Crosslinked polyurea-co-polyurethane aerogels with hierarchical structures and low stiffness

Abstract

Resilient poly(urethane urea) aerogels are synthesized from aromatic diisocyanates, aromatic diamines, aliphatic polyols, and a trifunctional amine crosslinker. Polyurethane prepolymers are synthesized with isocyanate end groups by reacting the polyol with 4,4′-diphenylmethane diisocyanate in anhydrous N-methyl-2-pyrrolidone (NMP). The isocyanate-capped polyurethane segments are reacted with 4,4′-oxydianiline to obtain isocyanate end-capped poly(urethane urea) copolymers which are then crosslinked using 1,3,5 triaminophenoxylbenzene (TAB) to obtain the gel networks. The gel networks are tailored by the choice of the polyol and by varying the crosslink density. The gels are dried under supercritical condition after exchanging NMP with acetone and acetone with liquid carbon dioxide. The resulting aerogels show density between 0.20 and 0.35 g/cm3, porosity between 71 and 85%, and surface area between 47 and 163 m2/g. The data suggest that the polyol weight fraction can be used as a parameter to control the shrinkage of aerogel specimens. The shrinkage data also correlate well with the extent of hydrogen bonding involving urea and urethane groups. A reduction of the amount of TAB leads to reduction of the extent of urea hydrogen bonding and an increase of urethane hydrogen bonding. These materials exhibit an onset of thermal decomposition at about 300 °C and offer compressive moduli between 12 and 52 MPa. The compressive modulus shows strong dependence on aerogel density.