8 Nanocomposites Based on Thermosetting Polyurethane Matrix. . .
137
interfacial area. When 0.25% MWCNT-ox is added, the main maximum at 10 ◦ C in
the spectrum of neat PU displaces to 90 ◦ C in the spectrum of the nanocomposite
and, additionally, the spectral contour shows a tendency to manifest relaxation
(curve bend) at 120–140 ◦ C.
Thus, covalent bonding of some isocyanate groups (in fact the PU network
junctions) with the functional groups covalently bound to MWCNT surface provides
the strongest interfacial interactions in the nanocomposites with MWCNT-ox. As a
result, the largest constrained dynamics effect is observed. This was attained even
in spite of some MWCNT aggregation and disrupting of the initial (hard and soft
domains) matrix structure [26].
Figure 8.9d shows that the incorporation of MWCNT-red particles, which have
a strongly decreased number of surface carboxyl groups (Table 8.1), into the PU
matrix results in the reduction of changes in the tan δ (T) curve, probably due
to weakening of the interfacial covalent interactions. It may be also assumed that
carboxylic groups attached to the MWCNT surface are more reactive than alcohol
and phenol groups in the reaction with isocyanate groups. Of interest, in this case
the maximal effect from filling is observed at 0.1% MWCNT-red, whereas at 0.25%
nanofiller effect of constrained dynamics disappears. The latter could, obviously, be
associated with the enhanced nanotube aggregation [26].
Thus, DMA data obtained for neat PU and the studied nanocomposites suggest the pronounced dynamic heterogeneity within their glass transitions. Such
supposition was confirmed by the dependencies of the effective activation energy
Q of segmental motion within the glass transition as a function of temperature,
as obtained from the DMA experiments performed at different frequencies and
also from the discrete creep rate spectra. These results allowed a comprehensive
understanding of the origin of the complicated glass transition dynamics in these
materials to be elucidated.
Figure 8.10 shows the DMA spectra obtained at 0.1, 1 and 10 Hz for neat PU
and for the nanocomposite with 0.25% MWCNT-ox, and Fig. 8.11 presents the
Q(T) dependencies obtained using these experimental data and the calculations
by formula (8.5). In general, the peculiar Q(T) plots directly indicate a very
broad dispersion of the effective activation barriers Q varying from 110 to
300 kJ mol −1 , where a few Q levels characterize the glass transitions of neat PU and
the nanocomposite based thereon. This means that the dynamics within their glass
transitions is formed by several constituent relaxations (dynamic modes) over the
temperature range from −60 to +150 ◦ C. The substantial discrepancy between both
dependencies obtained is observed only at temperatures between 50 and 150 ◦ C.
PU network includes the flexible PPG cross-links with a molecular weight of
2.000 g mol −1 , that is, each cross-link includes about five Kuhn segments, and
the more rigid PU network junctions. Judging by the Q and temperature values,
the Q(T) plots manifest the following dynamic modes with increasing temperature.
First, the “normal” cooperative glass transition in PPG moieties between −60
and −50 ◦ C is practically undisturbed by the network junctions or being located
at some distance from them. This is confirmed by a high, cooperative value of
Q = 280–300 kJ mol −1 and the temperature position of this relaxation which is
137
interfacial area. When 0.25% MWCNT-ox is added, the main maximum at 10 ◦ C in
the spectrum of neat PU displaces to 90 ◦ C in the spectrum of the nanocomposite
and, additionally, the spectral contour shows a tendency to manifest relaxation
(curve bend) at 120–140 ◦ C.
Thus, covalent bonding of some isocyanate groups (in fact the PU network
junctions) with the functional groups covalently bound to MWCNT surface provides
the strongest interfacial interactions in the nanocomposites with MWCNT-ox. As a
result, the largest constrained dynamics effect is observed. This was attained even
in spite of some MWCNT aggregation and disrupting of the initial (hard and soft
domains) matrix structure [26].
Figure 8.9d shows that the incorporation of MWCNT-red particles, which have
a strongly decreased number of surface carboxyl groups (Table 8.1), into the PU
matrix results in the reduction of changes in the tan δ (T) curve, probably due
to weakening of the interfacial covalent interactions. It may be also assumed that
carboxylic groups attached to the MWCNT surface are more reactive than alcohol
and phenol groups in the reaction with isocyanate groups. Of interest, in this case
the maximal effect from filling is observed at 0.1% MWCNT-red, whereas at 0.25%
nanofiller effect of constrained dynamics disappears. The latter could, obviously, be
associated with the enhanced nanotube aggregation [26].
Thus, DMA data obtained for neat PU and the studied nanocomposites suggest the pronounced dynamic heterogeneity within their glass transitions. Such
supposition was confirmed by the dependencies of the effective activation energy
Q of segmental motion within the glass transition as a function of temperature,
as obtained from the DMA experiments performed at different frequencies and
also from the discrete creep rate spectra. These results allowed a comprehensive
understanding of the origin of the complicated glass transition dynamics in these
materials to be elucidated.
Figure 8.10 shows the DMA spectra obtained at 0.1, 1 and 10 Hz for neat PU
and for the nanocomposite with 0.25% MWCNT-ox, and Fig. 8.11 presents the
Q(T) dependencies obtained using these experimental data and the calculations
by formula (8.5). In general, the peculiar Q(T) plots directly indicate a very
broad dispersion of the effective activation barriers Q varying from 110 to
300 kJ mol −1 , where a few Q levels characterize the glass transitions of neat PU and
the nanocomposite based thereon. This means that the dynamics within their glass
transitions is formed by several constituent relaxations (dynamic modes) over the
temperature range from −60 to +150 ◦ C. The substantial discrepancy between both
dependencies obtained is observed only at temperatures between 50 and 150 ◦ C.
PU network includes the flexible PPG cross-links with a molecular weight of
2.000 g mol −1 , that is, each cross-link includes about five Kuhn segments, and
the more rigid PU network junctions. Judging by the Q and temperature values,
the Q(T) plots manifest the following dynamic modes with increasing temperature.
First, the “normal” cooperative glass transition in PPG moieties between −60
and −50 ◦ C is practically undisturbed by the network junctions or being located
at some distance from them. This is confirmed by a high, cooperative value of
Q = 280–300 kJ mol −1 and the temperature position of this relaxation which is