Strong, Machinable, and Insulating Chitosan–Urea Aerogels: Toward Ambient Pressure Drying of Biopolymer Aerogel Monoliths

Biopolymer aerogels
are an emerging class of materials with potential applications in
drug delivery, thermal insulation, separation, and filtration. Chitosan
is of particular interest as a sustainable, biocompatible, and abundant
raw material. Here, we present urea-modified chitosan aerogels with
a high surface area and excellent thermal and mechanical properties.
The irreversible gelation of an acidic chitosan solution is triggered
by the thermal decomposition of urea at 80 °C through an increase
in pH and, more importantly, the formation of abundant ureido terminal
groups. The hydrogels are dried using either supercritical CO2 drying (SCD)
or ambient pressure drying (APD) methods to elucidate the influence
of the drying process on the final aerogel properties. The hydrogels
are exchanged into ethanol prior to SCD, and into ethanol and then
heptane prior to APD. The surface chemistry and microstructure are
monitored by solid-state NMR and Fourier transform infrared spectroscopy,
scanning electron microscopy, and nitrogen sorption. Surprisingly,
large monolithic aerogel plates (70 × 70 mm2) can
be produced by APD, albeit at a somewhat higher density (0.17–0.42
g/cm3). The as prepared aerogels have thermal conductivities
of ∼24 and ∼31 mW/(m·K) and surface areas of 160–170
and 85–230 m2/g, for SCD and APD, respectively.
For a primarily biopolymer-based material, these aerogels are exceptionally
stable at elevated temperature (TGA) and char and self-extinguish
after direct flame exposure. The urea-modified chitosan aerogels display
superior mechanical properties compared to traditional silica aerogels,
with no brittle rupture up to at least 80% strain, and depending on
the chitosan concentration, relatively high E-moduli
(1.0–11.6 MPa), and stress at 80% strain values (σ80 of 3.5–17.9 MPa). Remarkably, the aerogel monoliths
can be shaped and machined with standard tools, for example, drilling
and sawing. This first demonstration to produce monolithic and machinable,
mesoporous aerogels from bio-sourced, renewable, and nontoxic precursors,
combined with the potential for reduced production cost by means of
simple APD, opens up new opportunities for biopolymer aerogel applications
and marks an important step toward commercialization of biopolymer
aerogels.