Effects of skin layers on air permeability in macroporous polymer aerogels

Skin layers are ubiquitous in synthesis of macro- and mesoporous polymer aerogel monoliths, are much denser than the bulk materials, and can potentially impact several properties of aerogels. This study evaluates the effects of skin layers on the values of air permeability in aerogel monoliths of syndiotactic polystyrene (sPS) prepared from thermo-reversible gelation of sPS solutions followed by supercritical drying in carbon dioxide. The sPS aerogel monoliths are primarily macroporous and conducive to provide air permeability of the order of 10(-10) m(2). However, the overall air permeability is substantially reduced due to the denser skin layers. A composite model is used to estimate the thickness, bulk density, and air permeability of skin layers from experimentally measured air permeability data. It is found that skin layers of the order of 0.5 mm in thickness form during synthesis of sPS monoliths from solution in tetrahydrofuran. The air permeability of the skin layer is about half that of the bulk material. (C) 2017 Elsevier Ltd. All rights reserved.