For many practical applications of carbon nanotubes (CNTs), especially single-walled carbon nanotubes (SWNTs), the fabrication of SWNT superstructures with well-defined morphology, density, and direction is necessary to enhance the physical properties of SWNTs collectively or to create new functionalities. By utilizing the self-assembly of block copolymer, which shows rich phase behavior depending on the external conditions, such as concentration or temperature, we may achieve the fabrication of well-ordered SWNT superstructures in a simple and easy way. In this dissertation, as the first step toward this goal, the SWNT-induced phase behavior of block copolymer systems was studied and SWNT distribution was investigated using small-angle x-ray and neutron scattering. To the best of our knowledge, this is the first report of SWNT-induced lyotropic phase behavior study, so it can provide key information in the design of self-assembled SWNT superstructures.
Among various kinds of Pluronic polymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), simply PEO-PPO-PEO), F108 (PEO130-PPO60-PEO130) and F127(PEO100-PPO70-PEO100) can be the best dispersant of SWNT due to the long PPO region (which helps stabilize hydrophobic interaction) as well as long PEO (which enables SWNT dispersion by facing water). By 1 hr tip sonication, the PPO of Pluronics can adsorb on the surface of exfoliated SWNT by hydrophobic interaction. After ultracentrifugation (4 hr, 110000 g) and decanting of upper ca. 70% solution, F108-SWNT and F127-SWNT solutions can be obtained that maintained their isolation condition for more than two months.
After the redispersion process of both freeze-dried F108-SWNT and F127-SWNT in H2O, respectively, the stock solutions were divided into 20 conical tubes and evaporated to make 5 ~ 100 wt% concentration in a 5 wt% step. To observe the SWNT effect, F108-only and F127-only samples in aqueous solution were prepared using the same process. Using small-angle x-ray scattering (SAXS), the thermotropic/lyotropic phase of F108-SWNT as well as the lyotropic phase of F127-SWNT were measured. In an F108 system, as the concentration increases from 5 wt% to 100 wt%, the F108-SWNT in water exhibits a sequence of lyotropic phase transitions, isotropic-hexagonal-FCC-BCC-lamellar transitions, which is in clear contrast with that of the F108-only samples, isotropic-BCC-lamellar transitions. This clearly indicates that the hexagonal and the FCC phases are newly induced by the presence of long one-dimensional SWNTs. The SWNTs maintain their individuality or very small bundle state up to 80 wt% before the lamellar phase is reached. These suggest that, in the hexagonal phase, the SWNTs may be still located in the hydrophobic core of F108 cylinders. The epitaxial transitions between the hexagonal and the FCC phases, and between the FCC and the BCC phases, allowed us to identify the possible orientations of SWNTs in the FCC and the BCC phases: [110] in the FCC and either <100> or <111> in the BCC. In the lamellar phase, the SWNTs exist most likely in the hydrophobic layers forming aggregations among them. In thermotropic phase behavior, SWNT-induced hexagonal phases were also observed, and the phase diagram of both F108 and F108-SWNT were drawn with their lattice parameters.
The F127-SWNT sample, where F127 originally has a hexagonal phase, shows a re-entrant hexagonal phase (i.e. two hexagonal phase regions) during lyotropic phase behavior in water. The SAXS measurements show that as the concentration is increased from 5 to 100 wt% by evaporation, the F127-SWNT/water system exhibits isotropic-hexagonal-(FCC/hex)-BCC-hexagonal-lamellar phase transitions, which is in contrast with the F127/water system showing isotropic-FCC-BCC-hexagonal-lamellar phase transitions. This clearly indicates that, in the F127-SWNT/water system, the 1st hexagonal phase is induced by the presence of long one-dimensional SWNTs and the 2nd hexagonal phase originates from the phase behavior of F127 itself. The presence of SWNTs also induced the shifts of phase transition concentrations and the change of domain sizes. The SWNTs in the hexagonal phases are located in the hydrophobic core (PPO domain) of F127 cylinders, making their orientation parallel to the [001] direction. The possible orientations of SWNTs (which maintain their individuality or very small bundle state) in the FCC and BCC phases were identified based on the epitaxial transitions between corresponding phases (hexagonal to FCC, FCC to BCC, and BCC to hexagonal): [110] in the FCC and <111> in the BCC.
The selective distribution of p-SWNTs, which utilizes a noncovalently functionalization method by in-situ free radical polymerization of surfactant (cetyltrimethylammonium 4-vinylbenzoate, CTVB), in a Pluronic P84(PEO19-PPO43-PEO19)/water/oil ternary system was also investigated. Prepared p-SWNT, which is stable for few months, has 5 nm diameter (1 nm SWNT with 2 nm surfactant shell thickness) with ca. 500 nm length. It was reported that a P84/w/o ternary system has nine different phases at 25oC by controlling the mass ratio of P84, water, and p-xylene. Here, the reverse hexagonal phase (H2), by mixing 46/18/36 composition in weight ratio, was adopted for the nanotemplate of the selectively-distributed SWNT system. Here, p-SWNTs from 0 to 8 wt% with respect to water were added and mixed using an alternative centrifuge method (~200 times). To verify whether the hydrophilically functionalized SWNTs are located in polar or apolar region, two kinds of contrast sets were prepared and measured by small-angle neutron scattering (SANS) with contrast variation. By small-angle neutron scattering, clear 1:V3 :2 was obtained for all sets, indicating typical hexagonal phases, and the first-order Bragg peak intensity change was plotted against the p-SWNT concentration in water. As expected, the scattering intensity increases in a positive contrast set, where the average SLD of the apolar domain is larger than that of the polar domain, while the intensity decreases in a negative contrast set where the average SLD of the apolar domain is smaller than that of the polar domain. The difference between simulation result and experimental result can be understood by the defect effect (due to the insertion of p-SWNT).
