F. Putz, R. Morak, M. Elsässer, C. Balzer, S. Braxmeier, J. Bernardi, O. Paris, G. Reichenauer, N. Hüsing:
"Setting Directions: Anisotropy in Hierarchically Organized Porous Silica";
Chemistry of Materials,
Structural hierarchy, porosity, and isotropy/anisotropy are highly
relevant factors for mechanical properties and thereby the functionality of porous
materials. However, even though anisotropic and hierarchically organized, porous
materials are well known in nature, such as bone or wood, producing the synthetic
counterparts in the laboratory is difficult. We report for the first time a straightforward
combination of sol−gel processing and shear-induced alignment to create hierarchical
silica monoliths exhibiting anisotropy on the levels of both, meso- and macropores.
The resulting material consists of an anisotropic macroporous network of struts
comprising 2D hexagonally organized cylindrical mesopores. While the anisotropy of
the mesopores is an inherent feature of the pores formed by liquid crystal templating,
the anisotropy of the macropores is induced by shearing of the network. Scanning
electron microscopy and small-angle X-ray scattering show that the majority of
network forming struts is oriented towards the shearing direction; a quantitative
analysis of scattering data confirms that roughly 40% of the strut volume exhibits a
preferred orientation. The anisotropy of the material´s macroporosity is also reflected in its mechanical properties; i.e., the
Young´s modulus differs by nearly a factor of 2 between the directions of shear application and perpendicular to it. Unexpectedly,
the adsorption-induced strain of the material exhibits little to no anisotropy.
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