Insights into the Micromechanics of Organic Aerogels Based on Experimental and Modeling Results

Abstract

While the characteristics of the macroscopic mechanical behavior of organic aerogels are well known, the mechanisms responsible for the substructural evolution of their networks under mechanical deformation are not fully understood. Herein, organic aerogels from the aqueous sol−gel polymerization of resorcinol with formaldehyde are first prepared. Specifically, the resorcinol to water (R:W) molar ratio is varied for obtaining diverse highly open‐cellular porous structures with mean pore sizes ranging between 30 and 50 nm. The corresponding network structures are then characterized and exhibit different morphological and mechanical properties. Furthermore, a micromechanical constitutive model based on the pore‐wall kinematics is proposed. While the arrays of particles forming the pore walls are moderately connected, the pore walls are considered to behave as solid beams under mechanical deformation. Moreover, the damage mechanisms in the pore walls that result in the network collapse are defined. All model parameters are shown to be physically derived, and their sensitivity to the macroscopic network behavior is analyzed. The model predictions are shown to be in good agreement with the experimental stress−strain data of the different aerogels.

Acoustic Properties of Aerogels: Current Status and Prospects

“…This may lead to one order of magnitude increase in the modulus (Figure 4). [34][35][36] Synthetic polymer aerogels, for example, based on resorcinolformaldehyde, are not brittle and follow foam-like stress-strain dependence (Figure 4b); [37] similar trends were reported for other polymer aerogels. [33,38,39] Compressive modulus of synthetic polymer aerogels can reach several tens of MPa at density around 0.1-0.2 g cm À3 .…”

“…Illustration of the mechanical properties of aerogels: a) compressive modulus of various aerogels as a function of density (TAB is 1,3,5-benzenetricarbonyl trichloride, POSS is polyhedral oligomeric silsesquioxane), data taken from;[20,35,97,118] b) stress-strain curves stress-strain for resorcinol-formaldehyde aerogels under compression displaying the dependence on the resorcinol (R):water (W) ratio. Reproduced with permission [37]. Copyright 2021, The Authors, published by Wiley-VCH.…”

The Effect of Particle Necks on the Mechanical Properties of Aerogels

“…Dargazany and Itskov [13,14] studied the mechanical behavior of interconnected arrays of colloidal particles assuming that at the touching points the bonds can be described as non-linear springs. In many aerogels, the network of particles consist of moderately up to heavily connected bonds and therefore the Gibson and Ashby [1] model has been applied for aerogels [5,15]. All studies performed so far have shown that the pore collapse occurs as a result of a critical stress arising from buckling or bending of the pore walls or better the connected struts.…”

Aerogel Spring‐Back Correlates with Strain Recovery: Effect of Silica Concentration and Aging

“…[43,63] The value for α derived from the current dataset is 4.3, somewhat higher than the previously reported for silica aerogels, e.g., 3.6 [43] and 3.7 [63] from experiments or 3.61 from simulations, [64] and significantly higher than what is typical for many other aerogel systems, e.g., 1.75 for alumina aerogels, [60] 1.8 for cellulose aerogels, [61] or 2.7-3.7 for resorcinol-formaldehyde aerogels. [65,66] The E-modulus of the 2 h-aged aerogels is lower by a factor of 1.5-2.5 compared with the 24 h-aged counterparts of similar density, a direct consequence of the formation of thicker interparticle necks during the extended aging time, which increases the structural strength to the aerogel network. [35] Many industries use secant moduli σ 10 , σ 30 , σ 50 , and σ 80 , [67] i.e., the stress values for a given percentage of strain, rather than E-moduli to describe the sample deformation under stress.…”