This new SWNT-induced lyotropic phase behavior in a block copolymer system and selective distribution of SWNTs in desired distribution may provide a new scalable route to fabricate SWNT superstructures with well-defined architecture and new functionalities. Furthermore, this may provide new possibilities for self-assembling other one-dimensional nanoparticles into highly ordered superstructures using the phase behavior of block copolymer systems.
탄소나노튜브 - 특히 단일벽 탄소나노튜브(SWNT) - 의 우수한 물리적 특성을 잘 활용할 수 있는 실용적인 응용을 위해서, 잘 정의된 형태, 밀도, 방향성을 갖는 탄소나노튜브 초구조체의 제조가 매우 절실하다. 이를 위해서는 탄소나노튜브에 의해 유도되는 상변화 현상을 이해하는 과정이 반드시 선행되어야 한다. 이 학위논문에서는, 블록공중합체를 나노템플릿으로 활용한 자기조립 탄소나노튜브 초구조체의 핵심이 될 수 있는, 탄소나노튜브에 의해 유도되는 블록공중합체 시스템의 상변화 현상에 대한 내용을 담았다. 추측하건대, 이는 아마도 탄소나노튜브에 의해 유도되는 저농도(20~25%) 블록코폴리머 수용액에서 육방정계 구조를 발견한 첫 번째 보고가 될 것이라 생각한다.
A-B-A 형태의 삼블록공중합체의 대표적인 형태 중 하나인 Pluronic 고분자는, 소수성으로 구성된 Poly(propylene oxide) (PPO) 사슬이 가운데를 이루고 있으며, 양 옆으로 Poly(ethylene oxide) (PEO) 사슬이 연결되어 있는 구조를 가지고 있다. Pluronic 뒤에 붙는 이름은 고분자의 타입(Flake, Paste, or Liquidlike), 분자량(molecular weight), 그리고 고분자 내 PEO가 차지하는 부피비에 따라 명명된다. 수많은 Pluronic 고분자들 중 테스트를 거쳐 F108 (PEO130-PPO60-PEO130)과 F127(PEO100-PPO70-PEO100)이 탄소나노튜브 분산에 탁월한 성능을 보임을 확인하였으며, 초음파 분쇄 - 초원심 분리 - 디칸팅(decanting) 과정을 통해 얻어진 F108-SWNT와 F127-SWNT 용액을 확보하였으며, 수용액 상에서 두 달 이상 침전물 없이 안정적인 고립(isolated) 분산 상태를 유지하는 것으로 확인되었다.
냉동건조를 통해 얻은 F108-SWNT와 F127-SWNT 가루는 물(H2O)에 5 wt% 농도가 되도록 각각 재분산하고, 이를 20개로 나누어 담아 각각의 농도가 5~100 wt%가 되도록 증발하였다. 이에 대한 대조군으로써, F108과 F127만 물에 녹아있는 시스템 또한 동일한 방법으로 제조하였다. 소각X-선 산란(SAXS)를 통해, F108 시리즈(F108, F108-SWNT)의 농도/온도 변화 및 F127 시리즈(F127, F127-SWNT)의 농도 변화에 따른 상변화를 관측하였다. 모든 경우에 대하여 1차원 나노입자인 탄소나노튜브에 의하여 유도되는 hexagonal 구조를 발견하였으며, F108-SWNT의 농도 변화에서는 추가적으로 FCC구조가, F127-SWNT의 경우 re-entrant hexagonal phase가 관측되었다. 증발을 통해 준비된 샘플이기 때문에 epitaxial transition과 Bain distortion을 따를 것으로 생각하여, 이를 바탕으로 탄소나노튜브 array가 어떤 방향을 갖고 있을지에 대해 예상하였다.
마지막으로 비공유 기능화 방법을 통해 친수성으로 표면개질된 탄소나노튜브(p-SWNT)를 Pluronic/물/기름의 삼성분계(ternary system)에 삽입하려는 시도를 하였다. Pluronic P84(PEO19-PPO43-PEO19)/water/p-xylene)는 46/18/36의 질량비로 혼합되면 역육방정계(reverse hexagonal, H2)상을 만들 수 있다. 여기에 p-SWNT를 혼합하게 되면 친수성 cylindrical core 안으로 탄소나노튜브가 삽입되는 선택적 분포를 보일 것으로 예상하였으며, 그 검증을 위하여 contrast-varied 소각 중성자 산란(SANS) 기법을 도입하였다. 탄소나노튜브의 삽입에 따라 친수영역과 소수영역 간의 산란 길이 밀도(scattering length density : SLD)차가 커지거나 작아지도록 하고, intensity가 SLD의 제곱에 비례하는 점을 이용하여 p-SWNT 삽입에 따른 intensity의 변화를 추적하였다. 실험 결과, 친수성 cylindrical core의 지름과 비슷한 지름을 갖는 p-SWNT가 삽입됨에 따른 defect 효과 및 탄소나노튜브를 가지고 있는 cylinder의 분포에 따른 intensity의 변화 효과를 고려했을 때, p-SWNT는 예상했던 바와 같이 친수성 cylindrical core안에 속해 있음을 검증하였다.
이러한 형태의 탄소나노튜브를 삽입하려는 시도 및 탄소나노튜브에 의해 유도되는 상변화를 이해하는 과정은, 잘 정의된 구조를 갖는 탄소나노튜브 초구조체를 디자인하여 제조하는데 핵심적인 정보를 제공할 것으로 기대한다. 또한, 블록공중합체의 상변화를 통해 1차원 나노입자가 자기조립을 통해 초구조체 안에 고정렬(highly ordered)되어 있는 가능성을 제시할 것으로 생각한다.
