Modulating the Mechanical Properties of Supercrystalline Nanocomposite Materials via Solvent–Ligand Interactions

Abstract

Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.

Mapping the Mechanical Properties of Hierarchical Supercrystalline Ceramic-Organic Nanocomposites

“…To obtain a non-hierarchical SC material, the organic content needs to be reduced to 8 wt%—the quantity needed to obtain a single ligand monolayer on the NPs’ surface [ 30 ]. In the following, this material will be called “homogeneous SC” (NP diameter 18.4 ± 0.1 nm, assessed via SAXS) [ 29 ]. After pressing the self-assembled materials uniaxially in a rigid die to form bulk pellets (at 150 °C) [ 19 , 29 , 30 ], crosslinking of the organic phase is induced via heat treatment in inert atmosphere (nitrogen flow).…”

“…In the following, this material will be called “homogeneous SC” (NP diameter 18.4 ± 0.1 nm, assessed via SAXS) [ 29 ]. After pressing the self-assembled materials uniaxially in a rigid die to form bulk pellets (at 150 °C) [ 19 , 29 , 30 ], crosslinking of the organic phase is induced via heat treatment in inert atmosphere (nitrogen flow). All details are given in the Materials and Methods section.…”

“…As shown in Figure 2 , nanoindentation of the hierarchical nanocomposites heat-treated at increasing temperatures show the expected increase in hardness. Crosslinking takes place, and additionally the organic content decreases not only in the organic-rich matrix, but likely also within the supercrystalline grains (due to OPh decomposition with temperature [ 19 , 29 ]), leading to a maximization of the volume occupied by the hard ceramic phase. The hardness dependence on indentation depth is shown not only for the hierarchical nanocomposites, but also for the corresponding homogeneous SC material.…”

“…In both hierarchical and homogeneous SC cases, the nanocomposites are obtained through a sequence of self-assembly via evaporation of the toluene, pressing in a rigid die to form a bulk pellet (50 MPa at 150 °C), and heat treatment in inert atmosphere (N 2 ). The heat treatment (HT) temperatures are chosen to be 250 and 350 °C, to induce various degrees of crosslinking of the organic phase [ 19 , 29 ]. Three samples per processing condition were considered.…”

“…Supercrystalline arrangements enable a variety of emergent functionalities [ 20 , 22 ], but also the production of bricks that are very suitable to build hierarchical structural materials. Even though initially developed in micro-sizes and featuring relatively low mechanical properties [ 23 , 24 ], recent progress has been made towards the production of bulk macro-scale supercrystalline ceramic-organic nanocomposites, with mechanical properties boosted thanks to an annealing-induced crosslinking of the organic phase [ 19 , 25 , 26 , 27 , 28 , 29 ].…”