Abstract:
The purpose of this work was to investigate the effect of
multifunctionality on material properties of synthetic polymer aerogels.
For this purpose, we present the synthesis and characterization of
monolithic dendritic-type urethane-acrylate monomers based on an
aliphatic/flexible (Desmodur N3300), or an aromatic/rigid (Desmodur RE)
triisocyanate core. The terminal acrylate groups (three at the tip of
each of the three branches, nine in total) were polymerized with
2,2’-azobis(isobutyronitrile) (AIBN) via free radical chemistry. The
resulting wet-gels were dried with supercritical fluid (SCF) CO2.
Aerogels were characterized with ATR-FTIR and solid-state C-13 NMR. The
porous network was probed with N-2-sorption and scanning electron
microscopy (SEM). The thermal stability of aerogels was studied with
thermogravimetric analysis (TGA). Most aerogels were macroporous
materials (porosity > 80%), with high thermal stability (up to 300
degrees C). Aerogels were softer at low monomer concentrations and more
rigid at higher concentrations. The material properties were compared
with those of analogous aerogels bearing only one acrylate moiety at the
tip of each branch and the same cores, and with those of analogous
aerogels bearing norbornene instead of acrylate moieties. The
nine-terminal acrylate-based monomers of this study caused rapid
decrease of the solubility of the growing polymer and made possible
aerogels with much smaller particles and much higher surface areas. For
the first time, aliphatic/flexible triisocyanate-based materials could
be made with similar properties in terms of particle size and surface
areas to their aromatic/rigid analogues. Finally, it was found that with
monomers with a high number of crosslinkable groups, material properties
are determined by multifunctionality and thus aerogels based on
9-acrylate- and 9-norbornene-terminated monomers were similar. Materials
with aromatic cores are carbonizable with satisfactory yields (20-30%
w/w) to mostly microporous materials (BET surface areas: 640-740 m(2)
g(-1); micropore surface areas: 360-430 m(2) g(-1)).
