Résumé
For several decades now, mesoporous silica has been a major player for many functional applications, as well as an important model material, representative of the mesostructured family. Its sol-gel synthesis by micelle soft-templating is a method of choice for the deliberate tailoring of particle size, shape and pore texture. Among the variety of related materials, cage-type mesoporous silica has shown remarkable and fascinating structures, including low-dimensional nanostructures and clathrate analogs [1,2]. In these materials, the pores are all interconnected through windows, forming cages with well-defined symmetries. This implies that structure directing micelles remain discrete during, hence offering a unique opportunity to control materials dimensionality by either restricting, directing or confining the self-assembly of multiple micelles during the silica growth. In this context, polyion complex (PIC) micelles are emerging as alternative structure directing and functionalizing agents to traditional surfactant or amphiphilic polymer micelles. PIC micelles are formed by complexation of a multi-hydrophilic block copolymer containing a polyacid block with a micellization partner of opposite charge such as oligochitosan. These PIC micelles offer a versatile platform to modulate the structure of mesoporous silica materials by adjusting the nature, size, and functionality of each block. This approach, however, had so far been limited to the production of bulky material in the form of powders. While most previous studies have focused on double-hydrophilic block copolymers, we used for the first time a triple-hydrophilic block copolymer, namely polyacrylamide-poly(acrylic acid)-poly(poly (ethylene oxide) methyl ether acrylate), PAm-b-PAA-b-POEGMEA. In this contribution, we will show how using this copolymer for the synthesis of hybrid silica remarkably led to the formation of 0D cage-type nanoparticles formed around individual micelles and hence without 3D periodicity by restricting multi-micelle assembly in all directions [3]. The structure of these cages was resolved by single particle reconstruction from TEM images, revealing a well-defined dodecahedral symmetry. From this point, we were able to selectively release one or two directions of restriction for the self-assembly of multiple micelles, resulting in the formation of well-defined 1D and 2D mesoporous silica nanostructures, by playing with the copolymer composition and synthesis conditions. We will show, through a combination of TEM and in situ SAXS measurements, how a precise engineering of these multi-hydrophilic block copolymers allowed to modulate the micelle topology and the intermicellar interactions during their co-assembly with silica, and how it allowed to control the dimensionality (0D, 1D, 2D) of the nanomaterials. Beyond their fundamental interest, these low-dimensional structures also constitute well-suited building blocks for the bottom-up assembly of structured materials. The 1D nanostructures, in particular, have a strong impact on the rheological properties of colloidal suspension due to their high aspect ratio. We will showcase how this particular feature can be exploited for the formation of hierarchical and functional monoliths of high value for environmental or catalytic applications.[1] K. Ma, Y. Gong, T. Aubert, M.Z. Turker, T. Kao, P.C. Doerschuk, U. Wiesner, Self-assembly of highly symmetrical, ultrasmall inorganic cages directed by surfactant micelles, Nature 2018, 558, 577.[2] T. Aubert, K. Ma, K.W. Tan, U. Wiesner, Two-Dimensional Superstructures of Silica Cages, Adv. Mater. 2020, 32, 1908362.[3] A. Vashishtha, A. Phimphachanh, T. Gaillard, J. Schmitt, C. Gerardin, G. Rydzek, T. Aubert, Hybrid Silica Cage-Type Nanostructures Made from Triply Hydrophilic Block Copolymers Single Micelles, ACS Nano 2024, doi: 10.1021/acsnano.4c09887.