Mesoscale Simulation Of Amphiphilic Block Copolymer And Its Tethered Nanoparticle Assemblies In Solution:from Micelles To Vesicles

Amphiphilic block copolymer can self-assembly into micelles and vesicles in a selective solvent. Polymer vesicles (or polymersomes) are hollow structures with a hydrophobic bilayer membrane and hydrophilic coronas on both the inside and outside of the hydrophobic core. Their unique closed bilayer structure has attracted considerable attention because it can provide potential or practical applications in many fields such as drug delivery system, biomembrane model, etc. Moreover, the presence of various domains, such as cores, interfaces, and coronas, in block copolymer assemblies allows for selective localization of nanoparticles in different regions, which may critically affect the resulting properties and applications of the nanoparticles. In this thesis, we use dissipative particle dynamic (DPD) method to investigate various morphologies, such as large compound micelles, spheres, cylinders or disk-like micelles, formed by A1BnA1amphiphilic triblock copolymers in dilute solution. Specifically we mainly focus on the different formation mechanisms and tunable wall thicknesses of block copolymer vesicles. Then we continue to investigate morphologies formed by (AB3)3-P amphiphilic block copolymer tethered nanoparticles in dilute solution, we found that hydrophobic nanoparticle can selectively locate in cores of micelles or vesicle walls.1. The mechanisms of vesicle formation depend greatly on the hydrophobic block length, hydrophobic bead-solvent repulsive interaction, and copolymer concentration. Two typical intermediate states are obtained:(1) bilayer-type membrane such as rod-like, disk-like, or bowl-like micelle (mechanism I),(2) semivesicle originating from the rearranged hydrophilic blocks movement into the center or the trapped hydrophilic blocks during merging (mechanism II). Most importantly, during the transition period from mechanism I to mechanism II with increasing the degree of the hydrophobicity of the blocks (including the hydrophobic block length and hydrophobic bead-solvent repulsive interaction), they do not only lead to these two unique intermediate structures. This pathway is so-called an in-between pathway:the intermediate structures are either semivesicles that are nearly-disk shape or bilayer membranes which are nearly orbicular in shape. This mechanism transition can also be reflected in the radius of gyration and the asphericity parameters. Thus we propose that a crucial balance between the segregation of inner-hydrophobic beads and the attraction of outer-hydrophilic beads drastically affects the self-assembly pathways of amphiphilic block copolymer into vesicles from one mechanism over the other. The larger degree of the hydrophobicity of the blocks leads to the more spherical shape and less hydrophobic blocks contacting to the solvent. Finally, the copolymer concentration also affects the pathways of the vesicle formation. It is clearly understood that vesicles formed through mechanism I at a high concentration because the cluster of aggregation is larger and the probability of the adhesive amphiphile collisions is higher than that at a low concentration.2. There are two types of dependence between the wall thickness and size of block copolymer vesicles. By varying the hydrophobic block length and the hydrophobic bead-solvent repulsive interaction, plots of relationship between wall thickness and size of the vesicles were obtained from the A1BnA1series block copolymers, which include the strong-behavior regions, weak-behavior regions, and both strong-and weak-behavior regions with a crossover point. Note that in the previous experimental work of Ma and Eisenberg they also reported a similar relationship between wall thickness and size in block copolymer vesicles. For the copolymers with shorter hydrophobic block length, the relationship between wall thickness and vesicle size is fairly linear (strong-behavior region); and for the copolymers with longer hydrophobic block length, only a weak size distribution is obtained, and the wall thickness becomes essentially insensitive to the size of the vesicle (weak-behavior regions). The effect of increasing the hydrophobic-bead solvent repulsive interaction is equal to increasing the hydrophobic block length. As the degree of hydrophobicity of the blocks increases, from a totally strong-behavior to a totally weak-behavior relationship, the transformation is observed in large sized vesicles first and then in small sized vesicles. Two characteristics, the chain compaction of the vesicles and the area densities of the inner corona, are thought to be important in controlling the membrane thickness, which are proposed to explain the size-dependent behaviors of bilayer thickness. The compact core-forming chains and the sparse corona-forming chains balance in a way that maintains a weak dependence of the wall thickness on the vesicle size. These results have important significance for the biomembrane model.3. By varying the repulsive interactions between the hydrophobic nanoparticles and hydrophilic/hydrophobic blocks, three types of micelles (1) large mixed spherical micelles,(2) bilayers (vesicles or disk-like micelles), and (3) rods can be obtained in the tethered nanoparticle assemblies. At the definite polymer-nanoparticle interaction, hydrophobic nanoparticles can locate in the cores of the rods and micelles, or in the vesicle walls, which is consistent with the Eisenberg’s group experimental results.