Hierarchically organized structures, both in natural and synthetic materials, play an important role for a wide range of applications, including separation, catalysis, energy storage and conversion, sensing and actuation, biomedicine or tissue engineering. The multilevel porous architectures provide unique properties depending on the features present, e.g. micro/meso/macro- or meso/macro-pores, etc. A simultaneous control over pore sizes from angstroms to micrometers, pore shape as well as spatial distribution potentially enables the fabrication of porous structures exhibiting novel properties and multiple functions.
Starting from well-established synthesis protocols using glycolated silanes towards hierarchically organized silica monoliths with anisotropic mesoporosity and isotropic macroporosity , two approaches based on extrusion and 3D printing were applied to achieve an even better control of the morphology of the final network on different length scales.
From extrusion of a preformed wet gel body, monolithic materials with a preferential orientation of the macroporous network comprising anisotropic mesoporosity were obtained by shear-induced alignment . Possible applications as artificial mechanosensors or actuators that can be derived from similar structures present in biological materials [3,4] were investigated by sorption experiments.
In a second approach, 3D printing (direct ink writing) of the highly viscous precursor solution allows for the creation of complex macroporous shapes. The resulting material combines the features of the hierarchically organized network with meso- and macropores plus an additional hierarchical level arising from the printed superstructure.
We present structural data of these novel morphologies, which have been investigated in detail by scanning electron microscopy, small angle X-ray scattering and N2 sorption. In addition to these standard analytical techniques, we present experimental results of the sorption-induced deformation on different length scales of the well-defined hierarchically organized porous networks using in-situ SAXS or SANS and in-situ dilatometry .
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