Computer Simulation On The Self-assembly Of Miktoarm Star-like Block Copolymers In Dilute Solution

Soft matters, which can response to external environment sensitively, have at-tracted much attention of many researchers. As an important branch of “soft mat-ters”, block copolymers have performed some novel behaviors in many areas. Blockcopolymers can assemble into ordered structures, like micelles and vesicles, whichhave been applied in many scientific and industry areas, when they reach a concen-tration higher than critical micelle concentration （CMC）. Dendritic block copolymers,like star-like block copolymers and hyperbranched copolymers, is different from linearblockcopolymersinphysicalandchemicalproperties, sotheself-assemblyofdendriticblock copolymers may have some special behaviors.Nowadays, asthehardwareandsoftwareofcomputershavebeendeveloped, com-puter simulation performs an important role in scientific research. As a bridge betweentheory and experiment, computer simulation for real experiments can be performed viaappropriatemethods, andverifythetheory. Inthisstudy, weusecoarse-graineddynam-ics simulation methods to investigate the self-assembly of star-like block copolymersin dilute solution.The main research content of this thesis includes:（1） The self-assembly of miktoarm star-like block copolymers A_m（B_n）₂in dilutesolution is studied by dissipative particle dynamics （DPD） simulations. The pathwayof self-assembly, as well as the influence of molecular architecture of the block copoly-mers on the self-assembly structures are studied in detail. The miktoarm star-like blockcopolymers aggregate into disk-like micelles, then the micelles can close to form avesicle structure. For the system with short hydrophilic branch, it is easy to form the vesicle structure, and the thickness of the bilayer membrane of vesicle is larger sincethe hydrophobic branches of the miktoarm star-like block copolymers are longer. Theresults show that it is easier for the miktoarm star-like block copolymers to assembleinto vesicles than the corresponding linear diblock copolymers.（2） We study the self-assembly of symmetric star-like block copolymers（A_x）_y（B_x）_yC in dilute solution by using Brownian dynamics simulations. In the star-like block copolymer, incompatible A and B components are both solvophobic, andconnected to the center bead C of the polymer. Therefore, this star-like block copoly-mer can be taken as a representative of soft and deformable Janus particles. In ourBrownian dynamics simulations, these “soft Janus particles” are found to self-assembleinto worm-like lamellar structures, loose aggregates and so on. By systematically vary-ing solvent conditions and temperature, we build up the phase diagram to illustrate theeffects of polymer structure and temperature on the aggregate structures. At lowertemperatures, we can observe large worm-like lamellar aggregates. Upon increasingthe temperature, some block copolymers detach from the aggregate; this phenomenonis especially sensitive for the polymers with less arms. The aggregate structure will bequite disordered when the temperature is high. The incompatibility between the twoparts in the star-like block copolymer also affects the self-assembled structures. Wefind that the worm-like structure is longer and narrower as the incompatibility betweenthe two parts is stronger.（3） We study the self-assembly of star-like block copolymers （A_x）_y（B_x）_yC in dilutesolution as the solvent condition for component A varies from good to poor by usingBrownian dynamics simulations. The miktoarm star-like block copolymers can self-assembled into spherical micelles when the solvent is good for component A. The num-berofblockcopolymersinthemicellesissmallerasthelengthofthebranchesislonger.We then tune the solvent condition for component A, and find that the miktoarm star- like block copolymer can assemble into cylindrical micelles or disk-like micelles whenthe length of branches is shorter, and they can assemble into spherical micelles moreeasily when the length of branches is longer. The miktoarm star-like block copolymerscan self-assemble into vesicles when the length of A-branches is shorter. We explainthe mechanism of the phenomenon by using the concept of critical packing parameter.（4） We study the miktoarm star-like copolymers PS₅-PEO₅by using the coarse-grained model and force field parameter of PS-PEO block copolymers developed byChen et al. We find PS₅-PEO₅would take longer time to reach the equilibrium stateof spherical micelles than PS-PEO diblock copolymers. The size of the aggregatesassembled by PS₅-PEO₅is roughly the half of that for PS-PEO diblock copolymers.The size distribution of the micelles assembled by PS₅-PEO₅is also narrower, whichimplies that the miktoarm star-like copolymers can self-assemble into the micelles withhomogeneous size. We find the chain conformation of PS₅-PEO₅is more stretched inthe beginning. But the chains in PS-PEO will be more stretched suddenly during thequenching process, and they will be also more stretched than those of PS₅-PEO₅at theend of simulations.