Monte Carlo Simulation And Rheological Study On Self-assembly Of Block Copolymer In Solution

Due to the unique structure and property of block copolymer, various micelles withordered structures were fabricated by their self-assemblies in solution. Since these micelleshave great potential applications in nanotechnology, it becomes very important tounderstand the effects of the control parameters on the self-assembly of block copolymer.As self-assembly is a complex process related to many aspects, computer simulation hasbecome a convenient and efficient method to study the self-assembly of copolymer. Boththe predictions of new self-assembled structures and the statistics of regularities could beimplemented by computer simulation, which make ones understand the processes of theself-assembly of copolymers more comprehensively and provide theoretical supporting forexperiments. Using Monte Carlo simulation, we study the self-assembly of AB/BC mixture,ABC/AC mixture, copolymer solution under cylindrical confinement and soft confinementrespectively, and the formation mechanization of these systems. On the other hand,morphological transition is a common phenomenon in self-assembly, while on-linetechnique is very helpful to reveal the details during the morphological transition.Therefore, we applied the on-line rheological method to study the morphological transitionof block copolymer in solution. Detailed results are summarized as follows:1. Monte Carlo simulation study on the self-assembly of AB/BC copolymer mixture inA and C selective solvent. Using Monte Carlo simulation, we found that two different typesof diblock copolymers could co-assemble together to form various Janus micelles withoutadditional interaction between hydrophobic B block. From the morphological phasediagram related to the hydrophobicity of B blocks and incompatibility between hydrophilicA and C blocks, it is clear that the micellar shape is mainly controlled by thehydrophobicity of B blocks, while the formation of Janus architecture is dominated by theincompatibility between hydrophilic A and C blocks. Furthermore, we found that themicellization of hydrophobic B block always occurred before the microphase separation between the hydrophilic A and C blocks by monitoring the formation pathways of typicalJanus micelles.2. Self-assembly of ABC/AC copolymer mixture in C-selective solvent. Using MonteCarlo method to study the self-assembly of ABC/AC mixture, it was found that AC diblockcopolymer acted as the surfacants in the mixture, which could modulate the interactionbetween ABC triblock copolymer and solvent, resulting in various multicompartmentnanoparticles. By increasing the length of solvophobic middle B block, morphologieschanged from hamburger-like to ring-coiled-rod, and then to reversed hamburger-likenanoparticles. With the increase of fAC, the number of small spherical micelles formed byAC copolymers increased gradually in the micelles. Moreover, the micellar structures weremainly determined by the hydrophobicity of solvophobic blocks, and changing theincompatibility between different polymer monomers would affect the micellarmorphologies. From the results in these two sections, it is clear that mixing two differentblock copolymers is an effective method to turn the micellar structures.3. Study on the self-assembly of diblock copolymer in dilute solution confined incylindrical pore. Simulation results revealed that the interaction between copolymer andwall was a key factor in controlling micellar structures in cylindrical pore, and the size ofthe nanoparticles formed in cylindrical pore was relatively uniform. Increasing the diameterof cylindrical pore would cause the micellar morphological transition. However, only thelengths of micelles were increased along with the z axis of cylindrical pore by increasingthe concentration of copolymer, which was also proved by calculation of the contactnumber between polymer monomers. The results of mean-square radius of gyration showedthat the confinement effect increased with the increase of le/D, which induced the decreaseof extension degree of the chain.4. Self-assembly of ABA triblock copolymers under soft confinement. Using MonteCarlo method, it was found that ABA triblock copolymer under soft confinement couldself-assemble into novel nanostructures, such as the droplets with inner single helix, doublehelixes, stacked rings and cage-like structure. As the increase of the length of B block,more and more B domains were exposed to solvent when the solvent qualities for A and B were the same, while the inner domain formed by B block became more continous whenthe solvent qualities of A and B were different. As the increase of the hydrophobicity of Bblocks, the morphology for A5B8A5triblock copolymers changed from stacked lamella tobud-like structure, and then to onion-like structure, while the inner structure of theaggregate for A7B4A7triblock copolymers transformed from spherical to cylindrical phases.As the increase of the incompatibility between A and B, the domain formed by B blocktransitioned into a stretched structure. Moreover, the formation mechanisms of sometypical aggregates were also studied by miniorring the formation pathways of theaggregates.5. On-line rheological study on ion-induced micellar transition for self-assembly ofcopolymer in solution. By monitoring the viscosity of the solution in real time, we achievedon-line investigation on morphological transition induced by ion. In experiment, themicelles transformed from spheres to cylinders, and then to lamellae, and the viscosityincreased and then declined correspondingly; when the morphology changed from theaggregates of cylinders or lamellae to the vesicles or spheres, the viscosity showed agradual decline. By adding different concentrations of ion solution to induce the micellartransitions, different transition pathways could be visulized from the different viscositycurves. Our results confirmed that the rheological technique could be an effective on-linemethod for the micellar transition in dilute solution.