Theoretical Simulation Of Self-assembly Of Amphiphilic Block Copolymers Induced By Liquid-liquid Phase Separation

Amphiphilic molecules can spontaneously aggregate into clusters in selective solvents to form stable,ordered,structurally defined aggregates with hydrophilic heads outside and hydrophobic tails inside.Such ordered aggregates can self-assemble into a series of morphological structures,such as micelle structures with spherical,columnar,flaky and tubular shapes,as well as vesicles with spherical,ellipsoidal and oblate outer shapes and/or with complex inner cavities such as single-,multi-,annulated cavities and multilayered membrane structures.Since the incompatibility of hydrophilic and hydrophobic parts of amphiphilic polymers,they can undergo microscopic phase separation in the bulk or in the solvent.Therefore,they often have special physical,chemical and electrical properties that a single molecule does not possess.They are used to construct new structures,realize new functions and create new materials,and are of extensive use in the most concerned biomedical fields.Liquid-Liquid Phase Separation(LLPS)exists in biology,synthetic chemistry,crystallization kinetics and other fields widely,and it is very important to realize its related functions.Research on the competition between LLPS and micellization/vesiculation has made great progress recently.However,the way to effectively control the formation paths from homogeneous state to aggregates has not been completely solved,which is vital to dominate its structure and properties and even its future functions.In this thesis,the LLPS phenomenon of amphiphilic block copolymers(BCPs)and its influence on the self-assembly kinetics process are described by using dissipative particle dynamics(DPD).First of all,we understand its influence on the self-assembled morphology by changing the length and sequence of the block itself,the interaction between the blocks and the solvent,and the concentration of the polymers,and explored the occurrence conditions of LLPS.A₂B₇ is selected as the main research object,we constructed an equilibrium phase diagram,and divided the phase diagram into five characteristic regimes:homogeneous polymer solution(HS),phase-separated polymer-rich liquid droplets(LL),transition state(TS),large compound micelles(LCM)and multi-compartment vesicles(MCV)by analyzing the radial distribution of hydrophilic beads?_A(r),hydrophobic beads?_B(r)and solvent beads?_S(r)inside the aggregates.LL contains polymer-rich dense phase large droplets and polymer-poor dilute phase solution.We also established the scaling relationship between the concentration and the size and density of the large droplets in LL.Then,on the basis of the above-mentioned intermediate state of LLPS,namely the HS is quenched to the unstable two-phase region with respect to copolymer-solvent demixing and the large droplets in the stable LL obtained after the prenucleation time are used as the core,and the interaction is adjusted so that to make it grow stably into vesicles until almost all copolymers in solution are exhausted.We make the stepwise self-assembly from LL follow the nucleation growth mechanism(mechanism II),which is different from the traditional experience of the whole bilayer membrane closure mechanism(mechanism I)from HS.It has been proved by quantitative statistics of the solvents inside the vesicles formed by mechanism II have a larger size and a significantly larger cavity in the early stage.In summary,the self-assembly of amphiphilic BCPs can be regulated and controlled,and the range of LLPS can be effectively adjusted through thermodynamic properties of amphiphilic BCPs.The consideration of LLPS in the design of amphiphilic assemblies will hopefully provide greater control over the structure and properties of the resulting materials.This work is of great guiding significance for further exploring the LLPS process towards determining the structural properties of vesicles and the dynamics of the self-assembly process.The detailed research of LLPS and its self-assembly simulation has contributed to completing its theoretical basis and practical applications in the future in various fields.