Limited Space, The Block Copolymer Self-assembly Behavior Of Simulated Annealing Study

The self-assembly of block copolymers has attracted much scientific interest due to the formation of rich microstructures and potential applications of these microstructures. For the simplest case of diblock copolymers which are linear polymers composed of two different subchains, their bulk phase behavior is well known from both experiments and theories. A variety of ordered bulk phases, including lamellae, hexagonally packed cylinders, body-centered-cubic spheres, and a bicontinuous network structure called Gyroid are observed for diblock copolymers. In practice, confinement is frequently employed to induce long range ordering of structures resulting from self-assembly of block copolymers. Confinement effects, including the degree of structural frustration, confinement-induced entropy loss and surface-polymer interaction, influence the self-assembly process and can generate novel microstructures that may have potential novel applications.In this thesis, we systematically investigated the self-assembly behaviors of block copolymers under confinement using simulated annealing technique. The purposes of this thesis include predicting the underlying rules of self-assembled structures with the variation of various parameters, understanding the origin of the confinement-induced morphologies, elucidating the mechanism for the morphological transitions between novel structures and finally providing guidelines to design rich microstructures in experiments.In chapter one, we briefly introduced the background of block copolymer researches and computer simulation methods involved in the study. In the following chapters, we presented our researches on block copolymer melts in the bulk, confined between two parallel surfaces, and confined in cylindrical and spherical nanopores in detail.In chapter two, firstly, we systematically investigated the self-assembly of diblock copolymer melts in the bulk. Then, we calculated the characteristic periods for the various bulk equilibrium structures, which is crucial for the further studies of diblock copolymers under confinement. Furthermore, we studied the self-assembly of two kinds of cylinder-forming diblock copolymers with different volume fraction confined between two parallel surfaces. As a conclusion, we elucidated the influence of volume fraction on the self-assembled morphologies under confinementIn chapter three, we systematically investigated the self-assembled morphologies of diblock copolymers confined in cylindrical nanopores. The pore diameter and surface preference are systematically varied to examine their effects on the self-assembled morphologies and the chain conformations. A rich variety of novel structures under the two-dimensional confinement has been revealed. The morphological transitions between different structures are elucidated. For bulk lamella-forming diblock copolymers, perpendicular lamellae and concentric lamellae spontaneously form under the neutral and strongly preferential surface, respectively. When the surface preference is relatively weak, novel structures such as parallel lamellae, helices and catenarian structures are observed. A simple model is proposed for the concentric-cylindrical lamellae, which gives a reasonable description of the layer thickness. Good agreement between the computed value and the model predictions are obtained. For bulk cylinder-forming diblock copolymers, a series of novel structures such as helices and stacked toroids are identified which cannot form in bulk block copolymers. The formation of these novel structures is correlated with the ratio between the pore diameter and the bulk period, reflection the effects of confinement-induced structural frustration to the self-assembly. For bulk Gyroid-forming diblock copolymers, a series of novel morphologies including perpendicular, parallel, concentric and concentric perforated lamellae are observed with varying the pore diameter, surface preference and the volume fraction. Our simulation results are consistent with the available experiments.In chapter four, the self-assembly of diblock copolymers confined in channels of various shaped cross sections is studied. For the symmetric diblock copolymers, the cross sections of the confining channels are of different shapes including regular triangle, square and ellipse. For square and regularly triangular cross sections, a generic morphological transition sequence, from perpendicular lamellae to parallel lamellae to concentric lamellae or trifurcate lamellae is predicted with increasing the surface preference. For elliptic cross sections with weakly preferential surfaces, parallel lamellae occur when the ratio between the short axis length and the characteristic periods is close to an integer. For the bulk cylinder-forming diblock copolymers, the cross sections of the confining channels also include rectangules, regular hexagons and regular octagons besides the above mentioned shapes. Multiple packed cylinders and more complex structures such as helices or stacked toroids spontaneously form for low-symmetry and high-symmetry cross sections, respectively. Furthermore, the symmetry and domain spacing of these structures can be altered by the shape and size of the confining pore. Our studies indicate that the self-assembled morphologies of copolymers and their symmetry can be manipulated by imposing different shaped nanopores.In chapter five, we systematically investigated the self-assembly of diblock copolymers confined into spherical nanopores. For the bulk lamella-forming diblock copolymers, the observed sequence of stable structures is from perpendicular lamellae to helices or embedded structures and finally to concentric-spherical lamellae as the strength of the surface preference is increased gradually from neutral to weakly preferential and finally to strongly preferential. A new model is proposed which can describe the layer thichness for the simulated concentric-spherical and concentric-cylindrical lamellae better than the model proposed in chapter three. For the bulk cylinder-forming diblock copolymers, two systems of A₂B₁₀ and A₃B₉ are considered. In the former case, the minority blocks form curved cylinders packing into concentric layer in spherical pores. However, concentric-perforated lamellae are observed in the latter case. This different is attributed to the larger volume fraction of the minority blocks in the latter case. Furthermore, for the bulk body-centered-cubic sphere-forming diblock copolymers, a series of structures with dispersed spheres spontaneously form in confined spacing. Our studies elucidate that the self-assembled morphologies under these spherical-pore confinement are more complex than those under cylindrical-pore or under plane surface confinement.In chapter six, we systematically investigated the self-assembly of lamella-forming linear ABC triblock copolymers confined into cylindrical and spherical nanopores. In the cylindrical nanopores which prefer to the A-blocks, a series of novel lamellar morphologies are observed with increaing the pore diameter. And the results show the copolymers prefer forming concentric-lamellar structures with decreasing the interactions between different blocks when the pore diameter is relatively small. In the spherical nanopores, the pore diameter, surface preference and the volume fraction of the copolymers are systematically varied to examine their effects on the self-assembled morphologies. When the pore-surface is neutral or preferential for both of the terminal blocks, a variety of novel patchy structures spontaneously form. And the results show the number and the size of the patches in each structure can be tuned through varying the pore diameter and the strength of the surface preference.