Dissipative Particle Dynamics SimulatioN Study On Structure Transitions Of Vesicles Formed By Comb-Like Block Copolymers

Cells are common and essential structures in nature. They are closely related with the phenomenon of life and are basic units of organism. The primary roles of biomembranes are the separation between the inner and outer environments of the cell which is crucial for a variety of physiological functions, structural integrity and transmembrane transport of the cell. The complex biomembranes are deformed easily. So, it is very difficult to study the formation and structure transition of the biomembrane structures. However, the biomembranes are still membrane material, the physical and chemical nature of the self-assembly process is only connected with membrane characteristics. Therefore, it is very necessary to build a relatively simple membrane model to simulate some features of the biomembranes. Currently, the accepted model of membranemodel is vesicle obtained by molecular self-assembly. The characteristic of vesicle with hollow inner space has attracted much attention in the fields of drug and gene delivery, chemical and waste transport, and nanotechnology. The structure transitions of vesicle, such as fusion and fission, underlie the fundamental biological processes in normal and pathological conditions. Therefore, understanding the formation of various vesicle structures and elucidating the mechanism of their transitions has an important biological and technological significance.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 this dissertation, we carry out DPD simulations to study the thermodynamic influences on the vesicle structure transitions of a specific comb-like block copolymer. Within the DPD method, all the particles interact with each other through three pairwise forces: a conservative force, a dissipative force, and a random force. These forces are very soft, so the integration time steps can be very large. It is intrinsically embedded in the DPD model because of the pari-wise interactions which result the momentum of the system being conserved. DPD method has been applied on the study of microphase separation of the block copolymers, self-organizing of amphiphilic molecules into memberane, and the budding and fission of bionic micelles.The spontaneous vesicle structure transitions of comb-like block copolymers with semiflexible hydrophobic backbone are studied via Dissipative Particle Dynamics (DPD).simulations. The main results are as follows:(1) The study of spontaneous vesicle formation. We can observe a well-formed long tube vesicle from the comb-like block copolymer A₆ ( B₂ )₃ . It should be noted that for better comparing the influence of polymer architectures on the vesicle structures, we have also simulated a system composed of linear triblock copolymers A₃ B₆ A₃ and observe one well-generated spherical vesicle. Although the vesicle structures in these two systems are different, their vesicle formation pathways are rather the same.(2) The study of fission and reversal processes of vesicle. Line tension plays a decisive role on the fission of vesicles. We find two fission pathways: the first fission pathway is closely related to the domain budding outward from the parent vesicle, whereas the other fission pathway corresponds to the cleavage along the domain boundary. When we simulate long enough time, the separated small micelles or microvesicles are approaching to each other, and then fuse spontaneously. Finally, we can observe that the parent vesicle divides into two vesicles. We then study the vesicle reversal behavior by changing the solvent environment, and find that the tube-like vesicle forms special layered micelle structure and the onion-shape vesicle still form an onion-shape vesicle.