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블록공중합체를 이용한 나노다공성 구조의 제작과 그 분석

블록공중합체를 이용한 나노다공성 구조의 제작과 그 분석
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Block copolymers have been widely employed to develop functional materials with nanostructure due to its characteristic microphase behavior. Especially, polystyrene-block-poly(methyl methacrylate) copolymer has been used for various functional nanostructure because of its capability of selective removal of PMMA in block copolymer. In this thesis, interesting porous structures were fabricated based on PS-b-PMMA and analyzed with arising analytical techniques like electron tomography, X-ray tools, and optical analysis. And the porous materials are applied to antireflection coating. In chapter 2, cylindrical nanoporous structures were prepared by using a mixture film of PS-b-PMMA and PMMA homopolymer (hPMMA), and they were analyzed by transmission electron microtomography (TEMT), X-ray reflectivity (XR), and grazing incidence small-angle X-ray scattering. For this purpose, the mixture film was spin-coated onto a silicon wafer modified by a neutral brush for PS and PMMA blocks, which generates PMMA cylindrical microdomains oriented normal to the substrate. Two methods were employed to prepare nanoporous structures: (1) all the PMMA phase (PMMA block and PMMA homopolymer) in the film was removed by UV irradiation followed by rinsing with a selective solvent (acetic acid) to PMMA
and (2) only PMMA homopolymer was removed by the selective solvent etching without UV irradiation. We found via TEMT and XR that the nanoporous structure in the film prepared by UV irradiation exhibited almost perfect cylindrical shape throughout the entire film thickness. On the other hand, when the film was rinsed with a selective solvent, nanoporous structures were not straight cylinders, but exhibited a funnel shape that the diameter of nanopores located near the top of the film was larger than that located near the bottom of the film. In chapter 3, broadband antireflection (AR) covering from visible light to near infrared (NIR) wavelengths (400 ~ 2000 nm) was obtained out by using three sequential spin coatings of PS-b-PMMA with different volume fractions of PMMA block (fPMMA) on a glass. PS-b-PMMA having the lowest PMMA volume fraction (fPMMA ~ 0.3) among three PS-b-PMMAs was first spin-coated on a glass substrate. After spin-coating, the film was irradiated by ozone to prevent dissolution during the next spin-coating process. Then, PS-b-PMMA with the next larger volume fraction of PMMA block (fPMMA ~ 0.46) was spin-coated, and irradiated again by ozone. Finally, PS-b-PMMA with the largest volume fraction of PMMA block (fPMMA ~ 0.69) was spin-coated. After three sequential spin-coatings, the entire film was irradiated under UV followed by rinsing with acetic acid, which removed PMMA blocks. This process allowed us to have the triple layers with sponge-like nanoporous structures that the refractive index increases from the top to the bottom of the film. The morphology of the triple-layered nanoporous block copolymer films was investigated by scanning electron microscopy. The nanoporous film exhibited excellent broadband AR at wavelengths from 400 and 2000 nm. The measured reflectance curves are in good agreement with the calculation from the characteristic matrix theory. This AR coating would be used for the development of solar cells with high power convergence efficiency. In chapter 4, we prepared an inorganic nanoporous film with dense skin layer by using sponge-like nanoporous block copolymer template. After spin coating of PS-b-PMMA with 0.46 of PMMA volume fraction, PMMA block was selectively removed by UV irradiation. With this nanoporous template, silica precursor was spincoated followed by high temperature thermal treatment for silica conversion. The product showed interesting porous structure consisted of a dense silica dense skin layer on a sponge-like nanoporous layer. At visible light range, it showed low reflectance under 1%. The measured reflectance curves were good agreement with the calculation from characteristic matrix theory. And from pencil hardness test, this silica porous strcuture showed great increase of its mechanical strength than PS-b-PMMA porous film. We expect that this film could be used for antireflection coating film for flat display or touch panel required anti-scratch property.
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