The Coarse-grained Simulation Study On Amphiphilic Polymer Self-assembly System

Self-assembly is a process in which the structural units in a disordered system(such as molecules,nanoparticles or block copolymers)form to ordered structures in a system spontaneously.The micelles,membranes,vesicles and other structures obtained from the self-assembly process are widely used in the biology,medicine,catalysis and other fields.Most self-assembly experiments are relatively simple to operate,low cost and do not require special instruments.Computer simulation has also established from microscopic quantum chemical calculation to mesoscopic coarse-grained simulation and macroscopic finite element method to study self-assembly structure and its formation mechanism.Therefore,whether in experiment or in computer simulation,the self-assembly technology has always been studied,and many significant results have been reported.However,some microscopic details that are difficult to be observed in experiment,but often have an important impact on the understanding of the self-assembly process,have not been studied and reported systematically.In recent years,computer simulation has attracted more attention in the field of scientific research.By using computer simulation,we can not only visually observe some assembly processes that cannot be observed in experiment,but also predict some experimental results that have not yet been realized.With the development of computer software and hardware,the computational efficiency of computers has been greatly improved.In this project,we usecoarse-grained simulations method to study the microscopic details of the self-assembly process and the materials used in self-assembly,and we also synthesized some valuable structures through self-assembly.The main contents of this thesis include three aspects:(1)The influence of cosolvent on self-assembly structures and self-assembly mechanism is studied.Two solvents are often needed in the self-assembly process,one is selective solvent(usually use water)while the other is organic cosolvent.In previous experiments,it has been found that the cosolvent has an important effect on the self-assembly structures.In this project,we use the DPD method to systematically study the influence of cosolvent in the process of diblock copolymer self-assembly.We find that the cosolvent not only play the role of dissolving polymers,but also have a preferential adsorption in polymer aggregates during self-assembly process.This preferential adsorption results in various self-assembly structures in cases of different kinds of cosolvent or different concentration of cosolvent in the system.The preferential adsorption of cosolvent also affects the vesicle formation kinetic pathways.The vesicle formation in less cosolvent system is spherical micelle–wormlike micelle–membrane–vesicle pathway,while in more cosolvent system is the small micelles aggregates continuously till the vesicle forms.In the past simulation,the single solvent system in which has averaged solvation capability based on the two solvent components is often used to study self-assembly process.We compare the simulation taking single solvent system with two solvents components explicitly and find the self-assembly results in single solvent system may incorrect.(2)The influence of experimental conditions on the structure of self-assembled materials is studied.Janus nanoparticles have attracted considerable interest in self-assembly field because of the unique asymmetry chemical properties on both sides of nanoparticles.The “grafting-from” method is one of the most common methods for synthesis of Janus nanoparticles.In this project,we use DPD simulation and “grafting-from” method to synthesize the Janus nanoparticle at solvent interface,and we study the effects of initial conditions,reaction probability and interfacial tension on the microscopic characteristics of Janus nanoparticle.The results show that,the perfectly symmetric particles with an equal volume ratio between hydrophilic and hydrophobic parts can only be prepared in strictly symmetric systems by “grafting-from” method.In addition,a disordered domain can always be found at two immiscible solvents interface.The grafted chains of nanoparticles in this disordered domain are mostly copolymer chains,and the increase of interfacial tension between the water and oil phases may enhance the extent of disorder of the grafted chains.Using the asymmetry of initial monomer concentration and reaction probability,we can synthesize some interesting asymmetric Janus nanoparticles easily.(3)Moiré pattern structures with potential photoelectric properties are prepared by self-assembly of Janus nanoparticles.The preparation of superconductor materials by simple methods has always been a scientific research topic.In recent years,it has been found that the double layer of graphene with moiré pattern exhibits the characteristics of superconducting material.It is also found that the moiré pattern plays an important role in optical control and can be used in image transmission and information processing.Therefore,the preparation of moiré pattern structures has become an important research direction.However,most methods to make moiré pattern require sophisticated instruments or strict experiment conditions.In this project,we obtain some membrane structures with moiré pattern successfully by using Janus nanoparticles self-assembly spontaneously.We study these membrane structures and find that the formation of moiré patterns is attributed to the hydrophobicity of the nanoparticles(and so-induced \force strings" at the membrane rim)together with suitable grafted hydrophilic and hydrophobic chain lengths.Too long or too short grafted chains on the nanoparticles can make the moiré pattern disappear.In addition,the size of moiré pattern structure can be controlled directly in solution by adding stabilizers,some diblock copolymers,to the initial system.The spontaneous self-assembly of Janus nanoparticles into moiré pattern may also provide a new way for synthesising superconducting materials.