Frustrated Microparticle Morphologies of a Semicrystalline Triblock Terpolymer in 3D Soft Confinement

Abstract

Self-assembly of block copolymers (BCPs) in three-dimensional (3D) confinement of emulsion droplets has emerged as a versatile route for the formation of functional micro- and nanoparticles. While the self-assembly of amorphous coil–coil BCPs is fairly well documented, less is known about the behavior of crystalline–coil BCPs. Here, we demonstrate that confining a linear ABC triblock terpolymer with a crystallizable middle block in oil-in-water (O/W) emulsions results in a range of microparticles with frustrated inner structure originating from the conflict between crystallization and curved interfaces. Polystyrene-block-polyethylene-block-poly­(methyl methacrylate) (PS-b-PE-b-PMMA, S32E36M32 93) in toluene droplets was subjected to different preparation protocols. If evaporation was performed well above the bulk crystallization temperature of the PE block (T evap > T c), S32E36M32 93 first microphase-separated into microparticles with lamella morphology followed by crystallization into a variety of frustrated morphologies (e.g., bud-like, double staircase, spherocone). By evaporating at significantly lower temperatures that allow the PE block to crystallize from solution (T evap < T c), S32E36M32 93 underwent crystallization-driven self-assembly into patchy crystalline-core micelles, followed by confinement assembly into lenticular microparticles with compartmentalized hexagonal cylinder lattices. The frequency of these frustrated morphologies depends on polymer concentration and the evaporation protocol. These results provide a preliminary understanding of the morphological behavior of semicrystalline block copolymers in 3D soft confinement and may provide alternative routes to structure multicompartment microparticles from a broader range of polymer properties.

Block copolymer solution self‐assembly: Recent advances, emerging trends, and applications

“…This way it is possible to alter interfacial activity of the particles in response to specific stimuli and facilitate a reversible shape transformation 117 . As previously discussed, the morphology of amorphous BCPs in bulk mainly depends on the volume fraction and number of blocks, as well as their enthalpic interactions, while in emulsion confinement, the morphology is further controlled by the degree of confinement, the solvent evaporation rate, and the interfacial tension between the blocks and the surfactant/water interface 118 …”

“…Among the most typical examples of BCPs confined self‐assembly in emulsion droplets is the work of Gröschel's group 118–120 . They have reported on the formation of shape‐ and surface‐anisotropic “Janus” nanocups by cross‐linking the hemispherical lamellar nanostructures formed through the evaporation‐induced confinement assembly of a PS‐PB‐P t BMA triBCP in spherical oil‐in‐water droplets stabilized by cetyltrimethylammonium bromide (CTAB) surfactant, 119 while in a similar manner they prepared “Janus” nanorings from a PS‐PB‐PMMA terpolymer 120 .…”

“…They have reported on the formation of shape‐ and surface‐anisotropic “Janus” nanocups by cross‐linking the hemispherical lamellar nanostructures formed through the evaporation‐induced confinement assembly of a PS‐PB‐P t BMA triBCP in spherical oil‐in‐water droplets stabilized by cetyltrimethylammonium bromide (CTAB) surfactant, 119 while in a similar manner they prepared “Janus” nanorings from a PS‐PB‐PMMA terpolymer 120 . Their latest analogous study was on the confinement of a PS‐PE‐PMMA triBCP with a crystallizable middle PE block in toluene/water/CTAB emulsions followed by evaporation/crystallization protocols at temperatures promoting either evaporation or crystallization first 118 . Unsurprisingly, the preparation protocol determined the morphology of the resulting nanostructures.…”

“…Schematic illustration of two different routes for the oil‐in‐water emulsion confinement self‐assembly of a PS‐PE‐PMMA triBCP at different temperatures and corresponding transmission electron microscopy (TEM) images. Adapted with permission from Reference 118 copyright (2020) American Chemical Society…”

Patchy Micelles with a Crystalline Core: Self-Assembly Concepts, Properties, and Applications

“…Interestingly, patchy SEM wCCMs can also be hierarchically assembled by a confinement process through emulsification in a toluene-in-water emulsion and subsequent evaporation of the solvents. This leads to microparticles with a highly ordered hexagonal close-packed lattice structure [ 161 ].…”