Modular Construction of Complex-Architected Bottlebrush Block Copolymers and Their Self-Assembly Behaviors

Abstract

Microphase-separated block copolymers are attractive materials for self-assembled nanolithography, yet there is a disconnect between the simple patterns commonly formed by block copolymers and the complex patterns required for many nanoscale applications, particularly in microelectronics. To meet this challenge, researchers have sought to design and build copolymer systems at ever-increasing levels of complexity in the (macro)molecular level, which promises to show emergent intriguing properties that are otherwise absent. However, the synthetic challenge as well as the vastly increased parameter space have obscured the systematic study of such complex systems. An efficient, modular synthetic route is thus highly desired for Lego-like molecular construction of property-decoupled, individually-tunable target materials.

In this thesis, we will highlight the research endeavor in developing a multiblock Janus bottlebrush copolymer architecture as a novel platform for generation of diverse nanostructures that have been challenging to fabricate. The architecture, which features two orthogonal Janus domains, can be facilely constructed from corresponding building blocks by graft-through synthesis and can yield hierarchically engineerable phase-in-phase patterns.

Surprisingly, the two constituent domains, though relatively independent of each other, behave significantly differently when combined together under certain circumstances. Their collective behavior gives rise to two low-symmetry mesh-like network phases (monoclinic and tetragonal respectively) that have not been observed in other soft materials before, which are of both fundamental and technological interest. Through a suite of experimental and computational study, we show that this peculiar phenomenon is an outcome of intrinsic molecular confinement, an emergent effect unique to multi-body, multi-hierarchy complex architectures. This work demonstrates that intrinsic molecular confinement is a viable path to bottom-up assembly of new geometrical phases of soft matter, extending the capabilities of block copolymer nanofabrication.

As another example of modular synthesis, we will show an iterative polymerization methodology for controlled synthesis of bottlebrush copolymers with expanded compositional and architectural scope. When synergizing with other components, this strategy allows rapid access to functional materials that display different phase behavior when compared to the self-assembly of conventional copolymers.

Our work introduced here is expected to facilitate the synthesis of complex functional copolymers, spark interest in the exploration of their property-function relationship, and enable more opportunities for their application in nanopatterning and other advanced materials.