Simulated Annealing Study Of Self-Assenbly Of ABA Triblock Copolymer Brushes In Selective Solvents

Polymer brushes, that composed of flexible polymer chains which are tethered onto a surface or interface by one end, due to their extremely important role in the field of surface modifications, have become one of the most attractive branches in polymer science. Compared with homopolymer brushes, block copolymer brushes immersed in a selective solvent can form more complicated surface morphologies because of the increased number of species and thus the increased number of controlling parameters in the system. And therefore, block copolymer brushes have attracted considerable attentions in both theoretical and experimental studiesIn the bulk, it has been illustrated that, ABA triblock copolymer melts exhibit very similar phase behavior to their analogous AB diblock copolymer melts. However, a distinct difference between them is that in ABA triblock copolymers, the middle B-blocks can form bridges between two different A-domains, which are believed to significantly improve their bulk mechanical properties. In contrast to the AB diblock copolymer brushes, only a few studies have been focused on ABA triblock copolymer brushes. Therefore, it is desirable to investigate systematically the morphological dependence of the system on factors such as the grafting density and the lengths of different blocks.In this thesis, we investigate the morphologies and structural transitions of linear ABA triblock copolymer brushes immersed in selective solvent using the simulated annealing method. The influences of the grafting densities, the block lengths and interactions between different monomers on the morphology of the copolymer brushes have been investigated by constructing the phase diagram, showing snapshots of the morphologies, calculating the density profile of the monomers and the fraction of the folded chains.In chapter one, the research topics of the thesis, the model and method used in the research are introduced briefly, and the structural arrangement of the thesis is summarized. In chapter two to four, we study the situation where the solvent is assumed to be poor for the tethered and top A-blocks but good for the middle B-blocks. In chapter two, we focus on the self-assembly of linear ABA triblock copolymer brushes immersed in selective solvent in which the lengths of the tethered and top A-blocks are equal, i.e., symmetric ABA triblock copolymers. The phase diagrams for systems at three different grafting densities (σ=0.0625,0.111,0.25) have been constructed to illustrate the dependence of the morphological transitions on the length of the blocks. The phase diagrams exhibited two types of brush structures:structures with only one A-rich layer and structures with two A-rich layers. Morphological transitions from structures with only one A-rich layer to structures with two A-rich layers may occur at each grafting density when increasing the length of the middle soluble B-blocks. And the boundary between the two types of structures is sensitive to the grafting density of the copolymers. In particular, the boundary moves to a smaller value of NB so that the phase region of structures with only one A-rich layer becomes narrower at a higher grafting density, indicating that increasing the grafting density leads to an enhanced occurrence of structures with two A-rich layers.For both types of structures, the detailed morphologies of A-aggregates depend on the grafting density of the copolymers and the length of the blocks. For structures with only one A-rich layer, depending on the length of blocks and the grafting density, the grafted triblock copolymers can self-assemble to form a variety of A-micelles, including semispherical pinned micelles, wormlike micelles, stripes, a perforated layer and a layer. For structures with two A-rich layers, morphologies, such as semispherical pinned micelles, wormlike micelles, stripes and a perforated layer, and a complete layer in both the bottom A-rich layers and the top A-rich layers can be obtained by choosing the grafting density and the length of blocks. It is found that the structures with only one A-rich layer result from a situation in which all the triblock copolymer chains adopt a hairpin-like folded conformation to avoid the unfavorable contact of A-monomers with solvents. Furthermore, it is found that the conformation of chains plays an important role in the formation of self-assembled morphologies of the amphiphilic triblock copolymer systems. In addition, it is also found that with the increase of the interaction between A monomer and solvent, the system tends to form structures with only one A-rich layer; whereas, when we increase the attractive interaction between B monomer and solvent, or the interaction between A and B monomers, the system tends to form structures with two A-rich layers.In chapter three, we investigate the influence of the asymmetry of the copolymer chains on the morphologies of the ABA triblock copolymer bushes by two different means (fixing the lengths of two blocks and changing the length of the third one, or constructing the phase diagram of the system in the N1-N3space). In both of the two means, it is found that morphological transitions from structures with one A-rich layer to structures with two A-rich layers can be induced by increasing the length of the outer A-blocks as well as by decreasing the length of the tethered A-blocks. In addition, we found that the A-monomers of the bottom A-rich layer from different blocks (tethered or top A-blocks) of the chain are not mixed randomly. Specifically, when the length of the tethered A-block is relatively smaller, the A-monomers from the top A-blocks "folding down" to the bottom of the brush, distribute mainly in the center of the bottom A-rich layer; whereas, when the length of the tethered A-block is relatively longer, the top A-monomers of the folded chain will cover onto the top of the A-layer composed of the tethered A-blocks.In chapter four, we focus on the topic that "how the copolymer brushes self-assemble into classic layer structures" and investigated the influences of several factors, such as chain lengths and interactions between different spices, on the detailed morphologies of the layer structures. It is found that, with increasing the hydrophobicity of the A-monomers and the selectivity of the board to A-monomers, or decreasing the incompatibility between A and B monomers and the hydrophobicity of the B-monomers, several types of layer structures have been observed and discussed in the last of the chapter.Finally, in chapter five, we study the phase behavior of the ABA triblock copolymer brushes in selective solvents which is good for the A-block but poor for the middle B-block. The influence of the block lengths, for both the soluble A-block and the insoluble B-blocks, on the morphology of triblock copolymer brushes in solution is studied. It is shown that the morphological transitions of the aggregates, from spheres to stripes, to perforated layer, and then to layer with increasing the length of the B-block, and from stripes to wormlike micelles with increasing the length of the A-block, can be observed.