Block copolymers have the unique property to self-assemble into ordered microstructures with well-defined periodicity. Among the various attainable microdomain morphologies, complex morphologies are highly desirable due to their inherent structural connectivity. While such complex morphologies are difficult to obtain in monodisperse block copolymers, they may be stabilized by increasing block dispersity. In this thesis, we synthesize a series of poly(styrene-$b$-methyl methacrylate) (PS-$b$-PMMA) and poly(styrene-$b$-tert-butyl methacrylate) (PS-$b$-PtBMA) diblocks where dispersity of both blocks are varied. Morphology examination of the diblocks reveals that at fixed block composition, dispersity dictated phase transition is observed. Our results show that control of block dispersity can effectively stabilize energetically disfavored complex morphologies in block copolymers.
In chapter 3, the basic kinetics for atom transfer radical polymerization (ATRP) of PS and PMMA were investigated. Dispsersity of the PS and PMMA blocks are controlled by temporal regulation of initiator feed rate or addtion of PH modifier. These synthetic strategies were applied to synthesis a series of PS-$b$-PMMa and PS-$b$-PtBMA dibocks where dispersity of both blocks are varied.
In Chapter 4, Ps-$b$-PMMA diblock copolymers are prepared by ATRP. Dispersity of the PS block is controlled by temporal regulation of initiator feed rate, while dispersity of the PMMA block is controlled by addition of phenylhydrazine modifier. The morphological characteristics of the resulting diblocks, where dispersity of both blocks were systematically varied, were examined by combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). At fixed volume fraction of the PS block, dispersity dictated phase transition was observed. In particular, in the case where dispersity of both blocks were high, the polymers were found to exhibit perforated lamellae (PL) and disordered bicontinuous (BIC) morphologies. Dissipative particle dynamics (DPD) simulations are also carried out to confirm the stability of the observed morphologies.
In Chapter 5, composition windows for highly disperse diblock copolymers were explored. In order to improve the accessibility windows for wider range of composition, we investigate higher Flory-Huggins interaction parameter ($\chi$) block copolymer. Through creating compositional morphology diagrams, we have demonstrated that large dispersity value can lead to the expansion of composition windows for the complex morphologies, when compared with relatively narrow-disperse diblock copolymers.
블록 공중합체는 일정한 주기성을 가지는 규칙적인 미세구조로 자기조립 할 수 있는 특성이 있다. 얻어질 수 있는 다양한 미세도메인 모폴로지들 중에서 복잡한 모폴로지는 기본적으로 연속적 구조를 가지고 있어 가치가 높다. 이러한 복잡한 모폴로지들은 단분산 블록 공중합체에서는 얻기 어려운데 반해, 블록 다분산성이 증가하면 이들이 안정화 되는 것으로 알려져 있다. 본 학위논문에서는 각 블록의 다분산성이 다른 poly(styrene-$b$-methyl methacrylate) (PS-$b$-PMMA)와 poly(styrene-$b$-tert-butyl methacrylate) (PS-$b$-PtBMA) 이중블록 공중합체를 합성하였다. 이중블록의 모폴로지 확인 결과, 다분산성에 따른 상변화가 관찰되었다. 본 연구 결과는 열역학적으로 불안정한 복잡한 모폴로지를 블록 다분산성 조절을 통해 안정화 시킬 수 있다는 것을 보여준다.
