Study on the Phase Behavior of Block Copolymer Thin Film

Abstract

Block copolymers consist of two or more immiscible homopolymers linkedby covalent bonds, which exhibit a variety of ordered phases in nano-scale throughmicro-phase separation. Lamellar (LAM), hexagonally perforated lamellar (HPL),double gyroid (DG), hexagonally packed cylinder (HEX) and spheres arranged inbody centered cubic lattice (BCC) phases have been well investigated, and recentlyFddd phase was experimentally found.The phase behavior of a block copolymer is typically determined by thecomposition of a block copolymer and the incompatibility between blocks which isexpressed by the product of Flory-Huggins interaction parameter (χ) and degree ofpolymerization. Since χ is a function of temperature, various thermal phasetransitions can occur at a certain composition. And an epitaxial relationship betweentwo ordered phases is usually observed during the phase transition.The phase behavior of block copolymer thin film is different from that inbulk because block copolymer thin films are additionally influenced by interfacialinteractions (with a substrate, free surface, or both) and the morphologies aregeometrically confined. In this dissertation study, the phase behaviors in thin filmwere investigated to establish how the factors, such as film thickness and interfacialinteraction, affect the phase behavior of block copolymer.In chapter 1, General introduction of block copolymer and phase transitioninduced by changing temperature is briefly reviewed. Especially, most part isdevoted to the phase behavior of block copolymer in the form of thin film. And theprinciples of grazing incidence small angle X-ray scattering (GISAXS), transmissionelectron 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 diblockcopolymer 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 afunction of film thickness over the range of 150-2410 nm (7-107L0, L0: domainspacing of HPL) and temperature. The PS-b-PI (755 nm) exhibits a variety of orderedphases from HPL via DG to HEX before going to disordered phase (DIS) uponheating. 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 bedetermined unequivocally with in-situ heating process since the phase transition isslow and two phases coexist over a wide temperature range. Therefore, in an effort tofind an ‘equilibrium’ phase, we determined the long-term stable phase formed aftercooling the film from DIS phase to a target temperature and annealing for 24 hrs atthe temperature. The temperature windows of stable ordered phases are stronglyinfluenced by the film thickness. As the film thickness decreases, the temperaturewindow of layer-like structures such as HPL and HEX becomes wider whereas thatof the DG stable region decreases. For the films thinner than 160 nm (8L0), only HPLphase was found. In the films exhibiting DG phase, HPL at the free surface wasfound, which gradually converts to the internal DG structure. It seems that layerstructure which can minimize surface energy is preferred. The relief of interfacialtension by preferential wetting appears to play an important role to control themorphology in very thin films.In chapter 3, the pathway of phase transition upon cooling from DIS to DGstable region was investigated for PS-b-PI (Mn = 32,300, fPI=0.670) in thin film (755nm thick) on silicon wafer. The transition from DIS to DG was monitored byGISAXS, TEM and TEMT. The transition pathway was found to be affected byquench depth and cooling rate. For a slow cooling to a shallow quench depth, thephase transition occurred in the reverse order of heating (DIS→HEX→DG). On theother hand, when the thin film was deep-quenched into the DG region (close to thephase boundary of DG and HPL), a transient HPL phase was observed before thefinal DG phase was formed

i.e., DIS→HEX→HPL→DG. HPL start to develop fromthe interfacial regions and the transformation from HEX to HPL is verified by the 3different 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 quenchdepth. The pathway via HPL as transient was not found in bulk. It indicates that HPLis a kinetically favored phase with respect to DG in thin film. In thin film, layer-likestructure, HPL alleviates interfacial tension due to its structure, and it leads the phasetransition pathway in the direction of forming a transient phase prior to reaching thethermodynamic stable phase, DG.In chapter 4, the epitaxial phase transition between DG and HEX in PS-b-PI thinfilm on Si wafer was investigated. The thermal transition occurred reversibly and itstransitional structure was visualized using TEMT. The epitaxial transition of DG andHEX is affected by the transition direction. It was shown that one epitaxy dominatedduring 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. Althoughdimensional mismatch occurs in a lateral plane in this epitaxial relationship, all locihave the same path and the arms parallel to film plane of DG mostly contribute toform 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 thetransitional region, supporting the transition mechanism predicted by Matsen. In thisepitaxy, two phases match in orientation and domain spacing, but cylinders areformed through different paths involving two different structures of DG. The latterepitaxy was suppressed in the transition from DG to HEX. This would be explainedthat the arms of DG parallel to the film plane, which could be stabilized bypreferential 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 650nm thick film of the PS-b-PI thin film on a silicon wafer, Fddd phase was developedafter annealing at 215oC for 24 h. The single network structure consists of theconnected tripodal units of minor PS block domains. The {111}Fddd plane, the densestplane of minor PS phase, was found to orient parallel to the film plane. Thetransitional structure from the wetting layer at the free surface to the internal{111}Fddd plane via perforated layer structure was also observed.