Dissipative Particle Dynamics Simulations Of Vesicle Formation Mechanism And Structure Regulation

Vesicles are important assemblies.They possess hollow structure with a hydrophobic bilayer membrane and hydrophihe internal and external coronas.Because of the unique structure,vesicles have attracted considerable attention for their potential applications,such as drug delivery,microcapsules,nanreactors and so forth.Reseachers finds that the material exchange through the vesicle membrane is similar to cell membrane,so the vesicles can be viewed as valuable systems for mimicking cell machinery.In 2013,the research on the mechanism of vesicle transport had won the Nobel prize for medicine.This result proves that the research of vesicle is important in the field of biology and chemistry.Understanding of the vesicular structure and formation process is benefit for vesicle potential applications.In this paper,disspative particle dynamics(DPD)simulation was used to study the vesicles self-assembly mechanism of amphiphilic poly(acrylic acid)-b-polystyrene(PAA-b-PS)copolymers at different concentrations and PS/PAA block ratio.Then the terminal groups were connected to the polymer chain,the effect of hydrophobicity,location and number of terminal groups on the vesicle structure had been revealed.Firstly,we studied the effect of polymer block ratio and concentration on polymer self-assembly morphology and size.The simulation results show,at the same polymer concentration,with the increase ratio of the hydrophobic block,the self-assemblies transfer from spheres,rods,membrane and vesicles to compound micelles.This changing tendency is good consistent with the experiment result.Furthermore,concentration had no effect on the morphology of spheres,rods and compound micelles.But the vesicles will be further fusion to form spherical and tube-like vesicles with the increasing concentration.Through the analysis of spherical and tube-like vesicles,we found that the ratio of polymers in outer and inner layer is difference during the fusion process.So we come to the conclusion that the polymer movements between the inner and outer layer is the key factor governing the vesicle morphology.The structure of vesicle is the key factor to determine its application as a drug carrier.Recently researchers show that the terminal group can affect the structure of the vesicles,but the research in this area is still at an early stage.Through the study of this paper,we found that when the terminal group connected to the hydrophobic block,with the hydrophobicity of the terminal group increased,the vesicular wall thickness increases and cavity size decreases.Reversely,changing the location of the teriminal group and connected to the hydrophilic block,the vesicular cavity size increased but membrane nearly unchanged.Increasing the number of the terminal groups that connected to the hydrophobic block,the vesicular wall thickness also increase and cavity size decrease.When the number of the terminal group increased to 3,The aggregates become complicatedthat are multi-compartment vesicles.In this paper,the DPD simulation is employed to study the effect of polymer block ratio and concentration on polymer self-assembly.The difference of vesicle morphology was revealed in molecular level.What's more,through the investigation of hydrophobicity,location and number of terminal groups,we summarized the change regulation of vesicle structure.The simulation findings can provide a reference for vesicle structure regulation and application.