26 Bifunctional Silicas with Immobilized Lignin
419
Considering all extrema as endothermic ones, we can correlate them with the
following processes: the effect appearing at 427 ◦ C is related to degradation of
the aromatic ring; the peaks near 150 and 250 ◦ C can correspond to destruction
of different links in macromolecules. Namely, the syringyl, as well as the guaiacyl
units, is built into the lignin macromolecule mainly by ether bonds, and the
ether bonds between syringyl units are easier to split than those between guaiacyl
units [23]. Moreover, the guaiacyl units easily undergo condensation and coupling
reactions. The thermal effect at 320 ◦ C occurred due to destruction of main lignin
chain. It should be noted that the highest degradation rate of lignin is observed over
the temperature range from 290 to 430 ◦ C where the DSC curve has both the endo
and the exo direction [24]. And the endo-/exothermal transitions with extrema at
427 and 473 ◦ C can be attributed to lignin carbonization (Fig. 26.4c).
The differences in the inherent structures of lignin could account for the
diversities of thermal degradation behaviors. For lignin immobilized on silica,
the curves differ significantly from the curve of the initial polymer: only two
intense peaks of about 200 and 320 ◦ C were detected (Fig. 26.5). It is known that
the destruction of the cross-links in the lignin structure leads to a displacement
of the basic extrema to the lower temperatures, which allows us to assume the
inverse effect will be observed at formation of additional bonds. The DSC data
obtained were accumulated in Table 26.2. As can be seen, the first endotherm of
the decomposition is broadened, while the endothermic process at 320 ◦ C passes
with the same enthalpy. A general tendency is violated by samples based on silicon
hydride-containing silica. The DSC curve for Lig-A4 demonstrates β-relaxation at
114 ◦ C appeared during sample heating. This conformation transition (β-relaxation)
is the local rotational motion of the segments of the flexible macromolecules in the
interlayer between lamellar phases. At the stronger macromolecules, bending this
transition is more significant and appears at the lower temperature. For composites
under study, β-relaxation was found also for sample Lig-A8 but less apparent and
at the lower temperature, 107 ◦ C. In the absence of local tension, this transition
was not observed, the endotherm being narrower. For the Lig-A10 the narrow
endothermal peak at 156 ◦ C occurred which can be corresponded to the melting
process. Moreover, the endotherm has extremum at the lowest temperature and
includes two overlapped bands which can be caused by degradation of new bonds
formed in the polymer or with the silica surface. The endotherm’s broadening
usually occurs when polymer structure becomes loose. In this case we can assume
that the polymeric surface layer is structured. A possible explanation is that the
less homogeneous structure of the sample was caused by different orientation of
macromolecules near the particle surface and in the bulk. This leads to discreteness
of the thermodynamic and thermokinetic parameters of structure.
In order to investigate self-organized structures on the silica surface, the DSC
study was carried out in the interval −50–350 ◦ C with the different heating regimes.
When the components of the lignin sample are amorphous or semicrystalline
polymers during the degradation process, the melting point at 250 ◦ C depression is
indicative of favored breakage interactions between the polymer chains. The lower
H and T endo values may also be associated with the breakup of aromatic polymers
419
Considering all extrema as endothermic ones, we can correlate them with the
following processes: the effect appearing at 427 ◦ C is related to degradation of
the aromatic ring; the peaks near 150 and 250 ◦ C can correspond to destruction
of different links in macromolecules. Namely, the syringyl, as well as the guaiacyl
units, is built into the lignin macromolecule mainly by ether bonds, and the
ether bonds between syringyl units are easier to split than those between guaiacyl
units [23]. Moreover, the guaiacyl units easily undergo condensation and coupling
reactions. The thermal effect at 320 ◦ C occurred due to destruction of main lignin
chain. It should be noted that the highest degradation rate of lignin is observed over
the temperature range from 290 to 430 ◦ C where the DSC curve has both the endo
and the exo direction [24]. And the endo-/exothermal transitions with extrema at
427 and 473 ◦ C can be attributed to lignin carbonization (Fig. 26.4c).
The differences in the inherent structures of lignin could account for the
diversities of thermal degradation behaviors. For lignin immobilized on silica,
the curves differ significantly from the curve of the initial polymer: only two
intense peaks of about 200 and 320 ◦ C were detected (Fig. 26.5). It is known that
the destruction of the cross-links in the lignin structure leads to a displacement
of the basic extrema to the lower temperatures, which allows us to assume the
inverse effect will be observed at formation of additional bonds. The DSC data
obtained were accumulated in Table 26.2. As can be seen, the first endotherm of
the decomposition is broadened, while the endothermic process at 320 ◦ C passes
with the same enthalpy. A general tendency is violated by samples based on silicon
hydride-containing silica. The DSC curve for Lig-A4 demonstrates β-relaxation at
114 ◦ C appeared during sample heating. This conformation transition (β-relaxation)
is the local rotational motion of the segments of the flexible macromolecules in the
interlayer between lamellar phases. At the stronger macromolecules, bending this
transition is more significant and appears at the lower temperature. For composites
under study, β-relaxation was found also for sample Lig-A8 but less apparent and
at the lower temperature, 107 ◦ C. In the absence of local tension, this transition
was not observed, the endotherm being narrower. For the Lig-A10 the narrow
endothermal peak at 156 ◦ C occurred which can be corresponded to the melting
process. Moreover, the endotherm has extremum at the lowest temperature and
includes two overlapped bands which can be caused by degradation of new bonds
formed in the polymer or with the silica surface. The endotherm’s broadening
usually occurs when polymer structure becomes loose. In this case we can assume
that the polymeric surface layer is structured. A possible explanation is that the
less homogeneous structure of the sample was caused by different orientation of
macromolecules near the particle surface and in the bulk. This leads to discreteness
of the thermodynamic and thermokinetic parameters of structure.
In order to investigate self-organized structures on the silica surface, the DSC
study was carried out in the interval −50–350 ◦ C with the different heating regimes.
When the components of the lignin sample are amorphous or semicrystalline
polymers during the degradation process, the melting point at 250 ◦ C depression is
indicative of favored breakage interactions between the polymer chains. The lower
H and T endo values may also be associated with the breakup of aromatic polymers