자외선 오존처리 다중벽 카본 나노 튜브의 분산 안정성 평가

Abstract

탄소 나노 튜브(Carbon nanotube)는 1991년 lijima박사에 의해 발견된 이후 국내 외 산업전반에 걸쳐 많은 관심을 받아오고 있으며 평면 표시소자, 고집적 메모리소자, 2차 전지 및 초고용량 캐패시터(supercapacitor), 수소저장 물질, 화학 sensor, 고강도/초경량 복합재료, 전자파차폐(EMI/RFI shielding)등 많은 용용분야에 적용성이 뛰어나며 기존의 소자들이 가지고 있는 한계를 넘어 설수 있는 가능성을 가지고 있다. 이는 기존의 나노재료들에서는 쉽게 볼 수 없었던 1TPa이 넘는 강한 기계적 성질과 106S/m에 이르는 높은 전기적 성질, 형태학적으로 100~1000에 이르는 높은 종횡비(aspect ratio), 높은 비표면적(surface area)때문이다. 이러한 많은 장점에도 불구하고 탄소 나노 튜브의 낮은 분산성 때문에 상업적 응용에 있어서 많은 제약을 받고 있다. 이는 탄소 나노 튜브의 표면이 비활성이고 기능기(functional group)가 없을 뿐만 아니라 탄소 나노 튜브 각각은 분자간의 힘인 반데르발스 힘(van der waals force)때문에 강하게 응집되고 번들현상이 나타나기 때문이다. 즉 탄소 나노 튜브의 특성들을 잘 발현시키기 위해서는 이러한 응집 현상을 해결하고, 다발 형태의 튜브를 각각의 탄소 나노 튜브나 작은 다발로 잘 나누어야 한다. 이러한 문제를 해결하고 탄소 나노 튜브를 분산시키기 위해 많은 연구가 진행되어 왔다. 그 중에 대표적인 방법이 물리적 표면 개질과 화학적 표면 개질 방법이다. 하지만 이러한 방법들에도 단점이 있다. 물리적 개질 방법의 대표적인 계면활성제(surfactant)를 사용하는 방법은 추후에 계면활성제의 제거가 쉽지 않다는 단점이 있고, 화학적 표면 개질의 대표적인 방법인 산(acid) 처리를 이용하여 수산화기(OCH)를 도입하는 방법은 공정 자체가 친환경 적이지 못하고 결정적으로 강력한 산에 의해 탄소 나노 튜브가 절단 되어 구조적 성질이 변하게 된다는 점이 있다. 이러한 문제를 해결하고자 본 연구에서는 유기물의 표면 개질 방법으로 많이 쓰이고 있으며, 공정이 간단하고 친환경적인 자외선/오존(UV/O3)처리 방법을 사용하여 탄소 나노 튜브의 표면을 개질, 표면의 화학적 변화를 살펴보았고 다양한 분산성 평가 방법을 통해 DMF(dimethyl formamide)용매 내에서의 분산 안정성을 평가하였다. 또한 탄소 나노 튜브를 다양한 분야에 응용을 위해서는 다양한 용매에서 분산시키고 분산된 정도를 평가하는 기준이 필요하다. 이를 위해 본 연구에서는 다양한 용매에서의 자외선/오존 처리한 탄소 나노 튜브의 분산 정도를 파악하기 위해 한센 용해도 지수(Hansen solubility parameter)라는 개념을 이용하여 32가지 용매에서의 분산정도를 평가 하였다. 그 결과 자외선/오존 처리한 탄소 나노 튜브의 분산안정성은 미처리한 탄소 나노 튜브에 비해 훨씬 뛰어남을 확인 할 수 있었고, 자외선/오존 처리 시간이 증가함에 따라 더욱더 분산안정성이 향상됨을 알 수 있었다. 또한 자외선/오존 처리시간이 길어질수록 탄소 나노 튜브는 더욱 많은 종류의 용매에서 향상된 분산성을 보였다. 마지막으로 탄소 나노 튜브의 분산성과 구조적 결함이 고분자/탄소 나노 튜브 복합체 필름의 기계적 성질과 전기적 성질에 미치는 영향을 관찰하기 위하여 미처리, 산, 자외선/오존 처리한 탄소 나노 튜브를 이용, 용액 캐스팅 방법을 사용하여 PMMA/MWCNT 나노 복합체 필름을 제조 하였다. 제조한 필름을 이용하여 기계적 성질과 동적 기계적 성질 그리고 전기적 성질을 측정한 결과 자외선/오존 처리한 탄소 나노 튜브로 강화한 나노 복합체 필름의 인장특성은 미처리한 나노 튜브로 제작한 나노복합체 필름과 비교 하였을 때는 높게 나왔지만, 산 처리로 만든 나노 복합체 필름과 비교하였을 때는 약간 낮은 거동을 보였고, 동적 점탄성성질 또한 산 처리 한 탄소 나노 튜브로 강화한 나노복합체 필름이 높은 저장 탄성률과 Tg 값을 나타내었다. 즉 복합체의 기계적 특성에 있어서 구조적 결함의 영향보다는 분산성의 영향이 더 큰 것으로 사료된다. 이와는 다르게 전기적 특성의 경우는 산 처리한 탄소 나노 튜브로 제작한 나노 복합체 필름의 경우 자외선/오존 처리한 탄소 나노 튜브로 제작한 복합체 필름에 비해 높은 면 저항 값을 나타냈다. 이는 산 처리에 의해 나노 튜브가 절단되면서 종횡비(aspect ratio)가 감소하였고 이로 인하여 나노 튜브 개개의 다발들의 전기적 path가 줄어서 나타나는 현상으로 사료된다.|Carbon nanotubes (CNTs) have attracted considerable attention since their discovery due to a host of unique mechanical, thermal, and electrical properties. Because of these characteristics, CNTs are often employed as an important material for a variety application in the industries. However, the functionalities of CNTs are frequently limited by their insolubility in most organic solvent. The CNTs have strong van der Waals interaction, high aspect ratio and specific surface area, which lead to aggregation and bundles. The simple method to overcome these problems is introducing various functional groups by the chemical modification on the surface of the CNTs. The functionalized groups on CNT surface were typically enhanced the limited solubility. Many various surface modification and functionalization techniques have been studied for effective introduction to functional group on CNTs surface. Among these methods, high