Tuning of block dispersity and broad dispersity stabilized complex morphologies in AB diblock copolymers

Abstract

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.