Biomedical Engineering Reference
In-Depth Information
C
= O + H
C -
+
+
⇔
catalytic site [36]: CH
O, then the
carbocation attacks resorcinol which then attracts the furfural
ligand. The carbocation can be linked on the 2, 4, and 6 positions
of the aromatic ring in resorcinol, which cross-links with furfural
ligands. The consequent condensation reaction in resorcinol-furfural
precursors generates a three-dimensional network via formation of
methylene and methylene ether bridges [37]. The dried gels, heated
at 75°C for a gelation time of 42 h, were then collected for NMR
measurements as shown in Fig. 3.6.
COOH + H
H
3
3
2
Figure 3.5
Reaction mechanism of the sol-gel polymerization of resorcinol
with furfural catalyzed by acetic acid.
The acetic acid catalyzed CAs (CA4.8 and CA4.1) as well as
the uncatalyzed CA all show broad NMR lines, indicating that the
resorcinol-furfural clusters are highly cross-linked. The NMR peaks
were assigned to aromatic carbons with an -OH (150 ppm) shifted to
152 ppm and aromatic carbons ortho- to an -OH (120 ppm) shifted to
122 ppm similar to resorcinol-formaldehyde clusters [38]. However,
aliphatic carbons adjacent to oxygen (60 ppm) and methylene
carbons (20 ppm) that are present in clusters are not identified in
our samples. The peak at 110 ppm can be assigned to -COH from
furfural ligands. The peaks at 35 ppm (Fig. 3.6a) and 36 ppm (Fig.
3.6b and c) represent the bridges between aromatic rings and
furfural ligands. When the acetic acid content is increased (reducing
