Solvent Induced Morphology And Transitions Of PS-b-PEO

Crystalline block copolymers, which contain at least one crystalline block and one amorphous block, have been widely studied in recent years. The incompatibility between the different constituent blocks will result in microphase separation, and therefore various self-assembled structures are fabricated. Crystallization will further modify the microphase-separated morphology, which can either be confined in phase-separated nanostructures or breakout. So the phase behavior of semicrystalline block copolymers is more complex and interesting. Crystalline-amorphous diblock copolymers, which are relatively simple, are the most widely studied crystalline diblock copolymers. So far, little knowledge is about the crystalline behavior when crystallization and microphase separation happen simultaneously, especially to the evolution of morphology and rearrangement of structures in the film state by solvent induced.This thesis focused on studying the self-assembly behavior in solution and the crystalline behavior by solvent induced of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin films via AFM and TEM. The effect of the inorganic slats toward self-assembly behavior was also investigated.The diblock copolymer was dissolved in the non-selective solvent tetrahydrofuran (THF). Thin film was prepared by spin-coating from PS-b-PEO THF solution onto the Si wafers, and then exposed to the saturated THF, toluene and the mixed water/toluene vapor in the closed vessels for different periods. Then the samples were moved from the vessel for drying and characterized by AFM. When thin film was induced by THF, the surface relief of the membrane increased. While the final formed morphology was breakout crystallization when thin film was induced by toluene. However, when thin film was induced by the mixed solvent, the“holes”were observed and the final morphology was controlled by the coupling of crystallization and microphase separation.The PS-b-PEO diblock copolymers could self-assemble micelle cluster and grow“sandwich”square slice in dilute toluene solution. The evolution from spherical micelle to square slice in solution and structure transitions from spherical micelle and square slice to cylindrical microdomains annealed by the solvent were both discussed in detail. The structure transitions of the formed ordered and anisotropic self-assembly structures induced by the solvent were complex. In mixed water/toluene, spherical micelle and square slice would both undergo the rearrangement of the copolymer chains to construct a new order state (arrays of cylindrical microdomains). Furthermore, the addition of water in mixed solvent was the key factor to the rearrangement, which helped to overcome the energy barrier of the restructure. The induced morphology of the sample with 24h aging is more orderly than that with 2 months aging.The influence of cadmium acetate on the self-assembly behavior of PS-b-PEO diblock copolymers was also investigated. We also considered other factors, such as the initial polymer concentration and aging time. With the decreasing of the concentration of copolymer solution, the morphology of micelles changed from toroidal micelles to rod-like micelles. There was no significant effect about aging time on toroidal micelles, when the concentration of copolymer solution was high. While the axial length of rod-like micelles increased with adding aging time, the concentration of copolymer solution was low. When the concentration of copolymer solution was 5mg/mL, the morphology was breakout crystallization with the addition of cadmium acetate. And different contents of salt led to different crystal structures. However, when the concentration of copolymer solution was 1mg/mL, the addition of cadmium acetate was benefit for the forming of rod-like micelles and different contents of salt led to different diameters.