Aspect Ratio-Controlled Synthesis of Uniform Colloidal Block Copolymer Ellipsoids from Evaporative Emulsions

Abstract

Block copolymers (BCPs) confined in evaporative emulsions can assemble into ellipsoidal particles when solvent evaporation is faster than polymer diffusion within the droplet. Here, we report the synthesis of monodisperse, ellipsoidal polystyrene-block-1,4-polybutadiene (PS-b-PB) BCP particles with tunable aspect ratios (ARs) ranging from 1.0 to 2.2 and particle sizes ranging from 0.1 to 5 μm by membrane emulsification and subsequent solvent evaporation. The ratio of the propagation distance (l p) of ordered BCP domains perpendicular to the particle surface to the particle size (r BCP), or l p/r BCP, was found to be a critical parameter dictating the particle shape, where l p/r BCP > 1 yielded ellipsoids. We show that the AR of colloidal BCP ellipsoids can be precisely controlled by varying particle size (i.e., membrane pore size) and BCP molecular weight, as predicted by theoretical calculations of the free energy of particle elongation including three terms: (1) the interfacial energy between the two blocks of the BCP, (2) the entropic penalty associated with stretching of the BCP chains upon elongation of the particles, and (3) the surface energy between the BCP particles and the surrounding medium. Finally, using the resulting AR-controlled and highly monodisperse colloidal ellipsoids, we systematically investigated the effects of AR on the homogeneity of colloidal coatings obtained by drop-casting the ellipsoids into films.

Responsive Colloidal Polymer Particles with Ordered Mesostructures

“…A recent study reported that the aspect ratio of pupa‐like colloidal particles was well tuned by varying the evaporation rate of the solvent under a neutral wetting interface. [ 14 ]…”

“…The stimuli‐responsive colloidal polymer particles have potential as smart materials for various applications. [ 4c,d,14 ]…”

Self‐Assembly of Multiblock Copolymers

“…In‐situ incorporation or co‐assembly of nanoparticles (NPs) with BCPs is another effective way to change the particle shape and composition . The particle aspect ratio (AR) depends on the molecular weight of the BCP and pore size of the membrane when using the SPG emulsification technique . It was found that the variation of particle size has a more significant influence on the AR of oblate particles than prolate particles …”

“…[75] The particle aspect ratio (AR) depends on the molecular weight of the BCP and pore size of the membrane when using the SPG emulsification technique. [78,84] It was found that the variation of particle size has a more significant influence on the AR of oblate particles than prolate particles. [84] Regarding ABA triblock copolymer the soft confinement self-assembly was investigated with Monte Carlo (MC) simulations.…”

“…With continuous droplet shrinking, BCP microphase separation propagates from the droplet surface to the droplet center yielding well‐ordered microparticles after complete drying. Traditional emulsification methods such as homogenizer, vortex, and membrane‐extrusion produce microparticles with a rather broad size distribution, while new methods such as microfluidics, and the Shirasu porous glass (SPG) membrane emulsification technique can generate quite monodisperse particles.…”