Multiscale Computer Simulation of Tensile and Compressive Strain in Polymer-Coated Silica Aerogels
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Multiscale Computer Simulation of Tensile and Compressive Strain in PolymerCoated Silica Aerogels. Brian Good Materials and Structures Division, NASA Glenn Research Center, Cleveland, Ohio. ABSTRACT While the low thermal conductivities of silica aerogels have made them of interest to the aerospace community as lightweight thermal insulation, the application of conformal polymer coatings to these gels increases their strength significantly, making them potentially useful as structural materials as well. In this work we perform multiscale computer simulations to investigate the tensile and compressive strain behavior of silica and polymer-coated silica aerogels. Aerogels are made up of clusters of interconnected particles of amorphous silica of less than bulk density. We simulate gel nanostructure using a Diffusion Limited Cluster Aggregation (DLCA) procedure, which produces aggregates that exhibit fractal dimensions similar to those observed in real aerogels. We have previously found that model gels obtained via DLCA exhibited stress-strain curves characteristic of the experimentally observed brittle failure. However, the strain energetics near the expected point of failure were not consistent with such failure. This shortcoming may be due to the fact that the DLCA process produces model gels that are lacking in closed-loop substructures, compared with real gels. Our model gels therefore contain an excess of dangling strands, which tend to unravel under tensile strain, producing non-brittle failure. To address this problem, we have incorporated a modification to the DLCA algorithm that specifically produces closed loops in the model gels. We obtain the strain energetics of interparticle connections via atomistic molecular statics, and abstract the collective energy of the atomic bonds into a Morse potential scaled to describe gel particle interactions. Polymer coatings are similarly described. We apply repeated small uniaxial strains to DLCA clusters, and allow relaxation of the center eighty percent of the cluster between strains. The simulations produce energetics and stress-strain curves for looped and nonlooped clusters, for a variety of densities and interaction parameters. INTRODUCTION Silica aerogels are low-density, highly porous materials possessing thermal properties that have made them of interest for a wide variety of applications [1-3]. Notably, the low thermal conductivities characteristic of such gels have led to the aerospace community’s interest in these materials as lightweight thermal insulation. While these aerogels’ fragility limits their utility in many applications, researchers in our laboratory have developed a method for monomer-coating aerogels and cross-linking the coatings, so as to greatly improve the gels' strength while not greatly impacting their insulating properties [4]. Such coated gels may prove suitable for use as lightweight structural
materials. In order to provide an understanding of the mechanical behavior of the gels, and to provide predictive tools of use in
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