Ameya Rege

Mechanics of Nanostructured Porous Silica Aerogel Resulting from Molecular Dynamics Simulations

Silica aerogels are nanostructured, highly porous solids which have, compared to other soft materials, special mechanical properties, such as extremely low densities. In the present work, the mechanical properties of silica aerogels have been studied with molecular dynamics (MD) simulations. The aerogel model of 192 000 atoms was created with different densities by direct expansion of β-cristobalite and subjected to series of thermal treatments. Because of the high number of atoms and improved modeling procedure, the proposed model was more stable and showed significant improvement in the smoothness of the resulting stress-strain curves in comparison to previous models. Resulting Poissons ratio values for silica aerogels lie between 0.18 and 0.21. The elasticity moduli display a power law dependence on the density, with the exponent estimated to be 3.25 ± 0.1. These results are in excellent agreement with reported experimental as well as computational values. Two different deformation scenarios have been discussed. Under tension, the low-density aerogels were more ductile while the denser ones behaved rather brittle. In the compression simulations of low-density aerogels, deformation occurred without significant increase in stress. However, for high densities, atoms offer a higher resistance to the deformation, resulting in a more stiff response and an early densification. The relationship between different mechanical parameters has been found in the cyclic loading simulations of silica aerogels with different densities. The residual strain grows linearly with the applied strain (≥0.16) and can be approximated by a phenomenological relation ϵp = 1.09ϵmax - 0.12. The dissipation energy also varies with the compressive strain according to a power law with an exponent of 2.31 ± 0.07. Moreover, the tangent modulus under cyclic loading varies exponentially with the compressive strain. The results of the study pave the way toward multiscale modeling of silica as well as reinforced silica aerogels.

The three-dimensional structure of flexible resorcinol-formaldehyde aerogels investigated by means of holotomography

Organic aerogels based on resorcinol-formaldehyde gels display remarkable properties due to their pronounced nanoporosity. Therefore, studies towards the understanding of their structure-property-relationship are of high value for the design of improved materials. X-ray tomography is a technique that has been used for the structural elucidation of porous materials, but so far no highly resolved three-dimensional structures of resorcinol-formaldehyde gels have been obtained under the classical absorption-based experimental X-ray setup. This paper reports on the successful analysis of a superflexible resorcinol-formaldehyde aerogel using zoom holotomography that yielded images with an unprecedented resolution in the sub-micrometer range. The preparation of suitable powder from monolithic superflexible resorcinol-formaldehyde, the experimental conditions for tomography, and data-processing to obtain a 3D-image of the dried gel sample are described. Macropores above ca. 75 nm could be identified and visualized. They were shown to adopt almost spherical shape and to display a low connectivity. A quantitative analysis of the pore space revealed that most of the identified pores are small macropores (diameter < 0.5 µm), yet most pore volume is located in larger macropores of 1–4 µm diameter.Graphical Abstract

Correlating Synthesis Parameters to Morphological Entities: Predictive Modeling of Biopolymer Aerogels

In the past decade, biopolymer aerogels have gained significant research attention due to their typical properties, such as low density and thermal insulation, which are reinforced with excellent biocompatibility, biodegradability, and ease of functionalization. Mechanical properties of these aerogels play an important role in several applications and should be evaluated based on synthesis parameters. To this end, preparation and characterization of polysaccharide-based aerogels, such as pectin, cellulose and k-carrageenan, is first discussed. An interrelationship between their synthesis parameters and morphological entities is established. Such aerogels are usually characterized by a cellular morphology, and under compression undergo large deformations. Therefore, a nonlinear constitutive model is proposed based on large deflections in microcell walls of the aerogel network. Different sizes of the microcells within the network are identified via nitrogen desorption isotherms. Damage is initiated upon pore collapse, which is shown to result from the failure of the microcell wall fibrils. Finally, the model predictions are validated against experimental data of pectin, cellulose, and k-carrageenan aerogels. Given the micromechanical nature of the model, a clear correlation—qualitative and quantitative—between synthesis parameters and the model parameters is also substantiated. The proposed model is shown to be useful in tailoring the mechanical properties of biopolymer aerogels subject to changes in synthesis parameters.