Dissipative Particle Dynamics Simulation Study On The Solution Self-assembly Of Dendritic Multiarm Copolymer

Dendritic polymers, including dendrimer and hyperbranched polymer, have demonstrated great property advanteges of a large population of terminal functional groups, better solubility, lower solution or melt viscosity and huge amounts of interspace between molecules, especially the synthesis of hyperbranched polymers can be performed in a “one-pot” reaction which is much easier than that of dendrimers. Due to their unique molecular structures and properties, dendritic polymers are receiving more and more attention in the area of self-assembly. In experiment there are abundant morphologies obtained through the solution self-assembly, interfacial self-assembly and hybrid self-assembly of hyperbranched polymers, which initiative implementing the multidimensional and multiple scales self-assembly. But, the dynamic process of hyperbranched polymers self-assembly is too rapid to observe in experiment, the dynamic process of self-assembly and micro phase separation structure are limited to conjecture or deduction. Therefore, if through theoretical simulation to explain the above problems, and then predict the structure and performance of the self-assembly of hyperbranched polymers, it will be very conducive for experimental scientists to design and investigate the self-assembly structure of hyperbranched polymer.Nowadays, as the development of hardware and software of computers, the simulation performs an important role in the scientific research. As a bridge between the theory and experiment, computer simulation can perform a real system via appropriate methods, and then further verify the theory. Up to now, there have been extensive reports on the experimental study of hyperbranched polymers, while rarely reports on computer simulations of hyperbranched polymers due to the complexity of the structure. Therefore, it is significant for the further improve of the theory of the dendritic polymer self-assembly and reveal the assembly rules through computer simulation.In this study, we use dissipative particle dynamics method to simulate the self-assembly of dendritic polymers, especially the hyperbranched multiarm copolymer. The main research content of this thesis includes:1. Dissipative particle dynamics simulation study of the micellization behaviors of dendritic multiarm copolymers;Through the simulation, both the self-assembly mechanisms and dynamic self-assembly processes for the formation of unimolecular micelles, microphase-separated small micelles, and large multimolecular micelles have been disclosed. Most importantly, the work has proved the large multimolecular micelles are a kind of multimicelle aggregate(MMA) with two formation mechanisms. One is the unimolecular micelle aggregate(UMA) mechanism, which describes the dendritic multiarm copolymers form the unimolecular micelles first, and then the unimolecular micelles further aggregate into UMAs without any microphase separations; the other is named the small micelle aggregate(SMA) mechanism, which shows that the dendritic multiarm copolymers first self-assemble into small micelles, and then the small micelles further aggregate into SMAs. Through the simulation we analyze the characteries, dynamic process and the influence factors of the two mechanism.2. Dissipative particle dynamics simulation study on vesicles self-assembled from amphiphilic hyperbranched multiarm copolymers;The simulation disclosed both the self-assembly mechanisms of normal and reverse vesicles from hyperbranched multiarm copolymers, and the micro-phase separation structure of HMCs in vesicle. We find that the self-assembly of HMCs involves several steps, such as from randomly distributed unimolecular micelles to small spherical micelles, to membrane-like micelles, and to small vesicles. It is noticed that the membranes are formed through the direct aggregation and lateral fusion of small micelles, and then the bending and closing of the membranes give rise to small vesicles. Finally, large and steady vesicles are formed through the fusion of small vesicles. The results indicate some differences in the self-assembly mechanism for branched polymersomes and conventional polymersomes from linear block copolymers. No “sphere-to-rod” and “rod-to-membrane” transitions have been observed. By contrast, a direct “sphere-to-membrane” transition has been found. In addition, the bilayer or monolayer molecular packing modes of HMCs in normal or reverse vesicles have also been studied.We find that normal branched polymersomes can have either a bilayer or a monolayer structure depending on the hydrophilic fraction, while reverse branched polymersomes only possess a bilayer structure. These simulation results provide some details on the self-assembly of branched polymersomes, which not only validate the experimental data but also give new insight into the self-assembly of hyperbranched multiarm copolymers.3. Dissipative particle dynamics simulation study on amphiphilic janus micelles from hyperbranched oligoarm copolymers;The hyperbranched oligoarm copolymer HPG-star-PDMAEMA with a hydrophilic hyperbranched polyglycerol(HPG) core and average three pH-responsive poly[2-(dimethylamino)ethyl methacrylate](PDMAEMA) arms was synthesized from sequential cationic ring-opening polymerization(CROP) and atom transfer radical polymerization(ATRP). The obtained HPG-star-PDMAEMA copolymer is pH responsive. At the neutral condition, large multicompartment micelles are generated. When we increase the pH, i.e. pH 8.0, the hierarchical structures from linear or branched micelle chains to 3D micelle networks are obtained. For the further study, we adopt dissipative particle dynamics to simulate the anisotropic self-assembly process of hyperbranched oligoarm copolymers. At a neutral condition, due to the existence of weak “hydrophobic interaction” among the PDMAEMA chains, the interchain overlapping and entangling of PDMAEMA arms might happen, which drives the HPG-star-PDMAEMA copolymers to self-aggregate into LMCMs. With the increase of solution pH to 8.0, the hydrophobicity of PDMAEMA segments greatly increases and they collapse onto the surface of HPG cores to form hydrophobic domains, which leads to the formation of Janus micelles. Then these Janus micelles further aggregate into chain or network structure dominated by the anisotropic interactions. The simulation results not only validate the mechanism put forwarded in experiment, nut also reveal the phase separation mechanism and the influence factors during the self-assembly process.4. Dissipative particle dynamics simulation study on the micelle-to-vesicle morphologies transition of hyperbranched multiarm copolymers;The hyperbranched multiarm copolymer HBPO-star-PDEAEMA with a hydrophobic hyperbranched hyperbranched poly(3-ethyl-3-(hydroxymethyl)oxetane)(HBPO) core and pH-responsive poly[2-(diethylamino)ethyl methacrylate](PDEAEMA) arms was synthesized via atom transfer radical polymerization(ATRP). The HMC was observed to present water-addition induced aggregation in component solvent. With the addition of water, the assembly morphologies could take place a serious transitions. First, in dioxane HBPO-star-PDEAEMA could self-assemble into vesicles. When adding water, the solubility of PDEAEMA first increase, so the molecules dissociated from vesicles forming unimolecular micelles. While further increase the water content, the solubility of PDEAEMA decrease, which induced the aggregation of unimolecular micelles to form multimicelle aggregates. Then further increase the water content leading the collapse of PDEAEMA, and hollow spheres appear. Further increase the water content once more, the thickness of hollow spheres membrane decrease, so vesicles appear. For the further study, we adopt dissipative particle dynamics to simulate the water-addition induced morphologies transition of HMCs. The driving force of the morphologies transition is the non-linearlity change of the solubility of PDEAEMA arms induced by the linearity increase of water content. We successfully simulate the HMCs first self-assemble into micelles and multimicelle aggregates, and then the micelles in multimicelle aggregates rearrange and collapse into semi-vesicles with hollow structure, and finally diffuse into vesicles. The simulation results validate the mechanism put forwarded in experiment.