Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

Abstract

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion.

In Operando μCT Imaging of Silylated Silica Aerogels during Ambient Pressure Drying and Spring-Back

“…The calculated densities were in the range of typical values for silica aerogels reported in the literature by helium pycnometry. 20,24 These findings emphasized the reliability of the μCT quantitative imaging procedure and the accuracy of the underlying model for the modified gels. It is pertinent to be aware about the impact of potential closed pores in the calculation of the skeleton volume.…”

“…This suggested that the drying rate was only limited by the convection of hexane vapor within the drying chamber for the first 6 h. A previous study reported an increasing evaporation rate in similar gels, which may be related to a difference in the ambient conditions compared to this study. 24 Interestingly, we calculated a significant amount of gas phase in all gels already before the maximum shrinkage, making for up to 37 vol % of the gel's volume (Figures 6 and 7 and Figure S17). This suggested that the liquid−gas interface already receded into the pores, whereas common drying models predict that the liquid recedes in the pores once shrinkage stops.…”

“…As a consequence of the limited spring-back efficiency, the produced aerogels were denser than those reported in the literature for similar synthesis and drying conditions. 14,24 This effect can arise from one or more of the following: condensation reactions (chemical lock), plastic deformation, entanglement of the solid network, and residual liquid phase (mechanical locks). It is notably possible that differences in the TMCS/SiO 2 molar ratio affected the capability of the gels to spring-back.…”

“…The skeletal density was fixed and set at 1.9 g cm −3 for the modified gels and at 2.4 g cm −3 for the xerogels. 24 Despite the additional constraint, the resulting system had two more unknowns than equations and the simulations generated an infinite number of solutions as 2D surfaces. We chose to evaluate the change of the following quantities: volume V skel d .…”

“…To this regard, small-angle X-ray scattering (SAXS) and X-ray micro-computed tomography (μCT) methods are well suited and provide enough space for experiments on gels contained in a drying chamber. Zemke et al notably used synchrotron-based SAXS and monitored the drying of silica aerogels enclosed in a custom chamber under ambient pressure conditions . While SAXS provides structural information from 2 to 500 nm, μCT can generate 3D volumes of samples of various sizes with a maximum resolution of about 1 μm while also providing the reconstructed attenuation coefficient (RAC) values of a sample. , This distinct advantage over SAXS potentially permits to extract quantitative compositional information of hybrid materials such as aerogels but does not allow us to resolve aerogels structural heterogeneities, which extend to about 100 nm .…”