Light scattering and haze in TMOS-co-APTES silica aerogels

TMOS-co-APTES wet-gels have been used as templates for polymer crosslinked aerogels, which in turn have been used as strong materials and as starting materials for ceramic aerogels. Such wet-gels are prepared in acetonitrile (CH3CN) via co-gelation of tetramethylorthosilicate (TMOS) and 3-aminopropyl silane (APTES) in a 3:1 v/v ratio. Qualitatively, native monolithic TMOS-co-APTES aerogels appear more clear than the corresponding straight TMOS-derived materials, suggesting applications in thermally insulating daylighting. For that application, it is important to understand and minimize haze. Haze from the bulk of silica aerogels may be due to scattering from particles or pores, but study of its origin may be complicated by scattering and reflections from the surface of the monolith. In this report, using wet-gels as proxy materials for aerogels, haze from TMOS-co-APTES was investigated as a function of density, which is controlled by the amount of TMOS + APTES in the sol. The wet-gel approach to the study of haze of aerogels has the advantages of eliminating surface scattering issues from aerogel monoliths, and owing to better refractive index matching between the skeletal framework and the surrounding medium (CH3CN), bulk scattering is also reduced, facilitating study of lower-density materials. Haze from the bulk was traced to Rayleigh scattering down to 0.017 g cm−3 aerogels. The opacity of even lower-density samples (down to 0.006 g cm−3) was attributed to the Mie scattering. The scatterer size was obtained via Rayleigh’s law, and matched the secondary particle sizes of silica. The size of those particles increased as the density decreased, producing higher haze. Results suggest that in order to keep haze low at lower densities, one must keep the particle size low, for example, by increasing the rate of gelation.