1 between NN or NNN A-A beads. While the interaction parameterεBB andεABwere assumed to be zero for A-A and A-B interactions. For A3B1 chains, a single vesicle was obtained when the segment concentration of polymer chain Cp = 7%. A segments were found to locate in the wall of vesicle, while B segments concentrated at its interior and exterior surfaces. The formation process of vesicle was investigated in detail, a bilayer disk was aggregated in a randomly dispersed system, it then bended and encapsulated solvents, and finally closed up to form a vesicle. The formation mechanism of vesicle agreed with the previous reports of MD, BD, DPD, and density functional simulations. However, the bending of the bilayer disk was observed for the first time by using a lattice chain model.The influence of the chain concentration, chain structure and the solvent property on the vesicle size was investigated in chapter 4. Vesicular structure could be formed when m≥3 with n = 1. The vesicle size increased with the segment concentration when the segment concentration of A3B1 chains was in the range of 2% and 15%. The solvent property, which was represented by the repulsive interactionεBS between bead B and solvent, was found to affect the vesicle formation process. For a smallεBS < 0.05, the vesicle formation pathway was the same as that discovered in chapter 3. While forεBS≥0.05, the vesicle was formed through another mechanism: The randomly distributed chains quickly assembled into spherical aggregates, which further grew through the coalescence of aggregates or the evaporation-condensation-like process. When the spherical aggregate size reached an enough big value, A segments and solvents entered into the center of the sphere, resulting in the formation of vesicle. We also found that the vesicle size decreased with the increase ofεBS. That means we can control the vesicle size by adjusting the solvent property.Chapter 5 investigated the aggregation of nanoparticles in the presence of diblock copolymer. Due to the hydrophobic property, nanoparticles intended to form a big and compact aggregate in absence of block copolymer. The aggregate behavior of nanoparticles changed once upon addition of A3B1 diblock copolymers, where an additional attractive interaction between nanoparticles and A segment was introduced. It was observed that nanoparticles dispersed in the wall of vesicle. It is easy to image that a hollow sphere will be fabricated by nanoparticles after the calcination of copolymers. The loading of mixed nanoparticles in vesicle was also studied. We introduced an attraction between nanoparticle I and A segment, and a smaller attraction between nanoparticle II and B segment. After a long time movement, we found that nanoparticle I located in the vesicle wall, and nanoparticle II was loaded into the core of vesicle. Therefore, we will obtain the core-shell structure of mixed nanoparticles after the calcination of copolymer. The simulation demonstrates that addition of block copolymer can effectively control the aggregation of inorganic particles and lead to formation of a variety of nanostructures.