Epoxy Molecular Structure Alteration in Graphene …
463
on lifetime components. Source contribution was evidenced at the level of 12%,
allowing compensation of input from positrons annihilated in the Kaptons foil (τ s =
0.352 ns). Other details of the measuring technique can be found elsewhere [51, 52].
3 Results and Discussion
3.1 Data Obtained by Dielectric Spectroscopy
Dynamic dielectric properties represent the response of the material under study,
the disturbance caused by an ac-electric field applied externally. Quantitatively, the
dielectric properties can be described in the terms of the complex dielectric permittivity ε ∗ (ω, T ) = ε 1 (ω, T ) + jε 2 (ω, T ), where ε 1 is the dielectric permittivity and
ε 2 is the dielectric loss factor. In the general case, both ambient temperature (T ) and
frequency of an external electric field (ω) have the strongest impact on the dielectric
properties of materials. Therefore, studying frequency-temperature dependencies of
both the dielectric constant and dielectric loss factor is one of the most convenient
and sensitive methods of studying material’s inner structure [45].
It is advisable to begin studying MLG-loading impact on epoxy dielectric properties at low-frequency region where dipole-related relaxation processes do not
contribute to the dielectric response of the materials studied. In the neat resin, such
response may originate from either absorbed water molecules or short-scale local
motions of lateral and terminal moieties the polymer macromolecules, whereas in
the nanocomposites—from both the polymer itself and the nanoplatelets (namely,
from local motion of the residual surface hydroxyls and other dipole contaminant
molecules populating the surfaces).
From the physical point of view, complex dielectric permittivity of the neat epoxy
over the frequency range under study is governed by two basic polarization mechanisms, namely the orientation and the hopping ones [53], and also by the related
effects of the dielectric relaxation, because, in accordance with the classic Debye
theory of polarization [53], the orientation (thermal) mechanism gives its contribution into the permittivity in the frequency region (ω/2π ) ranging from 10
5 to 10
9
Hz, whereas the hopping mechanism contributes at the frequencies below 10
5 Hz.
Frequency dependences of ε 1 for the nanocomposites are similar to those for the
unfilled resin and ε 1 decrease monotonously with increasing frequency in the range
from ~40 Hz to 1 MHz regardless of filler’s loading. The decreasing occurs due to
delay of hopping and reorientation displacement of less movable dipole moieties
[53].
Frequency-temperature dependences of both ε 1 and ε 2 are essentially dependent
on the loading. Figure 2 shows low-temperature (T = 95 K) loading dependences
of the dielectric permittivity ε 1 (Fig. 2a) and the dielectric loss factor ε 2 (Fig. 2b) at
fixed frequencies.
463
on lifetime components. Source contribution was evidenced at the level of 12%,
allowing compensation of input from positrons annihilated in the Kaptons foil (τ s =
0.352 ns). Other details of the measuring technique can be found elsewhere [51, 52].
3 Results and Discussion
3.1 Data Obtained by Dielectric Spectroscopy
Dynamic dielectric properties represent the response of the material under study,
the disturbance caused by an ac-electric field applied externally. Quantitatively, the
dielectric properties can be described in the terms of the complex dielectric permittivity ε ∗ (ω, T ) = ε 1 (ω, T ) + jε 2 (ω, T ), where ε 1 is the dielectric permittivity and
ε 2 is the dielectric loss factor. In the general case, both ambient temperature (T ) and
frequency of an external electric field (ω) have the strongest impact on the dielectric
properties of materials. Therefore, studying frequency-temperature dependencies of
both the dielectric constant and dielectric loss factor is one of the most convenient
and sensitive methods of studying material’s inner structure [45].
It is advisable to begin studying MLG-loading impact on epoxy dielectric properties at low-frequency region where dipole-related relaxation processes do not
contribute to the dielectric response of the materials studied. In the neat resin, such
response may originate from either absorbed water molecules or short-scale local
motions of lateral and terminal moieties the polymer macromolecules, whereas in
the nanocomposites—from both the polymer itself and the nanoplatelets (namely,
from local motion of the residual surface hydroxyls and other dipole contaminant
molecules populating the surfaces).
From the physical point of view, complex dielectric permittivity of the neat epoxy
over the frequency range under study is governed by two basic polarization mechanisms, namely the orientation and the hopping ones [53], and also by the related
effects of the dielectric relaxation, because, in accordance with the classic Debye
theory of polarization [53], the orientation (thermal) mechanism gives its contribution into the permittivity in the frequency region (ω/2π ) ranging from 10
5 to 10
9
Hz, whereas the hopping mechanism contributes at the frequencies below 10
5 Hz.
Frequency dependences of ε 1 for the nanocomposites are similar to those for the
unfilled resin and ε 1 decrease monotonously with increasing frequency in the range
from ~40 Hz to 1 MHz regardless of filler’s loading. The decreasing occurs due to
delay of hopping and reorientation displacement of less movable dipole moieties
[53].
Frequency-temperature dependences of both ε 1 and ε 2 are essentially dependent
on the loading. Figure 2 shows low-temperature (T = 95 K) loading dependences
of the dielectric permittivity ε 1 (Fig. 2a) and the dielectric loss factor ε 2 (Fig. 2b) at
fixed frequencies.