Research Micellar Morphology Self Assembly From Triblock Copolymers In Solvents Using Annealed Simulation

One of the bottom-up strategies of creating structured materials for nanotechnology is to utilize spontaneous self-assembly of macromolecules. It has been proposed that the self-assembly of amphiphilic molecules into supramolecular assemblies and ordered structures can be used in the development of many new nanotechnological applications. The key to the success of this bottom-up strategy is the ability to predict and control the self-assembled nanostructures from the building molecules. One promising class of building molecules for nano-scale templates is block copolymers, which are macromolecules formed by covalently linking two or more chemically distinct polymeric blocks. In recent years, it has been demonstrated that block copolymers can be used to engineer a host of novel structures by tuning the block lengths, polymer architecture, and the type of monomers.The self-assembly of block copolymers in selective solvents is of fundamental interest because it offers tremendous promises for the creation of nano-structured materials in the form of micelles of different shape and internal structure. Especially, the possibility of forming multi-compartment micelles from block copolymers represents a significant step toward achieving hierarchical self-assembly with multiple functions and designed architectural features at several length scales. In this thesis, we report an extensive investigation of the solution-state self-assembly of amphiphilic terpolymers using computer simulations with simulated annealing technique. We have studied the influence of different parameters on the resulting micelle structures. A lot of multi-compartment micelles are observed. We have analyzed the chain packing in some representative micelles.Chapter 1 contains a brief introduction of the background of block copolymers, of the model and method used in the research, and of the framework of the thesis.Chapter 2 focuses on studies of self-assembly of ABC star terpolymers. Solution-state self-assembly of miktoarm star terpolymers provides a versatile and powerful route to obtain multi-compartment micelles. A variety of multi-compartment micelles are predicted from the simulations. Phase diagrams for typical star terpolymers are constructed. It is discovered that the overall micelle morphology is largely controlled by the volume fraction of the solvophilic A-arms, whereas the internal compartmented and/or segregated structures depend on the ratio between the volume fractions of the two solvophobic arms. The polymer-solvent and polymer-polymer interactions can be used to tune the effective volume fraction of the A-arm, and thereby induce morphological transitions. For terpolymers with equal or nearly equal length of B and C arms, several previously unknown structures, including vesicles with novel lateral structures (helices or stacked donuts), segmented semi-vesicles, and elliptic or triangular bilayer sheets, are discovered. When the lengths of B and C arms are not equal, novel micelles such as multi-compartment disks and onions are observed.Chapter 3 focuses on studies of solution-state self-assembly of linear triblock copolymers. The effects of polymer composition, polymer-solvents and polymer-polymer interaction on micellar morphology and morphological transitions are extensively investigated.First, the self-assembly of ABC linear terpolymers in solvents selective for the middle B-block is studied. We compared our results with those from ABC star terpolymers, and those from AB, BC diblock copolymer blending system. It is found that the spatial limitation of the middle solvophilic polymer on solvophobic sub-domains is conspicuous. With increasing the incompatibility between solvophobic polymers, a variety of complex morphologies such as vesicles, core containing vesicles, complex net-cage micelles and planar network micelles are observed in high copolymer concentration system. Furthermore, in high copolymer concentration system, ABC linear terpolymers can form complex micelles such as a big micelle contains several small micelles and a big vesicle with several layers.The self-assembly of ABA linear triblock copolymers in solvents selective for the middle B-block is also studied. The simulations reveal that a micellar sequence, ranging from spherical, rodlike, toroidal, disklike micelles, net-cage micelles to vesicles are always formed from the ABA copolymers in different systems, with increasing the A-solvent interactions. Phase diagrams are constructed by varying the A-solvent interaction and terpolymer concentration. It is demonstrated that higher terpolymer concentration can make the system to form complex micelles. A host of complex micellar morphologies and morphological transition are observed. Then, the self-assembly of ABC linear triblock copolymers in solvents selective for one end (C)-block is studied. A variety of novel micellar structures are discovered by varying the volume fraction of solvophobic A-polymer. It is found that the increase of the ratio of the volume fraction between two solvophobic polymers(A and B) can tune the micellar morphology from core-shell-corona structure to raspberry like micelles, and the copolymer concentration can control the complexity of the micelles. The detail structures are determined by the incompatibility between two solvophobic polymers. We found that the solvophilic C-polymer can largely determine the micellar morphology by controlling the packing (direction and location) of terpolymers.Finally, we studied the self-assembly of ABC linear triblock copolymers in solvents selective for the two end blocks. We also compared these results with those from AB and BAB block copolymers. It is found that the similarity of the morphologies among them is caused by the number of the solvophobic block in a copolymer.Chapter 4 focuses on the self-assembly of ABC linear triblock copolymers in solvents which are poor to all of the three blocks. The effects of the volume fraction of the three blocks, of the incompatibilities among different blocks and of the solvent quality on micellar morphology are examined systematically. A variety of novel micellar morphologies and several morphological transitions are discovered. It is revealed that the shape and structure of micelles are controlled by both of the terpolymer composition and the competition between solvent quality and the incompatibility between two solvophobic blocks.