This lesson illustrates the importance of elaboration processes in controlling the structure of materials at all scales, and consequently in optimizing their properties. Indeed, a material does not simply correspond to a compound, nor to a chemical composition. For example, the same chemical composition SiO2, known as silica, is the main constituent of sand, the superb crystals that make up quartz, window glass and the elegant protective frustule of diatoms. All these materials are made up of strings of SiO4 tetrahedra, but their properties differ because they have not been elaborated in the same way and, as a result, they can be crystallized or amorphous, and their "fine structures" are different. A material is the result of a combination of a chemical compound and a manufacturing process. A material's properties of use and its robustness are highly dependent on the quality of the coupling between its chemistry and its production process. The grand staircase of materials development, the one that takes us from the mesoscopic to the macroscopic world, can be taken either upwards or downwards. In the top-down mode, solid matter is divided or shaped by physical processes such as mechano-synthesis, laser ablation, ion etching, lithography or printing in the broadest sense. The bottom-up mode involves building matter by assembling molecular or nanometric objects. In this lecture, we present a review of the field based on a few relevant examples in which bottom-up or top-down elaboration processes and soft chemistry are cleverly coupled to build functional materials with meso- or hierarchical structures.
After a brief introduction to the elaboration of meso-structured porous films, whose walls can be amorphous or crystalline, obtained by soft chemistry and simple deposition, we placed particular emphasis on so-called "integrative" strategies. These provide access to materials with hierarchical structures, and are based on the coupling of soft chemistry, the physical chemistry of soft matter and a wide variety of elaboration processes. These multi-scale structures can be obtained in two main ways:
- the development of soft-chemistry synthesis modes in the presence of micelles - which partition the reaction space at the scale of a few nanometers (2-10 nm) -, combined with larger templates (such as latex beads, viruses, bacteria, etc.), water droplets in fog overhanging a surface on which a solvent is evaporating, a micron-scale but removable printed substrate, the micron structure of water-oil microemulsions, fibrous networks resulting from the organogelation of a solvent), and/or more generally to controlled phase separation phenomena.
- the coupling of soft chemistry along this first path with physical processes such as electrospinning, spraying, inkjet printing, UV optical lithography using two-photon absorption, and reactive ion etching.
We have used numerous examples to illustrate the properties of these new materials. These materials are of interest in fields such as optics (photonic crystals), energy (storage materials for hydrogen, membranes for fuel cells, batteries, etc.), the environment (photocatalysts to eliminate pollutants, catalysts for petrochemicals, permselective membranes with facilitated transport), medicine and health (optical or magnetic probes, implants, vectors, etc.).
