Investigating Structural Effects of Quaternizing Additives on Shape Transitions of Block Copolymer Particles

Abstract

Co-assembly of block copolymers (BCPs) and organic/inorganic additives affords the design of various hierarchical nanostructures. In this work, we investigate the shape-changing capabilities of poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) BCP particles upon quaternization with a series of bromoalkyl benzene additives with different alkyl spacer lengths. The bromoalkyl benzene additives exhibit different hydrolyzing and quaternizing behaviors depending on their chemical structures. When benzyl bromide (BB) is used, the PS-b-P2VP BCP particles exhibit dramatic shape transitions from ellipsoids to ellipsoids with swelled discs, swelled buds, and vesicles. These morphological transitions are attributed to the synergistic quaternization and protonation of the P2VP chains via the hydrolysis of BB in aqueous media. Upon increasing the molar ratio of BB to 2VP units, the pH of the surrounding aqueous solutions significantly decreases, and the protonated P2VP domains are swelled by the surrounding water, which eventually results in interfacial instability of the emulsion interface. When the additives contain longer alkyl spacers (e.g., ethyl, butyl, and hexyl), the additives lead to a narrower range of quaternization-dependent particle morphologies due to the absence of the hydrolysis of the additives. However, a broader spectrum of particle shapes is observed for additives with longer alkyl chains, due to their stronger quaternizing capabilities. We carefully investigate the structural effect of the quaternizing additives on the change of pH, degree of quaternization, and interfacial tension to elucidate the mechanism of the additive-driven particle morphology transitions.