Abstract:
Two types of preformed alginate wet gels, one with a low (30-35%) and
the other with a high (65-75%) content of glucuronic acid, were reacted
with an aliphatic triisocyanate that was priorly allowed to diffuse in
the pores. This reaction formed urethane groups on the surface of the
alginate framework and also formed a polyurea (PUA) network connecting
these urethane groups via respective reactions of the triisocyanate with
alginate surface -OH groups or with gelation water remaining adsorbed on
the inner surfaces of the wet gels. These processes formed a conformal
nanothin film of PUA around the alginate network. After drying the wet
gels with the supercritical fluid CO2, we obtained
PUA/polyurethane-cross-linked alginate (X-alginate) aerogels. Although
X-alginate aerogels are essentially copolymers, unlike all copolymers
mentioned in previous literature reports, the relative topology of the
alginate and the cross-linker is defined at the nanoscopic scale rather
than at the molecular level. For the systematic study of X-alginate
aerogels as a function of synthetic conditions, the experimental
protocol was designed according to the central circumscribed rotatory
design model using the alginate and the triisocyanate concentration as
independent variables. Empirical models were derived for all relevant
material properties by fitting experimental data to the two independent
variables. The chemical identity of all samples was confirmed with
attenuated total reflectance-Fourier transform infrared spectroscopy and
solid-state C-13 and N-15 cross-polarization magic angle spinning NMR
spectroscopy. The percentage of PUA uptake in X-alginate aerogels
(58-98%) was calculated from skeletal density data. Scanning electron
microscopy showed that all samples were nanofibrous, indicating that PUA
coated conformally the skeletal network of both alginates, and the
micromorphology remained the same as in the native (non-cross-linked)
samples. X-alginate aerogels are mechanically strong materials, in
contrast to their native counterparts, which are extremely weak
mechanically. Compared to various organic aerogels from the literature,
X-alginate aerogels can be as stiff as many other polymeric aerogels
with 2 or 3 times higher densities. In addition, X-alginate aerogels are
good candidates for sound insulation applications, as the speed of sound
in most samples was estimated to be significantly lower than the speed
of sound in dry air.
