Investigating Structural Effect of Quaternizing Additives on ShapeTransition of Block Copolymer Particles

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 a dramatic shape transition 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, resulting in interfacial instability of the emulsion interface. When the additives contain longer alkyl spacers, the additives lead to a narrower range of quaternization-dependent particle morphologies due to the absence of the hydrolysis of the additives. 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.