414
Y. Bolbukh et al.
Figure 26.2 presents the ATR spectra of the silica/lignin biocomposites obtained,
evidencing the presence of band characteristics of appropriate functional groups
both silica and lignin. Taking into account equal content of the polymer in samples,
the increasing (or decreasing) intensity of bands with changing of surface chemistry
of the silica matrix confirms different structure of the polymeric immobilized layer.
A band appearing in the spectra of samples with lignin in the range 950–720 cm −1
can be attributed to structure-sensitive one.
For the sample Lig-A1 with totally methylated silica, the bands 835 and
750 cm −1 appeared in spectra. These bands are characteristic for pure lignin and
attributed to guaiacyl units [20]. This means that on the hydrophobic surface, the
structure of the adsorbed layer is identical to the polymer structure in volume.
For samples Lig-A4 and Lig-A8 based on hydride- and amino-hydride-containing
silicas, respectively, the bands of stretching vibrations of C–H bonds intensified,
while the band at 940 cm −1 became more intensive in spectrum for the sample
Lig-A9 (contains 30% of silicon hydride groups), where the bands of stretching
vibrations of C–H bonds are less visible. Preliminarily, it can be concluded that the
silicon hydride groups on the silica surface have the greatest effect on the structure
of the immobilized polymer layer.
The UV–vis absorption spectra of dispersed samples at pH 7 are represented
in Fig. 26.3. The absorption bands located at 240 and 280 nm are assigned to Kand B-bands of π−π* transitions of the aromatic rings in lignin linked by ionized
phenolic hydroxyls and carboxylic groups [21]. After immobilization on silica, the
K-band disappears and the B-band shifts to 285 nm with decreased absorption
intensity. It is a result of change in protonation of phenolic hydroxyls. Exceptions are
samples Lig-A3, Lig-A5, and Lig-A10. Combining the results of UV analysis and
IR spectroscopy, one can conclude that a dense polymeric layer with a low degree of
freedom of –CH 3 (CH 2 ) groups (weak intensity of the bands in the region of 3000–
2800 cm −1 ) and a changed protonation degree of phenyl groups (Fig. 26.3, Lig-A1)
is formed on the hydrated surface of pristine silica; with a fully methylated surface
(Lig-A2), the interaction is carried out through the benzene rings of the polymer,
which changes the energy state of the phenolic groups, whereas the deformation
vibrations of the CH 3 (CH 2 ) groups are not hindered (Fig. 26.2, spectrum 2); on
a partially methylated surface (SiO 2 CH 3 30%), phenolic groups are not involved
in the interaction, which indicates a change in the orientation of macromolecules
depending on the degree of methylation of the silica surface.
The spectra of samples Lig-A5 and Lig-A10 (Fig. 26.3) indicate the presence
of K- and B-bands at 235 and 288 but with a significant decrease in intensity,
which indicates the interaction of the amino groups of silica with the acidic phenolic
groups of the polymer. In addition, for sample Lig-A5, relative intensification of the
Fig. 26.2 (continued) and aminated silica with modification degree 100% (5, Lig-A5); silica with
bifunctional amino-methyl (6, Lig-A6), hydride-methyl (7, Lig-A7), and hydride-amino surface
layer (8, Lig-A8); and silica with modification degree 30% with silicon hydride (9, Lig-A9) and
amino groups (10, Lig-A10). Sample description is present in Table 26.1
Y. Bolbukh et al.
Figure 26.2 presents the ATR spectra of the silica/lignin biocomposites obtained,
evidencing the presence of band characteristics of appropriate functional groups
both silica and lignin. Taking into account equal content of the polymer in samples,
the increasing (or decreasing) intensity of bands with changing of surface chemistry
of the silica matrix confirms different structure of the polymeric immobilized layer.
A band appearing in the spectra of samples with lignin in the range 950–720 cm −1
can be attributed to structure-sensitive one.
For the sample Lig-A1 with totally methylated silica, the bands 835 and
750 cm −1 appeared in spectra. These bands are characteristic for pure lignin and
attributed to guaiacyl units [20]. This means that on the hydrophobic surface, the
structure of the adsorbed layer is identical to the polymer structure in volume.
For samples Lig-A4 and Lig-A8 based on hydride- and amino-hydride-containing
silicas, respectively, the bands of stretching vibrations of C–H bonds intensified,
while the band at 940 cm −1 became more intensive in spectrum for the sample
Lig-A9 (contains 30% of silicon hydride groups), where the bands of stretching
vibrations of C–H bonds are less visible. Preliminarily, it can be concluded that the
silicon hydride groups on the silica surface have the greatest effect on the structure
of the immobilized polymer layer.
The UV–vis absorption spectra of dispersed samples at pH 7 are represented
in Fig. 26.3. The absorption bands located at 240 and 280 nm are assigned to Kand B-bands of π−π* transitions of the aromatic rings in lignin linked by ionized
phenolic hydroxyls and carboxylic groups [21]. After immobilization on silica, the
K-band disappears and the B-band shifts to 285 nm with decreased absorption
intensity. It is a result of change in protonation of phenolic hydroxyls. Exceptions are
samples Lig-A3, Lig-A5, and Lig-A10. Combining the results of UV analysis and
IR spectroscopy, one can conclude that a dense polymeric layer with a low degree of
freedom of –CH 3 (CH 2 ) groups (weak intensity of the bands in the region of 3000–
2800 cm −1 ) and a changed protonation degree of phenyl groups (Fig. 26.3, Lig-A1)
is formed on the hydrated surface of pristine silica; with a fully methylated surface
(Lig-A2), the interaction is carried out through the benzene rings of the polymer,
which changes the energy state of the phenolic groups, whereas the deformation
vibrations of the CH 3 (CH 2 ) groups are not hindered (Fig. 26.2, spectrum 2); on
a partially methylated surface (SiO 2 CH 3 30%), phenolic groups are not involved
in the interaction, which indicates a change in the orientation of macromolecules
depending on the degree of methylation of the silica surface.
The spectra of samples Lig-A5 and Lig-A10 (Fig. 26.3) indicate the presence
of K- and B-bands at 235 and 288 but with a significant decrease in intensity,
which indicates the interaction of the amino groups of silica with the acidic phenolic
groups of the polymer. In addition, for sample Lig-A5, relative intensification of the
Fig. 26.2 (continued) and aminated silica with modification degree 100% (5, Lig-A5); silica with
bifunctional amino-methyl (6, Lig-A6), hydride-methyl (7, Lig-A7), and hydride-amino surface
layer (8, Lig-A8); and silica with modification degree 30% with silicon hydride (9, Lig-A9) and
amino groups (10, Lig-A10). Sample description is present in Table 26.1