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Study on the Phase Behavior of Block Copolymer Thin Film

Study on the Phase Behavior of Block Copolymer Thin Film
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Block copolymers consist of two or more immiscible homopolymers linked by covalent bonds, which exhibit a variety of ordered phases in nano-scale through micro-phase separation. Lamellar (LAM), hexagonally perforated lamellar (HPL), double gyroid (DG), hexagonally packed cylinder (HEX) and spheres arranged in body centered cubic lattice (BCC) phases have been well investigated, and recently Fddd phase was experimentally found. The phase behavior of a block copolymer is typically determined by the composition of a block copolymer and the incompatibility between blocks which is expressed by the product of Flory-Huggins interaction parameter (χ) and degree of polymerization. Since χ is a function of temperature, various thermal phase transitions can occur at a certain composition. And an epitaxial relationship between two ordered phases is usually observed during the phase transition. The phase behavior of block copolymer thin film is different from that in bulk because block copolymer thin films are additionally influenced by interfacial interactions (with a substrate, free surface, or both) and the morphologies are geometrically confined. In this dissertation study, the phase behaviors in thin film were investigated to establish how the factors, such as film thickness and interfacial interaction, affect the phase behavior of block copolymer. In chapter 1, General introduction of block copolymer and phase transition induced by changing temperature is briefly reviewed. Especially, most part is devoted to the phase behavior of block copolymer in the form of thin film. And the principles of grazing incidence small angle X-ray scattering (GISAXS), transmission electron microscopy (TEM) and transmission electron microtomography (TEMT) used for morphological characterization of thin film are described. In chapter 2, the effect of film thickness on the phase behavior of diblock copolymer was investigated. The phase diagram was constructed for a polystyreneblock- polyisoprene (PS-b-PI, MW = 32,700, fPI = 0.670) in thin films on Si wafer as a function of film thickness over the range of 150-2410 nm (7-107L0, L0: domain spacing of HPL) and temperature. The PS-b-PI (755 nm) exhibits a variety of ordered phases from HPL via DG to HEX before going to disordered phase (DIS) upon heating. The morphology of the PS-b-PI in thin film was investigated by GISAXS, TEM and TEMT. In thin film, the phase transition temperature is difficult to be determined unequivocally with in-situ heating process since the phase transition is slow and two phases coexist over a wide temperature range. Therefore, in an effort to find an ‘equilibrium’ phase, we determined the long-term stable phase formed after cooling the film from DIS phase to a target temperature and annealing for 24 hrs at the temperature. The temperature windows of stable ordered phases are strongly influenced by the film thickness. As the film thickness decreases, the temperature window of layer-like structures such as HPL and HEX becomes wider whereas that of the DG stable region decreases. For the films thinner than 160 nm (8L0), only HPL phase was found. In the films exhibiting DG phase, HPL at the free surface was found, which gradually converts to the internal DG structure. It seems that layer structure which can minimize surface energy is preferred. The relief of interfacial tension by preferential wetting appears to play an important role to control the morphology in very thin films. In chapter 3, the pathway of phase transition upon cooling from DIS to DG stable region was investigated for PS-b-PI (Mn = 32,300, fPI=0.670) in thin film (755 nm thick) on silicon wafer. The transition from DIS to DG was monitored by GISAXS, TEM and TEMT. The transition pathway was found to be affected by quench depth and cooling rate. For a slow cooling to a shallow quench depth, the phase transition occurred in the reverse order of heating (DIS→HEX→DG). On the other hand, when the thin film was deep-quenched into the DG region (close to the phase boundary of DG and HPL), a transient HPL phase was observed before the final DG phase was formed
i.e., DIS→HEX→HPL→DG. HPL start to develop from the interfacial regions and the transformation from HEX to HPL is verified by the 3 different orientations of HPL layers which epitaxially grows from the three sets of {10}HEX. In the fast cooling, HPL occurs as a transient phase regardless of quench depth. The pathway via HPL as transient was not found in bulk. It indicates that HPL is a kinetically favored phase with respect to DG in thin film. In thin film, layer-like structure, HPL alleviates interfacial tension due to its structure, and it leads the phase transition pathway in the direction of forming a transient phase prior to reaching the thermodynamic stable phase, DG. In chapter 4, the epitaxial phase transition between DG and HEX in PS-b-PI thin film on Si wafer was investigated. The thermal transition occurred reversibly and its transitional structure was visualized using TEMT. The epitaxial transition of DG and HEX is affected by the transition direction. It was shown that one epitaxy dominated during the phase transition from DG to HEX, where the {121}DG, {111}DG and {220}DG are converted to {100}HEX, {110}HEX and {001}HEX, respectively. Although dimensional mismatch occurs in a lateral plane in this epitaxial relationship, all loci have the same path and the arms parallel to film plane of DG mostly contribute to form cylinders. When the transition starts from HEX, the other epitaxial relationship, where {100}HEX, {110}HEX and {001}HEX are changed to {121}DG, {220}DG and {111}DG ,respectively, was also observed. A 5-fold junction was detected at the transitional region, supporting the transition mechanism predicted by Matsen. In this epitaxy, two phases match in orientation and domain spacing, but cylinders are formed through different paths involving two different structures of DG. The latter epitaxy was suppressed in the transition from DG to HEX. This would be explained that the arms of DG parallel to the film plane, which could be stabilized by preferential wetting, does not contribute to the forming a cylinder in the latter epitaxy. In chapter 5, 3-dimensional Fddd network structure of PS-b-PI (Mn = 31,500, fPI = 0.645) was observed for the first time in real space by TEMT. In a 650 nm thick film of the PS-b-PI thin film on a silicon wafer, Fddd phase was developed after annealing at 215oC for 24 h. The single network structure consists of the connected tripodal units of minor PS block domains. The {111}Fddd plane, the densest plane of minor PS phase, was found to orient parallel to the film plane. The transitional structure from the wetting layer at the free surface to the internal {111}Fddd plane via perforated layer structure was also observed.
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