블록공중합체 박막의 용매증기처리법에 대한 연구

Abstract

Block copolymers (BCPs) are composed of chemically different polymers linked by covalent bond and can self-assemble to form various nanometer-scale ordered structures. Segregation power which induces microphase separation is represented by χN where χ is the Flory-Huggins interaction parameter and N is the degree of polymerization. A large number of intriguing microstructures have been reported. For the block copolymers at the glassy state, a proper phase development process (annealing) is essential to obtain a well-ordered microstructure. Thermal annealing (TA) and solvent vapor annealing (SVA) are the two representative methods for the purpose. In TA, block copolymers are annealed at a temperature higher than the glass transition temperature (Tg) for good chain mobility. Although TA has been widely used to develop a thermodynamically stable phase at the annealing temperature, it takes a long annealing time at high temperature that may cause degradation of polymer chains, particularly for high MW or thermally vulnerable polymers. On the other hand, SVA provides the chain mobility by solvent swelling of block copolymers and it can be carried out under a milder temperature condition and in a shorter annealing time than TA. However, the microstructure formation process in SVA is far more complicated than TA since it involves a large swelling and deswelling process, and the detailed microstructure development mechanism is not fully understood. In this dissertation study, the critical factors affecting SVA process were investigated to gain more insight on the process. And the phase behaviors of well-characterized diblock/triblock copolymer were estimated systematically. In chapter 1, general introduction of block copolymers and microphase development methods are described. In addition, morphological analysis methods, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) are described briefly. In chapter 2, several factors affecting the swelling of polymer films upon SVA are investigated such as MW of polymer, solvent type, initial pressure, and temperature. MW of polymer does not affect the swelling process significantly except that the dewetting phenomena becomes more serious as MW decreases. Swelling of the polymer thin films is greatly affected by the solvent type, but the solubility parameter is found not a good parameter to predict the swelling property. Individual swelling experiment appears necessary to predict the selectivity of a solvent to the individual blocks of block copolymers. Initial air pressure of the annealing chamber influences the swelling and annealing speed of SVA greatly. When the annealing chamber is pre-evacuated before introducing the solvent, the swelling/annealing rate is greatly enhanced. And a comparative study of TA and SVA on the ordering of a polystyrene-block-polyisoprene (PS-b-PI) (Mn = 35.6 k, Mw/Mn = 1.01, fPI = 0.67) film was performed. The HPL structure obtained by SVA shows a poorer ordering than TA. This is in part ascribed to the volume change of the polymer film during TA and SVA. In a TA, thermal expansion occurs about a few % at 100~200℃ only and the developed microstructure is pinned by rapid temperature quenching with a small volume change. Therefore, the developed microstructures are preserved without significant distortion. On the other hand, a SVA usually involves a large volume change (sometimes more than 100 %) and a significant distortion of the microstructure developed in the swollen state during the deswelling process. In chapter 3, temperature effect on SVA of a PS-b-PI film was investigated. In most of the reports in the literature, the TA and SVA combined experiments were conducted after raising the temperature above Tg to reduce the annealing time and/or to reduce defects. As a result, the annealing time could be reduced much to a better defect free morphology. However, the annealing temperature was still high enough to expose the polymer to the thermal degradation situation and the swelling degree of the polymer was not well controlled. This study shows that SVA efficiency can be greatly enhanced by a mild increase of the annealing temperature. For example, when the temperature of the annealing chamber is increased by 20oC above the room temperature, the degree of swelling decreases significantly (ca. 30%) at the same solvent vapor pressure. Nonetheless, the morphology of PS-b-PI develops much faster to a better ordered structure. It was demonstrated with two other PS-b-PI BCPs with HPL and HEX structures. This effect seems to be due to the facts that the low swelling of the polymer film at an elevated temperature renders high enough chain mobility as well as higher incompatibility of the blocks than a SVA at room temperature. In addition, the low swelling helps reducing the distortion during the deswelling process. In chapter 4, morphology of well-purified triblock terpolymers was investigated by X-ray scattering and transmission electron microscopy after SVA and compared with that developed after thermal annealing. Three polystyrene-block-polyisoprene-block-poly(methyl methacrylate) (PS-b-PI-b-PMMA, SIM-1, 2 and 3) of different compositions were employed after purification by high performance liquid chromatography. SIM-1, 2 and 3 show cylinder-on-lamellar (col), col, and 3-phase-4-layer morphologies after thermal annealing, respectively. CHCl3 and THF were used for SVA, known as neutral solvents for all block components of the triblock terpolymers. After SVA under CHCl3 vapor, SIM-1, 2 and 3 show microphases far better developed than thermal annealing, but exhibit different morphologies, cylinder-in-cylinder (cic), cylinder-on-cylinder (coc) and col phase, respectively. On the other hand, SVA under THF vapor did not change the morphology that was obtained by thermal annealing. The interesting phase behavior can be reasonably explained by the difference in the swelling ratio of the polymer blocks in the SVA process. Separate solvent swelling experiments show that CHCl3 swell PMMA substantially more than PS or PI while THF swells the three homo-polymers almost equally. Therefore, the volume fraction increase of PMMA in the SVA with CHCl3 seems to result in different morphologies. Considering that CHCl3 and THF have very similar solubility parameters, this result indicates that the phase behavior during the SVA process is not simply predictable by solubility parameters and separate swelling measurements seem necessary for a better prediction of the solvent selectivity in SVA. In addition, by virtue of the well-developed morphology of the col phase after SVA in CHCl3, it is confirmed that the PI block forming cylinder domains at the interface between PS and PMMA lamellar domains are located in the PS domain with a staggered configuration. The orientation effect of SVA is also confirmed that cic and coc phases developed perpendicular to the substrate in SIM thin film upon SVA with CHCl3.