Interfacial Instability-Driven Morphological Transition of Prolate Block Copolymer Particles: Striped Football, Larva to Sphere

Abstract

Shape- and structure-controlled block copolymer particles have received great attention due to their unusual physical properties, hence achieving structural diversity of the particles is highly desirable. Herein, we report a confined assembly of poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) and dibromoalkyl molecular additives in emulsion droplets, which allows for significant changes in particle shape and structure from striped football to larva and sphere. The morphological transitions are dependent on the volume fraction of dibromoalkyl additives, exhibiting three distinct phases including (1) elongation of striped football particles (prolate ellipsoids) as a consequence of increased domain spacing, (2) transformation of the striped internal morphology into sandwiched lamellae to result in larva-like particle, and (3) disassembly of larva into spheres. The key driving force to achieve such structural transition is the quaternization of P2VP chains by dibromoalkyl additives. Consequently, significant changes in swelling/deswelling of P2VP domains of PS-b-P2VP particles are induced in concert with a favorable interaction with the surrounding aqueous phase depending on the volume fraction of the additives. Detailed observation of the morphological evolution is performed by freeze-drying the emulsions as a function of evaporation time to gain a deeper understanding of these structural transitions.