A Dissipative Particle Dynamics Simulation Study On The Structures Of Comblike Block Copolymer

As an important branch of"soft materials", block copolymers have arouse many physicists and material scientists' interests for its many novel behavior, abundant physical connotation and its extensive application background. It had been proved that comb-like block copolymer can be tailored to display a range of novel mechanical, rheological and optical properties; In melt, block copolymer can form lots of ordered nano-materials, which usually present unique properties, such as in the field of optics and spectroscopy. In upper concentration, these block copolymers will form 3D network like gel structures which compose bracket for mesoporous material; In dilute solution, block copolymers have excellent self-assembly ability. They can assembly into complex structures, such as micelle and vesicle because of the selectivity of solvent. Block copolymers assembly to aggregates in selective solvents or micro-phase structure in bulk have great potential application and development prospects in many emerging areas, the most important in which is controlling the release of biologically active molecules. With the rapid development of polymer material and bio-medical disciplines, combine research of drug delivery technology and nano-scale materials became increasingly important in scientific research area. Block copolymer can form micelles or vesicle structure, which can be used in the biomedical field as a carrier, or involved in the control of drug release for its PH value and temperature-sensitive.Computer simulation performed by establishing model based on real problems, select appropriate simulation method to depict the real process, and then obtain reinforce results for experiment and theoretical. In principle, the computer can acquire all information of system. Computer simulation can not only provide complement for the experiment data but also to make predictions for the theoretical results. At present, computer simulation, theoretical approaches and experimental science have become three kinds of means to understand the objective world. We carry out the dissipative particle dynamics (DPD) simulations to study the topics we interested in detail. In DPD method, all the particles interact with each other through three pairwise forces: conservative force, dissipative force, and random force. These forces are so soft that the integration time step is larger than that in MD, and the time scale in DPD simulation can be at milliseconds. Some molecules or polymer segments can be coarse grained into one DPD particle due to the soft repulsion potential, thus the DPD method can be used to study the systems at micron length scale.The microphase separations of comb-like block copolymer, the influence of solvent size on the structural properties of bottle-brush polymers, and the spontaneous vesicle formation and fusion of comb-like block copolymers with semiflexible hydrophobic backbone are studied via Dissipative Particle Dynamics (DPD).simulations. The main results are as follows:(1) Dissipative particle dynamics simulations are carried out to study the microphase separation of comb-like block copolymer in two dimensions. By systematically varying the side chain length and the repulsion strength between the two components, we find various morphologies. For comparison, the linear and the star-like block copolymers are also studied in the simulations. The results reveal the main factors that affect the microphase separation of comb-like block copolymers in melts.(2) Dissipative particle dynamics had been successfully applied to study the polymer systems with various complexities. However, because of the soft nature of the interactions, DPD method can not avoid bond-crossing which is important to obtain real physical properties in the simulations. Here we propose a modified three dimensional DPD model based on the traditional DPD to avoid bond crossing. We find that this simulation model can effectively avoid the bond-crossing problem. Based on this model we investigate the effect of solvent size on the structural properties of bottle-brush polymers in dilute solution. We find that with increasing solvent size, the radius of gyration of the bottle-brush polymer decreases considerably in athermal solvent but increases in selective solvent favoring the backbone, respectively.(3)The spontaneous vesicle formation and fusion of comb-like block copolymers with semiflexible hydrophobic backbone are studied via DPD simulations. By systemically varying the solvent condition, we construct a phase diagram to indicate the thermodynamically stable region for vesicles. The spontaneous fusion between the vesicles is studied, whose mechanism is as follows: first, a stalk is formed between the vesicles; then, the holes appear in both vesicles near the feet of the stalk; finally, the stalk bends to circle the holes and the fusion process is completed. This fusion pathway is similar to that observed in Monte Carlo simulations and dynamic self-consistent filed theory but different from those reported in coarse-grained molecular dynamics and DPD simulations. The main reason for the difference may be attributed to the molecular structures used in different simulation techniques.