Theoretical Simulation Of Amphiphilic Block Copolymer With Different Topology In Solutions

The self-assembly of amphiphilic block copolymers (BCP) in selective solvents can form various morphologies, including spherical micelles, cylindrical micelles, disk-like micelles and vesicles, among others. Compared to small molecule aggregates assembled by amphiphilic surfactants, copolymer aggregates exhibit higher stability and durability due to their mechanical and physical properties. Therefore, copolymer self-assembly has attracted considerable attention, not only out of academic interest but also because of their potential applications in many fields, such as biomedicine, biomaterial, microelectronics, photoelectric materials, catalysts, etc. Many studies have focused on the co-assembly of nanoparticles (NPs) and block copolymers, which can form various structural hybrid aggregates. These hybrid aggregates can combine the properties from the parent constituents and generate new properties to meet the requirements in applications such as labeled materials, photonic nanodevices, or chemical sensors. Selective localization of the NPs in different domains of BCP aggregates of various morphologies may critically affect their resulting properties and possible applications.In this thesis, we use dissipative particle dynamics (DPD) method to investigate the self-assembled behaviors of amphiphilic block copolymer with different topology, including thermosensitive amphiphilic multiblock copolymers, surfactant analogue, and diblock copolymer tethered nanoparticles. The main contents and results as follows:The self-assembly of thermosensitive multiblock copolymer in selective solvents. The formation process of vesicle through nucleation and growth pathway is observed by varying the temperature. The simulation results show that spherical micelle takes shape at high temperature. As temperature decreases, vesicles with small aqueous cavity appear and the cavity expands as well as the membrane thickness decreases with the temperature further decreasing. This finding is in agreement with the experimental observation. Furthermore, by continuously varying the temperature and the length of the hydrophobic block, a phase diagram is constructed, which can indicate the thermodynamically stable region for vesicles. The morphological phase diagram shows that vesicles can form in a larger parameter scope. The relationship between the hydrophilic and hydrophobic block length versus the aqueous cavity size and vesicle size are revealed. Simulation results demonstrate that the copolymers with shorter hydrophobic blocks length or the higher hydrophilicity are more likely to form vesicles with larger aqueous cavity size and vesicle size as well as thinner wall thickness. However, the increase in hydrophilic block length results to form vesicles with smaller aqueous cavity size and larger vesicle size. Simulation results also show that the formation of vesicles precedes a nucleation and growth pathway with short hydrophilic blocks length, and it changes to a bilayer membrane closing pathway with longer hydrophilic blocks length.The self-assembly of surfactant analogue copolymer in selective solvents. Our simulation model is based on the experimental investigations about the giant surfactants PS-APOSS and PS-AC60, which possess a hydrophilic head with definite shape and size and one or more linear, hydrophobic polymeric tail. Dependent on the hydrophobic tail length (n), size of the hydrophilic head, and number of hydrophobic tails, copolymers can form a rich variety of morphological structures including spheres, worm-like cylinders, vesicles, disk-like micelles, pupa-like micelles, segmented rod-like micelles and large compound micelles. The properties of copolymers with smaller hydrophilic head are similar with diblock copolymers, however the properties of hydrophilic heads show the hydrophilicity when the size of hydrophilic head grows large. It is important that although the pupa-like and segmented rod-like micelles have been obtained in simulation and experimental studies, they are all formed by linear triblock copolymers or miktoarm star-like copolymers. To the best of our knowledge, we obtained segmented rod-like micelles using polymer tethered molecular nanoparticle amphiphiles for the first time. Then, we investigate the formation process of vesicles, pupa-like micelles and segmented rod-like micelles. We find bilayer membrane-closing mechanism for vesicles formation. However, the formation of pupa-like micelles and segmented rod-like micelles includes three steps, including aggregation, coalescence and growth, adjustment. These results have important significance for us to further understand the self-assembled behaviors of these shape amphiphiles.The self-assembly of diblock copolymer tethered nanoparticles in dilute solutions and the selective localization of nanoparticles in micelles. Different morphological aggregates, including spherical micelles, vesicles, disk-like micelles and rod-like micelles, were observed by varying the interaction parameters between the nanoparticle and the copolymer, number of tethered copolymer chains and length of hydrophobic block. Most importantly, the nanoparticles can selectively localize in the different domains within the aggregates. When the repulsive interaction between block copolymer and nanoparticle aPA=aPB=25, the nanoparticles evenly distributed within the spherical micelles, while aPA or aPB increases, the nanoparticles gradually aggregate and separate from copolymers, then localize in the center of vesicular wall. As increasing the number of tethered copolymer chains and length of hydrophobic block, nanoparticles localized within the micelles aggregate and form nanowires, nanorings and nanoclusters. The degree of stretching of the tethered copolymer chains gradually grows with the increase of aPA or sPB, while the degree of stretching of solvophobic block B decreases when the morphologies change from spherical to disk-like micelles and further to rod-like micelles. We use the theory of free energy to interpret the morphology transition of the aggregates. The surface free energy (EInterface) and the elastic stretching of core (Ecore) mainly affects the morphology of aggregates and actuates the morphology transform from spherical micelles to disk-like micelles and further to rod-like micelles.