3 research outputs found
Compressible, Thermally Insulating, and Fire Retardant Aerogels through Self-Assembling Silk Fibroin Biopolymers Inside a Silica StructureAn Approach towards 3D Printing of Aerogels
Thanks
to the exceptional materials properties of silica aerogels, this fascinating
highly porous material has found high-performance and real-life applications
in various modern industries. However, a requirement for a broadening
of these applications is based on the further improvement of the aerogel
properties, especially with regard to mechanical strength and postsynthesis
processability with minimum compromise to the other physical properties.
Here, we report an entirely novel, simple, and aqueous-based synthesis
approach to prepare mechanically robust aerogel hybrids by cogelation
of silk fibroin (SF) biopolymer extracted from silkworm cocoons.
The synthesis is based on sequential processes of acid
catalyzed (physical) cross-linking of the SF biopolymer and simultaneous
polycondensation of tetramethylorthosilicate (TMOS) in the presence
of 5-(trimethoxysilyl)pentanoic acid (TMSPA) as a coupling agent and
subsequent solvent exchange and supercritical drying. Extensive characterization
by solid-state <sup>1</sup>H NMR, <sup>29</sup>Si NMR, and 2D <sup>1</sup>H–<sup>29</sup>Si heteronuclear correlation (HETCOR)
MAS NMR spectroscopy as well as various microscopic techniques (SEM,
TEM) and mechanical assessment confirmed the molecular-level homogeneity
of the hybrid nanostructure. The developed silica–SF aerogel
hybrids contained an improved set of material properties, such as
low density (ρ<sub>b,average</sub> = 0.11–0.2 g cm<sup>–3</sup>), high porosity (∼90%), high specific surface
area (∼400–800 m<sup>2</sup> g<sup>–1</sup>),
and excellent flexibility in compression (up to 80% of strain) with
three orders of magnitude improvement in the Young’s modulus
over that of pristine silica aerogels. In addition, the silica–SF
hybrid aerogels are fire retardant and demonstrated excellent thermal
insulation performance with thermal conductivities (λ) of 0.033–0.039
W m<sup>–1</sup> K<sup>–1</sup>. As a further advantage,
the formulated hybrid silica–SF aerogel showed an excellent
printability in the wet state using a microextrusion-based 3D printing
approach. The printed structures had comparable properties to their
monolith counterparts, improving postsynthesis processing or shaping
of the silica aerogels significantly. Finally, the hybrid silica–SF
aerogels reported here represent significant progress for a mechanically
customized and robust aerogel for multipurpose applications, namely,
as a customized thermal insulation material or as a dual porous open-cell
biomaterial used in regenerative medicine
Setting Directions: Anisotropy in Hierarchically Organized Porous Silica
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
Adsorption-Induced Deformation of Hierarchically Structured Mesoporous SilicaEffect of Pore-Level Anisotropy
The
goal of this work is to understand adsorption-induced deformation
of hierarchically structured porous silica exhibiting well-defined
cylindrical mesopores. For this purpose, we performed an in situ dilatometry
measurement on a calcined and sintered monolithic silica sample during
the adsorption of N<sub>2</sub> at 77 K. To analyze the experimental
data, we extended the adsorption stress model to account for the anisotropy
of cylindrical mesopores, i.e., we explicitly derived the adsorption
stress tensor components in the axial and radial direction of the
pore. For quantitative predictions of stresses and strains, we applied
the theoretical framework of Derjaguin, Broekhoff, and de Boer for
adsorption in mesopores and two mechanical models of silica rods with
axially aligned pore channels: an idealized cylindrical tube model,
which can be described analytically, and an ordered hexagonal array
of cylindrical mesopores, whose mechanical response to adsorption
stress was evaluated by 3D finite element calculations. The adsorption-induced
strains predicted by both mechanical models are in good quantitative
agreement making the cylindrical tube the preferable model for adsorption-induced
strains due to its simple analytical nature. The theoretical results
are compared with the in situ dilatometry data on a hierarchically
structured silica monolith composed by a network of mesoporous struts
of MCM-41 type morphology. Analyzing the experimental adsorption and
strain data with the proposed theoretical framework, we find the adsorption-induced
deformation of the monolithic sample being reasonably described by
a superposition of axial and radial strains calculated on the mesopore
level. The structural and mechanical parameters obtained from the
model are in good agreement with expectations from independent measurements
and literature, respectively