concentration acids treatment using HNO3/H2SO4 has representative method of introducing oxygen-containing groups such as carboxyl and hydroxyl group onto the CNT surface. The presence of oxygen- containing groups facilitates the exfoliation of CNT bundles and increases the solubility in polar media. This method, however, not only creates to defects on the CNTs sidewalls, but also decreases the aspect ratio of the CNTs because the strong acids cut the CNTs into short pieces during the oxidative process, which may degrade the original mechanical properties of the CNTs. For this reason, a method of mild oxidation processing has attention to functionalization of the CNT surface. UV/ozone treatment of the CNTs has been developed as an alternative, milder method. It has many advantages, in which oxidation process can be carried out at air atmosphere, when compared with a high vacuum plasma process, the equipment and running cost are competitively low. This method is also an environment-friendly method that can easily induce chemical functionality of the CNT surface without changing the unique characteristics of them and enhance dispersion stability in polar solvent by inducing the oxygen-containing functional groups such as acid treatment. The solubility evaluation of CNTs is one of the essential processes in various studies of the CNTs, in order to apply to many fields easily. The studies of solubility of the CNTs in various organic solvents have steadily investigated. Recently, solubility parameters utilized in the polymer solubility theory have been considered to describe the behavior of CNTs-solvent mixtures. Hansen Solubility Parameters (HSPs) are widely used for predicting compatibility between two materials. The solubility of the CNTs at various solvents such as N,N-dimethylformamide (DMF), chloroform, acetone, toluene, benzene, etc., was studied in terms of HSPs. The HSPs defined solvent as point in 3-dimensional space. If a solvent point is within the boundaries of solute space, we can qualitatively apprehend the solubility behavior of CNTs in various solvents. In this study, we evaluate the dispersion stability of UV/ozone treated MWCNT in DMF(Dimethylformamide)solvent using various analysis methods. Moreover, we studied the solubility behaviors of the UV/ozone treated MWCNTs through the application of the hansen solubility parameters using 32 kinds of solvents. In addition, we report a systematic evaluation of the influence of dispersion and structural defect of carbon nanotube by acid treatment in Polymer/MWCNT composite. To determine this concept, the PMMA/MWCNT composite films were prepared using UV/ozone and acid treatment by solution casting method and then studied their mechanical, electrical properties.; Carbon nanotubes (CNTs) have attracted considerable attention since their discovery due to a host of unique mechanical, thermal, and electrical properties. Because of these characteristics, CNTs are often employed as an important material for a variety application in the industries. However, the functionalities of CNTs are frequently limited by their insolubility in most organic solvent. The CNTs have strong van der Waals interaction, high aspect ratio and specific