Geometry induced sequence of nanoscale Frank–Kasper and quasicrystal mesophases in giant surfactants

Abstract

Frank-Kasper (F-K) and quasicrystal phases were originally identified in metal alloys and only sporadically reported in soft materials. These unconventional sphere-packing schemes open up possibilities to design materials with different properties. The challenge in soft materials is how to correlate complex phases built from spheres with the tunable parameters of chemical composition and molecular architecture. Here, we report a complete sequence of various highly ordered mesophases by the self-assembly of specifically designed and synthesized giant surfactants, which are conjugates of hydrophilic polyhedral oligomeric silsesquioxane cages tethered with hydrophobic polystyrene tails. We show that the occurrence of these mesophases results from nanophase separation between the heads and tails and thus is critically dependent on molecular geometry. Variations in molecular geometry achieved by changing the number of tails from one to four not only shift compositional phase boundaries but also stabilize F-K and quasicrystal phases in regions where simple phases of spheroidal micelles are typically observed. These complex self-assembled nanostructures have been identified by combining X-ray scattering techniques and real-space electron microscopy images. Brownian dynamics simulations based on a simplified molecular model confirm the architecture-induced sequence of phases. Our results demonstrate the critical role of molecular architecture in dictating the formation of supramolecular crystals with "soft" spheroidal motifs and provide guidelines to the design of unconventional self-assembled nanostructures.self-assembly | Frank-Kasper phases | quasicrystal phases | giant surfactants | POSS I n addition to the close-packing schemes of identical atoms (such as hexagonal close-packing and face-centered cubic), atoms with different radii and electronic states in metal alloys are able to pack into more complex phases composed of spheres, such as the Frank-Kasper (F-K) phases (1, 2), which combine the Frank lattice (icosahedron with a coordination number of 12) and the Kasper lattice (with higher coordination numbers of 14, 15, and 16). A few F-K phases such as the A15-(space group of Pm 3n) and σ-(space group of P4 2 /mnm) phases are periodic approximants of different quasicrystals. Quasicrystals, first identified in supercooled metal alloys, are aperiodic, and possess 5-, 7-, 8-, 10-, or 12-fold rotational symmetry but no long-range translational periodicity (3-5). Stabilization of these phases in metals originates from both geometric factors and the tendency to enhance low orbital electron sharing due to fewer surface contacts among the atoms (6).F-K phases have also been identified in soft-matter systems, including small-molecule surfactants (7-9), block copolymers (10-12), dendrimers (13-15), liquid crystals (16, 17), colloidal particles (18), and, very recently, molecular giant tetrahedra (19). In contrast to metal alloys that use atoms as the motifs, organic/hybrid molecules first self-assemble into spheroidal motifs...

Low-symmetry sphere packings of simple surfactant micelles induced by ionic sphericity

“…Thus, this discovery bridges previous reports of FK phases neutral soft materials (2, 18, 19, 21-23, 31, 35) and metal alloys. However, the ubiquity of icosahedral QCs in multicomponent metal alloys (36) starkly contrasts the observation of only dodecagonal QCs in soft materials to date (23)(24)(25)(26). Icosahedral QCs, which are quasiperiodic in 3D, typically form in ternary alloys wherein the metal sites exhibit decoupled variations in both particle mass (atomic number) and charge distribution.…”

“…The first FK A15 (15) and C15 (16) phases in ionic surfactant and lipidic LLCs were identified over 30 y ago, yet the principles governing their formation remain poorly understood (17). More recently, FK A15 and σ phases were documented in thermotropic LCs of wedge-shaped dendrons (18)(19)(20), linear diblock and multiblock polymers (21,22), and giant shape amphiphiles (2,23). These studies culminated in the discovery of soft, dodecagonal QCs (23)(24)(25)(26), for which the A15 and σ phases are 3D periodic approximants.…”

Breaking Parallel Orientation of Rods via a Dendritic Architecture toward Diverse Supramolecular Structures

“…Adrastic change of the phase behavior is observed when the number of the OF rods is increased to three.F or 3OH-3OF 1 (3,4,5) with the shortest OF rods length, the positions of the SAXS peaks shown in Figure 2B(i)h ave q ratios of . This pattern could be assigned to the F-K A15 phase with aspace group of Pm3 n (see detailed peak assignments in the Supporting Information, Figure S13 and Table S3).…”

“…In the past forty years,great efforts on the investigation of structural self-assemblies have been made in various material systems such as block copolymers, [1] dendrons, [2] giant molecules, [3] hybrids, [4] and many others. [5] In flexible AB diblock copolymers,t he competition between the repulsive interaction between the Aa nd Bb locks,q uantified by the Flory-Huggins interaction parameter, the overall degree of polymerization, and the chain connectivity lead to microphase separation of the two blocks forming polymeric domains.The shape of the polymeric domains (for example,l amellae, cylinders,o rs pheres) with different preferred interfacial curvatures is determined by the composition (volume fraction) of the blocks and allows various ordered phases in block copolymers.…”

Direct AFM observation of individual micelles, tile decorations and tiling rules of a dodecagonal liquid quasicrystal

“…In addition to self-assembled spherical micelles from monodendrons [1,9], columns and spheres formed from micro-phase segregation of block copolymers (BCPs) [10][11][12], nanoparticles [13], porous silica [14], and surfactants [15] have all recently been found to be able to form quasicrystalline structures. The majority of these show dodecagonal quasicrystalline symmetry.…”