The effects of ambient humidity on the mechanical properties and surface chemistry of hygroscopic silica aerogel

The stress vs. strain relation of hygroscopic aerogels was tested in a controlled humidity chamber to study how the Young’s modulus and non-recoverable strain are affected by the adsorption of water in the gel. Mass gains over desiccated conditions varied from 4% at 32% relative humidity (RH) to 12% at 70% RH. Above 70% RH, the samples failed catastrophically. The Young’s modulus increased from 0.51 to 0.70 MPa and non-recoverable strain increased from 0.047 to 0.059 over the same humidity range. Yield stress did not show significant changes. The samples were also studied with near-infrared transmission measurements to determine if the adsorbed water was chemically as well as physically bound to the surface. Several absorption lines indicative of hydrogen bonding between water and silica were seen to increase with increasing humidity while surface silanol lines decreased.