聚合物接枝纳米粒子两亲性分子在溶液中自组装行为的模拟研究

Self-assembly of Polymer-grafted Nanoparticle Amphiphiles in Selective Solvents

利用布朗动力学模拟方法,研究了聚合物接枝纳米粒子组成的两亲性分子在溶液中的自组装行为.这种两亲性分子可通过自组装形成多种结构.考察了两亲性分子浓度、疏溶剂纳米粒子直径、聚合物链和纳米粒子之间的相互作用、聚合物链的链长以及溶剂性质对组装结构的影响.构建了随不同参数变化的形态相图.我们观察到2种囊泡形成路径,并且通过控制两亲性分子浓度,能实现2种路径之间的转变,并将本研究的模拟结果与已报道的相关实验观测和模拟结果做了比较.

We performed Brownian dynamics simulations with implicit solvent to study the self-assembly of polymer-grafted nanoparticle amphiphiles in selective solvents. Each model amphiphile consists of one hydrophobic nanoparticle（H） bead and one hydrophilic polymer chain composed of P-beads. The diameter of each H-bead is varied from one to several times that of each P-bead. The influences of experimental conditions on the self-assembled morphologies are investigated. The experimental conditions studied include the amphiphile concentration, the size of the hydrophobic head, the interaction parameters between the hydrophilic bead and hydrophobic bead, the polymer chain length and the solvent. Various self-assembled morphologies are obtained including conventional spherical micelles, cylindrical micelles, cylindrical networks, large compound micelles,thin sheets, spherical vesicles, and novel ones of tubular vesicles, cylindrical multicompartment vesicles, and spherical multicompartment vesicles. The morphological phase diagrams are constructed as a function of different parameters. Mechanisms of morphological formation are discussed. Two pathways, mechanisms I and II, of vesicle formation are identified. In mechanism I, the model amphiphiles first self-assemble into spherical micelles,which transform into cylindrical micelles, further into bilayer-sheets, and finally the sheets bend around and close up to form vesicles. In mechanism II, in the initial stage of simulation, the model amphiphiles first self-assemble into many small spherical aggregates, inside which the hydrophilic P-beads are mixing with hydrophobic H-beads.Subsequently, neighboring aggregates coalesce together, and microphase separation between H and P beads occurs in the interior of the aggregates, resulting in a concentration of P-beads at the center of the aggregates, i.e., the formation of semivesicles. As simulation proceeds further, the semivesicles grow larger, and more and more Pbeads enter into the inner of the semivesicles, and finally semivesicles expand outward, forming vesicles.Furthermore, transition from mechanism II to mechanism I can occur by increasing amphiphile concentration. At low amphiphile concentration, the attractions among the hydrophobic H-beads are dominant in the system. In this case, mechanism II occurs during vesicle formation. At high amphiphile concentration, repulsion among the hydrophilic P-beads dominates the system where bilayer-sheets occur as an intermediate state of the vesicle formation and thus mechanism I occurs. The simulation results are compared with available experimental and simulation results obtained from related systems.