surface area, which lead to aggregation and bundles. The simple method to overcome these problems is introducing various functional groups by the chemical modification on the surface of the CNTs. The functionalized groups on CNT surface were typically enhanced the limited solubility. Many various surface modification and functionalization techniques have been studied for effective introduction to functional group on CNTs surface. Among these methods, high concentration acids treatment using HNO3/H2SO4 has representative method of introducing oxygen-containing groups such as carboxyl and hydroxyl group onto the CNT surface. The presence of oxygen- containing groups facilitates the exfoliation of CNT bundles and increases the solubility in polar media. This method, however, not only creates to defects on the CNTs sidewalls, but also decreases the aspect ratio of the CNTs because the strong acids cut the CNTs into short pieces during the oxidative process, which may degrade the original mechanical properties of the CNTs. For this reason, a method of mild oxidation processing has attention to functionalization of the CNT surface. UV/ozone treatment of the CNTs has been developed as an alternative, milder method. It has many advantages, in which oxidation process can be carried out at air atmosphere, when compared with a high vacuum plasma process, the equipment and running cost are competitively low. This method is also an environment-friendly method that can easily induce chemical functionality of the CNT surface without changing the unique characteristics of them and enhance dispersion stability in polar solvent by inducing the oxygen-containing functional groups such as acid treatment. The solubility evaluation of CNTs is one of the essential processes in various studies of the CNTs, in order to apply to many fields easily. The studies of solubility of the CNTs in various organic solvents have steadily investigated. Recently, solubility parameters utilized in the polymer solubility theory have been considered to describe the behavior of CNTs-solvent mixtures. Hansen Solubility Parameters (HSPs) are widely used for predicting compatibility between two materials. The solubility of the CNTs at various solvents such as N,N-dimethylformamide (DMF), chloroform, acetone, toluene, benzene, etc., was studied in terms of HSPs. The HSPs defined solvent as point in 3-dimensional space. If a solvent point is within the boundaries of solute space, we can qualitatively apprehend the solubility behavior of CNTs in various solvents. In this study, we evaluate the dispersion stability of UV/ozone treated MWCNT in DMF(Dimethylformamide)solvent using various analysis methods. Moreover, we studied the solubility behaviors of the UV/ozone treated MWCNTs through the application of the hansen solubility parameters using 32 kinds of solvents. In addition, we report a systematic evaluation of the influence of dispersion and structural defect of carbon nanotube by acid treatment in Polymer/MWCNT composite. To determine this concept, the PMMA/MWCNT composite films were prepared using UV/ozone and acid treatment by solution casting method and then studied their mechanical, electrical properties.