Polyurethane aerogels
were prepared from a rigid aromatic triisocyanate (tris(4-isocyanatophenyl)methane)
and cage-shaped α- and β-cyclodextrins as rigid polyols.
Gelation was carried out in DMF using dibutyltin dilaurate as catalyst.
Wet-gels were dried to aerogels (abbreviated as α- or β-CDPU-xx) with supercritical fluid CO2. “xx” stands for the percent weight of the two monomers
in the sol and was varied at two levels for each cyclodextrin: 2.5%
and 15%. All aerogels were characterized with solid-state 13C and 15N NMR, CHN analysis, FTIR, XPS, SEM, and gas (N2 and CO2) sorption porosimetry. α- and β-CDPU-xx aerogels were investigated as desiccants at room temperature.
All materials had relatively higher capacities for water adsorption
from high-humidity environments (99%) than typical commercial desiccants
like silica or Drierite. However, α-CDPU-2.5 aerogels did stand
out with a water uptake capacity reaching 1 g of H2O per
gram of material. Most importantly though, adsorbed water could be
released quantitatively without heating, by just reducing the relative
humidity of the environment to 10%. All α- and β-CDPU-xx aerogel samples were cycled between humid and dry environments
10 times. Their unusual behavior was traced to filling smaller mesopores
with water and was attributed to a delicate balance of enthalpic (H-bonding)
and entropic factors, whereas the latter are a function of pore sizes.