In
this work, polyurethane aerogels are fabricated in re-entrant
honeycomb forms to achieve auxeticity and much higher flexibility
than corresponding aerogel monoliths. For this purpose, a set of re-entrant
honeycomb-shaped hollow molds is first printed from high-impact polystyrene
(HIPS) using a fused filament fabrication technique and subsequently
filled with polyurethane sol synthesized from an aliphatic triisocyanate
and a diol selected from among butanediol, pentanediol, hexanediol,
or octanediol. A sol–gel transition process yields the gel
in a re-entrant honeycomb shape, which is isolated by dissolving the
HIPS mold and supercritically dried using carbon dioxide. The effects
of diol chain length on the properties of aerogels, such as bulk density,
pore size, and tensile properties are investigated. The auxetic aerogels
show different deformation behavior and much higher elongation at
break than dogbone-shaped aerogel monoliths. Such unique properties
of auxetic structures can extend applications of aerogels as acoustic
absorbers and smart bandages that require high porosity, tunable pore
structures, and high flexibility.
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