Setting Directions: Anisotropy in Hierarchically Organized Porous Silica.

Florian Putz,Oskar Paris,Michael S Elsaesser,Stephan Braxmeier,Gudrun Reichenauer,Nicola Hüsing,Johannes Bernardi,Christian Balzer,Roland Morak

doi:10.1021/acs.chemmater.7b03032

Abstract

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.

Highlights

Natural materials frequently exhibit multiple levels of structural hierarchy combined with a directional orientation of the constituting components, resulting in exceptional properties, e.g., hydration-triggered morphology changes as found in pine cones or anisotropic mechanical properties as found in trabecular bone.[1−7] The pine cone example[3] demonstrates impressively how the combination of large accessible surface area and tailored hierarchical structuring leads to active or responsive properties
We present an investigation of shear-induced macroscopic anisotropy in self-assembled, hierarchically organized silica gels in comparison to their nonsheared counterparts, taking advantage of the dynamics of the sol−gel transition and of chemical reactions continuing after gelation
Organized, highly porous silica monoliths can be fabricated by using a combination of polymerization-induced phase separation and sol−gel processing of tetrakis(2hydroxyethyl)orthosilicate, a tailor-made silane that allows for hydrolysis and condensation reactions in a purely aqueous environment

Summary

INTRODUCTION

Natural materials frequently exhibit multiple levels of structural hierarchy combined with a directional orientation of the constituting components, resulting in exceptional properties, e.g., hydration-triggered morphology changes as found in pine cones or anisotropic mechanical properties as found in trabecular bone.[1−7] The pine cone example[3] demonstrates impressively how the combination of large accessible surface area and tailored hierarchical structuring leads to active or responsive properties. In this context, anisotropy is a fundamental parameter for the functionality of many biological materials. This knowledge is intended as the basis for future applications of hierarchically organized porous systems with anisotropic features in separation science, and in designed switchable components, for instance as actuators, thermal insulation, or acoustic mechanical components

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

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