Sol-Gel Strategies for Optimized Hierarchical Materials

Natural systems have evolved to solve complex engineering problems, like optimizing weight and stiffness, self-cleaning, adhesion, and water purification, using hierarchical structures composed of multiple, often disparate, materials arranged on differing prioritized length scales. Mimicking these proven materials designs in robust engineering materials with efficient processing strategies to achieve synergistic, optimized properties and combinations of properties is a current grand challenge. This talk highlights several simple, evaporation-driven sol-gel processing strategies enabling efficient formation of hierarchical porous and composite structures with optimized property combinations. As a first example, we show that by simple chemical modification of a siloxane gel network, we achieve completely reversible drying shrinkage, allowing the formation of hydrophobic aerogel thin film coatings on arbitrary substrates. These coatings combine superhydrophobicity and nearly perfect optical clarity. A second class of materials combines sol-gel processing and molecular self-assembly in a process we refer to as evaporation-induced self-assembly (EISA). EISA starts with a homogenous solution of surfactant plus hydrophilic oligosilicic acid precursors. Solvent evaporation accompanying spin- or dip-coating or printing concentrates the depositing film in precursors and surfactant inducing micelle self-assembly and further self-organization into 3D periodic silica/surfactant mesophases. This simple process has been used to realize a theoretical materials design that optimizes the combination of porosity and mechanical modulus. We used EISA to form cubic (C), hexagonal (H), and worm-like disordered (D) nanoporous silica films. Over the relative density range, 0.5 to 0.65, Young's modulus scales as (density)n where n(C) < n(H) < n(D) < 2, indicating that nanostructured porous silicas exhibit a structure-specific hierarchy of modulus values D < H < C. Scaling exponents less than 2 emphasize that the moduli are less sensitive to porosity than those of natural cellular solids, which possess extremal moduli based on linear elasticity theory. Using Raman spectroscopy and molecular modeling, we show that nanoscale confinement causes the silica framework to contain a higher portion of small, stiff rings (than found in other forms of amorphous silica). The nanostructure-specific hierarchy and systematic increase in framework modulus we observe, when decreasing the dimensional scale of the nanostructure below 2-nm as bulk density is reduced, provides a new ability to maximize mechanical properties at a given density needed for nanoporous materials integration.